from an “it’s alright I guess,” nova?

Hey everyone, Julian here for DNews. The sun is a miasma of incandescent plasma that’s

been burning for about 4.6 billion years. In another 5 billion it’ll run out of hydrogen

fuel and after going through some expansions and contractions will eventually lose its

outer layer, leaving a little glowing core called a White Dwarf to cool off in space.

As star deaths go it’ll be a pretty peaceful one, but sometimes stars say YOLO and go out

in a blaze of glory. They explode with a flash brighter than entire galaxies and send shock

wave shooting out in all directions. We call these explosions supernovae.

So what separates our sun’s fade to black from a grand finale? Something called the

Chandrasekhar limit, named for the Indian physicist Subrahmanyan Chandrasekhar who figured

out why some stars go boom, and he did it at the age of 19. Feeling old yet?

Chandrasekhar calculated that if a white dwarf had a mass about 1.4 times that of our sun’s

it would not be able to fend off the force of gravity. It would collapse, but as it collapsed

it would ignite a runaway chain of fusion reactions and Bam! Supernova.

So mass is the key to the galaxy’s greatest fireworks show, and there are two ways stars

can reach that Chandrasekhar limit. The first way is by leaching off another nearby star.

If a white dwarf is in a binary system it can pull matter from its partner until, like

all unhealthy relationships, it ends with a fiery explosion. This is called a Type Ia

supernova.

A Type II supernova begins with a single star that was huge in the first place. A star would

have to be at least 8 times more massive than our sun to have a core heavy enough to light

that giant space candle. Stars that massive don’t stop at fusing carbon and helium into

oxygen like ours will. They’ll burn neon, oxygen, and silicon to keep the fusion going.

But once they create iron, they’re done for because iron uses more energy to fuse

than it puts out. When the fusion can’t be maintained gravity wins out and the star

contracts, cramming protons and electrons together into neutrons, unleashing a wave

of neutrinos that would exert a huge outward pressure. The contracting outer layers would

also rebound off the dense inner ones. When these layers slam into each other heavier

elements are created and distributed through space. The explosion also frees up elements

like carbon and oxygen that would otherwise have been locked up in the star’s core,

so from a star’s death we get life. Once the explosion dissipates as a nebula, it leaves

behind a ball of densely packed neutrons called a neutron star that’s only a few miles across.

If the star was really massive, the neutrons will be crushed and form a black hole. And

if the star was really really massive, it leaves behind something called your mom.

Because stars have to be so huge to explode, supernovae don’t happen very often. In our

galaxy they only happen about twice a century. One star on supernova watch is Betelgeuse,

burning off the shoulder of Orion. Betelgeuse is a red supergiant at least 430 light years

away. When it does go, it’ll be visible in daylight, but it probably won’t explode

for another 100,000 years, unless someone says its name 3 times.

Betelgeuse is far from the most massive star ever discovered. The snappily named R136a1

holds that distinction, at 265 solar masses. Stars like this are pushing the upper limits

of size, and their supernovae will be mind blowing. The largest supernova ever observed

was seen by the All Sky Automated Survey for SuperNovae, or ASAS-SN. Designated ASASSN-15lh,

the explosion was 20 times brighter than all the stars in our galaxy combined. The explosion

released 10 times more energy than the sun will in its entire lifetime. Luckily it was

3.8 billion light years away.

Knowing how massive a star is gives us a good idea of what it’ll do. But how do we figure

that out in the first place? Trace explains that here.