Comment by dotancohen
17 hours ago
Is this a non-sequiter or just poorly phrased?
> Disk galaxies like the Milky Way form stars “inside-out” — starting from the center and working outwards through the disk. So, as a general rule, the farther out astronomers look, the younger the stars are.
Do they meant looking out from Earth (which is actually nearer to the center of a spiral arm than to either end) or out from the galactic bulge. Either way doesn't make sense.
To use an analogy, to add to everybody else: it's like rings on a tree stump. The innermost part of the stump is the oldest; the outer the youngest. Earth is on one of those in-between rings, neither the oldest nor the newest - doesn't matter which of the in-betweens, to be honest.
Suppose now that you're an ant on the middle ring of that tree stump. No matter which way you're looking from Earth's middle-ring, either the rings will get gradually older and then younger with increasing distance (if you're looking towards the center-ish), or the rings will get strictly younger (if you're looking away from the center-ish).
This analogy obviously breaks down if you delve into details but that should give a better intuition to what's going on.
When does "starting in the center" mean anything besides "starting in the center"?
The earth is not the center of the galaxy
The Earth is where the astronomers are. If they are looking outward (away from the center) then the further they look, the younger the stars.
He didn't say that. He said earth is nearer to the center of a spiral arm.
What does the center of a spiral arm have to do with the center of the galaxy?
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the location of earth has precisely zero to do with the topic
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I actually am not following what the ambiguity is - stars farther out from the center are younger, no?
The Earth isn't the center of the galaxy, so this feels confusing/confused:
> So, as a general rule, the farther out astronomers look, the younger the stars are.
The father out from the center of the galaxy they look, the younger the stars are.
The location of the Earth is completely irrelevant. "closer" and "farther" refer to the center of the galaxy.
Poorly phrased. The most recent stars are on the edges. The inner stars were first, hence the “working outwards”.
But why? Star forms when enough hydrogen (or maybe also helium) clusters together with gravity to spark fusion IIRC. The center of Milky way ain't some ultra dense place where stars are just trillions of kms from each other to support somehow earlier star formation.
Or did ie dark matter/energy somehow coalesce on the outer edge later? Milky way is supposed to be very old place, almost as old as universe itself so one would expect more homogeneous distribution, at least as a layperson.
You need certain density to start formation progress. Then you get more density as it drags more stuff via gravity from intergalactic void. So new stars form at edges when there is finally enough stuff pulled by gravity of whole galaxy to there for the formation to happen.
It seems that you need quite large concentrations(as in scale of whole universe average) to actually get to star formation. Otherwise stars would be uniform trough the universe.
Then again I am not astronomer.
Looking from Earth at the stars closer to the center of a galaxy, they are found to be older. Looking from Earth at the stars closer to the edge of a galaxy, they are found to be younger.
Earth isn't relevant. The stars at the center of the galaxy developed first, and development proceeded from the inside out, so the youngest stars are on the edge ... then they get older from there on out, as the stars beyond the edge broke away from the galaxy. The bottom of the age U is the location of the formative edge.
From Earth (where astronomers are) looking "out"ward (away from the bulge).
Try: "the farther out [from the center] astronomers look"
It is beyond obvious what they mean.
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