by Ann » Thu May 24, 2012 3:15 am
This multi-spectral view shows that both young and old stars are evenly distributed along M101s tightly-wound spiral arms.
Very interesting, but I don't think I entirely agree.
This composite image does not, in fact, trace the old yellow stars in M101. The infrared filter at 8.0 µm doesn't see the old yellow or red stars very well, since this wavelength is in the far infrared, and cool red stars emit copious amounts of red and near infrared light at about 700 nanometers (or 7.0 µm), but not a lot of far infrared light. The far infrared filter is much better at tracing dust, but dust is more strongly associated with young stars and star formation than with old red stars.
What about the Hubble image that traces optical light? In this case, the filter used for the Hubble image was centered at 435 nm, which is far into the blue part of the spectrum. Cool old stars will barely show up at all through such a filter. They will be even more invisible when seen through an ultraviolet filter, particularly a fairly far ultraviolet filter centered at 150 nm. Finally, there is no reason to believe that old red stars will show up particularly strongly through an X-ray detector.
So in my opinion, while the picture of M101 through four different filters is interesting and worthwhile, it doesn't say much about the distribution of old stars in M101.
Ann
[quote]This multi-spectral view shows that both young and old stars are evenly distributed along M101s tightly-wound spiral arms.[/quote]
Very interesting, but I don't think I entirely agree.
This composite image does not, in fact, trace the old yellow stars in M101. The infrared filter at 8.0 µm doesn't see the old yellow or red stars very well, since this wavelength is in the far infrared, and cool red stars emit copious amounts of red and near infrared light at about 700 nanometers (or 7.0 µm), but not a lot of far infrared light. The far infrared filter is much better at tracing dust, but dust is more strongly associated with young stars and star formation than with old red stars.
What about the Hubble image that traces optical light? In this case, the filter used for the Hubble image was centered at 435 nm, which is far into the blue part of the spectrum. Cool old stars will barely show up at all through such a filter. They will be even more invisible when seen through an ultraviolet filter, particularly a fairly far ultraviolet filter centered at 150 nm. Finally, there is no reason to believe that old red stars will show up particularly strongly through an X-ray detector.
So in my opinion, while the picture of M101 through four different filters is interesting and worthwhile, it doesn't say much about the distribution of old stars in M101.
Ann