Chapter 14: The Milky Way

Novae and supernovae are yet another case of astronomical phenomena that have been given similar names — because they looked similar to the astronomers who named them — but represent a number of distinct processes. They both represent the sudden and temporary brightening in the sky of a star. For ancient people who were very familiar with the night sky, these events were easily noticed and could be occasions for awe or fear.

The word nova comes from the Latin root for new and was the term used for all "new stars" that appeared in the sky in past centuries. In ancient Chinese records, the novae were called "guest stars." In this century, astronomers discovered from their rate of light variation and other properties that there were two types of "new stars" — the novae and a much more energetic type called a supernova (plural form: supernovae). These rapidly brightening or "exploding" stars were important historically as examples that the heavens are not immutable, as once thought, but are actually evolving, even as we watch.

Accretion Disk Binary System

Nova in NGC 1316

What are the differences between a nova and a supernova? A nova involves the transfer of mass onto a white dwarf, resulting in a rapid ignition of nuclear fusion on the surface that brightens the star for a while. Consider the case of a binary where the more massive star has filled its Roche lobe and the smaller star is a white dwarf of nearly 1.4 solar masses. If the larger star dumps hydrogen onto the surface of a white dwarf, the hydrogen will be compressed by the intense gravity of the white dwarf. It is necessary for the star to be a white dwarf because the surface gravity of a main sequence or larger star is too small to ignite nuclear reactions. The gas is heated as it is compressed, and the hydrogen may ignite in nuclear reactions that brighten the surface and blow excess gas outward. This type of explosion is called a nova (plural: novae).

The nova phenomenon can occur in cycles. Hydrogen may accumulate on a dwarf until nova explosions occur anywhere from 100 to 10,000 years apart. Since the individual explosions blow off only a small fraction of the star's mass, maybe only 0.01%, leaving the rest intact, the process can start over. The energy of the detonation accelerates the gas to speeds of thousands of kilometers per second, and deposits enough energy in the gas to cause the star to brighten by factors of hundreds or thousands. Among novae that are closest to Earth, the cloud of expanding debris can sometimes be seen in telescopes a few years after the explosion.

GK Persei Nova Blast/ Remnants

A nova can flare up repeatedly although there is not a fixed interval between outbursts. Of the 200 billion stars in our galaxy, 30 to 50 explode as novae each year. In December 1999 two amateur astronomers (one in Portugal and one in the United States) independently discovered a "new" star in the constellation Aquila using binoculars. A Follow-up study of this object revealed that it was not, in fact, a new star but that it had been too faint to be seen with binoculars before. This was a modern-day nova outburst. Recurrent novae with short intervals between outbursts are rare. Two that are close enough and bright enough to be visible to the naked eye are Rho Ophiuchus, which last erupted in 1967, 1985, and 2006, and T Coronae Borealis, which last erupted in 1866 and 1946.

 

Three rings of glowing gas around supernova 1987a

Supernovae are brighter than novae. One type of supernova results from the death of a single massive star, when the central core has been burned to iron and can no longer support the star against collapse. In this the material above the core collapses, and hits the dense iron core, being blown off in the resulting rebound and heating. This is labeled Type 2 by astronomers, who distinguish it by the strong hydrogen lines in the spectrum. However, a supernova can also occur in a binary system, when the mass transfer from a giant or supergiant pushes a white dwarf over the Chandrasekhar limit. The white dwarf collapses to form a neutron star or a black hole, blowing off a fraction of the excess mass in a titanic explosion. This is labeled Type 1 by astronomers, and it shows no hydrogen lines in the spectrum. In both processes, a supernova marks the brilliant death of a star. For a short time, a supernova releases enough light to rival the light of the entire Milky Way. These spectacular explosions are rare, with a supernova in the Milky Way occurring only once every 40 or 50 years. A supernova explosion is therefore about 1000 times rarer than a nova event.

Supernovae have undoubtedly been observed throughout human history; we can only imagine the reaction of our nomadic ancestors to a star that brightened enough to appear in the daytime shy! There's evidence that Hipparchus may have had his interest in astronomy triggered by a supernova. The earliest record of a supernova is an event in 185 A.D. noted by Chinese astronomers. The brightest supernova in recorded history was the event in 1006 A.D., and the supernova that led to the Crab Nebula went off in 1054 A.D. The last two supernovae to be observed in our galaxy were at their peak brightness in 1572 and 1604. Given the expected supernova rate of a couple per century, we are long overdue for a spectacular light show!