Download life cycle of stars notes

The Lives of the
Stars
What’s Out there in Space?
1. Space itself
2. Gases
a. Hydrogen (~73%)
b. Helium (~25%)
c. All other elements (<~2%)
3. Solids – ‘spacedust’ or ‘stardust’ –
grains of heavier elements, like sand
Start with a Nebula
A 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 Somehow
Areas 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 Born
1. 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
million years to 10 or more
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 He
Initial Composition
70% H
27% He
He
After 5 Billion years of fusion
Core Composition
65% H
35% He
What happens when Hydrogen runs out?
Main 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 RED GIANT
Red Giant
Core temp = 100 Million K
then
Helium Flash!!!
Helium Fusion Starts
and the star has a
‘second life’! Star
fuses Helium into C & O
He C, O
HHe
C, O
Then the HELIUM RUNS OUT
Core collapses again – becomes hotter &
denser
then
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
Then the HELIUM RUNS OUT – Take 2
Core collapses again – becomes hotter & denser
then
For a Massive/SuperMassive Stars
(starting at 100x more mass than Sun):
1. Fusion begins again
1. C fuses to O
2. O fuses to S, Si, and Ar
3. 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
4. Outer layers
of star are
blown off
spectacularly
in a
supernova.
5. Massive star
becomes
neutron star
6. Supermassiv
e 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!
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).
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)
Tarantula Nebula in LMC (constellation Dorado, southern hemisphere)
size: ~2000ly (1ly ~ 6 trillion miles), distance: ~180000 ly
Supernova Remnants
Hot gas cloud left behind remains
hot for a long time
Sometimes visible in x-rays, many
visible in radio
Size of remnant and expansion
velocity tell us the age
HST picture
Crab nebula
SN July 1054 AD
Dist: 6500 ly
Diam: 10 ly,
pic size: 3 ly
Expansion: 3 mill. Mph
(1700 km/s)
A SUPERNOVA LEADS TO
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
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
So, To Summarize….
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
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