Supernovae are exploding stars. They represent the very final stages of evolution for some stars. Supernovae, as celestial events, are huge releases of tremendous energy, as the star ceases to exist, with about 1020 times as much energy produced in the supernova explosion as our Sun releases every second. Our Sun, fortunately, will not end its life as a supernova.

Currently, supernovae are only seen in galaxies other than the Milky Way. We know that supernovae have occurred in our Galaxy in the past, since both Tycho Brahe and his protege, Johannes Kepler, discovered bright supernovae occurring in the Milky Way in 1572 and 1604, respectively. And, the Chinese, and others, have records of a "guest star" occurring in 1054 in the present constellation Taurus. Today, we see remnants of all three supernovae, which appear as expanding clouds of gas, where each was originally discovered. However, no supernova has been seen in our Galaxy since Kepler's.

Supernovae, when they are discovered, are designated by the year in which they are discovered, and the order in which they are discovered during that year, by using members of the alphabet. For instance, the fourth supernova discovered 1998 was named SN 1998D, which occurred in the galaxy NGC 5440.

The brightest supernova since Kepler's supernova was discovered on February 23, 1987, in the nearby galaxy, the Large Magellanic Cloud (LMC). This supernova was easily seen with the naked eye throughout 1987 in the Southern Hemisphere. This supernova was named SN 1987A. This supernova is still being observed by a number of telescopes, particularly, the Hubble Space Telescope. Another bright recent supernova, observable from the Northern Hemisphere, was SN 1993J in the galaxy Messier 81 (M81).

When astronomers observe supernovae, they do so today using telescopes working at various wavelengths. With optical telescopes, with which most of us are familiar, astronomers measure the amount of light being emitted by a supernova, as seen from Earth, usually through a number of light filters. From these measurements, they can determine how the luminosity, or brightness, and color of a supernova evolves, or, varies with time. Supernovae generally brighten to a maximum brightness, then decline slowly in brightness over many weeks or months.

Astronomers also pass the light through a device, like a prism, which breaks the light from the supernova into its component colors. This is known as a spectrum. A spectrum shows how the brightness of light depends on the wavelength of light. Light is not equally bright at all wavelengths for supernovae. In fact, the spectra of supernovae vary over many weeks or months, as well.

Both the "light curves," as they are known, and the spectra of supernovae tell astronomers about the physics that is occurring during, and after, the explosion. It is the nature of the explosion that is vitally important in understanding supernovae and learning which stars in galaxies blow up. Supernovae are responsible for the production of many of the chemical elements in Nature, and astronomers can study how these elements are produced, as well as estimating the amount of energy liberated in the explosion and its effects on the star.

The appearance of the spectrum allows astronomers to classify supernovae into two main types: Type I and Type II. Basically, supernovae arise from two very different classes of stars: massive ones and old, non-massive ones. The Type II supernovae very strongly show the presence of the element hydrogen in their spectra. Type I supernovae do not show any hydrogen in their spectra. The astronomer Rudolf Minkowski discovered this distinction in 1941, and this classification scheme was used for about five decades. It was thought that Type II supernovae are the explosions of massive stars, whereas Type I supernovae arise from old, low-mass stars.

In about 1985, things got a little more complicated. Some Type I supernovae discovered and studied in the early 1980s appeared to be peculiar in nature. They did not exhibit a characteristic spectral signature, thought to be due to the presence of silicon, seen in many other Type I supernovae spectra. Additionally, a few of these peculiar supernovae showed very strongly the presence of helium. Furthermore, these supernovae appeared to be occurring among populations of massive stars in galaxies. For these reasons, it was realized that Type I supernovae can be further subclassified into those with the silicon spectral feature, and these were called Type Ia supernovae, and those that do not show this feature; this latter group were called Type Ib supernovae.

Making affairs even more complicated, not all of the Type Ib supernovae since 1985 have showed the presence of helium in their spectra. These first cousins of Type Ib supernovae are today called Type Ic supernovae. More and more, supernova researchers have realized that the Type Ib/Type Ic distinction involves splitting hairs, and so, many such supernova pundits put both of these Type I subtypes into one main category: Type Ibc.

Supernovae are seen to occur in galaxies all over the Universe. Galaxies are basically classified into three major groups: spirals, ellipticals, and irregulars. Now, Type II and Type Ibc supernovae are seen to occur only in spiral and irregular galaxies, and these supernovae also tend to be discovered in regions of these galaxies where star formation, particularly the formation of massive stars, most certainly has recently occurred in the last 10 million years or so. These supernovae have not been seen in elliptical galaxies. It is therefore thought that these supernovae arise from the explosions of massive stars in galaxies.

Type Ia supernovae are discovered in all three types of galaxies. But, Type Ia supernovae are generally not found near massive star formation. Since very little, if any, star formation occurs today in elliptical galaxies, it is thought that Type Ia supernovae arise from older, less massive stars.

In conjunction with this "environmental" evidence for the nature of supernovae, astronomers, who develop physical theories to explain celestial phenomena, and are therefore generally called theorists by their colleagues (as opposed to the other group of astronomers, who are usually, more purely, observers), develop theoretical models to explain supernova explosions. Today, these models involve sophisticated and complex computer simulations of the explosions. What the theorists tend to find is that stars more massive than about 8 solar masses, or, in other words, 8 times the mass of our Sun, become Type II and Type Ibc supernovae. These are young, relatively massive stars, which form in spiral and irregular galaxies. They also find that the Type Ia supernovae can best be explained by the explosion of somewhat exotic low-mass stars known as white dwarfs.

Stellar evolution is the study of how stars evolve and change, both internally and externally, throughout their lives. Stars generate their own energy during their lives by the process of nuclear fusion. The nuclei of lighter elements, such as hydrogen and helium, are forced to fuse, or combine, under the tremendous pressures and temperatures at or near the centers of stars, into the nuclei of heavier elements. (The nucleus of an atom is the central body which generally contains protons and neutrons; for atoms and ions, electrons orbit the central nucleus. At the temperatures and pressures within stars, electrons are totally ripped free from the nuclei.)

As Albert Einstein discovered, in his famous mass-energy equivalence principle that everyone knows (but not nearly as many understand), E=mc², energy can be produced in large quantities from matter. When nuclear reactions occur inside stars, these reactions liberate huge amounts of energy, which inevitably trickles out from the star's interior to its surface, resulting in the light we see from the stars, their starshine.

In massive stars, those more massive than about 8 solar masses, the sequence of nuclear fusion progresses from the very simplest reaction of hydrogen nuclei to form helium nuclei, to more complicated reactions, involving the synthesis, as it is known, of silicon nuclei into iron nuclei. The iron nucleus is the most stable nucleus in Nature, and it resists fusing into any heavier nuclei, unless it is forced to do so with the input of truly formidable amounts of energy. As a result, when the central core, as it is known, of a star becomes pure iron nuclei, the core, which is generally the site of most of a star's energy production, is no longer able to produce energy and therefore support the star. The core can no longer support the crushing force of gravity, resulting from all of the matter above the core, and the core therefore collapses under its own weight.

Some really exotic physics takes place during this core collapse. But, basically, only neutrons can generally survive the collapse, and when the neutrons act together under truly unimaginable crushing pressure to resist the collapse, the core becomes what is known as a neutron star. The core then becomes stable, but the rest of the massive star is left in limbo. The core collapse suddenly stops, and the core, like a squeezed sponge, bounces back, releasing a huge amount of energy, which rips through the outer layers of the star. The original massive star dies in a fiery explosion, with only the newly-formed neutron star surviving this huge explosion.

The star has ended its life as a Type II or Type Ibc supernova. And the death throes of this star occur extremely rapidly, over only a time of several milliseconds! This, compared to a star that, up to that point, had existed for several million years!

If the star began its life with a really large amount of mass, the theorists say that not even the neutrons at the star's core can hold back the crushing force of gravity. At that point, as the star ends its life, the core becomes a black hole. Possibly, the result of the formation of the black hole is a supernova explosion, but some questions remain if this is really the chain of events for such very massive stars.

Now, this sort of evolution will not occur for the Sun. The Sun will continue to very quietly fuse its central hydrogen into helium for the next five billion years or so. The core will become pure helium, which will then fuse to carbon in a relatively short time. Finally, the carbon at the core cannot get hot enough to fuse into other type of nucleus. The carbon core can no longer sustain the Sun's energy and collapses under its own weight, much as the more complex cores of massive stars do. However, electrons in the core act to resist the collapse, and the core of the Sun will become what is known as a white dwarf. As the formation of the central white dwarf occurs, the outer layers of the Sun will be sloughed off into space to form a planetary nebula. As the nebula disperses over many thousands of years, the skeletal white dwarf remnant of the previous Sun will sit in the Galaxy and glow away its residual heat over many billions of years.

White dwarfs, as you may suspect, are not very massive, since one will form from the core of the Sun, which today contains, by definition, one solar mass. In 1938 the Indian astronomer, S. Chandrasekhar, determined that white dwarfs cannot be more massive in the Universe than 1.4 solar masses. If a white dwarf were to exceed this limit, called the Chandrasekhar limit (in his honor), the star would cease to exist. So, if a white dwarf finds itself in a binary star system, where the two stars are close enough that their mutual gravity results in their interaction, then the binary companion may dump matter onto the white dwarf. The white dwarf's mass slowly and steadily increases, to the point that it may exceed the Chandrasekhar limit. If this happens, then... poof! The white dwarf explodes in a Type Ia supernova and is completely destroyed. The matter that once was the white dwarf gets incinerated into radioactive elements, which decay over time, and continue to power the light curve of the supernova.

When supernovae explode, they have profound effects on their surroundings in galaxies. The tremendous energy that is liberated affects the gas in its environment, pushing on it and compressing it. If the gas was originally fairly dense, then the compressed denser gas can actually go on to collapse and form new stars. The energy of the explosion also synthesizes new elements, particularly those heavier than iron. These fresh, new elements are then sprinkled into the surrounding gaseous medium, enriching it. Therefore, later generations of stars formed after the supernova contain more heavy elements than previous generations. In fact, the enrichment of the gas in our region of the Milky Way reached such a point that a sufficient quantity of heavy elements existed to give rise to life, as we know it, here on Earth. Supernovae are thought to be directly responsible for us all!

Supernovae also likely through small atomic and subatomic particles out into the galaxies, which we call cosmic rays. These particles, moving through the Milky Way Galaxy, pass through space and impinge on the Earth; it is thought that these high-speed, high-energy cosmic rays might be partially responsible for genetic mutation and, therefore, evolution of life here on Earth.

Supernovae, particularly Type Ia supernovae, are intrinsically very bright, among the brightest objects in the Universe. As such, they can serve as beacons of light that can act as signposts indicating distances within space. Currently, astronomers are actively exploiting this fact about Type Ia supernovae, to measure the distances to very remote galaxies. It is thought that by determining these distances fairly accurately, and combining that information with the speeds at which the host galaxies are receding from us, due to the expansion of the Universe, originally studied most intently by Edwin Hubble, we can determine how much matter there is in the Universe, and, therefore, the Universe's ultimate fate. That's because, according to Einstein's theory of general relativity, the total amount of matter in the Universe determines what geometrical shape the Universe has. According to Einstein, matter curves the space and time around it. All of the matter in the Universe, of course, curves the entire Universe. The more matter, the more the curvature. The more curved the Universe, the more likely it is that the current expansion, resulting from the original Big Bang, will halt, due to the force of gravity, and the Universe will collapse back on itself in a Big Crunch. Alternatively, if there's not enough matter to cause a Big Crunch, then the Universe will expand forever, with essentially no end.

Astronomers are locating these supernovae by observing distant galaxies over and over. Quite often, they find bright, new objects appearing on their images. By taking the spectra of and producing light curves for enough distant supernovae, the astronomers can place constraints on the value of the mass of the Universe, and therefore determine whether it will collapse on itself or expand forever. Currently, new results seem to indicate that the amount of matter in the Universe is not enough to halt the expansion. But more results need to be obtained to verify these findings. The ultimate fate of the Universe is a profound question that humans have tried to answer. For creatures so used to beginnings and endings, having something last forever boggles the imagination. But, then, we're talking about the Universe.