I had a question via email asking about Comet Holmes. I thought that by answering it on the blog, maybe others would also have some questions answered. so, thanks to Marycie for her question.

Comet Holmes was a very dim, and expected comet. Until October last year. During the period October 23rd to 24th 2007, it suddenly brightened. If you want to know more about why it suddenly became visible, I’d recommend reading Astroprof’s post on Integrated Magnitudes.

The comet was visible to the naked eye after that date, and on about the 26th of October it began to look more like a classic comet, with a tail and nucleus. Comets have a nucleus, a tail and a coma, or halo. The nucleus is the hard, chewy centre. This is rocky bit and really ‘is’ the comet, if you like. The tail and coma are produced when the object passes closer to the Sun and particles of ice and dust begin to sublimate. A cloud of material (the coma) appears to boil off the nucleus and becomes the glowing, cloudy ball that makes comets familiar to most people.

The tail is produced by interaction with the Sun. Particles from the coma and nucleus are blasted back from the comet by the Sun and so the tail always points away from the Sun.

Comet Holmes’ coma grew enormously in size and this became interesting later on in October. Late in the month, the coma had grown to be about half the width of the Moon on the sky. However it was lying about twice as far from the Earth as the Sun is (2 AU), so the true size was around 1 million km. That’s two-thirds the diameter of the Sun.

Here’s an animation from the Comet Holmes Wikipedia entry, showing the location of the comet and the size of the coma on different dates.

In November 2007 the coma became even larger and was in fact bigger than the Sun. However because of this, the coma has become so diffuse that it was hardly visible to the naked eye at all. Although much of the internet was talking about Comet Holmes being bigger than the Sun, most general news and media didn’t care because they couldn’t see it well enough to show the public a good picture.

The final part of Marycie’s question was asking where in the solar system the comet was located. Well I mentioned earlier that it was 2AU away from us. For the time it was visible, Comet Holmes sat just beyond the orbit of Mars. You see its location in this video I’ve made using the excellent Starry Night software.

This video covers the period October 2007 to April 2008. You’ll note that the Earth moves half an orbit during the video. Apologies for the low quality of YouTube.

There is an excellent composite photo of Comet Holmes over the course of the 2007-2008 outburst on the Wikipedia page covering the topic. This image is shown below.

It was a little while ago now that Comet Holmes first came into our sights in Perseus. It flared from magnitude 17 to 2.8 in only two days and has since been picked up by many amateur and professionals alike, including a very recent Hubble press release.

So here is a collection of my favourite images of the Comet as it now begins to fade. We’ll begin with Sky & Telescope’s excellent finder chart for locating the comet:

On October 25th the first images of Comet Holmes were taken and the internet was buzzing with news and pictures. One image that seemed to really get around was this one from Cloudbait Observatory in Colorado.

We then started to see the comet expanding and getting even brighter. It was now clearly visible and many noted that it was larger than Jupiter in the sky. This animation from the Observatoire du Cégep de Trois-Rivières shows the first few days as it changed and grew.

In early November the comet began to change, now forming a tail. This striking image from Don Goldman from Nov 3rd shows that it no longer looks spherical and can start to be discerned as a comet.

Astronomy Picture of the Day featured a wonderful image on November 5th of a very long exposure, colour Comet Holmes. It was takenin Spain by Vincent Peris. It appears the comet has a greenish hue and a faint tail that is more pronounce than first thought.

As the nights get darker and the preparation becomes more intense, photographers are able to get even better images of the now much more comet-like comet. VC Astro’s beautiful Nov 14th photo is a really lovely example.

Finally in this collection, Hubble has been turned to view the comet’s nucleus. With its powerful optics, Hubble has taken high resolution images of the central part of the comet.

They have also put together a video which shows you the scale of the comet on the sky and so iI’ll finish with that video and hope you’ve enjoyed this small collection of Comet Holmes snaps. If you want to downloada higher resolution version of this video do so here. The file is approx 11MB.

Chris Lintott is reporting on his blog on Comet Holmes, which ought to be a magnitude 17 object but recently clocked in at mag 10 and then mag 3, making it a naked-eye visible comet (even from Cardiff if you’re lucky).

So can you see it? Why not have a go? Go and find Perseus (map below from BBC’s Night Sky pages) and see if you can spot a medium brightness star that shouldn’t be there (detailed map from Wikipedia). If you can, aim your telescope or binoculars at it and see if you spot something fuzzy.

Apparently the comet has a notorious reputation for being unpredictable. In fact a New York Times article from February 1893 tells how the comet changed in the space of a day from being a star-like point to a faint nebulous mass.

Back in the present there are already shots of it on Flickr: