What's that star in NGC 3312?

[Ann]

On Saturday, May 12, the Astronomy Picture of the Day showed the Hydra Cluster. Prominent among the galaxies in that cluster (or just possibly slightly in front of that cluster, but never mind) is large spiral NGC 3312. Mactavish asked for a good picture of NGC 3312, and bystander found three such images. Check out this thread.

The best picture that bystander could find was one by Stefan Binnewies and Rainer Sparenberg. Bystander also found two other good pictures of NGC 3312, one by Another Eye on Flickr and one by Eddie Trimarchi and AstroShed.

Here's the deal. I, too, found the image by Stefan Binnewies and Rainer Sparenberg, although it was an annotated version. Because of my extreme interest in all things that are blue, I scrutinized Binnewies and Sparenberg's image for blue knots in NGC 3312. I found four obvious blue knots, which are definitely young clusters. I also found a small number - make that two - non-blue point sources in NGC 3312. Due to their color, I didn't think they were clusters, although at least one of them just might be an old globular cluster, perhaps similar to Omega Centauri.

As for the blue knots in NGC 3312 in the picture by Binnewies and Sparenberg, three of them line up along a spiral arm. In the picture found by bystander, the three blue knots are lined up to the upper right of the yellow nucleus. No other bright point source can be seen in this area.

Okay, but now check out the picture posted by Another Eye on Flickr. You can see the three blue knots lined up along the spiral arm, just like in the picture by Binnewies and Sparenberg. But hey, something is different. In the picture by Another Eye, a non-blue point source, seemingly exactly as bright as the three blue knots, is seen to the left of the bottom one of the blue knots located at almost right angles to the line of knots.

This non-blue point source can also be seen in the picture by Eddie Trimarchi and AstroShed, although in that picture it can't be seen that the point source is different in color from the other nearby knots. The non-blue source is there, however, that much is certain.

And in the APOD by Angus Lau, the non-blue point source is there once again (although once again the color of this point source appears to be no different from the blue knots).

In my opinion, it is obvious that the blue knots are young clusters, and it is also obvious that they are bluer than the point source that is seen at almost right angles to them. Due to its color, I find it most unlikely that the non-blue point source is a young cluster.

So what the heck is it?

And why couldn't it be seen in the picture by Binnewies and Sparenberg?

Any thoughts?

Ann

[Chris Peterson]

So what the heck is it?
And why couldn't it be seen in the picture by Binnewies and Sparenberg?
Any thoughts?

Just speculating here- perhaps it's a star with a high IR output. The B&S image was made with an LRGB filter set, which is designed to block near IR. The Another Eye image was made with I and V filters only. The Trimarchi image was shot as LRGB, but the filters used are not specified. It is common when shooting LRGB to not use any L filter at all, in which case IR sources are bright because of the intrinsic sensitivity of the sensor in near IR. If Eddie didn't use an IR blocker for his luminance data, then the only image that shows nothing is the one that definitely did block IR.

[Ann]

Thanks, Chris. It makes sense. Do you think it is a faint, distant IR-bright star in the Milky Way (whose proper motion should however make it seem to move across the disk of NGC 3312) or could it be a humongously bright IR-source in NGC 3312 itself?

Ann

[Chris Peterson]

Thanks, Chris. It makes sense. Do you think it is a faint, distant IR-bright star in the Milky Way (whose proper motion should however make it seem to move across the disk of NGC 3312) or could it be a humongously bright IR-source in NGC 3312 itself?

That's a good question. It would be unusual, although not impossible, to detect a single star in a distant galaxy with a small telescope. But it's statistically unlikely for a foreground star to overlap a small object like this. Hard to say what it is.