Written in the Stars
So astronomers turn to exploding stars, called supernovae. Supernovae, which occur within a galaxy about every 100 years, are among the brightest events in the sky. When a star explodes, it releases so much energy that it can briefly outshine all the stars in its galaxy. In fact, we can sometimes see a supernova occur even if we can't see its home galaxy.
To determine distances, astronomers use a certain type of exploding star called a Type Ia supernova. Type Ia supernovae occur in a binary system — two stars orbiting one another. One of the stars in the system must be a white dwarf star, the dense, carbon remains of a star that was about the size of our Sun. The other can be a giant star or even a smaller white dwarf.
White dwarf stars are one of the densest forms of matter, second only to neutron stars and black holes. Just a teaspoon of matter from a white dwarf would weigh five tons. Because white dwarf stars are so dense, their gravity is particularly intense. The white dwarf will begin to pull material off its companion star, adding that matter to itself.
When the white dwarf reaches 1.4 solar masses, or about 40 percent more massive than our Sun, a nuclear chain reaction occurs, causing the white dwarf to explode. The resulting light is 5 billion times brighter than the Sun.
Because the chain reaction always happens in the same way, and at the same mass, the brightness of these Type Ia supernovae are also always the same. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the astronomer who discovered it.
To find the distance to the galaxy that contains the supernova, scientists just have to compare how bright they know the explosion should be with how bright the explosion appears. Using the inverse square law, they can compute the distance to the supernova and thus to the supernova's home galaxy.Back to Top
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