The Lives of The Lives of the Starsthe Stars

1. Space itself2. Gases

a. Hydrogen (~73%)b. Helium (~25%)c. All other elements (<~2%)

3. Solids – ‘spacedust’ or ‘stardust’ – grains of heavier elements, like sand

What’s Out there in Space?

Start with a NebulaA Large Cloud!

• Mass 100-1000 M¤ (solar masses) (BIG)

• A temperature of 20K to 100K (-279 F) (pretty cool)

• A density of about 10 atoms/cm3 (that’s not many!)

Disturb the Nebula SomehowAreas in the nebula collapse due to:

Cloud to Cloud Collisions: Two clouds collide and interfere with each other

Supernova Shock Waves: The violent death of a nearby star blasts the cloud and sends it swirling

Density Wave: dense areas in the galaxy interact with the cloud

No good reason at all: The cloud just finds the conditions right for collapse

A Star is Born1. Cloud begins to collapse. The density and

temp begin to rise!

2. Core of the cloud heats up to about 1000K (1340 F) 2000K (3140 F). Density rises further.

3. The cloud begins to glow as it gets hot. The protostar now has a luminosity.

4. Core collapses until Temp = 10-15 million K FUSION begins and the star Ignites.

5. A star is born!!

Stars live on average from a few Stars live on average from a few million years to 10 or more million years to 10 or more

billion years.billion years.

How a star lives and dies depends on how much mass it has.

Stars fuse Hydrogen into Helium during their Main Sequence life….

H HeInitial Composition

70% H27% He

After 5 Billion years of fusionCore Composition

65% H35% He

He

What happens when Hydrogen runs out?

Main Sequence Phase EndsMain Sequence Phase Ends• Core is hot & helium rich.• Energy output down – no fusion in the core • Core begins to collapse under gravity – this

makes the core hotter and denser• Hotter core causes star to expand up to 100x

original size due to ‘radiative pressure’ • Surface temp gets cooler – star becomes red• Core becomes “degenerate” - can’t be crushed

any more.

the star becomes a the star becomes a RED RED GIANTGIANT

He C, O

C, O

HHe

Red Giant

Core temp = 100 Million Kthen

Helium Flash!!!

Helium Fusion Starts

and the star has a

‘second life’! Star

fuses Helium into C & O

Core collapses again – becomes hotter & denser

then

Then the HELIUM RUNS OUT

For a Sun-Sized Star:

1. Fusion Ends

2. Core gets degenerate

3. Outer layers of star are blown off, forming a planetary nebula

4. Star becomes white dwarf

5. Cools to a black dwarf

Planetary Nebulas

Hour Glass Nebula

P:\temp\Astro\S0612plannebu.mov

Then the HELIUM RUNS OUT – Take 2

For a Massive/SuperMassive Stars (starting at 100x more mass than Sun):

1. Fusion begins again1. C fuses to O2. O fuses to S, Si, and Ar3. Si fuses to Fe, Cr

2. Heat from new fusion causes 2nd red giant phase – Red Supergiant.

3. After Fe, fusion must stop. Core collapses and gets degenerate

Core collapses again – becomes hotter & denserthen

4. Outer layers of star are blown off spectacularly in a supernova.

5. Massive star becomes neutron star

6. Supermassive star becomes a black hole

SUPERNOVA

Brightest objects in the universe

Can outshine an entire galaxy for a few weeks

Fairly rare – 1-10 per century per galaxy.

Supernova’s are important!Supernova’s are important!They:• Are very bright - visible over a

great distance, for a long time• spread new material out –

“stardust” that goes into making new stars

• can trigger new star formation• Produce the heavy elements – all

the elements from Iron (Fe) up to Uranium (U).

Tarantula Nebula in LMC (constellation Dorado, southern hemisphere) size: ~2000ly (1ly ~ 6 trillion miles), distance: ~180000 ly

Then one day in 1987 (February 23, 1987 to be exact)

Watch This area

Tarantula Nebula in LMC (constellation Dorado, southern hemisphere) size: ~2000ly (1ly ~ 6 trillion miles), distance: ~180000 ly

Then one day in 1987 (February 23, 1987 to be exact)

Supernova RemnantsHot gas cloud left behind remains hot for a long timeSometimes visible in x-rays, many visible in radioSize of remnant and expansion velocity tell us the age

HST picture

Crab nebulaSN July 1054 ADDist: 6500 lyDiam: 10 ly, pic size: 3 lyExpansion: 3 mill. Mph (1700 km/s)

A SUPERNOVA LEADS TO A SUPERNOVA LEADS TO ONE OF TWO ENDS…ONE OF TWO ENDS…

1. Massive stars – the core of the star collapses into a neutron star – an incredibly dense star made only of neutrons.

Supernova remnants – neutron stars

SN remnant Puppis A (Rosat)

Iisolated neutron star seen with Hubble Space Telescope

Supernova remnants – neutron stars

Supernova remnants – neutron stars

A SUPERNOVA LEADS TO A SUPERNOVA LEADS TO ONE OF TWO ENDS…ONE OF TWO ENDS…

2. Supermassive stars – the core of the star collapses into a black hole, a dead star so dense and massive that nothing can escape its gravity, not even light.

Supernova remnant – black hole

Supernova remnant – black hole

Average stars with a mass up to about 8 Solar Masses (8x the mass

of the Sun)

Nebula Protostar Main Sequence Star Red Giant Planetary Nebula White Dwarf

Black Dwarf

So, To Summarize….So, To Summarize….

Massive stars with a masses between 8 and 25 Solar Masses

Nebula Protostar Main Sequence Star Red Giant SuperGiant

Supernova Explosion Neutron Star

Supermassive stars with a masses greater than

25 solar masses

Nebula Protostar Main Sequence Star Red Giant SuperGiant Supernova Explosion Black Hole