Star?

by karthikeyan.d

I thought that the faint, blurry object next to the central galaxy is another galaxy but SkyServer labels it as a star. However its magnitude (+24) is higher than the galaxy (+18). Anyone knows what this is?

Posted July 20, 2013 11:21 AM

by JeanTate

There are actually five objects in the bluish blob; the one I think you are referring to is DR8 ObjId 1237667448339300579:

enter image description here

In astronomy, a higher magnitude means a fainter object; for example the brightest stars in the sky have ~0 magnitude, and the faintest most people can see (under good conditions) is 6.

However the main neighbor is 1237667448339300578, which is identified as a galaxy, and is much brighter; for example, its r-band magnitude is 18.47:

enter image description here

Posted July 20, 2013 1:04 PM

by karthikeyan.d

Thanks Jean. I failed to notice that there are five objects there. So, the star there is really fainter (+24) than the more distant (central) galaxy (+18)?

Posted July 20, 2013 2:02 PM

by JeanTate in response to karthikeyan.d's comment.

In this case, probably not. The SDSS photometric pipeline has an automated routine for breaking up faint, extended objects - like the blue galaxy - into pieces, and checking whether any are consistent with point objects (which are usually stars, but can also be quasars). Unfortunately, it makes mistakes, particularly with faint, blotchy galaxies like this one; it says there are stars where, in fact, there are none.

Posted July 21, 2013 2:31 PM