Type Ia Supernovae
There are different types of supernovae. Type Ia are also known as thermonuclear supernovae. This type of explosion does not take place when the core of a massive star collapses. They instead occur in a binary (or double) star system. To trigger a type Ia supernova, one of the two stars must be a white dwarf. The other star is often a low-mass star, like our Sun, or can be a red giant star.
White dwarfs are very dense stars. They do not have much more mass than our Sun, but that mass has been squashed into an Earth-sized sphere. This means the surface of a white dwarf has very strong gravity. This gravity pulls in material from the nearby star onto the white dwarf. The white dwarf gets more and more massive as it pulls in material from its companion star. This process is known as accretion.
Something called electron degeneracy pressure stops the white dwarf from collapsing. This means the electrons inside each atom repel each other. This prevents the white dwarf from shrinking any further.
During accretion, the mass of the white dwarf increases. If the white dwarf grows to over 1.44 times the mass of the Sun, the electrons are no longer strong enough to prevent the star from collapsing. At this point, the star explodes as a type Ia supernova. This mass limit is called the Chandrasekhar limit. During this type of explosion, the star is completely destroyed.
Type Ia supernovae are one of the brightest events in the Universe. They are many times brighter other kinds of supernovae. Type Ia supernovae are always the same brightness. This is because the explosion always takes place when the white dwarf reaches a set mass. We call objects of known brightness, standard candles. We use standard candles - like type Ia explosions - to measure distances in space. These observations have helped prove the Universe is expanding and accelerating.