Extra credit! Get out your clickers. The following questions are worth 2 points each.
What direction does the Sun rise in Sydney? • In the East • In the West • In the North • In the South • Cannot conclude
Looking south in Sydney, what type of stars would you see? • Seasonal • Circumpolar • Neither
Looking North in Sydney, what type of stars would you see? • Seasonal • Circumpolar • neither
Sydney’s circumpolar stars rotate • Counterclockwise • Clockwise • Rise in east, set in west • 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 Thousands to millions of stars NASA
Open cluster: Pleiades Hundreds of stars Case Western
Open Cluster: Jewel box Jordell Observatory
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 HR diagram of a cluster
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? B. A. 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!
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 ~12 billion years (Sun) 50 million years 1 million years! Large stars live and die very quickly! 200 billion years! Initial stellar mass (MSun)
H runs out: star becomes a red giant Exterior expanding H fusing He core contracting This is how stars leave the main sequence!
Our star is tiny compared to a red giant! Wikipedia
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