Outline of Ch 11: The H-R Diagram (cont.) Star Clusters: Confirmation of Stellar Evolution Open and Globular Clusters Ages of Clusters

Star Clusters: Confirmation of Stellar Evolution: 1. What is special about star clusters? All stars formed at same time, so plotting clusters with different ages on H-R diagrams we can see how stars evolve This confirms our theories of stellar evolution without having to wait billions of years observing how a single star evolves 2. Two types of clusters: Open and Globular 3. Ages of Clusters

Open cluster: A few thousand loosely packed stars

Useful movies to download: Globular cluster visualization http://terpsichore.stsci.edu/~summers/viz/scviz/spz.html Stellar collision simulation: http://www.ifa.hawaii.edu/faculty/barnes/research/stellar_collisions/ Globular cluster: Up to a million stars in a dense ball bound together by gravity

Two types of star clusters Open clusters: young, contain up to several thousand stars and are found in the disk of the galaxy. Globular clusters: old, contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy.

Our Galaxy

Which part of our galaxy is older?

How do we measure the age of a star cluster?

Theoretical Evolution of a star cluster

Massive blue stars die first, followed by white, yellow, orange, and red stars Star_cluster_evolving.swf

How do we know that this theoretical evolution is correct?

How do we know that this theoretical evolution is correct? We plot observations of actual clusters on the H-R diagram

H-R Diagram of Young Stellar Cluster

H-R Diagram of Young Stellar Cluster How do we know this cluster is Young?

H-R Diagram of Old Stellar Cluster

H-R Diagram of Old Stellar Cluster How do we know this cluster is Old?

Pleiades cluster now has no stars with life expectancy less than around 100 million years Main-sequence turnoff

Main-sequence turnoff point of a cluster tells us its age

To determine accurate ages, we compare models of stellar evolution to the cluster data Hr_diagr_and_age_of_cluster.swf

Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old (The universe is about 13.7billion years old)

What have we learned? How do we measure the age of a star cluster? Because all of a cluster’s stars we born at the same time, we can measure a cluster’s age by finding the main sequence turnoff point on an H–R diagram of its stars. The cluster’s age is equal to the hydrogen-burning lifetime of the hottest, most luminous stars that remain on the main sequence.

Question 1 If the brightest main sequence star in cluster 1 is a B star and the brightest main sequence star in cluster 2 is an M star. What can we say about the age of these two clusters?

Question 1 If the brightest main sequence star in cluster 1 is a B star and the brightest main sequence star in cluster 2 is an M star. What can we say about the age of these two clusters? Nothing, there is not enough information Cluster 1 is older than cluster 2 Cluster 2 is older than cluster 1 None of the answers are correct

Chapter 12. Star Stuff (mostly different from book) Birth of Stars from Interstellar Clouds •Young stars near clouds of gas and dust •Contraction and heating of clouds into protostars • Hydrogen fusion stops collapse II. Leaving the Main Sequence: Hydrogen fusion stops 1. Low mass stars (M < 0.4 solar masses) Not enough mass to ever fuse any element heavier than Hydrogen → white dwarf 2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun) He fusion, red giant, ejects outer layers → white dwarf 3.High mass Stars (M > 4 solar masses) Fusion of He,C,O,…..but not Fe (Iron) fusion Faster and faster → Core collapses → Supernova blows up and produces all elements heavier than Fe

Chapter 12. Star Stuff Part I Birth of Stars Birth of Stars from Interstellar Clouds •Young stars near clouds of gas and dust •Contraction and heating of clouds • Hydrogen fusion stops collapse

12.1 Star Birth Our Goals for Learning • How do stars form? • How massive are newborn stars?

We are “star stuff” because the elements necessary for life were made in stars

How do stars form?

I. Birth of Stars and Interstellar Clouds •Young stars are always found near clouds of gas and dust ● The gas and dust between the stars is called the interstellar medium. •Stars are born in intesrtellar molecular clouds consisting mostly of hydrogen molecules and dust • Stars form in places where gravity can make a cloud collapse

Orion Nebula is one of the closest star-forming clouds Infrared light from Orion

Summary of Star Birth Stars are born in cold, relatively dense molecular clouds. Gravity causes gas cloud to shrink Core of shrinking cloud collapses under gravity and heats up, it becomes a protostar surrounded by a spinning disk of gas. When core gets hot enough (10 million K), fusion of hydrogen begins and stops the shrinking New star achieves long-lasting state of balance (main sequence thermostat)

Hubble Space Telescope Image of an edge-on protostar and its jets

Protostar to Main Sequence (in book) Protostar contracts and heats until core temperature is sufficient for hydrogen fusion. Contraction ends when energy released by hydrogen fusion balances the gravity Takes less time for more massive stars to reach the Main Sequence (more massive stars evolve faster)

I. Birth of Stars and Interstellar Clouds • Protostar in the H-R diagram

I. Birth of Stars and Interstellar Clouds • Protostar in the H-R diagram This is the track of a collapsing and heating protostar but we do not see most of them because they are inside dense clouds of gas and dust

I. Birth of Stars and Interstellar Clouds • Protostar’s T-Tauri phase: a very active phase of protostars that clears the gas and dust from the surrounding disk

Question 2 What happens after an interstellar cloud of gas and dust is compressed and collapses?

Question 2 What happens after an interstellar cloud of gas and dust is compressed and collapses? It will heat and contract If its core gets hot enough (10 million K) it can produce energy through hydrogen fusion It can produce main sequence stars All of the answers are correct

Main Sequence ( Hydrogen Fusion) Main sequence Thermostat : very stable phase

How massive are newborn stars?

A cluster of many stars can form out of a single cloud.

Very massive stars are rare Low-mass stars are common. Minimum mass needed to become a star: 0.08 solar masses Luminosity Temperature

• How massive are newborn stars? Low mass stars are more numerous than high mass stars Newborn stars come in a range of masses, but cannot be less massive than 0.08MSun. Below this mass, pressure in the core is not enough (10 million K) for hydrogen fusion, and the object becomes a “failed star” known as a brown dwarf.

Equilibrium inside M.S. stars

Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core stays at same temperature D. Helium fusion immediately begins

Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core stays at same temperature D. Helium fusion immediately begins

Ch. 12 Part II (not like book) Ch. 12 Part II (not like book). Leaving the Main Sequence: Hydrogen fusion stops 1. Low mass stars (M < 0.4 solar masses) Not enough mass to ever fuse any element heavier than Hydrogen  white dwarf 2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun) He fusion, red giant, ejects outer layers  white dwarf 3.High mass Stars (M > 4 solar masses) Fusion of He,C,O,…..but not Fe (Iron) fusion Faster and faster  Core collapses  Supernova  Blows up and produces all elements heavier than Fe

Outline of Chapter 12 Part II Evolution and Death of Stars Leaving the Main Sequence: BEWARE THAT THE BOOK DOES NOT USE THE SAME DEFINITIONS OF LOW, INTERMEDIATE AND HIGH MASS STARS. AS MENTIONED, THE EXAM WILL BE BASED ON THE LECTURES AND NOT ON THE BOOK

Remember: Stellar Masses

Composition inside M.S. stars Eventually the core fills up with helium and hydrogen fusion stops

Leaving the Main Sequence: Hydrogen fusion stops 1. Low mass stars (M < 0.4 solar masses) Not enough mass to ever fuse any element heavier than Hydrogen  white dwarf White Dwarfs

I. Leaving the Main Sequence: Hydrogen fusion stops 2. Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun) He fusion, red giant, ejects outer layers  white dwarf

Helium fusion requires much higher temperatures than hydrogen fusion because larger charge leads to greater repulsion

Stars like our Sun become Red Giants after they leave the M. S Stars like our Sun become Red Giants after they leave the M.S. and eventually White Dwarfs

Most red giants stars eject their outer layers

Only a white dwarf is left behind A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind Great planetary-nebula movies can be downloaded from Press Release 2003-11 (Helix Nebula) of the Space Telescope Science Institute (see hubble.stsci.edu)

A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind

A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind

A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind

II. Leaving the Main Sequence: Hydrogen fusion stops 3.High mass Stars (M > 4 solar masses) Fusion of He,C,O,…..but not Fe (Iron) fusion Faster and faster  Core collapses  Supernova  Produces all elements heavier than Fe and blows up

Supernovas 3. High mass star (M > 4 solar masses) Fusion of He,C,O,…..but not Fe (Iron) fusion Faster and faster  Core collapses  Supernova Produces all elements heavier than Fe and blows envelope apart ejecting to interstellar space most of its mass Supernova Remnants: Crab nebula and others

An evolved massive star (M > 4 Msun)

An evolved massive star (M > 4 Msun)

before after Supernova 1987A in a nearby galaxy is the nearest supernova observed in the last 400 years

Crab Nebula: Remnant of a supernova observed in 1054 A.D.

Pulsar (a kind if neutron star) at center of Crab nebula

Older Supernova Remnant