I am aware of Pat Durrell’s work, of course. I read the literature and know Pat, even discussed my results with him at the meeting in Austin! Just one detail, he has not looked at this particular region and he has not used Hubble’s ACS to resolve the blue blobs that we have. We found 2000 stars in that region. Previous work done by Makarova and collaborators used the old WFPC2 camera but missed most of the blue blobs and detect only 200-300 stars. So, yes, we are the first ones to ever see the blue blobs in between M81 and M82 in detail. There are plenty of other areas around M81, Pat has seen them and published them. His results are very important too and help us build a better picture of what happens in the outer disk and halos of spiral galaxies. Anyway, just thought I should clarify this and not let people think that we have rediscovered the wheel…

http://adsabs.harvard.edu/abs/2004IAUS..217…90D

(yes, I pointed to an IAU abstract and not an AAS abstract, but the abstract from the AAS meeting didn’t say what they had discovered, but when they presented the poster and issued their press release they presented what they say they discovered in the IAU abstract, that is, blue stars in the gaseous tidal tails).

So as neat as I find this result, it is disappointing to see that no one seems to have realized that essentially this exact same result was first publicized five years ago.

And a tip for everyone for writing on the internet:
You can create some       spacing with the Non-Breaking Space entity:  

Thanks for the explanation. Fascinating stuff.

That’s actually quite a good question. Star ages are estimated more by their whole spectra than just by the predominant color seen in optical light. Stars of different ages have different abundances of chemical elements present in their outer layers, these elements having been synthesized by its history of nuclear fusion in the core.

And the spacing of spectral lines is constant even with a red or blue shift, although those lines are of course shifted over by an amount appropriate for the star’s relative radial velocity. It’s like this:

Hypothetical star with zero velocity:
Red orange yellow green blue violet
| || | ||| | | ||| |

Same star with some receding velocity:
Red orange yellow green blue violet
| || | ||| | | ||| |

So if the precise spacings of the lines can be matched up with a known mixture of chemical elements, the composition of the star is determined and its age estimated. Assuming that the match is accurate, the amount of redshift is also of course determined directly from the shift in the lines.

An independent way of determining the shift is by passing the starlight through a sample of a known gas on its way to the astronomer’s photographic apparatus. If that gas is present and emitting light in the star’s atmosphere, then it’ll make an absorption spectrum as it interacts with the gas sample here on Earth. This in turn will be shifted over by some amount, from which the velocity is determined.