by Ann » Mon Jan 21, 2019 1:43 pm
Thanks for an interesting picture of a rarely seen galaxy.
My impression is that IC 5201 is relatively large, but quite faint. Its surface brightness and overall luminosity seems to be fairly comparable to M33, meaning that its total luminosity is about 0.3 times that of the Milky Way.
But in other respects IC 5201 seems to be quite different from M33. For one thing, M33 has no bar, while there is a yellowish bar in IC 5201. More interestingly, M33 and IC 5201 have comparable B-V indexes - they are both quite blue galaxies - but their U-B indexes are
very different. M33 has a healthy negative U-B index, a testament to its rather high levels star formation. IC 5201, by contrast, has a weak
positive U-B index! There is not a lot of star formation going on in that galaxy at all, which is obvious from the picture, too. (Although there are a few bright blue-white knots in one arm of IC 5201, which I take to be sites of star formation.)
And by the way, that red foreground star at left (Pi 1 Gruis) is
very red! It's one of those "type S" stars, and I'm not quite sure exactly what that means, since I'm no huge fan of red stars, as some of you may know. Anyway, this star's B-V index is +2.23, which is very respectably but not amazingly red - the Garnet star, Mu Cephei, has a B-V index of +2.24 - but the V-I index of Pi 1 Gruis is 4.61. That is very infrared, and and more infrared than the Garnet star, whose V-I index is +3.57.
On the other hand, Pi 1 Gruis appears to be a relatively modest star, perhaps 60 times as bright as the Sun, whereas the Garnet Star is a stupendous whopper. Its parallax is uncertain, but it may be as bright in optical light as some 50,000 times the Sun or so. And seeing how infrared the Garnet Star is, you have to realize what a super duper cosmic lighthouse the Garnet Star might be, perhaps approaching half a million times the Sun in bolometric (total) luminosity.
Jim Kaler has called the Garnet stars one of the biggest stars in the Milky Way galaxy.
The Garnet Star owes its brilliance and red color to its high mass, cool temperature and enormous size. Pi 1 Gruis is another sort of beast, a star of "normal K0III size", and its red color must be due to internal processes, perhaps comparable to dumping "pollution" in its upper atmosphere.
Ann
[quote=starsurfer post_id=289114 time=1548005678 user_id=137896]
[size=120][b]IC 5201[/b][/size]
[url]http://www.astrophoton.com/IC5201.htm[/url]
Copyright: CEDIC
Processing: Markus Blauensteiner
[img2]http://www.astrophoton.com/images/I5201-1.jpg[/img2]
[/quote]
Thanks for an interesting picture of a rarely seen galaxy.
My impression is that IC 5201 is relatively large, but quite faint. Its surface brightness and overall luminosity seems to be fairly comparable to M33, meaning that its total luminosity is about 0.3 times that of the Milky Way.
But in other respects IC 5201 seems to be quite different from M33. For one thing, M33 has no bar, while there is a yellowish bar in IC 5201. More interestingly, M33 and IC 5201 have comparable B-V indexes - they are both quite blue galaxies - but their U-B indexes are [i]very[/i] different. M33 has a healthy negative U-B index, a testament to its rather high levels star formation. IC 5201, by contrast, has a weak [i]positive[/i] U-B index! There is not a lot of star formation going on in that galaxy at all, which is obvious from the picture, too. (Although there are a few bright blue-white knots in one arm of IC 5201, which I take to be sites of star formation.)
And by the way, that red foreground star at left (Pi 1 Gruis) is [i]very[/i] red! It's one of those "type S" stars, and I'm not quite sure exactly what that means, since I'm no huge fan of red stars, as some of you may know. Anyway, this star's B-V index is +2.23, which is very respectably but not amazingly red - the Garnet star, Mu Cephei, has a B-V index of +2.24 - but the V-I index of Pi 1 Gruis is 4.61. That is very infrared, and and more infrared than the Garnet star, whose V-I index is +3.57.
On the other hand, Pi 1 Gruis appears to be a relatively modest star, perhaps 60 times as bright as the Sun, whereas the Garnet Star is a stupendous whopper. Its parallax is uncertain, but it may be as bright in optical light as some 50,000 times the Sun or so. And seeing how infrared the Garnet Star is, you have to realize what a super duper cosmic lighthouse the Garnet Star might be, perhaps approaching half a million times the Sun in bolometric (total) luminosity. [url=http://stars.astro.illinois.edu/sow/garnet.html]Jim Kaler[/url] has called the Garnet stars one of the biggest stars in the Milky Way galaxy.
The Garnet Star owes its brilliance and red color to its high mass, cool temperature and enormous size. Pi 1 Gruis is another sort of beast, a star of "normal K0III size", and its red color must be due to internal processes, perhaps comparable to dumping "pollution" in its upper atmosphere.
Ann