APOD: Messier 24: Sagittarius Star Cloud (2018 Jun 29)

[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 intertellar 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 the left side of this gorgeous starscape. Covering about 4 degrees or the width of 8 full moons in the constellation Sagittarius, the telescopic field of view contains many small, dense clouds of dust and nebulae toward the center of the Milky Way, including reddish emission from IC 1284 near the top of the frame.

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[MuluTrail89]

"...a view of the distant stars in the Sagittarius spiral arm...." I may be out of date, but I've always remembered being taught that when we look at the summer Milky Way from our position in the Orion Spur we're looking across a gulf of 3,000-3,500 L.Y. to the Sagittarius Arm. Then when we look at its Small Sagittarius Star Cloud we're peering through a gap (window) in its obscuring dust clouds and seeing the next arm inward toward the Galactic Core: the Scutum-Centaurus Arm, and a region in it of rapid star formation with many supergiants, all at 10,000-12,000 L.Y. distant.

[BobStein-VisiBone]

What makes the view yellowish on the right? Other views of M24 don't show this. What is scattering longer wavelengths?

[orin stepanek]

Kind of reminds me of sand on the beach!

[neufer]

"...a view of the distant stars in the Sagittarius spiral arm...." I may be out of date, but I've always remembered being taught that when we look at the summer Milky Way from our position in the Orion Spur we're looking across a gulf of 3,000-3,500 L.Y. to the Sagittarius Arm. Then when we look at its Small Sagittarius Star Cloud we're peering through a gap (window) in its obscuring dust clouds and seeing the next arm inward toward the Galactic Core: the Scutum-Centaurus Arm, and a region in it of rapid star formation with many supergiants, all at 10,000-12,000 L.Y. distant.

---

<<The Scutum–Centaurus Arm, also known as Scutum-Crux arm, is a long, diffuse curving streamer of stars, gas and dust that spirals outward from the proximate end of the Milky Way's central bar. The Milky Way has been assumed since the 1950s to have four spiral arms although the evidence for this has never been strong. In 2008, observations using the Spitzer Space Telescope failed to show the expected density of red clump giants in the direction of the Sagittarius and Norma arms. In January 2014, a 12-year study into the distribution and lifespan of massive stars and a study of the distribution of masers and open clusters both found evidence for four spiral arms. The Scutum–Centaurus Arm lies between the minor Carina–Sagittarius Arm and the major Perseus Arm. The Scutum–Centaurus Arm starts near the core as the Scutum Arm, then gradually turns into the Centaurus Arm.

The region where the Scutum–Centaurus Arm connects to the bar of the galaxy is rich in star-forming regions. In 2006 a large cluster of new stars containing 14 red supergiant stars was discovered there and named RSGC1. In 2007 a cluster of approximately 50,000 newly formed stars named RSGC2 was located only a few hundred light years from RSGC1. It is thought to be less than 20 million years old and contains 26 red supergiant stars, the largest grouping of such stars known. Other clusters in this region include RSGC3 and Alicante 8.>>

[neufer]

What makes the view yellowish on the right? Other views of M24 don't show this. What is scattering longer wavelengths?

M24 (the Small Sagittarius Cloud) and nearby Messier Objects. Credit: Wikisky

---

<<In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is on the order of 1.8 magnitudes per kiloparsec.

Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons (in the optical, light that is redder), leaving the spectroscopic lines unchanged.>>

[heehaw]

Them darn intertellar dust clouds!

[MuluTrail89]

"...a view of the distant stars in the Sagittarius spiral arm...." I may be out of date, but I've always remembered being taught that when we look at the summer Milky Way from our position in the Orion Spur we're looking across a gulf of 3,000-3,500 L.Y. to the Sagittarius Arm. Then when we look at its Small Sagittarius Star Cloud we're peering through a gap (window) in its obscuring dust clouds and seeing the next arm inward toward the Galactic Core: the Scutum-Centaurus Arm, and a region in it of rapid star formation with many supergiants, all at 10,000-12,000 L.Y. distant.

---

<<The Scutum–Centaurus Arm, also known as Scutum-Crux arm, is a long, diffuse curving streamer of stars, gas and dust that spirals outward from the proximate end of the Milky Way's central bar. The Milky Way has been assumed since the 1950s to have four spiral arms although the evidence for this has never been strong. In 2008, observations using the Spitzer Space Telescope failed to show the expected density of red clump giants in the direction of the Sagittarius and Norma arms. In January 2014, a 12-year study into the distribution and lifespan of massive stars and a study of the distribution of masers and open clusters both found evidence for four spiral arms. The Scutum–Centaurus Arm lies between the minor Carina–Sagittarius Arm and the major Perseus Arm. The Scutum–Centaurus Arm starts near the core as the Scutum Arm, then gradually turns into the Centaurus Arm.

The region where the Scutum–Centaurus Arm connects to the bar of the galaxy is rich in star-forming regions. In 2006 a large cluster of new stars containing 14 red supergiant stars was discovered there and named RSGC1. In 2007 a cluster of approximately 50,000 newly formed stars named RSGC2 was located only a few hundred light years from RSGC1. It is thought to be less than 20 million years old and contains 26 red supergiant stars, the largest grouping of such stars known. Other clusters in this region include RSGC3 and Alicante 8.>>

Thank you, Neufer. This seems to confirm what I was taught, which was that when I look through the dust gap sometimes called the Small Sagittarius Star Cloud the view is inward toward the Galactic Core and 10,000 l.y. away stars of the Scutum-Centaurus Arm are viewed, not stars of the Sagittarius Arm as stated in the APOD text. Except that in my day we just called it the Centaurus Arm. The scale of the graphic you posted was especially helpful to show Scutum-Centaurus at the distance from the sun that I remembered. And what we called the Orion Spur (our home!) is now the Orion-Cygnus Arm. So many of the names have changed since five decades ago! At least on this modern graph the name of the Perseus Arm remains the same.
Thanks again.

[MarkBour]

...

Thank you, Neufer. This seems to confirm what I was taught, which was that when I look through the dust gap sometimes called the Small Sagittarius Star Cloud the view is inward toward the Galactic Core and 10,000 l.y. away stars of the Scutum-Centaurus Arm are viewed, not stars of the Sagittarius Arm as stated in the APOD text. Except that in my day we just called it the Centaurus Arm. The scale of the graphic you posted was especially helpful to show Scutum-Centaurus at the distance from the sun that I remembered. And what we called the Orion Spur (our home!) is now the Orion-Cygnus Arm. So many of the names have changed since five decades ago! At least on this modern graph the name of the Perseus Arm remains the same.
Thanks again.

I appreciated looking at this diagram today and learning from it as well.

I visited http://galaxymap.org/drupal/node/171 and the page "Basic plan of the Milky Way", which discusses this diagram, produced by Robert Hurt in 2008.

If MuluTrail89 is a bit disoriented with re-naming of the arms, I guess he or she should not feel alone! That article puts it colorfully here:

Unfortunately astronomers agree on the name of only one of these arms. This is called the Perseus arm and was discovered in the 1950s. It is the spiral arm that one sees first when looking into the outer galaxy (180°). As one might expect, it is most obvious at visual frequencies from the Earth in the direction of the constellations Perseus and Cassiopeia.

The other arms have been treated a bit like those roads in European cities that change their names every few blocks.

It would help if we had a giant mirror about 1 million light years above the Milky Way disk, so we could get a "selfie". But that would be too easy. Instead, I actually think it is great for humanity to have the challenge of figuring our what our galaxy looks like from the outside. And we're obviously making rapid progress on that.

[Ann]

What makes the view yellowish on the right? Other views of M24 don't show this. What is scattering longer wavelengths?

M24 (the Small Sagittarius Cloud) and nearby Messier Objects. Credit: Wikisky

---

<<In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is on the order of 1.8 magnitudes per kiloparsec.

Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons (in the optical, light that is redder), leaving the spectroscopic lines unchanged.>>

Click to view full size image

Small cluster M24 inside large star cloud M24.
Photo: Fred Espenak.

Exactly so, Art.

The so called Large or Great Sagittarius Star Cloud or Baade's Window is indeed a gap in nearby, obscuring interstellar dust clouds that allows a view of distant stars. For myself, I have always thought that Baade's Window offered a relatively unobscured view of our galaxy's old yellow bulge.

M24 is clearly something else, a distant region that is rich in young stars and star formation. Interestingly, I think that the designation "M24" has two meanings in the world of astronomy - one, the large bluish star cloud that fills the entire lower left half of today's superbly beautiful APOD, and two, a small cluster inside the large star cloud.

Ann

[Ann]

What makes the view yellowish on the right? Other views of M24 don't show this. What is scattering longer wavelengths?

Click to view full size image

Stars and dust in Andromeda.
Photo: Bob and Janice Fera.

There is generally more dust in the upper right than in the lower left part of today's APOD, and this dust causes obscuration and reddening in the orange-brown part of the APOD.

But it is also true that M24 is an extended "association" of young bright stars, whose bluish light dominate an area that is certainly thousands of light-years in diameter.

Take a look at the picture at left of about half of the Andromeda galaxy's disk. It is clear that the disk is generally dominated by old yellow stars, and dust lanes hide much of the star light from below and makes it look dark and brownish.

But there are blue areas in Andromeda as well, dominated by young massive stars. The brightest and most famous of these is is called NGC 206, and it can be seen at dead center of Bob and Janice Fera's beautiful picture at left.

In my opinion, M24 can probably be compared to NGC 206. Admittedly, we must assume that NGC 206 is brighter and richer in massive young stars than M24. But both are extended areas (rather than compact clusters) dominated by the light of young stars, so I think that these two regions are comparable.

Ann