Stellar Life and Death
Terms to Knowmass-lifetime relation
hydrogen shell burning
Chandrasekhar limit (1.4 MSun)
1. Stellar StructureYou can use simple equations that describe the mass, gas, and energy in stars to calculate their structure. For an average star like the Sun, you find that there is a
2. The Life of a StarThe mass of a star determines where on the Main Sequence it will spend most (typically 90%) of its life. High mass hot, bright, blue = upper left; low mass cool, dim, red = lower right.
As stars are born, evolve, and die, their luminosities and surface temperatures change -- so they move around on the H-R Diagram. NOTE: This does not mean that they move physically in space, just that their appearance changes with time.
Massive stars consume their fuel much faster than cool stars. Massive stars are like firecrackers, while low-mass stars are like slow-burning embers.
More precisely, since the Mass-Luminosity relation (L M3.5) is so steep (exponent = 3.5), the lifetime of a Main Sequence star is = (fuel available/rate of consumption) = M/L M/M3.5 or M-2.5. (Low mass long life, high mass short life.)
Example: A main sequence star with mass M=5 MSun has lifetime = 5-2.5 = 0.018 times as long as the Sun. It will burn itself out in about 180 million years, much less than it took for life to evolve on the Earth.
Stars with mass M=0.85 MSun have Main Sequence lifetimes about 15 billion years -- about the age of the entire Universe.
The combination of
3. Stellar Evolution in a NutshellStars are born when huge clouds of gas and dust get nudged by a passing shock wave (from e.g., a nearby supernova) and collapse under their own gravity. Eventually, the collapse leads to temperatures and pressures high enough to ignite nuclear fusion in the core of the protostar. Leftover gas from the cloud either clumps together to form planets, or else gets blown away by the new star's intense radiation and stellar wind.
The star soon settles into a stable life converting H to He in its core, with gravity and pressure balanced by the "thermostat" of hydrostatic equilibrium. Stars in this long middle-age stage lie on the main sequence of the Hertzprung-Russell Diagram.
What happens when a star uses up its fuel, i.e., converts all the hydrogen in its core to helium? It all depends on the mass of the star:
4. The Smallest Stars: Brown and Red DwarfsSome protostars don't even quite make it to star-hood: if its mass is less than about 0.08 MSun, a ball of H and He gas won't have enough gravity to produce the temperature and pressure necessary for nuclear fusion. These luke-warm failed stars are called brown dwarfs.
If the star is just massive enough to ignite nuclear fusion in its core, but not much more, what happens when it uses up its hydrogen? It will contract and heat up -- but not enough to fuse helium atoms together -- winding up as a small, hot, glowing helium ember, emitting black-body radiation and cooling over billions of years: a white dwarf.
Note that very low-mass stars like red dwarfs are
fully convective: they mix up their insides constantly like
boiling water. Therefore H gets used up throughout the star, not just in
5. Sun-Like Stars --> Red Giants, then White DwarfsIf a star has a mass between about 0.4 MSun and 7 MSun, something different happens when the H runs out in a star's core:
NOW what happens? After the helium is all used up, that's it. Core contracts to degenerate state again, and helium fuses into Carbon in a shell around the carbon core; another red giant phase. Eventually the rest of star blows away in stellar wind, forming a planetary nebula (so called because early astronomers thought they resembled planets; now we know they have nothing to do with each other). The Sun will die this way in about 5-6 billion years.
Note that the maximum mass allowed for white dwarfs
is 1.4 MSun; this is the Chandrasekhar limit. If the white
dwarf has more mass than that, it will collapse to a neutron star. So stars
that begin with more than 1.4 MSun must lose all but that 1.4
MSun during their lifetime if they are going to finish up as white
6. Stars with M>7 MSun: Main Sequence --> Supernova!Really massive stars, like firecrackers, are spectacular but short-lived, and they too go out in a blaze of glory.
When the hydrogen is used up in the core of a massive star (which takes only a few million years for a star with M=20MSun), the star goes through the same stages (core contraction, hydrogen shell burning, envelope expansion, core contraction, helium flash) as solar-type stars. But it keeps on going: the crushing force of gravity is so strong that it can heat up the core enough to fuse carbon into oxygen, oxygen into neon, followed by magnesium, silicon, sulfur, and finally iron, in multiple layers like an onion's.
But when the very interior fuses sulfur into iron, it has reached the end of the line . Further fusion will actually consume energy, not release it, because iron is the most strongly bound of all the elements. The iron core is a dead end.
When fusion in the core stops, pressure support disappears, and gravity takes over once again, crushing the core of the star in less than a second. This is too fast! The tremendous rush towards the center of the star results in a rebound, like a superball thrown against the floor, and most of the star is hurled out into space in a spectacular explosion, a supernova.
What is left behind?
What about the material that is blown away by the
supernova? It drifts about between the stars, possibly collecting into clouds
that eventually form new stars later on. This gas consists of lots of "heavy
elements" such as carbon, oxygen, silicon, and iron. This is where we
came from: every atom in your body has been processed by a massive star!
7. How stellar evolution looks on the H-R DiagramRemember: the mass of a star determines almost everything in its life: its lifetime, luminosity, temperature, eventual composition, and ultimate fate.