The Yardstick of the Universe
How astronomers use supernovae to measure distance in space
Most of you probably know that the Universe is big. Really big. Distances melt away in the face of the sheer scale of the incomprehensible black vastness.
But life gets a whole lot more complicated once you start asking questions about how far distant objects are away. How do you measure the distance to something that is so far away and faint that even the strongest telescopes can barely make it out?
This is why astronomers are always looking for so-called “standard candles”. These are objects that consistently emit the same amount of light, no matter where in the Universe they are. These objects are also ideally really, really bright so they can be spotted from even the remotest corners of the Universe.
“A white dwarf no longer has any active fusion going on, so nothing stops gravity from causing the star to collapse in on itself.”
This is where type Ia supernovae come in. A supernova, as most of you probably know, is what happens when a giant star burns through all its fuel — a gargantuan explosion of hot gas and stardust. But a type Ia supernova is different from a normal supernova in that it isn’t a giant star that’s exploding — it’s a tiny one.
These supernovae only happen in binary star systems — where two stars are orbiting each other. More specifically, they need a situation in which one of the stars is a small white dwarf that has exhausted all its fuel and is slowly cooling down. If the other star is close enough, then the white dwarf will start ‘stealing’ hot gas from its bigger brothers’ atmosphere through gravitational attraction. This means the white dwarf will start to get increase in mass, getting heavier and heavier.
In a normal star this wouldn’t be a problem because the outward pressure produced by fusion in its core will balance out the inward gravitational force. But a white dwarf no longer has any active fusion going on, so nothing stops gravity from causing the star to collapse in on itself.
credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team
Nothing, that is, except for a strange quirk in quantum mechanics called the Pauli exclusion principle. This basically states that it’s impossible for the electrons in the star to come really close to each other, counteracting the gravitational pressure of the star and keeping it alive. But it can only do so much.
If the star keeps on sucking in hot gas, it eventually reaches a certain threshold called the “Chandrasekhar limit”. At this point, even the Pauli exclusion principle can’t keep up with the increasing gravitational force and the only option left is catastrophic collapse. This releases an enormous amount of energy, devastating the white dwarf and catapulting a massive burst of impossibly bright light in all directions.
The great thing about these explosions is that no matter how the white dwarf looked originally, it will always explode at the same mass threshold — the Chandrasekhar limit — and thus always release the same amount of energy into the Universe. This means that if you see a type Ia supernova, then you immediately know how far away it is from how bright it appears to you.
The most famous example of the use of these supernovae to calculate distance was the High-Z Supernovae Search Team which used these standard candles to prove that the Universe was not only expanding but that this expansion was taking place at an ever accelerating pace.
So the next time you lie awake at night, thinking about the ultimate fate of the Universe, remember the little white dwarfs to whom we owe this important piece of fundamental knowledge about these mysterious place we live in.
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