Extra credit!

Get out your clickers.

The following questions are worth 2 points each.

What direction does the Sun rise in Sydney?

A. In the East

B. In the West

C. In the North

D. In the South

E. Cannot conclude

Looking south in Sydney, what type of stars would you see?

A. Seasonal

B. Circumpolar

C. Neither

Looking North in Sydney, what type of stars would you see?

A. Seasonal

B. Circumpolar

C. neither

Sydney’s circumpolar stars rotate

A. Counterclockwise

B. Clockwise

C. Rise in east, set in west

D. Rise in west, set in east

Agenda

Reading: Finish Unit 5, if you haven’t already.

Star clusters Stellar life cycles (stellar evolution)

Star clusters

Easier to observe overall evolution than of one star

Stars in a cluster Formed at the same time Have similar composition They will differ only in mass

Globular clusterM 15

NASA

Thousands to millionsof stars

Globular cluster Tucanae 47

NASA

Open cluster: Pleiades

Hundreds of stars

Case Western

Open Cluster: Jewel box

Jordell Observatory

HR diagram of a cluster

Color (B – V) on horizontal axis Equivalent to

OBAFGKM Apparent magnitude

(V) on vertical axis All stars at the same

distance Easy to convert to

absolute magnitude

Evolution and the HR diagram

High mass (higher luminosity) stars progress through life more quickly

Lower mass stars take longer to be born, consume their fuel more slowly.

Which HR diagram shows the older cluster?

A. B.

C. Cannot conclude

Young cluster ~80 million yrs

U. of Sheffield

Older cluster

U. of Oregon

Compare the HR diagrams

Many older red giantsMany young, hot (blue) stars

Star clusters — summary

Stars in one cluster are of different types but the same age.

Observing many clusters tells us about star life cycles HR diagram Old stars leave the main sequence Cluster age <=> turnoff point

Stellar “evolution” (first part)

What we found in star clusters: Small stars live longer Very massive stars live hard and die

young Old stars leave the main sequence to

become red giants.

Raw materials for star birth

Interstellar clouds.

This is a star cluster in the making!

Raw materials for star birth

In a Stellar nursery

Raw materials collapse Protostar begins to spin Eventually, fusion of H into He begins

Life as a star

Zero-age main sequence when a star first starts fusing H into He

Stars do this for 90% of their lives

Big stars don’t live long!

Massive stars burn very fast.

They soon run out of fuel!

Wikipedia

Time on main sequence versus initial stellar mass

0.4 0.8 1.1 1.7 3.3 16 40

Large stars live and die very quickly!200 billion years!

Initial stellar mass (MSun)

~12 billion years (Sun)

50 million years

1 million years!

He core contracting

H fusing

Exterior expanding

H runs out: star becomes a red giant

This is how starsleave the main sequence!

Wikipedia

Our star is tiny comparedto a red giant!

Further evolution

Helium all used up, gravity takes over again

Much mass is spewed into space

Mass now determines death

Low mass: White dwarf Medium mass: Neutron star or pulsar High mass: Black hole

Summary

Stars are born from Hydrogen Stars spend 90% of their lifetime

fusing Hydrogen into Helium Stars leave the main sequence and

become red giants

Next time

Death of stars:

Black holes, neutron stars, Relativity