by Ann » Mon Jan 04, 2021 4:10 am
VictorBorun wrote: ↑Mon Jan 04, 2021 3:11 am
Ann wrote: ↑Sun Jan 03, 2021 5:55 am
Check out
this video, which shows the formation of a spiral galaxy. Note how the galaxy gradually evolves a bar.
Ann
Video is great, thanks.
But now I don't understand how come that the core, though low-gas and populated mostly with long-living stars, is solid enough to cunduct a density wave.
A galaxy arm, with many shining blue nebules, is one thing. Blue supergiants manage to form and discharge while an arm is propagating across the media.
But the core? The bar? Are there this many stars forming and discharging as the bar's ends move across the disk?
The bar structure is not solid, but it is dense as stellar structures go. That is to say, it contains a very large number of stars.
Also remember that almost all bars are yellow. Galactic features that are bright and yellow always contain a very high number of stars. That is because the individual stars that make up bright yellow features in galaxies are not bright themselves, and in fact, most of them are fainter - or indeed, a
lot fainter - than the Sun. That is because there is almost never star formation in the bar, and the vast majority of long-lived stars are little red dwarfs.
But although the red dwarfs are faint, they are surprisingly massive. They are less massive than the Sun, of course, but they are not
that much less massive.
Consider
Lacaille 8760 (or AX Microscopii), the brightest red dwarf in the sky. From a distance of a little less than 13 light-years, it is still too faint to be seen with the naked eye. Its mass is 60% solar, which is not so bad. Two of these stars would weight more than the Sun.
Yes, but the luminosity of Lacaille 8760 is only 7% solar. And now we are talking bolometric luminosity, which means all the energy that Lacaille 8760 emits at all wavelengths. The visual luminosity of Lacaille 8760 is only 3% solar. So a star which is 60% of the Sun's mass generates only 3% of its visual light.
The average star in a galactic bar is likely to contain less mass than Lacaille 8760, and they are therefore going to be
very much less luminous than Lacaille 8760. Or to put it differently: A galactic bar is likely to be made up of stars whose average mass is (perhaps) 40-50% of the Sun's mass, but whose average luminosity is going to be far less than 1% of the Sun's luminosity.
So in order to be as bright as bars often are, they have to be very full of small stars and therefore really massive. Through density waves, the bars rotate as solid bodies, even though they are not solid. As they rotate, they sweep up gas and channel it into two places: At the ends of the bar, which often see increased star formation, and into a small central ring surrounding the nucleus.
Not all barred galaxies have central rings, but many do. Sometimes these rings are really bright and full of young stars.
Ann
[quote=VictorBorun post_id=309455 time=1609729892 user_id=145500]
[quote=Ann post_id=309432 time=1609653324 user_id=129702]
Check out [url=https://www.youtube.com/watch?v=hVNuwAtnKeg]this video[/url], which shows the formation of a spiral galaxy. Note how the galaxy gradually evolves a bar.
Ann
[/quote]
Video is great, thanks.
But now I don't understand how come that the core, though low-gas and populated mostly with long-living stars, is solid enough to cunduct a density wave.
A galaxy arm, with many shining blue nebules, is one thing. Blue supergiants manage to form and discharge while an arm is propagating across the media.
But the core? The bar? Are there this many stars forming and discharging as the bar's ends move across the disk?
[/quote]
[float=right][img3="NGC 1512, a barred galaxy with an outer and an inner ring. The outer ring displays an increased amount of star formation on its 'left' side. The inner ring is quite bright and contains many young blue stars. Photo: NASA, ESA, Hubble Space Telescope."]https://apod.nasa.gov/apod/image/1708/NGC1512inner_Hubble_960.jpg[/img3]
[img3="NGC 1300, a barred galaxy with an inner ring. The inner ring is not bright, however, and contains little or no star formation. The galaxy displays an increased amount of star formation particularly at the 'right' end of the bar. Photo: NASA, ESA, and The Hubble Heritage Team STScI/AURA)."]https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg/1920px-Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg[/img3][/float]The bar structure is not solid, but it is dense as stellar structures go. That is to say, it contains a very large number of stars.
Also remember that almost all bars are yellow. Galactic features that are bright and yellow always contain a very high number of stars. That is because the individual stars that make up bright yellow features in galaxies are not bright themselves, and in fact, most of them are fainter - or indeed, a [i][b]lot[/b][/i] fainter - than the Sun. That is because there is almost never star formation in the bar, and the vast majority of long-lived stars are little red dwarfs.
But although the red dwarfs are faint, they are surprisingly massive. They are less massive than the Sun, of course, but they are not [b][i]that[/i][/b] much less massive.
Consider [url=https://en.wikipedia.org/wiki/Lacaille_8760]Lacaille 8760[/url] (or AX Microscopii), the brightest red dwarf in the sky. From a distance of a little less than 13 light-years, it is still too faint to be seen with the naked eye. Its mass is 60% solar, which is not so bad. Two of these stars would weight more than the Sun.
Yes, but the luminosity of Lacaille 8760 is only 7% solar. And now we are talking bolometric luminosity, which means all the energy that Lacaille 8760 emits at all wavelengths. The visual luminosity of Lacaille 8760 is only 3% solar. So a star which is 60% of the Sun's mass generates only 3% of its visual light.
The average star in a galactic bar is likely to contain less mass than Lacaille 8760, and they are therefore going to be [i]very[/i] much less luminous than Lacaille 8760. Or to put it differently: A galactic bar is likely to be made up of stars whose average mass is (perhaps) 40-50% of the Sun's mass, but whose average luminosity is going to be far less than 1% of the Sun's luminosity.
So in order to be as bright as bars often are, they have to be very full of small stars and therefore really massive. Through density waves, the bars rotate as solid bodies, even though they are not solid. As they rotate, they sweep up gas and channel it into two places: At the ends of the bar, which often see increased star formation, and into a small central ring surrounding the nucleus.
Not all barred galaxies have central rings, but many do. Sometimes these rings are really bright and full of young stars.
Ann