Date: July 12, 2011
Title: A Bigger Boom
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
Description: A new class of supernova that has been identified over the last few years, a class of supernova that may eventually be able to tell us about the first stars in the early Universe.
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.
Sponsor: This episode of “365 Days of Astronomy” is sponsored by midnightmartian.com: 3D apps for the iPhone, iPad and iPod Touch.
Transcript:
A BIGGER BOOM
Hello, and thank you for listening to 365 Days of Astronomy! I am Rob Knop, Professor of Physical Science at Quest University Canada.
Today, I’m going to talk about supernova. In particular, I’m going to talk about a new class of supernova that has been identified over the last few years, a class of supernova that may eventually be able to tell us about the first stars in the early Universe. But before we get to that, I want to talk about supernovae in general. And, before we get to supernovae, I want to say a few words about how knowledge progresses in astronomy.
Unlike a laboratory science, astronomy is entirely observational. We can’t set up controlled experiments to test an idea. All we can do is look at the Universe that is out there, and try to use those observations to understand what is going on. Just like any other science, knowledge never comes to us fully formed. We observe a new effect or a new kind of object, and we do our best to try to understand what we’re seeing. Over time, as more information comes in, we learn more and more about what it is, revising our ideas of the physics behind our observations.
For example, consider stars. Humans have observed stars as long as there have been humans. For most of that history, they’ve just been pinpoints of light in the sky. In the last couple of centuries, the technique of spectroscopy allowed us to learn much more about the diversity of stars. Astronomical spectroscopy is just a refined version of allowing light to pass through a prism. If you’ve ever allowed sunlight to pass through a prism, you see that the prism breaks it into a rainbow. If you were to carefully measure the light of that rainbow, you could figure out how bright the sunlight is at all of those different colors. The “spectrum” is the name you give to the measurement of the brightness of the light as a function of the color, or wavelength of the light. If you take a spectrum at a high enough resolution, you can begin to see individual features that correspond to the effects of specific atomic or molecular species in the outer atmosphere of the star.
Before the 20th century, astronomers had begun to classify stars based on the strength of features in their spectrum. They assigned a letter to each class of stars: A, B, C, etc. Originally, this classification was entirely observational. It did not directly address the nature of the stars, only what they looked like. It was Annie Jump Cannon, following the work of a few others, who derived the more modern scheme of stellar classification, in which stars are ordered in the non-alphabetic sequence OBAFGKM. Today, we understand that when stars originally classified otherwise are ordered in that sequence, it’s a sequence of decreasing surface temperature for the stars. However, because of the historical classification, we ended up with the letters out of order.
The term “nova” was invented to describe a “new star”. This was a star that didn’t “wander” across the sky as the planets did, but that wasn’t one of the “fixed” stars that are always there. It was a new fixed star, hence the term nova. In the 20th century, it was recognized that some novae were much more luminous than others, and the term “supernova” was invented to describe those extremely luminous novae. Today, we understand that a nova is an explosion on the surface of a white dwarf star, whereas a supernova is a catastrophic event that represents the death of a star.
Observationally, supernovae are divided into two classes, unimaginatively called Type I and Type II supernovae. Today, that division of classes is inconvenient. As we’ve learned more about supernovae, we’ve come to understand that some Type I supernovae are physically more like Type II supernovae than the are like other Type I supernovae. However, the original classification, just like the letters assigned to stars, was based entirely on what we saw. A Type II supernova is one that shows features associated with Hydrogen in its spectrum. A Type I supernova has no such features. This is significant, because Hydrogen is the most common element in the Universe. Our Sun, for example, is approximately 98% Hydrogen.
If we’re talking about the physics of what is going on in a supernova, rather than just their spectra, today we talk about thermonuclear and core-collapse supernova. We have also created a zoo of subtypes of supernova. Thermonuclear supernovae, also known as Type Ia supernova, represent the death of a white dwarf star. This is the same type of star that is the source of a plain old nova. In a plain old nova, there is a thermonuclear explosion on the surface of the star. In a supernova, the entire white dwarf star is blown away in a massive thermonuclear explosion. It is reasonable to say that a Type Ia supernova is a thermonuclear bomb nearly one and a half times the mass of the Sun.
A core-collapse supernova represents the death of a massive star. Stars power themselves through nuclear fusion at their core. When a massive star uses up the fuel available, the core collapses. The shock resulting from that collapse blows away the outer layers of the star, leaving behind a neutron star or a black hole. The supernova we observe is the expanding hot gasses thrown off by the shock of that collapse.
Until the 1990s, when supernovae were found they were found by chance. However, in the last two decades, we’ve been able to systematically find supernovae, both using robotic telescopes scanning nearby galaxies and using wide-field surveys where each image includes thousands of distant galaxies. As the number of supernovae we’ve discovered each year has increased, we’ve been able to find and identify rare types of supernovae that either we hadn’t seen before, or that hadn’t been seen in enough numbers to be identified as a class.
Last month, Caltech astronomer Robert Quimby and his collaborators at Caltech published a paper in Nature that described a new class of supernovae. These supernovae show no lines of Hydrogen, so nominally they would be classed as some sort of Type I supernovae. However, they are also much brighter than the thermonuclear supernovae we’ve observed. What’s more, the rate at which these supernovae brighten from initial explosion to maximum light, and then decay away, is quite different from what is observed in thermonuclear supernovae. However, the observed characteristics of these supernova, both in terms of brightness and spectrum, do not match our models for how the light comes out from the traditional core-collapse supernovae that we observe all the time.
Ultimately, the power source of these supernovae is almost certainly the collapse of a stellar core, just like in a more traditional core-collapse supernova. However, the way in which the energy released in that core collapse is then converted into the light we see from the hot expanding gasses must be quite different. The mechanisms that Quimby and his collaborators suggest may power this light emission suggests that the stars responsible for these explosions must be extremely massive– perhaps 100 times the mass of the Sun. What’s more, these supernovae have mostly been observed from small, underluminous galaxies, rather than big galaxies like our own.
Put these two facts together– very massive stars in underluminous galaxies– and it is likely that the explosions we’re seeing are from stars in an earlier generation than most of the stars in our galaxy. Unlike people, the amount of time corresponding to a generation of stars depends on where those stars are. At the core or in the disk of a big galaxy, it’s possible to go through a lot of generations fast, whereas in a smaller galaxy one generation might take a long time. However, in the very early Universe, all of the stars were from an early generation. Indeed, one of the holy grails of astronomy right now is to understand the very first generation of stars, stars that formed from the gas left behind by the Big Bang, gas that had not been modified by being processed through a previous generation of stars.
This new class of supernova may well help us understand some of the earliest generations of stars. First, because they’re so luminous, they’ll be possible to spot even further away than more traditional supernovae (which we can already see to great distances). Second, because they probably do represent explosions of stars that are more like early-generation stars than the most of the exploding stars we see today, we’re directly probing an earlier sort of star. Finally, we can use them as lighthouses to illuminate the material around them; as the supernova fades, we can look for features in the supernova that result not from the supernova itself, but from the interaction of that light with other material between us and the supernova.
It is significant that we can do all of this for objects at extreme distances, because the farther away an object is, the longer the light took to reach you, and therefore the further back in time we’re looking. Looking very far away means probing the Universe as it was in its early stages.
This new type of supernova represents a bigger boom, at least as far as we’ve been able to see so far. It will be interesting to see what knowledge about the early Universe results from continued study of them. Thank you for listening to 365 Days of Astronomy!
End of podcast:
365 Days of Astronomy
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