APOD: Messier 20 and 21 (2020 Aug 06)

[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 (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]

APOD Robot wrote:

Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old.

I'd say it is the ionizing star itself, HD 164492, that is probably only 300,000 years old.

The innermost part of the Trifid Nebula with its main ionizing star, HD 164492. NASA, ESA, and The Hubble Heritage Team (AURA/STScI); Acknowledgment: F. Yusef-Zadeh (Northwestern Univ.)

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Central star of the Lagoon Nebula. Infrared image by NASA/ESA/Hubble.

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An O-type star that is involved in dust and surrounded by a red emission nebula is probably very young. In a few million years (or less) the harsh ultraviolet light and strong stellar wind from the massive O-type star will blow the nebula away.

Interestingly, the central star of the Trifid Nebula seems to be a first-generation massive star. The O-type star surrounded by a hurricane of dust inside inside the Lagoon Nebula, by contrast, seems to be a second or maybe even a third generation of massive stars forming inside the Lagoon. The first generation of the Lagoon Nebula is likely NGC 6530, now a loose open cluster probably located in front of the dusty and red-glowing nebula, as the nebula front has receded over time.

The Trifid Nebula and cluster M21, with three striking blue stars located between them. Photo: Stanislav Volskiy, Chilescope Team.

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One thing I have long wondered about is the three bright blue stars seemingly located between the Trifid Nebula and M21. They are not involved in any nebulosity, as you can see. I checked them (HD 164402, mag 5.7, HD 164637, mag 6.7 and HD 164833, mag 7.1) with Simbad, and the two latter had parallaxes around 0.9 milliarcseconds per year, while HD 164402 had a parallax closer to 0.6 mas/year, but with a very large margin of error (0.23 mas/year). Someone else is welcome to figure out how far away those parallaxes put these stars!

Anyway, the stars of M21 seem to be located farther away. Simbad gave the parallax of the central star, HD 164863, as 0.345 mas/year, with an error of 0.1 mas/year.

So the striking bright blue stars located between M20 and M21 may be foreground objects, and M20 and M21 are not related. Still, together these shining beacons make up a splendid cosmic composition!

Ann

[orin stepanek]

Messier 20 and 21
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.

Actually; isn't 8 million years quite young for stars; even
though they are a lot older than the 300,000 years for trifid?

[Ann]

Messier 20 and 21
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.

trifidnebulaM20M21_1024.jpg


Actually; isn't 8 million years quite young for stars; even
though they are a lot older than the 300,000 years for trifid?

The Pleiades, a young cluster at a 100 million years old. Photo: Marco Lorenzi.

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Absolutely, yes, it is! Consider the Pleiades! They are around 100 million years old, and they are considered young!

But it's all a matter of mass, as you know. The more massive a star is, the shorter is its lifetime. The mass of stars of the Pleiades is only a few times that of the Sun (the brightest member of the Pleiades, Alcyone, has a mass of 6 solar, according to Jim Kaler).

The brightest members of M21 are certainly more massive. But how massive? I'm not sure if the nature of the stars in M21 is really known. My guess is that the brightest member of M21, HD 164863, has a mass of at least 12 times that of the Sun, maybe more. And if so, an age of 8 million years is not that young.

Why did I say that the brightest member of M21 is about 12 times as massive as the Sun? Well, I'm totally guessing. But Jim Kaler has said that Betelgeuse, which as we all know is an extremely evolved supergiant star, probably started out with a mass 18-19 times solar.

And according to Wikipedia, Betelgeuse is probably about the same age as the stars of M21, some 8 million years. So if Betelgeuse and the stars of M21 are contemporaries, we should assume that Betelgeuse started out more massive than any of the stars in M21, since it is so much more highly evolved.

The stars of M21 are "unevolved", at least in the respect that they are still blue. No red giants can be seen in the cluster. This, interestingly, is an aspect they have in common with the Pleiades. But the Pleiades is suspected to have had at least one red giant in the past, because this cluster does seem contain a white dwarf, named EG 25 (LB 1497).

Ann

[orin stepanek]

Messier 20 and 21
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.

trifidnebulaM20M21_1024.jpg


Actually; isn't 8 million years quite young for stars; even
though they are a lot older than the 300,000 years for trifid?

The Pleiades, a young cluster at a 100 million years old. Photo: Marco Lorenzi.

---

Absolutely, yes, it is! Consider the Pleiades! They are around 100 million years old, and they are considered young!

But it's all a matter of mass, as you know. The more massive a star is, the shorter is its lifetime. The mass of stars of the Pleiades is only a few times that of the Sun (the brightest member of the Pleiades, Alcyone, has a mass of 6 solar, according to Jim Kaler).

The brightest members of M21 are certainly more massive. But how massive? I'm not sure if the nature of the stars in M21 is really known. My guess is that the brightest member of M21, HD 164863, has a mass of at least 12 times that of the Sun, maybe more. And if so, an age of 8 million years is not that young.

Why did I say that the brightest member of M21 is about 12 times as massive as the Sun? Well, I'm totally guessing. But Jim Kaler has said that Betelgeuse, which as we all know is an extremely evolved supergiant star, probably started out with a mass 18-19 times solar.

And according to Wikipedia, Betelgeuse is probably about the same age as the stars of M21, some 8 million years. So if Betelgeuse and the stars of M21 are contemporaries, we should assume that Betelgeuse started out more massive than any of the stars in M21, since it is so much more highly evolved.

The stars of M21 are "unevolved", at least in the respect that they are still blue. No red giants can be seen in the cluster. This, interestingly, is an aspect they have in common with the Pleiades. But the Pleiades is suspected to have had at least one red giant in the past, because this cluster does seem contain a white dwarf, named EG 25 (LB 1497).

Ann

I know that the sun is supposed to be middle aged at about 4 1/2 to 5 billion years in age! It is supposedly called a 3rd generation star! I guess that means it was made up from the leftovers of preceding stars! So I'm Thinking 8 million years makes them pretty young! Maybe not?

[neufer]

<<The [Trifid] nebula's distance is rather uncertain, with values between 2,200 light years (Mallas/Kreimer; Glyn Jones has 2,300) and about 7,600 light years (C.R. O'Dell 1963). The Sky Catalog 2000 gives 5,200 light years, a value which is also used by Archinal and Hynes (2003), and which we adopt here. The WEBDA database has 3140, the Hubble Press Release of Jeff Hester (STScI-PRC99-42) gives "about 9000" light years.>>

The [Trifid] nebula's distance: 4100 ± 200 ly

Kinematics in Young Star Clusters and Associations with Gaia DR2

Michael A. Kuhn (1), Lynne A. Hillenbrand (1), Alison Sills (2), Eric D. Feigelson (3), Konstantin V. Getman (3) ((1) California Institute of Technology, (2) McMaster University, (3) Pennsylvania State University)

Submitted on 5 Jul 2018

<<The Gaia mission has opened a new window into the internal kinematics of young star clusters at the sub-km/s level, with implications for our understanding of how star clusters form and evolve. We use a sample of 28 clusters and associations with ages from 1-5 Myr, where lists of members are available from previous X-ray, optical, and infrared studies. Proper motions from Gaia DR2 reveals that at least 75% of these systems are expanding; however, rotation is only detected in one system. Typical expansion velocities are on the order of ~0.5 km/s, and, in several systems, there is a positive radial gradient in expansion velocity. Systems that are still embedded in molecular clouds are less likely to be expanding than those that are partially or fully revealed. One-dimensional velocity dispersions, which range from 1 to 3 km/s, imply that most of the stellar systems in our sample are supervirial and that some are unbound. In star-forming regions that contain multiple clusters or subclusters, we find no evidence that these groups are coalescing, implying that hierarchical cluster assembly, if it occurs, must happen rapidly during the embedded stage.>>

[Ann]

I know that the sun is supposed to be middle aged at about 4 1/2 to 5 billion years in age! It is supposedly called a 3rd generation star! I guess that means it was made up from the leftovers of preceding stars! So I'm Thinking 8 million years makes them pretty young! Maybe not?

Yes, 8 million years is certainly young for stars! I was just making the point that all massive stars are young, because they don't live to be old.

But you are right, M21 is a young cluster!

Ann

[neufer]

I know that the sun is supposed to be middle aged at about 4 1/2 to 5 billion years in age! It is supposedly called a 3rd generation star! I guess that means it was made up from the leftovers of preceding stars! So I'm Thinking 8 million years makes them pretty young! Maybe not?

Yes, 8 million years is certainly young for stars!

I was just making the point that all massive stars are young, because they don't live to be old.

But you are right, M21 is a young cluster!

At 8 million years M21 is just a toddler in many respects.

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<<An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years.

Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, their properties (such as distance, age, metallicity, extinction, and velocity) are more easily determined than they are for isolated stars. A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye.>>

[Ann]

I know that the sun is supposed to be middle aged at about 4 1/2 to 5 billion years in age! It is supposedly called a 3rd generation star! I guess that means it was made up from the leftovers of preceding stars! So I'm Thinking 8 million years makes them pretty young! Maybe not?

Yes, 8 million years is certainly young for stars!

I was just making the point that all massive stars are young, because they don't live to be old.

But you are right, M21 is a young cluster!

At 8 million years M21 is just a toddler in many respects.

---

<<An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years.

Betelgeuse. Photo: ESO.

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Globular cluster OMega Centauri. Photo: ESO.

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Massive clusters live for a long time, but massive stars don't!

There are no stars like circa 18 solar mass star Betelgeuse in 12-billion-year-old circa one million star-rich globular cluster Omega Centauri!

Ann

[orin stepanek]

I know that the sun is supposed to be middle aged at about 4 1/2 to 5 billion years in age! It is supposedly called a 3rd generation star! I guess that means it was made up from the leftovers of preceding stars! So I'm Thinking 8 million years makes them pretty young! Maybe not?

Yes, 8 million years is certainly young for stars! I was just making the point that all massive stars are young, because they don't live to be old.

But you are right, M21 is a young cluster!

Ann

Thanks Ann; fabulous!

[orin stepanek]

I know that the sun is supposed to be middle aged at about 4 1/2 to 5 billion years in age! It is supposedly called a 3rd generation star! I guess that means it was made up from the leftovers of preceding stars! So I'm Thinking 8 million years makes them pretty young! Maybe not?

Yes, 8 million years is certainly young for stars!

I was just making the point that all massive stars are young, because they don't live to be old.

But you are right, M21 is a young cluster!

At 8 million years M21 is just a toddler in many respects.

---

<<An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years.

Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, their properties (such as distance, age, metallicity, extinction, and velocity) are more easily determined than they are for isolated stars. A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye.>>

Thanks Art!

Isn't Methuselah A supermassive star? Andyet it is very old! https://video.search.yahoo.com/yhs/sear ... tion=click

[johnnydeep]

Isn't Methuselah A supermassive star? Andyet it is very old! https://video.search.yahoo.com/yhs/sear ... tion=click

Wikipedia says it's only .8 times the mass of the sun, with twice the radius and 5 times the luminosity. See https://en.wikipedia.org/wiki/HD_140283

The sun will be come a red giant in about 5 billion years, at which point it will be 9.5 billion years old. The Methuselah star is estimated to be about 13.5 billion years old, but is not a red giant...(yet?). Not sure if that makes total sense to me, but here is some more from that Wikipedia page:

Because HD 140283 is neither on the main sequence nor a red giant, its early position in the Hertzsprung-Russell diagram has been interpreted with its data and theoretical models of stellar evolution based on quantum mechanics and the observations of processes in millions of stars to infer its old age. For field stars (as opposed to stars in clusters) it is rare to know a star's luminosity, surface temperature and composition precisely enough to get a well-constrained value for their age; because of their relative scarcity, this is even rarer for a Population II star like HD 140283. A study published in 2013[13][14] used the Fine Guidance Sensors of NASA's Hubble Space Telescope to measure a precise parallax (and therefore distance and luminosity) for the star,[1] and employ this information to estimate an age for the star of 14.46 ± 0.8 billion years.[1] Due to the uncertainty in the value, this age for the star may or may not conflict with the calculated age of the Universe as determined by the final 2015 Planck satellite results of 13.799 ± 0.021 billion years.[1][15]

Once dubbed the "Methuselah Star" by the popular press due to its age, if the assumptions of stellar evolution are correct in the report, the star must have formed soon after the Big Bang[1] and is one of the oldest stars known.[citation needed] The search for such very iron-poor stars has shown they are almost all anomalies in globular clusters and the Galactic Halo. This concords with a narrative that they are rare survivors of their generation.

[Ann]

This is what I think I can say about HD 140283:

According to Hipparcos data, this star is about 3.72 times brighter than the Sun. Its B-V index is 0.484 ± 0.01, which makes it noticeably bluer than the Sun, whose B-V index is 0.656 ± 0.005. My software Guide classifies it as an F-type star of type sdF3 (whatever that means). But the mass of HD 140283 is lower than the mass of the Sun, about 0.8 solar. So how can it be brighter than the Sun if it is less massive?

The way I understand it, at least part of the answer is that HD 140283 is so metal-poor. In other words, there are only minuscule amounts of trace elements more massive than hydrogen and helium in HD 140283. Our own Sun is much, much more metal-rich.

Color-magnitude diagram of globular cluster M55. Globular clusters contain metal-poor stars somewhat similar to HD 140283. Illustration: B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope

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Very metal-poor stars stay longer on the "blue part of the spectrum" than metal-rich stars like the Sun. When the Sun leaves the main sequence it is going to turn much redder than it is, and it's not going to become blue until it turns into a white dwarf. But metal-poor stars turn first red, then blue, then red again when they evolve.

The way I understand it, practically all very old stars must be metal-poor, because they formed at a time when the gas in the Universe was mostly pristine and hardly mixed with the by-products of supernovas at all.

My guess is that HD 140283 is located either on the horizontal branch (which is where you find the signature RR Lyrae stars of globular clusters) or else it is a subgiant. If it is a horizontal branch star, then it has already been a red giant. It has thereafter undergone the so called helium flash and turned bluer (possibly much bluer) than it was when it was a red giant. If it is a subgiant, then I don't know how it has evolved.

Personally I suspect that it may well take longer than the present age of the Universe for any sub-Solar-mass stars to become "asymtotic branch stars", which is the stage where the star is bloated and red and poised to shed its outer layers and become a white dwarf. I suspect, too, that most sub-Solar-mass stars have not even made it onto the red giant branch yet. I suspect that the vast majority of all red giants that have ever existed in the Universe started out more massive than the Sun.

Ann

[neufer]

Isn't Methuselah A supermassive star? And yet it is very old! https://video.search.yahoo.com/yhs/sear ... tion=click

Wikipedia says it's only .8 times the mass of the sun, with twice the radius and 5 times the luminosity. See https://en.wikipedia.org/wiki/HD_140283

The sun will be come a red giant in about 5 billion years, at which point it will be 9.5 billion years old. The Methuselah star is estimated to be about 13.5 billion years old, but is not a red giant...(yet?).

lower-mass tracks show very short long-lasting subgiant branches

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Methuselah was (for a long time) a low mass low luminosity star that is so old that it has recently left the main sequence and become relatively large bright subgiant star.

Methuselah star age: 13.66-15.26 Gyrs.
Planck Universe age: 13.78-14.02 Gyrs.

(Close enough for government work.)

Since Methuselah is suffering from iron-deficiency anemia it is unfortunate that a radial velocity of −169 km/s indicates that it is not willing to stay socially distanced (at 202.4 ly).

[orin stepanek]

Isn't Methuselah A supermassive star? Andyet it is very old! https://video.search.yahoo.com/yhs/sear ... tion=click

Wikipedia says it's only .8 times the mass of the sun, with twice the radius and 5 times the luminosity. See https://en.wikipedia.org/wiki/HD_140283

The sun will be come a red giant in about 5 billion years, at which point it will be 9.5 billion years old. The Methuselah star is estimated to be about 13.5 billion years old, but is not a red giant...(yet?). Not sure if that makes total sense to me, but here is some more from that Wikipedia page:

Because HD 140283 is neither on the main sequence nor a red giant, its early position in the Hertzsprung-Russell diagram has been interpreted with its data and theoretical models of stellar evolution based on quantum mechanics and the observations of processes in millions of stars to infer its old age. For field stars (as opposed to stars in clusters) it is rare to know a star's luminosity, surface temperature and composition precisely enough to get a well-constrained value for their age; because of their relative scarcity, this is even rarer for a Population II star like HD 140283. A study published in 2013[13][14] used the Fine Guidance Sensors of NASA's Hubble Space Telescope to measure a precise parallax (and therefore distance and luminosity) for the star,[1] and employ this information to estimate an age for the star of 14.46 ± 0.8 billion years.[1] Due to the uncertainty in the value, this age for the star may or may not conflict with the calculated age of the Universe as determined by the final 2015 Planck satellite results of 13.799 ± 0.021 billion years.[1][15]

Once dubbed the "Methuselah Star" by the popular press due to its age, if the assumptions of stellar evolution are correct in the report, the star must have formed soon after the Big Bang[1] and is one of the oldest stars known.[citation needed] The search for such very iron-poor stars has shown they are almost all anomalies in globular clusters and the Galactic Halo. This concords with a narrative that they are rare survivors of their generation.

Must be made of light materials then as I read that it had a radius of 1.7 that of sol! Maybe I read wrong! Whatever; it is an old cronie like me!

[orin stepanek]

Isn't Methuselah A supermassive star? And yet it is very old! https://video.search.yahoo.com/yhs/sear ... tion=click

Wikipedia says it's only .8 times the mass of the sun, with twice the radius and 5 times the luminosity. See https://en.wikipedia.org/wiki/HD_140283

The sun will be come a red giant in about 5 billion years, at which point it will be 9.5 billion years old. The Methuselah star is estimated to be about 13.5 billion years old, but is not a red giant...(yet?).

lower-mass tracks show very short long-lasting subgiant branches

---

Methuselah was (for a long time) a low mass low luminosity star that is so old that it has recently left the main sequence and become relatively large bright subgiant star.

Methuselah star age: 13.66-15.26 Gyrs.
Planck Universe age: 13.78-14.02 Gyrs.

(Close enough for government work.)

Since Methuselah is suffering from iron-deficiency anemia it is unfortunate that a radial velocity of −169 km/s indicates that it is not willing to stay socially distanced (at 202.4 ly).

Wow; It must have mellowed with age!

[Boomer12k]

Great wide angle shot... my close up shot...with my 10" Meade LX-200... which I recently sold...sigh... taken in 2013.

:---[===] *