APOD: M31: The Andromeda Galaxy (2023 Mar 22)

[APOD Robot]

M31: The Andromeda Galaxy

Explanation: How far can you see? The most distant object easily visible to the unaided eye is M31, the great Andromeda Galaxy, over two million light-years away. Without a telescope, even this immense spiral galaxy appears as an unremarkable, faint, nebulous cloud in the constellation Andromeda. But a bright white nucleus, dark winding dust lanes, luminous blue spiral arms, and bright red emission nebulas are recorded in this stunning fifteen-hour telescopic digital mosaic of our closest major galactic neighbor. But how do we know this spiral nebula is really so far away? This question was central to the famous Shapley-Curtis debate of 1920. M31's great distance was determined in the 1920s by observations that resolved individual stars that changed their brightness in a way that gave up their true distance. The result proved that Andromeda is just like our Milky Way Galaxy -- a conclusion making the rest of the universe much more vast than had ever been previously imagined.

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[RocketRon]

What a magnificent photo of a magnificent object. !

[johnnydeep]

I'm always amazed that even a "lowly" Askar 600 refractor, with an aperture of 108 mm (4.25 in) can take stunning images like this! Granted, this was a 15 hour exposure, but still!

[De58te]

That makes sense. We know a star's distance away when they change their brightness. Just like when you are out driving at night on a highway and an oncoming car with low beams suddenly flashes their high beams at you. You can then assume that car now is much closer to you since high beams are a sudden change of brightness.

[Chris Peterson]

That makes sense. We know a star's distance away when they change their brightness. Just like when you are out driving at night on a highway and an oncoming car with low beams suddenly flashes their high beams at you. You can then assume that car now is much closer to you since high beams are a sudden change of brightness.

No, it's not like that. This is about a special class of stars whose cyclic rate of variation can be used to computer their absolute magnitude. And if you know the absolute magnitude and the apparent magnitude, you can compute the distance (because of the inverse square law). Most variable stars are of no use in assessing distance at all, because their variability is unrelated to their actual brightness.

[MelvzLuster]

Great & wonderful, after Milky Way explorations, we are going to explore Andromeda Galaxy & find something extraordinary in this mysterious galaxy.

[Ann]

That makes sense. We know a star's distance away when they change their brightness. Just like when you are out driving at night on a highway and an oncoming car with low beams suddenly flashes their high beams at you. You can then assume that car now is much closer to you since high beams are a sudden change of brightness.

Edwin Hubble discovered a Cepheid variable star in Andromeda and realized that Andromeda is a separate galaxy, not a part of the Milky Way. Cepheid variables are "standard candles" whose variability is intimately linked to their true brightness. The Cepheid star in Andromeda told Hubble that Andromeda is much farther away than the outer reaches of the disk of the Milky Way.

Edwin Hubble's photographic plate of Andromeda from October 6, 1923, when Hubble discovered a Cepheid variable star in Andromeda. 'Var!' means 'Hooray, I've found a variable star in Andromeda!'

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Click to play embedded YouTube video.

Understanding Cepheid stars.

Cepheid variable star in galaxy M100. Credit: Dr. Wendy L. Freedman, Observatories of the Carnegie Institution of Washington, and NASA/ESA

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Cepheid variable star in galaxy M100. You take many pictures of a galaxy, find a star that varies and find out the period it has, calculate how bright it really is (from its period of variability), measure how bright it appears to be, and calculate how far away it really is.

Ann

[AVAO]

Edwin Hubble's photographic plate of Andromeda from October 6, 1923, when Hubble discovered a Cepheid variable star in Andromeda. 'Var!' means 'Hooray, I've found a variable star in Andromeda!'"]

Ann

...Interesting that Hubble was able to figure this out from this inconspicuous little star...

Click to view full size image 1 or image 2

jac berne (flickr)

[Avalon]

I cannot see any bars in the structure of the Andromeda Galaxy. Is it a barred galaxy? Do we assume that there is a super massive black hole in the galactic center as is in the center of our Milky Way?

[Ann]

I cannot see any bars in the structure of the Andromeda Galaxy. Is it a barred galaxy? Do we assume that there is a super massive black hole in the galactic center as is in the center of our Milky Way?

I could have sworn that Andromeda is an unbarred galaxy, because it has always been described that way when I have come across a written description of it. My own software, Guide, describes it as an unbarred galaxy of Hubble class Sb, not as a barred galaxy of Hubble class SBb.

But lo and behold, now some Chinese astronomers have found evidence that Andromeda is indeed barred!

The bar structure in Andromeda. Image: Hubble Space Telescope/Subaru Telescope/Mayall Telescope/Zi-Xuan Feng

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AAS Nova wrote:

Even though the Andromeda Galaxy is among our nearest galactic neighbors, there’s still much about it that we don’t know. Since the 1950s, astronomers have debated whether Andromeda, similar to the Milky Way, hosts a central bar of stars. Discerning Andromeda’s structure is key to understanding how it formed and evolved, but its tilted orientation makes it difficult to do so from our vantage point. Now, a team led by Zi-Xuan Feng (Shanghai Astronomical Observatory and University of the Chinese Academy of Sciences) has presented new evidence that shows Andromeda is indeed a barred galaxy. The above image shows the new results superimposed atop observations from the Hubble Space Telescope and the Subaru and Mayall ground-based telescopes. The red and blue symbols indicate the locations of velocity jumps — shocks — identified in emission from oxygen and hydrogen gas. Using simulations, Feng and collaborators show that shocks of this type cannot form without a rotating bar of stars.

And Andromeda does indeed have a supermassive central black hole!

Wikipedia wrote:

The nucleus consists of two concentrations separated by 1.5 pc (4.9 ly). The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 3–5 × 10⁷ M_☉ in 1993, and at 1.1–2.3 × 10⁸ M_☉ in 2005.

So the mass of the black hole of Andromeda was measured to be 30-50 million solar masses in 1993, and it was measured to be 110-230 million solar masses in 2005. In either case, the black hole of Andromeda is a lot more massive than the black hole of the Milky Way, which is some 4 million solar masses.

Ann

[VictorBorun]

Wikipedia wrote:
The nucleus consists of two concentrations separated by 1.5 pc (4.9 ly). The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 3–5 × 10⁷ M_☉ in 1993, and at 1.1–2.3 × 10⁸ M_☉ in 2005.

So the mass of the black hole of Andromeda was measured to be 30-50 million solar masses in 1993, and it was measured to be 110-230 million solar masses in 2005. In either case, the black hole of Andromeda is a lot more massive than the black hole of the Milky Way, which is some 4 million solar masses.

Ann

Now that the first runaway super massive central black hole is found

https://arxiv.org/abs/2302.04888

it's pretty sure that a two galaxy merger leads to their central BHs merger emitting 10% of the smaller BH's mass as gravitational wave energy and, if their spins happen to be codirectional, then "pinching the kernel of a cherry" and firing the gravitational wave packet mostly in one direction; the recoil is huge. Suppose the smaller BH's mass was 10% of the greater's; then kick velocity is 1/200 c = 1500 km/s, no chance for the merged BH to stay in the merged galaxy.

Look with new eyes at the range of central BH's mass of a galaxy. It may well be telling the story of loss and then growing a substitute central BH from scratch