What conclusions can you draw from Hubble images?
Posted: Sun Sep 20, 2015 8:09 am
Recently in the Found Images forum, bystander posted a Hubble picture of NGC 3921, brilliantly processed by Judy Schmidt. A previous Hubble picture of the same galaxy is said to show 1,000 bright young clusters at the heart of this pair of colliding galaxies. You can see the picture that claims to show 1,000 clusters here.
I'm baffled. I think I can see one cluster, at about 4 o'clock, at the edge of the bright central part.
Perhaps Hubble has measured the brightness of the center of this galaxy and concluded that only 1,000 young clusters could produce this much light? But if so, I'm surprised by this galaxy's colors. Its B-V index is 0.680 and its U-B index is 0.250. There is no suggestion here that the galaxy is full of young clusters, rich in blue and ultraviolet light.
Perhaps the galaxy is very dusty? That would make sense for a pair of merging galaxies that have formed a lot of hot new stars. But NGC 3921 really isn't very dusty. Its far infrared magnitude, which is a measure of its dust content, is about the same as its blue magnitude. Compare that with M82, the Cigar galaxy, which may contain about 150 bright clusters in its core, and which is 4.5 magnitudes brighter in the far infrared than in blue light.
It is possible that there are 1,000 young clusters in the core of NGC 3921, but they may not be young enough to have very blue and ultraviolet colors. But isn't it equally likely, then, that the central bulge of NGC 3921 is a mixture of old yellow stars and young blue stars that were born in clusters, but many of the clusters have now been disrupted due to the crowded conditions in the bulge?
And in any case, I'm still baffled as to how anyone can conclude that NGC 3921 contains 1,000 young clusters in its core, based on a Hubble picture showing a very bright central part and one definite cluster at the edge of it. Unless, of course, that is a foreground star.
Ann
I'm baffled. I think I can see one cluster, at about 4 o'clock, at the edge of the bright central part.
Perhaps Hubble has measured the brightness of the center of this galaxy and concluded that only 1,000 young clusters could produce this much light? But if so, I'm surprised by this galaxy's colors. Its B-V index is 0.680 and its U-B index is 0.250. There is no suggestion here that the galaxy is full of young clusters, rich in blue and ultraviolet light.
Perhaps the galaxy is very dusty? That would make sense for a pair of merging galaxies that have formed a lot of hot new stars. But NGC 3921 really isn't very dusty. Its far infrared magnitude, which is a measure of its dust content, is about the same as its blue magnitude. Compare that with M82, the Cigar galaxy, which may contain about 150 bright clusters in its core, and which is 4.5 magnitudes brighter in the far infrared than in blue light.
It is possible that there are 1,000 young clusters in the core of NGC 3921, but they may not be young enough to have very blue and ultraviolet colors. But isn't it equally likely, then, that the central bulge of NGC 3921 is a mixture of old yellow stars and young blue stars that were born in clusters, but many of the clusters have now been disrupted due to the crowded conditions in the bulge?
And in any case, I'm still baffled as to how anyone can conclude that NGC 3921 contains 1,000 young clusters in its core, based on a Hubble picture showing a very bright central part and one definite cluster at the edge of it. Unless, of course, that is a foreground star.
Ann