APOD: M31 versus M33 (2015 Sep 26)

[APOD Robot]

M31 versus M33

Explanation: Separated by about 14 degrees (28 Full Moons) in planet Earth's sky, spiral galaxies M31 at left, and M33 are both large members of the Local Group, along with our own Milky Way galaxy. This narrow- and wide-angle, multi-camera composite finds details of spiral structure in both, while the massive neighboring galaxies seem to be balanced in starry fields either side of bright Mirach, beta star in the constellation Andromeda. Mirach is just 200 light-years from the Sun. But M31, the Andromeda Galaxy, is really 2.5 million light-years distant and M33, the Triangulum Galaxy, is also about 3 million light years away. Although they look far apart, M31 and M33 are engaged in a gravitational struggle. In fact, radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.

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[Ann]

What a fine picture!

M31 and M33 are certainly "equidistant" from bright star Mirach as seen from the Earth.

This is a picture full of the letter "M". There is M31, M33 and Mirach, which is an M-type star.

As a color commentator I find the colors very interesting. The bright yellow color of Mirach is very striking. We are often told that M-type stars are very red, but when I looked at M-type stars through a telescope, I often thought they looked this sort of yellow.

But while Mirach doesn't look red (very far from it), its yellow brilliance makes the color of the galaxies pale to a faded white in comparison. The picture clearly shows us that while Andromeda and M33 are different sort of galaxies - just look at the large bulge, long straight dust lane and disky flatness of Andromeda compared with the somewhat messy spiral nature of small M33 - the color difference between these two galaxies is visible but by no means striking.

Mirach, an M-type star, is neither fusing hydrogen nor helium in its core.

Jim Kaler wrote:
It is difficult to say just what state the star is in. It is clearly massive, having three or four times the mass of the Sun, but it may have a core made of helium or one made of carbon.

If MIrach has a helium core, it is fusing hydrogen in a shell around its core, and if it has a carbon core, it is fusing helium in a shell around its core.

But as an M-type star, Mirach is redder - make that yellower - than a galaxy will ever be. Not even "red and dead" galaxies have integrated colors as red as an M-type star. Even the light of galaxies without any star formation at all is dominated by stars less red (or yellow) than M-type stars.

For comparison, the B-V index of Mirach is 1.576 ± 0.010. The overall integrated B-V index of Andromeda is 0.920, and the B-V of M33 is 0.550. Even though M31 is dominated by cool stars (and its large bulge is completely made up of cool stars) while M33 is dominated by hot stars, the "odd man out" when it comes to color of the threesome in today's APOD is the cool star, not one of the two galaxies.

Ann

[Boomer12k]

Picture reminded me of the old Space Wars arcade game...so, Two Galaxies, locked in mortal combat with a lone star in the middle.
On the Commodore 64 it was called Stellar Triumph. Note bright yellow star...

If these mergers take place 10+ billion years in the future, the Sun will probably be a fading Planetary Nebula...with us long gone, and Mirach long Exploded,....they don't have a crying emoticon.
(>)(<) Eyes with tears...

Prepare to repel boarders, Arrrrggghhh...

:---[===] *

The page would not keep my Colons as tears, as it would not preserve the SPACES needed...

[neufer]

Picture reminded me of the old Space Wars arcade game...so, Two Galaxies, locked in mortal combat with a lone star in the middle.
On the Commodore 64 it was called Stellar Triumph. Note bright yellow star...

If these mergers take place 10+ billion years in the future, the Sun will probably be a fading Planetary Nebula...with us long gone, and Mirach long Exploded,....they don't have a crying emoticon.
(>)(<) Eyes with tears...

Prepare to repel boarders, Arrrrggghhh...

• The Sun will probably be a white dwarf (though we could all petition that they then drop the "dwarf" status):

---

<<After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blown away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB. As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling once the hydrogen shell is exhausted through fusion and mass loss. In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the star.>>

[Cousin Ricky]

As a color commentator I find the colors very interesting. The bright yellow color of Mirach is very striking. We are often told that M-type stars are very red, but when I looked at M-type stars through a telescope, I often thought they looked this sort of yellow.

To me, M stars have appeared from butterscotch to copper-colored. I'll try to remember to get a glimpse of Mirach tomorrow night.

According to a tool I have, a black body at 3842 K should be pale orange (#FFD09D █████ in sRGB). I'm not sure what effect our atmosphere has on Mirach's appearance, though.

I suspect the yellow color may be an artifact of image processing. (I also find it odd that Mirach is the only star with diffraction spikes. Mu and nu Andromedae are only two magnitudes dimmer, and should show something.) But Mr. Park hasn't included processing information with the image.

[Cousin Ricky]

If these mergers take place 10+ billion years in the future, the Sun will probably be a fading Planetary Nebula...with us long gone, and Mirach long Exploded,....they don't have a crying emoticon.

As Neufer mentioned, the Sun will have long passed its pretty death shroud phase. Also, at 3 to 4 solar masses, Mirach does not have sufficient mass to explode.

[Ann]

As a color commentator I find the colors very interesting. The bright yellow color of Mirach is very striking. We are often told that M-type stars are very red, but when I looked at M-type stars through a telescope, I often thought they looked this sort of yellow.

To me, M stars have appeared from butterscotch to copper-colored. I'll try to remember to get a glimpse of Mirach tomorrow night.

According to a tool I have, a black body at 3842 K should be pale orange (#FFD09D █████ in sRGB). I'm not sure what effect our atmosphere has on Mirach's appearance, though.

I suspect the yellow color may be an artifact of image processing. (I also find it odd that Mirach is the only star with diffraction spikes. Mu and nu Andromedae are only two magnitudes dimmer, and should show something.) But Mr. Park hasn't included processing information with the image.

I've seen Betelgeuse look the kind of yellow that is sported by Mirach in today's APOD. I have seen one unbelievably red carbon star - V Aquilae - but the only star looking the #FFD09D kind of orange I've ever seen is Mu Cephei.

Ann

[Cousin Ricky]

Mu Cephei looks like a glowing ember to me&mdash;bright orange. Of course, my black body tool would be useless on that star.

I don't have a tool to model the effect of our atmosphere on the color of extraterrestrial objects. I guess I could create one by comparing the extraterrestrial solar spectrum to an Earth-based solar spectrum, but the latter would surely vary according to local atmospheric conditions.

[saturno2]

M31 Andromeda Galaxy
M33 Triangulum Galaxy
Milky Way
are in the Local Group of Galaxies.
A day, they will be a only galaxy .
They will fusion

[Yep]

M31 Andromeda Galaxy
M33 Triangulum Galaxy
Milky Way
are in the Local Group of Galaxies.
A day, they will be a only galaxy .
They will fusion

Sweet

[Visual_Astronomer]

An absolutely stunning picture!

[starsurfer]

M31 Andromeda Galaxy
M33 Triangulum Galaxy
Milky Way
are in the Local Group of Galaxies.
A day, they will be a only galaxy .
They will fusion

That's a beautiful poem!

[Ron-Astro Pharmacist]

It seems like it might be in the realm of possibilities to reflect back just the light of Mirah wave flipped by 180 degrees to cancel out its light from entering the telescope thus getting to see just those objects in close proximity to the star.

http://phys.org/news/2015-03-particle.html

Maybe you can cancel the wave part but still have the particle part. What does one do with a left over "icle" - cancel them with anticles?

[Cousin Ricky]

For some reason, the full Moon went quite dim last night, so I took advantage of this turn of events to look at Mirach. It appeared a bright orange yellow in the center, a bit more on the orange side than in Mr. Park's photo, with copper-colored fringes. Unfortunately, my telescope was poorly collimated, so it was difficult to get a good focus, or even a good smear. I may try again some other time.

[Ann]

For some reason, the full Moon went quite dim last night, so I took advantage of this turn of events to look at Mirach. It appeared a bright orange yellow in the center, a bit more on the orange side than in Mr. Park's photo, with copper-colored fringes. Unfortunately, my telescope was poorly collimated, so it was difficult to get a good focus, or even a good smear. I may try again some other time.

Better luck next time!

In any case, I think it is quite true that different people see star colors differently. Not only that, but I think that perceived star colors of the same stars vary for the individual stargazer on different nights, for different reasons. Most of it has to do with external factors such as seeing conditions, subtly different colors of the background sky and the like, but some of it may just have to do with the stargazers own different "disposition" on different nights.

To return to the APOD in question, please note that you can actually see how the disk of Andromeda stretches considerably wider than the obviously bright part of it. No similar effect, or not much of it, can be seen for M33.

Ann

[Cousin Ricky]

Mu Cephei looks like a glowing ember to me&mdash;bright orange. Of course, my black body tool would be useless on that star.

Not necessarily so; I had misremembered mu Cep's spectral class when I wrote this. As it turns out, a published surface temperature of mu Cep yields a black body color similar to what I posted for Mirach, conforming to Ann's observation. However, using a formula that correlates B-V values to surface temperature yields a stronger orange color, similar to my impression of mu Cep.