Neutron stars are some of the densest objects in existence, only beaten out by black holes. Neutron stars are something of a paradox. They are inherently simple, their formation being governed by a rather straightforward competition between quantum mechanics and gravity. This clean system and its resulting dynamics make them a model object for both quantum mechanics and astrophysics courses, where students can be expected to come up with analytical models of neutron stars as part of a three-hour, six-question exam.

On the other hand, there is a lot of subtle variation in the details that provide a rich vein of discovery for observational and theoretical physics. Combine that with modern observatories that collect data faster than we can analyze it, and astrophysicists with an interest in neutron stars are sitting on a lifetime's worth of interesting results. In this week's Nature, researchers present the first ever observational evidence of the atmosphere of a very young neutron star, which implies that, even for neutron stars, youth is smooth skinned and old age is crusty.

A pair of researchers from the Universities of Southampton and Alberta, looked at measurements of a neutron star called Cassiopeia A, made by the Chandra X-ray observatory in 2004 and 2006. Cassiopeia A was produced by a supernova observed in 1680, but no one knew that a neutron star had formed at its heart until Chandra turned its orbiting eye upon the remnants.

These two observations had looked at the spectrum of X-Rays emitted by the neutron star—a spectrum is a measure of how much energy is emitted at each measured wavelength. In the case of neutron stars, the spectrum can be difficult to interpret. For instance, if a star has a really strong magnetic field, it can accelerate the hell out of charged particles, causing the star to appear very bright at very short wavelengths, even though the star itself is not producing much light there.

Chandra's observations revealed a star that was relatively calm, giving the researchers confidence that the observed spectrum really did correspond to the emission spectrum from the stellar surface.

They used this data, combined with mass and temperature information, to determine what the star's 1cm thick atmosphere consisted of. The nice thing about neutron stars is that the gravity is so immense that it sorts atoms by mass, meaning that the atmosphere should only consist of a single atomic species. So the researchers "simply" had to compare emission spectra between the star and various elements on the periodic table.

They chose hydrogen and helium—elements thought to be very common in neutron stars where the atmosphere results from accreting material—as well as iron, the end result for neutron stars with atmospheres that come from thermonuclear reactions. Although many thermonuclear reactions follow a chain that terminates at iron, they also compared the atmosphere to the emission spectrum of the intermediates on the way to iron, like carbon, oxygen, and nitrogen. They also considered a black body—a blackbody spectrum would indicate a mix of elements in the atmosphere.

It should be noted that there is one free parameter that the researchers could not constrain at the beginning: the area of emission. Basically, they could not be sure that Chandra's observations were not dominated by a hotspot on the surface of the star. Indeed, both helium and hydrogen atmospheres could fit the data if the emission was dominated by a hot-spot about a quarter of the area of the entire neutron star surface. The blackbody, spectrum could also fit the data, but only if the star was much smaller than it was known to be.

In the end, only carbon was consistent for all the data. You might be thinking that it could still he a hydrogen or helium hotspot, but the thing to remember is that neutron stars rotate quite fast, so Chandra should have seen Cassiopeia A blinking. It didn't, ruling out a small hot-spot

What does all this mean? Well, this is a very young star, at just 330 years old (at least from the perspective of the light now reaching us). It isn't accreting material, so we can watch how the star evolves over time.

We expected that very old neutron stars should have hydrogen or helium atmospheres. But earlier, when the temperature is higher, the hydrogen gets consumed by nuclear reactions at a very fast rate—a hydrogen atom landing in the atmosphere of a neutron star can expect to last one year. Likewise, helium is also consumed, albeit at a slower pace. However, no one is really sure how long this process lasts or if it always continues until only iron is left.

With continued observations, this smooth skinned youngster might be able to tell us if it is destined to become an ironclad old crusty.

Nature, 2009, DOI: 10.1038/nature08525

Listing image by NASA