APOD: Blue Straggler Stars in Globular M53... (2021 Feb 07)

[johnnydeep]

To add to (or perhaps misunderstand!) Ann's posts, I like the pic of M53 from https://en.wikipedia.org/wiki/Messier_53 better, since it seems to more clearly show individual "blue stragglers":


M53.JPG


But are they actually "blue stragglers"? And frankly, I seem to see the same number of blue stars as orange stars, both of which seem to be fewer in number than the majority of whiter stars. The filters used no doubt play a role...

No, the blue stars in that picture are blue horizontal stars, not blue stragglers.

We can tell that they are blue horizontal stars because

a) They are "medium-bright" (they stand out, but they are not as bright as the brightest red stars)

d) They are all more of less the same brightness

c) There are a lot of them

d) They are spread evenly all over the globular


As for the blue stragglers...

a) They are practically always fainter than the blue horizontal stars, which are a kind of "giants"

b) They are not all the same brightness, because they are not all the same mass

c) They are often less blue than the blue horizontal branch stars

d) They are not evenly spread throughout the cluster, because they are comparatively massive, so they tend to sink towards the cluster center

Blue straggler stars (center) in NGC 6397, and blue horizontal branch stars scattered over the globular. Photo: ESA/Hubble and NASA.

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Closeup of the blue straggler stars in NGC 6397.

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Ann

P.S. I'm sure you are right that there are a lot more white stars than either red or blue. The white stars are in all probability main sequence Sun-like stars, or else they are Sun-like in temperature but more lightweight than the Sun in mass. That is a possibility in metal-poor globulars.

P.P.S. I missed my nice number 11111 posts. A pity.

Ann, You are tooooooooooooooo kind to take efforts to explain again what you explained in deep detailed thoughts above already. Seems JohneyDepp didn't read carefully your detailed thoughts/analysis

And thanks for dissing me, shaileshs. I'll keep that in mind when reading your future contributions.

[Chris Peterson]

The article was too detailed for me. Does it actually say that GC rotation is what prevents them from collapsing sooner? And does it say how much sooner collapse would happen without rotation? And how fast do GCs rotate on average?

Yes, the orbital rotation of all those stars in globular clusters is precisely what prevents them from just plunging to the center and collapsing into one monstrous black hole.

Note that it is the individual stars in the globular that must rotate to prevent a huge collapse of the globular.

Frankly, everything in space rotates, orbits, or is on the move in other ways. What prevents the Earth from just plunging into the Sun and getting heated and fried into blisteringly hot silicon gas?

You know the answer. It is the Earth's orbit, its rotation around the Sun. We don't plunge into the Sun because we orbit around it.

We orbit, therefore we are.

Ann

I thought we were distinguishing the random orbital motions of individual stars in a GC (which is indisputable) from any overall common motion of all the stars in a GC about a shared axis (which the paper was about).

Yes. But in terms of that rotation slightly flattening the GCs. Not impacting their stability.

[VictorBorun]

I learn metallicity in steps.
First I got to know that low metallicity interstellar medium is a poor radiator and makes for larger stars formation.
Now I learn that low metallicity stars after fusioning all the hydrogen in their cores turn to hotter "red giant" phase, and are in fact blue.

And the reason for both features is the same: pure H/He medium is a poor radiator. It absorbs the photons it has emitted.
A low metallicity star with a mass of 1 Sun and the same energy flow through the same size sphere as of the Sun
has more shallow photoshere (the outer layer that sends photons to the outer space) and therefore hotter and bluer.

"A blue horizontal branch star" sounds prettier than "a blue red giant phase star". Thank you, Ann!

[Ann]

I learn metallicity in steps.
First I got to know that low metallicity interstellar medium is a poor radiator and makes for larger stars formation.
Now I learn that low metallicity stars after fusioning all the hydrogen in their cores turn to hotter "red giant" phase, and are in fact blue.

And the reason for both features is the same: pure H/He medium is a poor radiator. It absorbs the photons it has emitted.
A low metallicity star with a mass of 1 Sun and the same energy flow through the same size sphere as of the Sun
has more shallow photoshere (the outer layer that sends photons to the outer space) and therefore hotter and bluer.

"A blue horizontal branch star" sounds prettier than "a blue red giant phase star". Thank you, Ann!

Victor, you understand math, and I don't. But I think of low metallicity stars as more "transparent" than high metallicity stars.

Okay, Chris, you don't have to tell me - stars are opaque. I know. But maybe we see deeper inside the star if it is low in metals. Or else, as Victor suggested, maybe the photosphere is thinner in such stars.

Ann

P.S. Victor, I'm glad you like the expression "blue horizontal branch stars"!