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

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)

Open Clusters NGC 290 M35 NGC 2158 Pleiades M6 M36

Globular Clusters M3 M80 M10 M2 M13

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:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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