A “Standard Candle” of the Universe

September 8, 2009

Over the spring and summer, we finished our series on the lives of stars by describing the most violent event in the universe: a massive dying star exploding as a Type II supernovae.

But there is another type of supernova with a completely different physical cause.  This is the Type Ia supernovae, which also turns out to be indispensable to astronomers for measuring the size of the universe.

Like their Type II counterparts, Type Ia supernovae show up in our galaxy and in others.  Like Type II, they are immensely bright… about 5 billion times as bright as the Sun.  Many Type Ia supernovae are discovered each year, some by amateur astronomers.

Type II supernova occur in a single massive star that’s collapsed after running out of fuel in its core.  But Type Ia supernovae occur in a double or multiple-star system.  One star in the system is a white dwarf, which pulls gas from its companion main sequence or red giant star onto its surface.   When the white dwarf collects enough new mass to approach the Chandresekhar limit of 1.4 solar masses (see White Dwarfs) the star collapses and explodes as a supernova.  Ka-boom!


SN1994D

Supernova 1994D in galaxy NGC 4526

A more recent view suggests the white dwarf doesn’t collapse, but gets hot enough to burn all its carbon in an immense runaway nuclear fusion reaction that we see as a supernova explosion.

But here’s the important thing… all white dwarfs collapse or explode at about 1.4 solar masses, which means they make the same kind of explosion with the same intrinsic brightness.  And if all Type Ia supernova have roughly the same intrinsic brightness, they serve as excellent distance markers to the galaxies in which they occur.  The fainter a Type Ia appears, the farther away it must be.  In this way, astronomers can measure the distances to far away galaxies for which there is no other practical distance marker.

How can astronomers tell a Type Ia from a Type II?  Type Ia supernova have an optical spectrum that shows almost no hydrogen in the explosion… this is very different from Type II blasts, which contain a lot of leftover hydrogen from the exploded star.  Type Ia supernova also have a distinct “light curve, a way of brightening and fading over time.

Posted in Science of Astronomy