1. Clusters Clusters and Age Stars are born from molecular clouds
Usually many stars born, not just one
A cluster is a group of stars that are together and were probably born at the same time
Some stars leave the cluster later – probably our Sun
Because they were all born at the same time, they will be the same age
By comparing the H-R diagram of a cluster to theoretical models, we can deduce the approximate age
The turn off point

2. Types of Clusters There are two types of clusters
Open clusters are loosely bound together
Up to a thousand stars
Generally younger stars
In the disk of galaxies (more on this later)
Globular clusters are more tightly bound
A few thousand to one million stars
Generally older stars
Not confined to disks of galaxies (more on this later)

3. Open Clusters
NGC 290
M35
NGC 2158
Pleiades M6
M36

4. Globular Clusters M3
M80
M10 M2
M13

5. Cluster Diagrams
A cluster diagram is a Hertzsprung Russell diagram showing all the stars in a cluster
Recall:
Stars are “born” as Main Sequence Stars
Massive stars are the hot luminous ones
The most massive stars die first
Over time, the cluster diagram will change:

6. Cluster Diagram: 1 Million years
At 1 million years old:
Some stars aren’t even main sequence yet
The brightest stars, though rare, dominate the light
O and B stars
Blueish tint to the cluster
The Sun

7. Cluster Diagram: 10 Million Years
At 10 million years old:
Almost all stars are now main sequence
Some of the heaviest are in their supergiant phases
The transition determines the turnoff point
Some of them have died
Turnoff

8. Cluster Diagram: 30 Million Years
At 30 million years old:
More stars are supergiants
Turnoff point has moved
Mix of stars now
White color to cluster
Turnoff

9. Cluster Diagram: 200 Million Years
At 200 million years old:
Red giants, core helium- burning, and double shell-burning
Turnoff point moved farther
Yellow tint to cluster
Turnoff

10. Cluster Diagram: 2 Billion Years
At 2 billion years old:
G, K, M stars dominate
Yellow/orange tint to cluster
Turnoff

11. 10 Billion Years Q. 83: Identifying Age of a Cluster
At 10 billion years old:
K, M stars dominate
Red tint to cluster
Sun is about to turn off
Turnoff
Q. 83: Identifying Age of a Cluster

12. The Turnoff Point Clusters move around on H-R diagram over time
The turnoff point tells us how old the cluster is
The color of a cluster changes from blue (young) to red (old) over time
The Sun is not in a cluster

13. Binary Stellar Evolution
How Stars are Arranged
When stars form, common for two or more to end up in orbit
Multiples more common than singles
Binaries are the most common multiples
Most higher multiples are “hierarchical binaries”
When binaries are far apart (more than a few AU) nothing unusual happens
First star lives, ages, dies
Second star lives, ages, dies

14. Close Binary Evolution
When binaries are close together (AU or less), they can interact extensively
During giant stages, gas can be transferred from one star to the other
This can affect evolution or appearance of star
There will be two distinct periods when something interesting happens
First, when the larger star becomes a giant
Second, when the smaller star becomes a giant

15. Roche Lobes No man’s land Star A’s Roche Lobe Star B’s Roche Lobe
In a binary system, the region of space gravitationally controlled by each star is called the Roche lobe of that star
Anything within the Roche lobe is likely to be absorbed by the star
The more massive star has a bigger Roche lobe
No man’s land
Star A’s Roche Lobe
Star B’s Roche Lobe

16. Close Binary Evolution: Act I
During main sequence, nothing interesting happens
When the first star becomes a giant, it expands
If it expands enough, it can fill its Roche lobe
Any more expansion leads to mass transfer
This can change the mass balance
The star that was initially lighter may become heavier
Q. 84: The Algol Paradox

17. Accretion Disks Second star
Incoming gas is rotating - from revolution of two stars
Gravity pulling it towards object
Gravity vs. rotation = disk
The system changes
Stars may merge or separate
Second star may become more massive star

18. Close Binary Evolution: Act II, Intro
Compact object: any of the three types of stellar corpses
White dwarf
Neutron star
Black hole
When second star becomes a giant, evolution gets interesting
Q. 85: Triple Star System Interesting

19. Close Binary Evolution: Act II, Outline
We now have a giant star/compact object binary
What you get depends on which type of compact object you have:
White dwarf:
Nova
White dwarf supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster

20. How to Make a Nova White Dwarf White dwarf: Nova White dwarf supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster
White Dwarf
White dwarf
Hydrogen gets added to carbon/oxygen layer
Builds up
Ignites and explodes
Cycle repeats

21. How to Make a White Dwarf Supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster
During each cycle the white dwarf gains mass
Shrinks slightly
Reaches Chandrasekhar mass
Star begins to collapse
Heats up
Fusion begins
Whole star burns - explodes
Star is completely destroyed
Burns mostly to iron

22. How to Make a Black Hole X-Ray Binary
White dwarf:
Nova
White dwarf supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster
X-Rays
Black Hole
Accretion disk, as always
Gas is going super fast - enormous gravity
Friction heats up the disk
X-rays from hot gas
Most efficient way to make energy
Even more efficient than fusion
Q. 86: Fate of Gas in Black Hole X-Ray Binary

23. How to Make a Neutron Star X-Ray Binary
Neutron star with gas flowing in
Magnetic field of neutron star channels gas to magnetic poles
Large gravity – gas slams into neutron star
Spot on neutron star gets very hot
White dwarf:
Nova
White dwarf supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster
X-rays

24. How to Make an X-Ray Pulsar
The neutron star is rotating as well
As viewed from here, hot spot appears and disappears – it pulses
Over time, the neutron star gains mass
White dwarf:
Nova
White dwarf supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster

25. How to Make an X-Ray Burster
Hydrogen flows onto surface of neutron star
Hot enough, it burns to Helium
Helium accumulates
Burns explosively, producing X-rays
Cycle repeats
If it passes the maximum mass for a neutron star (2 – 3 MSun) it collapses to a black hole
White dwarf:
Nova
White dwarf supernova
Black hole:
X-Ray Binary
Neutron star:
X-Ray Pulsar
X-Ray Burster