Date: March 26, 2010
Title: Whence Supernovae?
Podcaster: Rob Knop
Organization: My home page: http://www.sonic.net/~rknop
MICA: http://www.mica-vw.org
MICA Public Events: http://www.mica-vw.org/wiki/index.php/MICA_Events
Description: What are the different types of supernovae, and what kinds of stars make the different types?
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. This semester, he’s teaching physics at Belmont University in Nashville, and next fall will join the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Adrian Tillich and dedicated to Dr. Erich Übelacker who directed the Planetarium Hamburg (Germany) for a quarter of a century and whose remarkable voice and whose passion for Astronomy still stay with me after all these years.
Transcript:
I’m Dr. Rob Knop. I’m associated with the Meta-Institute of Computational Astronomy (on the web at http://www.mica-vw.org), and next fall I will be joining the faculty of Quest University in British Columbia.
A supernova is an exploding star. It’s different from a nova, in that it’s many, many times brighter. In a nova, there’s an explosion of the layers right on the surface of a star. In a supernova, the entire star explodes, leaving behind nothing but a very energetic cloud of expanding gas, and perhaps an exotic stellar cinder.
There are two basic types of supernova. Astronomers have a classification system, whereby supernovae are divided into “Type I” and “Type II” supernovae, but the system is very old, and doesn’t completely match up with the true natures of the supernovae. A more modern way to look at it is to just use the names; there are “core-collapse” supernovae and “thermonuclear” supernovae.
A core-collapse supernova occurs when the core of a very massive star collapses– the name is a good one! The core of every “living” star (not counting stellar remnants such as neutron stars and white dwarves) is powered by nuclear fusion. This is true of our Sun. Most stars have a fusion engine at their core that is fusing Hydrogen together to make Helium. This, of course, uses up the Hydrogen fuel– but don’t worry, our Sun has a few billion years of fuel left in its core. Eventually, as the star goes into the later stages of its life and becomes a red giant, there’s an inert core of extremely dense Helium, surrounded by a shell where more Hydrogen is fusing into Helium, continuing to power the star. Eventually, the Helium core builds up enough density that it starts to fuse, creating carbon, and then perhaps oxygen.
That’s as far as most stars get. They never build up enough density to get the carbon and oxygen to fuse. They’ll eventually slough off their outer layers, leaving behind the inert carbon or carbon and oxygen core as a white dwarf– more about them later.
The most massive stars, however– stars more massive than eight times the mass of our Sun– build up enough carbon at their core to eventually ignite fusion in the carbon. Nuclear fusion will continue at the cores of these most massive stars, creating ever heavier elements, until iron. However, if you want to fuse iron with something else to make a heavier element, you have to put energy into it; you can’t get energy out by using iron as a nuclear fusion fuel. Once a very massive star has built up an inert iron core, it’s life is almost over.
There is a maximum size that the inert core of a star can have; that size is about one and a half times the mass of the Sun. When the inert iron core of a massive star reaches this mass, it can no longer hold itself up against its own gravity. There’s no fusion to support it, and the forces between the electrons in the iron atoms can no longer support it. Electrons start to combine with protons to make neutrons. The entire core of the star collapses, very rapidly.
The inert iron core of a star is roughly the size of the Earth; after the collapse, the neutron star that is left behind is only something like 20 kilometers across. When you drop something, you release energy. The more massive something you drop, and the further you drop it, the more energy is released. The dinosaurs could tell you all about this… if they hadn’t been killed off by the apocalypse that was caused when an asteroid hit the Earth. Now, imagine that you drop the mass of the Sun through 6,000 kilometers. That’s a lot, a whole lot, a tremendously huge lot of energy. That’s how much energy is released when the inert iron core of a massive star collapses. The energy that is released produces a core-collapse supernova. Galaxies like our own have one of these events about once every hundred years. One occurred in the Large Magellanic Cloud, a satellite galaxy to our own, in 1987.
A core-collapse supernova blows away most of the star. What is left behind is a very exotic object. From most supernovae, there will be a neutron star. This is an object that’s up to a few times the mass of our sun, but only 10km across; the entire object has a density comparable to an atomic nucleus! The very most massive stars, when they go supernova, may even leave behind a black hole.
The other type of supernova is known as a thermonuclear supernova. Remember white dwarves? That’s what is left behind when the vast majority of stars– stars less than eight times the mass of our Sun– end their lives. It’s an inert ball of carbon, or carbon and oxygen. Most white dwarves have a mass that’s about 60% of the mass of our Sun, and they’re roughly the size of the Earth. That’s quite dense! Not as dense as a neutron star, but still more dense, by a lot, than anything you run into during your life.
The maximum mass for the inert iron core of a massive star applies to white dwarves. A white dwarf can’t be more massive than about one and a half times the mass of our sun. If, somehow, the white dwarf achieves that mass, it will start to collapse, just like the iron core of a highly massive star collapses. There’s a difference, though. Whereas you can’t get energy out by using iron as the fuel for nuclear fusion, you can get energy out by fusing carbon. So, when the white dwarf starts its collapse, the collapse is halted. Runaway nuclear fusion of the carbon is triggered, and the entire star is blown away all at once in a tremendous thermonuclear explosion. This is a thermonuclear supernova. It is, effectively, a thermonuclear bomb, where the warhead has a mass that’s one and a half times the Sun’s mass. The total amount of energy released in this explosion is comparable to the total amount of energy released in a core-collapse supernova. Thermonuclear supernovae happen in galaxies like our own about only once every 500 years, although exactly when they happen is random. The last one we know of in our own Galaxy was Kepler’s Supernova, which exploded in 1604. Ironically, another thermonuclear supernova, known as Tycho’s Supernova, exploded just a few decades previously in 1572.
The question, then, is how to get a white dwarf up to this critical mass. It can’t have started at this critical mass, for the star that left it behind wasn’t massive enough to ever build up a core that would be dense enough to fuse carbon. There are two primary models astronomers have for how a white dwarf might get to the critical mass. One is that the white dwarf has a companion star which is either a normal star, or a red giant star (which is just a normal star in the late stages of its life). If the white dwarf is close enough to the red giant star, the gravity of the white dwarf competes with the gravity of the red giant itself for the gas right at the outside edge of the red giant. The gas will get pulled off of the red giant, and will build up in a disk swirling around the white dwarf. The gas from the center of this disk will settle on to the white dwarf, building up the mass of the white dwarf. Sometimes, the gas that has built up will explode in a nova, leaving the white dwarf intact. However, if enough gas can build up and stay on the white dwarf, eventually the white dwarf will attain the critical mass and explode in a thermonuclear supernova.
The other model for getting a white dwarf to the critical mass is the collision of two white dwarf stars. When two white dwarf stars that are less than the critical mass run into each other and start to merge, at some point you’ll have a combined star that is at the critical mass. That merger product will then start to collapse, and will undergo the runaway nuclear fusion that makes a thermonuclear supernova.
For a long time, many astronomers believed that the white dwarf pulling matter off of a neighboring star was the mechanism that produced many or most thermonuclear supernovae. However, a paper published just a month ago by Gilfanov and Bogd√°n, two astronomers at the Max Planck Institute for Astronomy in Germany, presented a result that indicated that most thermonuclear supernovae, at least in elliptical galaxies, can’t come from this kind of system. It turns out that a white dwarf accreting matter from a companion will emit X-rays as a result of the high temperatures in the disk of material around the white dwarf. This paper used archival data from the Chandra X-ray telescope to show that there can’t be enough systems like this to produce the observed rate of thermonuclear supernovae. This strongly suggests the majority of thermonuclear supernovae come from stellar collisions, rather than from stars more sedately pulling matter off of a companion.
Whether thermonuclear or core-collapse, the supernova signals the final death of a star. Supernovae are extremely rare, but they are also extremely bright. This has allowed astronomers to use them not only to understand the deaths of stars, but as celestial beacons that tell us things about the extremely distant universe. But that’s another whole story.
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.
Trackbacks/Pingbacks