What do you know about supernovae? You will probably say that a supernova is a grandiose explosion of a star, in the place of which a neutron star or black hole remains.

However, not all supernovae are actually the final stage in the life of massive stars. The modern classification of supernova explosions, in addition to supergiant explosions, also includes some other phenomena.

Novas and supernovae

The term “supernova” migrated from the term “nova”. “Novae” were called stars that appeared in the sky almost from scratch, after which they gradually faded away. The first “new” ones are known from Chinese chronicles dating back to the second millennium BC. Interestingly, among these novae there were often supernovae. For example, it was a supernova in 1571 that was observed by Tycho Brahe, who subsequently coined the term “nova.” Now we know that in both cases we are not talking about the birth of new luminaries in the literal sense.

Novas and supernovae indicate a sharp increase in the brightness of a star or group of stars. As a rule, previously people did not have the opportunity to observe the stars that gave rise to these flares. These were objects too dim for the naked eye or astronomical instrument of that time. They were observed already at the moment of the flare, which naturally resembled the birth of a new star.

Despite the similarity of these phenomena, today there is a sharp difference in their definitions. The peak luminosity of supernovae is thousands and hundreds of thousands of times greater than the peak luminosity of novae. This discrepancy is explained by the fundamental difference in the nature of these phenomena.

The Birth of New Stars

The new flares are thermonuclear explosions occurring in some close star systems. Such systems also consist of a larger companion star (main sequence star, subgiant or). The white dwarf's powerful gravity pulls material from its companion star, causing an accretion disk to form around it. Thermonuclear processes occurring in the accretion disk at times lose stability and become explosive.

As a result of such an explosion, the brightness of the star system increases by thousands, or even hundreds of thousands of times. This is how a new star is born. A hitherto dim or even invisible object to an earthly observer acquires noticeable brightness. As a rule, such an outbreak reaches its peak in just a few days, and can fade away for years. Often such outbursts are repeated in the same system every few decades, i.e. are periodic. An expanding gas envelope is also observed around the new star.

Supernova explosions have a completely different and more diverse nature of their origin.

Supernovae are usually divided into two main classes (I and II). These classes can be called spectral, because they are distinguished by the presence and absence of hydrogen lines in their spectra. These classes are also noticeably different visually. All class I supernovae are similar both in explosion power and in the dynamics of brightness changes. Class II supernovae are very diverse in this regard. The power of their explosion and the dynamics of brightness changes lie in a very wide range.

All class II supernovae are generated by gravitational collapse in the interior of massive stars. In other words, this is the same explosion of supergiants that is familiar to us. Among the supernovae of the first class, there are those whose explosion mechanism is more likely to be similar to the explosion of new stars.

Death of the Supergiants

Stars whose mass exceeds 8-10 solar masses become supernovae. The cores of such stars, having exhausted hydrogen, proceed to thermonuclear reactions involving helium. Having exhausted helium, the nucleus proceeds to synthesize increasingly heavier elements. In the depths of the star, more and more layers are created, each of which has its own type of thermonuclear fusion. At the final stage of its evolution, such a star turns into a “layered” supergiant. The synthesis of iron occurs in its core, while closer to the surface the synthesis of helium from hydrogen continues.

The fusion of iron nuclei and heavier elements occurs with the absorption of energy. Therefore, having become iron, the supergiant core is no longer able to release energy to compensate for gravitational forces. The core loses its hydrodynamic equilibrium and begins to undergo random compression. The remaining layers of the star continue to maintain this equilibrium until the core contracts to a certain critical size. Now the remaining layers and the star as a whole are losing hydrodynamic equilibrium. Only in this case, it is not the compression that “wins,” but the energy released during the collapse and further chaotic reactions. The outer shell is released - a supernova explosion.

Class differences

The different classes and subclasses of supernovae are explained by what the star was like before the explosion. For example, the absence of hydrogen in class I supernovae (subclasses Ib, Ic) is a consequence of the fact that the star itself did not have hydrogen. Most likely, part of its outer shell was lost during evolution in a close binary system. The spectrum of subclass Ic differs from Ib in the absence of helium.

In any case, supernovae of such classes occur in stars that do not have an outer hydrogen-helium shell. The remaining layers lie within fairly strict limits of their size and mass. This is explained by the fact that thermonuclear reactions replace each other with the onset of a certain critical stage. This is why the explosions of class Ic and class Ib stars are so similar. Their peak luminosity is approximately 1.5 billion times that of the Sun. They reach this luminosity in 2-3 days. After this, their brightness weakens by 5-7 times per month and slowly decreases in subsequent months.

Type II supernova stars had a hydrogen-helium envelope. Depending on the mass of the star and its other features, this shell may have different boundaries. This explains the wide range in supernova patterns. Their brightness can range from tens of millions to tens of billions of solar luminosities (excluding gamma-ray bursts - see below). And the dynamics of changes in brightness have a very different character.

White dwarf transformation

A special category of supernovae are flares. This is the only class of supernovae that can occur in elliptical galaxies. This feature suggests that these flares are not the product of the death of supergiants. Supergiants do not live to see their galaxies “grow old,” i.e. will become elliptical. Also, all flashes in this class have almost the same brightness. Thanks to this, type Ia supernovae are the “standard candles” of the Universe.

They arise according to a distinctively different pattern. As noted earlier, these explosions are somewhat similar in nature to new explosions. One scheme for their origin suggests that they also originate in a close system of a white dwarf and its companion star. However, unlike new stars, detonation of a different, more catastrophic type occurs here.

As it "devours" its companion, the white dwarf increases in mass until it reaches the Chandrasekhar limit. This limit, approximately equal to 1.38 solar masses, is the upper limit of the mass of a white dwarf, after which it turns into a neutron star. Such an event is accompanied by a thermonuclear explosion with a colossal release of energy, many orders of magnitude higher than a normal new explosion. The almost constant value of the Chandrasekhar limit explains such a small discrepancy in the brightness of various flares of this subclass. This brightness is almost 6 billion times higher than solar luminosity, and the dynamics of its change are the same as those of class Ib, Ic supernovae.

Hypernova explosions

Hypernovae are explosions whose energy is several orders of magnitude higher than the energy of typical supernovae. That is, in fact, they are hypernovae, very bright supernovae.

Typically, a hypernova is considered to be an explosion of supermassive stars, also called . The mass of such stars starts at 80 and often exceeds the theoretical limit of 150 solar masses. There are also versions that hypernovae can form during the annihilation of antimatter, the formation of a quark star, or the collision of two massive stars.

Hypernovae are remarkable in that they are the main cause of perhaps the most energy-intensive and rarest events in the Universe - gamma-ray bursts. The duration of gamma bursts ranges from hundredths of seconds to several hours. But most often they last 1-2 seconds. In these seconds, they emit energy similar to the energy of the Sun for all 10 billion years of its life! The nature of gamma-ray bursts is still largely unknown.

Progenitors of life

Despite all their catastrophic nature, supernovae can rightfully be called the progenitors of life in the Universe. The power of their explosion pushes the interstellar medium into the formation of gas and dust clouds and nebulae, in which stars are subsequently born. Another feature of them is that supernovae saturate the interstellar medium with heavy elements.

It is supernovae that give rise to all chemical elements that are heavier than iron. After all, as noted earlier, the synthesis of such elements requires energy. Only supernovae are capable of “charging” compound nuclei and neutrons for the energy-intensive production of new elements. The kinetic energy of the explosion carries them throughout space along with the elements formed in the depths of the exploding star. These include carbon, nitrogen and oxygen and other elements without which organic life is impossible.

Supernova Observation

Supernova explosions are extremely rare phenomena. Our galaxy, which contains more than a hundred billion stars, experiences only a few flares per century. According to chronicles and medieval astronomical sources, over the past two thousand years, only six supernovae visible to the naked eye have been recorded. Modern astronomers have never observed supernovae in our galaxy. The closest one occurred in 1987 in the Large Magellanic Cloud, one of the satellites of the Milky Way. Every year, scientists observe up to 60 supernovae occurring in other galaxies.

It is because of this rarity that supernovae are almost always observed already at the moment of their outburst. Events preceding it have almost never been observed, so the nature of supernovae still remains largely mysterious. Modern science is not able to accurately predict supernovae. Any candidate star can flare up only after millions of years. The most interesting in this regard is Betelgeuse, which has a very real opportunity to illuminate the earth’s sky in our lifetime.

Universal flares

Hypernova explosions are even rarer. In our galaxy, such an event occurs once every hundreds of thousands of years. However, gamma-ray bursts generated by hypernovae are observed almost daily. They are so powerful that they are recorded from almost all corners of the Universe.

For example, one of the gamma-ray bursts, located 7.5 billion light years away, could be seen with the naked eye. It happened in the Andromeda galaxy, and for a couple of seconds the earth’s sky was illuminated by a star with the brightness of the full moon. If it happened on the other side of our galaxy, a second Sun would appear against the background of the Milky Way! It turns out that the brightness of the flare is quadrillion times brighter than the Sun and millions of times brighter than our Galaxy. Considering that there are billions of galaxies in the Universe, it is not surprising why such events are recorded every day.

Impact on our planet

It is unlikely that supernovae could pose a threat to modern humanity and in any way affect our planet. Even a Betelgeuse explosion would only light up our sky for a few months. However, they certainly influenced us decisively in the past. An example of this is the first of five mass extinctions on Earth, which occurred 440 million years ago. According to one version, the cause of this extinction was a gamma-ray burst that occurred in our Galaxy.

More noteworthy is the completely different role of supernovae. As already noted, it is supernovae that create the chemical elements necessary for the emergence of carbon-based life. The earth's biosphere was no exception. The solar system was formed in a gas cloud that contained fragments of past explosions. It turns out that we all owe our appearance to the supernova.

Moreover, supernovae continued to influence the evolution of life on Earth. By increasing the radiation background of the planet, they forced organisms to mutate. We should also not forget about major extinctions. Surely supernovae have “made adjustments” to the earth’s biosphere more than once. After all, if it weren’t for those global extinctions, completely different species would now dominate the Earth.

The scale of stellar explosions

To clearly understand how much energy supernova explosions have, let us turn to the equation of mass and energy equivalent. According to him, every gram of matter contains a colossal amount of energy. So 1 gram of the substance is equivalent to the explosion of an atomic bomb detonated over Hiroshima. The energy of the Tsar Bomb is equivalent to three kilograms of matter.

Every second during thermonuclear processes in the depths of the Sun, 764 million tons of hydrogen are converted into 760 million tons of helium. Those. Every second the Sun emits energy equivalent to 4 million tons of matter. Only one two-billionth of the total energy of the Sun reaches the Earth, this is equivalent to two kilograms of mass. Therefore, they say that the explosion of the Tsar Bomba could be observed from Mars. By the way, the Sun delivers to Earth several hundred times more energy than humanity consumes. That is, in order to cover the annual energy needs of all modern humanity, only a few tons of matter need to be converted into energy.

Considering the above, imagine that the average supernova at its peak “burns” quadrillions of tons of matter. This corresponds to the mass of a large asteroid. The total energy of a supernova is equivalent to the mass of a planet or even a low-mass star. Finally, a gamma-ray burst, in seconds, or even a fraction of a second of its life, splashes out energy equivalent to the mass of the Sun!

Such different supernovae

The term "supernova" should not be associated solely with the explosion of stars. These phenomena are perhaps as diverse as the stars themselves are diverse. Science has yet to understand many of their secrets.

Stars don't live forever. They are also born and die. Some of them, like the Sun, exist for several billion years, calmly reach old age, and then slowly fade away. Others live much shorter and more turbulent lives and are also doomed to catastrophic death. Their existence is interrupted by a giant explosion, and then the star turns into a supernova. The light of a supernova illuminates space: its explosion is visible at a distance of many billions of light years. Suddenly a star appears in the sky where before, it would seem, there was nothing. Hence the name. The ancients believed that in such cases a new star actually lights up. Today we know that in fact a star is not born, but dies, but the name remains the same, supernova.

SUPERNOVA 1987A

On the night of February 23-24, 1987, in one of the galaxies closest to us. In the Large Magellanic Cloud, only 163,000 light years away, a supernova appeared in the constellation Doradus. It became visible even to the naked eye, in May it reached visible magnitude +3, and in subsequent months it gradually lost its brightness until it again became invisible without a telescope or binoculars.

Present and past

Supernova 1987A, as its name suggests, was the first supernova observed in 1987 and the first to be visible to the naked eye since the dawn of the telescope era. The fact is that the last supernova explosion in our Galaxy was observed back in 1604, when the telescope had not yet been invented.

But more importantly, star* 1987A gave modern agronomists the first opportunity to observe a supernova at a relatively short distance.

What was there before?

A study of supernova 1987A showed that it was a Type II supernova. That is, the progenitor star or predecessor star, which was discovered in earlier photographs of this part of the sky, turned out to be a blue supergiant, whose mass was almost 20 times the mass of the Sun. Thus, it was a very hot star that quickly ran out of its nuclear fuel.

The only thing left after the gigantic explosion was a rapidly expanding gas cloud, inside which no one had yet been able to discern a neutron star, whose appearance theoretically should have been expected. Some astronomers argue that the star is still shrouded in released gases, while others have hypothesized that a black hole is forming instead of a star.

LIFE OF A STAR

Stars are born as a result of gravitational compression of a cloud of interstellar matter, which, when heated, brings its central core to temperatures sufficient to initiate thermonuclear reactions. The subsequent development of an already ignited star depends on two factors: the initial mass and chemical composition, the first, in particular, determining the combustion rate. Stars with larger masses are hotter and lighter, but that's why they burn out earlier. Thus, the life of a massive star is shorter compared to a low-mass star.

Red giants

A star that burns hydrogen is said to be in its “primary phase.” Most of the life of any star coincides with this phase. For example, the Sun has been in the main phase for 5 billion years and will remain there for a long time, and when this period ends, our star will go into a short phase of instability, after which it will stabilize again, this time in the form of a red giant. The red giant is incomparably larger and brighter than the stars in the main phase, but also much cooler. Antares in the constellation Scorpio or Betelgeuse in the constellation Orion are prime examples of red giants. Their color can be immediately recognized even with the naked eye.

When the Sun turns into a red giant, its outer layers will “absorb” the planets Mercury and Venus and reach the Earth’s orbit. In the red giant phase, stars lose a significant part of the outer layers of their atmosphere, and these layers form a planetary nebula like M57, the Ring Nebula in the constellation Lyra, or M27, the Dumbbell Nebula in the constellation Vulpecula. Both are great for viewing through your telescope.

Road to the final

From this moment on, the further fate of the star inevitably depends on its mass. If it is less than 1.4 solar masses, then after the end of nuclear combustion such a star will be freed from its outer layers and will shrink to a white dwarf, the final stage of the evolution of a star with a small mass. It will take billions of years for the white dwarf to cool down and become invisible. In contrast, a high-mass star (at least 8 times more massive than the Sun), once it runs out of hydrogen, survives by burning gases heavier than hydrogen, such as helium and carbon. Having gone through a series of phases of compression and expansion, such a star after several million years experiences a catastrophic supernova explosion, ejecting a gigantic amount of its own matter into space, and turns into a supernova remnant. Within about a week, the supernova exceeds the brightness of all the stars in its galaxy, and then quickly darkens. A neutron star remains in the center, a small object with a gigantic density. If the mass of the star is even greater, as a result of the supernova explosion, not stars, but black holes appear.

TYPES OF SUPERNOVA

By studying the light coming from supernovae, astronomers have found that they are not all the same and can be classified depending on the chemical elements represented in their spectra. Hydrogen plays a special role here: if the spectrum of a supernova contains lines confirming the presence of hydrogen, then it is classified as type II; if there are no such lines, it is classified as type I. Type I supernovae are divided into subclasses la, lb and l, taking into account other elements of the spectrum.

Different nature of explosions

The classification of types and subtypes reflects the variety of mechanisms underlying the explosion and the different types of progenitor stars. Supernova explosions such as SN 1987A occur in the last evolutionary stage of a star with a large mass (more than 8 times the mass of the Sun).

Type lb and lc supernovae result from the collapse of the central parts of massive stars that have lost a significant part of their hydrogen envelope due to strong stellar winds or due to the transfer of matter to another star in a binary system.

Various predecessors

All supernovae of types lb, lc and II originate from Population I stars, that is, from young stars concentrated in the disks of spiral galaxies. Type la supernovae, in turn, originate from old Population II stars and can be observed in both elliptical galaxies and the cores of spiral galaxies. This type of supernova comes from a white dwarf that is part of a binary system and is pulling material from its neighbor. When the mass of a white dwarf reaches its stability limit (called the Chandrasekhar limit), a rapid process of fusion of carbon nuclei begins and an explosion occurs, as a result of which the star throws out most of its mass.

Different luminosity

Different classes of supernovae differ from each other not only in their spectrum, but also in the maximum luminosity they achieve in the explosion, and in how exactly this luminosity decreases over time. Type I supernovae are generally much brighter than Type II supernovae, but they also dim much faster. Type I supernovae last for a few hours to a few days at peak brightness, while Type II supernovae can last up to several months. A hypothesis was put forward according to which stars with a very large mass (several tens of times the mass of the Sun) explode even more violently, like “hypernovas,” and their core turns into a black hole.

SUPERNOVES IN HISTORY

Astronomers believe that on average one supernova explodes in our Galaxy every 100 years. However, the number of supernovae historically documented in the last two millennia does not reach even 10. One reason for this may be due to the fact that supernovae, especially type II, explode in spiral arms, where interstellar dust is much denser and, accordingly, can dim the glow supernova.

The first one I saw

Although scientists are considering other candidates, today it is generally accepted that the first observation of a supernova explosion in history dates back to 185 AD. It was documented by Chinese astronomers. In China, galactic supernova explosions were also observed in 386 and 393. Then more than 600 years passed, and finally, another supernova appeared in the sky: in 1006, a new star shone in the constellation Wolf, this time recorded, among other things, by Arab and European astronomers. This brightest star (whose apparent magnitude at its peak brightness reached -7.5) remained visible in the sky for more than a year.
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Crab Nebula

The supernova of 1054 was also exceptionally bright (maximum magnitude -6), but again it was noticed only by Chinese astronomers, and perhaps also by American Indians. This is probably the most famous supernova, since its remnant is the Crab Nebula in the constellation Taurus, which Charles Messier included in his catalog under number 1.

We also owe Chinese astronomers information about the appearance of a supernova in the constellation Cassiopeia in 1181. Another supernova exploded there, this time in 1572. This supernova was also noticed by European astronomers, including Tycho Brahe, who described both its appearance and the subsequent change in its brightness in his book “On the New Star,” whose name gave rise to the term that is commonly used to designate such stars.

Supernova Quiet

32 years later, in 1604, another supernova appeared in the sky. Tycho Brahe passed this information on to his student Johannes Kepler, who began tracking the “new star” and dedicated the book “On the New Star at the Foot of Ophiuchus” to it. This star, also observed by Galileo Galilei, remains today the last supernova visible to the naked eye to explode in our Galaxy.

However, there is no doubt that another supernova has exploded in the Milky Way, again in the constellation Cassiopeia (the constellation that holds the record for three galactic supernovae). Although there is no visual evidence of this event, astronomers have found a remnant of the star and calculate that it must correspond to an explosion that occurred in 1667.

Outside the Milky Way, in addition to supernova 1987A, astronomers also observed a second supernova, 1885, which exploded in the Andromeda galaxy.

Supernova Observation

Hunting for supernovae requires patience and the right method.

The first is necessary, since no one guarantees that you will be able to discover a supernova on the first evening. You can't do without the second one if you don't want to waste time and really want to increase your chances of discovering a supernova. The main problem is that it is physically impossible to predict when and where a supernova explosion will occur in one of the distant galaxies. So a supernova hunter must scan the sky every night, checking dozens of galaxies carefully selected for this purpose.

What do we have to do

One of the most common techniques is to point a telescope at a particular galaxy and compare its appearance with an earlier image (drawing, photograph, digital image), ideally at approximately the same magnification as the telescope with which the observations are made . If a supernova appeared there, it will immediately catch your eye. Today, many amateur astronomers have equipment worthy of a professional observatory, such as computer-controlled telescopes and CCD cameras that allow them to take photographs of the starry sky directly in digital format. But even today, many observers hunt for supernovae by simply pointing a telescope at a particular galaxy and looking through the eyepiece, hoping to see if another star appears somewhere.

Necessary equipment

Supernova hunting doesn't require overly sophisticated equipment. Of course, you need to consider the power of your telescope. The fact is that each instrument has a limiting magnitude, which depends on various factors, and the most important of them is the diameter of the lens (however, the brightness of the sky is also important, depending on light pollution: the smaller it is, the higher the limiting value). With your telescope, you can look at hundreds of galaxies looking for supernovae. However, before you begin observing, it is very important to have on hand celestial maps to identify galaxies, as well as drawings and photographs of the galaxies you plan to observe (there are dozens of resources on the Internet for supernova hunters), and, finally, an observation log where you will record data for each observation session.

Night difficulties

The more supernova hunters there are, the greater the chances of noticing their appearance immediately at the moment of explosion, which makes it possible to track their entire light curve. From this point of view, amateur astronomers provide extremely valuable assistance to professionals.

Supernova hunters must be prepared to endure the cold and humidity of the night. In addition, they will have to fight sleepiness (a thermos with hot coffee is always included in the basic equipment of lovers of night astronomical observations). But sooner or later their patience will be rewarded!

Supernova or supernova explosion- a phenomenon during which a star sharply changes its brightness by 4-8 orders of magnitude (a dozen magnitudes) followed by a relatively slow attenuation of the flare. It is the result of a cataclysmic process that occurs at the end of the evolution of some stars and is accompanied by the release of enormous energy.

As a rule, supernovae are observed after the fact, that is, when the event has already occurred and its radiation has reached the Earth. Therefore, the nature of supernovae was unclear for a long time. But now quite a few scenarios are proposed that lead to outbreaks of this kind, although the main provisions are already quite clear.

The explosion is accompanied by the ejection of a significant mass of matter from the outer shell of the star into interstellar space, and from the remaining part of the matter from the core of the exploded star, as a rule, a compact object is formed - a neutron star, if the mass of the star before the explosion was more than 8 solar masses (M ☉), or a black star a hole with a star mass over 20 M ☉ (the mass of the core remaining after the explosion is over 5 M ☉). Together they form a supernova remnant.

A comprehensive study of previously obtained spectra and light curves in combination with the study of remnants and possible progenitor stars makes it possible to build more detailed models and study the conditions that existed at the time of the outburst.

Among other things, the substance ejected during the flare largely contains products of thermonuclear fusion that occurred throughout the life of the star. It is thanks to supernovae that the Universe as a whole and each galaxy in particular chemically evolves.

The name reflects the historical process of studying stars whose brightness changes significantly over time, the so-called novae.

The name is made up of the label SN, followed by the year of opening, followed by a one- or two-letter designation. The first 26 supernovae of the current year receive single-letter designations, at the end of the name, from capital letters from A before Z. The remaining supernovae receive two-letter designations from lowercase letters: aa, ab, and so on. Unconfirmed supernovae are designated by letters PSN(eng. possible supernova) with celestial coordinates in the format: Jhhmmssss+ddmmsss.

The big picture

Light curves

The light curves for type I are highly similar: there is a sharp increase for 2-3 days, then it is replaced by a significant drop (by 3 magnitudes) for 25-40 days, followed by a slow weakening, almost linear on the magnitude scale. The average absolute magnitude of the maximum for Ia flares is M B = − 19.5 m (\textstyle M_(B)=-19.5^(m)), for Ib\c - .

But the light curves of type II are quite varied. For some, the curves resembled those for type I, only with a slower and longer decline in brightness until the linear stage began. Others, having reached a peak, stayed at it for up to 100 days, and then the brightness dropped sharply and reached a linear “tail.” The absolute magnitude of the maximum varies widely from − 20 m (\textstyle -20^(m)) before − 13 m (\textstyle -13^(m)). Average value for IIp - M B = − 18 m (\textstyle M_(B)=-18^(m)), for II-L M B = − 17 m (\textstyle M_(B)=-17^(m)).

Spectra

The above classification already contains some basic features of the spectra of supernovae of various types; let us dwell on what is not included. The first and very important feature, which for a long time prevented the interpretation of the obtained spectra, is that the main lines are very broad.

The spectra of type II and Ib\c supernovae are characterized by:

• The presence of narrow absorption features near the maximum brightness and narrow undisplaced emission components.
• Lines , , , observed in ultraviolet radiation.

Observations outside the optical range

Flash rate

The frequency of flares depends on the number of stars in the galaxy or, which is the same for ordinary galaxies, luminosity. A generally accepted quantity characterizing the frequency of flares in different types of galaxies is SNu:

1 S N u = 1 S N 10 10 L ⊙ (B) ∗ 100 y e a r (\displaystyle 1SNu=(\frac (1SN)(10^(10)L_(\odot )(B)*100year))),

Where L ⊙ (B) (\textstyle L_(\odot )(B))- luminosity of the Sun in filter B. For different types of flares its value is:

In this case, supernovae Ib/c and II gravitate toward spiral arms.

Observing supernova remnants

The canonical scheme of the young remainder is as follows:

1. Possible compact remainder; usually a pulsar, but possibly a black hole
2. External shock wave propagating in interstellar matter.
3. A return wave propagating in the supernova ejecta material.
4. Secondary, propagating in clumps of the interstellar medium and in dense supernova emissions.

Together they form the following picture: behind the front of the external shock wave, the gas is heated to temperatures T S ≥ 10 7 K and emits in the X-ray range with a photon energy of 0.1-20 keV; similarly, the gas behind the front of the return wave forms a second region of X-ray radiation. Lines of highly ionized Fe, Si, S, etc. indicate the thermal nature of the radiation from both layers.

Optical radiation from the young remnant creates gas in clumps behind the front of the secondary wave. Since the propagation speed in them is higher, which means the gas cools faster and the radiation passes from the X-ray range to the optical range. The impact origin of the optical radiation is confirmed by the relative intensity of the lines.

Theoretical description

Decomposition of observations

The nature of supernovae Ia is different from the nature of other explosions. This is clearly evidenced by the absence of type Ib\c and type II flares in elliptical galaxies. From general information about the latter, it is known that there is little gas and blue stars there, and star formation ended 10 10 years ago. This means that all massive stars have already completed their evolution, and only stars with a mass less than the solar mass remain, and no more. From the theory of stellar evolution it is known that stars of this type cannot be exploded, and therefore a life extension mechanism is needed for stars with masses of 1-2M ⊙.

The absence of hydrogen lines in the Ia\Iax spectra indicates that there is extremely little hydrogen in the atmosphere of the original star. The mass of the ejected substance is quite large - 1M ⊙, mainly containing carbon, oxygen and other heavy elements. And the shifted Si II lines indicate that nuclear reactions are actively occurring during the ejection. All this convinces that the predecessor star is a white dwarf, most likely carbon-oxygen.

The attraction to the spiral arms of type Ib\c and type II supernovae indicates that the progenitor star is short-lived O-stars with a mass of 8-10M ⊙ .

Thermonuclear explosion

One of the ways to release the required amount of energy is a sharp increase in the mass of the substance involved in thermonuclear combustion, that is, a thermonuclear explosion. However, the physics of single stars does not allow this. Processes in stars located on the main sequence are in equilibrium. Therefore, all models consider the final stage of stellar evolution - white dwarfs. However, the latter itself is a stable star, and everything can change only when approaching the Chandrasekhar limit. This leads to the unambiguous conclusion that a thermonuclear explosion is possible only in multiple star systems, most likely in the so-called double stars.

In this scheme, there are two variables that influence the state, chemical composition and final mass of the substance involved in the explosion.

• The second companion is an ordinary star, from which matter flows to the first.
• The second companion is the same white dwarf. This scenario is called double degeneracy.

• An explosion occurs when the Chandrasekhar limit is exceeded.
• The explosion occurs before him.

What all supernova Ia scenarios have in common is that the exploding dwarf is most likely carbon-oxygen. In the explosive combustion wave traveling from the center to the surface, the following reactions occur:

The mass of the reacting substance determines the energy of the explosion and, accordingly, the maximum brightness. If we assume that the entire mass of the white dwarf reacts, then the energy of the explosion will be 2.2 10 51 erg.

The further behavior of the light curve is mainly determined by the decay chain:

The isotope 56 Ni is unstable and has a half-life of 6.1 days. Further e-capture leads to the formation of a 56 Co nucleus predominantly in an excited state with an energy of 1.72 MeV. This level is unstable, and the transition of the electron to the ground state is accompanied by the emission of a cascade of γ quanta with energies from 0.163 MeV to 1.56 MeV. These quanta experience Compton scattering, and their energy quickly decreases to ~100 keV. Such quanta are already effectively absorbed by the photoelectric effect, and, as a result, heat the substance. As the star expands, the density of matter in the star decreases, the number of photon collisions decreases, and the material on the star's surface becomes transparent to radiation. As theoretical calculations show, this situation occurs approximately 20-30 days after the star reaches its maximum luminosity.

60 days after the onset, the substance becomes transparent to γ-radiation. The light curve begins to decay exponentially. By this time, the 56 Ni isotope has already decayed, and the energy release is due to the β-decay of 56 Co to 56 Fe (T 1/2 = 77 days) with excitation energies up to 4.2 MeV.

Gravitational core collapse

The second scenario for the release of the necessary energy is the collapse of the star's core. Its mass should be exactly equal to the mass of its remnant - a neutron star, substituting typical values ​​we get:

where M = 0, and R = 10 km, G is the gravitational constant. The characteristic time for this is:

where ρ 12 is the density of the star, normalized to 10 12 g/cm 3 .

The resulting value is two orders of magnitude greater than the kinetic energy of the shell. A carrier is needed that, on the one hand, must carry away the released energy, and on the other, not interact with the substance. Neutrinos are suitable for the role of such a carrier.

Several processes are responsible for their formation. The first and most important for the destabilization of a star and the beginning of compression is the process of neutronization:

3 H e + e − → 3 H + ν e (\displaystyle ()^(3)He+e^(-)\to ()^(3)H+\nu _(e))

4 H e + e − → 3 H + n + ν e (\displaystyle ()^(4)He+e^(-)\to ()^(3)H+n+\nu _(e))

56 F e + e − → 56 M n + ν e (\displaystyle ()^(56)Fe+e^(-)\to ()^(56)Mn+\nu _(e))

Neutrinos from these reactions carry away 10%. The main role in cooling is played by URKA processes (neutrino cooling):

E + + n → ν ~ e + p (\displaystyle e^(+)+n\to (\tilde (\nu ))_(e)+p)

E − + p → ν e + n (\displaystyle e^(-)+p\to \nu _(e)+n)

Instead of protons and neutrons, atomic nuclei can also act, forming an unstable isotope that experiences beta decay:

E − + (A , Z) → (A , Z − 1) + ν e , (\displaystyle e^(-)+(A,Z)\to (A,Z-1)+\nu _(e) ,)

(A , Z − 1) → (A , Z) + e − + ν ~ e . (\displaystyle (A,Z-1)\to (A,Z)+e^(-)+(\tilde (\nu ))_(e).)

The intensity of these processes increases with compression, thereby accelerating it. This process is stopped by the scattering of neutrinos on degenerate electrons, during which they are thermolyzed and locked inside the substance. A sufficient concentration of degenerate electrons is achieved at densities ρ n u c = 2, 8 ⋅ 10 14 (\textstyle \rho _(nuc)=2,8\cdot 10^(14)) g/cm 3 .

Note that neutronization processes occur only at densities of 10 11 /cm 3, achievable only in the stellar core. This means that hydrodynamic equilibrium is disturbed only in it. The outer layers are in local hydrodynamic equilibrium, and collapse begins only after the central core contracts and forms a solid surface. The rebound from this surface ensures the release of the shell.

Model of a young supernova remnant

Supernova remnant evolution theory

There are three stages in the evolution of the supernova remnant:

The expansion of the shell stops at the moment when the pressure of the gas in the remnant equals the pressure of the gas in the interstellar medium. After this, the residue begins to dissipate, colliding with chaotically moving clouds. Resorption time reaches:

T m a x = 7 E 51 0.32 n 0 0.34 P ~ 0 , 4 − 0.7 (\displaystyle t_(max)=7E_(51)^(0.32)n_(0)^(0.34)(\tilde (P))_( 0.4)^(-0.7)) years

Theory of the occurrence of synchrotron radiation

Construction of a detailed description

Search for supernova remnants

Search for precursor stars

Supernova Ia theory

In addition to the uncertainties in the supernova Ia theories described above, the mechanism of the explosion itself has been a source of much controversy. Most often, models can be divided into the following groups:

• Instant detonation
• Delayed detonation
• Pulsating delayed detonation
• Turbulent fast combustion

At least for each combination of initial conditions, the listed mechanisms can be found in one variation or another. But the range of proposed models is not limited to this. An example is a model where two white dwarfs detonate at once. Naturally, this is only possible in scenarios where both components have evolved.

Chemical evolution and impact on the interstellar medium

Chemical evolution of the Universe. Origin of elements with atomic number higher than iron

Supernova explosions are the main source of replenishment of the interstellar medium with elements with atomic numbers greater (or as they say heavier) He . However, the processes that gave rise to them are different for different groups of elements and even isotopes.

R process

r-process is the process of the formation of heavier nuclei from lighter ones through the sequential capture of neutrons during ( n,γ) reactions and continues until the rate of neutron capture is higher than the rate of β− decay of the isotope. In other words, the average time of capture of n neutrons τ(n,γ) should be:

where τ β is the average time of β-decay of nuclei forming a chain of the r-process. This condition imposes a limitation on the neutron density, because:

Where (σ n γ , v n) ¯ (\displaystyle (\overline ((\sigma _(n\gamma),v_(n)))))- product of the reaction cross section ( n,γ) on the neutron velocity relative to the target nucleus, averaged over the Maxwellian spectrum of the velocity distribution. Considering that the r-process occurs in heavy and medium nuclei, 0.1 s< τ β < 100 с, то для n ~ 10 и температуры среды T = 10 9 , получим характерную плотность

Such conditions are achieved in:

ν-process

Main article: ν-process

ν-process is a process of nucleosynthesis through the interaction of neutrinos with atomic nuclei. It may be responsible for the appearance of the isotopes 7 Li, 11 B, 19 F, 138 La and 180 Ta

Impact on the large-scale structure of the galaxy's interstellar gas

Observation history

Hipparchus's interest in the fixed stars may have been inspired by the observation of a supernova (according to Pliny). Earliest record identified as supernova SN 185 (English), was made by Chinese astronomers in 185 AD. The brightest known supernova, SN 1006, has been described in detail by Chinese and Arab astronomers. The supernova SN 1054, which gave birth to the Crab Nebula, was well observed. Supernovae SN 1572 and SN 1604 were visible to the naked eye and were of great importance in the development of astronomy in Europe, as they were used as an argument against the Aristotelian idea that the world beyond the Moon and the Solar System is unchanging. Johannes Kepler began observing SN 1604 on October 17, 1604. This was the second supernova that was recorded at the stage of increasing brightness (after SN 1572, observed by Tycho Brahe in the constellation Cassiopeia).

With the development of telescopes, it became possible to observe supernovae in other galaxies, starting with observations of the supernova S Andromeda in the Andromeda Nebula in 1885. During the twentieth century, successful models for each type of supernova were developed and understanding of their role in star formation increased. In 1941, American astronomers Rudolf Minkowski and Fritz Zwicky developed a modern classification scheme for supernovae.

In the 1960s, astronomers discovered that the maximum luminosity of supernova explosions could be used as a standard candle, hence a measure of astronomical distances. Supernovae now provide important information about cosmological distances. The most distant supernovae turned out to be fainter than expected, which, according to modern ideas, shows that the expansion of the Universe is accelerating.

Methods have been developed to reconstruct the history of supernova explosions that have no written observational records. The date of the supernova Cassiopeia A was determined from the light echo from the nebula, while the age of the supernova remnant RX J0852.0-4622 (English) estimated by measuring temperature and γ emissions from the decay of titanium-44. In 2009, nitrates were discovered in Antarctic ice, consistent with the timing of the supernova explosion.

On February 23, 1987, supernova SN 1987A, the closest to Earth observed since the invention of the telescope, erupted in the Large Magellanic Cloud at a distance of 168 thousand light years from Earth. For the first time, the neutrino flux from the flare was recorded. The flare was intensively studied using astronomical satellites in the ultraviolet, X-ray and gamma-ray ranges. The supernova remnant was studied using ALMA, Hubble and Chandra. Neither a neutron star nor a black hole, which, according to some models, should be located at the site of the flare, have yet been discovered.

On January 22, 2014, the supernova SN 2014J erupted in the M82 galaxy, located in the constellation Ursa Major. Galaxy M82 is located 12 million light-years from our galaxy and has an apparent magnitude of just under 9. This supernova is the closest to Earth since 1987 (SN 1987A).

The most famous supernovae and their remnants

• Supernova SN 1604 (Kepler Supernova)
• Supernova G1.9+0.3 (The youngest known in our Galaxy)

Historical supernovae in our Galaxy (observed)

How many impressions are associated with these words among amateurs and professionals - space explorers. The word “new” itself carries a positive meaning, and “super” has a super positive meaning, but, unfortunately, it deceives the very essence. Supernovae can rather be called super-old stars, because they are practically the last stage of the development of a Star. So to speak, a bright eccentric apotheosis of star life. The flare sometimes eclipses the entire galaxy in which the dying star is located, and ends with its complete extinction.
Scientists have identified 2 types of Supernovae. One is affectionately nicknamed the explosion of a white dwarf (type I) which, compared to our sun, is denser, and at the same time much smaller in radius. Small, heavy White dwarf is the penultimate normal stage of the evolution of many stars. There is practically no hydrogen in the optical spectrum anymore. And if a white dwarf exists in a symbiosis of a binary system with another star, it draws its matter until it exceeds its limit. S. Chandresekhar in the 30s of the 20th century said that each dwarf has a clear limit of density and mass, exceeding which collapse occurs. It is impossible to shrink endlessly and sooner or later an explosion must occur! The second type of supernova formation is caused by the process of thermonuclear fusion, which forms heavy metals and contracts into itself, causing the temperature in the center of the star to rise. The core of the star is compressed more and more and neutronization processes (“grating” protons and electrons, during which both turn into neutrons) begin to occur in it, which leads to a loss of energy and cooling of the center of the star. All this provokes a rarefied atmosphere, and the shell rushes towards the core. Explosion! Myriads of small pieces of a star scatter throughout space, and a bright glow from a distant galaxy, where millions of years ago (the number of zeros in years of visibility of a star depends on its distance from Earth) the star exploded, is visible today to scientists of planet Earth. News of the tragedy of the past, another life cut short, a sad beauty that we can sometimes observe for centuries.

For example, the Crab Nebula, which can be seen through the telescope eye of modern observatories, is the consequences of a supernova explosion, which was seen by Chinese astronomers in 1054. It is so interesting to realize that what you are looking at today was admired for almost 1000 years by a person who no longer existed on Earth for a long time. This is the whole mystery of the Universe, its slow, drawn-out existence, which makes our life like a flash of a spark from a fire, it amazes and leads to some awe. Scientists have identified several of the most famous supernova explosions, which are designated according to a clearly defined scheme. Latin SuperNova is abbreviated to the characters SN, followed by the year of observation, and at the end the serial number in the year is written. Thus, the following names of famous supernovae can be seen:
The Crab Nebula - as mentioned earlier, it is the result of a supernova explosion, which is located at a distance of 6,500 light years from Earth, with a diameter today of 6,000 light years. This nebula continues to fly apart in different directions, although the explosion occurred just under 1000 years ago. And in its center there is a neutron star-pulsar, which rotates around its axis. Interestingly, at high brightness this nebula has a constant flow of energy, which allows it to be used as a reference point in the calibration of X-ray astronomy. Another find was supernova SN1572; as the name suggests, scientists observed the explosion in November 1572. By all indications, this star was a white dwarf. In 1604, for a whole year, Chinese, Korean, and then European astrologers could observe the explosion-glow of supernova SN1604, which was located in the constellation Ophiuchus. Johannes Kepler devoted his main work to its study, “On a new star in the constellation Ophiuchus,” and therefore the supernova was named after the scientist - SuperNova Kepler. The closest supernova explosion occurred in 1987 - SN1987A, located in the Large Magellanic Cloud 50 parsecs from our Sun, a dwarf galaxy - a satellite of the Milky Way. This explosion overturned some of the already established theories of stellar evolution. It was supposed that only red giants could flare up, but then, inopportunely, a blue one exploded! Blue supergiant (mass more than 17 solar masses) Sanduleak. The very beautiful remains of the planet form two unusual connecting rings, which scientists are studying today. The next supernova amazed scientists in 1993 - SN1993J, which before the explosion was a red supergiant. But the surprising thing is that the remnants, which were supposed to fade out after the explosion, on the contrary, began to gain brightness. Why?

A few years later, a satellite planet was discovered that was not damaged by the supernova explosion of its neighbor and created the conditions for the glow of the shell of the companion star that was torn off shortly before the explosion (neighbors are neighbors, but you can’t argue with gravity...), observed by scientists. This star is also predicted to become a red giant and a supernova. The explosion of the next supernova in 2006 (SN206gy) is recognized as the brightest glow in the entire history of observing these phenomena. This allowed scientists to put forward new theories of supernova explosions (such as quark stars, the collision of two massive planets, and others) and call this explosion a hypernova explosion! And the last interesting supernova is G1.9+0.3. For the first time, its signals as a radio source of the Galaxy were caught by the VLA radio telescope. And today the Chandra Observatory is studying it. The rate of expansion of the remains of the exploded star is amazing; it is 15,000 km per hour! Which is 5% of the speed of light!
In addition to these most interesting supernova explosions and their remnants, of course, there are other “everyday” events in space. But the fact remains that everything that surrounds us today is the result of supernova explosions. Indeed, in theory, at the beginning of its existence, the Universe consisted of light gases of helium and hydrogen, which, during the burning of stars, were transformed into other “building” elements for all currently existing planets. In other words, the Stars gave their lives for the birth of a new life!

Ancient annals and chronicles tell us that occasionally stars of exceptionally great brightness suddenly appeared in the sky. They quickly increased in brightness, and then slowly, over several months, faded away and ceased to be visible. Near maximum brightness, these stars were visible even during the day. The most striking outbreaks were in 1006 and 1054, information about which is contained in Chinese and Japanese treatises. In 1572, such a star flared up in the constellation Cassiopeia and was observed by the outstanding astronomer Tycho Brahe, and in 1604, a similar flare in the constellation Ophiuchus was observed by Johannes Kepler. Since then, during the four centuries of the “telescopic” era in astronomy, no such flares have been observed. However, with the development of observational astronomy, researchers began to detect a fairly large number of similar flares, although they did not reach very high brightness. These stars, suddenly appearing and soon disappearing as if without a trace, began to be called “novae”. It seemed that the stars of 1006 and 1054, the stars of Tycho and Kepler, were the same flares, only very close and therefore brighter. But it turned out that this was not the case. In 1885, astronomer Hartwig at the Tartu Observatory noticed the appearance of a new star in the well-known Andromeda nebula. This star reached the 6th visible magnitude, that is, the power of its radiation was only 4 times less than that of the entire nebula. Then this did not surprise astronomers: after all, the nature of the Andromeda nebula was unknown, it was assumed that it was just a cloud of dust and gas quite close to the Sun. Only in the 20s of the twentieth century did it finally become clear that the Andromeda nebula and other spiral nebulae are huge star systems, consisting of hundreds of billions of stars and millions of light years away from us. Flashes of ordinary novae, visible as objects of 17-18 magnitude, were also discovered in the Andromeda nebula. It became clear that the star of 1885 exceeded the Novaya Stars in radiation power tens of thousands of times; for a short time its brilliance was almost equal to the brilliance of a huge star system! Obviously, the nature of these outbreaks must be different. Later, these most powerful flares were called “Supernovae,” in which the prefix “super” meant their greater radiation power, and not their greater “novelty.”

Supernova Search and Observations

Supernova explosions began to be noticed quite often in photographs of distant galaxies, but these discoveries were accidental and could not provide the information necessary to explain the cause and mechanism of these grandiose outbreaks. However, in 1936, astronomers Baade and Zwicky, working at the Palomar Observatory in the USA, began a systematic systematic search for supernovae. They had at their disposal a telescope of the Schmidt system, which made it possible to photograph areas of several tens of square degrees and gave very clear images of even faint stars and galaxies. By comparing photographs of one area of ​​the sky taken several weeks later, one could easily notice the appearance of new stars in galaxies clearly visible in the photographs. Regions of the sky that were richest in nearby galaxies were selected for photography, where their number in one image could reach several dozen and the probability of detecting supernovae was greatest.

In 1937, Baada and Zwicky managed to discover 6 supernovae. Among them were quite bright stars 1937C and 1937D (astronomers decided to designate supernovae by adding letters to the year of discovery, showing the order of discovery in the current year), which reached a maximum of 8 and 12 magnitudes, respectively. For them, light curves were obtained - the dependence of the change in brightness over time - and a large number of spectrograms - photographs of the spectra of the star, showing the dependence of the radiation intensity on the wavelength. For several decades, this material became the basis for all researchers trying to unravel the causes of supernova explosions.

Unfortunately, the Second World War interrupted the observation program that had begun so successfully. The systematic search for supernovae at the Palomar Observatory was resumed only in 1958, but with a larger telescope of the Schmidt system, which made it possible to photograph stars up to magnitude 22-23. Since 1960, this work has been joined by a number of other observatories around the world where suitable telescopes were available. In the USSR, such work was carried out at the Crimean station of the SAI, where an astrograph telescope with a lens diameter of 40 cm and a very large field of view - almost 100 square degrees, was installed, and at the Abastumani Astrophysical Observatory in Georgia - on a Schmidt telescope with an entrance hole of 36 cm. And in Crimea, and in Abastumani, many supernova discoveries were made. Of the other observatories, the largest number of discoveries occurred at the Asiago Observatory in Italy, where two telescopes of the Schmidt system operated. But still, the Palomar Observatory remained a leader both in the number of discoveries and in the maximum magnitude of stars available for detection. Together, in the 60s and 70s, up to 20 supernovae were discovered per year, and their number began to grow rapidly. Immediately after the discovery, photometric and spectroscopic observations began on large telescopes.

In 1974, F. Zwicky died, and soon the search for supernovae at the Palomar Observatory was stopped. The number of supernovae discovered has decreased, but has begun to increase again since the early 1980s. New search programs were launched in the southern sky at the Cerro el Roble Observatory in Chile, and astronomy enthusiasts began to discover supernovae. It turned out that using small amateur telescopes with 20-30 cm lenses, one can quite successfully search for bright supernova explosions, systematically observing visually a specific set of galaxies. The greatest success was achieved by a priest from Australia, Robert Evans, who managed to discover up to 6 supernovae per year since the early 80s. It is not surprising that professional astronomers joked about its “direct connection with the heavens.”

In 1987, the brightest supernova of the 20th century was discovered - SN 1987A in the Large Magellanic Cloud galaxy, which is a “satellite” of our Galaxy and is only 55 kiloparsecs distant from us. For some time, this supernova was visible even to the naked eye, reaching a maximum brightness of about 4 magnitude. However, it could only be observed in the southern hemisphere. A series of photometric and spectral observations that were unique in their accuracy and duration were obtained for this supernova, and now astronomers continue to monitor how the process of transforming the supernova into an expanding gas nebula develops.

Supernova 1987A. Top left is a photograph of the area where the supernova exploded, taken long before the explosion. The soon-to-explode star is indicated by an arrow. Top right is a photograph of the same area of ​​the sky when the supernova was near maximum brightness. Below is what a supernova looks like 12 years after the explosion. The rings around the supernova are interstellar gas (partially ejected by the pre-supernova star before the outburst), ionized during the outburst and continuing to glow.

In the mid-80s, it became clear that the era of photography in astronomy was ending. Rapidly improved CCD receivers were many times superior to photographic emulsion in sensitivity and recorded wavelength range, while being practically equal in resolution. The image obtained by a CCD camera could be immediately seen on the computer screen and compared with those obtained earlier, but for photography the process of developing, drying and comparison took at best a day. The only remaining advantage of photographic plates - the ability to photograph large areas of the sky - also turned out to be insignificant for the search for supernovae: a telescope with a CCD camera could separately obtain images of all the galaxies falling on the photographic plate, in a time comparable to a photographic exposure. Projects of fully automated supernova search programs have appeared, in which the telescope is pointed at selected galaxies according to a pre-entered program, and the resulting images are compared by computer with those obtained previously. Only if a new object is detected does the computer send a signal to the astronomer, who finds out whether a supernova explosion has actually been detected. In the 90s, such a system, using an 80-cm reflecting telescope, began to operate at the Lick Observatory (USA).

The availability of simple CCD cameras for astronomy enthusiasts has led to the fact that they are moving from visual observations to CCD observations, and then stars up to 18th and even 19th magnitude become available for telescopes with 20-30 cm lenses. The introduction of automated searches and the growing number of amateur astronomers searching for supernovae using CCD cameras has led to an explosion in the number of discoveries: there are now more than 100 supernovae discovered per year, and the total number of discoveries has exceeded 1,500. In recent years, a search has also been launched for very distant and faint supernovae on the largest telescopes with a mirror diameter of 3-4 meters. It turned out that studies of supernovae, reaching a maximum brightness of 23-24 magnitudes, can provide answers to many questions about the structure and fate of the entire Universe. In one night of observations with such telescopes equipped with the most advanced CCD cameras, more than 10 distant supernovae can be discovered! Several images of such supernovae are shown in the figure below.

For almost all supernovae currently being discovered, it is possible to obtain at least one spectrum, and for many the light curves are known (this is also a great merit of amateur astronomers). So the volume of observational material available for analysis is very large, and it would seem that all questions about the nature of these grandiose phenomena must be resolved. Unfortunately, this is not the case yet. Let us take a closer look at the main questions facing supernova researchers and the most likely answers to them today.

Supernova classification, light curves and spectra

Before drawing any conclusions about the physical nature of a phenomenon, it is necessary to have a complete understanding of its observable manifestations, which must be properly classified. Naturally, the very first question that arose before supernova researchers was whether they were the same, and if not, how different they were and whether they could be classified. Already the first supernovae discovered by Baade and Zwicky showed significant differences in light curves and spectra. In 1941, R. Minkowski proposed dividing supernovae into two main types based on the nature of their spectra. He classified supernovae as type I, the spectra of which were completely different from the spectra of all objects known at that time. The lines of the most common element in the Universe - hydrogen - were completely absent, the entire spectrum consisted of broad maxima and minima that could not be identified, the ultraviolet part of the spectrum was very weak. Supernovae were classified as type II, the spectra of which showed some similarity with “ordinary” novae in the presence of very intense hydrogen emission lines; the ultraviolet part of their spectrum is bright.

The spectra of Type I supernovae remained mysterious for three decades. Only after Yu.P. Pskovsky showed that the bands in the spectra are nothing more than sections of the continuous spectrum between wide and rather deep absorption lines, did the identification of the spectra of type I supernovae move forward. A number of absorption lines were identified, primarily the most intense lines of singly ionized calcium and silicon. The wavelengths of these lines are shifted to the violet side of the spectrum due to the Doppler effect in the shell expanding at a speed of 10-15 thousand km per second. It is extremely difficult to identify all the lines in the spectra of type I supernovae, since they are greatly expanded and overlap each other; In addition to the mentioned calcium and silicon, it was possible to identify the lines of magnesium and iron.

Analysis of supernova spectra allowed us to draw important conclusions: there is almost no hydrogen in the shells ejected during a type I supernova explosion; while the composition of type II supernova shells is almost the same as that of the solar atmosphere. The expansion speed of the shells is from 5 to 15-20 thousand km/s, the temperature of the photosphere is around the maximum - 10-20 thousand degrees. The temperature drops quickly and after 1-2 months reaches 5-6 thousand degrees.

The light curves of supernovae also differed: for type I they were all very similar, they have a characteristic shape with a very rapid increase in brightness to the maximum, which lasts no more than 2-3 days, a rapid decrease in brightness by 3 magnitudes in 25-40 days and subsequent slow decay, almost linear on the magnitude scale, which corresponds to an exponential decay of luminosity.

The light curves of type II supernovae turned out to be much more diverse. Some were similar to the light curves of type I supernovae, only with a slower and longer decline in brightness until the beginning of a linear “tail”; for others, immediately after the maximum, a region of almost constant brightness began - the so-called “plateau”, which can last up to 100 days. Then the shine drops sharply and reaches a linear “tail”. All early light curves were obtained from photographic observations in the so-called photographic magnitude system, corresponding to the sensitivity of conventional photographic plates (wavelength range 3500-5000 A). The use of a photovisual system (5000-6000 A) in addition to it made it possible to obtain important information about the change in the color index (or simply “color”) of supernovae: it turned out that after the maximum, supernovae of both types continuously “turn red,” that is, the main part of the radiation shifts towards longer waves. This reddening stops at the stage of linear decline in brightness and may even be replaced by the “blueness” of supernovae.

In addition, type I and type II supernovae differed in the types of galaxies in which they exploded. Type II supernovae have only been discovered in spiral galaxies where stars are currently still forming and there are both old, low-mass stars and young, massive, and “short-lived” (only a few million years) stars. Type I supernovae occur in both spiral and elliptical galaxies, where intense star formation is not thought to have occurred for billions of years.

In this form, the classification of supernovae was maintained until the mid-80s. The beginning of widespread use of CCD receivers in astronomy has made it possible to significantly increase the quantity and quality of observational material. Modern equipment made it possible to obtain spectrograms for faint, previously inaccessible objects; with much greater accuracy it was possible to determine the intensities and widths of lines and register weaker lines in the spectra. CCD receivers, infrared detectors, and instruments mounted on spacecraft have made it possible to observe supernovae across the entire range of optical radiation from ultraviolet to far-infrared; Gamma-ray, X-ray and radio observations of supernovae were also carried out.

As a result, the seemingly established binary classification of supernovae began to quickly change and become more complex. It turned out that type I supernovae are not nearly as homogeneous as it seemed. Significant differences were found in the spectra of these supernovae, the most significant of which was the intensity of the singly ionized silicon line, observed at a wavelength of about 6100 A. For most type I supernovae, this absorption line near maximum brightness was the most noticeable feature in the spectrum, but for some supernovae it was practically absent, and the helium absorption lines were the most intense.

These supernovae were designated Ib, and the “classical” Type I supernovae became designated Ia. Later it turned out that some Ib supernovae also lack helium lines, and they were called type Ic. These new types of supernovae differed from the “classical” Ia ones in their light curves, which turned out to be quite diverse, although similar in shape to the light curves of Ia supernovae. Type Ib/c supernovae also turned out to be sources of radio emission. All of them were discovered in spiral galaxies, in regions where star formation may have recently occurred and fairly massive stars still exist.

The light curves of supernovae Ia in the red and infrared spectral ranges (R, I, J, H, K bands) were very different from the previously studied curves in the B and V bands. If a “shoulder” is noticeable on the curve in R 20 days after the maximum, then in filter I and longer wavelength ranges a real second maximum appears. However, some Ia supernovae do not have this second maximum. These supernovae are also distinguished by their red color at maximum brightness, reduced luminosity, and some spectral features. The first such supernova was SN 1991bg, and objects similar to it are still called peculiar supernovae Ia or “type 1991bg supernovae.” Another type of supernova Ia, on the contrary, is characterized by increased luminosity at maximum. They are characterized by lower intensities of absorption lines in the spectra. The "prototype" for them is SN 1991T.

Back in the 1970s, type II supernovae were divided according to the nature of their light curves into “linear” (II-L) and those with a “plateau” (II-P). Subsequently, more and more supernovae II began to be discovered, showing certain features in their light curves and spectra. Thus, in their light curves, the two brightest supernovae of recent years sharply differ from other type II supernovae: 1987A and 1993J. Both had two maxima in their light curves: after the flare, the brightness quickly fell, then began to increase again, and only after the second maximum did the final weakening of luminosity begin. Unlike supernovae Ia, the second maximum was observed in all spectral ranges, and for SN 1987A it was much brighter than the first in longer wavelength ranges.

Among the spectral features, the most frequent and noticeable was the presence, along with broad emission lines characteristic of expanding shells, also of a system of narrow emission or absorption lines. This phenomenon is most likely due to the presence of a dense shell surrounding the star before the outburst; such supernovae are designated II-n.

Supernova Statistics

How often do supernovae occur and how are they distributed in galaxies? Statistical studies of supernovae should answer these questions.

It would seem that the answer to the first question is quite simple: you need to observe several galaxies for a sufficiently long time, count the supernovae observed in them and divide the number of supernovae by the observation time. But it turned out that the time covered by fairly regular observations was still too short for definite conclusions for individual galaxies: in most, only one or two flares were observed. True, a fairly large number of supernovae have already been registered in some galaxies: the record holder is the galaxy NGC 6946, in which 6 supernovae have been discovered since 1917. However, these data do not provide accurate data on the frequency of outbreaks. Firstly, the exact time of observations of this galaxy is unknown, and secondly, the almost simultaneous outbursts for us could actually be separated by fairly large periods of time: after all, light from supernovae travels a different path inside the galaxy, and its size in light years is much larger than the observation time. It is currently possible to estimate the flare frequency only for a certain set of galaxies. To do this, it is necessary to use observational data from the search for supernovae: each observation gives some “effective tracking time” for each galaxy, which depends on the distance to the galaxy, on the limiting magnitude of the search and on the nature of the supernova light curve. For different types of supernovae, the observation time of the same galaxy will be different. When combining results for several galaxies, it is necessary to take into account their differences in mass and luminosity, as well as in morphological type. Currently, it is customary to normalize the results to the luminosity of galaxies and combine data only for galaxies with similar types. Recent work, based on combining data from several supernova search programs, has yielded the following results: only type Ia supernovae are observed in elliptical galaxies, and in an “average” galaxy with a luminosity of 10 10 solar luminosities, one supernova erupts approximately once every 500 years. In a spiral galaxy of the same luminosity, Ia supernovae explode at only a slightly higher frequency, but Type II and Ib/c supernovae are added to them, and the total burst rate is approximately once every 100 years. The frequency of flares is approximately proportional to the luminosity of galaxies, that is, in giant galaxies it is much higher: in particular, NGC 6946 is a spiral galaxy with a luminosity of 2.8 10 10 solar luminosities, therefore, about three flares can be expected in it per 100 years, and 6 supernovae observed in it can be considered not a very large deviation from the average frequency. Our Galaxy is smaller than NGC 6946, and one outburst can be expected in it on average every 50 years. However, it is known that only four supernovae have been observed in the Galaxy over the past millennium. Is there a contradiction here? It turns out not - after all, most of the Galaxy is hidden from us by layers of gas and dust, and the vicinity of the Sun, in which these 4 supernovae were observed, constitutes only a small part of the Galaxy.

How are supernovae distributed within galaxies? Of course, for now it is possible to study only summary distributions reduced to some “average” galaxy, as well as distributions relative to the details of the structure of spiral galaxies. These parts include, first of all, spiral sleeves; in fairly close galaxies, regions of active star formation are also clearly visible, identified by clouds of ionized hydrogen - the H II region, or by clusters of bright blue stars - the OB association. Studies of the spatial distribution, repeated many times as the number of discovered supernovae increased, yielded the following results. The distributions of supernovae of all types by distance from the centers of galaxies differ little from each other and are similar to the distribution of luminosity - the density decreases from the center to the edges according to an exponential law. The differences between the types of supernovae are manifested in the distribution relative to the star formation regions: if supernovae of all types are concentrated in the spiral arms, then only supernovae of types II and Ib/c are concentrated in the H II regions. We can conclude that the lifetime of a star producing a type II or Ib/c flare is from 10 6 to 10 7 years, and for type Ia it is about 10 8 years. However, supernovae Ia are also observed in elliptical galaxies, where it is believed that there are no stars younger than 10 9 years. There are two possible explanations for this contradiction - either the nature of supernova Ia explosions in spiral and elliptical galaxies is different, or star formation still continues in some elliptical galaxies and younger stars are present.

Theoretical models

Based on the totality of observational data, the researchers came to the conclusion that a supernova explosion should be the last stage in the evolution of a star, after which it ceases to exist in its previous form. Indeed, the supernova explosion energy is estimated as 10 50 - 10 51 erg, which exceeds the typical values ​​of the gravitational binding energy of stars. The energy released during a supernova explosion is more than enough to completely disperse the star's matter in space. What kind of stars and when do they end their lives with a supernova explosion, what is the nature of the processes leading to such a gigantic release of energy?

Observational data show that supernovae are divided into several types, differing in the chemical composition of the shells and their masses, in the nature of energy release and in their connection with different types of stellar populations. Type II supernovae are clearly associated with young, massive stars, and their shells contain large amounts of hydrogen. Therefore, their flares are considered the final stage of the evolution of stars whose initial mass is more than 8-10 solar masses. In the central parts of such stars, energy is released during nuclear fusion reactions, ranging from the simplest - the formation of helium during the fusion of hydrogen nuclei, and ending with the formation of iron nuclei from silicon. Iron nuclei are the most stable in nature, and no energy is released when they fuse. Thus, when the core of a star becomes iron, the release of energy in it stops. The core cannot resist gravitational forces and quickly contracts - collapses. The processes occurring during collapse are still far from being fully explained. However, it is known that if all the matter in the core of a star turns into neutrons, then it can resist the forces of gravity. The star's core turns into a "neutron star" and the collapse stops. In this case, enormous energy is released, entering the shell of the star and causing it to begin expansion, which we see as a supernova explosion. If the evolution of the star had previously occurred “quietly,” then its envelope should have a radius hundreds of times greater than the radius of the Sun, and retain a sufficient amount of hydrogen to explain the spectrum of type II supernovae. If most of the shell was lost during evolution in a close binary system or in some other way, then there will be no hydrogen lines in the spectrum - we will see a type Ib or Ic supernova.

In less massive stars, evolution proceeds differently. After burning hydrogen, the core becomes helium, and the reaction of converting helium into carbon begins. However, the core does not heat up to such a high temperature that fusion reactions involving carbon begin. The nucleus cannot release enough energy and contracts, but in this case the compression is stopped by the electrons located in the nucleus. The core of the star turns into a so-called “white dwarf”, and the shell dissipates in space in the form of a planetary nebula. Indian astrophysicist S. Chandrasekhar showed that a white dwarf can only exist if its mass is less than about 1.4 solar masses. If the white dwarf is located in a sufficiently close binary system, then matter may begin to flow from the ordinary star to the white dwarf. The mass of the white dwarf gradually increases, and when it exceeds the limit, an explosion occurs, during which rapid thermonuclear combustion of carbon and oxygen occurs, turning into radioactive nickel. The star is completely destroyed, and in the expanding shell there is radioactive decay of nickel into cobalt and then into iron, which provides energy for the glow of the shell. This is how Type Ia supernovae explode.

Modern theoretical studies of supernovae are mainly calculations on the most powerful computers of models of exploding stars. Unfortunately, it has not yet been possible to create a model that, from a late stage of star evolution, would lead to a supernova explosion and its observable manifestations. However, existing models describe the light curves and spectra of the vast majority of supernovae quite well. Usually this is a model of the shell of a star, into which the energy of the explosion is “manually” invested, after which its expansion and heating begins. Despite the great difficulties associated with the complexity and diversity of physical processes, great progress has been made in this area of ​​research in recent years.

Impact of Supernovae on the Environment

Supernova explosions have a strong and diverse impact on the surrounding interstellar medium. The supernova envelope, ejected at enormous speed, scoops up and compresses the gas surrounding it. Perhaps this could give rise to the formation of new stars from clouds of gas. The energy of the explosion is so great that the synthesis of new elements occurs, especially those heavier than iron. Material enriched in heavy elements is scattered by supernova explosions throughout the galaxy, resulting in stars formed after supernova explosions containing more heavy elements. The interstellar medium in “our” region of the Milky Way turned out to be so enriched in heavy elements that the emergence of life on Earth became possible. Supernovae are directly responsible for this! Supernovae, apparently, also generate streams of particles with very high energy - cosmic rays. These particles, penetrating to the Earth's surface through the atmosphere, can cause genetic mutations, due to which the evolution of life on Earth occurs.

Supernovae tell us about the fate of the Universe

Supernovae, and especially Type Ia supernovae, are among the brightest star-shaped objects in the Universe. Therefore, even very distant supernovae can be studied with currently available equipment.

Many supernovae Ia have been discovered in fairly close galaxies, the distance to which can be determined in several ways. Currently, the most accurate method is considered to be the determination of distances based on the apparent brightness of bright variable stars of a certain type - Cepheids. Using the Space Telescope. Hubble discovered and studied a large number of Cepheids in galaxies distant from us at a distance of about 20 megaparsecs. Sufficiently accurate estimates of the distances to these galaxies made it possible to determine the luminosity of type Ia supernovae that erupted in them. If we assume that distant supernovae Ia have on average the same luminosity, then the distance to them can be estimated from the observed magnitude at maximum brightness.