Star Clusters: Confirmation of Stellar Evolution Open and Globular Clusters Ages of Clusters

- HW Ch. 10, EXTENDED Mon. Nov. 8 - HW Ch. 11 & 12, due Mon. Nov. 15- HW Ch. 13 & 14 due Mon. Nov. 22

Exam 3 on Tuesday Nov. 23

IV. Star Clusters: Confirmation of Stellar Evolution

• Open and Globular Clusters

• Ages of Clusters

Outline of Ch 11: The H-R Diagram (cont.)

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

Star Clusters: Confirmation of Stellar Evolution:

2. Two types of clusters: Open and Globular

3. Ages of Clusters

Open cluster: A few thousand loosely packed stars

Globular cluster: Up to a million stars in a dense ball bound together by gravity

Two types of star clustersOpen 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

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

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

Young Stellar Cluster

H-R Diagram of Young

Stellar Cluster

Young Stellar Cluster

H-R Diagram of Young

Stellar Cluster

How do we know this cluster is Young?

Old Stellar Cluster

H-R Diagram of Old Stellar Cluster

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-sequenceturnoff

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

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 1If 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 1If 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?

A. Nothing, there is not enough information

B. Cluster 1 is older than cluster 2

C. Cluster 2 is older than cluster 1

D. None of the answers are correct

Chapter 12. Star Stuff (mostly different from book)I. 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 stops1. 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

I. 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

Infrared light from Orion

Orion Nebula is one of the closest star-forming clouds

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 2What happens after an interstellar cloud of gas

and dust is compressed and collapses?

Question 2What happens after an interstellar cloud of gas

and dust is compressed and collapses?

A. It will heat and contract

B. If its core gets hot enough (10 million K) it can produce energy through hydrogen fusion

C. It can produce main sequence stars

D. 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.

Temperature

Lu

min

osi

ty•Very massive stars are rare

•Low-mass stars are common.

•Minimum mass needed to become a star: 0.08 solar masses

• 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 offB. Core shrinks and heats upC. Core stays at same temperatureD. Helium fusion immediately begins

Question

What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools offB. Core shrinks and heats upC. Core stays at same temperatureD. Helium fusion immediately begins

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) fusionFaster and faster Core collapses Supernova Blows up and produces all elements heavier than Fe

Ch. 12 Part II (not like book). Leaving the Main Sequence: Hydrogen fusion stops

I. 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

Outline of Chapter 12 Part II Evolution and Death of Stars

Remember: Stellar Masses

Composition inside M.S. stars

Eventually the core

fills up with helium and 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

Leaving the Main Sequence: Hydrogen fusion stops

White Dwarfs

2. Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)He fusion, red giant, ejects outer layers white dwarf

I. Leaving the Main Sequence: Hydrogen fusion stops

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. and eventually White Dwarfs

Most red giants stars eject their outer layers

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

A star like our sun dies by puffing off its outer layers, creating a planetary nebula.

Only a white dwarf is left behind

3.High mass Stars (M > 4 solar masses)Fusion of He,C,O,…..but not Fe (Iron) fusionFaster and faster Core collapses Supernova Produces all elements heavier than Fe and blows up• •

II. Leaving the Main Sequence: Hydrogen fusion stops

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

Supernovas

An evolved massive star (M > 4 Msun)

An evolved massive star (M > 4 Msun)

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

before after

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