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 starsHotter

Protostars evolve into main-sequence stars

Hotter

Brig

hter

• 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 RSun

T = TSun

R = RSun

T = TSun

Which star is more luminous?

1 2

Luminosity versus radius and temperature

R = 2 RSun

T = TSun

R = RSun

T = TSun

• Each cm2 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 = RSun

T = TSun

R = RSun

T = 2TSun

1 21

Which star is more luminous?

The hotter star is more luminous.

Luminosity varies as T4 (Stefan-Boltzmann Law)

Luminosity Law

1 2

L R2 T4

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 24 = 16 times as luminous.

Protostars evolve into main-sequence stars

Hotter

Brig

hter

• 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 sequence2. More massive : shorter lifetimes : less massive3. More luminous : larger radii : less luminous4. Holllywood : worse tans : Broadway

5.25.3

1MM

MLMt

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