johnnydeep wrote: ↑
Tue Dec 01, 2020 4:33 pm
APOD Robot wrote: ↑
Tue Dec 01, 2020 5:05 am
NGC 346: Star Forming Cluster in the SMC
Are stars still forming in the Milky Way's satellite galaxies? Found among the Small Magellanic Cloud's (SMC's) clusters and nebulas, NGC 346
is a star forming region about 200 light-years across, pictured here
in the center of a Hubble Space Telescope
image. A satellite galaxy
of the Milky Way, the Small Magellanic Cloud (SMC
) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation of the Toucan
). Exploring NGC 346
, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here
on the right. Still collapsing within their natal clouds
, the stellar infants'
light is reddened by the intervening dust. Toward the top of the frame is another star cluster with intrinsically older and redder stars. A small
, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe
. These small galaxies, though, are thought to be building blocks
for the larger galaxies present today.
So just what "cluster" of stars is being referred to here? Consider these 5 apparent concentrations of stars:
I gather that number 4 is the APOD subject NGC 346 and number 5 is the "another star cluster with intrinsically older and redder stars", but are the other groupings I've circled anything in particular? Are 2 and 3 really part of 4? And what of the redder 1?
I'd say all of them are clusters, that is, 1, 2, 3, 4 and 5 are clusters. In some cases, the clusters are partly hidden behind dust and gas (1 and 3), but they are clusters, or rather, they contain
clusters nevertheless. For a comparison with a Milky Way "starless nebula", consider M17, the Omega Nebula:
You asked if clusters 2 and 3 in your annotated image is really part of cluster number 4. No, I don't think so. Consider the Large Magellanic Cloud and the Tarantula Nebula:
As you can see, there are two clusters in the Tarantula Nebula, and they are both part of the Tarantula Nebula region. But they are certainly not one and the same cluster. And only one of them, NGC 2070 with its central condensation R136, does almost all of the ionizing of the nebula.
In the same way, the different clusters that we can see in today's APOD could all be part of the same nebular region. Perhaps they are all part of NGC 346. But as clusters, they are distinct from one another.
Number 5 in your annotated image is different, because it is an old cluster. You can see that although there are many stars present, none of them is particularly bright. That's because all the really massive stars have died, because such stars use up their fuel at a furious rate. Also note that the stars of cluster 5 look yellow or even orange. The color suggests that the clusters is old, so that many of its stars have turned into red giants. But the red color also suggests to me that the cluster is somewhat reddened by dust. Also the nebula that this cluster was once born from was dispersed a long, long time ago.
Because the old cluster of red stars was born such a long time ago, whereas all the other clusters in NGC 346 are young, I'd say that cluster number 5 in your annotated image is not part of NGC 346.
Finally, you asked why your annotated nebula number 1 in today's APOD is so red in color. I don't know... maybe this region of star formation is reddened by dust, just like the Flame Nebula in Orion?
Oh, I misunderstood! You asked if the annotated nebula number 1 is part of the larger NGC 346 region. Well, why not? This object is clearly an emission nebula and a site of star birth in the general direction of NGC 346.
The nebula may possibly be an example of "triggered star formation", in that strong stellar winds from other clusters in the NGC 346 region may have compressed some gas and dust in this location and made it collapse into a site of star formation of its own.