A supernova is born as a result. Model of a young supernova remnant

right after the explosion depends a lot on luck. It is she who determines whether it will be possible to study the processes of the birth of a supernova, or whether one will have to guess about them in the wake of an explosion - a planetary nebula propagating from a former star. The number of telescopes built by man is not large enough to constantly observe the entire sky, especially in all regions of the electromagnetic radiation spectrum. Often, amateur astronomers come to the aid of scientists, directing their telescopes wherever they please, and not at interesting and important objects for study. But a supernova explosion can happen anywhere!

An example of help from amateur astronomers is a supernova in the spiral galaxy M51. Known as the Pinwheel Galaxy, it is very popular among lovers of observing the universe. The galaxy is located at a distance of 25 million light-years from us and is turned directly towards us with its plane, due to which it is very convenient to observe it. The galaxy has a satellite that is in contact with one of the arms of M51. Light from a star that exploded in the galaxy reached Earth in March 2011 and was recorded by amateur astronomers. The supernova soon received the official designation 2011dh and became the focus of both professional and amateur astronomers. “M51 is one of the closest galaxies to us, it is extremely beautiful and therefore widely known,” says Caltech employee Sheeler van Dyck.

The supernova 2011dh considered in detail turned out to belong to a rare type IIb class of explosions. Such explosions occur when a massive star is stripped of virtually all of its outer garb of hydrogen fuel, which is likely to be pulled over by its binary companion. After that, due to the lack of fuel, thermonuclear fusion stops, the star's radiation cannot resist gravity, which tends to compress the star, and it falls towards the center. This is one of the two ways of supernova explosions, and in such a scenario (a star falling on itself under the influence of gravity), only every tenth star gives rise to a type IIb explosion.

There are several well-established hypotheses regarding the general scheme for the birth of a Type IIb supernova, but reconstructing the exact chain of events is very difficult. Since a star cannot be said to go supernova very soon, it is impossible to prepare for its careful observation. Of course, studying the state of a star can suggest that it will soon become a supernova, but this is on the time scale of the Universe in millions of years, while observation requires knowing the time of the explosion with an accuracy of several years. Only occasionally do astronomers get lucky and have detailed pictures of the star before the explosion. In the case of the M51 galaxy, this situation takes place - due to the popularity of the galaxy, there are many images of it in which 2011dh has not yet exploded. “Within days of the discovery of the supernova, we turned to the archives of the Hubble Space Telescope. As it turns out, this telescope previously created a detailed mosaic of the M51 galaxy at different wavelengths,” says van Dyck. In 2005, when the Hubble telescope photographed the 2011dh region, there was only an inconspicuous yellow giant star in its place.

Observations of supernova 2011dh have shown that it does not fit well with the standard idea of ​​an explosion of a huge star. On the contrary, it is more suitable as the result of the explosion of a small star, for example, the yellow supergiant companion from Hubble images, which has lost almost all of its atmosphere. Under the influence of the gravity of a nearby giant, only its core remained from the star, which exploded. “We decided that the precursor to the supernova was an almost completely stripped star, blue and therefore invisible to Hubble,” says van Dyck. - The yellow giant hid its small blue companion with its radiation until it exploded. That is our conclusion."

Another team of researchers studying the star 2011dh came to the opposite conclusion, which coincides with the classical theory. It was the yellow giant that was the precursor of the supernova, according to Justin Mound, an employee of the Queen's University in Belfast. However, in March of this year, a supernova revealed a mystery to both teams. The problem was first noticed by van Dyck, who decided to collect additional information about 2011dh using the Hubble telescope. However, the device did not find a large yellow star in the old place. "We just wanted to watch the evolution of a supernova again," says van Dyck. “We could never have imagined that the yellow star would go somewhere.” Another team came to the same conclusion using ground-based telescopes: the giant has disappeared.

The disappearance of the yellow giant points to it as the true supernova precursor. Van Dyk's post resolves this controversy: "The other team was completely right, we were wrong." However, the study of supernova 2011dh does not end there. As the brightness of 2011dh fades, M51 will return to its pre-explosion state (albeit without one bright star). By the end of this year, the supernova should have dimmed enough to reveal the yellow supergiant's companion - if there was one, as the classic Type IIb supernova theory suggests. Several groups of astronomers have already reserved Hubble observation time to study the evolution of 2011dh. "We need to find a binary companion for the supernova," says van Dyck. “If it is discovered, there will be a confident understanding of the origin of such explosions.”

Ancient chronicles and chronicles tell us that occasionally stars of exceptionally high brightness suddenly appeared in the sky. They quickly increased in brightness, and then slowly, over several months, faded and ceased to be visible. Near maximum brightness, these stars were visible even during the day. The brightest 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 of 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, for four centuries of the "telescopic" era in astronomy, no such outbreaks have been observed. However, with the development of observational astronomy, researchers began to detect a fairly large number of similar flashes, although they did not reach very high brightness. These stars, suddenly appearing and soon as if disappearing without a trace, began to be called "New". It seemed that the stars of 1006 and 1054, the stars of Tycho and Kepler, were the same outbursts, only very close and therefore brighter. But it turned out that this was not the case. In 1885, the astronomer Hartwig at the observatory in Tartu noticed the appearance of a new star in the well-known Andromeda Nebula. This star reached the 6th apparent magnitude, that is, the power of its radiation was only 4 times less than from 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 1920s it finally became 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. In the Andromeda Nebula, flashes of ordinary New Stars, visible as objects of 17-18 magnitudes, were also detected. It became clear that the star of 1885 surpassed the New Stars in terms of radiation power by tens of thousands of times, for a short time its brightness was almost equal to the brightness of a huge star system! Obviously, the nature of these outbreaks must be different. Later, these most powerful flashes were called "Supernovae", in which the prefix "super" meant their greater radiation power, and not their greater "novelty".

Search and observations of supernovae

In photographs of distant galaxies, supernova explosions began to be noticed quite often, but these discoveries were accidental and could not provide the information necessary to explain the cause and mechanism of these grandiose flares. However, in 1936, astronomers Baade and Zwicky, who worked at the Palomar Observatory in the United States, began a systematic systematic search for supernovae. They had a Schmidt telescope at their disposal, which made it possible to photograph areas of several tens of square degrees and gave very clear images of even faint stars and galaxies. Comparing photographs of one region of the sky taken a few weeks later, one could easily notice the appearance of new stars in galaxies that are clearly visible in the photographs. The areas of the sky that were richest in nearby galaxies were selected for photographing, where their number in one image could reach several tens and the probability of detecting supernovae was the highest.

In 1937, Baade and Zwicky managed to discover 6 supernovae. Among them were rather bright stars 1937C and 1937D (astronomers decided to designate supernovae by adding letters to the year of discovery indicating 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 with 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 main one for all researchers who tried to unravel the causes of supernova explosions.

Unfortunately, World War II interrupted the observational 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 22-23 magnitudes. 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 inlet of 36 cm. Crimea, and in Abastumani, many supernova discoveries were made. Of the other observatories, the largest number of discoveries was made at the Asiago Observatory in Italy, where two telescopes of the Schmidt system were operating. But still, the Palomar Observatory remained the 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 per year were discovered, and their number began to grow rapidly. Immediately after the discovery, photometric and spectroscopic observations began with large telescopes.

In 1974, F. Zwicky died, and soon the search for supernovae at the Palomar Observatory was discontinued. The number of discovered supernovae has decreased, but since the beginning of the 1980s it has begun to grow again. New search programs were launched in the southern sky - at the Cerro el Roble Observatory in Chile, and astronomers began to discover supernovae. It turned out that with the help of small amateur telescopes with lenses of 20-30 cm, one can quite successfully search for bursts of bright supernovae by systematically observing a visually defined set of galaxies. The greatest success was achieved by the priest from Australia, Robert Evans, who managed to discover up to 6 supernovas a year since the early 80s. No wonder professional astronomers joked about his "direct connection to the heavens."

In 1987, the brightest supernova of the 20th century, SN 1987A, was discovered in the Large Magellanic Cloud galaxy, which is a "satellite" of our Galaxy and is only 55 kiloparsecs away 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. For this supernova, series of photometric and spectral observations, unique in accuracy and duration, were obtained, and now astronomers continue to monitor how the process of transformation of a supernova into an expanding gaseous nebula develops.

In the mid-80s, it became clear that the era of photography in astronomy was coming to an end. The rapidly improving CCD receivers were many times superior to the photographic emulsion in sensitivity and the recorded wavelength range, practically not inferior to it in resolution. The image obtained by the CCD camera could be immediately seen on the computer screen and compared with those obtained earlier, and for photography, the process of development, 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 image all the galaxies falling on a photographic plate in a time comparable to a photographic exposure. Projects of fully automated supernova search programs have appeared, in which the telescope, according to a previously entered program, is aimed at selected galaxies, and the obtained images are compared by a computer with those obtained earlier. Only if a new object is detected, the computer sends a signal to the astronomer, who finds out whether a supernova explosion has indeed been recorded. In the 1990s, such a system, using an 80-cm reflecting telescope, began to operate at the Lick Observatory (USA).

The availability of simple CCD cameras for amateur astronomers has led to the fact that they move from visual observations to CCD observations, and then stars up to 18 and even 19 magnitude become available for telescopes with lenses of 20-30 cm. The introduction of automated searches and the growth in the number of amateur astronomers searching for supernovae using CCD cameras has led to an explosion in the number of discoveries: now more than 100 supernovae are discovered per year, and the total number of discoveries has exceeded 1500. In recent years, searches for very distant and weak 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.

Nearly all currently discovered supernovae can obtain at least one spectrum, and many have known light curves (to the credit of amateur astronomers, too). So the amount of observational material available for analysis is very large, and it would seem that all questions about the nature of these grandiose phenomena should be resolved. Unfortunately, this is not the case yet. Let us consider in more detail the main questions facing supernova researchers and the most probable 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 observed manifestations, which must be properly classified. Naturally, the very first question that confronted supernova researchers was whether they are the same, and if not, how different and whether they can be classified. Already the first supernovae, discovered by Baade and Zwicky, showed significant differences in their light curves and spectra. In 1941, R. Minkowski proposed to divide supernovae into two main types according to the nature of the spectra. He attributed supernovae to 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 assigned to type II, the spectra of which showed some similarity with "ordinary" novae by the presence of very intense emission lines of hydrogen, the ultraviolet part of their spectrum is bright.

The spectra of Type I supernovae remained enigmatic for three decades. Only after Yu.P. Pskovskii showed that the bands in the spectra are nothing but segments of the continuous spectrum between broad and rather deep absorption lines, the identification of the spectra of Type I supernovae moved 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 superimposed on each other; in addition to the mentioned calcium and silicon, it was possible to identify the lines of magnesium and iron.

An analysis of the spectra of supernovae made it possible to draw important conclusions: there is almost no hydrogen in the shells ejected during a type I supernovae; while the composition of the shells of type II supernovae is almost the same as that of the solar atmosphere. The expansion rates of the shells are from 5 to 15-20 thousand km / s, the temperature of the photosphere is about the maximum - 10-20 thousand degrees. The temperature drops rapidly and after 1-2 months it reaches 5-6 thousand degrees.

The light curves of supernovae also differed: for type I, they were all very similar, have a characteristic shape with a very rapid increase in brightness to a maximum that 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 in the scale of stellar magnitudes, which corresponds to an exponential decay in luminosity.

The light curves of type II supernovae have turned out to be much more diverse. Some were similar to the light curves of type I supernovae, only with a slower and more prolonged fall in brightness to the beginning of a linear "tail", in others, immediately after the maximum, a region of almost constant brightness begins - the so-called "plateau", which can last up to 100 days. Then the brilliance drops sharply and enters a linear "tail". All early light curves were obtained from photographic observations in the so-called photographic magnitude system, corresponding to the sensitivity of ordinary photographic plates (wavelength interval 3500-5000 A). The use of a photovisual system in addition to it (5000-6000 A) 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, both types of supernovae continuously "redden", that is, the main part of the radiation shifts towards longer wavelengths. This reddening stops at the stage of a linear decrease in brightness and may even be replaced by a "bluer" supernovae.

In addition, type I and II supernovae differed in the types of galaxies in which they flared up. Type II supernovae have only been detected in spiral galaxies, where stars continue to form at the present time and where both low-mass old stars and young, massive and "short-lived" (only a few million years) stars are present. Type I supernovae erupt in both spiral and elliptical galaxies, where star formation is not thought to have been intense for billions of years.

The classification of supernovae remained in this form until the mid-1980s. The beginning of the widespread use of CCD receivers in astronomy made it possible to significantly increase the quantity and quality of observational material. Modern equipment made it possible to obtain spectrograms for faint, hitherto inaccessible objects; with much greater accuracy it was possible to determine the intensities and widths of the lines, to register weaker lines in the spectra. CCD receivers, infrared detectors and spacecraft-mounted instruments have made it possible to observe supernovae in the entire range of optical radiation from the ultraviolet to the far infrared; gamma, x-ray and radio observations of supernovae were also carried out.

As a result, the apparently established binary classification of supernovae began to rapidly change and become more complex. It turned out that type I supernovae are far from being 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 line of singly ionized silicon, observed at a wavelength of about 6100 A. For most type I supernovae, this absorption line near the brightness maximum 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 were designated Ia. Subsequently, 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 they are 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 have been found in spiral galaxies, in regions where star formation may have recently taken place and fairly massive stars still exist today.

The light curves of supernovae Ia in the red and infrared spectral ranges (bands R, I, J, H, K) differed greatly from the previously studied curves in the B and V bands. in the I filter and longer wavelengths, a true 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 like it are still called peculiar Ia supernovae or "1991bg-type supernovae". Another type of supernova Ia, on the contrary, is characterized by an increased luminosity at the maximum. They are characterized by lower intensities of absorption lines in the spectra. The "prototype" for them is SN 1991T.

As far back as the 1970s, Type II supernovae were divided according to the nature of their light curves into "linear" (II-L) and "plateau" (II-P) ones. In the future, more and more supernova II began to be discovered, showing certain features in the light curves and spectra. Thus, according to the light curves, two of the brightest supernovae of recent years, 1987A and 1993J, sharply differ from other type II supernovae. Both had two maxima in the light curves: after the outburst, the brightness rapidly dropped, then began to rise again, and only after the second maximum did the final luminosity decrease begin. In contrast to supernovae Ia, the second maximum was observed in all ranges of the spectrum, 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 the broad emission lines characteristic of expanding shells, also 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 outbreak, such supernovae were designated II-n.

Supernova statistics

How often do supernovae break out and how are they distributed in galaxies? These questions must be answered by statistical studies of supernovae.

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 is still too short for definite conclusions for individual galaxies: in most, only one or two outbursts were observed. True, in some galaxies a fairly large number of supernovae have already been registered: 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 outbursts, which are almost simultaneous for us, could actually be separated by quite large time intervals: after all, the light from supernovae travels different paths inside the galaxy, and its dimensions in light years are much larger than the observation time. So far, it is possible to obtain an estimate of the flare frequency only for a certain set of galaxies. To do this, it is necessary to use observational data on 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 supernovae of different types, the time of observation of the same galaxy will be different. Combining the results for several galaxies, one must take into account their difference in mass and luminosity, as well as in morphological type. At present, 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 given 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 flares up about once every 500 years. In a spiral galaxy of the same luminosity, supernovae Ia flare with only a slightly higher frequency, but supernovae of types II and Ib / c are added to them, and the total frequency of flares is about once every 100 years. The flare frequency 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 per 100 years can be expected in it, and 6 supernovae observed in it can be considered as not a very large deviation from the average frequency. Our Galaxy is smaller than NGC 6946, and one outburst can be expected in it every 50 years on average. However, only four supernovae in the Galaxy are known to have been observed in the last millennium. Is there a contradiction here? It turns out not - after all, most of the Galaxy is closed from us by layers of gas and dust, and the vicinity of the Sun, in which these 4 supernovae were observed, make up only a small part of the Galaxy.

How are supernovae distributed within galaxies? Of course, so far 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 arms; in fairly close galaxies, regions of active star formation are also clearly visible, distinguished by clouds of ionized hydrogen - the H II region, or by clusters of bright blue stars - the OB association. Repeatedly repeated as the number of discovered supernovae increases, studies of the spatial distribution have 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-forming regions: if supernovae of all types are concentrated towards the spiral arms, only supernovae of types II and Ib/c are concentrated towards the H II regions. It can be concluded 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, Ia supernovae are also observed in elliptical galaxies, where no stars are thought to be younger than 10 9 years old. There are two possible explanations for this contradiction - either the nature of Ia supernova 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 entire set 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 energy of a supernova explosion is estimated at 10 50 - 10 51 erg, which exceeds the typical values ​​of the gravitational binding energy of stars. The energy released during the supernova explosion is more than enough to completely disperse the matter of the star 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 shells and their masses, in the nature of energy release, and in connection with various types of stellar populations. Type II supernovae are clearly associated with young, massive stars, and hydrogen is present in large quantities in their shells. Therefore, their flares are considered the final stage in the evolution of stars, the initial mass of which 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 merge. Thus, when the core of a star becomes iron, the release of energy in it stops. The core cannot resist gravitational forces and quickly shrinks - collapses. The processes occurring during the collapse are still far from a complete explanation. 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 attraction. The core of the star turns into a "neutron star" and the collapse stops. In this case, a huge energy is released, which enters the shell of the star and causes it to start expanding, which we see as a supernova explosion. If the evolution of the star before that happened "quietly", then its shell should have a radius hundreds of times greater than the radius of the Sun, and retain enough 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 is not heated to such a high temperature that fusion reactions involving carbon begin. The nucleus cannot release enough energy and shrinks, however, in this case, the compression is stopped by the electrons in the substance of the nucleus. The core of the star turns into the 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 exist only if its mass is less than about 1.4 solar masses. If the white dwarf is in a sufficiently close binary system, then matter can begin to flow from an ordinary star to a white dwarf. The mass of the white dwarf gradually increases, and when it exceeds the limit, an explosion occurs, during which there is a rapid thermonuclear combustion of carbon and oxygen, which turn into radioactive nickel. The star is completely destroyed, and in the expanding shell there is a 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 would lead to a supernova explosion and its observable manifestations from the late stage of stellar evolution. However, the existing models adequately describe the light curves and spectra of the vast majority of supernovae. 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 success has been achieved in this direction of research in recent years.

The impact of supernovae on the environment

Supernova explosions have a strong and diverse effect on the surrounding interstellar medium. The shell of a supernova thrown off at a tremendous speed scoops up and compresses the gas surrounding it. Perhaps this can give impetus to the formation of new stars from clouds of gas. The energy of the explosion is so great that new elements are synthesized, especially those heavier than iron. Material enriched in heavy elements is scattered throughout the galaxy by supernova explosions, as a result, stars formed after supernova explosions contain 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 the surface of the Earth through the atmosphere, can cause genetic mutations, due to which the evolution of life on Earth takes place.

Supernovae tell us about the fate of the universe

Supernovae, and in particular Type Ia supernovae, are among the brightest starlike objects in the universe. Therefore, even very distant supernovae can be studied with currently available equipment.

Many Ia supernovae have been discovered in galaxies close enough that the distance to them can be determined in several ways. The most accurate is currently considered to be the determination of distances by the apparent brightness of bright variable stars of a certain type - Cepheids. With the help of the Space Telescope Hubble discovered and studied a large number of Cepheids in galaxies that are up to about 20 megaparsecs away from us. Sufficiently accurate estimates of the distances to these galaxies made it possible to determine the luminosity of type Ia supernovae that flared in them. If we assume that distant supernovae Ia have the same average luminosity, then the observed magnitude at maximum brightness can be used to estimate the distance to them.

One of the important achievements of the 20th century was the understanding of the fact that almost all elements heavier than hydrogen and helium are formed in the inner parts of stars and enter the interstellar medium as a result of supernova explosions, one of the most powerful phenomena in the universe.

Pictured: Brilliant stars and wisps of gas provide a breathtaking backdrop to the self-destruction of a massive star dubbed Supernova 1987A. Its explosion was observed by astronomers in the Southern Hemisphere on February 23, 1987. This Hubble image shows a supernova remnant surrounded by inner and outer rings of matter in diffuse clouds of gas. This three-color image is a composite of several photographs of the supernova and its neighboring region taken in September 1994, February 1996, and July 1997. Numerous bright blue stars near the supernova are massive stars, each of which is about 12 million years old and 6 times heavier than the Sun. They all belong to the same generation of stars as the one that exploded. The presence of bright gas clouds is another sign of the youth of this region, which is still fertile ground for the birth of new stars.

Initially, all stars whose brightness suddenly increased by more than 1,000 times were called novae. Flashing, such stars suddenly appeared in the sky, breaking the usual configuration of the constellation, and increased their brightness at the maximum, several thousand times, then their brightness began to drop sharply, and after a few years they became as weak as they were before the outbreak. The recurrence of flares, during each of which the star ejects up to one thousandth of its mass at high speed, is characteristic of new stars. And yet, for all the grandeur of the phenomenon of such a flash, it is not associated either with a radical change in the structure of the star, or with its destruction.

For five thousand years, information has been preserved about more than 200 bright outbursts of stars, if we restrict ourselves to those that did not exceed the brilliance of the 3rd magnitude. But when the extragalactic nature of the nebulae was established, it became clear that the novae that flared in them surpassed ordinary novae in their characteristics, since their luminosity often turned out to be equal to the luminosity of the entire galaxy in which they flared. The unusual nature of such phenomena led astronomers to the idea that such events are something completely different from ordinary new stars, and therefore, in 1934, at the suggestion of the American astronomers Fritz Zwicky and Walter Baade, those stars whose flashes reach the luminosities of normal galaxies at their maximum brightness were isolated into a separate, brightest in luminosity and rare class of supernovae.

In contrast to the outbursts of ordinary new stars, supernova outbursts in the current state of our Galaxy are an extremely rare phenomenon, occurring no more than once every 100 years. The most striking outbreaks were in 1006 and 1054; information about them is contained in Chinese and Japanese treatises. In 1572, the outstanding astronomer Tycho Brahe observed the outbreak of such a star in the constellation of Cassiopeia, while Johannes Kepler was the last to follow the supernova in the constellation of Ophiuchus in 1604. For four centuries of the "telescopic" era in astronomy, no such flares were observed in our Galaxy. The position of the solar system in it is such that observations of supernovae are optically available to us in about half of its volume, and in the rest of it the brightness of the outbreaks is muted by interstellar absorption. IN AND. Krasovsky and I.S. Shklovsky calculated that supernova explosions in our galaxy occur on average once every 100 years. In other galaxies, these processes occur with approximately the same frequency; therefore, the main information about supernovae in the optical outburst stage was obtained from observations of them in other galaxies.

Realizing the importance of studying such powerful phenomena, astronomers W. Baade and F. Zwicky, who worked at the Palomar Observatory in the USA, began a systematic systematic search for supernovae in 1936. They had a Schmidt telescope at their disposal, which made it possible to photograph areas of several tens of square degrees and gave very clear images of even faint stars and galaxies. Over the course of three years, they discovered 12 supernova explosions in different galaxies, which were then studied using photometry and spectroscopy. As observational technology improved, the number of newly discovered supernovae steadily increased, and the subsequent introduction of automated search led to an avalanche-like increase in the number of discoveries (more than 100 supernovae per year, with a total number of 1,500). In recent years, large telescopes have also begun searching for very distant and faint supernovae, since their research can provide answers to many questions about the structure and fate of the entire Universe. In one night of observations with such telescopes, more than 10 distant supernovae can be discovered.

As a result of the explosion of a star, which is observed as a supernova phenomenon, a nebula is formed around it, expanding at a tremendous speed (about 10,000 km / s). The high expansion rate is the main feature by which supernova remnants are distinguished from other nebulae. In the remnants of supernovae, everything speaks of an explosion of enormous power, which scattered the outer layers of the star and imparted enormous speeds to individual pieces of the ejected shell.

crab nebula

Not a single space object has given astronomers as much valuable information as the relatively small Crab Nebula observed in the constellation Taurus and consisting of a gaseous diffuse substance expanding at high speed. This nebula, which is the remnant of a supernova observed in 1054, was the first galactic object with which a radio source was identified. It turned out that the nature of radio emission has nothing to do with thermal radiation: its intensity systematically increases with wavelength. Soon it was possible to explain the nature of this phenomenon. There must be a strong magnetic field in the supernova remnant, which holds the cosmic rays (electrons, positrons, atomic nuclei) created by it, which have speeds close to the speed of light. In a magnetic field, they radiate electromagnetic energy in a narrow beam in the direction of motion. The discovery of non-thermal radio emission from the Crab Nebula prompted astronomers to search for supernova remnants precisely on this basis.

The nebula located in the constellation Cassiopeia turned out to be a particularly powerful source of radio emission; at meter wavelengths, the radio emission flux from it is 10 times higher than the flux from the Crab Nebula, although it is much farther than the latter. In optical beams this rapidly expanding nebula is very weak. The Cassiopeia Nebula is believed to be the remnant of a supernova explosion that took place about 300 years ago.

A system of filamentous nebulae in the constellation Cygnus also showed radio emission characteristic of old supernova remnants. Radio astronomy has helped to find many other non-thermal radio sources, which turned out to be the remnants of supernovae of different ages. Thus, it was concluded that the remnants of supernovae, even tens of thousands of years ago, stand out among other nebulae with their powerful non-thermal radio emission.

As already mentioned, the Crab Nebula was the first object in which X-ray emission was detected. In 1964, it was possible to discover that the source of X-ray radiation emanating from it is extended, although its angular dimensions are 5 times smaller than the angular dimensions of the Crab Nebula itself. From which it was concluded that X-rays are emitted not by a star that once erupted as a supernova, but by the nebula itself.

Supernova influence

On February 23, 1987, a supernova exploded in our neighboring galaxy, the Large Magellanic Cloud, which became extremely important for astronomers because it was the first one that they, armed with modern astronomical instruments, could study in detail. And this star gave confirmation of a whole series of predictions. Simultaneously with the optical flash, special detectors installed in Japan and Ohio (USA) registered a stream of neutrinos - elementary particles that are born at very high temperatures during the collapse of the star's core and easily penetrate through its shell. These observations confirmed the earlier assumption that about 10% of the mass of the collapsing stellar core is emitted as neutrinos at the moment when the core itself collapses into a neutron star. In very massive stars, during a supernova explosion, the cores are compressed to even greater densities and, probably, turn into black holes, but the outer layers of the star are still thrown off. In recent years, indications have appeared that some cosmic gamma-ray bursts are related to supernovae. It is possible that the nature of cosmic gamma-ray bursts is related to the nature of explosions.

Supernova explosions have a strong and diverse effect on the surrounding interstellar medium. The supernova shell, which is thrown off at a tremendous speed, scoops up and compresses the gas surrounding it, which can give impetus to the formation of new stars from the gas clouds. A team of astronomers led by Dr. John Hughes of Rutgers University, using observations from NASA's Chandra X-ray Observatory, has made an important discovery that sheds light on how silicon, iron, and other elements are formed in supernova explosions. An X-ray image of the supernova remnant Cassiopeia A (Cas A) reveals clumps of silicon, sulfur and iron ejected from the star's interior during the explosion.

The high quality, clarity and information content of the images of the Cas A supernova remnant obtained by the Chandra observatory allowed astronomers not only to determine the chemical composition of many nodes of this remnant, but also to find out exactly where these nodes were formed. For example, the most compact and bright nodes are composed mainly of silicon and sulfur with very little iron. This indicates that they formed deep inside the star, where temperatures reached three billion degrees during the collapse that ended in a supernova explosion. In other nodes, astronomers found a very high content of iron with impurities of a certain amount of silicon and sulfur. This substance was formed even deeper in those parts where the temperature during the explosion reached higher values ​​from four to five billion degrees. A comparison of the arrangements in the supernova remnant Cas A of both bright silicon-rich and fainter iron-rich nodes revealed that "iron" features originating from the deepest layers of the star are located at the outer edges of the remnant. This means that the explosion threw the "iron" nodes farther than all the others. And even now, they seem to be moving away from the center of the explosion at a faster rate. The study of the data obtained by Chandra will make it possible to dwell on one of several mechanisms proposed by theorists that explain the nature of a supernova explosion, the dynamics of the process, and the origin of new elements.

SN I supernovae have very similar spectra (with no hydrogen lines) and light curve shapes, while SN II spectra contain bright hydrogen lines and are distinguished by a variety of both spectra and light curves. In this form, the classification of supernovae existed until the mid-1980s. And with the beginning of the widespread use of CCD receivers, the quantity and quality of observational material increased significantly, which made it possible to obtain spectrograms for previously inaccessible faint objects, determine the intensity and width of lines with much greater accuracy, and also record weaker lines in the spectra. As a result, the apparently established binary classification of supernovae began to rapidly change and become more complex.

Supernovae are also distinguished by the types of galaxies in which they flare up. In spiral galaxies, supernovae of both types flare up, but in elliptical galaxies, where there is almost no interstellar medium and the star formation process has ended, only SN I type supernovae are observed, obviously, before the explosion these are very old stars, whose masses are close to solar. And since the spectra and light curves of supernovae of this type are very similar, it means that the same stars explode in spiral galaxies. The natural end of the evolutionary path of stars with masses close to the sun is the transformation into a white dwarf with the simultaneous formation of a planetary nebula. There is almost no hydrogen in the composition of a white dwarf, since it is the end product of the evolution of a normal star.

Several planetary nebulae are formed in our Galaxy every year, therefore, most of the stars of such a mass quietly complete their life, and only once every hundred years does an SN I type supernova burst. What reasons determine a very special ending, not similar to the fate of other stars of the same kind? The famous Indian astrophysicist S. Chandrasekhar showed that in the event that a white dwarf has a mass less than about 1.4 solar masses, it will calmly “live out” its life. But if it is in a sufficiently close binary system, its powerful gravity is able to "pull" matter from the companion star, which leads to a gradual increase in mass, and when it passes the allowable limit, a powerful explosion occurs, leading to the death of the star.

Supernovae SN II are clearly associated with young, massive stars, in the shells of which hydrogen is present in large quantities. Explosions of this type of supernovae are considered the final stage in the evolution of stars with an initial mass of more than 810 solar masses. In general, the evolution of such stars proceeds quite quickly in a few million years they burn their hydrogen, then helium, which turns into carbon, and then carbon atoms begin to transform into atoms with higher atomic numbers.

In nature, the transformations of elements with a large release of energy end in iron, the nuclei of which are the most stable, and no energy is released during their fusion. Thus, when the core of a star becomes iron, the release of energy in it stops, it can no longer resist gravitational forces, and therefore begins to quickly shrink, or collapse.

The processes that occur during collapse are still far from being fully understood. However, it is known that if all the matter of the core turns into neutrons, then it can resist the forces of attraction the core of the star turns into a "neutron star", and the collapse stops. In this case, huge energy is released, which enters the shell of the star and causes expansion, which we see as a supernova explosion.

From this, one would expect a genetic link between supernova explosions and the formation of neutron stars and black holes. If the evolution of the star before this happened “quietly”, then its shell should have a radius hundreds of times greater than the radius of the Sun, and also retain enough hydrogen to explain the spectrum of SN II supernovae.

Supernovae and pulsars

The fact that after a supernova explosion, in addition to an expanding shell and various types of radiation, other objects remain, it became known in 1968 due to the fact that a year earlier, radio astronomers discovered pulsars - radio sources, the radiation of which is concentrated in separate pulses, repeating after a strictly defined period of time. Scientists were struck by the strict periodicity of the pulses and the shortness of their periods. The pulsar attracted the most attention, the coordinates of which were close to the coordinates of a very interesting nebula for astronomers, located in the southern constellation of the Sails, which is considered the remnant of a supernova explosion its period was only 0.089 seconds. And after the discovery of a pulsar in the center of the Crab Nebula (its period was 1/30 of a second), it became clear that pulsars are somehow connected with supernova explosions. In January 1969, a pulsar from the Crab Nebula was identified with a faint 16th-magnitude star that changes its brightness with the same period, and in 1977, a pulsar in the constellation of Sails was also identified with a star.

The periodicity of the emission of pulsars is associated with their rapid rotation, but not a single ordinary star, even a white dwarf, could rotate with a period characteristic of pulsars it would be immediately torn apart by centrifugal forces, and only a neutron star, very dense and compact, could stand before them. As a result of analyzing many options, scientists came to the conclusion that supernova explosions are accompanied by the formation of neutron stars, a qualitatively new type of objects, the existence of which was predicted by the theory of evolution of large mass stars.

Supernovae and black holes

The first proof of a direct connection between a supernova explosion and the formation of a black hole was obtained by Spanish astronomers. As a result of the study of radiation emitted by a star orbiting a black hole in the Nova Scorpii 1994 binary system, it was found that it contains large amounts of oxygen, magnesium, silicon and sulfur. There is an assumption that these elements were captured by it when a nearby star, having survived a supernova explosion, turned into a black hole.

Supernovae (particularly Type Ia supernovae) are among the brightest stellar objects in the universe, so even the most distant ones can be explored with currently available equipment. Many Type Ia supernovae have been discovered in relatively nearby galaxies. Sufficiently accurate estimates of the distances to these galaxies made it possible to determine the luminosity of supernovae that burst out in them. If we assume that distant supernovae have the same average luminosity, then the observed magnitude at maximum brightness can also be used to estimate the distance to them. Comparison of the distance to the supernova with the removal rate (redshift) of the galaxy in which it exploded makes it possible to determine the main quantity characterizing the expansion of the Universe, the so-called Hubble constant.

Even 10 years ago, values ​​for it were obtained that differed by almost two times from 55 to 100 km/s Mpc, today the accuracy has been significantly increased, as a result of which a value of 72 km/s Mpc is accepted (with an error of about 10%) . For distant supernovae, the redshift of which is close to 1, the relationship between the distance and the redshift also makes it possible to determine quantities that depend on the density of matter in the Universe. According to Einstein's general theory of relativity, it is the density of matter that determines the curvature of space, and, consequently, the future fate of the universe. Namely: will it expand indefinitely or will this process ever stop and be replaced by contraction. Recent studies of supernovae have shown that most likely the density of matter in the universe is insufficient to stop the expansion, and it will continue. And in order to confirm this conclusion, new observations of supernovae are needed.

supernovae- stars ending their evolution in a catastrophic explosive process. This term was given to stars that flared much (by orders of magnitude) stronger than the so-called "new stars". In fact, neither one nor the other is physically new, already existing stars always flare up. But in several historical cases, those stars that were previously almost or completely invisible in the sky flared up, which created the effect of the appearance of a new star.

Type II supernovae

According to modern concepts, thermonuclear fusion eventually leads to the enrichment of the composition of the inner regions of the star with heavy elements. In the process of thermonuclear fusion and the formation of heavy elements, the star contracts, and the temperature in its center rises. (The effect of the negative heat capacity of gravitating non-degenerate matter.) If the mass of the star is large enough, then the process of thermonuclear fusion reaches its logical conclusion with the formation of iron and nickel nuclei, and the contraction continues. In this case, thermonuclear reactions will continue only in a certain layer of the star around the central core - where there is still unburned thermonuclear fuel. The central core contracts more and more, and at some point, due to pressure, neutronization reactions begin to take place in it - protons begin to absorb electrons, turning into neutrons. This causes a rapid loss of energy carried away by the resulting neutrinos (so-called neutrino cooling), so that the core of the star contracts and cools. The process of collapse of the central core is so fast that a rarefaction wave forms around it. Then, following the core, the shell also rushes to the center of the star. Further, the shell material rebounds from the nucleus and a shock wave propagating outward is formed, initiating thermonuclear reactions. In this case, sufficient energy is released to eject the supernova shell at high speed. Of great importance is the process of feeding the shock wave with the energy of neutrinos emerging from the central region. Such an explosion mechanism belongs to type II supernovae (SN II). Numerical simulations show that the rebound shock wave does not lead to a supernova explosion. It stops at a distance of about 100-200 km from the center of the star. Accounting for rotation and the presence of a magnetic field makes it possible to numerically simulate a supernova explosion (the magnetorotational mechanism of a supernova explosion with a collapsing core). It is believed that the formation of a type II supernova ends the evolution of all stars whose initial mass exceeds 8–10 solar masses. After the explosion, a neutron star or a black hole remains, and around these objects in space for some time there are remnants of the shells of the exploded star in the form of an expanding gaseous nebula.

Type Ia supernovae

The mechanism of bursts of supernovae of type Ia (SN Ia) looks somewhat different. This is the so-called thermonuclear supernova, the explosion mechanism of which is based on the process of thermonuclear fusion in the dense carbon-oxygen core of a star. The precursors of SN Ia are white dwarfs with a mass close to the Chandrasekhar limit. It is generally accepted that such stars can be formed by the flow of matter from the second component of a binary star system. This happens if the second star of the system goes beyond its Roche lobe or belongs to the class of stars with a superintense stellar wind. As the mass of a white dwarf increases, its density and temperature gradually increase. Finally, when the temperature reaches about 3×10 8 K, conditions arise for thermonuclear ignition of the carbon-oxygen mixture. From the center to the outer layers, the combustion front begins to spread, leaving behind combustion products - the cores of the iron group. The propagation of the combustion front occurs in a slow deflagration regime and is unstable to various types of disturbances. Of greatest importance is the Rayleigh-Taylor instability, which arises due to the action of the Archimedean force on light and less dense combustion products, compared with a dense carbon-oxygen shell. Intensive large-scale convective processes begin, leading to an even greater intensification of thermonuclear reactions and the release of supernova energy (~ 10 51 erg) necessary for the ejection of the shell. The speed of the combustion front increases, turbulence of the flame and the formation of a shock wave in the outer layers of the star are possible.

Other types of supernovae

There are also SN Ib and Ic, whose precursors are massive stars in binary systems, in contrast to SN II, whose precursors are single stars.

Supernova theory

There is no complete theory of supernovae yet. All proposed models are simplified and have free parameters that must be adjusted to obtain the required explosion pattern. At present, it is impossible to take into account all the physical processes that occur in stars and are important for the development of a flare in numerical models. There is also no complete theory of stellar evolution.

Note that the known supernova SN 1987A, assigned to the second type, is a blue supergiant, and not a red one, as was assumed before 1987 in SN II models. It is also likely that there is no compact object such as a neutron star or a black hole in its remnant, as can be seen from observations.

The place of supernovae in the universe

According to numerous studies, after the birth of the Universe, it was filled only with light substances - hydrogen and helium. All other chemical elements could be formed only in the process of burning stars. This means that our planet (and you and me) consists of matter formed in the depths of a prehistoric star and thrown out sometime in a supernova explosion.

A supernova explosion is an extremely rare phenomenon. According to modern concepts, a supernova explosion should occur in our Galaxy approximately every 50 years. Most of these explosions are hidden from us by the opaque dust subsystem of our Galaxy. Therefore, most supernovae are observed in other galaxies. Deep sky surveys on automatic cameras connected to telescopes now allow astronomers to discover more than 300 flares per year.

supernova observations

To designate supernovae, astronomers use the following system: first, the letters SN are written (from the Latin S uper N ova), then the year of discovery, and then in Latin letters - the serial number of the supernova in the year. For example, SN 1997cj denotes a supernova discovered 26 * 3 ( c) + 10 (j) = 88th in a row in 1997.

The most famous supernovas

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

Historical supernovae in our Galaxy (observed)

Astronomers have officially announced one of the most high-profile events in the scientific world: in 2022, from the Earth with the naked eye, we will be able to see a unique phenomenon - one of the brightest supernova explosions. According to forecasts, it will outshine the radiance of most stars in our galaxy with its light.

We are talking about a close binary system KIC 9832227 in the constellation Cygnus, which is separated from us by 1800 light years. The stars in this system are located so close to each other that they have a common atmosphere, and the speed of their rotation is constantly increasing (now the rotation period is 11 hours).

About a possible collision, which is expected in about five years (plus or minus one year), said at the annual meeting of the American Astronomical Society Professor Larry Molnar (Larry Molnar) from Calvin College in the United States. According to him, it is quite difficult to predict such cosmic catastrophes - it took several years to study (astronomers began to study the stellar pair back in 2013).

The first such forecast was made by Daniel Van Noord, a researcher at Molnar (still a student at that time).

"He studied how the color of a star correlates with its brightness, and suggested that we are dealing with a double object, moreover, with a close binary system - one where two stars have a common atmosphere, like two peanut kernels under one shell," Molnar explains in a press release.

In 2015, Molnar, after several years of observation, told his colleagues about the forecast: astronomers are likely to experience an explosion similar to the birth of supernova V1309 in the constellation Scorpio in 2008. Not all scientists took his statement seriously, but now, after new observations, Larry Molnar again touched on this topic, presenting even more data. Spectroscopic observations and processing of more than 32 thousand images obtained from different telescopes ruled out other scenarios for the development of events.

Astronomers believe that when the stars crash into each other, both will die, but before that they will emit a lot of light and energy, forming a red supernova and increasing the brightness of the double star ten thousand times. The supernova will be visible in the sky as part of the constellation Cygnus and the Northern Cross. This will be the first time that experts and even amateurs will be able to track binary stars directly at the moment of their death.

“It will be a very dramatic change in the sky, and anyone can see it. You won’t need a telescope to tell me in 2023 if I was right or not. Although the absence of an explosion will disappoint me, any alternative outcome will be no less interesting,” — adds Molner.

According to astronomers, the forecast really cannot be taken lightly: for the first time, experts have the opportunity to observe the last few years of the life of stars before their merger.

Future research will help to learn a lot about such binary systems and their internal processes, as well as the consequences of a large-scale collision. "Explosions" of this kind, according to statistics, occur about once every ten years, but this is the first time that a collision of stars will occur on. Previously, for example, scientists observed an explosion.

A preprint of a possible future paper by Molnar (PDF document) can be read on the College website.

By the way, in 2015, ESA astronomers discovered a unique one in the Tarantula Nebula, whose orbits are at an incredibly small distance from each other. Scientists predicted that at some point such a neighborhood would end tragically: celestial bodies would either merge into a single giant star, or a supernova explosion would occur, which would give rise to a binary system.

We also recall that earlier we talked about how supernova explosions.