I didn't have time yesterday, but I can't resist posting a few clusters and commenting on their ages!
Yeah, yeah, we all know about the Hyades and the Pleiades. Nevertheless, it is still interesting to see a good color photo like Alson Wong's, where you can really see that the stars of the Pleiades, some 100 million years old, are blue, whereas the stars of the Hyades, some 650 (or possibly 790) million years old, don't look blue in this picture. (Admittedly a number of them are much bluer than the Sun.) Also note that even though the Pleiades stars are more than twice as far away as the stars of the Hyades - some 440 light-years versus 153 light-years - the stars of the Pleiades look just as bright, or brighter, than the stars of the Hyades. You've got to remember that bright red giant Aldebaran isn't a member of the Hyades, but instead a foreground star. So not only are the stars of the Pleiades bluer, but they are intrinsically much brighter, too.
To me it is a bit of a mystery that there ar no red giants in the Pleiades. They are certainly old enough for at least one of their members to have turned into a red giant, and moreover, the Pleiades is a sufficiently rich cluster that we have reason to believe it started out with at least one or two members that were more massive than the present luminary of the Seven Sisters, Alcyone, whose mass according to Wikipedia is 5.9-6.1 M☉
. If the Pleiades originally contained a star of 8 M☉
, then that star should have evolved into a white dwarf by now, after first having gone through its red giant stage.
Perhaps that is what happened. Perhaps there once was an 8 M☉
star in the Pleiades that turned into a bright red giant and then shrank into a tiny and faint white dwarf. I don't know if there are any confirmed white dwarfs in the Pleiades, but there are definitely white dwarfs in the Hyades
All right. Let's look at Alson Wong's picture again. Can you see that there is a small, faint cluster to the upper left (northeast) of the Hyades? That is NGC 1657. How old is that cluster?
When you first look at this image, you may ask yourself, where's the cluster? But if you enlarge the picture you will see it in the center of the image. Roberto Mura's image is a good color picture, and if you compare the color of stars in the cluster with the yellow and reddish stars in the foreground, you can clearly see that the stars in NGC 1647 are not red or even yellow. You must bear in mind, too, that NGC 1647 is quite distant, some 1800 light-years, some 4 times more distant than the Pleiades and more than 10 times more distant than the Hyades. Therefore we should expect the stars of NGC 1647 to be reddened, and they are, but they still don't look yellow.
So we should conclude that this cluster is young, and it is, according to Wikipedia: It is some 150 million years, so it's a bit older than the Pleiades, but much younger than the Hyades. But there are indeed orange stars seen at the outskirts of NGC 1647 that may be members of the clusters, and if so, stellar evolution into red gianthood has started in NGC 1647.
Let's compare the Hyades and the Beehive Cluster, M44!
The Hyades. Photo: Bijan Moravej alahkami
M44. Photo: Jakub Korbel.
Can you see that these two clusters, the Hyades at left and the Beehive Cluster, M44, at right, are of similar ages? M44, the Beehive Cluster, is estimated to be 600–700 million years old, according to Wikipedia
, similar to the Hyades. But M44 is a little more massive than the Hyades.
It is harder to "see" the Hyades than M44, because bright foreground star Aldebaran is so distracting, and there is quite some dust along our line of sight to the Hyades, and there is a really non-negligent population of background Milky Way stars there, too. M44 is much easier, because it is much more detached from the band of the Milky Way, and there is no dust in the vicinity and no bright foreground stars. But can you see that the proportion of bluish and orange bright stars is about the same in both M44 and the Hyades? Also the bluish and the orange stars are about equally bright - this is again easier to see in the picture of M44.
Jakub Korbel's picture of M44 is great. Do note the profusion of faintly reddish stars peppering the image. In my opinion, these are most likely faint and low-mass members of M44.
Let's look at another one!
Take a look at NGC 3293 in Carina and the Double Cluster of Perseus! Can you see, or guess, that these cluster are of smilar ages?
All the red nebulosity in the vicinity of NGC 3293, plus the cluster's compact size, high number of bright blue stars, plus one bright red member, strongly suggests that this is a cluster of really massive stars, and therefore it must be young. According to Wikipedia it is about 12 million years old, and its brightest, hottest members are blue supergiants of spectral class B0, which would be similar to the the bright blue stars in Orion's Belt. But there is also an M-type supergiant in the cluster, which would be similar to Betelgeuse.
The Double Cluster in Perseus looks different, particularly because there is no nebulosity in its vicinity. But the central parts of the member clusters are compact and bright, and there are a few bright red supergiants scattered among the blue stars, which have been reddened to a white color.
According to Wikipedia, the two clusters of the Double Cluster, NGC 869 and NGC 884, are both 14 million years old. Their hottest main sequence members are said to be of spectral class B0. In NGC 3293, the brightest star is a supergiant of spectral class B0. Since blue supergiant stars are always a little cooler than the main sequence stars they evolved from, this means that NGC 869 and NGC 884 are a little further along in their evolution than NGC 3293.
NGC 869 and NGC 884 are very massive:
NGC 869 has a mass of 4,700 solar masses and NGC 884 weighs in at 3,700 solar masses; both clusters are surrounded with a very extensive halo of stars, with a total mass for the complex of at least 20,000 solar masses.
That's a lot! NGC 3293 is less massive: Wikipedia
puts its mass at 1457 M☉
. Perhaps the higher mass of the Double Cluster explains why all the gas in the vicinity has either been used up or "blown away".
The most remarkable cluster in the Local Group of galaxies is R136 in the Large Magellanic Cloud:
R136 is an incredible cluster. Not only are there hundreds of blue stars in it, but there are hundreds of O-type stars!
R136 is thought to be less than 2 million years old
. None of the member stars is significantly evolved and none is thought to have exploded as supernova. The brightest stars are WNh, O supergiants, and OIf/WN slash stars, all extremely massive fully convective stars. There are no red supergiants, blue hypergiants, or luminous blue variables within the cluster. A small number of class B stars have been detected in the outskirts of the cluster, but less massive and less luminous stars cannot be resolved from the dense cluster core at the large distance of the LMC.
The cluster contains many of the most massive and luminous stars known, including R136a1. Within the central 5 parsecs there are 32 of the hottest type O stars (O2.0–3.5), 40 other O stars, and 12 Wolf-Rayet stars, mostly of the extremely luminous WNh type. Within 150 parsecs there are a further 325 O stars and 19 Wolf-Rayet stars
We can indeed see that R136 is a massive, fantastic cluster. We can see from the Hubble image that it contains huge numbers of stars that are all blue, except one red star near the top of the "cavity"(?) to the left of R136 proper in the Hubble image. Otherwise, though, there is a myriad of blue stars, and at the center of the extended cluster is a very tight and compact core. No lightweight or old cluster can ever look like that!
Well, I could go on - I'd really love to discuss one of the greatest clusters in the Milky Way, NGC 3603, but hey - I'll just post a picture of it and leave it for you to google it, okay?
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