Call it the story of the young and the quarkless: Astronomers have a surprising new take on the youngest supernova remnant in our corner of the Milky Way, and it may solve a long standing mystery.

Supernova remnant Cassiopeia A

Located 10,000 light-years away, Cassiopeia A is the remnant of a once massive star that died in a violent supernova explosion seen 325 years ago. This view combines infrared observations from the Spitzer Space Telescope (colored red); visible data from the Hubble Space Telescope (yellow); and X-ray data from the Chandra X-ray Observatory (green and blue).

NASA / JPL / Caltech / O. Krause

The object in questions is Cas A (rhymes with passé) a glowing wreath of energized gas that was discovered years ago in the constellation Cassiopeia. Cas A was created when a massive star reached the end of its nuclear rope about three centuries ago and blew itself to smithereens. What’s left at the center is a tiny nugget of superdense matter called a neutron star, the youngest example of one we know.

So far, so good. But there’s always been something weird about the neutron star in Cas A since it was first spotted by the Chandra X-ray Observatory in 1999. Now it looks like there’s an explanation.

First, the weirdness: Based on its brightness in the X-ray spectrum and its distance from Earth, astronomers initially calculated that the neutron star in Cas A is no more than about 10 km across. That’s too small to be a neutron star, according to what physics tells us a neutron star should be like.

One suggestion to account for this is that the X-ray emission is not coming from the entire neutron star but from a hot spot that is relatively small in size. The problem is the spot isn’t pulsing or blinking, which is what you’d expect from a neutron star that’s spinning around really fast (which neutron stars are wont to do).

Too strange by half? The neutron star at the heart of the supernova remnant Cas A has always been an oddball.


An even stranger suggestion is that the object at the center of Cas A isn’t a neutron star at all but rather a hypothetical “quark star.” To become a quark star, the object’s gravity has to be so strong that it causes the neutrons in a regular neutron star to lose their individual identities and merge into one giant ball of quarks — including “strange quarks,” which are heavier than the “up” and “down” quarks that exist within individual neutrons. The resulting strange quark star would be more compact than a neutron star but not quite a black hole. Just a few months ago, the Astrophysical Journal published an interpretation of the X-ray data from Cas A as evidence for a strange quark star.

Now come Wynn Ho (Southhampton University) and Craig Heinke with a different way of approaching the problem.

In the November 5th edition of Nature, Ho and Heinke report that if you assume the object at the heart of Cas A is shining through an atmosphere of carbon atoms, its brightness corresponds to a neutron star about 24 to 30 km across — basically normal size, if you can call a neutron star “normal.”

So why a carbon atmosphere? First of all, it’s not unusual to think of a neutron star with a hydrogen atmosphere, since that’s the material that surrounds it after the massive star blows up. Most of the glowing gas in Cas A is hydrogen.

Heinke points out that neutron stars are hot when they’re young — so hot that a surrounding envelope of hydrogen might fuse into helium and then into carbon. The carbon layer is only ankle high, but it’s enough to radically change the way the neutron star looks. Over time, that carbon would settle into the body of the neutron star and be replaced by fresh hydrogen from above. By then the neutron star has cooled enough that it can no longer fuse hydrogen above its surface, so the hydrogen remains as a residual atmosphere.

It’s a nice story that seems to explain why Cas A is different — not because it’s full of quark matter, but because of its relative youth.

“It’s immensely satisfying,” to have come up with such a tidy solution, says Heinke, “and it fits the data beautifully.”

Heinke admits that it would have been fun to verify something as strange as the existence of strange quark stars. On the other hand the universe is plenty weird enough without them.

Ivan Semeniuk is host of the podcast The Universe in Mind and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.


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Stephen Lawrence

November 13, 2009 at 3:30 pm

As a professional astronomer who conducted his PhD research on Cas A, I'd like to point out a minor quibble with the sentence "Most of the glowing gas in Cas A is hydrogen."

At optical wavelengths most of the emission from Cas A comes from the "fast moving knots" or FMKs. The FMKs are glowing with atomic line emission from shocked gas, and their spectra show that they are astoundingly depleted in both hydrogen and helium. This is remarkably unlike nearly every other nebula in the galaxy. The most abundant elements revealed by optical emission lines are oxygen and sulfur. This is interpreted as the FMKs being debris from the mantle of an extremely massive Wolf-Rayet star that had already shed its hydrogen and helium envelopes before it exploded; the FMKs are heated by the reverse shock plowing backwards into the ejecta.

The only features emitting visible light that are demonstrated to have hydrogen are the "quasi-stationary flocculi" or QSFs. They do show moderate hydrogen emission lines, but also very strong nitrogen lines. These are interpreted as clumps of pre-supernova mass loss shocked by the forward blast wave. But the QSFs are fewer and fainter than the FMKs.

I'm less of an expert on the X-ray emission, but I do believe it also mostly comes from stellar debris shocked by the reverse shock. Only the thin blue outermost spherical layer is interpretted as the forward shock plowing out through the star's circumstellar envelope and/or local ISM. These could features could contain H-rich gas. They also appear moderately stong in infrared emission, possibly from shocked molecular lines from molecular hydrogen, but also possibly from thermal continuum from shock-heated dust.

So I don't think the statement that "most of the emission" comes from hydrogen is justifiable; certainly not at visible wavelengths.

Prof. Steve Lawrence, Hofstra University

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Tim S.

November 14, 2009 at 7:25 am

The article states that the supernova occurred 3 centuries ago, however it is 10,000 light years away. Perhaps the article meant to say that we observed the suprnova 3 centuries ago.

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kate sisco

January 30, 2012 at 2:00 pm

I was wool-gathering and I thought that this mass requirement (mass is after all entirely different in the electric universe theory) for black holes dependent upon the size of the star collapsing does not have to stand. If the mass really didn't matter and the gas pressure did, then we could see LT (my name) stars of tiny size, say a couple of miles d, exist as the first initial form after the collapse. After all it was proposed that Tungusta was due to a mini black hols. So, if LT stars appears first, the appearance would be cloaked by its ability to pull in and use photons. This article proposes a 'quark' star where all is quanta mix. I am curious to know just how the black hole quanta would be arranged to create an even more dense star but.........the most fascinating aspect is that it does not present an interruped glow. So, if the LT star were sufficiently pressurized magnetically, it would at first not be visible. This state could only exist until it shed sufficient energy to rearrange the interior quanta into a neutron state, and when neutron, shed neutrinos until it reaches a state of normal atomic arrangement.

It makes so much more sense that way. It is not the size and gravity but the magnetic field, pressure and sound waves.

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