Date: September 21, 2011
Title: A Backyard Boom: Thermonuclear Supernova in M101
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
Description: On August 23, a star exploded in nearby galaxy M101. This is a supernova you could see with a modest backyard telescope and a dark sky. Astronomers caught it less than a day after it exploded. A supernova this nearby, and caught this early, provides us with a unique opportunity to observe the explosion as it’s happening, and will hopefully allow us to learn a lot about just what these thermonuclear supernovae really are.
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. He now teaches physics 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.
Sponsors: This episode of “365 Days of Astronomy” is sponsored by Victor Kamutzki. On the occasion of our anniversary, dedicated to my wife Sara, who accepted my proposal under the most beautiful star-filled sky either of us have ever seen.
This episode of “365 Days Of Astronomy” has also been sponsored anonymously and is dedicated to people who like to look up in the night sky and get goosebumps.
Transcript:
A Backyard Boom: Thermonuclear Supernova in M101
Hello, and thank you for listening to 365 Days of Astronomy! I am Rob Knop, professor of physical science at Quest University Canada.
Just a few weeks ago, a star exploded in the galaxy known as M101. If you’re an amateur astronomer, you’ll recognize the naming of this galaxy. Galaxies whose names start with “M” are part of the Messier catalog of deep-sky objects, including star clusters, nebulae, and galaxies. For a galaxy to be on this list of about 110 objects, it must be pretty bright. So, the fact that this galaxy has an M name tells you that this must be a pretty bright galaxy. And, for a galaxy to be pretty bright, it must be pretty nearby.
Of course, when an astronomer says nearby, he means something different from the guy at the gas station from whom you’re asking directions. The galaxy M101 is about 21 million light-years away. That may sound like a long way to you, but to a modern extragalactic astronomer, that’s basically in our backyard. I will try to put this distance in context.
We live in the Milky Way Galaxy, sometimes just called “the Galaxy” with a capital G. It’s a bit difficult to precisely describe the size of the Galaxy, because it doesn’t have a hard edge. The density of stars gets thinner and thinner as you get farther and farther from the center of the Galaxy, until it sort of peters out. However, by looking at where most of the visible stellar disk is, we can say that the Sun is in the disk of the Galaxy, about halfway out. It’s about 30 million light-years from the Sun to the center of the Galaxy, so it’d be accurate to say that the disk of the Galaxy is about 100 light-years across.
The nearest big galaxy to our own is the Andromeda Galaxy. The Andromeda Galaxy is not the nearest galaxy; there are a bunch of dwarf galaxies, including satellites of our own Galaxy such as the Large and Small Magellanic Clouds. But the Andromeda Galaxy is another big spiral galaxy like our own. It is about two and a half million light-years away. You can get a sense for what this is like by putting two quarters down on the table 60cm, or 2 feet, apart from each other. Each quarter represents the stellar disk of one of the two galaxies.
The “Local Group” of galaxies includes our Galaxy, the Andromeda Galaxy, another smaller spiral galaxy known as M33, and then a whole bunch of dwarf galaxies. All of the galaxies of the Local Group are within about three million light-years of our own Galaxy.
In comparison, the nearest big cluster of galaxies, the Virgo Cluster, is 65 million light-years away. But even the Virgo Cluster is in our back yard, cosmologically speaking. Indeed, nowadays, extragalactic astronomers would easily consider anything closer than about a billion light years part of the “local Universe”. So, at a mere 21 million light-years, M101 is indeed quite a nearby neighbor. No, it’s not in our own Local Group, but it’s not as far away even as the nearest cluster.
In the sky, M101 is found fairly far to the North, inside the Big Dipper. Right now isn’t really the best time of year for observing the Big Dipper, but if you look to the north/northwestern sky right after sunset, and you live far enough north of the Equator, you should still be able to grab it with a small backyard telescope. M101 is called the “Pinwheel Galaxy” because of the nice spiral arms that show up in long exposure photographs. In a smallish telescope, you’ll just make out a fuzz-patch that is the center of the galaxy. Maybe, if you have a big enough telescope and a really dark sky, you’ll be able to just make out some nearby dim fuzz-patches that are the brighter parts of one or two of the spiral arms.
The supernova, officially named SN2011fe, exploded on August 23, and was discovered within a day of the explosion by the Palomar Transient Factory. The Palomar Transient Factory is a project using the 1.2 meter telescope at Palomar Observatory in southern California. Using an array of CCD cameras, they regularly scan large swaths of the sky, looking for new bright spots– or, so-called transients. Among the things they find are supernovae, including this supernova. An image of M101 on the evening of August 22 showed nothing out of the ordinary. A day later, there was a new dim spot, and two days later, the spot was brighter still. Early data on this new transient showed that it was indeed a thermonuclear supernova. A plot of the brightness of the supernova versus time allowed the astronomers at the Palomar Transient Factory to figure out the exact time on August 23 that the supernova exploded, to within a fraction of an hour. In Pacific Daylight Time, the current time zone for Palomar Observatory, that time of explosion was between 9AM and 10AM on August 23.
So what’s the big deal? The truth is that astronomers find and observe thermonuclear supernovae all the time nowadays. However, this supernova is rare for two reasons. First, because of how close it is. It’s the closest thermonuclear supernova observed in the last couple of decades. While famous supernova SN1987A happened in the Large Magellanic Cloud, a satellite galaxy to our own Galaxy, SN1987A was one of the other type of supernova, known as a core-collapse supernova. The last really nearby thermonuclear supernova was in Centaurus A, and it exploded in 1986. This was still at the dawn of the CCD era of astronomy, when digital cameras were just first being used regularly on big telescopes.
This supernova in M101 is also important not only because of how close it is, but because of how early it was caught. There have been supernovae observed within a few days of their explosion before, but often they were not identified as supernovae until much later. Only then would astronomers realize that they had earlier images where the supernovae had just barely exploded. This has often happened with very distant supernovae. With distant supernovae, they’re often dim enough that the nominal observations of them just after explosion are too dim to make out from the background galaxy. What’s more, when the supernovae are only identified later, you can’t plan follow-up observations to catch the supernova on the rise, because, of course, it’s too late.
This supernovae in M101, however, is a bonanza. It was identified early enough that other astronomers were able to redirect telescopes and start looking at it within a day of its explosion. It’s a unique opportunity to look at and to try to figure out what’s going on with this sort of supernova so soon after the explosion.
And just what might we be looking for? Thermonuclear supernovae are important partly because they have been used as lighthouses to probe the expansion of the Universe. Every time one of these supernovae explodes, it reaches about the same maximum intrinsic luminosity. That means that we can figure out how far away it is just from observing how bright it gets. Measuring the brightness of thermonuclear supernovae is one of the most reliable ways to measure very large distances in astronomy. It was observations of these types of supernovae that allowed us to discover 13 years ago that the expansion of the Universe is accelerating. As such, understanding these objects that have been used to make grand deductions about our Universe is important. However, the reliable luminosity of these objects is what’s most important for cosmological purposes, and that’s pretty well established. What’s more important about this nearby supernova is for understanding the supernovae themselves.
One of the big questions right now is exactly what sort of star system leads to these explosions. The standard model, a model that seems to hold together very well, is that a thermonuclear supernova is a white dwarf star that reaches a critical mass and blows itself away in a runaway nuclear explosion. White dwarf stars are small stars, usually made up of Carbon and Oxygen. They are only about the size of the Earth, but approximately the mass of the Sun. They’re the end stage of the life of stars similar to our Sun, up to stars about eight times the mass of our Sun. There are a huge number of white dwarf stars hanging around our Galaxy. Most of them have a mass just a bit more than half of the mass of our Sun. The stars they came from had more mass than that, but much of that mass was shed out into the interstellar medium in the last stages of the star’s life before it became a white dwarf. In a sense, a white dwarf is a dead star.
There is a strict limit on the size of a white dwarf, however, given by the fundamental physics of electrons. A white dwarf that is more massive than 1.4 times the mass of the Sun– a mass known as the Chandrasekhar Limit– will collapse. That collapse makes it dense enough to trigger runaway fusion of the Carbon and Oxygen that makes it up, and boom! You have yourself a thermonuclear supernova.
The outstanding question, however, is just how a white dwarf gains the mass necessary to bring it up to this 1.4 solar mass limit. The traditional picture– the picture you’ll see in the images if you look up thermonuclear supernova on Wikipedia, for example– is that the white dwarf star has a companion star that’s a normal star, or perhaps more likely a giant star. The white dwarf star then pulls some of the very outer layers of the other star off; that matter builds up into a disk around the white dwarf, and that disk spirals in and builds up the mass of the white dwarf. When it’s gained enough mass, the white dwarf explodes.
A second idea is that two white dwarfs, both less than the Chandrasekhar mass, are orbiting each other. If they’re in a close enough orbit, over long periods of time they will eventually spiral into each other and merge. When they merge, if the result is more than the Chandrasekhar mass, they will explode in a runaway thermonuclear supernova.
Most astronomers, until recently, have at least informally favored the model of a single white dwarf accreting matter from a companion star, probably a giant star. However, some recent observations have suggested that this model doesn’t really work, and that the model of two merging white dwarfs might be more likely. It’s safe to say that this is an open question, with some indicators pointing away from what many of us thought was most likely. A thermonuclear supernova observed so nearby as this one in M101, and also caught so early, will give us the opportunity to test the predictions of different models. Indeed, a team lead by several astronomers at Caltech were able to make observations of this supernova with both radio telescopes and the orbiting Swift X-ray telescope, starting a day after the explosion. In both cases, they didn’t see anything. While that might seem not very interesting, in fact the lack of detectable light does tell them something. Unfortunately, it can’t unambiguously differentiate between the single white dwarf and the double white dwarf models. However, the lack of detection is inconsistent with plausible models of a white dwarf accreting from a giant star. It’s still possible that the progenitor white dwarf was accreting from a smaller companion star.
This supernova in M101 is still in its early days. It will have reached maximum light on around September 10th. That means that at the time of this podcast, September 21, it’s still just under two weeks after maximum light. If you’ve got a dark sky, and an 8-inch telescope, you might just be able to see the supernova, as right now it should be around 11th magnitude. Go to the Palomar Transient Factory’s web site to see a finding chart for the supernova. Over the course of the next year, indeed over the next several years, I fully expect that we’ll hear about a lot more science coming from observations of this supernova.
As one final note, I said that the supernova exploded on August 23, 2011. In fact, that’s not quite right. I’ve made a small error of… 21 million years. Yes, the supernova really exploded 21 million years ago. It took the intervening 21 million years for the light to reach us, because M101 is 21 million light-years away. August 23 is the day on which the very first few photons of light from the explosion reached Earth.
End of podcast:
365 Days of Astronomy
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