Stellar Evolution The birth, Life and Death of stars Assigned Reading: up to Chapter 12

A main sequence star is the one which is supported by hydrogen fusion The main sequence exists because stars balance their weight with energy outflows, produced by nuclear fusion in their core

41H --> 4He + energy ( E = mc2 ) mass(4He) = 0.993{mass(H)+mass(H)+mass(H)+mass(H)} mass loss is 0.007{mass(4H)} = 5 x 10-29 kg E=mc2=(5 x 10-29)(3 x 108)2 = 4 x 10-12 joules How many fusions per second? Solor Luminosity = 4 x 1026 joules/sec 4 x 1026 joules/sec ----------------------- = 1038 fusions/sec = 200 million tons/sec!!! 4 x 10-12 joules Actually, about 500 million tons/sec are needed!

A main-sequence star can hold its structure for a very long time. Why A main-sequence star can hold its structure for a very long time. Why? Time = c2 M / L = c2 M / M3.5 = 1 / M2.5 Gravitational Contraction Thermal Pressure

How does a star hold itself? This balance between weight and pressure is called hydrostatic equilibrium. Four key equations govern how stars work (shell by shell) Hydrostatic Equilibrium Weight on each shell balanced by pressure Energy transport Energy moves from hotter to cooler places by radiation, convection,conduction Conservations of mass Sum of all shells equals total mass of star Conservation of Energy Total luminosity equals sum of energy produced in each shell

The Solar Thermostat Outward thermal pressure of core is larger than inward gravitational pressure Core expands Nuclear fusion rate rises dramatically Expanding core cools Contracting core heats up Nuclear fusion rate drops dramatically Core contracts Outward thermal pressure of core drops (and becomes smaller than inward grav. pressure)

Pressure and Temperature of a Gas

Main Sequence Stars Main Sequence stars are all fusing H to He in their cores. Life time of a star is determined by its mass. Nature makes more low-mass stars than high-mass stars. Low-mass stars also live longer. That is why there are a lot more low-mass stars. What happens after the main sequence (when hydrogen in the core runs out)?

Low-End of Main Sequence Very common stars, but very hard to see This one is CHRX 73 A+B, a 0.3 Mo red dwarf plus a 15 MJ brown dwarf

High-End of Main Sequence Very luminous byt very rare Stars. Very hard to measure the mass Also, very hard to find stars with M>100 Mo. Large mass ejection This one is Eta Carinae: two Stars, one of 60 Mo and the The other of 70 Mo.

When core hydrogen fusion ceases, a main-sequence star becomes a giant The thermal pressure in the core can no longer support the weight of the outer layers. The enormous weight from the outer layers compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion. This fusion takes place at very high temperatures and the new thermal pressure causes the outer layers to expand into a giant star. Both the cooling/collapsing inert He core and the H-burning shell contributes to energy output. Star overproduces energy: it expands, surface cools, and becomes a luminous giant

Anatomy of a Star that is leaving the Main Sequence Hydrogen burning core shell Hydrogen fuel ABSOLUTELY NOT IN SCALE: In a 5 Mo star, if core has size of a quarter, envelope has size of a baseball diamond. Yet, core contains 12% of mass Helium “ash”

Up the red giant branch Eventually, hydrogen will burn only in the outer parts of the mostly-helium core. The star will swell to enormous size and luminosity, and its temperature will drop, becoming a red giant. Sun in ~5 Gyr Sun today

How does the Helium core push back? As matter compresses, it becomes denser (and heats up!) Eventually, the electrons are forced to be too close together. A quantum mechanical law called the Pauli Exclusion Principle restricts electrons from being in the same state (i.e., keeps them from being too close together). The resulting outward pressure which keeps the electrons apart is called electron degeneracy pressure – this is what supports the core Stars with M > 3 Mo never develop degenerate He core Indistinguishable particles are not allowed to stay in the same quantum state.

Helium fusion begins at the center of a giant While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon (if M > 0.5 Mo) Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion. He fuses through a number of reactions, generally referred to as the “3-a” reactions He + He + He = C + energy … and produces an element “crucial” to our existence: CARBON For a star with M<Msun, the carbon core never gets hot enough to ignite nuclear fusion (star needs 600,000,000 K to do so).

After helium fusion gets going… The Sun will expand and cool again, becoming a red (super) giant. Earth, cooked to a cinder during the red giant phase, will be engulfed and vaporized within the Sun. At the end of this stage, the Sun’s core will consist mostly of carbon, with a little oxygen.

For low mass stars

Planetary Nebula At the center of the nebula there is the dying star. Destiny of stars with roughly M < 8Mo M <0.4 Mo He WD M < 4 Mo, C WD M < 8 Mo, C + O + Si WD

Nuclear burning in massive stars (>4 Mo)

The lead-up to disaster in massive stars Iron cores do not immediately collapse due to electron degeneracy pressure. If the density continues to rise, eventually the electrons are forced to combine with the protons – resulting in neutrons. What comes next … is core collapse.

Massive Star Explosions: Supernovae The gravitational collapse of the core releases an enormous amount of energy. All the shells ignite, and the stars literally explodes It can fully disintegrates, nothing is left of it (Type Ia) Or a neutron star or black hole (core cadaver) is left (Type II) 100 times the total amount of energy produced by the Sun in its lifetime is released in a matter of seconds. For a few days, the star is ~as luminous as a whole galaxy!!! Then luminosity decays in following months: E.g. A Type Ia SN dims by a factor of 100 in about 170 days Chart of light intensity versus time is called “Light Curve” (see fig13-13, page 300).

Supernova 1987a before/after

Supernova Remnant Cassiopeia A

End Products of Stars M > 8 Msun  Supernova + neutron star or a black hole 0.08 Msun < M < 8 Msun  White dwarf M < 0.08 Msun  Brown dwarf (fusion never starts)

Stellar Evolution in a Nutshell Mass controls the evolution of a star! M < 8 MSun M > 8 MSun Mcore < 3MSun Mcore > 3MSun

O

All of the Heavy Elements are Made During Supernovae

The Key Point in the Production of Elements in the Universe Hydrogen and Helium are initially created in the Big Bang Stars process Hydrogen and Helium into heavier elements (elements lighter than iron) during their lives. Elements heavier than iron are generated only in the deaths of high mass stars (supernovae). We were all once fuel for a stellar furnace. Parts of us were formed in a supernova.

The evolution of stars Points to remember: How is the helium core of a star supported? What causes the expansion of a star to become a red giant? What is a supernova? When does a massive star explode as a supernova?.

Where does the energy come from in a star like the Sun? Why? Nuclear fusion. What elements can such a star produce? Carbon and Oxygen. Why cannot the star produce heavier element? not enough mass to reach the temperature. Why more massive stars have higher central temperatures? high pressure to balance the gravity. What is the heaviest element that can be fused into in a star? Why? Iron, which is the most bound nucleus.

The H-R diagram O Which is the faintest? the sun, an O star, a white dwarf, or a red giant? Which of these star is the hottest? What do Sun-like stars (0.4 Msun < M < 8 Msun) have in common? What about red dwarfs (0.08 Msun < M < 0.4 Msun) ? Where do stars spend most of their time? What is the order of stellar evolution of a star like the Sun?