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.
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).
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.