Introduction to Supernovae

The onion skin model (for type II SN and possibly one component of a type Ib SN) serves to illustrate the layered structure of massive stars with lighter gases on the outside becoming heavier gases on the inside through nuclear fusion. Image adapted from Stars, James B. Kaler, Scientific American Library, 1992.

What are supernovae?

Supernovae are massive exploding giant stars. When the explosion occurs, the resulting illumination can be as bright as an entire galaxy!

"One of the most energetic explosive events known is a supernova. These occur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star's envelope into interstellar space. The result of the collapse may be, in some cases, a rapidly rotating neutron star that can be observed many years later as a radio pulsar."

What causes these stars to explode?

As a result of gravitational forces acting against the nuclear structure of the core of a fuel depleted star, tremendous shock waves are generated which cause the outside layers of the star to be blown away from the core. This can happen in one of two ways depending on the type of supernova.

Type II (Core Collapse) Supernovae. Gravitational forces condensing hydrogen gas raises the temperature at the center of the star to the point where nuclear fusion is initiated. According to the Onion Skin Model (illustrated above), the following sequence occurs. Hydrogen is fused into helium and energy is given off in the process. As more helium accumulates at the center, the temperature rises due to compression until another nuclear fusion is initiated. This time helium is converted to carbon and oxygen and additional energy is given off during the nuclear fusion. A similar process continues with carbon and oxygen fusing to neon, magnesium, and oxygen. These elements then undergo another fusion process as the temperature and pressure increase to produce silicon and sulfur. The latter two elements then fuse into iron. During each nuclear fusion, energy is given off. This takes two orders of magnitude less time to happen than on the previous fusion. However, nuclear fusion stops at iron because energy is no longer produced by fusion. The iron core collapses very quickly (within hours or less). Since the iron core can collapse only so far and can no longer undergo fusion, it becomes extremely hot and now begins to expand rapidly. This occurs while the star's outer shells are rushing in to fill the void left by the collapsed iron core. The expanding iron and the collapsing outer gases collide with each other producing tremendous shock waves which blow the outer layers away from the core, thus causing the supernova's gigantic explosion.

Type I Supernovae. These type of supernovae involve two stars, one of them being a white dwarf whose gravitational attraction is so intense that it is capable of siphoning off material from its companion. Unfortunately for the star (but fortunately for us at a long distance!), the white dwarf exceeds its Chandrasekhar limit of stability causing it to go into thermonuclear instability and produces one of the largest explosions known in the Universe, the Type I SN. There are currently three types of Type I SNe accepted by the astronomical community in general. The subclass types (Ia, Ib, and Ic) are basically determined by the state of the white dwarf's companion star, though to qualify as a Type I SN the companion should have expelled its hydrogen layer. Mike Richmond's SN Taxonomy table gives a good schematic idea about the (more or less) current thinking on the topic.

Remember that these are only models attempting to explain these massive explosions. They can change at any time! It is fun attending a conference on the topic of SNe. The research astronomers exchange videos with each other to show how their current model actually accomplishes the explosive event.

What are the characteristics of the various types of supernovae?

The types of supernovae are characterized by their spectral lines which indicate their chemical composition. Dr. Mike Richmond at the Astrophysics Department of Princeton University categorized the various types of supernovae and their characteristics in his informative SN Taxonomy chart.

How do the light curves differ between type I and type II supernovae?

The Supernovae Light Curve shown at the right shows a typical comparison between the light intensity of type I supernovae with that of type II supernovae as a function of the number of days since peak. Type I supernovae (red) is much brighter, but decays much faster than type II supernovae (blue). Curves from Stars, James B. Kaler; Scientific American Library, 1992.

What happens after the explosion?

What happens after the explosion depends on the type and mass of the progenitor stars. Mostly they produce a gas cloud called a supernova remnant which initially expands at a rate of about 10,000 km/s. Gradually the expansion rate slows down while dissipating into the interstellar medium, seeding the neighborhood with heavy elements and providing the necessary shock waves for new stellar formation. The Crab Nebula, M1 (image), is a remnant of the supernova of 1054 (which occurred within our Milky Way Galaxy).

Could our sun become a supernova?

Not a chance! Our sun is not large enough to become a type II supernova and it's not coupled with a white dwarf to become a type I supernova. Besides, it will take another 5 billion years before our sun's supply of hydrogen is depleted. At that time it will begin its dying process and eventually become a white dwarf with a surrounding shell of material much like the Ring Nebula (M57) in the constellation of Lyra!

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