Type Ia supernova

A white dwarf that explodes, the nickel that makes it shine for weeks, and how its brightness measured the accelerating universe

Watch it in 3D

Step by step

A dead star that explodes

A Type Ia supernova is a white dwarf, the ember of a dead Sun-like star, blowing itself apart. This one is being fed by a companion star.

Some white dwarfs have a companion close by. Gas streams from it onto the white dwarf and settles in a hot disc, so the white dwarf slowly gains mass. In the picture the white dwarf is drawn many times larger than to scale, so that it can be seen next to its companion.

The tipping point

Before it reaches the limit, the centre becomes dense and hot enough for carbon to start fusing. Where the extra mass comes from is not settled: gas from a companion, or a merger with another white dwarf. SN 2011fe, caught within hours of exploding in 2011, showed no sign of a giant companion (Li et al., 2011).

Runaway

Fusion in a white dwarf cannot make the star expand and cool, as it would in a Sun-like star, so it runs away: the flame heats the gas and the hotter gas burns faster. Models suggest the flame starts slowly and then turns into a supersonic detonation. Exactly how is still debated.

Nickel, cobalt, iron

Nickel-56 is radioactive, and its decay, then cobalt-56's, sends out gamma rays that heat the thick debris and make it shine. Without them the explosion would fade quickly. Much of the iron around us, including in our blood, is thought to have been made in explosions like this.

The light curve

Real Type Ia supernovae differ a little: the brighter ones fade more slowly, a relation found by Mark Phillips in 1993 that lets astronomers correct for it. This simple model assumes all the gamma rays stay trapped, so it fades more slowly than real ones do after about two months.