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Stellar Evolution. The Formation, Life and Death of the Stars. Star Birth I. Protostar Evolutionary Paths.

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Stellar Evolution

The Formation, Life and Death of the Stars


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Star Birth I. Protostar Evolutionary Paths

Stars form from the fragmentation and gravitational collapse of clouds of gas and dust (recall these “Giant Molecular Clouds”). As the protostar shrinks and gets hotter, it follows an initial evolutionary path on the HR diagram that hooks onto the main sequence.

I

V

III

10 solar mass

protostar

.

Luminosity

Sun

1 solar mass

protostar

Where the protostar “lands” on the main sequence and its entire subsequent course of existence, is entirely determined by the star’s mass and chemical composition at the time it is “born”.

WD

O B A F G K M


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Star Birth II.

A protostar becomes a star when the pressure and temperature in the central core of the object become sufficiently high to trigger thermonuclear fusion, a process in which the positively charged nuclei of light atoms are forced to combine to form heavier elements. The high pressure and temperature overcome the repulsive electrical forces arising from the protons in the nuclei, and the nuclear strong force takes over and pulls nuclei together to form a new, heavier nucleus.

It turns out that the new nucleus, formed from the combination of two lighter nuclei, weighs somewhat less than the total weight of the two nuclei combined under the intense heat and pressure. Einstein’s famous equation E = mc2 states that this mass discrepancy is actually converted into energy in an amount given by the equation. Because c2 is a very large number, the amount of energy resulting from a small amount of mass discrepancy is enormous.


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Eagle Nebula (M 16)

Kitt Peak National Observatory Image


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Eagle Nebula (M 16)- Star Formation & Gaseous Pillars

“Elephant Trunks”

Photoevaporating Gaseous

Pillars

Evaporating Gaseous Globules

“EGGs”

Hubble Space Telescope Images


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Trifid Nebula

(M 20)

in Sagittarius

Kitt Peak National Observatory Image


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Orion Nebula (M 42)

Kitt Peak National Observatory Image


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Horsehead Nebula in Orion

Kitt Peak National Observatory Image


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Orion Nebula (M 42)Star Formation & Protoplanetary Disks

100 AU

(13 light hours)

2.5 light years

The “Trapezium”

Hubble Space Telescope Images


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The “Life” of a Star

Stars are gaseous throughout, and the light we see from them comes from a thin layer of their outer atmosphere called the photosphere. Stars support themselves against their own gravity by natural processes in their interiors that produce an outward force of pressure to balance the inward pull of gravity. When this balance between gravity and pressure is achieved, the star is said to be in hydrostatic equilibrium.

Normal stars on the main sequence and in most other locations of the HR Diagram achieve hydrostatic equilibrium by generating a tremendous pressure in their interiors by thermonuclear fusion. When the star runs out of thermonuclear fuel this is no longer possible, and the star does what it can to achieve a new balance between gravity and pressure.

Sometimes this balance late in a star’s life can be achieved in exotic ways, and sometimes not at all. In the last case, gravity will win out.


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The Proton-Proton Chain

Stars of mass similar to the sun and located on the main sequence of the HR Diagram derive their energy in their cores through a simple thermonuclear process called the proton-proton chain in which hydrogen is converted to helium and other light particles. This process liberates a vast amount of energy.

Step One: 1H1 + 1H1 2H1 + e+ + n

neutrino

proton

proton

Deuteron

(neatron+proton)

positron

Step Two: 2H1 + 1H1 3He2 + g-ray

photon

simple

Helium

Step Three: 3He2 + 3He2 4He2 + 1H1 + 1H1

normal

Helium


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The Proton-Proton Chain – Net Result

The net result is that four protons (hydrogen nuclei) are converted to one helium nucleus plus two positrons (positively charged electrons that are annihilated when they combine with normal electrons to produce high-energy photons), two neutrinos (tiny nearly massless particles that travel close to the speed of light), two high-energy gamma ray photons, and a whopping amount of heat energy.

The proton-proton chain in the sun’s core is the source for all the radiant energy leaving the sun’s surface. The sun’s luminosity is about

4 x 1033 ergs/second

and the sun consumes about 600 million tons of hydrogen per second to maintain that luminosity. (An erg is the amount of energy required to raise one cm3 of water one degree Celsius in temperature.)

At this rate, the sun can generate its energy from the proton-proton chain for about 10 billion years. It is now about halfway through that stage.


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Main Sequence Lifetimes

The main sequence lifetime of a star during which it derives its energy from hydrogen fusion is inversely related to the star’s mass. Thus, the most massive stars have relatively brief main-sequence lifetimes, and the lowest mass stars remain there for very long times.

  • Spectral Mass Lifetime

  • Type (x sun’s) on MS

  • O5 40 106

  • B0 16 107

  • A0 3.3 5x108

  • F0 1.7 3x109

  • G0 1.1 9x109

  • K0 0.8 14x109

  • M0 0.4 2x1011


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Intermediate Evolution of the Sun

Once fusion ignites in the sun’s core, the sun will move very slowly away from its initial position on the main sequence towards somewhat higher luminosity and somewhat lower temperature as it stably “burns” hydrogen into helium. Towards the end of this 10 billion year process, the solar core becomes depleted in H and rich in He.

As the sun’s H supply in the core becomes depleted, the core will shrink and bring a surrounding shell of H into pressure and temperature extremes at which the H will burn. The pressure and temperature in the He-rich core will also become high enough to burn helium which quickly ignites in the helium flash, and the sun settles into a period of stable helium core burning lasting several hundred million years. The byproduct of helium burning is carbon.

As helium fusion replaces hydrogen fusion, the outer atmosphere will expand to several times its present diameter, the surface temperature will drop, and the sun will become a red giant star. It has moved well away from the main sequence and is now in the upper right part of the HR Diagram.


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Final Evolution of the Sun

After a few hundred million years as a red giant, core He will become depleted. Stars of the sun’s mass cannot produce sufficient pressure and temperature in their cores to burn carbon, and the aging sun finds itself without the means to generate additional energy.

As the core cools, the sun will shrink on itself producing heat from gravitational contraction. It will increase in temperature but its shrinking surface will keep its luminosity from increasing. The sun will move to the upper left of the HR Diagram, probably blowing off a portion of its mass in the form of a planetary nebula as it moves horizontally across the HR Diagram. This shrinking will occur until election degeneracy sets in, stopping any further collapse, and the sun has become a white dwarf star.

White dwarf stars no longer produce energy and slowly fade from view as they cool and slip far downward and to the right on the HR Diagram. At some point, white dwarfs are so cool and so weakly glowing they are essentially lost from view and are called black dwarf stars.


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The Helix Nebula in AquariusColliding Gases Surrounding a Dying Star

“Cometary”

Tail

1000

AU

Gaseous Knot

Central Star

HST Close-up Image

Ground-based Photograph


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Planetary Nebulae

Episodes of Dying Stars

NGC 2392 – “Eskimo Nebula”

IC 418

Hubble Space Telescope Images


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More Planetary Nebulae

M 57 – “Ring Nebula” in Lyra

NGC 6543

Hubble Space Telescope Images

Mz 3


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Overall Evolution of the Sun

The sun will have spent about 10 billion years on the Main Sequence before depleting its H and quickly evolving into a Red Giant. After a few hundred million years, its core He will be gone and it will evolve leftward in the HR Diagram, losing part of its mass in a Planetary Nebula eruption before beginning a swift contraction to its final White Dwarf stage.

I

V

P.N.

R.G

III

He flash

.

Luminosity

Sun

1 solar mass

protostar

WD

O B A F G K M


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The Peculiar Properties of White Dwarfs

White dwarfs are the natural endpoints of stars whose mass is less than 1.4 solar masses, a limit known as the Chandrasekhar Limit. White dwarfs support themselves against further collapse by packing electrons very tightly into a so-called degenerate structure. Their material is so dense that one cm3 would weigh about a ton.

A white dwarf star is about the diameter of the Earth but has the mass of the sun. Since the sun is about 330,000 times more massive than the Earth, this means that the gravity at the surface would be about 330,000 times that of the Earth’s surface gravity. A person weighing 150 lbs on Earth would weigh about 25,000 tons (yes, tons) on a star like Sirius B, the white dwarf companion to the bright star Sirius A, first seen visually in 1862.

Sirius A & B from NASA’s Chandra X-ray satellite


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Evolution of Massive Stars

Stars whose masses exceed 1.4 solar masses can successfully burn carbon and continue to derive energy from thermonuclear fusion of heavier and heavier elements until at last they find themselves with a Fe-rich core.

Prior to the buildup of iron, the fusion process is exothermic, producing energy in larges quantities. But the fusion of iron is endothermic, and absorbs energy rather than generates it. Thus, when a star attempts to burn iron, energy is sucked from its interior, pressure drops drastically, and the star implodes in a stellar catastrophe known as a supernova event. The star explodes in a process that produces many heavy elements, and ejects the majority of its mass into the interstellar medium enriching that material with newly synthesized heavy elements that will show up in future generations of stars.

The supernova may leave behind a tiny compact object only twenty or so kilometers in diameter but with more mass than the sun. This is a neutron star.


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Neutron Stars

Stars whose final masses are between 1.4 and several solar masses can halt their gravitational collapse by the phenomenon of neutron degeneracy. Such stars are incredibly dense and have forced the electrons and protons of their interiors to combine to form neutrons.

Neutron stars are more than a hundred million times denser and have a nearly a thousand times the surface gravity of white dwarf stars.

Neutron stars are the source for pulsars, discovered by radio astronomers some thirty years ago.


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Pulsars – Rapidly Rotating Neutron Stars

In 1967, Jocelyn Bell discovered a source of radio energy flashing 30 times per second with extremely high repeatability in the Crab Nebula. The Crab Pulsar was soon identified as a flashing star like object, and theoretical models were developed that explain this and other pulsars as rapidly rotating neutron stars that beam energy into space from their magnetic poles which, as in the case of the Earth, do not correspond with their rotational poles.

For us to see the “lighthouse” effect of this, we must be in a direction so that the radiation beam sweeps across our line of sight as the pulsar rotates.

Some 400 pulsars are now known.

rotation

axis

radiation beam

magnetic pole

magnetic pole


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Pulsars – Starquakes and Radiation Braking

Pulsars are observed to have their periods slowly increasing interrupted by occasional abrupt decrease in pulse period.

The continuous increase in period is due to radiation carrying away energy that is ultimately caused by the object’s rotation. This loss of energy effectively acts as a brake on the spin rate. In general, you would expect younger pulsars to be rotating faster than older ones.

The abrupt decreases in period, or increases in rotation rate, are thought to be due to small slippages in the outer “crust” of the neutron star. Hence, the name starquake.

Pulse Period

Time


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Messier 1 - Crab Nebula

Supernova Remnant with a Pulsar

National Optical Astronomy Observatory Image


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Other Views of the Crab Nebula

Still Expanding Since the Supernova Explosion of 1054

Chandra X-ray image

ESO VLT image


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Supernova 1987A – With Luminous Rings

Hubble Space Telescope Image


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Black Holes – The Ultimate Victory of Gravity

A dying star whose mass exceed several times that of the sun will produce an inward pull of gravity so strong that not even neutron degeneracy can stop its collapse. Such a star is destined to become a black hole.

Black holes are described by Einstein’s General Theory of Relativity, which is an alternate and more accurate theory of gravity than Newton’s theory. Einstein described the effects of gravity in terms of the bending of space and time rather than as a force. Hence, photons (particles of light) will be affected by gravity. Newtonian gravity maintains that photons are not affected since they do not possess mass. Experiments have shown Einstein to be correct.

Because gravity under extreme conditions affects the behavior of photons, black holes are described in terms of how photons react in their vicinity.


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Collapsing to a Black Hole I.

Imagine a not particularly bright person who has volunteered to stand on the surface of a black hole as it collapses. He takes with him a laser pointer. We’ll watch what happens to the beam of light as the star shrinks.

(By the way, try to ignore the fact that the temperature at the surface of the collapsing star would vaporize this guy and its gravity would crush him.)


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Collapsing to a Black Hole II.

As he shines his laser in various directions around the sky, we notice that the beam is bent and may even hit the surface when he lowers it sufficiently. If he is pointing within a certain distance from his zenith, the light escapes. If he tilts the point further away from the zenith, the light hits the surface of the object and doesn’t escape.

The cone of escape of light around the man’s zenith out of which light can escape is called the light cone.


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Collapsing to a Black Hole III.

As the radius of the collapsing star gets smaller, we see that the light cone shrinks.

Eventually the radius reaches a value, called the Schwarzschild Radius, at which the cone closes entirely. Light can no longer escape at all and the collapsing surfaces is said to have crossed the event horizon. Inside this horizon, a particle requires an infinite amount of energy to escape, thus nothing, (not even light) escapes, and the object has become a black hole.

shrinking

light

cone


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Black Hole Structure

A black hole can be pictured as a singularity, where all the mass of the black hole is concentrated in zero volume, surrounded by the event horizon within which nothing escapes.

The radius of the event horizon is directly proportional to the mass of the black hole. A 5 solar mass black holes has an event horizon 30kilometers in diameter.

By the way, black holes are not celestial vacuum cleaners sucking up material from space around them. Another star, or even a spaceship, can comfortably and stably orbit around one as long as it stays outside the event horizon.

event horizon

+

singularity


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Do Black Holes Actually Exist? – Yes!

Black holes have been shown to exist in massive binary star systems that also emit X-rays. We can see light from the normal star in the binary pair and can infer the mass of the unseen companion (the black hole) from the orbital motion of the visible star. The X-rays come from material compressed into a very hot and dense accretion disk surrounding the event horizon.

Observations of stars in the center of the Milky Way Galaxy have shown them moving in a swarming motion about a central point at which nothing is seen. By studying the speeds of the swarming stars, it has been calculated that a super-massive black hole, with 3 to 4 million solar masses, is located at the center of our galaxy. Similar evidence indicates that such objects may be at the cores of many galaxies.


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The Cygnus X-1 Binary

The Unseen Companion Must be a Black Hole

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkbin.html


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The Milky Way’s Supermassive Black Hole

Located at Sgr A*

European Southern Observatory Image


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Endpoints of Stellar Evolution – A Summary

A given star’s ultimate fate is determined by the mass it has left after completing evolution along the main sequence and giant branches.

If the mass is less than 1.4 solar masses (the Chandrasekhar limit), the star becomes a white dwarf.

If the mass is between 1.4 and ~3 solar masses, the star becomes a neutron star.

If the final mass exceeds several solar masses, the star will become a black hole.

On the way to these endpoints, stars can produce fantastic celestial shows as they erupt into planetary nebulae or even explode into supernovae.


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