Tag: Supernova

Bit by bit Astronomers are learning the secrets of the massive stellar explosions called Supernovas, even if they do occasionally come across evidence that doesn’t fit their theories.

Everyone has heard a little bit about Supernovas, you know those stars who destroy themselves in explosions that for a few weeks can outshine many billions of normal stars. Supernovas are very rare events, happening only every couple hundred years in our galaxy of one hundred billion stars. In fact supernovas are so rare that most of what we’ve learned about them comes from observing ones that happen in other galaxies. It works like this, if supernova only occur once every 100 years per big galaxy then if you keep an eye on 1,000 galaxies you should see about 10 a year!

Three images, taken on successive nights, of the Galaxy M101. On August 22 (l) there was no supernova, green arrow shows location. SN2011fe was discovered on the night of the 23rd (m) while by the night of the 24th (r) it was considerably brighter. By keeping an eye on a thousand such galaxies astronomers manage to observe a dozen or so supernova ever year. (Credit: Space.com)

The very first studies of supernovas, conducted more than 70 years ago now, used spectral analysis to show that there were two basic types. One type, not surprisingly called Type 1, had virtually no hydrogen in the spectra obtained from their light. Now hydrogen is the most common element in the Universe so for Type 1 supernovas to be completely lacking in it is really significant. Type 2 supernovas are just the opposite, their spectra shows plenty of hydrogen. One thing both types have in common is that they are very rare which indicates that only a small percentage of stars ever go supernova.

In time astrophysicists came up with two rather different models of supernovas. Type 1 begin with a white dwarf, the superdense corpse of a once normal star, for example our Sun will become a white dwarf in about 6-7 billion years when it runs out of its hydrogen fuel. A typical white dwarf has a mass about that of our Sun but its size is only that of the Earth. The surfaces of dwarfs are extremely hot but because of their small size they are much dimmer than a normal star like our Sun.

If a white dwarf star steals mass from a companion star it can grow too massive leading to a collapse that triggers a Type 1 Supernova. (Credit: Phys.org)

Now if a white dwarf happens to have a companion star, and there are many examples of binary star systems, the dwarf can start pulling material away from its companion. This stealing of matter can only go on so long however because there is a maximum limit to the mass of a white dwarf. This maximum mass is about 1.4 times the mass of our Sun and if a dwarf exceeds this limit it begins to collapse triggering the Type 1 supernova. After the explosion all that’s left of the star is a neutron star or even a black hole.

The famous Crab Nebula M1 is the remains of a Type 2 Supernova. At the center of the nebula is a neutron star that emits radio signals as a pulsar. (Credit: Wikipedia)

Type 2 supernova however start as huge, very massive stars, at least ten times the mass of the Sun. The fusion reactions in such stars use up their hydrogen fuel in only a few million years. The star will then begin to fuse helium into carbon and oxygen, which is as far as our Sun will ever get. Massive stars however have enough energy to keep going, fusing carbon and oxygen into heavier elements all the way up to iron.

The supermassive star Eta Carinae, seen here in an image from Hubble, is destined to explode in a few million years as a Type 2 supernova. (Credit: Medium)

Iron is a brick wall however, fusing iron into a heavier element doesn’t produce energy it consumes it. The fusion reactor of this huge, massive, intensely hot star suddenly comes to a screeching halt and the star begins to collapse upon itself. This collapse triggers the supernova but unlike a Type 1 supernova there is still some hydrogen left in the star’s outermost regions, which shows up in the explosion’s spectra. After the explosion all that remains of the star is a neutron star or black hole.

Nuclear binding energy per nucleon. Nuclei that are less massive than iron can be fused to produce energy while more massive than iron can be split to produce energy. Whatever you do to iron however will require energy. (Credit: Conceptual Physics)

Those are the theories, but to be certain they’re right we would have to observe a star before it goes supernova and that’s not an easy thing to do. After all there are literally trillions of stars in our galaxy and nearby ones, while only a couple of dozen of those stars will go supernova each year. The question is then, which ones? Well what astronomers have tried to do is to get observations of as many stars as possible. Then when a supernova does occur they check their archives to see if they have any prior images of it.

The star Sanduleak -69degrees 202 (r) before it exploded as SN1987A (l). This was the first time astronomers were able to identify the progenitor star to a supernova. (Credit: David Malin, Anglo-Australian Observatory)

The first time that this technique worked was the Type 2 Supernova SN 1987A, in the Large Magellanic Cloud, which is a satellite galaxy orbiting the Milky Way. Almost as soon as SN 1987A was detected astronomers quickly began looking through their past observations and succeeded in finding a few observations of the star, catalog name SK-69º202 before it exploded. Although there were a few surprises SK-69º202 turned out to be pretty much what astronomers had expected, with 15 times the mass of our Sun and a very hot surface. The data gained from SN1987A taught astrophysicists a great deal about Type 2 supernovas, but of course they wanted more, and in particular, they wanted a Type 1 supernova progenitor.

SN 2019yvr in the galaxy NGC 4666 at a distance of 46 million light years. (Credit: Remote astrophotography using Slooh.com)

Now they may have one, and it’s not what they expected. Back in December of 2019 astronomers spotted a supernova, designated SN 2019yvr in the galaxy NGC 4666 which is about 46 million light years away in the Virgo super cluster of galaxies. Even as observations were showing that SN 2019yvr was a Type 1 supernova astronomers associated with the Hubble Space Telescope were rummaging through earlier images of NGC 4666 to try to see if Hubble had ever made any observations of the star.

The Hubble space telescope has made many discoveries during its lifespan. The image of the progenitor star for SN2019yvr may be its latest. (Credit: Science Focus)

The astronomers spent more than a year of checking and crosschecking between the measurements made after the supernova began to those that had been taken earlier. Nevertheless they think they may have found the supernova’s progenitor in a series of images taken some 2.6 years before the explosion, problem is, the star they’ve identified is not the kind the theory says it should be.

Instead of a tiny, dense, extremely hot white dwarf the star that’s been identified is a fairly cool orange-yellow star more than 300 times the width of our Sun. A star like that should have plenty of hydrogen left in it but the spectra of the supernova showed none, it’s just a mystery.

The type of star Hubble found however appears to be an Orange ‘K’ type star shown here with a larger ‘G’ type star, our Sun, and a smaller red ‘M’ type star. (Credit: EarthSky)

The astronomers have already come up with several explanations for the disagreement with the theory. First of all they could simply have identified the wrong star. They can’t check to make certain right now because the debris of the supernova is currently obscuring that region of space and it will take 5-10 years before they are able to see if the orange-yellow star is still there.

The expanding debris field of SN1987A makes it difficult to observe the remaining neutron star at the center. (Credit: CEA-Irfu)

Then there’s also the possibility that the companion star, remember Type 1 supernovas require a companion star, could have given off enough material to form a cool gaseous shroud around a white dwarf that was the actual supernova progenitor. Again checking this possibility will have to wait for the debris to clear a good deal.

Of course there’s also the possibility that our theories are just wrong and have to be adjusted. Whichever possibility turns out to be true astronomers are bit by bit learning the secrets of what are some of the most spectacular events in the Universe, Supernovas. 

The red super giant star Betelgeuse in the constellation Orion has lost half its brilliance. Could that mean it is about to explode as a Supernova?

One of the most familiar stars in the night sky is Betelgeuse, the star that marks the right shoulder of Orion the hunter. (Right shoulder, that means we see it on the hunter’s left!). Generally Betelgeuse is the eleventh brightest star in the sky but because there are several other very bright stars nearby Betelgeuse is very easy to find. Not only is Rigel, Orion’s left foot, slightly brighter but to Betelgeuse’s upper right is Aldebaran the eye of Taurus the Bull while to the lower left is the brightest of all true stars Sirius. I my opinion these stars together make Orion and the region around it one of the most interesting parts of the night sky, and the simplest to find.

The Constellation of Orion the Hunter. Betelgeuse is Orion’s Shoulder and is a red star. (Credit: EarthSky)

Lately however Betelgeuse has not been looking as strong and bright as usual. Astronomers have known for centuries that Betelgeuse varies in its intensity by as much as a factor of two but for the past month the star’s energy output has been the lowest it’s been for over a hundred years.

A red giant star, Betelgeuse is so large that if it replaced our Sun it would swallow all of the planets out to Jupiter. (Credit:WWW.Severe-Weather.eu)

So what’s happening to Betelgeuse? Could its recent convulsions be a prelude to something extraordinary, perhaps even the star’s soon exploding as a supernova?

Betelgeuse often goes into convulsions but its recent activity is abnormal. (Credit: Discover Magazine)

It’s worth considering; our current theories about Type 2 supernovas tell us that Betelgeuse is a prime candidate. Only very heavy stars that have used up all of their nuclear fuel end their lives as type 2 supernovas. At an estimated mass of twelve times that of our own Sun and with a bloated red sphere as large as the orbit of Jupiter indicating that both its hydrogen and helium resources are gone Betelgeuse seems to be ready to go at any time.

A Supernova explosion is so powerful that for a couple of weeks it can outshine all of the stars in its galaxy combined! (Credit: Harvard Gazette – Harvard University)

Of course at any time for an object as long lived as a star could mean sometime in the next million years or more. However a recent paper has suggested that Betelgeuse’s end might come any time in the next 100,000 years so there is a slight chance it could be happening soon.

So what would Betelgeuse going supernova mean to us here on Earth? Would there be any danger? Well at an estimated distance of 700 light years Betelgeuse is too far away for its burst of radiation, mostly gamma and X-rays, to do any damage to our atmosphere. However since the total amount of light coming from Betelgeuse could be as much as that of a full Moon, all squeezed into a single point of light in the sky, that point would be intensely bright, easily seen in daytime. It is possible that anyone staring at that point for too long could suffer some eye damage. Still, all in all there’s no reason to get too excited, but a nearby supernova would be something to see.

The Crab Nebula is the remnant of a Supernova that was seen in the year 1054. (Credit: YouTube)

Especially for astronomers, over the last century there have been hundreds of observations of supernova in other galaxies but those are so far away that precise measurements of what is happening are difficult to make. Even worse, when a star goes supernova in another galaxy astronomers almost never have any observations of the star before it went nova.

Only once, back in 1987 when a star in the Large Magellanic Cloud, a satellite galaxy to our Milky Way, went supernova have astronomers had any observations of a star before it exploded. The last star to go supernova in our own Milky Way galaxy was way back 1604 when telescopes were nothing more than cardboard tubes with a lens at each end. If Betelgeuse or another nearby, well-known star were to explode it would allow astronomers to test many of their theories about supernovas and star evolution in general.

A Hubble space telescope image of the Supernova 1987a as seen today. The rings are a part of the expanding shell of the explosion. (Credit: SolStation.com)

Personally I’ve been hoping to see a naked eye supernova most of my life so I’ll be keeping my fingers crossed the next few months. You know, writing this post about the possibility of Betelgeuse going supernova has made me realize that I ought to write a post just about supernova. After all type 1 supernova are an entirely different kind of animal from type 2 so I ought to describe them. Maybe I’ll do so here in the next few months, so keep coming back.