APOD: Messier 24: Sagittarius Star Cloud (2022 Apr 07)

[APOD Robot]

Messier 24: Sagittarius Star Cloud

Explanation: Unlike most entries in Charles Messier's famous catalog of deep sky objects, M24 is not a bright galaxy, star cluster, or nebula. It's a gap in nearby, obscuring interstellar dust clouds that allows a view of the distant stars in the Sagittarius spiral arm of our Milky Way galaxy. When you gaze at the star cloud with binoculars or small telescope you are looking through a window over 300 light-years wide at stars some 10,000 light-years or more from Earth. Sometimes called the Small Sagittarius Star Cloud, M24's luminous stars fill this gorgeous starscape. Covering over 3 degrees or the width of 6 full moons in the constellation Sagittarius, the telescopic field of view includes dark markings B92 and B93 just above center, along with other clouds of dust and glowing nebulae toward the center of the Milky Way.

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[Ann]

M24, the Small Sagittarius Star Cloud. Photo: Gabriel Rodrigues Santos.

Gorgeous image. The colors and details are superb. I would have had a field day with it if I had had my software, but I don't.

Note the faint reddish Hα light visible to the upper right (north and west) of M24. Also note the evaporating dark dust clouds. To me, this signals the remnants of a region of spent star formation. Or to put it differently:

This patch of sky once held a huge molecular cloud that contracted and and gave rise to large-scale star formation, probably from several "cores". After all, we don't see a compact young "super star cluster" in this region, noting like NGC 3603 or Westerlund 2 (although Westerlund 1 is supposedly an even larger young cluster than Westerlund 2, it's just not so photogenic).

Now star formation has come to an end, and the faint remaining Hα emission and evaporating dust clouds pay witness to the feast of dusty and chaotic star birth that happened here not so long ago. There are O-type stars in M24 - I know, because I checked the brightest blue stars there when I still had my software - so star formation must have ended here not that many million years ago.

Herschel Space Observatory infrared image of high mass star formation in Cassiopeia. If you ask me, what we see here is the Heart and Soul nebulas. The Soul is to the left.

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The M24 region may have looked like the Heart and Soul nebulas not that many million years ago.

Ann

[orin stepanek]

Almost looks like a blurred photo of someone lying down!

[pclarkmusic2@gmail.com]

Are there stars within that amazing starscape that are closer together than our sun is to our closest neighbor? If so, how much closer?

[Ann]

Are there stars within that amazing starscape that are closer together than our sun is to our closest neighbor? If so, how much closer?

Yes, there are indeed stars in this picture that are closer to each other than our Sun is to Alpha Centauri.

Star cluster NGC 6603 in Messier 24.

Position of the Sun in the Milky Way. Artist: I don't know.

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For one thing, there is at least one star cluster in Messier 24, NGC 6603, and we always expect stars in a cluster to be relatively close to one another.

But you must also consider the position of the Sun and the solar system inside the Milky Way. We are not located in the most densely populated part of our galaxy by a long shot.

So when we look at Messier 24, we are actuality looking through a "clearing" in the dust that hides our view from the inner parts of our galaxy, and we are looking at parts of the Milky Way where the star density is considerably higher, and the stars are on average closer together, than in the neighborhood of the Solar system.

How much closer the stars are, however, I couldn't say.

[De58te]

Are there stars within that amazing starscape that are closer together than our sun is to our closest neighbor? If so, how much closer?

It should be interesting to note that although the closest star to our Sun is Proxima Centauri at a distance of about 4.25 light years, note that our Sun is NOT the closest star to Proxima Centauri. That would be Alpha and Beta Centauri which are only about one fifth of a light year away from Proxima Centauri. And to top it off, the distance between Alpha Centauri and Beta Centauri is about the same distance as the former planet Pluto or Neptune is away from the Sun.

[ddorn777]

So, with all the talk of proximity to other stars, what is the "sweet spot" wherein a sun-like star would be likely to host planets capable of sustaining life? I've been told that within most star clusters, the nearness of other starts and all their "cosmic radiation" would prevent long-lasting atmospheres, or it would be so strong as to prevent life from occurring. Is there a galactic sweet spot, within the Milky Way, where we are more likely to see life arise? Is there a range of area within our galaxy (separate from clusters) more toward the center, before things get too crowded?

I'm just curious. No real need for an in-depth explanation, but maybe a link to a paper/article/book?

[Chris Peterson]

So, with all the talk of proximity to other stars, what is the "sweet spot" wherein a sun-like star would be likely to host planets capable of sustaining life? I've been told that within most star clusters, the nearness of other starts and all their "cosmic radiation" would prevent long-lasting atmospheres, or it would be so strong as to prevent life from occurring. Is there a galactic sweet spot, within the Milky Way, where we are more likely to see life arise? Is there a range of area within our galaxy (separate from clusters) more toward the center, before things get too crowded?

I'm just curious. No real need for an in-depth explanation, but maybe a link to a paper/article/book?

Once you get regions where stars are within a light year or so of each other, it's doubtful that they will have stable planetary systems, especially for planets far enough from the star for liquid water to exist. So that would suggest the most likely places are at the edges of arms or between them.

[Ann]

So, with all the talk of proximity to other stars, what is the "sweet spot" wherein a sun-like star would be likely to host planets capable of sustaining life? I've been told that within most star clusters, the nearness of other starts and all their "cosmic radiation" would prevent long-lasting atmospheres, or it would be so strong as to prevent life from occurring. Is there a galactic sweet spot, within the Milky Way, where we are more likely to see life arise? Is there a range of area within our galaxy (separate from clusters) more toward the center, before things get too crowded?

I'm just curious. No real need for an in-depth explanation, but maybe a link to a paper/article/book?

What you are referring to might be called the galactic habitable zone. Wikipedia has an article about it.

The galactic habitable zone is often viewed as an annulus 7-9 kpc from the galactic center, shown in green here, though recent research has called this into question. Milky Way image: NASA/Caltech. Green overlay: Unknown.

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Wikipedia wrote:

In astrobiology and planetary astrophysics, the galactic habitable zone is the region of a galaxy in which life might most likely develop. The concept of a galactic habitable zone analyzes various factors, such as metallicity (the presence of elements heavier than hydrogen and helium) and the rate and density of major catastrophes such as supernovae, and uses these to calculate which regions of a galaxy are more likely to form terrestrial planets, initially develop simple life, and provide a suitable environment for this life to evolve and advance. According to research published in August 2015, very large galaxies may favor the birth and development of habitable planets more than smaller galaxies such as the Milky Way. In the case of the Milky Way, its galactic habitable zone is commonly believed to be an annulus (chubby ring) with an outer radius of about 10 kiloparsecs (33,000 ly) and an inner radius close to the Galactic Center (with both radii lacking hard boundaries).

Galactic habitable-zone theory has been criticized due to an inability to accurately quantify the factors making a region of a galaxy favorable for the emergence of life. In addition, computer simulations suggest that stars may change their orbits around the galactic center significantly, therefore challenging at least part of the view that some galactic areas are necessarily more life-supporting than others.

Ann

[Fred the Cat]

The brightest star in the foreground of Barnard 92 caught my eye.

Interestingly B 92 was nicknamed “The Black Hole” way before those other objects. HD 312872 really pinpoints the nebulosity.

[Devil Particle]

I bet if you took a straight line from left to right through the center of this image there would be at least 10,000 stars. So if the width of the image is 300ly, that would make the stars less than 0.03 ly apart from each other. Which is only like 2 hundred billion miles between stars. And I'm sure some of them are much closer than that!

[VictorBorun]

I bet if you took a straight line from left to right through the center of this image there would be at least 10,000 stars. So if the width of the image is 300ly, that would make the stars less than 0.03 ly apart from each other. Which is only like 2 hundred billion miles between stars. And I'm sure some of them are much closer than that!

The stars, clusters and other objects of M24 form a portion of a spiral arm (the Sagittarius or Sagittarius-Carina arm) which fills a space of significant depth, at a distance of 10,000 to 16,000 light-years.
If we want to evaluate the density of the stellar population, we need to count all the stars in the window and divide the number by 6000 ly depth × 300 ly width × 200 ly height.
If we want to evaluate the mean distance between the neighbours, we need to calculate ∛(the volume, occupied by one star) = 1/∛(the density of the stellar population).

Let us compare this with 2D calculations. If we want to evaluate the 2D density of the stellar population, we need to count all the stars in the window and divide the number by 300 ly width × 200 ly height.

If we want to evaluate the mean 2D distance between the neighbours, we need to calculate √(the area, occupied by one star) = 1/√(the 2D density of the stellar population).

So the mean distance can be expressed as ∛((the mean 2D distance between the neighbours)² × 6000 ly depth).

So if we set the mean 2D distance between the neighbours to 0.03 ly, we get the mean distance of ∛((0.03 ly)² × 6000 ly depth) = ∛(5.4 ly³) = 1,8 ly.
That would be 2 times closer than Sun to Alpha Centauri.
Nothing close to shockingly small value of 0.03 ly