5.2STELLAR EVOLUTION

Stellar Models

Star Birth Recall stars are born in a rotating

cloud of interstellar gas and dust in space known as a nebula. The nebula is rotating due to angular

momentum and gravity enables the in-falling of material to the center of the nebula.

The contraction of a dense core as matter falls in results in heat production and the object gradually behaves more like a star.

This collapsing cloud of gas and dust, destined to become a star is known as a protostar, and marks the first stage of star development. Only detectable at IR wavelengths.

Upper End of Main Sequence

Astronomers don’t find stars more massive than 100 M (solar masses). Massive clouds fragment into smaller pieces during

star formation. Very massive stars lose mass in strong stellar winds.

Eta Carinae is a binary star system of 60 Mο

and 70 Mο

Lower End of Main Sequence

In stars below 0.08 M, stellar progenitors do not get hot enough in their cores to ignite Hydrogen fusion.

These objects are known as brown dwarfs and slowly contract, converting their gravitational energy into thermal energy and radiating it away star or planet debate?

Brown Dwarfs Hard to detect because they are very faint and

cool (2500 K dull, muddy-red/brown color); mainly emitting at infrared wavelengths.

Evolution on the Main Sequence

A normal main sequence star supports its weight by fusing H into He. Chemical composition in

its core changes and the star evolves.

Core contracts and temperature and density increase outer layers expand and cool (larger, brighter, cooler).

When a star begins its life, it settles on the lower edge of a band called the zero-age main sequence (ZAMS).

Evolution on the Main Sequence

How long a star spends on the main sequence is dependent upon its mass. Massive stars lots of fuel, but they use it rapidly and

live short lives. A 25 M will exhaust its H and die in ~ 7 million years.

Recall the Sun can fuse H into He for ~ 10 billion years.

Red dwarfs, which will be discussed later, have low masses and use their fuel so slowly they should survive for hundreds of billions of years. Since the universe is estimated to be only about 14

billion years old, red dwarfs must still be in their infancy.

Trouble Looms for Earth When the Sun began its life

approximately 5 BYA, it had only about 70% of its present luminosity.

By the time it leaves the main sequence in another 5 billion years, it will have twice its present luminosity. Average temperatures on Earth

are estimated to climb 100°F and above.

Polar ice caps melt, oceans evaporate, atmosphere will vanish into space.

Life Expectancy of Stars The amount of fuel a star

has is proportional to its mass; the rate at which it burns its fuel is proportional to its luminosity you could make a first estimate of the star’s life expectancy by dividing its mass by its luminosity. Recall the mass-luminosity

relation: L = M3.5

Therefore: T = M/L = M/M3.5

Star Life Expectancy

Formula

T = 1/M2.5

T = life expectancy, in solar lifetimes

M = mass, in solar masses

Life Expectancy of Stars For example, how long can a 4 solar-mass

star live, relative to the Sun?

1/32 solar lifetimes

~10 billion/32

~ 310 million

years

Post-Main Sequence Evolution

When the H in a star’s core is completely converted into He, “Hydrogen Burning” (fusion of H into He) stops in the core. H burning continues in a shell

around the core. The He core contracts and

heats up, producing more energy than it needs to balance its own gravity.

Outer layers expand and cool producing a red giant.

Degenerate Matter

He core continues to contract gravity squeezes it tighter and it becomes very small.

When a gas is so dense that its electrons are not free to change their energy, astronomers call it degenerate matter.

Red Giant Evolution As temperatures

continue to increase in the He core, approaching 100,000,000 K, He nuclei begin to fuse together to make Carbon. Three He nuclei are

needed to make a C nucleus, and a He nucleus is called an alpha particle, therefore astronomers refer to He fusion as the triple-alpha process:

4He + 4He 8Be + g8Be + 4He 12C + g

Onset of He fusion occurs very rapidly in an event known as a

Helium flash.

Fusion Into Heavier Elements

Fusion into elements heavier than C and O require very high temperatures; occurs only in very massive stars ( > 8 M).

Death of Low-Mass Stars

Stars < 0.4 M are known as red dwarfs, and can survive a long time. Small mass little weight to

support. H and He well mixed

throughout. No phase of “shell burning”

to expansion of red giant. Slow conversion of H into He;

not hot enough to ignite He fusion leads to slow death contracting into white dwarf (black dwarf eventual fate).

Death of Sun-like Stars Medium-mass stars like the Sun, roughly

between 0.4 – 4 M, fuse H and later He, but cannot get hot enough to ignite C, the next fuel in the sequence.

Interiors contract while their envelopes expand, turning them into red giants. Develops a core rich in C and O. Not hot enough to fuse C can’t resist weight

so it contracts; energy released makes the star’s surrounding envelope expand and cool further.

Such stars can lose large amounts of mass from their surfaces.

Death of Sun-like Stars The “final breath” of

medium-mass stars like our Sun, may come when the outward pressure of an aging giant expels its outer atmosphere in repeated surges in what is known as a planetary nebula.

Despite the name, it has nothing to do with planets.

Most scientists believe the Sun will form a planetary nebula, but it is still debated.

STAGE 1

STAGE 2

Dumbbell Nebula

Ring Nebula

Eskimo Nebula

Cat’s Eye Nebula

Helix Nebula

Hourglass Nebula

Ant Nebula

White Dwarfs Among the most

common stars. Sirius B is an example of a

white dwarf star.

Interior mainly C and O nuclei floating among a whirling storm of degenerate electrons (crystal formation?)

Doesn’t generate any nuclear energy simply degenerate material. Instead of calling a white

dwarf a star, you can refer to it as a compact object.

White Dwarfs Very dense 1 tsp ~ 16 tons

Chunk of white dwarf material the size of a beach ball would outweigh an ocean liner.

Because white dwarfs contain a tremendous amount of heat, they need billions of years to radiate the heat through its small surface area.

Eventually, such objects may become cold and dark, known as black dwarfs. Our galaxy is not old enough to

contain black dwarfs. The coolest white dwarfs in our

galaxy are just a bit cooler than the Sun.

Chandrasekhar Limit Math equations predict if you add

mass to a white dwarf, its radius would shrink due to an increase in its gravity, causing it to squeeze tighter together. If you added enough to raise its total

to about 1.4 M, its radius would shrink to zero.

This is called the Chandrasekhar Limit after Subrahmanyan Chandrasekhar. Therefore, a star’s collapsing core

that is more massive than 1.4 M cannot become a white dwarf.

Theoretical models show an 8 M star should be able to reduce its mass to 1.4 M before it collapses.

Evolution of Binary Stars

Binary stars can sometimes interact by transferring mass from one star to another.

The gravitational fields of the two stars, combined with the rotation of the binary system, define a dumbbell-shaped volume around the pair of stars called the Roche lobes.

Nova Explosions

An explosion on the surface of a white dwarf in a binary system is called a nova. Occurs when mass transfers from a normal star through

the Inner Lagrangian point into an accretion disk around a white dwarf.

Mass transfer resumes shortly after new layer of fuel begins to accumulate.

Death of Massive Stars High-mass stars > 8 M live

short, violent lives and have much too much mass to die as white dwarfs. Evolution similar to lower-mass

stars (consume H in their cores and expand into supergiants).

Heavy element fusion ends with an Iron (Fe) core being produced; ultimately collapses triggering an explosion destroying the star known as a supernova.

Crab Nebula

Types of Supernovae Supernovae are rare and only a few have been seen

in our galaxy along with some other galaxies.

Astronomers have noticed 2 main types: Type I supernovae have spectra containing no Hydrogen

lines. Type II supernovae have spectra containing Hydrogen lines.

Neutron Stars When a star > 8 M explodes

(supernova), its central core usually collapses into a compact object more massive than the Chandrasekhar Limit (1.4 M) not able to form a white dwarf.

The pressure becomes so high at this point that electrons and protons combine to form stable neutrons throughout the compact object now termed a neutron star.

p + e- n + ne

Neutron Stars

Neutron Stars A piece of neutron star material the size of a sugar cube

would have a mass ~ 100 million tons!

The principal of conservation of angular momentum predicts that neutron stars should spin rapidly.

All stars rotate because they form from swirling clouds of interstellar matter.

The 1967 detection of regularly spaced pulses in the output of a radio telescope

led to the discovery of pulsars.

Pulsars Pulsars are thought to be spinning neutron stars.

Emit winds and jets of high-energy particles; slows rotation slightly.

Lighthouse Model of Pulsars

A pulsar’s magnetic field has a dipole, similar to that of Earth’s.

Radiation emitted mainly along the poles.

Black Holes Similar to how collapsing cores of

white dwarfs have a mass limit (Chandrasekhar Limit), there is a mass limit to neutron stars (3 M). If the core of a star contains > 3 M

when it collapses, no force can stop it.

If an object collapses to zero radius, its density and gravity become infinite in a point known as the singularity. Both space and time will stop.

If the contracting core becomes small enough, the escape velocity in the region of space around it is so large not even light can escape and is known as a black hole.

Black Holes Since light cannot even escape,

the inside of a black holes is totally beyond the view of an outside observer.

The event horizon is the boundary between the isolated volume of space-time and the rest of the universe.

The radius of the event horizon is called the Schwarzschild radius. Limiting radius where escape

velocity reaches the speed of light (c).

Named after Karl Schwarzschild

Schwarzschild Radius Formula

Rs = 2GM/c2G = universal gravitational constant ~ 6.67 x 10-11

m3/kgs2

M = mass, in kg

c = speed of light ~ 3 x 108

m/s

Schwarzschild Radius

General Relativity Effects

At a distance, the gravitational fields of a black hole and a star of the same mass are virtually identical.

At smaller distances however, the much deeper gravitational potential will become noticeable.

General Relativity Effects An astronaut descending

down towards the event horizon of a black hole will be stretched vertically and squeezed laterally in an effect known as spaghettification.

Black Hole Candidates