Stellar Evolution The birth, Life and Death of stars Assigned Reading: Chapter 12 + Chapter 13

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

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

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. KEY QUESTION: What happens after the main sequence (when hydrogen in the core runs out)?

Low-End of Main Sequence Most 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 star leaves the main sequence and 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

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

End Products of Stars M < 0.08 Msun  Brown dwarf (fusion never starts) 0.08 Msun < M < 8 Msun  White dwarf Helium White Dwarf: 0.08 Msun < M < 0.4 Msun Carbon White Dwarf: 0.4 Msun < M < 4 Msun Oxygen-Neon White Dwarf: 4 Msun < M < 8 Msun M > 8 Msun  Supernova II+ neutron star or a black hole If Supernova Ia (explosion of the degenerate core), the whole star disintegrates, nothing is left of it

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 the nnuclear-burning phase of 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? (electron degenerate pressure) What causes the expansion of a star to become a red giant? (shell burning, energy over-production) What is a supernova? (the consequence of an unstoppable gravitational collapse) When does a massive star explode as a supernova? (once it cannot support itself anymore)

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.

Observing Stellar Evolution How can we see stellar evolution in action? Stellar Clusters, a group of coeval stars, I.e. all born at the same time, but with different masses (hence different life time) How can one estimate the age of a stellar cluster? By looking at the HR diagram of the cluster, namely at the evolutionary phase of stars of the same age but with different mass

Our First Measurement of Age Star Clusters Open cluster: 103 stars, up to 30 pc in size, found in disk of galaxy. All have mostly young stars Globular cluster: up to 106 stars and 150 pc in size, in disk and halo of galaxy. All have old stars

Why are clusters useful to astronomers? All stars in a cluster are at about same distance from Earth. All stars in a cluster are of about the same age. Clusters therefore are natural laboratory in which mass, rather than age, of stars is only significant variable.

The Hertzsprung-Russell Diagram More mass, more fuel, very fast burning. Shorter Lifetime of Star Less mass, less fuel, slow, steady burning. Longer How do we know the age of a star?

The H-R Diagram of a Cluster We can date a cluster by observing its population of stars. All these stars in the cluster have burned themselves out! Turn-Off point: Age indicator The oldest clusters known have been measured to be ~14 billion years old.

Variable Stars Chepeids RR-Lyrae Variability due to Instability Variability is PERIODIC Instability caused by presence of ionized He More luminous variable stars have large Period Variability is EXTREMELY USEFUL, because it is an absolute distance indicator

Cepheid Variable Stars Cepheid variable stars have variable brightness that is very regular. The period of the variation can be from days to weeks Pulsation due to instability: He ionization layer acts a energy sponge it seems to be a reliable indication of the star’s luminosity!

Cepheids: the Period-Luminosity Relation Henrietta Leavitt Henrietta Leavitt (1868-1921). Luminosity=4D2B

Standard Candles If we know an object’s true luminosity, we can measure its distance by measuring its apparent brightness. An object that has a known luminosity is called a standard candle.

What is burning in stars? Gasoline Nuclear fission Nuclear fusion Natural gas

Survey Questions 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?.

The death of stars and stellar remnants He white dwarfs M<0.4 Mo C White dwarfs (planetary nebulae) 0.4<M<4 Mo Carbon-Neon-Silicon White Dwarfs 4<M<8 Mo Two types of Supernovae M>8 Mo Type Ia, the exploding stars disintegrates Type II (core collapse), the star leaves remnants: Neutron stars (basically, a neutron white dwarf, I.e. degenerate gas of neutrons) Black holes

Stellar evolution can be deeply altered in a binary system: mass transfer An originally massive star can loose mass and become less massive (longer life) A nearly dead star can be rejuvenated by accretion of fresh fuel

A Type of Stellar Remnant: Planetary Nebulae At the center of the nebula there is the dying star. This is a white dwarf, where small and hot: it photo-ionizes the nebula The nebula formed out of the mass loss during the red super-giant phase. 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

NOVAE: an example of binary stars Novae are nuclear explosions on the surface of white dwarf and neutron stars Brightness changes by a factor of 4000!

Two basic types of supernovae Type Ia – from the thermonuclear detonation of a white dwarf with M ~ 1.4 Msun after accreting matter from its companion. (1.4 Msun is called Chandrasekhar limit) Type II – from core collapse of a massive star  neutron star or black hole.

Type Ia: White Dwarf Supernova If a White Dwarf accretes enough matter from a companion star, it will eventually nova. If, after the nova, it does not shed all the mass it gained, it will continue to accrete mass until it novas again. If this process continues (accretion, nova, accretion, nova, etc.) such that the WD continues to gain mass, once it has a mass of 1.4Msun, the core will collapse, carbon fusion will occur simultaneously throughout the core, and the WD will supernova.

How might it be possible for a White Dwarf to flare back to life?

Remnant from a Type Ia supernova: A lot of irons!

Another distance indicator: White Dwarf (Type Ia) Supernovae in distant galaxies. L=4D2 B

A 25 Mo star burns: H in 7 million years O in 6 month Si in 1 day Then… Booom Core collapse in ~0.001-0.01 sec

Remnant from a Type II supernova Crab Nebula The supernova explosion that created the Crab was seen on about July 4, 1054 AD.

Another Type of Stellar Remnant: Neutron stars A neutron star --- a giant nucleus --- is formed from the collapse of a massive star. Supported by neutron degeneracy pressure. Only about 10 km in radius. A teaspoon full would contain 108 tons! Very hot and with very strong magnetic field

Jocelyn Bell Neutron stars discovered as pulsar

Pulsar, as a light house A fast rotating, magnetized neutron star. Emits both strong radiation (radio) and jets pf high-energy particles. Jets not very well understood; their existence is due to the rotation and to the presence of magnetic fields

The Limit of Neutron Degeneracy The upper limit on the mass of stars supported by neutron degeneracy pressure is about 3.0 MSun (predicted by Lev Landau) If the remaining core contains more mass, neutron degeneracy pressure is insufficient to stop the collapse. In fact, nothing can stop the collapse, and the star becomes a black hole.

Black holes When the ball of neutrons collapses, it keep its mass, but shrinks to smaller and smaller sizes. No amount of pressure can stop the collapse, because in those extreme situations, pressure itself contributes more to gravity than it does of opposing it. It forms a singularity – a region in space with the mass of the parent material, but with virtually null volume and hence potentially infinite gravitational field. In a singularity gravity is so strong, i.e. the space is so tightly curved, that nothing can escape, even light! The most interesting aspects of a black hole are not what it’s made of, but what effect is has on the space and time around it.

Review Questions What are type-Ia supernovae? What do a type-II supernova leave behind? Why does a neutron star spin fast? What is a pulsar?