Supernovae in General, and Week 1 Updates

While my internship is starting up, I think it might be a good idea to get a few terms out there so the rest of the blog makes sense! The first of these is "supernova".

What Is a Supernova?

In general, the term "supernova" (abbreviated SN) refers to a star exploding. Not all stars do this, but when they do, it's big. Supernova explosions (supernovae, or SNe) are some of the most energetic phenomena in the observable universe. If a supernova were to occur where the sun is now, the amount of radiation and energy your face would receive from looking at it would exceed the amount from a typical nuclear bomb exploding while touching your nose. Stars that go supernova briefly rival the brightness of their host galaxies, expending all their energy in a final violent display. The material left behind in a supernova contains heavier elements than iron, because the high amount of energy present in the explosion is enough to trigger fusion of heavy elements. All elements heavier than nickel were created by supernovae phenomena (except laboratory elements, of course). Supernova remnants (the remains of these violent explosions) are some of the most beautiful visible-light objects in the sky.

The crab nebula, a supernova remnant. Courtesy Wikimedia Commons.

The most well-known kind of supernovae in the public eye are type II supernovae, or core-collapse supernovae. These supernovae occur in very massive stars, between 8 and 50 solar masses. Because of their large mass, these stars can continue the fusion chain past hydrogen and helium, fusing elements in concentric layers until eventually reaching a point where they fuse their cores into iron or nickel.

The onion-like layers of a star about to go supernova. Courtesy Wikimedia Commons.

Fusing iron and nickel provides no net energy output, so pressure from the core is lost and equilibrium is broken. The star is held up through only electron degeneracy pressure. When the mass of the core exceeds the Chandrasekhar limit of about 1.4 solar masses, electron degeneracy pressure gives way to gravity and a catastrophic implosion occurs. The outer core of the star reaches velocities of up to 20% the speed of light, while the inner core reaches temperatures in excess of 100 billion Kelvin. Eventually the star collapses far enough for neutron degeneracy pressure to come into play, and the collapse rebounds and explodes outwards. The core of the star is left behind as either a neutron star (if the star is less than 20 solar masses) or a black hole (if the star is more than 20 solar masses). A neutron star is composed almost entirely of neutrons held up by neutron degeneracy pressure, while a black hole is a singularity.

Although they are interesting, my research does not focus on type II supernovae (as the title of the blog may have hinted). Instead, I'm focusing on the slightly less popularized (but no less interesting!) type Ia supernovae. In general, type I supernovae occur from stars called white dwarfs. When stars on the so-called main sequence (stars like our sun) get very old, they expand to enormous stars called "red giants" before fluffing off their outer layers, leaving behind a tiny, dense ball of electron-degenerate matter called a "white dwarf" (because it is small and white).

The Sirius star system, containing a white dwarf, the faint dot below and to the left of the main star.

White dwarfs do not undergo fusion, and emit radiation because they are hot. Usually, these white dwarfs will simply live out their lives in relative peace, and are expected to eventually cool until they are no longer energetic enough to emit light or heat (becoming so-called "black dwarfs"). Some, however, are destined for a more exciting fate. However, if the white dwarf is in a binary system with another star (like in the above photograph), it's possible for it to end its life in a more spectacular manner. It can pull (or "accrete") mass from its companion star, and if enough mass is gained it will exceed the Chandrasekhar mass (1.44 solar masses, different than the Chandrasekhar limit mentioned above) and they will begin a runaway fusion reaction. Enough energy is released for the star to explode violently in a type Ia supernova.

Type Ia supernovae are particularly important to cosmology because of their uniform nature. They reach a consistent peak luminosity because of the uniformity of the white dwarf progenitor systems. This makes them ideal candidates for so-called "standard candles" that can be used to determine the distances to other galaxies. If a type Ia supernova goes off in NGC 5584, for example, we know what we observe to be it's peak magnitude (or its apparent magnitude), and what its actual (or absolute) magnitude should be, allowing us to calculate the distance to NGC 5584. Such calculations give critical information about the expansion of the universe, making type Ia supernovae critical to the study of the evolution of the universe.

Although type Ia supernovae are very important to modern astronomy, surprisingly little is known for certain about how they come about. The above model of accretion is called the "single degenerate" progenitor model, because it only contains one degenerate star (the white dwarf). Other models such as the double degenerate model propose that two white dwarfs can collide and produce a supernova explosion. Probing the progenitor systems of type Ia supernovae is important in determining how these remarkable events occur.

Internship: Week 1

The first week of my internship has been afflicted with a horrible disease known as "technical difficulty." Data transfer between computers, installing new operating systems, getting licensing for all of the software working, and finding out we don't have all the data we thought we had have all been problems encountered thus far. Despite these setbacks, much work has been done and much progress made. I have done considerable reading from the theses of two of Dr. Milne's graduate students, Dina Drozdov and Ginger Bryngelson, and read through several papers on previous light echoes in type Ia supernovae. Additionally, I have done work on refining and improving the computer simulations of the light echo in our 2007 supernova, with the goal of better understanding the geometry of dust around the progenitor system.