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It’s not every day you see a star explode. Unless you’re an
astronomer studying supernovas, and then you very well
might. When you look at the whole universe, they’re a
common cosmic catastrophe. And some can outshine an
entire galaxy. So it’s a good thing that they’re happening
well away from our solar system. While a few prominent
supernovas have been witnessed without the aid of telescopes, astronomers have used this
invention to learn more about exactly how
stars die spectacularly. And they’ve learned that there’s
more than one way to do it. So let’s dive into five of the
most explosive ways a star can end its life. On February 23,
1987, humanity witnessed a supernova on the Milky Way’s
cosmic porch. Supernova 1987A, as it’s known,
happened a mere 170,000 light years away, in a dwarf galaxy
called the Large Magellanic Cloud. As the closest observed
supernova in nearly four centuries,
it was visible to the naked eye for months. And astronomers have been watching it through their
telescopes ever since. Most agree that 1987A began as a
regular, blue supergiant star about twenty times the mass
of our Sun. Like all stars, it spent most of its life fusing
lighter elements into heavier ones, creating light in the
process. But after ten million years or so, it had run out of
fuel. There were still plenty of atoms hanging around, but it
wasn’t hot enough to fuse those elements, too. So the fusion engine shut down,
and the entire star started collapsing in on itself. With
the pressure of that contraction, temperatures at the core got
hot enough for the fusion furnace to ignite once more.
At least for a little while, until it ran out of fuel again. For a blue
supergiant, this cycle happens a few times,
until the core is filled with iron. And iron just so happens to be a special
tipping point for stars. Smaller atoms give off energy
by fusing together. Bigger atoms give off energy
by breaking apart. But giant stars have no good way to get energy from iron itself.
Instead, as the star’s mass descends upon the core one
final time, the iron is squeezed so tightly that physics
becomes a little fuzzy. Protons and electrons start merging
to form neutrons. The rest of the falling gas bounces off
this new neutron core and explodes outward, propelled by
a torrent of light and other particles into interstellar space.
The catastrophic result is what astronomers call a Type II,
or core collapse, supernova. In a fraction of a second,
1987A released more energy than our Sun will put out in
its entire ten billion year lifetime. But in destruction, there is
the opportunity for creation. The expanding clouds that
these supernovas leave behind are filled with the elements
that can construct a planet, as well as the curious
inhabitants that call it home. When people describe a
supernova, they often describe a core collapse scenario
like what happened to 1987A. Sometimes the explosion
leaves behind a super dense neutron star, and sometimes
it’s a black hole. But a collapsing core isn’t the only way for
a star to go boom. It’s just the simplest, which makes it
nice for summarizing what happens when big stars die. But
scientists love edge cases. And boy, do supernovas have some weird ones. On July 4, 1054, a new star
appeared and was so bright, you could see it during the
day for nearly a month. At night, you could see it for the
next two years. Today, we know this was the supernova
that produced the Crab Nebula and the neutron star pulsating at its heart, roughly
6500 light years from Earth. Astronomers spent years thinking
that it was just another core collapse supernova,
like 1987A, but there was one notable problem: the original
star wasn’t massive enough. Estimates put it somewhere
between eight and ten times the mass of the Sun, which is big,
sure, but not big enough for the core to build
up iron. It should have stopped when there was mostly oxygen, neon, and magnesium
surrounded by a sea of free-flying electrons, which serve
as a sort of scaffold to help support the stellar core from
collapsing further. And that mixture shouldn’t have blown
up with an energy equivalent to 1987A. So astronomers
put their heads together and realized something very
special had happened. If the star contained just the right
amount of oxygen, neon, and magnesium, the neon and
magnesium would absorb some of those free electrons into
their nuclei faster than they released them back into the
star. In other words, they removed pieces of stellar
scaffolding over time. And eventually, the crushing force of
gravity won out. All of a sudden, the collapsing core
became hot enough for one last frenzied round of fusion,
leading to an iron core, neutron star, and ultimately what
astronomers call an electron-capture supernova. This
kind of supernova was first predicted in the 1980s, but it
took until 2018 for astronomers to observe one in action,
and prove they were possible. After that, they could finally
solve the mystery of 1054, including why it shined so bright
for so long. It was an electron-capture supernova, and
appeared brighter than normal because the light from its
explosion lit up the clouds of gas that the star had thrown
off before it died. All the supernova’s light bounced around
that gas like a hall of mirrors, escaping over time instead of
all at once. Electron capture supernovas are a twist on the
usual core collapse model. But some supernovas go to
much greater lengths to stand out from the pack. Take our
next entry: Supernova 2007bi. It happened about 1.6 billion
light years away, and it first caught astronomers’ attention
because it wasn’t the typical flash in the pan. It brightened
over about two months, and then it just stayed there. A
year and a half later, it was barely any dimmer. But there
was another oddity. Certain elements had gone missing.
Before a supernova’s light can reach us, it has to travel
through layers of gas its star lost to space. And by passing
through layers made of different elements, the light records
the overall composition of what it had to travel through.
Typically, that tends to be mostly hydrogen and helium. But
2007bi’s light didn’t show the presence of either element.
Instead, it was marked by layers of heavier elements like
oxygen, sulfur, and nickel, four whole Suns’ worth of
radioactive nickel. Based on how much of these heavy
elements came out of the explosion, astronomers estimate
that the original star must have been about two hundred
times more massive than the Sun. And to explain the weird
pattern in its light, they think 2007bi was one of the first
conclusive examples of a pair instability supernova.
When such a massive star nears the end of its life, the
fusion in its core creates particles of light that are more
energetic than usual. In fact, this light has so much energy
that it can randomly transform into matter. Each particle of
light turns into a pair of subatomic particles, like electrons
and positrons. And that matter doesn’t transfer heat as well
as light does. So all of a sudden, the temperature in the
core starts dropping. And the core starts shrinking. It keeps
shrinking until it can literally shrink no more. The atoms get
shoved so close together, they fuse rapid-fire into iron,
leading to an explosion so powerful that the core is blown
to smithereens, leaving nothing behind. 2007bi was so
violent, the star kind of got turned inside-out. That would
explain the lack of a hydrogen and helium signal in the light
we see from Earth: The heavier layers swapped with them,
concealing their presence. 2007bi was wild, one of the
largest explosions humans have ever measured. But that’s
not the most extreme kind of pair instability out there. The
supernova 2016aps is in the running for the brightest
supernova ever detected, when you account for distance.
That title is hard to define, so the belt is always changing
hands. But all told, it released about fifteen times the
energy our Sun will emit over its whole lifetime. To shine so
brightly, it must have been surrounded by a dense cloud of
hydrogen, probably at least a few dozen Suns’ worth. All
that gas would help trap the light and matter bouncing
around inside, instead of letting it spread out and cool
everything down. If you’re keeping track, that’s a lot like
what happened with the supernova in 1054. To some astronomers, the story stops there.
Having lots of gas around a big, exploding
star keeps the party going longer than you’d expect. Meanwhile,
other astronomers think there must have been more to the story. One team
proposed it was thanks to pair instability, like we saw with
2007bi. But because the star behind 2016aps wasn’t as
massive as the one behind 2007bi, it took time for the
catastrophe to build up. Instead of happening all at once,
light would have transformed into pairs of subatomic
particles in fits and starts, maybe over hundreds or
thousands of years. After a series of super-bright flashes
that each had as much energy as a typical supernova, one
final bout of pair production would have done everything in.
Astronomers call it a pulsational pair instability
supernova. It’s the extreme version of an already extreme
cosmic event. We’re lucky 2016aps happened three billion
light years away. But for our final entry, we need to bring
things back a lot closer to home. In 1006, astronomers
around the world saw what was possibly the brightest
supernova in recorded history. At its peak, it outshone
every star and planet in the night sky, and it remained
visible for two and a half years. In the 1960s, astronomers
finally found the nebula that this explosion produced. It’s
called “Supernova Remnant SN 1006” and it’s about 7100
light years from us. That name isn’t as cool as “the Crab
Nebula”, but its origin story sure is. Because the supernova
of 1006 involved two stars, not one. Okay, technically one
of those stars was a white dwarf: the core of an already
dead star that wasn’t massive enough to go supernova on
its own. But if a white dwarf gets a little help from a friend,
it can jump over that hurdle spectacularly. When a white
dwarf explodes, it produces a Type Ia supernova. And the
key to blowing up white dwarfs is they have to break one
very important rule. White dwarfs can only get so heavy,
about 1.4 times the mass of the Sun. If one happens to
exceed that limit, it suddenly kicks off a round of nuclear
fusion that eventually leads to a massive explosion that
leaves nothing behind. Astronomers know the supernova
of 1006 was within the Milky Way, and based on the light it
produced they’re confident it was a Type Ia. But we don’t
know what the other star looked like. It could have been a
case of cosmic vampirism, where the white dwarf siphoned
off enough gas from a nearby companion star that it tipped
the scales and exploded. But the other object could have
also been another white dwarf, and the two overshot the
1.4 solar mass limit by smashing into one another.
Astronomers lean toward it being another white dwarf
because they can’t find a star near the remnant nebula that
looks like it could have been the companion, but no one’s
sure. What they do know is that, given how dramatic the
death was, and how close it happened to Earth, humanity
is unlikely to witnessan explosion as bright as 1006 for a
very long time. Is this the most extraordinary kind of
cosmic explosion humanity has ever witnessed? Only time,
and more research, will tell. But space is awesome, so we
shouldn’t be surprised when another supernova comes
along with an even more complicated origin story that
brings us another step closer to understanding the universe we live in. And to understand
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