APOD: Comet 45P Returns (2017 Jan 02)

[APOD Robot]

Comet 45P Returns

Explanation: An old comet has returned to the inner Solar System. Not only is Comet 45P/Honda–Mrkos–Pajdušáková physically ancient, it was first discovered 13 orbits ago in 1948. Comet 45P spends most of its time out near the orbit of Jupiter and last neared the Sun in 2011. Over the past few months, however, Comet 45P's new sunward plummet has brightened it considerably. Two days ago, the comet passed the closest part of its orbit to the Sun. The comet is currently visible with binoculars over the western horizon just after sunset, not far from the much brighter planet Venus. Pictured, Comet 45P was captured last week sporting a long ion tail with impressive structure. Comet 45P will pass relatively close to the Earth early next month.

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[Dad is watching]

The kids want to know if that is Saturn just a little up and to the left of the comet.

[Chris Peterson]

The kids want to know if that is Saturn just a little up and to the left of the comet.

No. Just some stars (or a single star and some artifacts; it's a little tricky making comet images because the comet moves against the background stars, so it requires careful stacking to construct an image).

[Ann]

Pictures of the same comet have been posted in the Recent Submissions forum. Can anyone identify any of the background stars in any of those photos?

Ann

[Chris Peterson]

Pictures of the same comet have been posted in the Recent Submissions forum. Can anyone identify any of the background stars in any of those photos?

You can always run the images through astrometry.net. For example, in the SpookyAstro submission, the bright star to the left of the end of the tail is eta Cap, and the distinctive line of stars on the lower right, pointing towards the head, starts with HIP 103114 at the bottom. You can view the solution page for more information.

[Ann]

Pictures of the same comet have been posted in the Recent Submissions forum. Can anyone identify any of the background stars in any of those photos?

You can always run the images through astrometry.net. For example, in the SpookyAstro submission, the bright star to the left of the end of the tail is eta Cap, and the distinctive line of stars on the lower right, pointing towards the head, starts with HIP 103114 at the bottom. You can view the solution page for more information.

Thanks a bunch, Chris!

The biggest reason why I asked was that one of the stars in SpookyAstro's image, immediately to the right of the comet's tail, looked bluer than any other object in the image. Thanks to you, I can now identify it. The star is 20 Cap, and it is indeed blue - its Johnson B-V index is −0.110 ± 0.008, but the Hipparcos value appears to be a little bluer still. 20 Cap is classified as an ApSi star, and they are often as hot as B-type stars, even though they are classified as A-type ones. I think that ApSi stars typically have weak helium lines and an assortment of interesting "metal lines", among them, of course, silicon.

I might add that SpookyAstro's image shows a strikingly orange star to the right of the comet's coma. The star in question is HD 198670, and it is indeed quite red, with a B-V index of 1.560 ± 0.041. The orange star enhances the beauty of the comet's aqua-green head.

I am full of admiration for SpookyAstro for bringing out the star colors in his images so splendidly. The comet itself also looks beautiful in his images.

And again, thank you, Chris, for identifying the field and eta Cap for me.

Ann