Type Ia Supernovae

Artists impression of Type Ia supernovae
Credit: NASA/CXC/M. Weiss

Type Ia supernovae are also known as thermonuclear supernovae. They involve a binary (or double) star system. Where one star is a white dwarf, and the other is usually a star like our Sun or a huge red giant star.

White dwarfs are very dense stars. Although they have masses comparable to our Sun, they are squeezed into a volume similar to that of the Earth. This means that they have a strong gravitational force which can pull material from the neighbouring star onto its own surface. The mass of the white dwarf gradually increases as it draws more and more material from its companion in a process is known as accretion.

The white dwarf doesn't collapse due to the pressure of the electrons inside each atom repelling each other. The increase in mass from accretion can however cause the white dwarf to become unstable. If the white dwarf reaches 1.44 times the mass of the Sun, (known as the Chandrasekhar limit), it is unable to accrete any more material - the electron pressure is no longer able to balance gravity and the star explodes. This explosion is known as a type Ia (or thermonuclear) supernova. These explosions completely destroy the star and leave nothing behind. They are some of the brightest events in the Universe, many times brighter than supernova explosions from massive stars.

Type Ia supernovae are always the same brightness due to these explosions occurring when the white dwarf reaches a set mass. This means that we can use them to measure distances to objects in space. They are known as standard candles. Observations of type Ia supernovae helped astronomers to prove that the Universe is expanding and accelerating.