M81 in ultraviolet (APOD 15 May 2007)

[AZJames]

I'm a little puzzled by the 15 May 2007 APOD.

Aside from the yellowish core of the galaxy, depicting cooler, older stars in the core of M81, I also see many yellow colored circles far from the core. A few of the circles are very large.

Are these displaced yellow circles representing old, cool stars in our own galaxy? I think this must be true. But, if so, why doesn't the commentary make this clear?

[orin stepanek]

http://antwrp.gsfc.nasa.gov/apod/ap070515.html

I believe they are stars within the Milky Way; they are much closer which is why they appear to be so large.
Orin

[Chris Peterson]

I'm a little puzzled by the 14 May 2007 APOD.
Aside from the yellowish core of the galaxy, depicting cooler, older stars in the core of M81, I also see many yellow colored circles far from the core. A few of the circles are very large.

In images of other galaxies, virtually every star you see is in our own galaxy, in the foreground. Very few individual stars are resolved in the distant galaxy; what appear to be stars are in most cases small clusters or active regions.

The apparent diameter of a star in an image is determined by its intensity: brighter is bigger (of course, the stars themselves are essentially points regardless of their distance or intensity, but diffraction broadens them into discs). All of the big yellow discs in the image are nearby cool stars.

[BMAONE23]

There is a region in the top center of the image that apperas to be a large cluster or small satalite galaxy associated with M81. I can't seem to locate it in any visible light image of the galaxy. Is it only visible in UV, is it a new feature, or is it only visible in a deeper image?

[orin stepanek]

There is a region in the top center of the image that apperas to be a large cluster or small satalite galaxy associated with M81. I can't seem to locate it in any visible light image of the galaxy. Is it only visible in UV, is it a new feature, or is it only visible in a deeper image?

http://antwrp.gsfc.nasa.gov/apod/ap070427.html
Could that be Holmberg IX that you are refering to?
Orin

[BMAONE23]

Sure enough, missed that one

[Maddad]

Either way, it sure is a gorgeous image. I am only guessing here, but I think the consensus is right. Those bright spots are stars in our own galaxy, and not part of M-81. Again, I am guessing, but I figure that Holmberg IX would be similar to our Magellanic Clouds.

[NoelC]

You have to go deep to capture Holmberg IX, but it's been done. I've always found it fascinating that it comes out very blue in amateur astroimages, and now I guess we know why.

From the APOD caption:

Visible above M81 is a satellite galaxy dubbed Holmberg IX. Studying the unexpectedly bright ultraviolet glow of this small irregular galaxy may help astronomers understand how the many satellites of our own Milky Way Galaxy developed.

It's also notable, though not seen in the APOD image, that there's a local (in our galaxy) nebulosity all around the M81 area called the "Integrated Flux Nebula".

-Noel