by Ann » Thu Dec 28, 2023 5:47 am
VictorBorun wrote: ↑Wed Dec 27, 2023 8:03 pm
Ann wrote: ↑Wed Dec 27, 2023 7:00 pm
Ann
But what if the Milky Way in Sagittarius is in fact the core looming from both sides of the disk? It would not be fair then to attribute all that yellow to the whole galaxy
In large galaxies, such as the Milky Way, we always expect the central bulge to be bright and yellow, and to provide most of the light of the galaxy.
There are exceptions. M101, the Pinwheel Galaxy, has a bright but small yellow bulge and very large, sprawling arms richly populated by young blue star clusters and pink emission nebulas. M101 is actually exceptionally "blue" as large galaxies go.
But by far most spiral galaxies have less impressive sets of arms than M101, and they have fewer young blue stars and less star formation. Instead, they have bright yellow bulges, which have been built up over billions of years as the galaxies have undergone burst after burst of star formation and one galactic merger after another.
Galaxies are blue or bluish because of recent high-mass star formation. That is the only mechanism by which they can be blue. But the bright blue stars quickly die, and unless they are replaced by new young clusters, the blue light of the galaxy will start to fade away.
- The Pleiades, at left, is a young cluster of some 100 million years. All the bright stars of the Pleiades are blue B-type stars of at least 12,000 K. By contrast, the Hyades, at right, is some 600 million years old, and all the B-type stars have died. A few A-type stars remain. The bluest star of the Hyades is delta 3 Tau, a blue straggler of spectral type A2 and a likely temperature of, at most, 9,000 K.
But when the blue stars die, the yellow and orange stars remain:
Trumpler 5. This cluster is possibly some 5 billion years old.
Credit: zirl/Photocommunity Deutschland.
In order to maintain its blue light, galaxies have to continually form new young clusters of massive blue stars. But each new star cluster also contains many low-mass, non-blue, typically yellow stars. These will live on when the blue stars die, adding to the yellow light of the galaxy. Every burst of star formation will eventually give the galaxy more yellow light.
One more thing. We are used to seeing pictures of spiral galaxies where the (typically blue) arms have been made to look brighter than they are, compared with the galactic bulge. The galaxies simply look better that way, but they don't show us the "true" appearance of the galaxies. A good example is M81:
Adam Block's portrait of M81 is gorgeous, but still, the spiral arms have been made to look brighter in his image than they "really" are, compared with the bulge of the galaxy. Also the bright light of the bulge has been somewhat muted, as we can see when we compare his image with the Hubble image. I love Adam Block's images and would never criticize them, but still, we might bear this detail in mind.
Ann
[quote=VictorBorun post_id=335924 time=1703707420 user_id=145500]
[quote=Ann post_id=335922 time=1703703652 user_id=129702]
[img3="The Milky Way in Sagittarius. Note the bright yellow light from the bulge of the Milky Way. It seems highly likely that this bright yellow light dominates the overall color of the Milky Way, and that the Milky Way is a relatively yellow galaxy. Credit:
Terrence Dickinson."]https://cdn.spacetelescope.org/archives/images/screen/opo9930e.jpg[/img3]
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Ann
[/quote]
But what if the Milky Way in Sagittarius is in fact the core looming from both sides of the disk? It would not be fair then to attribute all that yellow to the whole galaxy
[/quote]
In large galaxies, such as the Milky Way, we always expect the central bulge to be bright and yellow, and to provide most of the light of the galaxy.
There are exceptions. M101, the Pinwheel Galaxy, has a bright but small yellow bulge and very large, sprawling arms richly populated by young blue star clusters and pink emission nebulas. M101 is actually exceptionally "blue" as large galaxies go.
[img3="Messier 101. Credit: Hubble Legacy Archive, ESA, NASA; Processing and additional imaging - Robert Gendler"]https://apod.nasa.gov/apod/image/1104/M101HST-Gendler900.jpg[/img3]
But by far most spiral galaxies have less impressive sets of arms than M101, and they have fewer young blue stars and less star formation. Instead, they have bright yellow bulges, which have been built up over billions of years as the galaxies have undergone burst after burst of star formation and one galactic merger after another.
[img3="All these stellar streams are likely remnants of dwarf galaxies merging with the Milky Way over billions of years. "]https://i.ytimg.com/vi/6dtzfW6FO8I/maxresdefault.jpg[/img3]
Galaxies are blue or bluish because of recent high-mass star formation. That is the only mechanism by which they can be blue. But the bright blue stars quickly die, and unless they are replaced by new young clusters, the blue light of the galaxy will start to fade away.
[img3="The Pleiades, at left, is a young cluster of some 100 million years. All the bright stars of the Pleiades are blue B-type stars of at least 12,000 K. By contrast, the Hyades, at right, is some 600 million years old, and all the B-type stars have died. A few A-type stars remain. The bluest star of the Hyades is delta 3 Tau, a blue straggler of spectral type A2 and a likely temperature of, at most, 9,000 K. "]https://apod.nasa.gov/apod/image/1111/PleiadesHyades_andreo600h.jpg[/img3]
But when the blue stars die, the yellow and orange stars remain:
[float=left][attachment=0]Trumpler 5 Photocommunity Deutschland.png[/attachment][c][size=85][color=#FF4000]Trumpler 5. This cluster is possibly some 5 billion years old.
Credit: zirl/Photocommunity Deutschland.[/color][/size][/c][/float]
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In order to maintain its blue light, galaxies have to continually form new young clusters of massive blue stars. But each new star cluster also contains many low-mass, non-blue, typically yellow stars. These will live on when the blue stars die, adding to the yellow light of the galaxy. Every burst of star formation will eventually give the galaxy more yellow light.
[img3="This is the portrait of the Milky Way by ESA's Gaia telescope. You can see that most of the light from our galaxy comes from the yellow bulge."]https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2018/04/gaia_s_sky_in_colour2/17475368-10-eng-GB/Gaia_s_sky_in_colour_pillars.jpg[/img3]
One more thing. We are used to seeing pictures of spiral galaxies where the (typically blue) arms have been made to look brighter than they are, compared with the galactic bulge. The galaxies simply look better that way, but they don't show us the "true" appearance of the galaxies. A good example is M81:
[float=left][img3="M81. The arms look splendidly long, bright and blue. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona "]https://www.messier-objects.com/wp-content/uploads/2015/08/Messier-81-1024x1019.jpg[/img3][/float][float=right][img3="M81. The arms look nowhere near as impressive as in Adam Block's image. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)"]https://www.messier-objects.com/wp-content/uploads/2015/08/Bodes-Galaxy.jpg[/img3][/float]
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Adam Block's portrait of M81 is gorgeous, but still, the spiral arms have been made to look brighter in his image than they "really" are, compared with the bulge of the galaxy. Also the bright light of the bulge has been somewhat muted, as we can see when we compare his image with the Hubble image. I love Adam Block's images and would never criticize them, but still, we might bear this detail in mind.
Ann