by Ann » Tue Oct 06, 2020 5:46 am
orin stepanek wrote: ↑Mon Oct 05, 2020 8:45 pm
Thanks Ann;
I was thinking Star because I think red galaxies are {IMO) rare; and looked like a pretty dusty area! I was aware of the white Galaxy but didn't Question it!
I really like today's APOD; Pretty neat!
Absolutely right, Orin, you can say that red galaxies don't exist! But they can look right for two reasons.
1) Galaxies can look red because there is a lot of dust in front of them, so we see them through a curtain of dust. The dust in front of it filters away the short wavelengths of light (i.e., violet, blue, green) that were emitted by that galaxy, making it look yellow, orange or even red. (Or it may in fact become invisible in optical light, because if there is enough dust in front of the galaxy, not even the galaxy's reddest light can penetrate the dust.)
2) Galaxies can look red because they are very far away, and the light they emitted has literally been "stretched" by the expansion of the Universe. If the galaxy itself looked "white", all the light that the galaxy emitted has been stretched to longer, redder colors. Because as you know, longer wavelengths are redder and shorter wavelengths are bluer. Again, if the galaxy is sufficiently far away, it becomes invisible in optical light, because even its bluest light becomes stretched to infrared wavelengths.
(Amazingly though, some distant background galaxies look blue. Such galaxies are chock full of intensely hot massive stars, which emit copious amounts of far-ultraviolet light. When the light of these galaxies reaches the Earth, the far ultraviolet light has been redshifted to optical violet, blue and green. However, if these starbursting galaxies are sufficiently far away, their light too will be stretched first to red and then to invisible infrared light.)
Redshifted background galaxies behind the Tadpole Galaxy. The background
galaxies look red because their light has been "stretched" by the expansion of
the Universe. Photo: NASA, H. Ford (JHU), G. Illingworth (USCS/LO),
M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA.
So red galaxies don't exist, but they can look red anyway!
Ann
[quote="orin stepanek" post_id=306893 time=1601930726 user_id=100812]
Thanks Ann; [b][color=#FF0000]I was thinking Star because I think red galaxies are {IMO) rare[/color][/b]; and looked like a pretty dusty area! I was aware of the white Galaxy but didn't Question it! 8-) I really like today's APOD; Pretty neat! 8-)
[/quote]
Absolutely right, Orin, you can say that red galaxies don't exist! But they can look right for two reasons.
1) Galaxies can look red because there is a lot of dust in front of them, so we see them through a curtain of dust. The dust in front of it filters away the short wavelengths of light (i.e., violet, blue, green) that were emitted by that galaxy, making it look yellow, orange or even red. (Or it may in fact become invisible in optical light, because if there is enough dust in front of the galaxy, not even the galaxy's reddest light can penetrate the dust.)
2) Galaxies can look red because they are very far away, and the light they emitted has literally been "stretched" by the expansion of the Universe. If the galaxy itself looked "white", all the light that the galaxy emitted has been stretched to longer, redder colors. Because as you know, longer wavelengths are redder and shorter wavelengths are bluer. Again, if the galaxy is sufficiently far away, it becomes invisible in optical light, because even its bluest light becomes stretched to infrared wavelengths.
(Amazingly though, some distant background galaxies look blue. Such galaxies are chock full of intensely hot massive stars, which emit copious amounts of far-ultraviolet light. When the light of these galaxies reaches the Earth, the far ultraviolet light has been redshifted to optical violet, blue and green. However, if these starbursting galaxies are sufficiently far away, their light too will be stretched first to red and then to invisible infrared light.)
[float=left][img3="Dust-reddened dwarf galaxy IC 10. It is reddened by dust in the Milky Way, because we see it behind a lot of stars and dust in Cassiopeia. Photo: Mark Hanson."]https://images.squarespace-cdn.com/content/v1/57e97e6ab8a79be1e7ae0ae6/1495321093897-SSJ762WL8A7SH8EIP664/ke17ZwdGBToddI8pDm48kNiEM88mrzHRsd1mQ3bxVct7gQa3H78H3Y0txjaiv_0fDoOvxcdMmMKkDsyUqMSsMWxHk725yiiHCCLfrh8O1z4YTzHvnKhyp6Da-NYroOW3ZGjoBKy3azqku80C789l0s0XaMNjCqAzRibjnE_wBlkZ2axuMlPfqFLWy-3Tjp4nKScCHg1XF4aLsQJlo6oYbA/NGC1512New.jpg[/img3][/float] [float=right][attachment=0]Redshifted background galaxies behind Tadpole.png[/attachment][c][size=85]Redshifted background galaxies behind the Tadpole Galaxy. The background
galaxies look red because their light has been "stretched" by the expansion of
the Universe. Photo: NASA, H. Ford (JHU), G. Illingworth (USCS/LO),
M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA.[/size][/c][/float]
So red galaxies don't exist, but they can look red anyway!
Ann