Monday, July 19, 2021

Vampire Stars, Type Ia Supernovae, and How to Fathom Galactic Distances

Scientists can find distances to far-flung galaxies in the Universe by studying the dimness of Type Ia supernovae, which give off a standard amount of light.

Type Ia supernovae occur in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf while the other can vary from a giant star to an even smaller white dwarf. White dwarf stars are one of the densest forms of matter in the Universe. Just a teaspoon of matter from a white dwarf star would weigh five tons on Earth. Because a white dwarf star is so dense, its gravity is particularly intense, and it acts as a stellar vampire, sucking matter from its companion star and adding that matter to itself.

When the white dwarf reaches about 1.39 solar masses (the Chandrasekhar limit), a nuclear chain reaction occurs, causing the white dwarf to explode. The resulting light is 5 billion times brighter than the Sun. 

 According to a formula known as "the inverse square law," if we know the actual brightness of an object, we can measure how far it is from us by analyzing how dim it appears. Now, since the astronomers know how bright these objects truly are, they can measure their distance from us by analyzing how dim they appear.

Type Ia supernovae are of particular interest in cosmology, because they can be used as "standard candles" to measure distances to far-flung galaxies in the Universe. They can also be used to calibrate the accelerating expansion that is driven by dark energy.

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