Class 17 : Stellar evolution, Part I Evolution of stars of various masses Red giants. Planetary nebulae. White dwarfs. Supernovae. Neutron stars.

HR diagram gives clues to stellar evolution. Main Sequence (MS): Consists of stars living out the “normal” part of their lives… Stars on MS produce energy via steady hydrogen burning (i.e., converting hydrogen into helium). Stars of different mass lie at different points on the main sequence.  Mass-luminosity relation: L  M 4. Eventually, the hydrogen “fuel” runs out and the stars begin to die… they then leave the main sequence…

Life of a 0.05 M  “star” “Star” forms from rotating collapsing gas cloud (recall formation of solar system in class 2). Core heats up to few million K. Trace deuterium burns to form helium. That’s it… Temperature never gets high enough to initiate hydrogen burning. So never really becomes a proper star. Object becomes a “brown dwarf”. This is the case up to about 0.08 M .

Evolution of the Sun Same beginning… cloud collapses. This time, core is hot enough to initiate hydrogen burning (p-p chain). Steady hydrogen burning for 10 billion years (5 billion years more to go…). Then run out of hydrogen in core. Nuclear reactions slow then stop. Core gradually collapses; outer parts of Sun puff up tremendously – becomes red giant. He burning starts (forming carbon).

Once core helium is exhausted, the red giant blows off its outer layers into space. Produces a “planetary nebula”. Only the core of the star is left – becomes a white dwarf. Cools forever like a dying ember.

Evolution of a 10 M  star Star forms as before. H-burning much faster (“CNO cycle”) Only lasts a few million years. When H is exhausted, core contracts, gets hot enough for helium-burning (makes carbon). When He exhausted, core contracts and gets hot enough for carbon burning. And so on… until the core is turned into iron (the most stable element).

Get shell or “onion” structure. No more energy available when core becomes iron. Catastrophic core collapse… Core turns into neutron star Rest of star ejected in a supernova explosion.

Core-collapse (type-II) supernovae Very powerful explosion J released as radiation (VERY bright!). 100 more released in a neutrino pulse. SN1987A (LMC)

Cas-A remnant

Neutron stars The remnant of a SN explosion… Typical mass of 1.5 M  but radius only 10 km! Made of densely packed neutrons (10 18 kg/m 3 ) – a teaspoonful would weigh a million tons! Extreme properties… Very strong gravity on surface. Very strong magnetic fields on surface. Can spin very quickly (hundreds of times per second)… gives rise to pulsars.