“The diversity of the phenomena of nature is so great, and the treasures hidden in the heavens so rich, precisely in order that the human mind shall never be lacking in fresh nourishment.” -Johannes Kepler
So said the man who, in 1604, discovered the supernova that was the last to be seen, visually, within our own galaxy. Although it’s likely that two others occurred subsequently, they were not visible to human eyes, and only with powerful telescopes were their remnants discovered.
With a typical rate of about one supernova per galaxy per century, one can’t help but wonder, as one of our perennial commenters did, what we’d see — and how quickly we’d manage to see it — if a supernova went off in our own galaxy.
Remember, now, there are two ways we can have a supernova, but both ways involve a runaway nuclear fusion reaction, giving off a tremendous amount of light and energy. But most of the energy, perhaps surprisingly, isn’t in the form of light! Let’s take you inside a star that goes supernova during those critical few seconds.
Although there are shocks and heat that are produced, you’ll see that the interior reactions produce neutrinos, nearly all of which do not interact with the outer layers of the star! A few of them do, as do all of the protons, neutrons and electrons produced, and the overall production isn’t instantaneous. But while it takes some time — a couple of hours — for the shock to reach the outer surface of the dying star, the neutrinos make it out almost immediately!
What this means is that when we have a star go supernova, neutrinos get emitted from it before the light does! We actually discovered this, firsthand, back in 1987.
When supernova 1987a went off just 168,000 light years away, it was close enough — and we had enough neutrino detectors operating — that we detected 23 (anti)neutrinos over a timespan of about 13 seconds. The largest detector, Kamiokande-II, contained 3,000 tons of water and detected 11 antineutrinos.
Today, the detector that sits in the same spot, Super Kamiokande-III, contains 50,000 tons of water and contains over 11,000 photomultiplier tubes. (There are many other excellent neutrino detectors around the world, but I’m focusing on this one in particular as an example.)
This setup is so amazing because it can not only detect neutrinos, but it can reconstruct the direction, energy, and point-of-interaction of even a single neutrino fortunate enough to interact with any one particle in those 50,000 tons of water!
Depending on where a potential supernova goes off in our galaxy, we would expect Super Kamiokande-III to see anywhere from a few thousand antineutrinos (for something on the other side of the galaxy) to over ten million of them, all in the timespan of just 10-15 seconds!
Neutrino detectors from all over the world would see a flood of detections like this, all at the same time, all coming from the same direction. At that point, we’d have something on the order of two-to-three hours to identify the direction of origin of those neutrinos, and point our telescopes towards that direction to try and obtain an optical view of the supernova — for the very first time — from its very beginning!
The closest supernova since 1987 was this one from last year, which we managed to catch just half a day after ignition, which is remarkable.
We’ve gotten very close — mostly by good fortune — with a very intense hypernova back in 2002.
Even so, we didn’t get to first observe this one, SN 2002ap, until 3-4 hours after first ignition. If the supernova that eventually comes is a type Ia supernova — which originates from a white dwarf — we have no way of predicting where in the galaxy that will occur. White dwarfs are simply too abundant, and the locations of almost all of them are simply unknown, and thought to be distributed all over the galaxy.
But if this originates from a very massive star whose core collapses under its own gravity (i.e., a type II supernova), we have a number of really good candidates, and some outstanding places to look.
Most obvious is the galactic center, the location of the Milky Way’s last known supernova, and also the location of the most massive stars ever discovered within our galaxy. We’re certainly going to have many type II supernovae originating here over the next 100,000 years, but we have no way of knowing when we’ll see the next one. As you look at the above picture, take a moment to appreciate that it’s very likely already happened, and we’re just waiting for the neutrinos (and then the light) to get here!
But there are closer candidates than the galactic center.
Look inside one of the great, star forming nebulae in our galaxy, and you’re going to find some of the hottest, youngest stars you’re going to find anywhere in the Universe. This is where the ultra-massive stars live, and in particular, the Eagle Nebula, above, may be home to an extremely recent supernova. The Eagle Nebula, the Orion Nebula, and many other regions filled with new stars are all great places to anticipate the next supernova.
But what about known, individual stars? While there are many good candidates, we have two in particular that we can’t help but talk about.
Eta Carinae, in the very last stages of its life, could literally go supernova at any second. But it may also live hundreds, thousands, or even tens of thousands more years before it does so. Still, if we get a flood of antineutrinos originating from anywhere near its position in space, this will be the very first place we point our telescopes!
But unlike all of these candidates that are many thousands of light-years away, we have one good one that’s much closer. In fact, it’s the closest supernova candidate we have!
Say hello to Betelgeuse, a red supergiant just 640 light-years distant. Betelgeuse is so gigantic that it literally is the diameter of Saturn’s orbit around the Sun! If Betelgeuse went supernova, our neutrino detectors around the globe would detect — all told — somewhere in the vicinity of a hundred million (anti)neutrinos, which is more neutrinos of any type than have ever been detected in the history of the world, combined.
But unless it’s one of these known candidates that goes supernova, how will we tell whether it’s a type Ia or a type II supernova?
We can always wait, I suppose. Supernovae of different types have very distinct light-curves, and how the light dies off after it’s reached its peak brightness will tell us what type of supernova we had.
But if something this exciting happens, I’m not going to have that kind of patience. Luckily, I won’t need it, because a supernova within our galaxy would likely be the very first detectable observation for the newest type of astronomy: Gravitational-Wave Astronomy!
Undisturbed by the presence of, well, anything, gravitational waves from a supernova explosion should pass through the intervening star, any gas, dust, or matter completely undisturbed, arriving at the same time the front end of the (anti)neutrino pulse arrives! The wonderful thing is that — according to our best simulations of general relativity — type II (core-collapse) and type Ia (inspiraling white dwarfs) should give vast different signals for gravitational waves!
If we have a type Ia supernova, we expect to see three separate regions to our signal.
The inspiral phase should give a periodic pulse that increases in frequency and magnitude as the white dwarfs reach their final stage of their separation. As the ignition occurs, there should be a spike in the signal, followed by a “ringdown” phase as the ripples go away. Very distinctive.
But if we have a type II supernova, from a super-massive collapsing star, we’re only going to see two interesting things.
Just a huge spike — where the supernova itself occurs — just a tenth of a second after the core collapses, followed by a very rapidly dying (within 0.02 seconds) ringdown. And so if we want to know what we saw, all we need to do is extract the telltale signal from gravitational waves!
And if the galaxy’s next supernova were to happen today, this is what we’d see!