What is a Supernova Remnant?

A supernova remnant is simply what is left over (i.e. the remnant) of a supernova. That probably isn't a very helpful definition, so I'll try to explain what I mean. First, I should probably define what is meant by the term "supernova." A supernova is simply the explosion of a star. Keep in mind, the average star is a million times more massive than the entire Earth, so we're talking about an incredible explosion here. The energy released in a typical supernova is on the order of 1044 joules. (A joule is simply a unit of energy that physicists use). 1044 is a 1 followed by 44 zeroes! There is no comparison for this amount of energy that is even meaningful. Supernovae are so powerful that they can be seen from halfway across the universe (billions of light years!). Here we can draw a comparison. The farthest star you can see with your eye on a dark night is only a few thousand light-years away.

I'll need to talk about what causes a supernova, because it is somewhat relevant here. Stars live out their lives by burning (via nuclear fusion reactions) light elements like hydrogen into heavier elements like helium in their core. For a star like the sun, this process will go on for about 10 billion years before it runs out of fuel. More massive stars have more fuel to burn, but they go through it much more rapidly, so they actually live shorter lives. When a star runs out of hydrogen, it will try to burn helium into even heavier elements, like carbon, nitrogen, and oxygen. If those elements sound familiar, they should. You're a carbon-based lifeform, and you're breathing nitrogen and oxygen as we speak. All of those materials came from the core of some ancient star that exploded and spread its materials around the galaxy, before the Sun and the Earth were even formed! The cartoon on the right shows a deuterium nucleus combining with a tritium nucleus to for an alpha particle (and a stray neutron). Alpha particles are the nuclei of helium atoms, and can be combined with other particles to form all the elements that we commonly see around us.

Supernova Types

When a star burns all of its available fuel, one of a few things can happen. For a star like our sun, which is only an average sized star, death will be a relatively calm affair. In about 5 billion years, the sun will exhaust its fuel supply, and will puff up into a bloated star called a red giant (swallowing up Earth in the process). It will then shed it outer layers off into space, forming a beautiful phenomena known as a planetary nebula. At the center of the nebula will be the remains of what was once our sun, a cinder of a star known as a white-dwarf (white because it will be hot, dwarf because it will be very small, about the size of Earth). Planetary nebulae have nothing to do with planets, the name is something of a misnomer. To the left is an example of a planetary nebulae. This one is NGC 6543, which is more commonly known as the "Cat's Eye." These nebulae are the remains of stars that were like our sun that have recently "burned-out."

Left by themselves, the planetary nebula will fizzle out after a few million years and the white-dwarf star will continue to smolder for billions of years, before eventually cooling off and being nothing more than a cold hunk of dead star material. However, there is something more exciting that can happen. Many stars in the universe are in binary systems, which is a system of two stars that mutually orbit each other. If one of these stars turns into a white dwarf, it can actually suck material off of the other star, provided that it is close enough. If a white dwarf gets too massive, it becomes unstable, and can explode if pushed over a certain limit. The resulting explosion is known as a type Ia supernova. The details of the explosion are quite complicated, and are still being worked out by smart people with fancy computers! To the right is an artist's conception of what a binary system containing a white dwarf and a red giant would look like. The white dwarf on the right is pulling material off of the big star on the left, and the material is spiraling in to accrete onto the white dwarf. When the white dwarf pulls off enough matter and reaches a critical mass, it will explode!

For stars that are much larger and more massive than the Sun, an entirely different scenario unfolds at the end of their life. First of all, as I mentioned above, massive stars actually live shorter lives than smaller stars. So a star that is 10 or so times more massive than the Sun will live for only a few hundred million years. After it has run through all the fusion reactions it can, it starts to collapse in on itself. It does this because there are no more reactions going on to provide pressure outward to counteract the force of gravity, which is always pulling it inward. With one final spectacular burst the outer parts of the star bounce off of the core and the resulting shockwave explodes the star, leaving only the core of the star behind, as either a neutron star or a black hole. These are commonly known as core-collapse supernovae, and include type II, type Ib and type Ic supernovae.

Both types of supernovae are extraordinarily violent events, releasing more energy in a short amount of time than an entire galaxy (hundreds of billions of stars). You might be wondering if there is any danger to Earth, since this sounds like it could be the plot of a bad Hollywood doomsday movie. Not to worry. A supernova would need to be within a few dozen parsecs of Earth to be dangerous to human life. At this time, there are no stars in that immediate vicinity that are candidates to go supernova anytime soon. There are stars relatively close by in our galaxy that could explode within the next few million years (Eta Carina and Betelgeuse, to name two), but we should be far enough away that we will be protected. That's not a guarantee, but an educated guess.

Supernovae happen at the rate of about once every fifty to one hundred years for a given galaxy. Teams of astronomers using telescopes all over the world scan the skies every night looking for new supernovae. Up until a few years ago, it was relatively rare to find one, but thanks to advances in telescope and computer technology, as well as an increase in the number of people working on it, they are becoming commonplace as billions of galaxies are part of nightly automated surveys. We actually detect supernovae at the rate of more than one per night now! Most of these are far, far away, and require advanced telescopes to see them, but occasionally we'll get one in our own neighborhood. The image on the left is an example of what a distant supernova looks like. The fuzzy green part is the galaxy that the star was located in, and the bright spot is supernova 2006gy, a supernova discovered in 2006. The thing that everyone in the field is waiting for, though, is the next supernova in our own galaxy. We have no idea when it will happen. We haven't observed one in the Milky Way in over 300 years. If a star should meet its fate in our local section of the galaxy, the show will be spectacular. If it happens within a few thousand light-years of Earth, it will easily be the brightest object in the night sky, perhaps even bright enough to look like a star during the day! It will slowly fade over the next 6 months to a year, at which point it will only be visible through a telescope. What happens next leads us to our discussion of what a supernova remnant is.

Supernova Remnants

What happens after a star goes supernova? An explosion of that magnitude doesn't simply dissipate in a short amount of time (at least not by human standards). A supernova remnant is simply the expanding blast wave from the explosion plowing through outer space, as well as the remains of what was once the star following behind it. The image to the right is an example of a supernova remnant. It is Cassiopeia A, the last supernova in our galaxy. It is the remnant of a supernova that exploded in the late 17th-century, as seen from Earth. The colors in the image represent different things. The faint blue ring around the outside is the outer edge of the blast wave running out into space at a speed of about 6000 kilometers per second. It is analogous to the ripple wave created when a rock is dropped into a pond. It shows emission from gas located in the interstellar medium (a fancy term for space) that has been heated up to several million degrees when the blast wave slammed into it. The multiple colors on the inside (green, yellow, orange) show emission from material that was once contained in the star. This is called the ejecta, since it was ejected from the star when it exploded. The fuzzy red color represents emission from interstellar dust that has encountered the blast wave and has been heated up. This image combines images from the three Great Observatories that NASA currently has in operation in outer space: The Hubble Space Telescope (HST), the Chandra X-ray Observatory (CXO), and the Spitzer Space Telescope (SST). These telescopes all see the universe in different wavelengths of light, and all provide valuble pieces of information about what is going on when we examine a supernova remnant. For more examples (and pictures!) of supernova remnants that come directly from my personal work, see my page on LMC Supernova Remnants.

So why do we study supernova remnants? What can possibly be learned by studying the remains of an explosion that has already happened? We are primarily interested with the interaction of the shock wave and the ejecta with the interstellar medium, and how the medium evolves as a result of this interaction. The universe is nearly 14 billion years old, and at this point, every point in a galaxy has most likely been overrun by several supernova blast waves and has been mixed with ejecta from multiple stars. By studying present supernova remnants, we can understand how our local region of space got to be like it is. As I've mentioned above, all "heavy" elements in the universe are made either in stars or in supernova explosions. "Heavy," when referred to by astronomers, means anything heavier than hydrogen or helium.

The interaction of the shock wave with interstellar dust is also of interest. Dust is important in many areas of astrophysics.It is required to form planetary systems and star-forming regions, and its attenuation and extinction effects on starlight can be measured, giving us an idea of how much dust is in the universe. Studying supernova remnants in the infrared (which is how dust grains radiate) can also give us an idea of the amount of dust present. Recent work done by myself and my collaborators has shown a discrepancy between these two methods of determining dust content in the ISM. We don't know exactly why this is yet, but we're working on figuring it out. Perhaps it involves some new physics that we haven't considered yet, perhaps it is just a consequence of using rather indirect methods to come up with a measurement.

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