Everything we know about star clusters might be wrong!

One of the default assumptions about star clusters is that all of the
stars inside each of them are of nearly the same age as each other. So a
star cluster that is 10 billion years old, will have stars that are all
10 billion years old. This is such a well established astronomical rule,
that once the age of one star is known, then they use it to determine
the ages of all other stars inside the clusters.

Well a new discovery is showing that new stars are being born inside
some of these clusters, in at least one cluster so far. It's possible
that it happens inside most if not all other clusters too. It's also
looking like no new gas from the outside is entering these clusters, all
of the star formation comes from gas expelled from dying and dead
members of the cluster, producing new stars.

Yousuf Khan

https://phys.org/news/2017-03-star-c...applecart.html

On Thursday, March 9, 2017 at 8:07:54 AM UTC-5, Yousuf Khan wrote:
One of the default assumptions about star clusters is that all of the
stars inside each of them are of nearly the same age as each other. So a
star cluster that is 10 billion years old, will have stars that are all
10 billion years old. This is such a well established astronomical rule, ...

It's more of an reasoned assumption than a rule. Given that there are a only a handful of younger stars in the cluster that was studied, it might be fairer to say that "a very tiny part of what we know about star clusters might be wrong."

On 3/13/2017 7:50 AM, Craig Markwardt wrote:
On Thursday, March 9, 2017 at 8:07:54 AM UTC-5, Yousuf Khan wrote:
One of the default assumptions about star clusters is that all of
the stars inside each of them are of nearly the same age as each
other. So a star cluster that is 10 billion years old, will have
stars that are all 10 billion years old. This is such a well
established astronomical rule, ...

It's more of an reasoned assumption than a rule. Given that there
are a only a handful of younger stars in the cluster that was
studied, it might be fairer to say that "a very tiny part of what we
know about star clusters might be wrong."

Well, they only studied the one cluster so far, and it took a lot of
detective work to figure out which ones were younger than the rest of
the group. They will now expand the finding out to more clusters, and
other groups will probably also join the search and find even more
within these clusters. It's a handful of stars right now, but it's only
now been discovered. More might come after more careful analysis, even
within this one cluster?

Yousuf Khan

In article ,
Yousuf Khan writes:
Well a new discovery is showing that new stars are being born inside
some of these clusters,

As far as I can tell, the actual paper is at
https://academic.oup.com/mnrasl/article/3060925/A

in at least one cluster so far.

Which cluster is that? It looks to me as though the authors found 15
out of 738 star clusters (in the Large Magellanic Cloud) to have
young stellar objects (YSOs) within a projected distance of 10 pc
along the line of sight. My quick estimate from their Fig 2 is that
20 clusters should have had YSOs that close just by chance, so the
number found is lower than the chance expectation. However, the
authors claim the probability of finding 15 or more associations is
5%. I can't tell how they derived that number.

There are other arguments that make it seem likely that at least some
of the associations are real, but details are missing.

It's possible that it happens inside most if not all other
clusters too.

What makes you think that? For one thing, the clusters with YSO
associations are all less than 0.4 Gyr old, so taking the authors'
work at face value, that would be the maximum age spread.

It's also looking like no new gas from the outside is entering
these clusters, all of the star formation comes from gas expelled
from dying and dead members of the cluster, producing new stars.

Yes, that's the suggestion. If correct, it implies younger stars
will form only a tiny fraction of a cluster's population.

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