Supernovae (Part 2)

The second way in which a star can produce a supernova is less direct, and is barely understood by astronomers. Astrophysicists have come up with an explanation based on a few clues:

(1) An explosion occurs that has an energy comparable to that contained in the core of a star. Nothing else in nature contains that kind of energy, so the explosion must involve a star.
(2) In the aftermath of the explosion, one does not see hydrogen. Therefore, the explosion cannot be from an ordinary star. The star must have blown its hydrogen away long before exploding.
(3) All of these supernova seem to release the same energy.

Briefly, Type Ia supernova appear to result from the collapse of a white dwarf. White dwarves are the remains of the cores of stars like our sun, after they have finished burning their nuclear fuel. The intense heat produced in the later stages of the star blew away all of its outer layers, so that the white dwarf contains very little hydrogen. Instead, the white dwarf is composed of helium, carbon, oxygen, magnesium, neon, and other elements, each contributing a fraction of the star that depends upon when nuclear burning stopped in the original star (heavier stars produce heavier elements).

White dwarves are held up by the quantum-mechanical repulsion between electrons (in a similar way as neutron stars are held up by the repulsion between protons and neutrons). However, these quantum-mechanical forces can only resist gravity up to a fixed mass, about 1.4 times the mass of the Sun, which is referred to as the Chandresekhar limit in honor of the man who first predicted it (Subrahmanyan Chandrasekhar). If any more mass is added, the white dwarf will collapse. The collapse heats up the star, and causes thermonuclear burning to re-ignite catastrophically. If the conditions are right, the entire white dwarf can be consumed by thermonuclear burning. So much energy is produced that the white dwarf is blown apart. The explosion is seen by astronomers as a supernova. It always has the same energy, because the white dwarf that produces it is always the same mass, 1.4 times the mass of the sun. The image above is actually of one of these "Type Ia" (read, "type one a") supernova, the death of a white dwarf.

The main thing astrophysicists don't understand is how nature adds mass to a white dwarf. There are two competing ideas, both of which rely on the fact that stars often come in pairs. One idea is that the explosion is produced when the white dwarfs that descend from a pair of stars merge. However, only a few of these pairs of white dwarfs are known in our Galaxy, and they will take a very long time to merge. The mergers may not occur often enough to explain the number of Type Ia supernovae that are observed. The other idea is that some white dwarfs manage to shred the second star in their pairs, and collect those stars' matter on their surfaces. Thousands of these bizarre pairs have been identified in our Galaxy (depending upon how they are found, they are called cataclysmic variables, dwarf novae, or classical novae). However, for these systems, there is controversy as to whether matter will collect onto the white dwarf, or whether smaller explosions will eject all the collected matter from the surface. In the end, astronomers are pretty certain that white dwarfs can explode, but don't know which white dwarfs end up exploding.

Nonetheless, the supernova that result from exploding white dwarfs are a useful tool to astronomers, because they all put out about the same energy, and they can be seen from vast distances across the Universe. Astronomers use these supernova to measure distances. In the same way that one can tell how far away a ship is at night by the brightness of its running lights, one can tell how far a Type Ia supernova is by how bright it appears to Earth-bound astronomers. Astronomers use Type Ia supernova to map how far away the galaxies in which they occur are. They then measure how fast each galaxy is moving away from us, in order to map how space is expanding. Remarkably, observations of Type Ia supernova suggest that the expansion of the Universe is accelerating --- something is counteracting the force of gravity, which should make the expansion of the Universe slow down with time.

The accelerating Universe remains controversial. Many astronomers were willing to accept the above explanation for Type Ia supernova when they existed as a curiosity, or as a simple tool. However, that explanation is the start of a logical chain that implies that a new force exists in the Universe, driving the expansion of space to accelerate. Moreover, no indication for this new force had ever been seen before. Therefore, many astronomers feel that we should re-visit our assumptions about how Type Ia supernova occur, to see if something could be causing us to mis-estimate how far away they are.