Star Clusters
Stellar Evolution
We have spent considerable time in
understanding the evolution of stars.
At this point you should have realized
that there are only 3 properties and
3 forces that are all-important:
The properties: Mass Mass Mass
Gravity Gravity
The forces: Gravity
Of course, much of this is theory
How do we check our ideas?
As always, we OBSERVE
But stars stay on the Main Sequence
for most of their lives. It's hard to
find the ones who are in a brief,
transitory phase.
We need to find a sample where the
stars were born at nearly the same
time and are at nearly the same
distance away.
How do we check our ideas?
1.
2.
3.
4.
Runner
Cyclist
Roller Blader
Can't tell
How do we check our ideas?
We can't tell who
the best athlete is
because we don't
know where and
when they started
If they all started together in Philadelphia, we would
have to think the runner is the best. On the other hand,
perhaps the runner started from the marina several
hours before the photo was taken.
How do we check our ideas?
1.
2.
3.
4.
Purple
Red
Green
White
(Probably not, this is a Beer Garden)
How do we check our ideas?
Since this is a Triathelon, and all triathletes start at
the same place and the same time, the first one out of
the water must be the best athlete.
Observing Evolution
Betelgeuse
Which star has reached
the end of its
evolutionary path?
Betelgeuse, Mintaka or
Rigel?
Mintaka
Rigel
It’s hard to say, since the stars in Orion are all different
ages and distances it’s difficult to say which has been
evolving the longest.
Don’t confuse oldest with most evolved.
Star Clusters
The best candidates for observing evolution
are located in the Star Clusters
These are groups of stars that are
– close together in space
– bound together by gravity
– formed at the same time, from the same cloud
They contain stars of all different masses so
we can see the end point of evolution in the
massive ones, the beginning in the tiny ones
and all variations in between
Which Stars are most evolved?
The most evolved stars
are the red ones.
These are the Red
Giants which have left
the main sequence.
The blue and yellow
are main sequence
stars.
Since all the stars are formed at the same time, the more
massive have evolved faster. Thus a yellow G2 of the
same age is less evolved than an M2 Red Giant.
M13 – Globular Cluster
47 Tucanae – Globular Cluster
Omega Centauri-Globular Cluster
NGC 4755
The Jewel Box – an Open Cluster
The Double Cluster
Open Clusters
M7 – Ptolemy’s Cluster
Open Cluster
M45 - Pleiades
M11 – The Wild Duck Cluster
Hyades
The Hyades, in
Taurus, is an open
cluster.
Let’s remove
Aldebaran and
just look at the
cluster stars.
Comparisons
Type
Globular
Open
Association
Size
large
medium
small
Diameter (Ly)
50 - 300
less than 30
100 - 500
Mass (Msun)
104 – 108
100 - 1000
100 - 1000
Number of Stars
104 – 108
50 - 1000
5 - 50
Color
red
red or blue
blue
Luminosity (Lsun)
104 – 108
102 - 106
104 - 107
Age
Very old
Old
young
Number in Galaxy
150
1000's
1000's
Example
M13
Jewel Box
Pleiades
How can we test stellar evolution models?
H-R diagram for a
group of stars all
born at the same
time: hot, massive
stars evolve the
most rapidly.
Over time, main
sequence stars of
progressively lower
temps/masses peel
away to the giant
regions on the
diagram.
Testing Stellar Evolution
Theoretical H-R diagram for a star cluster with an age of
1 Million years. The red line is the ZAMS. Note that the
lower mass stars are still evolving toward the MS, while
some high-mass stars have already evolved off the MS.
Testing Stellar Evolution
A cluster age of 10 Million years.
Testing Stellar Evolution
Cluster age = 100 Million years. All lower
mass stars have reached the MS, but the
stars along the upper half of the MS have all
ended their lives.
Testing Stellar Evolution
Cluster age = 1 billion years.
Testing Stellar Evolution
Cluster age = 10 billion years.
Testing Stellar Evolution
The H-R diagram
for NGC 2264, a
nearby cluster.
Estimated age is
1 million years.
ZAMS
Testing Stellar Evolution
The H-R
diagram for the
Pleiades
Estimated age
of 100 million
years.
Testing Stellar Evolution
The H-R diagram
for M 67, a
cluster with an
estimated age of
4 billion years.
Testing Stellar Evolution
H-R diagram for
M13, a globular
cluster.
Estimated age is
13 billion years.
Testing Stellar Evolution
H-R diagrams of star
clusters verify our
models of stellar
evolution.
We can then use the
locations of cluster
turn-off points to
determine the ages of
clusters.