Tag: Pulsars

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. 

Astronomers think that they have finally solved the mystery of ‘Fast Radio Bursts’ (FRBs), and what are FRBs anyway?

Go out some clear night and look up at the night sky, it’s much better if you can get away from big city lights by the way. If you think about it there really aren’t that many different types of objects up there. Aside from the Moon, if it’s out, all there really is up there are a lot of points of light, stars. O’k some stars are certainly brighter than others, and if you look closely it is easy to see that there are some stars with distinct colour to them. Nevertheless, from here on Earth the Universe just looks like a lot of points of light, a lot of stars.

The night sky may look beautiful, but there really don’t appear to be a lot of different types of objects. Appearances can be deceptive however. (Credit: Forbes)

Of course we all know that’s not true. With the invention of the telescope we quickly learned that stars and planets are very different. We also learned that some stars are giants while some are dwarfs.

The visual differences between the stars is summed up in the Hertzsprung Russell Diagram. (Credit: Chandra X-ray Observatory)

Then, as new types of telescopes were invented, other stranger kinds of objects were discovered. Radio telescopes discovered both quasars and pulsars while X-ray telescopes discovered black holes. One very unusual discovery was made when CIA spy satellites were the first to observe Gamma Ray bursts.

Quasars are the brightest steady objects in the Universe. They are now known to actually be a supermassive Black Hole in the center of a galaxy that is feasting on nearby stars, releasing some of that energy. (Credit: Hubble Space Telescope)

Based upon the observations from those telescopes astrophysicists then had to figure out what those objects actually were. Quasars for example turned out to be supermassive black holes in the centers of distant galaxies that are devouring nearby stars and releasing some of that energy feast. Pulsars are the remains of stars that went supernova, been crushed down into neutron stars and are emitting radio waves like a lighthouse. Gamma Ray Bursts on the other hand are caused by giant stars collapsing into black holes.

A Gamma Ray Burst (GRB) is even brighter than a Quasar but it only lasts for a few seconds. When a supermassive star explodes as a supernova some of its energy is released as gamma rays creating the GRB.

Fast Radio Bursts (FRBs) are the newest member of the cosmic zoo. Now FRBs are exactly what they sound like.  Without any warning a powerful burst of radio waves occurs that only lasts for a tiny fraction of a second. Transient events like FRBs are a curse to scientists because you’re never looking right at it when it happens, and by the time you say ‘what was that’ and turn around to look at it it’s gone.

The Parkes radio telescope in Australia. Large reflector dishes such as Parkes can only observe a small region of the sky at any one time. That’s what makes finding transient objects such as an FRB so difficult. (Credit: Square Kilometre Array)

In fact the very first FRB was actually ‘discovered’ in an analysis of old data from the Parkes Observatory in Australia. The data had been collected in 2001 but the FRB wasn’t recognized until 2007. Then it wasn’t until the 15th of January in 2015 that an FRB was detected live, also at Parkes Observatory. Canada’s new Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope, which unlike other radio telescopes is designed to have a wide field of view, has detected dozens of FRBs since it first went online in 2018.

The CHIME telescope in Canada is a different type of radio telescope that has a wider field of view. That now makes it the go-to instrument for studying FRBs. (Credit: Phys.org)

One of the few things that we do known about FRBs is that the most common frequencies of the burst are from around 800-1400 Mega-Hertz (MHz), that’s not to far from the frequencies used by your cellphone. Also, when astronomers say fast they mean really fast, each event being a single spike of radio waves lasting no more than a few milli-seconds. And because FRBs are scattered evenly across our sky, rather than being concentrated along the Milky Way, they must come from intergalactic space, perhaps as far away as billions of light years. At those enormous distances the energy released during those few milliseconds must be more than our Sun emits over the course of more than a dozen years.

A typical observation of an FRB. The burst releases different amounts of power at different frequencies but the entire event only last a few milli-seconds. (Credit: Daily Mail)

The mystery of FRBs has generated a lot of attention in the astronomy field and with all of that interest it’s not surprising that new discoveries are being made every year. A few FRBs have been discovered that are irregular repeaters, that is they have erupted more than once but without a predictable pattern. On the other hand FRB 180916 has been found to repeat on a schedule of once every 16.35 days. (By the way FRBs are numbered by the year, month and day they were first observed hence FRB 180916 was observed on the 16th of September in 2018.)

Just this past April an FRB was definitively detected as coming from inside our own galaxy and astronomers believe that they can even identify the source as the known object magnetar SGR 1935+2154 which is located in the constellation Vulpecula at a distance of about 30,000 light years. This is significant because some astrophysicists have been promoting magnetars as a possible source for FRBs for the past several years.

A magnetar is the dead corpse of a star that blew up as a supernova. The remaining core has been chrushed so small that its magnetic fiend is super intensified! (Credit: Quanta Magazine)

Now magnetars are the dead corpses of massive stars that exploded as supernovas. What remains after that explosion is an object about as massive as our Sun crushed down to the size of a city, an object so dense that it has become composed mainly of neutrons, a neutron star. As the star was squeezed its magnetic field also got compressed. But while compressing a magnetic field may decrease its size it also increases its intensity and a neutron star with a particularly strong magnetic field is given the special name of magnetar.

But wait, didn’t I say near the top of this post that neutron stars are also known as pulsars. Yes indeed, in fact all of these creatures are so closely related that astrophysicists argue all of the time where one class ends and another begins, in fact many neutron stars may be both pulsars and magnetars at the same time.

Pulsars also are the dead core of supernovas, and they also have very strong magnetic fields. So is there any real difference between a pulsar and a magnetar? Perhaps not very much. (Credit: Pinterest)

And just because one FRB has been identified as coming from a magnetar doesn’t mean that they all do. There may be even more exotic animals in the cosmic zoo that are as yet completely unknown such as dark matter particles or even cosmic strings. So far we’ve figured out a bit about FRBs but there’s still a lot more to learn. But then isn’t that the whole fun of astronomy!