by Ann » Mon Aug 17, 2020 7:47 am
BaldEagle asked about the blinding yellow center of NGC 6814, and I can't resist the chance to talk a little about yellow and blue stars in spiral galaxies, and why we find them where we find them.
To start off, we need to talk about star formation. Stars form in bursts, forming clusters. Take a look at this Hubble image of a cluster (or two clusters) in nebula NGC 602 in the Small Magellanic Cloud:
As you can see, there are two clusters in the picture at left, one in the upper left corner, one at center right. There is a moderately equal number of bright blue stars in both clusters, but only the cluster at right contains huge numbers of small stars. Why is that?
At least one part of the answer is that the cluster at right is
very young. Small faint stars are born whenever stars form, but bright massive stars are formed only in massive clusters. There are always
more small faint stars than big bright stars formed in any cluster. So whenever there is a very young cluster of stars, there is always going to be large numbers of small stars in that cluster.
So why don't we see any small stars in the cluster at left?
Answer: They may have scattered, because of the violent tidal forces of the big bright (and massive) stars orbiting one another. Clusters also scatter when they encounter molecular clouds. The cluster at right is so young that the small stars have not yet had time to scatter. Another possibility, which is perhaps more likely, is that the cluster at right was photographed through an infrared filter, while the cluster at left was not. That would make a huge difference in whether or not you can see the small red and yellow stars in the cluster.
Click to play embedded YouTube video.
The more massive a cluster is, the better it will be at holding on to its stars. The globular clusters of the Milky Way were born with hundreds of thousands of stars, and the combined gravity of all these stars makes them stick together as clusters after, typically, 8-12 billion years.
Take a look at the video at right, where you can see the evolution of a (relatively) massive star cluster. You can see that some stars change color, become red, and then disappear. Why?
The answer is that the more massive (and bright and blue) a star is, the faster it uses up its core hydrogen. When the star's core hydrogen is gone, it swells and turns into a red giant. Now its supports itself by fuelling less energy-efficient elements than hydrogen. Relatively soon it runs out, and then it either explodes as a supernova (if it is massive enough), or else it sheds its outer layers and becomes a white dwarf.
What does a rich cluster look like when all its bright blue stars are gone?
At left you can see the old open cluster Trumpler 5. I have not managed to find out how old it is, but my guess is that it is 2-4 billion years old. At that age, all of its blue stars have died. The cluster still holds on to most of its stars, because the cluster was clearly born quite massive. It is also almost certain that the cluster is located in a "quiet" part of the Milky Way, where it is not disturbed by the passage of large molecular clouds. It is located behind the nebulosity that you can see in this image.
When clusters like Trumpler 5 finally disperse, where do their (small red and yellow) stars end up?
Well, the small red and yellow stars will quietly disperse and mostly follow the orbital motion of their parent cluster. The illustration at right shows the dispersal of a globular cluster. Open clusters will disperse in the same way, only faster, because they have less total gravity to hold them together.
Where will most of the small red and yellow stars that escaped from their parent cluster end up?
Take a look at spiral galaxy M74. Its center is yellow, and its arms are blue. Why is that?
The central part of M74, its bulge, is bright and yellow because it contains billions and billions of small red and yellow stars, which are the remnants of past star formation in this part of the galaxy. The formation of so many clusters in this part on the galaxy used up and scattered the available fuel for star formation, the cold and concentrated hydrogen gas, and no more stars are formed in the bulge. But the enormous number of small faint yellow stars in the bulge still makes the bulge shine bright and yellow.
While the bulge of M74 is yellow, its spiral arms are blue. They are blue because massive, bright and blue stars are born here. You can see individual clusters and possibly even individual bright stars in the arms. These bright stars are all young and will die soon, and they will leave behind the kind of yellow stars that crowd together in the bulge. But before even the arms of M74 turn yellow, they will turn a more dilute blue color from intermediate-mass stars like Sirius and Vega. Stars like Sirius and Vega last much longer than brilliant blue stars like the ones in Orion's Belt, but they still die much sooner than stars like the Sun.
You can see that the arms of M74 are bluer the farther away they are from the center of the galaxy. You can see, too, that the pink nebulas surrounding newborn stars are larger the farther away they are from the galactic center. You can also see that the galaxy gets ever darker the farther away we get from its center, because there are much fewer stars here. The fact that bright blue stars are born here can't compensate for the fact that the number of these stars is so small.
A great example of how galaxies form small numbers of hot bright and ultraviolet stars in an outer set of arms is the galaxy NGC 1512. In visual light, we see an oval-shaped galaxy, with a yellow bar-shaped bulge, a bright inner ring of hot stars surrounding a yellow nucleus, and a relatively faint outer ring. In ultraviolet light, however, we see long outer arms shining by the light of small numbers of hot bright stars lined up along elongated structures of dust and gas.
Finally, take a look at the picture at left of the very center of the Milky Way. The small bright white swirl of gas surrounds the black hole of our galaxy. Note the number of stars outside this gaseous swirl. There are young blue stars born here, and the white swirl is probably the brightest feature in the entire Milky Way.
Ann
[url=https://asterisk.apod.com/viewtopic.php?p=305215#p305215]BaldEagle[/url] asked about the blinding yellow center of NGC 6814, and I can't resist the chance to talk a little about yellow and blue stars in spiral galaxies, and why we find them where we find them.
To start off, we need to talk about star formation. Stars form in bursts, forming clusters. Take a look at this Hubble image of a cluster (or two clusters) in nebula NGC 602 in the Small Magellanic Cloud:
[float=left][img3="Clusters in NGC 602 in the Small Magellanic Cloud. NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration."]https://imgsrc.hubblesite.org/hvi/uploads/image_file/image_attachment/30545/_STScI-gallery-0704a-1024x361.jpg[/img3][/float]
As you can see, there are two clusters in the picture at left, one in the upper left corner, one at center right. There is a moderately equal number of bright blue stars in both clusters, but only the cluster at right contains huge numbers of small stars. Why is that?
At least one part of the answer is that the cluster at right is [b][i]very[/i][/b] young. Small faint stars are born whenever stars form, but bright massive stars are formed only in massive clusters. There are always [b][color=#FF0000]more small faint stars[/color][/b] [b][color=#0040FF]than big bright stars[/color][/b] formed in any cluster. So whenever there is a very young cluster of stars, there is always going to be large numbers of small stars in that cluster.
So why don't we see any small stars in the cluster at left?
Answer: They may have scattered, because of the violent tidal forces of the big bright (and massive) stars orbiting one another. Clusters also scatter when they encounter molecular clouds. The cluster at right is so young that the small stars have not yet had time to scatter. Another possibility, which is perhaps more likely, is that the cluster at right was photographed through an infrared filter, while the cluster at left was not. That would make a huge difference in whether or not you can see the small red and yellow stars in the cluster.
[float=right][youtube]https://www.youtube.com/watch?v=VF6J7P2cpn8[/youtube][/float]
The more massive a cluster is, the better it will be at holding on to its stars. The globular clusters of the Milky Way were born with hundreds of thousands of stars, and the combined gravity of all these stars makes them stick together as clusters after, typically, 8-12 billion years.
Take a look at the video at right, where you can see the evolution of a (relatively) massive star cluster. You can see that some stars change color, become red, and then disappear. Why?
[float=left][img3="Artis's impression of a supernova."]https://www.ox.ac.uk/sites/files/oxford/styles/ow_medium_feature/s3/field/field_image_main/Supernova.jpg?itok=UAEbSQOs[/img3][/float]
The answer is that the more massive (and bright and blue) a star is, the faster it uses up its core hydrogen. When the star's core hydrogen is gone, it swells and turns into a red giant. Now its supports itself by fuelling less energy-efficient elements than hydrogen. Relatively soon it runs out, and then it either explodes as a supernova (if it is massive enough), or else it sheds its outer layers and becomes a white dwarf.
[float=left][img3="Trumpler 5, a rich old cluster where all the bright blue stars have died. Photo: zirl."]https://img.fotocommunity.com/trumpler-5-ein-methusalem-unter-den-offenen-sternhaufen-6f817c60-4487-4911-862f-2007cd968333.jpg?width=1000[/img3][/float]
What does a rich cluster look like when all its bright blue stars are gone?
At left you can see the old open cluster Trumpler 5. I have not managed to find out how old it is, but my guess is that it is 2-4 billion years old. At that age, all of its blue stars have died. The cluster still holds on to most of its stars, because the cluster was clearly born quite massive. It is also almost certain that the cluster is located in a "quiet" part of the Milky Way, where it is not disturbed by the passage of large molecular clouds. It is located behind the nebulosity that you can see in this image.
When clusters like Trumpler 5 finally disperse, where do their (small red and yellow) stars end up?
[float=right][img3="Dispersal of a globular cluster. Illustration: Geraint F. Lewis and the S5 collaboration."]https://scitechdaily.com/images/Phoenix-Stream-777x680.jpg[/img3][/float]
Well, the small red and yellow stars will quietly disperse and mostly follow the orbital motion of their parent cluster. The illustration at right shows the dispersal of a globular cluster. Open clusters will disperse in the same way, only faster, because they have less total gravity to hold them together.
Where will most of the small red and yellow stars that escaped from their parent cluster end up?
[float=left][img3="Spiral galaxy M74. NASA/ESA/Hubble Heritage Team."]https://cdn.mos.cms.futurecdn.net/wWieWanunf3ct2xtaEVAwY-1200-80.jpg[/img3][/float]
Take a look at spiral galaxy M74. Its center is yellow, and its arms are blue. Why is that?
The central part of M74, its bulge, is bright and yellow because it contains billions and billions of small red and yellow stars, which are the remnants of past star formation in this part of the galaxy. The formation of so many clusters in this part on the galaxy used up and scattered the available fuel for star formation, the cold and concentrated hydrogen gas, and no more stars are formed in the bulge. But the enormous number of small faint yellow stars in the bulge still makes the bulge shine bright and yellow.
While the bulge of M74 is yellow, its spiral arms are blue. They are blue because massive, bright and blue stars are born here. You can see individual clusters and possibly even individual bright stars in the arms. These bright stars are all young and will die soon, and they will leave behind the kind of yellow stars that crowd together in the bulge. But before even the arms of M74 turn yellow, they will turn a more dilute blue color from intermediate-mass stars like Sirius and Vega. Stars like Sirius and Vega last much longer than brilliant blue stars like the ones in Orion's Belt, but they still die much sooner than stars like the Sun.
You can see that the arms of M74 are bluer the farther away they are from the center of the galaxy. You can see, too, that the pink nebulas surrounding newborn stars are larger the farther away they are from the galactic center. You can also see that the galaxy gets ever darker the farther away we get from its center, because there are much fewer stars here. The fact that bright blue stars are born here can't compensate for the fact that the number of these stars is so small.
[float=left][img3="NGC 1512 in visual light. Image Credit: NASA, ESA, Hubble Space Telescope"]https://apod.nasa.gov/apod/image/1708/NGC1512inner_Hubble_960.jpg[/img3][/float] [float=right][img3="NGC 1512 and its companion galaxy in ultraviolet light. NASA/JPL-Caltech."]https://upload.wikimedia.org/wikipedia/commons/6/65/Ngc1512_nasajpl.jpg[/img3][/float]
A great example of how galaxies form small numbers of hot bright and ultraviolet stars in an outer set of arms is the galaxy NGC 1512. In visual light, we see an oval-shaped galaxy, with a yellow bar-shaped bulge, a bright inner ring of hot stars surrounding a yellow nucleus, and a relatively faint outer ring. In ultraviolet light, however, we see long outer arms shining by the light of small numbers of hot bright stars lined up along elongated structures of dust and gas.
[float=left][img3="The center of the Milky Way. Note the white swirl of gas circling around the black hole of our galaxy, and note the large numbers of stars near this gaseous swirl. Hubble: NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech)"]https://upload.wikimedia.org/wikipedia/commons/thumb/1/15/An_Infrared_View_of_the_Galaxy.jpg/1280px-An_Infrared_View_of_the_Galaxy.jpg[/img3][/float]
Finally, take a look at the picture at left of the very center of the Milky Way. The small bright white swirl of gas surrounds the black hole of our galaxy. Note the number of stars outside this gaseous swirl. There are young blue stars born here, and the white swirl is probably the brightest feature in the entire Milky Way.
Ann