Annoucements Go observing! Soon! The next exam is on Friday, October 8. –That is only 9 days from today.

How Stars Evolve Evolution of protostars onto main sequence –Gravitational contraction –Luminosity versus temperature and radius The fate of the Sun –Red giant phase –Helium flash Reading: sections 19.4, 18.4, 19.1

Protostars evolve into main-sequence stars Hotter

Protostars evolve into main-sequence stars Hotter Brighter Protostar collapses under gravity. The collapse releases energy. The star is relatively transparent, so the energy is radiated away, temperature doesn’t change much. The star gets smaller so it get dimmer.

Luminosity versus radius and temperature R = 2 R Sun T = T Sun R = R Sun T = T Sun Which star is more luminous? 1 2

Luminosity versus radius and temperature R = 2 R Sun T = T Sun R = R Sun T = T Sun Each cm 2 of each surface emits the same amount of radiation. The larger stars emits more radiation because it has a larger surface. It emits 4 times as much radiation. 1 2

Luminosity versus radius and temperature R = R Sun T = T Sun R = R Sun T = 2T Sun Which star is more luminous? The hotter star is more luminous. Luminosity varies as T 4 (Stefan-Boltzmann Law)

Luminosity Law 1 2 L  R 2 T 4 Luminosity  surface area  (Radius) 2 Luminosity  (Temperature) 4 If star A is 2 times as hot as star B, and the same radius, then it will be 2 4 = 16 times as luminous.

Protostars evolve into main-sequence stars Hotter Brighter The star gets smaller so it get dimmer. Next the protostar becomes opaque, light no longer escapes easily. Heat is trapped and the protostar gets hotter. Finally, the core gets hot enough to start nuclear fusion, contraction stops.

More massive protostars evolve faster

Choose the best match: All ____stars have ___ than ___ stars. 1.Main-sequence : more mass : pre-main sequence 2.More massive : shorter lifetimes : less massive 3.More luminous : larger radii : less luminous 4.Holllywood : worse tans : Broadway

The Fate of the Sun How will the Sun evolve over time? What will be its eventual fate?

Sun’s Structure Core –Where nuclear fusion occurs Envelope –Supplies gravity to keep core hot and dense

Main Sequence Evolution Core starts with same fraction of hydrogen as whole star Fusion changes H  He Core gradually shrinks and Sun gets hotter and more luminous

Gradual change in size of Sun Now 40% brighter, 6% larger, 5% hotter

Main Sequence Evolution Fusion changes H  He Core depletes of H Eventually there is not enough H to maintain energy generation in the core Core starts to collapse

Red Giant Phase He core –No nuclear fusion –Gravitational contraction produces energy H layer –Nuclear fusion Envelope –Expands because of increased energy production –Cools because of increased surface area

Sun’s Red Giant Phase

HR diagram Giant phase is when core has been fully converted to Helium

Helium Flash He core –Eventually the core gets hot enough to fuse Helium into Carbon. –This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. –We don’t see this flash because it’s buried inside the Sun. H layer Envelope

Movement on HR diagram