When a massive star comes to the end of its life, it doesn’t go out with a whimper,
rather it explodes in majestic fashion in what we call a supernova.
This is one of the most energetic events that we know of.
At its peak brightness, a powerful supernova can be as luminous as an entire galaxy.
This luminosity doesn’t last long, as the energy is expended in a short space of time,
perhaps lasting only a couple of days at most.
On an astronomical timescale, that is just a heartbeat.
So how often do supernovae occur?
There’s certainly never been one mentioned in the media recently, right?
Well, let’s wait together for the next supernova to occur somewhere in the universe.
Start the clock now.
And stop.
Only one second.
As you can see, supernovae are pretty common, but they aren’t all that easy to spot.
For one, they have to occur somewhere in the observable universe, and be close enough that
we can see them and notice them.
But, in just the astronomically short space of time that humans have been able to observe
other galaxies, we have seen tens of supernova directly.
This image of galaxy NGC 4526, captured by Hubble in 1994, shows a supernova on the outskirts,
during its peak brightness.
Astonishingly, it is so bright, in this image it appears to be a star in our own galaxy.
But being in another galaxy means even though it was this bright, only strong telescopes
were able to observe it.
But what about naked-eye supernova events, how often do they occur, and what would they
look like?
Well it depends on the distance to us.
The most recent naked-eye supernova event was SN 1987A, a supernova that occurred in
the Large Magellanic Cloud in 1987.
This is the only supernova that’s been observed up close in our local group of galaxies, as
the Large Magellanic Cloud is a satellite galaxy to our own Milky Way at only 160,000
light years away.
At this supernova’s peak brightness, it reached an apparent magnitude of 3, meaning
that although it was visible to the naked-eye, it would have only appeared like another star
in the night sky.
However, this event was of particular interest to scientists as they had had the Large Magellanic
Cloud mapped out, meaning they could pinpoint the exact star that exploded, and view the
before and after shots.
And the after shots are spectacular.
This is the remnants of the star.
This ring around it is stellar material ejected by the star 20,000 years before the explosion,
and as the shockwave from the supernova expands you can see it impact the ring, lighting it
up through ionisation.
But with an apparent magnitude of 3, it’s not super impressive for those of us wanting
to look up and see something fantastic in the night sky.
The previous naked-eye supernova occurred in 1604, known as Kepler’s supernova.
Occurring only 20,000 light years away, it had an apparent magnitude of -2.5, much brighter
than any other star in the night sky and even visible during the day.
This brightness lasted a couple of weeks.
We can now observe the remnants of this supernova using modern day telescopes.
But perhaps the brightest supernova in recorded history was in 1006, with an apparent magnitude
of -7.5, meaning it was 16 times brighter than Venus, clearly visible during the day.
Thanks to historical records of early astronomers, we can pinpoint this supernova’s remnant
too.
But even SN 1006, the brightest supernova on record, was still 7,500 light years away.
Are there any stars even closer than that that might erupt in a supernova soon?
And if so, what would that be like?
As it happens, there are 12 stars that are approaching the end of their lives that would
erupt in a supernova less than 3,000 light years away from us.
This is the list of them.
One of them, one of the most famous stars in our night sky, is Betelgeuse.
Found in the Orion constellation on Orion’s shoulder, this star is distinctively reddish
in colour.
This is because it is a red supergiant, which is really near the end of its life.
We don’t know for sure when any supernova will occur, as we haven’t observed any close
enough to look out for tell-tale signs, but it is expected that Betelgeuse could go any
moment really, although it could also still take tens of thousands of years.
It is only a few hundred light years away from us, and it is estimated that it would
appear brighter than a full moon when it finally erupts in a supernova.
But don’t fret, even at this distance, it is expected that no real harm would come to
life on Earth.
A supernova would have to occur within 50 light years of us for any harm to befall us,
and we just don’t see any eligible candidates for that right now.
Although, it should be noted that a certain type of supernovae can also occur with White
Dwarves.
If a lot of mass gets fed into these tiny, dense stars by other nearby stars, this could
trigger a supernova.
Because of their tiny and dim nature, we don’t have a good understanding of how many there
are out there.
It is thought that there could be a few hundred of them within 50 light years from us that
we don’t know about.
We do seem to be overdue for a supernova somewhere in our galaxy though.
Supernovae are believed to occur every 50 years on average in the Milky Way, yet we
haven’t seen one with modern telescopes yet.
And you can be sure that when it does finally happen, that everything about it will be observed
for years to come.
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