Download every star in the cluster.

Star Clusters
Meet a Star Cluster!
Nature’s Helpful Gift!
The stars in a given cluster are all:

Co-eval (formed at the same time, near enough)

Formed of the same material (formed from an enormous

At the same distance from us (so differences in apparent
cloud of thoroughly mixed-up material: no significant compositional
differences, star-to-star)
brightness are meaningful! If one star looks brighter than its
neighbour, it really is! It’s not an accident of distance.)
How Do We Use Clusters?
You can’t watch a single star cluster evolve (or
any of the individual stars within it)
but you can still intercompare the properties of
stars within different clusters, and eventually
identify clusters of different ages.
This provides us with critical snapshots of stellar
evolution.
There are Two
Kinds of Clusters
1. OPEN:
 relatively
young
 often sparse (a few hundred stars)
 scattered about, no obvious symmetry
Example: the Pleiades
2.
GLOBULAR:
 contain
the oldest known stars
 very rich (up to a million stars)
 centrally concentrated in a round ball – like a
swarm of luminous bees!
Inside a Globular Cluster
Of course the stars are all moving! (If they were at rest,
they would fall together under the mutual pull of gravity.)
On the other hand, there is no dominant central object that
they orbit around. Instead, each star moves in response
to the combined gravitational effects of all the others.
Think of a swarm of luminous bees!
Watch this animation:
https://www.spacetelescope.org/videos/heic1017a/
The Reality of Clusters
Could they be chance clumpings of stars? There
are good reasons to think not:


Statistics (there are far too many stars in one area to be
a chance clumping. Look at a globular cluster!)
The stars share motions in common, moving ‘as a group’
Moving Clusters
Visit
http://www.astronexus.com/node/28
and watch the animations under `3D Universe’
(especially in the Orion region)
Now Consider Their H-R Diagrams
Pick an interesting cluster. Now measure the
brightness and colour (= temperature
indicator) for every star in the cluster. Plot
these data as an HR Diagram. What should we
expect?
The first thing to note is that you will not see the
same thing for all clusters. But why not?
What Do We Expect?
For a young cluster: we expect to see some
brighter blue stars, some intermediate
yellow (sun-like) ones, and some fainter
red ones – all of them recently formed
They will be spread out along the main
sequence, reflecting the birth of stars of a
variety of masses
On the Other Hand
For older clusters:
we should no longer see any bright blue stars
near the top of the main sequence
This is because such stars are the first to use up
their hydrogen fuel!
The question is: what will they have turned into?
Two Real Clusters:
The Hyades and The Pleiades
Their HR Diagrams
[‘B-V’ is a measure of colour, from blue to red]
Now Plot Them in the Same Figure
as apparent magnitudes
Compare Stars of Identical Colour
[say, B-V = 0.4]
These are main sequence stars, so those in the Hyades
must be essentially identical to those in the Pleiades.
A First Striking Difference
A More Significant Difference
What Else?
The Hyades contains four red giant stars;
there are none in the younger Pleiades.
That is what the
bright blue stars
have turned into!
‘Turnoff’ Ages
A cluster starts with stars all along the main sequence, but
gradually the bright blue ones disappear. The point above
which none are left is called the ‘turnoff.’ In this figure,
there are no O and B stars left, only A stars and cooler.
That defines the ‘turnoff age’.
Two Examples, with Ages
A Globular
Cluster
There once were main
sequence stars in the
region indicated by the
blue line. They have all
evolved away, and we
also see lots of red
giants!
How old must this
cluster be? All the stars
originally hotter than the
Sun are gone!
One Important Clarification:
Red Giants Aren’t Forever
As we have seen, once stars exhaust their hydrogen in the core,
they turn into red giants – but they don’t stay like that for long!
This is just one relatively brief stage on the way to stellar
demise.
The globular cluster in the figure just above has a lot of red
giants, continually forming from evolving stars near the turnoff.
But there were originally many stars that were even more
massive, that became red giants for a time, and that have
moved on to a different final form. The cluster contains a huge
number of ‘stellar remnants.’ [Details to follow!]
Myriads of Clusters
Our understanding is not based on a simple
comparison of just a few clusters of stars!
There are literally tens of thousands of star
clusters in the Milky Way galaxy, and we
have lots of observational evidence to
compare with our theoretical expectations!
Various Clusters
Superimposed
(showing a variety of
turnoff points)
Let’s Do a Specific Test of Our
Theoretical Understanding
Hypothesize:
a gas cloud a million times the mass of the sun,
mostly H and He gas;
and assume:
that it gravitationally condenses to form a
globular cluster containing a million stars (a
small number of extremely very massive ones,
very many of lower mass), each of which then
undergoes known sets of nuclear reactions
The Crucial Question
What kinds of stars would we expect to
see in such a cluster after, say, 10-15
billion years? What would its HR diagram
look like?
Use the Computer!

Calculate the nuclear reactions in each star (the
pp cycle, the CNO cycle, etc)

Work out the rate of fuel consumption (faster in
the more massive [hotter] stars, etc)

Calculate how the stars will change in internal
structure and external appearance

Predict what we would see!
Meet the Target – 47 Tuc
https://www.eso.org/public/videos/eso1302a/
Real Data Frames
the raw stuff of astronomical research
The Observed
HR Diagram
for the Cluster
Compare with
What Theory
Predicts
Great
Success!
One Final Question:
Did the Sun Itself Form in a Cluster?
..or is it perhaps even now in a loose
cluster?
(suggested reading: Nightfall, by Isaac Asimov)
Stars Do Form
in Rich Complexes
But Must
They Do So?
There are small
dark clouds of
gas and dust that
may form single
stars in isolation.
It’s A Challenging Question
Analogy: without extra info, you don’t know if your adult
friend was a single child, or part of a really big family.
Similarly in astronomy; even if born in a cluster, stars can
later wind up alone, for a couple of reasons:
1.
2.
Clusters can slowly‘evaporate’ (fast-moving stars
leave); or
Clusters can be slowly disrupted (tidal forces ‘pull’ stars
out)
So the sun may indeed have been in a cluster at one
time. We don’t know.