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Supernova

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A supernova is the explosion of a star. In an instant, a star with many times the mass of our own Sun can detonate with the energy of a billion suns. And then, within just a few hours or days, it dims down again. Some explode into a spray of gas and dust, while others become exotic objects like neutron stars or black holes.

Astronomers have classified supernovae into two broad classifications: Type I and Type II. Type I supernovae occur in binary systems, where one star pulls off mass from a second star until it reaches a certain amount of mass. This causes it to explode as a supernova. Type II supernovae are the explosions of massive stars which have reached the end of their lives.

All of the elements heavier than iron were created in supernova explosions. As a massive star runs out of hydrogen fuel, it starts to fuse together heavier and heavier elements. Helium into carbon and oxygen. And then oxygen into heavier elements. It goes up the periodic table this way, fusing heavier elements until it reaches iron. Once a star reaches iron, it’s no longer able to extract energy from the fusion process. The core collapses down into a black hole, and the material around it is fused together into the elements heavier than iron. If you’re wearing any gold jewelry, that was created in a supernova.

In 1054 Chinese astronomers saw a supernova explosion that was so bright it was visible in the middle of the day. The explosion of gas and dust is now visible as the Crab Nebula (that’s the picture at the top of this article). The most recent powerful supernova explosion occurred in 1987, when a star exploded in the Large Magellanic Cloud.

Astronomers use Type I supernovae to judge distances in the Universe. This is because they always explode with approximately the same amount of energy. When a white dwarf star collected approximately 1.4 times the mass of the Sun, it can’t support its mass and collapses. This amount is called the Chandrasekhar Limit. When an astronomer sees a Type I supernova, they know how bright it is, and so they can measure how far away it is.

We’ve written many articles about supernovae for Universe Today. Here’s an article about a slow motion supernova, and here’s an article about a theoretical supernova that was actually found to exist.

If you’d like to see a gallery of supernova photographs, check out this section of the Hubble Space Telescope site, and here’s NASA’s Photo Gallery of Nebulae.

We’ve also recorded several episodes of Astronomy Cast about supernovas. Check out this one, Episode 14: We’re All Made of Supernovae.

References:
http://www.cfa.harvard.edu/supernova//newdata/supernovae.html
http://en.wikipedia.org/wiki/Chandrasekhar_limit

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Neutron Star at Core of Cas A Has Carbon Atmosphere

A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. Credit: NASA/CXC

Supernova remnant Cassiopeia A (Cas A) has always been an enigma. While the explosion that created this supernova was obviously a powerful event, the visual brightness of the outburst that occurred over 300 years ago was much less than a normal supernova, — and in fact, was overlooked in the 1600’s — and astronomers don’t know why. Another mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. But in 1999, astronomers discovered an unknown bright object at the core of Cas A. Now, new observations with the Chandra X-Ray Observatory show this object is a neutron star. But the enigmas don’t end there: this neutron star has a carbon atmosphere. This is the first time this type of atmosphere has been detected around such a small, dense object.

A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A, with an artist's impression of the neutron star at the center of the remnant. The discovery of a carbon atmosphere on this neutron star resolves a ten-year old mystery surrounding this object. Credit: Chandra image: NASA/CXC/Southampton/W.Ho; illustration: NASA/CXC/M.Weiss
A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A, with an artist's impression of the neutron star at the center of the remnant. The discovery of a carbon atmosphere on this neutron star resolves a ten-year old mystery surrounding this object. Credit: Chandra image: NASA/CXC/Southampton/W.Ho; illustration: NASA/CXC/M.Weiss

The object at the core is very small – only about 20 km wide, which was key to identifying it as a neutron star, said Craig Heinke from the University of Alberta. Heinke is co-author with Wynn Ho of the University of Southampton, UK on a paper which appears in the Nov. 5 edition of Nature.

“The only two kinds of stars that we know of that are this small are neutron stars and black holes,” Heinke told Universe Today. “We can rule out that this is a black hole, because no light can escape from black holes, so any X-rays we see from black holes are actually from material falling down into the black hole. Such X-rays would be highly variable, since you never see the same material twice, but we don’t see any fluctuations in the brightness of this object.”

Heinke said the Chandra X-ray Observatory is the only telescope that has sharp enough vision to observe this object inside such a bright supernova remnant.

But the most unusual aspect of this neutron star is its carbon atmosphere. Neutron stars are mostly made of neutrons, but they have a thin layer of normal matter on the surface, including a thin–10 cm–very hot atmosphere. Previously studied neutron stars all have hydrogen atmospheres, which is expected, as the intense gravity of the neutron star stratifies the atmosphere, putting the lightest element, hydrogen, on top.

But not so with this object in Cas A.

“We were able to produce models for the X-ray radiation of a neutron star with several different possible atmospheres,” Heinke said in an email interview. “Only the carbon atmosphere can explain all the data we see, so we are pretty sure this neutron star has a carbon atmosphere, the first time we’ve seen a different atmosphere on a neutron star.”

An artist's impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth's atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss
An artist's impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth's atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss

An artist’s impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth’s atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss

So how does Heinke and his team explain the lack of hydrogen and helium on this neutron star? Think of Cas A as being a baby.

“We think we understand that as due to the really young age of this object–we see it at the tender age of only 330 years old, compared to other neutron stars that are thousands of years old,” he said. “During the supernova explosion that created this neutron star (as the core of the star collapses down to a city-sized object, with an incredibly high density higher than atomic nuclei), the neutron star was heated to high temperatures, up to a billion degrees. It’s now cooled down to a few million degrees, but we think its high temperatures were sufficient to produce nuclear fusion on the neutron star surface, fusing the hydrogen and helium to carbon.”

Because of this discovery, researchers now have access to the complete life cycle of a supernova, and will learn more about the role exploding stars play in the makeup of the universe. For example, most minerals found on Earth are the products of supernovae.

“This discovery helps us understand how neutron stars are born in violent supernova explosions,” said Heinke.

Source: Interview with Craig Heinke

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What Is The Crab Nebula?

The Crab Nebula, or M1 (the first object in Messier’s famous catalog), is a supernova remnant and pulsar wind nebula. The name – Crab Nebula – is due to the Earl of Rosse, who thought it looked like a crab; it’s not in the constellation Cancer (the Crab), rather Taurus (the Bull).

The supernova which gave rise to the Crab Nebula was seen widely here on Earth in 1054 (and so it’s called SN 1054 by astronomers); it is perhaps the most famous of the historical supernovae. It is certainly one of the brightest (estimated to be –7 at peak), partly because it is so close (only 6,300 light-years away), and partly because it’s not hidden by dust clouds. The expansion of the nebula – as in seen-to-be-getting-bigger, rather than the-gas-is-moving-very-fast – was first confirmed in 1930.

As it was a core collapse supernova (a massive star which ran out of fuel), it left behind a neutron star; by chance, we are in line with its ‘lighthouse beam’, so we see it as a pulsar (all young neutron stars are pulsars, but not all of them have beams which point to us in one part of the cycle). It’s a pretty fast pulsar; the neutron star rotates once every 33 milliseconds. Because it’s so young and so close, the Crab Nebula pulsar was the first to be detected in the visual waveband, and also in x-rays and gamma rays. Being the source of the tremendous output of energy, from both the pulsar wind nebula and the pulsar itself, and as energy is conserved, the pulsar is slowing down, at a rate of 15 microseconds per year.

The inner part of the Crab Nebula, the pulsar wind nebula, contains lots of really hot (‘relativistic’) electrons spiraling around magnetic fields; this creates the eerie blue glow … synchrotron radiation. This makes the Crab Nebula one of the brightest objects in the x-ray and gamma ray region of the electromagnetic spectrum, and as it is a relatively steady source (unlike most high energy objects) it has given its name to a new astronomical unit, the Crab. For example, a new x-ray source may be 2 mCrab (milli-Crab), meaning 0.002 times as strong an x-ray source as the Crab Nebula.

This SEDS page has a lot more information on the Crab Nebula, both historical and contemporary.

Such an intensively studied object, no wonder there are lots of Universe Today stories on it; for example Nearly a Thousand Years After the Death of a Star, Giant Hubble Mosaic of the Crab Nebula, The Peculiar Pulsar in the Crab Nebula, Astronomers Locate High Energy Emissions from the Crab Nebula, and Evidence of Supernovae Found in Ice Core Sample.

Astronomy Cast’s Neutron Stars and Their Exotic Cousins has more on pulsars, and Nebulae more on nebulae.

Sources: Caltech Astronomy, SEDS, Stanford University SLAC

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Supernova 1987A

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The first supernova in 1987 (that’s what the “A” means) was the brightest supernova in several centuries (and the first observed since the invention of the telescope), the first (and so far only) one to be detected by its neutrino emissions, and the only one in the LMC (Large Magellanic Cloud) observed directly.

Ian Shelton, then a research assistant with the University of Toronto, working at the university’s Las Campanas station, and Oscar Duhalde, a telescope operator at Las Campanas Observatory, were the first to spot it, on the night of 23/24 February 1987 (around midnight actually); over the next 24 hours several others also independently discovered it.

The IAU’s CBAT went wild! That’s the International Astronomical Union’s Central Bureau for Astronomical Telegrams, the clearing house for astronomers for breaking news. You can read the historic IAUC (C for Circular) 4416 here.

Once the discovery of SN 1987A became known, physicists examined the records from various neutrino detectors … and found three, independent, clear signals of a burst of neutrinos several hours before visual discovery, just as predicted by astrophysical models! Champagne flowed.

Not long afterwards, the star which blew up so spectacularly – the progenitor – was identified as Sanduleak -69° 202a, a blue supergiant. This was not what was expected for a Type II supernova (the models said red supergiants), but an explanation was quickly found (Sanduleak -69° 202a had a lower-than-modelled oxygen abundance, affecting the transparency of its outer envelope).

The iconic Hubble Space Telescope image (above) of SN 1987A shows the inner ring, where the debris from the explosion is colliding with matter expelled from the progenitor about 20,000 years ago; more from the Hubble here.

AAVSO (American Association of Variable Star Observers) has a nice write-up of SN 1987A.

No wonder, then, that SN 1987A features so often in Universe Today stories; for example Supernova Shockwave Slams into Stellar Bubble, XMM-Newton’s View of Supernova 1987A, Supernova Left No Core Behind, and Hubble Sees a Ring of Pearls Around 1987 Supernova.

SN 1987A figures prominently in Astronomy Cast The Search for Neutrinos, and in Nebulae.

References:
AAVSO
University of Oregon

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Chandrasekhar Limit

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When a human puts on too much weight, there is an increased risk of heart attack; when a white dwarf star puts on too much weight (i.e. adds mass), there is the mother of all fatal heart attacks, a supernova explosion. The greatest mass a white dwarf star can have before it goes supernova is called the Chandrasekhar limit, after astrophysicist Subrahmanyan Chandrasekhar, who worked it out in the 1930s. Its value is approx 1.4 sols, or 1.4 times the mass of our Sun (the exact value depends somewhat on the white dwarf’s composition how fast it’s spinning, etc).

White dwarfs are the end of the road for most stars; once they have used up all their available hydrogen ‘fuel’, low mass stars shed their outermost shells to form planetary nebulae, leaving a high density core of carbon, oxygen, and nitrogen (that’s a summary, it’s actually a bit more complicated). The star can’t collapse further because of electron degeneracy pressure, a quantum effect that comes from the fact that electrons are fermions (technically, only two fermions can occupy a given energy state, one spin up and one spin down).

So what happens in the core of a massive star, one whose core weighs in at more than 1.4 sols? As long as the star is still ‘burning’ nuclear fuel – helium, then carbon etc, then neon, then … – the core will not collapse because it is very hot (electron degeneracy pressure won’t hold it up ’cause it’s too massive). But once the core gets to iron, no more burning is possible, and the core will collapse, spectacularly, producing a core collapse supernova.

There is a way a white dwarf can go out with a bang rather than a whimper; by getting a little help from a friend. If the white dwarf has a close binary companion, and if that companion is a giant star, some of the hydrogen in its outer shell may end up on the white dwarf’s surface (there are several ways this can happen). The white dwarf thus adds mass, and every so often the thin hydrogen envelope blows up, and we see a nova. One day, though, the extra mass may put it over the limit, the Chandrasekhar limit … the temperature in its center gets high enough that the carbon ‘ignites’, the ‘flame’ spreads throughout the star, and it becomes a special kind of supernova, a Ia supernova.

For more technical details of the Chandrasekhar limit, Richard Fitzpatrick of the University of Texas at Austin has an online Thermodynamics & Statistical Mechanics course, which includes a page on the Chandrasekhar limit.

Supernovae are very important to astronomy, so you won’t be surprised to learn that there are lots of Universe Today stories on the Chandrasekhar limit! Some examples: White Dwarf Theories Get More Proof, White Dwarf “Close” to Exploding as Supernova, and Colliding White Dwarfs Caused a Powerful Supernova.

Astronomy Cast Episode 90 (The Scientific Method) includes a look at how Chandrasekhar worked out the limit that now bears his name, and Where Do Stars Go When They Die? also covers this topic.

References:
Wikipedia
http://www.bluffton.edu/~bergerd/NSC_111/stars.html