ISON Image of the Week

First Amateur Spectrum of Comet ISON  (Oct 14, 2013)

The first amateur spectrum of comet C/2012 S1 (ISON) in which we clearly see the C2 emission and a probable detection of CN. These are common features in comets, but this does not detract from the fantastic achievement of an amateur astronomer obtaining a spectrum of what is still a relatively small, faint and distant target!
[Image credit: Christian Buil/Castanet-Tolosan Observatory (France)]
Scientists study comets for many reasons including how they formed, what their mass and density is, what physical and chemical processes are occurring on their surface and within their core, and even what would happen if a large one impacted our planet! That list is far from exhaustive but fundamental to all of those questions, and many others we have, is this: what are comets made of?

By studying their composition, we can gain tremendous insight into comets and begin to find answers to many of these questions. However, performing these studies is not simple. We can send probes to comets, but that is expensive and lengthy, and thus is something we rarely can do. In the meantime we have to find other ways to study the composition of comets, and the primary method of doing that is with the scientific technique called spectroscopy.

Spectroscopy is a method in which we study the properties of an object based on the light that we see coming from it (including reflected light). From Earth, spectroscopy can be tricky as our own atmosphere blocks certain wavelengths of light, so most of this work is performed from space by telescopes such as Spitzer that have a clear view of the entire electromagnetic spectrum. A certain amount of spectroscopy can be done from Earth but it requires a lot of skill and specialized equipment, so remains primarily the domain of large astronomy facilities.

But there are exceptions to every rule, and this week week have an exceptional exception! Last week, amateur astronomer Christian Buil was able to obtain the first "amateur" ground based spectrum that we have seen for comet ISON using an "Alpy 600" spectrograph on a 28cm telescope at the Castanet-Tolosan Observatory, near Toulouse in the south of France. His data, which he reported to the CIOC via our CIOC Facebook Group, show quite nicely two C2 (diatomic carbon) emission lines and a probable faint signature of CN (cyanide radical). Both of these are common in comets, and the former in particular likely responsible for the green glow that some images of ISON are beginning to show. So there's no new great discovery here in that sense, but certainly this is a fabulous achievement for Christian to have obtained this data, particularly while ISON is still a relatively small and faint target (though really beginning to brighten up nicely now!).

When ISON gets closer to the Sun in the coming weeks, increasing amounts of its material will begin to heat up, sublimate, and give off their unique signatures of light. Sungrazing comets are particularly fascinating in this sense as they approach closer to the Sun - and thus get "hotter" - than any other type of comet, leading to the vaporization of material that may not ordinarily sublimate when further from the Sun. Pair this with the fact that ISON is a fresh chunk of primitive solar system material, and we have a recipe for some fantastic new science in the weeks to come!

Every week this year we will put up a new image related to Comet C/2012 S1 (ISON). If you have a cool image you'd like us to consider, please send it to, along with a description and any credits you would want applied. We'll contact you if we choose to use your image on the CIOC Website.

See our ISON Image of the Week Archives for earlier picks!