When some stars die they explode in a huge
supernova.
But what separates a supernova 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.
