by Ann » Tue Jan 01, 2013 9:26 am
It's a lovely picture indeed! The diffraction spikes around the brighter stars, which contrasts with the "modesty" of the multitude of smaller star, combined with the strong (and real) color contrast between the blue and the red stars, make this picture look like a celestial treasure trove full of greater and lesser precious stones. It is lovely indeed.
Note how you can tell at a glance that the clusters are young. They are relatively compact, especially NGC 869 (which is the cluster on the lower left, by the way), and they contain several bright stars that stand out because of their brightness. Also, most of their brightest stars are blue. There is a great color contrast between the blue and the red stars, which clearly suggests that these stars are very massive. Massive stars start out very blue and then turn very red when they become red giants. Even though the stars in the famous outline of
Orion don't form a "cluster", we can see, nevertheless, some similarities between Orion and the Double Cluster: the obvious brightness of the brightest stars, the preponderance of blue stars, the great color contrast between red and blue stars.
Because there are some very red stars in or near the Double Cluster, we can say that these clusters are old enough that some of their most massive stars have evolved into red giants. But the clusters are young enough that they still retain a lot of blue giants. We are almost certainly talking about an age of less than ten million years.
It is interesting to compare the appearance of clusters of different ages. Here is a huge picture (5.0 MB!) of
the Hyades and the Pleiades. You can see that the stars of the Hyades are neither very blue nor very red, which clearly suggests that there are no supergiants here. At the same time there are indeed small color contrasts between the redder and the bluer stars, which suggests that the brightest stars of the Hyades are just a bit more massive than the Sun, and they are close to the turning point when they exhaust the hydrogen in their cores and turn into (modest) red giants. Note that apart from bright star Aldebaran, which doesn't belong to the cluster, the brightest stars of the Hyades don't seem to be exceptionally bright. The cluster doesn't even stand out all that well against the background. All of this suggests that the Hyades is several hundred million years old. According to Wikipedia,
the age of the Hyades is about 625 million years.
Compare the Hyades with the Pleiades, which is so much bluer than the Hyades. All the brightest stars are blue, and the cluster stands out very clearly against the background. On the other hand, the blue color of the nebulosity around the cluster tells us that the stars in the Pleiades are not hot enough to ionize the gas and make it glow red. Also note that there is no "
Barnard's Loop" around the Pleiades. All this suggests that although the stars are blue and the cluster is young, the combined ultraviolet light from the cluster isn't enough to ionize the gas in the vicinity. The age of the Pleiades cluster is estimated to be
between 75 and 150 million years.
Note, by the way, the small and relatively rich cluster to the upper left of the Hyades, NGC 1647. The overall color of this cluster appears to be slightly blue, bluer than the Hyades. And indeed it is. According to my software, the brightest stars of this cluster are of spectral class late B and A. I know nothing about NGC 1647, but it might actually be about the same age as the Pleiades.
This paper says that its age is about 190 million years, older than the Pleiades but considerably younger than the Hyades.
Finally, let's take a look at rich cluster
M11. Note that apart from one yellowish star, there appear to be no excessively bright stars in the rich cluster. This suggests that the really massive stars that must have been here when the cluster was young have already exploded as supernovae, or at least sloughed off their outer envelopes and turned into tiny white dwarfs. Note, too, that there appears to be a preponderance of blue stars in the cluster, but these blue stars don't stand out because of their brightness. There are several yellow and reddish stars here, too. We can clearly tell from the visual appearance of this cluster that it is definitely older than the Pleiades. Because of the large number of blue stars we may guess that it is younger than the Hyades. The age of M11 is
estimated to 220 million years.
Ann
It's a lovely picture indeed! The diffraction spikes around the brighter stars, which contrasts with the "modesty" of the multitude of smaller star, combined with the strong (and real) color contrast between the blue and the red stars, make this picture look like a celestial treasure trove full of greater and lesser precious stones. It is lovely indeed.
Note how you can tell at a glance that the clusters are young. They are relatively compact, especially NGC 869 (which is the cluster on the lower left, by the way), and they contain several bright stars that stand out because of their brightness. Also, most of their brightest stars are blue. There is a great color contrast between the blue and the red stars, which clearly suggests that these stars are very massive. Massive stars start out very blue and then turn very red when they become red giants. Even though the stars in the famous outline of [url=http://www.davidmalin.com/fujii/image/Ori_www.jpg]Orion[/url] don't form a "cluster", we can see, nevertheless, some similarities between Orion and the Double Cluster: the obvious brightness of the brightest stars, the preponderance of blue stars, the great color contrast between red and blue stars.
Because there are some very red stars in or near the Double Cluster, we can say that these clusters are old enough that some of their most massive stars have evolved into red giants. But the clusters are young enough that they still retain a lot of blue giants. We are almost certainly talking about an age of less than ten million years.
It is interesting to compare the appearance of clusters of different ages. Here is a huge picture (5.0 MB!) of [url=http://www.alsonwongastro.com/images/Star_Clusters/M45-HyadesLRGB3_2638.jpg]the Hyades and the Pleiades[/url]. You can see that the stars of the Hyades are neither very blue nor very red, which clearly suggests that there are no supergiants here. At the same time there are indeed small color contrasts between the redder and the bluer stars, which suggests that the brightest stars of the Hyades are just a bit more massive than the Sun, and they are close to the turning point when they exhaust the hydrogen in their cores and turn into (modest) red giants. Note that apart from bright star Aldebaran, which doesn't belong to the cluster, the brightest stars of the Hyades don't seem to be exceptionally bright. The cluster doesn't even stand out all that well against the background. All of this suggests that the Hyades is several hundred million years old. According to Wikipedia, [url=http://en.wikipedia.org/wiki/Hyades_(star_cluster)]the age of the Hyades is about 625 million years[/url].
Compare the Hyades with the Pleiades, which is so much bluer than the Hyades. All the brightest stars are blue, and the cluster stands out very clearly against the background. On the other hand, the blue color of the nebulosity around the cluster tells us that the stars in the Pleiades are not hot enough to ionize the gas and make it glow red. Also note that there is no "[url=http://apod.nasa.gov/apod/image/0504/bigorion_wang.jpg]Barnard's Loop[/url]" around the Pleiades. All this suggests that although the stars are blue and the cluster is young, the combined ultraviolet light from the cluster isn't enough to ionize the gas in the vicinity. The age of the Pleiades cluster is estimated to be [url=http://en.wikipedia.org/wiki/Pleiades#Age_and_future_evolution]between 75 and 150 million years[/url].
Note, by the way, the small and relatively rich cluster to the upper left of the Hyades, NGC 1647. The overall color of this cluster appears to be slightly blue, bluer than the Hyades. And indeed it is. According to my software, the brightest stars of this cluster are of spectral class late B and A. I know nothing about NGC 1647, but it might actually be about the same age as the Pleiades. [url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1992AJ....104.1865T&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES]This paper[/url] says that its age is about 190 million years, older than the Pleiades but considerably younger than the Hyades.
Finally, let's take a look at rich cluster [url=http://www.astrosurf.com/jwisn/m11.jpg]M11[/url]. Note that apart from one yellowish star, there appear to be no excessively bright stars in the rich cluster. This suggests that the really massive stars that must have been here when the cluster was young have already exploded as supernovae, or at least sloughed off their outer envelopes and turned into tiny white dwarfs. Note, too, that there appears to be a preponderance of blue stars in the cluster, but these blue stars don't stand out because of their brightness. There are several yellow and reddish stars here, too. We can clearly tell from the visual appearance of this cluster that it is definitely older than the Pleiades. Because of the large number of blue stars we may guess that it is younger than the Hyades. The age of M11 is [url=http://en.wikipedia.org/wiki/Wild_Duck_Cluster]estimated to 220 million years[/url].
Ann