by Ann » Sun Aug 30, 2015 7:42 am
I really think that the starforming "spiral rings" of Andromeda in today's APOD truly look like breaking waves, spreading outwards after a bull's-eye "splash". Except M32 hasn't really hit yet, as far as I can understand. Maybe we will see some real fireworks when it hits for real - or our descendants may see it, if we are lucky.
NGC 1316. Photo: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Is Andromeda in the process of turning into a lenticular galaxy? Actually M31 is a quite red galaxy, and redder than most of us think it is. Our big sister galaxy is close enough that we can easily make out "her" star forming regions, especially when it is photographed through an Ha filter. We can also see the blue stars in the spiral "rings". But in reality M31 is rather poor in star formation, and its colors are red: 0.92 (B-V), and 0.50 (U-B). This is comparable to many elliptical galaxies. One example is NGC 1316, an elliptical galaxy that contains dust but no visible regions of star formation. Yet, the colors of NGC 1316 are actually very slightly
bluer than the integrated colors of Andromeda.
NGC 253. Photo: R. Jay GaBany.
Some spiral galaxies are also about as red as Andromeda, but they are usually quite dusty, and hide regions of star formation behind dust. On example is the Silver Dollar galaxy, NGC 253. The integrated colors of NGC 253 are somewhat bluer than the integrated colors of Andromeda, but while Andromeda is dust-poor - it is about 1.3 magnitudes fainter in the far infrared than in blue light - NGC 253 is almost 2.5 magnitudes
brighter in the far infrared than in blue light. There are many brilliant blue stars in NGC 253 that are hidden behind all that dust.
To summarize: M31 is a red galaxy, poor in star formation, with huge numbers of old red stars, but with rather little dust. Is it on the way to becoming a lenticular galaxy? I'd say its star forming ability is definitely winding down.
Ann
I really think that the starforming "spiral rings" of Andromeda in today's APOD truly look like breaking waves, spreading outwards after a bull's-eye "splash". Except M32 hasn't really hit yet, as far as I can understand. Maybe we will see some real fireworks when it hits for real - or our descendants may see it, if we are lucky.
[float=left][img2]https://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Ngc1316_hst.jpg/756px-Ngc1316_hst.jpg[/img2][c][size=80]NGC 1316. Photo: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)[/size][/c][/float]Is Andromeda in the process of turning into a lenticular galaxy? Actually M31 is a quite red galaxy, and redder than most of us think it is. Our big sister galaxy is close enough that we can easily make out "her" star forming regions, especially when it is photographed through an Ha filter. We can also see the blue stars in the spiral "rings". But in reality M31 is rather poor in star formation, and its colors are red: 0.92 (B-V), and 0.50 (U-B). This is comparable to many elliptical galaxies. One example is NGC 1316, an elliptical galaxy that contains dust but no visible regions of star formation. Yet, the colors of NGC 1316 are actually very slightly [i]bluer[/i] than the integrated colors of Andromeda.
[float=right][img2]http://www.personal.psu.edu/afr3/blogs/siowfa12/spiralgalaxysmaller.jpg[/img2][c][size=85]NGC 253. Photo: R. Jay GaBany.[/c][/size][/float]
Some spiral galaxies are also about as red as Andromeda, but they are usually quite dusty, and hide regions of star formation behind dust. On example is the Silver Dollar galaxy, NGC 253. The integrated colors of NGC 253 are somewhat bluer than the integrated colors of Andromeda, but while Andromeda is dust-poor - it is about 1.3 magnitudes fainter in the far infrared than in blue light - NGC 253 is almost 2.5 magnitudes [i]brighter[/i] in the far infrared than in blue light. There are many brilliant blue stars in NGC 253 that are hidden behind all that dust.
To summarize: M31 is a red galaxy, poor in star formation, with huge numbers of old red stars, but with rather little dust. Is it on the way to becoming a lenticular galaxy? I'd say its star forming ability is definitely winding down.
Ann