Igniting a Standard Candle Donald Serna-Grey A compact star the size of the Earth is supernova is mostly the same. However, destroyed in an explosion so bright it can be astronomers think there are two different seen from across the universe, but what is it? ways the supernova can occur which are In astronomy any object or event in the called the Single-Degenerate channel and universe that is used as a measuring stick for understanding the universe is called a standard candle. One such standard candle is known as a Type Ia Supernova which occurs when the core of a star like the Sun, called a white dwarf star, reaches a critical mass and explodes. A white dwarf is about the mass of the Sun though only the size of Earth and will become unstable when its mass is increased by accretion to about one and a half times that of the Sun, known as the Chandrasekhar Mass. When a white Illustration 1: Image from: dwarf reaches the Chandrasekhar Mass the http://blogs.discovermagazine.com/badastronomy/20 10/02/18/dwarf-merging-makes-for-an-explosive- white dwarf undergoes a thermonuclear combo/#.VmUKAnarRhE runaway resulting in an explosion, called a Type Ia Supernova, that completely destroys the white dwarf and will outshine its entire host galaxy for a few days. Since these stars explode in the same way every time they reach this critical mass it is easy for astronomers to detect and use Type Ia Supernovae because the light from each Illustration 1: ESO/VLT image of Henze 2-428 Planetary Nebula the Double-Degenerate channel. The Single-Degenerate channel is when the white dwarf has a close orbiting companion star that the white dwarf will pull mass from via gravity, and this accretion eventually leads to the white dwarf reaching it's critical Illustration 2: ESO/L. Calçada. Artist rendition of the Double-Degenerate white dwarf system in Henize mass, the Chandrasekhar Mass, causing the .2-428 detonation of a Type Ia Supernova. In the enough that they will merge within 114 2 Double-Degenerate channel there are two million years . This system is the first white dwarfs, with a total mass more than known Double-Degenerate Type Ia the Chandrasekhar Mass, orbiting each other Supernova progenitor found and research and slowly spiraling in towards each other. from this system is greatly helping in Type This inspiralling will eventually result in the Ia Supernova studies though other teams are two white dwarfs merging which then still looking for Single-Degenerate results in the detonation of both stars in a progenitor systems as well. This discovery Type Ia Supernova. There are a few ways to is only one step towards understanding the search for a potential Double-Degenerate puzzle of these standard candles and how Type Ia Supernova system, called a they are ignited. progenitor system. The easiest way is by looking at a planetary nebula, the remains of Sources Cited the atmosphere of a star that has collapsed into a white dwarf, and look for asymmetry 1/www.eso.org/public/news/eso1505 in its shape which could indicate the 2http://www.eso.org/public/archives/ presence of two white dwarfs in the nebula. releases/sciencepapers/eso1505/eso1505a. A team at the European Southern pdf/ Observatory did exactly that and found a system in the nebula Henize 2-428 using the Very Large Telescope1. The European team found that the pair of white dwarfs in this nebula do indeed have a total mass more than the Chandrasekhar Mass and are close .