johnnydeep wrote: ↑Sun Jul 18, 2021 12:51 pm
De58te wrote: ↑Sun Jul 18, 2021 10:52 am
Curious. The Andromeda galaxy in ultraviolet seems to be surrounded on all sides by more yellow stars than blue. All except for a portion near the central top frame of the photo where there appears to be a large void of stars. If Andromeda is twice as big as the Milky Way, this void seems to approach the size of the Milky Way. Could this be because the stars there don't shine in the ultraviolet, are blocked by a really dark cloud of dust, or that really is an area void of stars?
Not sure about the prevalence of yellow stars, but I'd guess it's because most stars simply appear yellow in this ultraviolet image? But the "void" you noticed is some sort of instrument or image processing artifact. Here are the voids exposed by edge detection in GIMP:
Andromeda is a very yellow galaxy
The U-B index of Andromeda is +0.500, and the B-V index is +0.920. That's very yellow. And it means that there is an amazing abundance of red and yellow stars in our big sister galaxy and rather few blue stars there. The superb image by u/_bar gives you an idea of how yellowish it is.
Andromeda is seen in a part of the sky which corresponds, I'd say, to the thick disk of the Milky Way. The thick disk is dominated by old yellow stars. Young blue star Nu Andromeda, located very close to where we see Andromeda in the sky, is an exception to the rule (and it belongs to the thin disk, too).
But the Milky Way itself is dominated by old yellow stars in the part of the sky where we see Andromeda. (And in my opinion, the Milky Way is probably a quite red galaxy too, although not as red as Andromeda.)
The ultraviolet image of Andromeda shows our sister galaxy to be a ring galaxy, which means it may have some similarities with the Cartwheel galaxy. IN the case of the Cartwheel, one of its satellite galaxies passed violently right through the disk of the larger galaxy, causing it to ripple like the waves of a pond. The outer ring of the Cartwheel is very active in star formation.
Andromeda, by contrast, is low in star formation, which is why it is so yellow in visible light. So why is it so blue in ultraviolet? The way I understand it, the far ultraviolet filter in the now-defunct GALEX telescope reacted to even modest A-type stars like Vega och Sirius. So you should probable think of the blue rings of Andromeda as conglomerates of hobbit stars
like Sirius rather than brilliant clusters of O- and B-type giants like Rigel and the three mighty stars in Orion's Belt.