Yes yes yes, this is such a beautiful APOD! Where is the "Jumping up and down" emoji?
NGC206_APOD1024[1].jpg
NGC 206 and the Star Clouds of Andromeda
Image Credit & Copyright: Roberto Marinoni
This is what I see in the picture:
APOD 28 November 2024 annotated.png
At far left, there is a huge, broad, vaguely L- or V-shaped, bluish "star cloud" - but this isn't NGC 206, but something much larger. However, while the overall color is bluish, there are relatively few individual bright stars in it. Most of the color comes from large numbers of A-type stars like Vega or Sirius, and also a number of modest B-type stars like Regulus.
To the upper right of this bluish star cloud, there is a vaguely U-shaped brown dust lane. Numerous pink or red emission nebulas cluster along the outer edge of this dust lane. Hot blue stars are born in these nebulas. Therefore, we can make an educated guess that this dust lane has helped populate the blue star cloud outside of it. Hot blue stars have been born here, and later the stars have drifted away from their birthplace. Relatively soon the hottest, brightest, most massive stars that were born from it have died in supernova explosions.
In the lower right part of this APOD, you can see numerous dust lanes with red emission nebulas attached to them.
What about NGC 206? It is sitting between the fertile U-shaped dust-lane that spawned so many emission nebulas and the numerous long dust lanes at lower right. If you look carefully, you can also see that there is dust above and below NGC 206, which I haven't shown in my annotated image. But in short: Dust (and more importantly, as Chris would tell us, gas) was fed from at least two dust lanes into the place where NGC 206 was born. Molecular hydrogen was clearly channeled into this particular region, and it set off an enormous and extremely widespread burst of star formation.
Let's take a look at starburst galaxy NGC 5253:
In NGC 5253, you can really see the dust lanes channeling gas into the center of the galaxy and feeding the central starburst. And because NGC 5253 is small, the starburst dominates the galaxy. Andromeda is huge, and NGC 206 looks small, but in real terms it isn't. And it contains very many hot massive OB stars:
Wikipedia wrote about NGC 206:
It contains hundreds of stars of spectral types O and B.
The interesting thing here is the sheer number of OB stars in NGC 206: There are hundreds of them.
However, according to Wikipedia, the radius of NGC 206 is 400 light-years, whereas according to the APOD caption it is
4,000 light-years. Which figure is correct?
I am reminded of the Sco-Cen OB association of the Milky Way.
The Sco-Cen association is a very large and rather scattered collection B-type stars in the constellations Scorpius, Ophiuchus, Lupus, Centaurus and Crux. What is the size of the Sco-Cen association of the Milky Way? It may be around 200 parsecs, or circa 600 light-years.
Sebastian Ratzenböck et al. wrote about the Sco-Cen association:
The Scorpius-Centaurus OB association (Sco-Cen; Blaauw 1946, 1964) is the closest OB association to Earth. The full association is a large, roughly 200-pc-wide complex that still includes molecular clouds with ongoing star formation.
The Sco-Cen association looks very large in the sky, but it looks large because the stars are nearby.
Almost all the scattered stars that seem to float above the band of the Milky Way to the right side of the image belong to the Sco-Cen association.
But there is another association - make that a star cloud - that is of interest to us when we compare NGC 206 of Andromeda with something that is found in the Milky Way, and that is M24:
Wikipedia wrote:
The Small Sagittarius Star Cloud (also known as Messier 24 and IC 4715) is a star cloud in the constellation of Sagittarius approximately 600 light years wide, which was catalogued by Charles Messier in 1764....
Messier 24 holds some similarities with NGC 206, a bright, large star cloud within the Andromeda Galaxy.
So what's the difference between M24 and NGC 206? Well, the number of hot massive stars. My software shows me a small select number of bright OB stars in M24, of which only one is an O-type star, HD 167771. In NGC 206, there are hundreds of really bright OB-stars!
Let's take a look at the really big and bright star clusters in the Local Group:
NGC 206 Roberto Marinoni.png
NGC 206 in Andromeda. Credit: Roberto Marinoni
We've got nothing that big! No, we don't, but let's see what tomorrow's APOD will show us. If we are lucky, it will feature a massive cluster in the Milky Way. Like... Westerlund 1? Maybe?
Ann
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NGC 206 and the Star Clouds of Andromeda
