by Ann » Tue Jun 14, 2022 5:26 am
beryllium732 wrote: ↑Mon Jun 13, 2022 11:24 pm
A beautiful image!
I have two questions. How come there's a lot of star forming regions of blue o-type stars and dust so near the black hole? Why doesn't it have the typical yellow bulge like many other galaxies have like our own or Andromedas? Does the galaxy really consist of so many blue star regions or is just a different colour made by Hubble?
The string of blue stars on the left side of the image is because it's been dragged by the gravitational force by the smaller galaxy consisting of a lot of yellow older stars?
We know that massive blue stars can form very near massive black holes. There are O-type stars quite close to the supermassive black hole of the Milky Way, Sgr A*, too.
Center of the Milky Way. Sgr A* is the supermassive black hole.
The Arches Cluster and the Quintuplet Cluster contain O-type stars.
Image: NASA, ESA and Q.D. Wang
In the Milky Way, the massive Arches Cluster and Quintuplet Cluster are located some 100 light-years from our galaxy's supermassive black hole, Sgr A*. But there are massive stars even closer to Sgr A*:
Click to play embedded YouTube video.
A star called S2 has been found orbiting the black hole of the Milky Way at a distance that brings it, at closest approach, some four times the distance between the Sun and Neptune. S2 is a hot star, not an O-type star but the next best thing, a star of spectral class B0-B3. Read about S2
here.
In a way you are asking why M51 is so blue, when many or most other galaxies - like, say, Andromeda - seem to be so yellow. The short answer is that M51 is small galaxy that hasn't undergone a lot of star formation in the past, and therefore it doesn't have a huge underlying population of long-lived faint red and yellow stars. It also hadn't used up much of its available gas before it encountered NGC 5194, the yellow elliptical galaxy that it is interacting with. The interaction "stirred" the gas of M51 and made it form a lot of bright blue stars, which has given the entire galaxy a bluish appearance.
I really want to bring home the fact that M51 is a smallish galaxy:
Wikipedia wrote:
The Whirlpool Galaxy lies 28 million light-years from Earth and has an estimated diameter of 76,000 light-years. Overall the galaxy is about 43% the size of the Milky Way. Its mass is estimated to be 160 billion solar masses, or around 10.3% of the mass of Milky Way Galaxy.
Some 10% of the mass of the Milky Way is not that much.
Let's compare M51 with Andromeda:
Compare the huge bright yellowish bulge of Andromeda with the small (or non-existent?) bulge of M51.
According to Wikipedia, astronomers still argue about whether or not Andromeda is more massive than the Milky Way. But the way I understand it, Andromeda really is believed to contain more stars in its disk than the Milky Way does, and our own galaxy may or may not contain more dark matter than Andromeda.
So Andromeda is a large bright massive star-rich galaxy whose stars are predominantly old and red or yellow:
Wikipedia wrote:
Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×109 years. The estimated luminosity of the Andromeda Galaxy, ~2.6×1010 L☉, is about 25% higher than that of our own galaxy...
The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way...
This suggests that the latter once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation...
According to recent studies, the Andromeda Galaxy lies in what in the Galaxy color–magnitude diagram is known as the "green valley"...
In simulated galaxies with similar properties to the Andromeda Galaxy, star formation is expected to extinguish within about five billion years, even accounting for the expected, short-term increase in the rate of star formation due to the collision between the Andromeda Galaxy and the Milky Way.
So Andromeda has experienced great star formation in the past, which left behind myriads of small long-lived red and yellow stars. But now its rate of star formation has run down, and it is expected to stop forming stars almost completely in 5 billion years' time.
Andromeda shines brightly in yellow because of its tremendous numbers of old red and yellow stars, whereas smallish M51 has not had much star formation in the past and therefore only has a small underlying old yellow population. Now M51 is bluish because of its present high rate of star formation.
Finally, you asked about a string of blue stars on the left side of the image. I think you mean this:
The part that I marked in blue (at lower left) is simply the arm of M51 that has been stretched due to tidal forces of the encounter with NGC 5194. The blue stars belong to M51. As you said, the blue stars have been dragged there by the gravitational forces of NGC 5194.
Ann
[quote=beryllium732 post_id=323364 time=1655162646 user_id=146004]
A beautiful image!
I have two questions. How come there's a lot of star forming regions of blue o-type stars and dust so near the black hole? Why doesn't it have the typical yellow bulge like many other galaxies have like our own or Andromedas? Does the galaxy really consist of so many blue star regions or is just a different colour made by Hubble?
The string of blue stars on the left side of the image is because it's been dragged by the gravitational force by the smaller galaxy consisting of a lot of yellow older stars?
[/quote]
We know that massive blue stars can form very near massive black holes. There are O-type stars quite close to the supermassive black hole of the Milky Way, Sgr A*, too.
[float=left][attachment=1]Annotated image of the center of the Milky Way NASA ESA Spitzer Q D Wang.png[/attachment][c][size=85][color=#0040FF]Center of the Milky Way. Sgr A* is the supermassive black hole.
The Arches Cluster and the Quintuplet Cluster contain O-type stars.
Image: NASA, ESA and Q.D. Wang[/color][/size][/c][/float]
[clear][/clear]
In the Milky Way, the massive Arches Cluster and Quintuplet Cluster are located some 100 light-years from our galaxy's supermassive black hole, Sgr A*. But there are massive stars even closer to Sgr A*:
[youtube]https://www.youtube.com/watch?v=B0QRpid5_QU[/youtube]
[clear][/clear]
A star called S2 has been found orbiting the black hole of the Milky Way at a distance that brings it, at closest approach, some four times the distance between the Sun and Neptune. S2 is a hot star, not an O-type star but the next best thing, a star of spectral class B0-B3. Read about S2 [url=https://en.wikipedia.org/wiki/S2_(star)]here[/url].
In a way you are asking why M51 is so blue, when many or most other galaxies - like, say, Andromeda - seem to be so yellow. The short answer is that M51 is small galaxy that hasn't undergone a lot of star formation in the past, and therefore it doesn't have a huge underlying population of long-lived faint red and yellow stars. It also hadn't used up much of its available gas before it encountered NGC 5194, the yellow elliptical galaxy that it is interacting with. The interaction "stirred" the gas of M51 and made it form a lot of bright blue stars, which has given the entire galaxy a bluish appearance.
I really want to bring home the fact that M51 is a smallish galaxy:
[quote][url=https://en.wikipedia.org/wiki/Whirlpool_Galaxy#Properties]Wikipedia[/url] wrote:
The Whirlpool Galaxy lies 28 million light-years from Earth and has an estimated diameter of 76,000 light-years. Overall the galaxy is about 43% the size of the Milky Way. Its mass is estimated to be 160 billion solar masses, or around 10.3% of the mass of Milky Way Galaxy.[/quote]
Some 10% of the mass of the Milky Way is not that much.
Let's compare M51 with Andromeda:
[float=left][img3="M51: The Whirlpool Galaxy from Hubble. Image Credit: NASA, ESA, Hubble, HLA; Processing & Copyright: Bernard Miller"]https://apod.nasa.gov/apod/image/2206/M51_HubbleMiller_1080.jpg[/img3][/float][float=right][img3="The Andromeda Galaxy (Messier 31). The small Messier 32 galaxy is seen above and slightly to the left (directly south) of the centre of M31, and Messier 110 is below and to the left. Image: David (Deddy) Dayag."]https://upload.wikimedia.org/wikipedia/commons/thumb/8/8c/Andromeda_Galaxy_560mm_FL.jpg/1280px-Andromeda_Galaxy_560mm_FL.jpg[/img3][/float]
[clear][/clear]
Compare the huge bright yellowish bulge of Andromeda with the small (or non-existent?) bulge of M51.
According to Wikipedia, astronomers still argue about whether or not Andromeda is more massive than the Milky Way. But the way I understand it, Andromeda really is believed to contain more stars in its disk than the Milky Way does, and our own galaxy may or may not contain more dark matter than Andromeda.
So Andromeda is a large bright massive star-rich galaxy whose stars are predominantly old and red or yellow:
[quote]Wikipedia wrote:
Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly [b][color=#FF8000]older stars[/color][/b] with ages >7×10[size=85][sup]9[/sup][/size] years. The [b][color=#FF0000]estimated luminosity[/color][/b] of the Andromeda Galaxy, ~2.6×10[size=85][sup]10[/sup][/size] L[size=85][sub]☉[/sub][/size],[b][color=#FF0000] is about 25% higher than that of our own galaxy[/color][/b]...
The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way...
This suggests that [b][color=#FF0000]the latter once experienced a great star formation phase, but is now in a relative state of quiescence[/color][/b], whereas the Milky Way is experiencing more active star formation...
According to recent studies, the Andromeda Galaxy lies in what in the Galaxy color–magnitude diagram is known as the "green valley"...
In simulated galaxies with similar properties to the Andromeda Galaxy, star formation is expected to extinguish within about five billion years, even accounting for the expected, short-term increase in the rate of star formation due to the collision between the Andromeda Galaxy and the Milky Way.[/quote]
So Andromeda has experienced great star formation in the past, which left behind myriads of small long-lived red and yellow stars. But now its rate of star formation has run down, and it is expected to stop forming stars almost completely in 5 billion years' time.
Andromeda shines brightly in yellow because of its tremendous numbers of old red and yellow stars, whereas smallish M51 has not had much star formation in the past and therefore only has a small underlying old yellow population. Now M51 is bluish because of its present high rate of star formation.
Finally, you asked about a string of blue stars on the left side of the image. I think you mean this:
[attachment=0]APOD 13 June 2022 slightly annotated.png[/attachment]
[clear][/clear]
The part that I marked in blue (at lower left) is simply the arm of M51 that has been stretched due to tidal forces of the encounter with NGC 5194. The blue stars belong to M51. As you said, the blue stars have been dragged there by the gravitational forces of NGC 5194.
Ann