APOD: The Once and Future Stars of Andromeda (2023 Oct 07)

[APOD Robot]

The Once and Future Stars of Andromeda

Explanation: This picture of Andromeda shows not only where stars are now, but where stars will be. The big, beautiful Andromeda Galaxy, M31, is a spiral galaxy a mere 2.5 million light-years away. Image data from space-based and ground-based observatories have been combined here to produce this intriguing composite view of Andromeda at wavelengths both inside and outside normally visible light. The visible light shows where M31's stars are now, highlighted in white and blue hues and imaged by the Hubble, Subaru, and Mayall telescopes. The infrared light shows where M31's future stars will soon form, highlighted in orange hues and imaged by NASA's Spitzer Space Telescope. The infrared light tracks enormous lanes of dust, warmed by stars, sweeping along Andromeda's spiral arms. This dust is a tracer of the galaxy's vast interstellar gas, raw material for future star formation. Of course, the new stars will likely form over the next hundred million years or so. That's well before Andromeda merges with our Milky Way Galaxy in about 5 billion years.

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[RocketRon]

Busy place !!

Any visualizations of what our sky might look like as Andromeda draws ever closer ?
Depending on the closeness / angle of approach, the Milky Way may then take a back seat in our night sky ??

Are we likely to actually merge, or is it just a pass-through fleeting visit ?
Answers on the back of a postage stamp - necessarily somewhat speculative ...

And, will our Sun peg out before this happens.
No Restaurant at the End of the Universe bookings necessary ...

[Ann]

The Once and Future Stars of Andromeda. Image Credit: NASA, NSF, NOAJ, Hubble, Subaru, Mayall, DSS, Spitzer; Processing & Copyright: Robert Gendler & Russell Croman

---

Dust lanes in Andromeda.

I don't agree that the presence of dust or dust lanes necessarily means that we can expect star formation there in the future. We see many, many galaxies with dust lanes where there are few signs of star formation.

Consider AM 1316-241, a pair of overlapping galaxies:

AM 1316-241. Credit: NASA, ESA, Hubble Heritage

You can see that a long dusty outer arm of the galaxy in the foreground is completely "barren", without star formation. And seriously, what about the Sombrero galaxy, M104?

The Sombrero Galaxy, M104. Credit: David Webster

---

Yes, dust lanes full of gas make stars. But no, dust lanes don't always make stars.

Far from it!

Ann

[RocketRon]

We are here.
Made out of dust.

How would we know what is 'enough' dust to make stars ?
The primary ingredient of which is Hydrogen
A clear colourless gas ...

[Ann]

We are here.
Made out of dust.

How would we know what is 'enough' dust to make stars ?
The primary ingredient of which is Hydrogen
A clear colourless gas ...

This is a quick and non-googled answer.

You need molecular, not atomic gas in the dust lane to make stars, which is to say that you need (I think) H₂. Also, the gas has to be "slow-moving" and very cold, so it can "settle down" and sink to the bottom of its own "gravitational well". Once it has reached this bottom of its gravitational well, it must keep accreting more and more gas, without acting up too much in the process to "scare the additional gas away".

So if the gas is too fast-moving, too hot, too turbulent, or too thin, it will not collapse into stellar cores, and it won't become full-blown stars.

Turbulent gas and dust in Cen A, NGC 2158. The gas here is very likely going to form some new stars, but probably not very many of them. Credit: ESO

---

Ann

[RocketRon]

Given that no-one has witnessed what it takes to make a bunch of free hydrogen gas ignite into a full-blown star,
its probably as good a start of an explanation as any ... !

[Chris Peterson]

Given that no-one has witnessed what it takes to make a bunch of free hydrogen gas ignite into a full-blown star,
its probably as good a start of an explanation as any ... !

There are a great many observations of stars in the earliest stages of formation, from pre- to post-ignition, with rich details of the environments they are forming in.

[Christian G.]

I find it impressive enough that Andromeda, although not hugely bigger than the Milky Way, contains a whopping trillion stars, and now this image suggests that there is PLENTY more where that came from! It's a trillion - and counting! (provided star births exceed star deaths)

[Fred the Cat]

Averaging 100 stars per year, Andromeda seems pretty good at star formation.

Prototypical?

[Ann]

Averaging 100 stars per year, Andromeda seems pretty good at star formation.

Prototypical?

Annette Ferguson of Astronomy Magazine wrote:

So, on average, we expect that roughly six to seven new stars form in the Milky Way every year.

While this might seem small, the star-formation rate of the Andromeda Galaxy (M31) has been estimated to be even smaller at roughly 0.4 solar mass per year (or just one to two stars on average).

Looking at other spiral galaxies in the nearby universe, we see a range of star-formation rates, but it seems like the rates measured for the Milky Way and Andromeda are fairly typical overall.

P.S. I read your links, but I couldn't find the claim that Andromeda is forming 100 new stars per year. Can you provide us with a quote?

Ann

[Fred the Cat]

Averaging 100 stars per year, Andromeda seems pretty good at star formation.

Prototypical?

Annette Ferguson of Astronomy Magazine wrote:

So, on average, we expect that roughly six to seven new stars form in the Milky Way every year.

While this might seem small, the star-formation rate of the Andromeda Galaxy (M31) has been estimated to be even smaller at roughly 0.4 solar mass per year (or just one to two stars on average).

Looking at other spiral galaxies in the nearby universe, we see a range of star-formation rates, but it seems like the rates measured for the Milky Way and Andromeda are fairly typical overall.

P.S. I read your links, but I couldn't find the claim that Andromeda is forming 100 new stars per year. Can you provide us with a quote?

Ann

Just averaging star estimate over age. I'd be curious about a more recent count too.

[orin stepanek]

Galaxies uniting!
No way that you could count the # of stars born or die! Heck; I
can't even count the stars in the MW let alone in Andromeda! We
have to live with estimates!

[Chris Peterson]

NGC3314_HubbleOstling_960.jpg
Galaxies uniting!
No way that you could count the # of stars born or die! Heck; I
can't even count the stars in the MW let alone in Andromeda! We
have to live with estimates! :mrgreen: 😁

But the estimates are derived from sampling methods, and are statistically robust.

[johnnydeep]

Averaging 100 stars per year, Andromeda seems pretty good at star formation.

Prototypical?

Annette Ferguson of Astronomy Magazine wrote:

So, on average, we expect that roughly six to seven new stars form in the Milky Way every year.

While this might seem small, the star-formation rate of the Andromeda Galaxy (M31) has been estimated to be even smaller at roughly 0.4 solar mass per year (or just one to two stars on average).

Looking at other spiral galaxies in the nearby universe, we see a range of star-formation rates, but it seems like the rates measured for the Milky Way and Andromeda are fairly typical overall.

P.S. I read your links, but I couldn't find the claim that Andromeda is forming 100 new stars per year. Can you provide us with a quote?

Ann

Just averaging star estimate over age. I'd be curious about a more recent count too.

So, for Andromeda, assuming 10 billion years old and 1 trillion stars now, that averages out to 100 new stars per year?

[ PS - I always thought that "thumbs up" emoji you often use was a guy blowing one of those extending "blowout whistles" / "party squawkers"! It would be better as a ]

[johnnydeep]

We are here.
Made out of dust.

How would we know what is 'enough' dust to make stars ?
The primary ingredient of which is Hydrogen
A clear colourless gas ...

This is a quick and non-googled answer.

You need molecular, not atomic gas in the dust lane to make stars, which is to say that you need (I think) H₂. Also, the gas has to be "slow-moving" and very cold, so it can "settle down" and sink to the bottom of its own "gravitational well". Once it has reached this bottom of its gravitational well, it must keep accreting more and more gas, without acting up too much in the process to "scare the additional gas away".

So if the gas is too fast-moving, too hot, too turbulent, or too thin, it will not collapse into stellar cores, and it won't become full-blown stars.
...

Ann

In addition, can we assume that the presence of dust usually helps to "seed" the star formation - at least initially - since it adds to the available mass? (and providing it's not too hot/fast/turbulent?

[Chris Peterson]

We are here.
Made out of dust.

How would we know what is 'enough' dust to make stars ?
The primary ingredient of which is Hydrogen
A clear colourless gas ...

This is a quick and non-googled answer.

You need molecular, not atomic gas in the dust lane to make stars, which is to say that you need (I think) H₂. Also, the gas has to be "slow-moving" and very cold, so it can "settle down" and sink to the bottom of its own "gravitational well". Once it has reached this bottom of its gravitational well, it must keep accreting more and more gas, without acting up too much in the process to "scare the additional gas away".

So if the gas is too fast-moving, too hot, too turbulent, or too thin, it will not collapse into stellar cores, and it won't become full-blown stars.
...

Ann

In addition, can we assume that the presence of dust usually helps to "seed" the star formation - at least initially - since it adds to the available mass? (and providing it's not too hot/fast/turbulent?

Yes... which is part of why star formation in the earliest universe (when there was not yet any dust) remains poorly understood.

[johnnydeep]

This is a quick and non-googled answer.

You need molecular, not atomic gas in the dust lane to make stars, which is to say that you need (I think) H₂. Also, the gas has to be "slow-moving" and very cold, so it can "settle down" and sink to the bottom of its own "gravitational well". Once it has reached this bottom of its gravitational well, it must keep accreting more and more gas, without acting up too much in the process to "scare the additional gas away".

So if the gas is too fast-moving, too hot, too turbulent, or too thin, it will not collapse into stellar cores, and it won't become full-blown stars.
...

Ann

In addition, can we assume that the presence of dust usually helps to "seed" the star formation - at least initially - since it adds to the available mass? (and providing it's not too hot/fast/turbulent?

Yes... which is part of why star formation in the earliest universe (when there was not yet any dust) remains poorly understood.

Aha!

[RocketRon]

There are a great many observations of stars in the earliest stages of formation, from pre- to post-ignition, with rich details of the environments they are forming in.

Glib answer to an almost totally unknown complete 'mystery' ?

How would we even see stars in a "pre-ignition" phase ?????
Some gravity effects may gave some indication - albeit at quite some distance

Even our own Solar System formation is quite an enigma.
Hence the intense study of samples from tektites, meteorites, moon, mars, bennu & psche16 etc.

[Chris Peterson]

There are a great many observations of stars in the earliest stages of formation, from pre- to post-ignition, with rich details of the environments they are forming in.

Glib answer to an almost totally unknown complete 'mystery' ?

How would we even see stars in a "pre-ignition" phase ?????
Some gravity effects may gave some indication - albeit at quite some distance

Even our own Solar System formation is quite an enigma.
Hence the intense study of samples from tektites, meteorites, moon, mars, bennu & psche16 etc.

We see stars in all stages of formation. There's a lot of well developed, well supported theory. It is far from an "enigma" even if we still have a lot to learn. Not remotely an "almost totally unknown complete mystery"!

[Ann]

Averaging 100 stars per year, Andromeda seems pretty good at star formation.

Prototypical?

Annette Ferguson of Astronomy Magazine wrote:

So, on average, we expect that roughly six to seven new stars form in the Milky Way every year.

While this might seem small, the star-formation rate of the Andromeda Galaxy (M31) has been estimated to be even smaller at roughly 0.4 solar mass per year (or just one to two stars on average).

Looking at other spiral galaxies in the nearby universe, we see a range of star-formation rates, but it seems like the rates measured for the Milky Way and Andromeda are fairly typical overall.

P.S. I read your links, but I couldn't find the claim that Andromeda is forming 100 new stars per year. Can you provide us with a quote?

Ann

Just averaging star estimate over age. I'd be curious about a more recent count too.

I'd say that Andromeda has not formed all those trillion stars by itself. Instead, it has probably "stolen" quite a few of them, by capturing a lot of hapless nearby dwarf galaxies and laying claim to their stars.

I didn't even try to find images or even much info on all the merging events of Andromeda, but there are some interesting tidbits about the merging history of the Milky Way (and we have no reason to believe that Andromeda has been any less diligent than the MW when it comes to eating galactic snacks):

Remnants of smaller galaxies that have merged with the Milky Way. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

---

ESA wrote:

Our galaxy, the Milky Way, began forming around 12 billion years ago. Since then, it has been growing in both mass and size through a sequence of mergers with other galaxies.
...

When a foreign galaxy falls into our own, great gravitational forces known as tidal forces pull it apart. If this process goes slowly, the stars from the merging galaxy will form a vast stellar stream that can be easily distinguished in the halo. If the process goes quickly, the merging galaxy’s stars will be more scattered throughout the halo and no clear signature will be visible.

But the merging galaxy may contain more than just stars. It could also be surrounded by a population of globular star cluster and small satellite galaxies.

So some of the globulars that now belong to the Milky Way may have originally come from smaller galaxies that were captured by the Milky Way. And the way I understand it, the number of globular clusters of a galaxy says something about its "merging history". As for Andromeda, it has more than twice as many globulars as the Milky Way (~150):

Wikipedia wrote:

There are approximately 460 globular clusters associated with the Andromeda Galaxy. The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the Local Group of galaxies.

Ann

[orin stepanek]

NGC3314_HubbleOstling_960.jpg
Galaxies uniting!
No way that you could count the # of stars born or die! Heck; I
can't even count the stars in the MW let alone in Andromeda! We
have to live with estimates!

But the estimates are derived from sampling methods, and are statistically robust.

Touche!