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 cluster M 15 NASA Thousands to millions of 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 giants Many 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 Large stars live and die very quickly! 200 billion years! Initial stellar mass (M Sun ) ~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 stars leave the main sequence!

Wikipedia Our star is tiny compared to 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