APOD: NGC 1850: Not Found in the Milky Way (2023 Feb 20)

[APOD Robot]

NGC 1850: Not Found in the Milky Way

Explanation: There is nothing like this ball of stars in our Milky Way Galaxy. This is surprising because, at first glance, this featured image by the Hubble Space Telescope suggests that star cluster NGC 1850's size and shape are reminiscent of the many ancient globular star clusters which roam our own Milky Way Galaxy's halo. But NGC 1850's stars are all too young, making it a type of star cluster with no known counterpart in the Milky Way. Moreover, NGC 1850 is also a double star cluster, with a second, compact cluster of stars visible here just to the right of the large cluster's center. Stars in the large cluster are estimated to be 50 million years young, while stars in the compact cluster are younger still, with an age of about 4 million years. A mere 168,000 light-years distant, NGC 1850 is located near the outskirts of the Large Magellanic Cloud galaxy. The glowing gas filaments across the image left, like supernova remnants in our own galaxy, testify to violent stellar explosions and indicate that short-lived massive stars have recently been present in the region.

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[Ann]

NGC 1850 as seen through two filters (left) and through five filters (right).

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The picture that is today's APOD is seen at left in the double portrait above. According to NASA, two filters were used for the image, one visible one and one near-infrared one. I'll have a guess and say that the visible one was probably centered at 656 nm, the wavelength of hydrogen alpha, whereas the near-infrared one was almost certainly Hubble's trusted 814 nm filter. And the red hydrogen alpha nebula surrounding the cluster was mapped as blue, and the mostly blue stars, detected by an infrared filter, were shown as yellow. I must say that the colors bother me.

The picture at right, constructed from two ultraviolet, two visible and one near infrared filter, gives a much "truer" portrait of NGC 1850. This young globular cluster (well, maybe Chris would call it a "populous cluster") is apparently only about 100 million years old, the same age as the Pleiades.We do expect to find a lot of at least moderately bright blue stars in such a cluster. And in a 100 million year-old cluster as large as NGC 1850, we do expect to find a lot of rd giant stars as well.

I will not say that Hubble's five filter portrait of NGC 1850 shows the cluster in "true color", since three of the five filters detect wavelengths that are not visible to the human eye. But the five filter portrait is still so much better than the two filter one.

Here is another version of NGC 1850, processad by our very own Judy Schmidt, Geckzilla, from Hubble data:

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The stars are not blue, but this is almost certainly due to dust reddening. At least the stars aren't as uniformly yellow as they are in the APOD. And the nebulosity is red, as it should be.

Go to this page to read Geck's comment and to see the full size of the image.

Ann

[johnnydeep]

So why is it that a cluster of young stars like this one can't (or hasn't) formed in the Milky Way? Or is it simply that we haven't discovered such a one yet? I realize that the "typical" globular clusters that "roam the M-W's halo" are much older, because the halo stars are all (or predominantly) metal poor, and hence old, but why couldn't there be a cluster of young stars elsewhere in the M-W (or even in an atypical part of the halo)?

[Chris Peterson]

So why is it that a cluster of young stars like this one can't (or hasn't) formed in the Milky Way? Or is it simply that we haven't discovered such a one yet? I realize that the "typical" globular clusters that "roam the M-W's halo" are much older, because the halo stars are all (or predominantly) metal poor, and hence old, but why couldn't there be a cluster of young stars elsewhere in the M-W (or even in an atypical part of the halo)?

Westerlund 1 is a similar cluster in the Milky Way, somewhat less massive. It's also worth noting that some parts of the Milky Way are not very accessible to observation, unlike many other galaxies.

[Ann]

So why is it that a cluster of young stars like this one can't (or hasn't) formed in the Milky Way? Or is it simply that we haven't discovered such a one yet? I realize that the "typical" globular clusters that "roam the M-W's halo" are much older, because the halo stars are all (or predominantly) metal poor, and hence old, but why couldn't there be a cluster of young stars elsewhere in the M-W (or even in an atypical part of the halo)?

I think these clusters are hard to form in the present universe. Conditions were different when these massive clusters formed in large numbers, some 10-12 billion years ago. If we are to find one in the Local Group of galaxies, then it is not surprising that we find it in the Large Magellanic Cloud, given all the star formation that is going on there.

An interesting example of a young globular cluster is an extremely massive cluster in NGC 6946. NGC 6946 is an interesting galaxy because it has produced no less than ten supernovas in the 20th and 21st century.

NGC 6946 contains one - just one - super star cluster (or, if you want to put it that way, a young globular cluster):

NGC 6946 in infrared light. The compact cluster is seen at upper right, at one o'clock, surrounded by a loose halo of stars. Credit: ESA/Hubble & NASA, A. Leroy, K. S. Long.

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Young globular cluster (or super star cluster) in NGC 6946. Credit: HST/NASA/S. Larsen.

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In all probability, it takes extremely special conditions to produce a really massive young super star cluster in the nearby universe, since these clusters are so truly rare.

Ann

[johnnydeep]

Thanks, Chris and Ann. So we know of at least one largish young cluster nearby in the M-W (Westerland 1), but they are likely rare and/or our view is obscured due to residing the the plane of the M-W.