Once Around Supernova 1987A

追加: 2026-05-08

[00:00:02]Once around supernova 1987a.

[00:00:06]So SN987A was the explosion of a giant star in a small dwarf galaxy 160,000 lighty years from the sun. It was seen on the 23rd of February for the first time as the star detonated. The core collapsed inwards and the outer layers were blasted away in a huge explosion. And you can see the explosion in the two images on the right there. A wide field image and a much more zoomed in if showing the shock waves emanating out from the center. It was located in the constellation of Dorado in the southern hemisphere inside the large melanic cloud near a feature called the tarantula nebula. Now on the map there, the large melanic cloud is the green ring where it says 2070.

[00:01:00]That is the center of the tarantula nebula.

[00:01:04]It was detected twice independently by Shelton and Duhal at the La Campanis Observatory in Chile and also by amateur astronomer Albert Jones in New Zealand.

[00:01:18]It's shown in the image on the right here as the very bright thing just to the right of the center. Up to the left there is the Tarantula Nebula and it was peaking at a magnitude of three which actually made it brighter than any of the stars in the constellation of Dorado.

[00:01:37]Having been discovered, investigations proceeded looking back through previous photographs of the area, trying to determine what it was that had exploded, and pre-covery images were made to show that the star was rapidly brightening during that month. Now, the large melanic cloud, 160,000 lighty years or so away from us, that's a small dwarf galaxy that's captured into orbit around the Milky Way. You can see this rather nice image of it in the sky, and I've seen it in the sky overhead in New Zealand when I had the pleasure to visit about 20 years ago. And then up to the upper left hand side of the galaxy, there is the bright area. That's the Tarantula Nebula. If we zoom right in on that, we get this fantastic image here showing a huge amount of gas and dust out of which many new stars have been formed and are still forming. And right in the center there, that hot young cluster, the the designation of which is NGC 2070, the 2070 on the star map that I showed you. So this contains many many hot young stars, many of which are giant and destined to live short violent lives and explode as supernovi. And uh the one that gave rise to supernova 1987a of course did so.

[00:03:11]It's actually the best observed and closest supernova to us since604.

[00:03:18]or Kepler's supernova of that year was much closer. That was inside our Milky Way, but of course it predated the invention of the telescope. So nobody was able to uh investigate that in detail.

[00:03:34]So 1987A is the first to have been studied with all our modern instruments, and we're still waiting for a new supernova inside the Milky Way to explode at some point.

[00:03:47]uh so it would be vastly closer to us and we'd get a much better view.

[00:03:53]When we look at supernovi we often follow the light curve. You observe how bright the object appears to be from an initial peak and then gradually decaying away in a classic curve as per the curve here from 1987a.

[00:04:14]And this is driven by the initial flash and then the radioactive decay of the isotopes that are created in the explosion.

[00:04:28]The radioactivity of those emits gamma rays and the gamma rays are then picked up by the gas and dust trapping the energy heating the material causing uh further expansion of course and causing it to glow very brightly indeed.

[00:04:44]But inevitably the radioactive isotopes decay away and the brightness drops and does so in this characteristic curve with an initial sharp peak. Um then a small shoulder where it says radioactive tail 56 cobalt at the top there uh caused by that isotope and then you get uh further processes dust formation and then a radioactive tail caused by longer lived isotopes.

[00:05:15]So the prime reactions are 56 nickel decaying to 56 cobalt giving out a gamma ray and an anti-elect electron a posetron. Um the anti-electron soon meets an ordinary electron and gives more gamma rays of course as it annihilates. And on average it seems to be around the 9 days of duration for that initial very rapid peak as that first reaction occurs.

[00:05:46]Then cobalt decays to iron much more slowly releasing energy again in particular gamma rays and again anti-electrons carrying away the positive charge. um that's much slower and gives you a prolongation of the light curve. And as it goes down, those isotopes decay and it's left to longer lived ones. In particular, the decay of things like isotope 44 of the element titanium, 44Ti, and that has a halflife of about 60 years. So now all the cobalt has decayed pretty much. And there is still most of the remaining power at a lower level is being emitted by the decay of isotopes like uh 44 titanium. We've detected that in the X-ray spectrum with the integral spacecraft and gamma rays have also been detected with gammaray telescopes and the gammaray spectrum is shown at the top there with a peak that is uh put down to 44 TI decay.

[00:06:56]So what happened? What came before the supernova? Where was the progenitor star? Well, that is it shining with the six defraction points around it just to the left of the center of the tarantula nebula and it was located just 4 days after the event. We went back and found images and worked out which star it must have been that had been responsible for the explosion. And it's called Sandulik 69202 and was a blue super giant star observed by Nicholas Sandulik in 1970 and documented. It was magnitude 12. So the great distance resulting in it being fairly faint but nevertheless a real powerhouse given how far away it was. a blue super giant around about 20 times the mass of our sun classed as a B3. So a fairly hot B-class star. Not the hottest and most massive. The Oclass or even the Wolf Ray stars, those are even hotter and more powerful and more massive, but nevertheless quite a giant temperature 16,000 Kelvin putting out a 100,000 times the output power of our sun. So somewhat similar in many ways to stars like Riel and Denb that we see in our night skies because they're dramatically closer to us. Of course, apparently a normal luminous super giant in the run-up to the explosion. It may well have been a luminous blue variable in recent past in the last few thousands of years or so. Uh we're not quite sure about that. But we just don't have very many observations of it. Wasn't really noted until before 1970 after all. But the blue super giants exploding was not something that anyone expected.

[00:09:03]We understood that it was red super giants, red hyper giants that exploded as core collapse supernovi.

[00:09:13]And to see a blue giant doing so directly was a surprise. So much so that people tried to explain what might have happened in other ways, suggesting that there had been a stellar collision or some very rare event. But based on the properties of super 19 supernova 1987a and it progenitor as a blue super giant a wide search was made for other supernovi and it seems that we have found several other cases of blue super giants exploding. So the models of ste stellar evolution needed a bit of an update and the image on the right there that is a computer simulation of a blue super giant explosion. So we now think that we understand what's going on after the explosion. We took lots of images and there's a time lapse here taken by the Hubble Space Telescope of the rings forming around the central explosion. And as the central explosion faded, the rings brightened and produced a very clumpy appearance.

[00:10:25]Now, this is not material that was ejected by the supernova. It's material ejected in the preceding 10,000 years, the stellar wind from the hot blue super giant pouring material out and forming into a rings and clouds and clumps around the star.

[00:10:48]about 2/3 of a lightyear away from the center. And this was then illuminated and energized by the flash of X-rays and ultraviolet light which took 8 months to cover that 2/3 of a lightyear to reach it and cause it to then begin to glow, ionizing the material. And we've been able to study the emission lines in the spectra of the rings there and work out what they were made of as well, revealing a lot of dirt and dust. And that's excellent because supernovi were thought to be a major source of cosmic dust, carbon grains, silicate grains, uh metal dust, that sort of thing. But this was the real first confirmation that this was a major source that a lot of the dust had been ejected by the star in the first place and then yet more was poured out in the supernova itself coming from the very central region. So sort of two sources of dust and that accounts for how much dust there is in between the stars as a result of all these previous generations exploding.

[00:12:07]Now we also saw the light bouncing back directly from these outer shells of materials in what's called a light echo.

[00:12:16]So we saw the flash and then 8 months later the light had traveled outwards and started penetrating the shells of overlying material lighting them up and some of the light just bouncing back our way and having taken a longer path. The direct line of sight had taken 160,000 years but to go sideways first and then come towards us took an extra eight months. And in fact, because we can see the size of the rings and we know the speed of light, um, we can work out a better estimate of the distance by a spot of triangulation.

[00:12:54]And doing that, we came out with an accurate distance of 168,000 light years out to uh, supernova 1987a.

[00:13:04]definitely inside the large melenic cloud. But refining our distance estimates, the truly fascinating thing about this was the neutrino burst at 7:35 universal time. three different h sorry 3 hours before the visible light arrived three different detectors saw a burst of neutrinos. Now when I say a burst it was 12 neutrinos detected by the cameo candandy 2 experiment in Japan imaged in the top right there with all those myriad detectors.

[00:13:49]Eight were detected in the Brook Haven detector shown at the very bottom right there with a diver swimming in the the water inside its pool and five by the back sand detector shown in the middle there. So not very many particles but enough to stand out from the relatively low background rate just during the 13se second burst that had arrived.

[00:14:15]And of course at that time nobody knew what it was but three hours later the visible supernova was detected and uh the uh two can then be associated. These detectors are reasonably able to back calculate the direction of the arrival of the neutrinos and so the the uh burst could be tied back to SN1 1987A.

[00:14:43]Now those neutrinos are released when the core of a star collapses to a neutron star. You get neutrinos at other times but you get an absolute blizzard when a supernova of this type collapses.

[00:14:58]As it does so, the enormous pressure in the core crushes the protons and electrons together to form neutrons and a burst of neutrinos is released carrying away the uh leptton number because the electrons are destroyed.

[00:15:15]Electrons are lepttons so are neutrinos and you have to keep the number of lepttons the same. So when you destroy an electron, the leptonness has to go somewhere and it's the neutrino that carries it away. But the visible light from the explosion has to fight its way out through the core of the remaining overlying layers of material and break out to the surface. And it's that process once the shock wave breaks the star apart then you get to see the visible flash. And that takes the 3 hours uh plus that uh results in the delay of the visible light arriving. Of course, the light is traveling towards us at the speed of light as fast as it can. The neutrinos are nearly going that fast. So the light gradually overhauls them and reduces that initial delay by a small amount uh depending on exactly how fast the neutrinos are going. But they are doing something like 99.9% the speed of light some of these. So they are terrifically energetic.

[00:16:24]Now our calculations suggest that the total energy output of the supernova was 10 ^ 46 jewels of energy and that 99% of that was carried away by the neutrinos.

[00:16:39]And this fits with our models. 10 ^ 58 neutrinos are created. a truly staggering number. And of course, they radiate out in all directions across the sphere, ever expanding around the explosion. And from a distance of 160,000 light years, it's no surprise that we only detected about 20 of them.

[00:17:04]Doing this and detecting the burst of neutrinos has helped us confirm some of our understanding of them. It's set an upper limit on the mass of the electron neutrino at 30,000 times smaller than that of an electron or less. And it's confirmed that there must be fewer than eight different types of neutrino. Now, that's fair enough. We think there are three. There might be more. We don't really know. Um, some theories like string theory demand that there are additional types of neutrino, but we've never seen any. So uh it puts some constraints on the standard model of physics confirming what we already guessed.

[00:17:49]But the mystery is that we couldn't find the neutron star. It looks like a neutron star had been created. The burst of neutrinos agrees with that. But searches including using the Hubble Space Telescope could not find it. Now that is a bit of a mystery. And although as time went on, we got some clues. In 2019, the ALMA uh the Atakama microwave telescope in the desert there in Chile with millimeter uh radiation was able to make detections that seem to point to the presence of a neutron star inside a dense clump.

[00:18:36]And in 2021, hard X-rays were detected, highly energetic X-rays that probably come from the Pulsar Wind Nebula, the nebula that is illuminated by all of the high energy magnetic field and jets that that creates from the rotating pulsar that's probably in the center. So, not a direct detection of the neutron star, the rotating neutron star being a pulsar, um, but clues.

[00:19:10]Enter James Webb, the James Webb Space Telescope. This, of course, has got fantastic resolution, but is coming to the party slightly late. So now this image that JWST produced shows this equatorial ring of material that was ejected tens of thousands of years before the supernova now glowing in infrared. Remember James Web is an infrared telescope. And in this image uh the blue color is 1.5 micron infrared.

[00:19:42]Cyan is slightly uh longer wavelengths of 1.64 64 to two and the yellow color is 2 uh sorry 3.2 to four microns and red is anything over four microns. So it's a composite multi-wavelength image and the bright spots again are those dense clumps of material heated by the expanding shock wave. But we couldn't really see into the center. Delving right down into the middle. Just there's too much dust. JWST was able to find ionized argon in there. And that also agrees with the uh presence of a neutron star. And you can see the outer rings just fading away there. Those two initial shock waves still moving outwards in the image. So, is the neutron star hiding behind a huge cloud of cosmic dust created by the supernova?

[00:20:43]Possibly. So, perhaps a lot of that material though fell back onto the neutron star, boosting its mass over the upper limit of the mass of a neutron star, the Tolman Oppenheimer Vulov limit of about 2.2 solar masses. And anything more massive than that is predicted to collapse all the way down to a black hole. But there was one odd result and it goes back to those neutrinos.

[00:21:15]Here's the actual data from the three different detectors. Yellow for the camei, the IMB at Brooklyn in red, and the Baxan in blue. And I've put the large ovals around them in two groups. There was an initial burst of neutrinos, nine from the Cameo Candai and some others from the other two detectors during that first few seconds, the first couple of seconds of the uh explosion. And then around the 6 to 10 second mark, there is a second weaker group of neutrinos that seem to have arrived perhaps with slightly lower energies.

[00:22:00]Maybe that's lower energy. Certainly the red ones seem lower. The yellow ones you could argue that on average they're lower and the blue ones may be about the same. But nevertheless, this is pointing to a two-phase collapse. As if it collapsed, produced a burst of neutrinos, quite an intense one, and then after a few seconds there was a second phase of collapse creating more neutrinos.

[00:22:30]And the possibility that remains has been discussed in a number of papers.

[00:22:35]There's one here by these Chinese authors from the university in Hong Kong there. Um, and there are several others discussing the idea that 1987a might have been a quark star in formation, the generation of a star made of strange matter, an intermediate between full collapse from a neutron star to a black hole. Maybe these quark stars can exist in the 2.2 to about four solar mass range as even more extreme objects. And I discussed that in one of my very earlier videos called Quark Stars and Strange Matter, which was actually perhaps my most popular um video that I produced during the uh lockdown period when I started making videos in order to keep the Cambridge Astronomical Association alive. Anyway, I hope you can enjoyed this one and I hope you can go and enjoy Quark Stars and Strange Matter as well. A lot of people have. Thanks very much for listening. I'll see you on the next one.