Date: March 17, 2009
Title: Echoes from a 430-year-old Supernova
Podcaster: Robert Knop
Organization: Meta-Institute of Computational Astronomy
www.mica-vw.org
Description: Tycho’s supernova was observed in 1572, more than 430 years ago. Today, we see its remnant, the expanding gasses left behind by this historical stellar explosion. We had already good evidence, from observing the remnant, and from Tycho’s visual observations, that this supernova was of a particular type, a “thermonuclear supernova”. Recently, however, astronomers were able to obtain a spectrum of this supernova at maximum light by observing the light echo of this supernova as it was reflected off of other gas clouds in the galaxy, hundreds of light years away from the supernova.
Bio: This is Dr. Rob Knop. I am associated with MICA, the Meta-Institute of Computational Astronomy. You can find us on the web at www.mica-vw.org, and in Second Life in the StellaNova region.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at www.astrocamp.org
Transcript:
In 1572, people looked into the sky and saw a new star. Except for the Moon, this new star may well have been the brightest thing in the night sky. It was in the northern sky, in the constellation Cassiopeia. Over the next couple of years, the star faded from the brightness of the planet Venus (which we often see as the very bright morning star or evening star), until it was no longer bright enough to see. This event, which we unimaginatively call SN1572, is also known as Tycho’s supernova, for the famous astronomer Tycho Brahe made many systematic visual observations of the supernova. Indeed this supernova event may have been a factor that convinced Tycho Brahe to dedicate his life to astronomy.
Tycho’s supernova is one of only about eight or so supernovae that have been observed with the naked eye by people in the last couple of thousand years. Supernovae are rare! They only occur in a galaxy like ours once every few hundred years. In fact, there are two different types of supernovae. A core-collapse supernova occurs when a massive star reaches the end of its life. It has fused elements in its core all the way up to iron, but it is not able to fuse iron to produce more energy to sustain itself. The iron core collapses down to become a neutron star, and the explosion that results from the collapse is a core-collapse supernova. The supernova observed 36 years after Tycho’s Supernova in 1608, dubbed Kepler’s supernova, was a core-collapse supernova. The supernova observed by the Chinese in 1054 was also a core-collapse supernova. Today, we see the remnant of that supernova as the Crab nebula, and we see the neutron star left behind by that supernova as the Crab pulsar. If we look at other galaxies– which we must, if we’re going to see more than just a few supernova– core-collapse supernovae are more common, and occur about once every hundred years or so in a galaxy like ours.
Tycho’s supernova was of the other type of supernova, known as a thermonuclear supernova. This type of supernova originates in a binary star system. One of the two stars is a white dwarf; the other type of star is probably a normal “main-sequence” star, or a red giant star. A white dwarf star is what is left behind when a star like our sun reaches the end of its lifetime. It sloughs off its outer layers, which glow for a few ten thousand years as a planetary nebula. It leaves behind a hot but inert core, composed of very dense Carbon, or Carbon and Oxygen. This core then just cools off, no longer producing new energy, but shedding the energy it has inside it from being at the very hot core of a star like our sun. This white dwarf star is approximately the mass of our sun, but is only the size of the Earth, so it’s millions of times denser than the Sun. There are many white dwarf stars in our galaxy; most of them are minding their own business, cooling off over the course of billions of years.
When a white dwarf has a companion star, though, that is just the right distance away, it can become a nova, or even a supernova. What happens is that the white dwarf pulls some of the matter off of the outer part of its companion star. That gas can then accumulate in an accretion disk around the white dwarf, and then build up on the white dwarf. In the case of a supernova, the white dwarf pulls off just enough mass to reach a critical mass, known as the Chandrasaekar mass; this mass is about 1.4 times the mass of our Sun. When a white dwarf reaches the Chandrasaekar mass, it becomes unstable, and starts to collapse. The increased density of the star from that collapse allows thermonuclear fusion of the Carbon that makes up the white dwarf to begin… and the entire white dwarf star blows itself away very quickly in a massive thermonuclear explosion. A thermonuclear supernova is basically a nuclear bomb one and a half times the mass of our Sun. This type of supernova throws off nearly as much energy as the other type of supernova, and in fact is brighter in visible light than a core-collapse supernova. However, this type of supernova is very rare. Although white dwarfs are common, it’s extremely rare for one to have a companion at just the right spot to allow the white dwarf to reach the critical mass and go supernova. A thermonuclear supernova only happens in a galaxy like ours something like once every 500 years.
Tycho’s supernova was a thermonuclear supernova. How do we know this? There are a few ways we can tell. If we look in Cassiopeia today, we can see the faint remnant of expanding gasses left over from this explosion 430 years ago. This remnant is easiest to observe with radio telescopes, but we’ve also seen it with optical and X-ray telescopes. An analysis of the light we see from that remnant can tell us what elements are in the expanding gasses. The different types of supernova produce elements in different ratios, so we can make a reasonable guess as to the type of the supernova that produced a given remnant by looking at the elements in it. With the case of Tycho’s supernova, we also have Tycho’s data. He recorded how bright the supernova was compared to other stars in the sky over the course of the months following the event, and the “light-curve”, or the plot of brightness versus time, matches the light curve we’d expect for a core-collapse supernova. Neither of these are truly definitive, however.
The way to truly categorize a supernova is to make an observation of its spectrum, ideally somewhere close to the time of peak brightness, which happens within a few weeks of the explosion. The spectrum of an object is what you see if you let its light shine through a prism. If you let sunlight shine through a prism, you see red light on one side, green light in the middle, and blue and purple light on the other side. If you look very carefully at the spectrum of the Sun, you will see there are some specific dark bands in it, which we call absorption lines. Features like absorption lines in a spectrum allow us to classify an object and understand something about its nature. Core-collapse and thermonuclear supernovae show different features in their spectra, and thus we can definitively distinguish the two types with a good spectrum. Alas, spectroscopy had not been invented in Tycho’s time, nor had the kind of emulsive or digital photography that we use today to record the data from spectroscopy.
However, we have been able to observe the spectrum of Tycho’s supernova at maximum light. How could we have done this if the supernova exploded 430 years ago? Well, first of all, in fact, the supernova is probably about 7,500 light-years away, so in reality it exploded nearly 8,000 years ago. But if the light of the supernova at the time of its explosion reached us 430 years ago, how can we get a spectrum of it now? The answer is that we’re seeing the echo of the light which has bounced off of other gas clouds in the galaxy.
When you look at the daytime sky, it looks bright, much brighter than the nighttime sky. In fact, the light from the sky you are seeing is the scattered light of the Sun. It turns out that the scattering of light tends to be more efficient for shorter wavelengths of light– that is, for bluer light. That’s why the sky looks blue, because more of the Sun’s blue light is scattered than red light. The atmosphere scatters the Sun’s light in all directions. Only a small amount of the light is scattered, and only the small fraction of that small amount that is headed in your direction is what you see. However, the Earth receives enough light from the Sun in the first place that this small amount of scattered light is enough for us to see the sky as a bright blue color.
Light will also scatter off of clouds of gas in dust in the galaxy. Imagine, if you will, a gas cloud that is 400 or so light-years away from the position of Tycho’s supernova, along a direction that’s different from the direction between that supernova and us. When the light of the supernova reached that gas cloud, the cloud scattered the light of the supernova. Some of that scattered light would then come towards us– only delayed 400 years, because it made the side trip to the gas cloud before it started its journey on the way to us. Also, the brightness of this scattered light would be a whole lot less than the brightness we’d observe directly from the supernova, because only a small fraction of the light is scattered, and only that small amount of that small fraction of scattered light which is heading in our direction will be seen by us. As such, while this supernova was extremely bright to start with– as bright as Venus in our sky!– it took one of the world’s largest telescopes to observe the spectrum of this object.
In late 2008, a team of astronomers from Germany and Japan led by Oliver Krause published a paper describing observations of the peak-brightness spectrum of Tycho’s supernova. They used the 8m Subaru telescope, which is on Mauna Kea on the big island of Hawaii, to make their observations. They obtained a very nice spectrum of the supernova that showed that this supernova was definitively a thermonuclear supernova.
Astronomers are very lucky to have a time machine; because light takes time to travel, the farther away something is, the further back in time you’re looking when you observe it. However, when an event is transient, like a supernova, you have to be looking at just the right time to see it. Even though you see it thousands (or even millions or billions) of years after it happened, you have to be looking at just the right time when the light is reaching us. In this case, however, astronomers were able to extend the power of that time-machine by taking advantage of light echos off of other objects in our Galaxy, confirming one of the most famous historical supernovae to be of the same type of supernovae that was used to discover the acceleration of the expansion of the Universe.
End of podcast:
365 Days of Astronomy
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