Clusters and Age Stars are born from molecular

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 M 35 NGC 290 NGC 2158 M 6 M 36 Pleiades

Globular Clusters M 80 M 3 M 10 M 2 M 13

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 heliumburning, and double shellburning • 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 At 10 billion years old: • K, M stars dominate • Red tint to cluster • Sun is about to turn off Q. 83: Identifying Age of a Cluster Turnoff

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 • 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 Star A’s Roche Lobe No man’s land 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 C B • When second star becomes a giant, evolution gets interesting Q. 85: Triple Star System Interesting A

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 Binary • X-Ray Pulsar • X-Ray Burster

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

How to Make a White Dwarf Supernova • 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 • White dwarf: – Nova – White dwarf supernova • Black hole: – X-Ray Binary • Neutron star: – X-Ray Binary – X-Ray Pulsar – X-Ray Burster

How to Make a Black Hole X-Ray Binary 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 • White dwarf: – Nova – White dwarf supernova • Black hole: – X-Ray Binary • Neutron star: – X-Ray Binary – X-Ray Pulsar – X-Ray Burster Q. 86: Fate of Gas in Black Hole XRay 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 X-rays • White dwarf: – Nova – White dwarf supernova • Black hole: – X-Ray Binary • Neutron star: – X-Ray Binary – X-Ray Pulsar – X-Ray Burster

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 Binary – 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 Binary – X-Ray Pulsar – X-Ray Burster