Issue 3.09

The Astrophysics Spectator

May 10, 2006

This fortnight The Astrophysics Spectator directs its attention to a pair of loosely-related topics: the various ways that a degenerate dwarf star might be detonated to produce a supernova, and the places we search for black hole candidates. The relationship between these topics is that both the supernova and the black hole are created in the collapse of a star.

Supernovae come in many flavors, but the theories for them run along only two tracks: supernovae are produced by the collapse of the core of a massive star to a neutron star, and supernovae are produced by the thermonuclear explosion of a degenerate dwarf star. The second mechanism is associated with a particularly useful type of supernova. Known as the type 1a supernova, these events are standard candles, meaning that one can derive their distance by measuring their brightness; they are used to measure the distance to the farthest galaxies in the observable universe. They are associated with degenerate white dwarfs because of the chemical composition portrayed by their spectra. With no evidence of hydrogen, but strong signatures of intermediate-mass elements such as silicon and heavier elements such as iron, the supernova remnant appears to be composed of the end products of carbon and oxygen fusion. For this reason, these supernovae are associated with degenerate dwarfs. The only real issue is what triggers the explosion? The leading theory is that the degenerate dwarf detonates when it begins to collapse after acquiring mass from a companion star.

Black holes are created in the collapse of large stars. Black holes should be larger than 3 solar masses, because a star somewhat smaller should collapse to a neutron star. Short-lived and common, the large stars should have created numerous black holes in our Galaxy over the past 16 billion years. But how do we see these objects? There is only one effective way of finding a black hole, and that is to search for the black holes that are pulling matter onto themselves. Gas falling into a black hole converts its gravitational potential energy into radiation. Very little gas is needed to make a black hole visible across the Galaxy. We find black holes accreting gas in two places: in compact binary star systems, and at the centers of galaxies.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for publication on May 24.

Stars

Degenerate Dwarf Supernoavae. Degenerate dwarfs composed of oxygen and carbon contain enough nuclear energy to blow themselves apart. Astrophysicists accept that degenerate dwarfs are the progenitor of the type 1a supernovae, because the thermonuclear explosion of a degenerate dwarf can match the characteristics of these supernovae. But how does one detonate a degenerate dwarf? Three theories exist to explain this, all involving a binary star system with at least one of the stars a degenerate dwarf. One idea is that helium from the companion star collects on the surface of the degenerate dwarf, ignites, and detonates the carbon and oxygen at the degenerate dwarf's core. A second idea is that two degenerate dwarf stars in a binary system merge together and explode. The most favored idea is that gas pulled from a companion star pushes the mass of a degenerate dwarf above its maximum stable mass; causing the degenerate dwarf to collapse until its oxygen and carbon detonate. (continue)

General Relativity

Finding Black Holes. A black hole is nothing more than gravitational field, and only through the effect of its gravitational field can a black hole be found. The observed black hole candidates fall into two categories: black holes in binary star systems with masses around 5 solar masses, and black holes at the centers of galaxies, including our own, with masses greater than a million solar masses. The black holes in both categories are very bright, because they pull gas onto themselves, causing the gas to heat and radiate light. Black holes outside of these two groups are effectively invisible to us. (continue)