Star Clusters and their stars
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Open clusters and globular clusters
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General characteristics of globular clusters
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Globular cluster stars in the H-R diagram
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Stellar interactions in globular clusters
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Exotic binaries in globular clusters
Star clusters
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Virtually all stars, Sun
included, are born in
star clusters.
At first, with lots of gas
and dust in the vicinity,
they look like they are
just a part of a
molecular cloud.
However, as the stars
start to shine, they blow
away the gas and dust
that helped create them
(through pressure
radiation).
Star clusters
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As the gas clears, we
observe an open
cluster. These
clusters always have
young stars, many of
which are blue.
The reason is that the
stars in open clusters
are not bound to the
cluster. Therefore,
they disperse while
they are still young.
What if the cluster is
massive (and dense)
enough to keep its
stars in its vicinity?
Globular clusters
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If the cluster is massive and dense
enough to hold its stars together,
then they will stay together – and
age together. We have therefore a
globular cluster.
In our Galaxy, all globular clusters
(i.e., the clusters where the stars
are bound) have very old stars.
You can tell by their color – all blue
stars have already left the main
sequence, leaving only long-lived,
cooler stars.
No open clusters are ever observed
with old stars in them.
Globular clusters
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Are there any young globular
clusters?
Not in our Galaxy. But in the
Large Magellanic clouds,
there is an ongoing burst of
star formation, caused by the
tidal interaction with the
Milky Way.
Many of the new open
clusters are very massive.
Could one of them be dense
and massive enough to stay
bound?
Globular clusters
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Yes! NGC 1818, in
the LMC, is one
example of a young
globular cluster.
General characteristics of GCs
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There are almost 200 globular
clusters orbiting our Galaxy.
They typically have 105 – 106 stars.
They are spherically distributed
around the center of our Galaxy.
They tend to concentrate towards
the center of the Galaxy, with many
in the constellations Sagittarius,
Scorpio and Ophiunchus
It was by studying the distribution of
globular clusters that astronomers
first suspected that the Sun was not
at the center of our Galaxy, that it
instead was somewhere in
Sagittatius.
General characteristics of GCs
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M87 is a giant elliptical
galaxy in Virgo. It has
an estimated 15,000
globular clusters.
Stellar evolution in GCs
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In the Galaxy,
particularly where there
is ongoing star
formation (in the spiral
arms), we see young
and old stars. The
Hertzprung-Russell
diagram is therefore
”filled”.
Stellar evolution in GCs
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In globular clusters, no new stars
have been formed after the
original episode of star formation
– the gas was blown out of the
cluster because of star light.
Therefore, the brightest (shorterlived) stars have left the main
sequence.
Cluster age can be determined
from the brightest stars still in the
main sequence. These are
generally similar to the Sun,
except for the lower metallicity
(the Universe was much younger
when these stars formed).
Therefore cluster age is about
1010 yr or a bit older.
Stellar evolution in GCs
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Because all stars in a globular
cluster were formed at a very
similar time, the curve they form
in the H-R diagram is called an
isochrone.
These curves are extremely
useful in finding out what
happens to stars after they leave
the main sequence.
Because most globular clusters
have ages similar to what the
Sun will have when it leaves the
main sequence, globular clusters
show us directly what will happen
at the end of the Sun's life.
Stellar interactions in GCs
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Stellar densities in GCs can
me much higher than in the
Solar vicinity – normally
1000 stars per cubic pc in
the cores of most GCs, but
106 stars per cubic pc in
clusters with collapsed cores
– example: M15
Such high densities lead to
stellar interactions that
produce objects that we can
never find in the Galaxy!
Stellar interactions in GCs
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Blue stragglers have been
considered a problem for
stellar evolution theory. They
appear as MS stars that look
brighter and bluer than the vast
majority of stars in the cluster.
It looks as if they might be the
result of stellar mergers – once
two stars merge, they will be
more massive, and therefore
hotter and fusing hydrogen at a
faster rate
Stellar interactions in GCs
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Some evidence of this can be obtained from studying the distribution of blue stragglers
– they are preferentially formed in the cores of dense globular clusters, precisely where
the rates of stellar collisions are higher.
Such stars are an example of the “exotic” objects formed only in GCs and never in the
Galaxy.
Stellar interactions in GCs
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Even more exotic
objects can be found in
X-rays. Globular
clusters have an
anomalous number of
X-ray binaries for their
mass.
The excess of X-ray
binaries compared to
the Galaxy is created by
the disruption of mainsequence binary
systems by compact
objects (white dwarfs
and neutron stars).
Stellar interactions in GCs
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If a massive compact star
approaches a binary
system to a distance
smaller than 3-4 orbital
separations, the binary
might be disrupted.
The most likely outcome is
the ejection of the lighter
star in the previous binary
system and the formation
of a binary consisting of
the massive compact star
and the intruder.
Such binary systems will
either disrupt or become
more compact as they
interact with other stars in
the cluster.
Exotic binaries in GCs
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Many of these newlyformed binaries with
compact objects will
become X-ray
binaries.
As the MS star leaves
the MS, it will grow to
fill its Roche lobe.
At this stage, matter
will be pulled into an
accretion disk orbiting
the compact star,
causing X-ray
emission.
Exotic binaries in GCs
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The compact star is a previously existing dead neutron star, the accretion of material
and angular momentum from the disk will spin it up – this stage is called a Low-Mass
X-ray Binary. An extreme case is MXB 1820-30, in the globular cluster NGC 6624.
This system has the shortest orbital period known for any astronomical object – only
11.4 minutes.
Relative to their mass, globular clusters have a disproportionately high number of
LMXBs.
Exotic binaries in GCs
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Once accretion into the NS
stops, we observe a
millisecond pulsar.
GCs are known to be
excellent breeding grounds
for millisecond pulsars –
more MSP are known there
than in the disk of the
Galaxy.