Requiem for a Star

Stellar Collapse

•Gravity

•Gravity is an inexorable force always trying to cause further collapse•Nebulae → Protostars•Protostars → Main Sequence Stars•Main Sequence Stars → ?

•At each stage stability is only achieved if there is a balancing force.

Protostar

Nebula Star

Main Sequence Stars

•Force of gravity is balanced by internal pressure of star•Pressure increases with depth

•Internal pressure is maintained by heat produced by the fusion of light elements into heavier elements•If fuel is exhausted then internal pressure is not maintained, and gravity causes collapse of star

Main Sequence StarsQuestion

Will anything other than hydrostatic equilbrium stop inevitable collapse?

It all depends on the mass

Options

• Fusion of elements other than hydrogen• Electrons• Neutrons• Nothing

Fusion Reactions• 4 H → He• 3 He → C• 2 C → Mg• C + He → O• O + He → Ne• O + O → Si• O + He → Ne• Si + 7 He → Ni• Ni → Co (radioactive decay)• Co → Fe (radioactive decay)• Cannot have fusion reactions for elements above Fe

Fusion Reactions

• Fusion of heavier elements becomes increasingly difficult– Impossible for elements above iron

• Requires increasingly higher temperatures and pressures– Requires more gravitational collapse = more mass

• All stars must eventually run out of fuel

Stars Comparable to Sun

• M up to about 3 or 4 solar masses• As a Main Sequence star can only use

hydrogen as a fuel• When hydrogen is exhausted collapse of

interior is inevitable• Increase in temperature caused by collapse

suddenly ignites unprocessed hydrogen, causing star to expand to become red giant

Stars Comparable to Sun• Red Giant star pushes its outer

atmosphere into space– Planetary nebula– Unused hydrogen

• Helium residue in core ‘ignites’ and rapidly turns to carbon

• Carbon core cannot be used. Remains as a hot carbon remnant, slowly cooling in space– White dwarf

Ring Nebula

White Dwarf Stars

What causes further collapse?

•Electrons•Electron degeneracy

•Can only be packed so far•Prevents further collapse when white dwarf is approximately the size of the Earth•Only possible if the mass is less than about 1.4 solar masses•Chandrasekhar limit•Above 1.4 solar masses electrons are not strong enough to balance out gravity

Heavier stars

• If mass > 3-4 solar masses the core left behind would be larger than Chandrasekhar limit– Cannot use electrons to stabilize collapse of star– Cannot form white dwarfs

• Can use elements heavier than hydrogen as fuel during main sequence life– Onion like structure

Cores of Heavier Stars

HeC

OSi

Fe

Final products fromDeath of Heavier Stars

• M < 3-4 solar masses– Core less than 1.4 solar masses– Collapse stabilized by electrons– White dwarf

• 3 < M < 6 solar masses– Core between 1.4 and 3 solar masses – Elements disappear, only neutrons remain– Collapse stabilized by neutrons– Neutron star = Pulsar

• M > 6 solar masses– Core greater than 3 solar masses– Collapse cannot be stabilized– Continues indefinitely– Black hole

← Black hole

Main sequence Star

Main sequence Star

Red Red GiantGiant

Red Red SuperSuperGiantGiant

Red Red SuperSuperGiantGiant

Planetary Planetary NebulaNebula

White White DwarfDwarf

Neutron Neutron StarStar

Black Black HoleHole

Planetary Planetary NebulaNebula

Planetary Planetary NebulaNebula

M = 3 M = 6