APOD: Messier 20 and 21 (2014 Aug 28)

[APOD Robot]

Messier 20 and 21

Explanation: The beautiful Trifid Nebula, also known as Messier 20, is easy to find with a small telescope in the nebula rich constellation Sagittarius. About 5,000 light-years away, the colorful study in cosmic contrasts shares this well-composed, nearly 1 degree wide field with open star cluster Messier 21 (top right). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. In fact, M21's stars are much older, about 8 million years old.

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[Ann]

That is indeed a gorgeous picture!

Is M21 about 8 million years old? Well, it is clearly much older than the stars of the Trifid Nebula, since there is no nebulosity around M21 at all. It is interesting, however, that there are no red giants in M21. Perhaps M21 is a more massive version of the the Pleiades cluster - we see these clusters at a time when none of their stars are massive and old enough to be red giants. (It is interesting to think that the Pleiades may in fact have had red giants in the past, but if so, those red giants have disappered by now. And not only are the red giants themselves gone, if they were ever there in the first place, but the planetary nebulas that they must have left behind if they were there have also evaporated now.)

But the stars of the Trifid Nebula are only 300,000 years old. They are baby stars, indeed. And how brilliantly beautiful their natal cloud is!

But really, is there no connection between M20 and M21? I find that somewhat hard to believe. Of course I realize that we are not talking about triggered star formation, but still - the visual appearance of these clusters strongly suggests a connection, and if they are at the same distance from us as well, then that makes it almost too much of a coincidence that they would not be at least distantly related.

Ann

[Boomer12k]

Oooooo...pretty Trifid nebula...This is really nice and sharp, and the colors really stand out...excellent.

I got a decent picture of it last year...but mine is way more close up...and not quite as colorful. Mine is more of the central region of pink...

:---[===] *

[starsurfer]

There is a small possibility that M20 and M21 might have formed out of the same molecular cloud but I don't know how long molecular clouds last for.

Also I have to say that I would much prefer this scene north up, its so weird seeing it sideways.

[Nitpicker]

M20 was one of the first nebulae I ever managed to photograph reasonably well (I think of my images as "not so deep sky" photography). But this APOD is quite amazing and puts my effort to shame. Thank you.

[Chris Peterson]

There is a possibility that the Trifid Nebula and star cluster M21 are connected in some way. However, there is no evidence that the Trifid Nebula is a part of the Milky Way galaxy.

I think you mean there is no evidence that the Trifid is not a part of the Milky Way galaxy. There is overwhelming evidence that it lies inside the galaxy- an understanding that is accepted without doubt.

The Ring Nebula will likely evolve into a binary star system with two G type stars separated by approximately 1 LY.

The Ring Nebula is a planetary nebula- the consequence of a star shedding some of its material late in evolution. The central star is now a white dwarf (it is no longer producing energy through fusion). It certainly isn't going to magically become two stars.

Your comments are bordering on pseudoscience, which is not well tolerated in this forum.

[geckzilla]

Yes, I banned croc. The next post he put in the thread sounded procedurally generated, to be honest. Both posts were removed in the process.

[Nitpicker]

Great, A.I. has now developed to the point of creating automatic crackpots on the interwebs.

[geckzilla]

With enough jargon, a science bot could easily fool many people. Some fiendishly clever individuals have done this with SCIgen. http://pdos.csail.mit.edu/scigen/

[DavidLeodis]

In the explanation it states M21's stars are "about 8 million years old". However, in the information brought up through the 'Messier 21' link it states "this group of 57 various magnitude stars all started life together about 4.6 million years ago". I appreciate that it can be difficult to date objects and that the difference between 8 and 4.6 may be well within errors, but even so the difference does seems a bit large. I wonder if there is an accepted age for M21

[rstevenson]

Most sites I checked say 4.6 million years, but many of them may be parroting just one source. One site said the estimated age of the stars in the cluster ranged from 4 to 8 million years.

One site I found says there are 110 stars in the cluster, another says there are only about 57. They estimate its distance from us at 3000 to 4200 ly, and the width of the cluster as 11 to 16 ly. So you can see that there's a lack of precision in almost everything we "know" about the cluster.

Rob

[geckzilla]

This happens a lot with astronomical objects. A good idea is to check the most recent few papers on them done with the most advanced data available. There will always be accuracy issues for star clusters because the edge of a cluster is subject to variation and because line-of-sight objects may also get counted accidentally but those error bars seem a bit broader than usual.

[DavidLeodis]

Thanks Rob and geck for your help, which is appreciated.

PS. Apologies geck if using that seems a bit impersonal but I used it rather than geckzilla because of your signature "Just call me "geck" because "zilla" is like a last name."

[starsurfer]

Thanks Rob and geck for your help, which is appreciated.

PS. Apologies geck if using that seems a bit impersonal but I used it rather than geckzilla because of your signature "Just call me "geck" because "zilla" is like a last name."

I like to call her geckie.

[geckzilla]

I like just "geck" or I could just change my name to Judy Schmidt but that would be boring.