Stellar Evolution

The Life Cycles of Stars

Warm Up

1. What is a star?

2. What do stars really do as they burn?

3. What happens to stars as they get older?

4. How do stars change as they get older?

5. Are their different kinds of stars?

6. What kind of star is our Sun?

Warm Up

1. What do you call the life cycle of a star from beginning to end?

2. Where do all stars originate?

3. What do stars do?

4. What is the first sign that a star is dying?

5. What is the definition of a high-mass star?

6. What is the definition of a low mass star?

7. What are the two major end products of high-mass stars?

8. What is a black hole?

9. Where do black holes come from?

Warm Up

1. List the 7 spectral classes of stars and estimate their temperatures.

2. What is a protostar?

3. What is infall and how is it important to forming stars?

4. Why are new stars variable stars?

5. What causes new stars to vary in brightness?

Warm Up

1. Describe the complete evolutionary cycles of a high-mass star.

2. Describe the evolutionary cycle of a low-mass star.

3. What is the most important factor in determining the evolutionary path of a star?

Warm Up 11/29/12

1. What is a white dwarf and what does it come from?

2. What is a neutron star and where does it come from?

3. What is a black hole and where does it come from?

Stellar EvolutionStars are similar to living things in that they are born, they live and eventually they die. This process that is a star’s life is called its stellar evolution.

Stellar Evolution

By human standards, stellar evolution happens incredibly slowly and requires millions or even billions of years to occur.

Stellar EvolutionThe overriding consideration in determining the evolutionary path and life-span of a star is its mass. A high-mass star has 10 or more times the mass of our Sun. A low-mass star has 9 or less solar masses.

The Origins of StarsStars begin as interstellar clouds. These clouds are stable until an event takes place to change their state of equilibrium. Some scientist think our interstellar cloud collided with another one. Others believe that an exploding star could have caused it.

Stellar FormationRegardless of the impetus, the cloud collapsed under its own gravity and began to spin. Angular velocity says that as the clouds radius got smaller, the cloud began to rotate faster, sweeping up material as it did so.

The Origins of StarsThe cloud begins to collapse under their own gravity. The denser the core becomes, the hotter it gets. In only a few million years, the sun begins to burn. The new star, or protostar, begins the process of turning hydrogen into helium.

What Do Stars Do?

Stars are chemical factories that change lighter elements into heavier ones. This is where all the elements come from that you find on the periodic table.

What Do Stars Do?

Like an onion, each successive layer of a star is hotter and creates a heavier element.

Small-Mass StarsSmall-mass stars live for billions of years. They are thrifty with their fuel. As their hydrogen becomes diminished, they begin to convert helium into yet heavier elements.

High-Mass Stars

High-mass stars are not frugal with their fuel and burn it up very quickly lasting only a few million years. Their large masses creates huge gravitational forces pulling in material. This compaction creates even more heat which burns even more fuel.

The Evolution of a High-Mass StarFollowing its interstellar cloud period and protostar period (where it reaches equilibrium) a high-mass star on the main sequence rapidly burns its fuel turning helium to carbon, to oxygen and ultimately silicon into iron.

The Evolution of a High-Mass Star

When iron fuses, it returns no energy. The star begins to lose its outward-pushing force. The star collapses, increasing its density until the core gets hotter, pushing the star outward. The star has become a red giant.

The Evolution of a High-Mass StarEventually gravity will overcome heat and the collapse of the star will occur in a fraction of a second. The resultant implosion will create either a neutron star or a mysterious black hole.

The Evolutionary Cycles of Stars

Evolution of a Low-Mass StarSmall-mass stars take longer to evolve because of how slowly they consume their fuel. But, they too form from interstellar clouds into protostars and then into red giants.

Evolution of a Low-Mass StarAs its helium begins to fuse, the star enters a yellow giant phase and eventually a second red giant phase. Radiation streaming from the star drives off most of the stars remaining fuel.

Evolution of a Low-Mass Star

What remains after this process is a ring-shaped gas halo (called a planetary nebula) surrounding a small, central white dwarf. The core is fiercely hot, but it has no remaining energy. It is dead.

Planetary Nebula

Non-uniformity of Clouds

Radio telescopes (infrared) have revealed that interstellar clouds are not uniform. Once pushed, they develop areas of higher and lower density/gravity. This makes portions of the cloud clump together, each

clump forming

a star.

Non-uniformity of CloudsThis is one reason stars tend to form in clusters and not alone. These clusters flatten as they spin. After about a million years, their cores begin to fuse, creating protostars.

The Pleiades

ProtostarsProtostars start out relatively cool, but not for long. Material, called infall, continues to fall into the star as it grows. The release of gravitational energy is greater than the star’s heat energy at this stage of its development.

Stars forming from nebulae

ProtostarsImagine dropping a brick into a box of ping-pong balls. It releases kinetic energy. In a gas, this kinetic energy is turned into thermal energy thus heating the star more.

ProtostarsThis infall creates tremendous changes in the star’s brightness as well as, strangely, an outflow of gas. This gas is not released randomly, but focused into a pair of jets. Theses jets produce bipolar flows.

Red flow is Blue flow is Doppler shifted Doppler shifted

away from us toward us

Bipolar Flows Bipolar flows are important because they clear gas and dust away from the stars allowing us to see them.

Protostars: Variable Stars

Bipolar flows clear only part of the material that surrounds a new star. What remains continues to fall into the star adding energy. This random addition of energy affects the star’s luminosity and it changes depending on the volume of in-fall material. These variables are called T Tauri stars.

Blok Globules

Small, dark protostars that have not starting giving off visible light are called Blok globules for the astronomer who first described them, Bert Blok.

Stellar Mass Limits

Extremely small stars (< 0.1 MSun) are rarely seen. This is because they lack the mass to begin fusion.

Extremely large stars (>100 MSun) are rarely seen as well. Large stars quickly deplete their fuel and the immense radiation they create blows much of their fuel out into space.

Living On The Main Sequence

Stars living on the main sequence are stars that are mostly converting hydrogen into helium. The time a star resides on the main sequence is called its main-sequence lifetime. The rate at which a star burns its fuel is related to the star’s luminosity.

Living On The Main SequenceLike a car, the more horsepower you demand from it, the faster you burn gas.

How Long Do Stars Burn

With some simple math, you can calculate the approximate main-sequence lifetime of a star. The equation is:

t = 1010 (M/L) years, where M is mass and L is luminosity. So, how long will our star remain on the main sequence? Well…

How Long Do Stars Burn

Using the formula t = 1010 (M/L) years, then:

t = 1010 (1 MSun/1 LSun) =

t = 1010 (1)

t = 1010 or about 10 billion years.

Warm Up: 11/15/13

1. The magnitude of Vega is 2X that of Sun. Its luminosity is 4X that of the Sun. How long will it burn?

2. Define electron degeneracy pressure.

3. Define neutron degeneracy pressure.

4. Define main sequence lifetime.

How Long Do Stars Burn

How about Sirius whose luminosity is twenty times our Sun’s and whose mass is twice our Sun’s:

t = 1010 (MSun/LSun) =

t = 1010 (2/20)

t = 109 or about 100 million years.

Giant Stars: Leaving the Main Sequence

When a star has exhausted most of its hydrogen and begins burning helium (in the triple alpha process), it begins to cool. Cooling reduces the outward pressure (to about 1/10th) what it formerly exerted and the star contracts inward.

Giant Stars: Leaving the Main Sequence

This inward contraction increases the star’s core temperature and it expands as it fuses heavier elements. Stars can expand from 5 to 100 times their main sequence size depending on their mass. These stars are called red giants.

Red Giants

Degeneracy Pressure

The compression of the gas in a star as it collapses, makes the gas behave like no ordinary gas. Physics says that no two bodies can occupy the same space. As gases in the star are compacted to the subatomic level, their electrons are pressed toward their nuclei. They resist compaction past a point and begin to exert degeneracy pressure or electron degeneracy pressure.

Degeneracy Pressure

Because of the increased temperature in the star, its fusion begins to occur outside its core and in the star’s outer shell. This is called hydrogen shell burning.

Degeneracy in Low-Mass StarsThe degenerate star, unable to release energy into this super compacted gas gets hotter and hotter. The release of this energy reaches explosive levels in only minutes until it is released in the helium flash.

Degeneracy in Low-Mass Stars• Release of this energy allows the gas

inside the star to return to a more normal state, destroying degeneracy. The star shrinks, expands and reheats into a new phase called the yellow giant phase. The helium flash marks the end of the red giant stage of a low-mass star.

Degeneracy in High-Mass Stars

High-mass stars do not have a helium flash. Their additional mass creates enough heat with very little contraction to fuse helium. High-mass stars do not reach degeneracy. They simply expand from the red giant phase to the yellow giant phase with little upheaval.

Warm Up1. Do more stars form in isolation or in groups? Why?2. What is infall?3. Why is infall important to protostars?4. When kinetic energy is transferred to a super-

compacted gas, it turns into what?5. You see a planetary nebula through a telescope. What

do you know about it and its history?6. You see a red giant through a telescope. What can

you say about this star and its history? What can you not say about it?

7. You see a star in a telescope that has a bi-polar flow. What can you say about this star?

8. What is a T Tauri star and what affects its luminosity?9. How do you calculate the main sequence life of a star?10. Explain electron degeneracy, photodisintegration and

neutron degeneracy.

Phase TransitionThe transition in both high- and low-mass stars as they change between burning hydrogen to burning helium is a unstable time. In many cases these changes make a star pulsate. Regardless of their mass, many yellow phase stars swell and shrink rhythmically.

Phase Transition

The shrinking and swelling is due to heat trapped beneath the star’s outer layers. The outer layer expands, releasing the heat and the star begins to contract again.

Think about a boiling pot of water as an analogy.

Variable StarsAs we saw previously, not all stars have constant luminosity. Some stars, called variable stars, change brightness over time. The time between the intervals of maximum brightness is called the star’s period. The graph of the star’s brightening and darkening cycle is called its light curve.

Variable StarsVariables come in two types:

RR Lyrae- Have periods of hours, normally about half a day. Named for the first of its kind in the constellation Lyra.

Variable Stars

Cepheid- Have periods ranging from a day to several months. This class is named for the first star identified, Delta Cephei.

The Period-Luminosity LawObservation has shown that the more slowly a star pulsates, the more luminous it becomes. Why? A brighter star has a larger radius with more surface area to produce more light.

The Period-Luminosity LawThe outer layers of larger stars are held more loosely by gravity. This allows them to expand farther which takes longer, increasing the period.

Death of a Star

Death of a Low-mass Star

The evolutionary cycles of low-mass stars end the same way, as planetary nebulae. As their external pressure continue to diminish, gravity compacts the stellar core more and more. The star’s core super heats.

Death of a Low-mass Star

• This heat drives off the stars outer layers of gas effectively removing the star’s fuel source. What evolves is a white dwarf surrounded by a halo of gas called a planetary nebula.

Death of a High-mass StarThe evolutionary cycles of high-mass stars end in one of two ways: as neutron stars or as black holes.

Neutron Star Black Hole

Neutron Stars

Because of their incredible density, neutron stars create tremendously large magnetic fields. These magnetic fields shine across space like beacons. Neutron star also rotate at tremendous rates.

The PulsarThis rotation along with the magnetic beacon create a phenomenon called a pulsar. Because the neutron star’s axis of rotation and direction of emission are oblique, they cast their signals into space like giant light houses.

The Pulsar

It is difficult to estimate the actual number of pulsars that exist. We are only able to “see” the ones whose radiative beams point toward the Earth at some point in their rotation.

Black Holes

Scientists began to speculate about the possibility of the existence of black holes since Einstein presented his beliefs on space-time.

Black HolesEinstein, flying in the face of Newton, proposed that satellites orbit planets due to warps in space-time created by the gravitational effects of the bodies masses.

Warm Up

1. Define degeneracy.

2. Describe electron and neutron degeneracy!

3. In a Cepheid variable star, what is the relation between its period and its magnitude.

Black HolesEinstein theorized that if enough gravitational force were to be places in a small enough area that it could actually break or “tear” the very fabric of time. Theoretically, a black hole as a large gravitational force concentrated in an infinitely small area.

Black Holes

Black holes are the densest, most massive singular objects in the universe. Formed in one of three main processes, they exert so much gravitational force that nothing - not even light - can escape their pull. Since nothing can ever come out, it is called a hole. Since not even light nor other electromagnetic radiation can escape, it is called a black hole.

How Black Holes FormCurrent theory holds that black holes form in three main ways. The first is that if a star has more than nine solar masses when it goes supernova, then it will collapse into a black hole. The reason that a neutron star stops collapsing is the strong nuclear force, the fundamental force that keeps the center of an atom from collapsing.

How Black Holes FormHowever, once a star is this big, the gravitational force is so strong that it overwhelms the strong nuclear and collapses the atom completely. Now there is nothing to hold back collapse of the star, and it collapses into a point (or, in theory, a ring) of infinite density.

How Black Holes FormA second way for black holes to form is that, in some rare instances, two neutron stars will be locked in a binary relationship. Because of energy lost through gravitational radiation, they will slowly spiral in towards each other, and merge. When they merge, they will almost always form a black hole.

How Black Holes FormFinally, a third way was proposed by quantum cosmologist Stephen Hawking. He theorized that trillions of black holes were produced in the Big Bang, with some still existing today. This theory is not as widely accepted as the other two.

Classification of Black Holes

• A black hole is classified by the only three properties that it possesses: Mass, Spin, and Magnetic Field.

• Currently, there are only two recognized mass classes of black hole: Stellar and Supermassive. The stellar black holes are star-sized and range in the 10-100 solar mass range. The supermassive black holes are at the cores of every large galaxy, including our Milky Way. These range in the millions to even billions of solar masses.

Schwarzschild Black Hole• The simplest black hole has no spin and

no magnetic field. This is called a Schwarzschild black hole. To begin with, a Schwarzschild black hole has two main components - a singularity and an event horizon.

Schwarzschild Black HoleThe singularity is what is left of the collapsed star, and is theoretically a point of 0 dimension with infinite density but finite mass. The event horizon is a region of space that is the "boundary" of the black hole. Within it, the escape velocity is faster than light, so it is past this point that nothing can escape.

Schwarzschild Black Hole: Event Horizons

Nothing made of matter can survive the trip across the black hole’s event horizon and through. In general relativity, it is a general term for a boundary in space-time, defined with respect to an observer, beyond which events cannot affect the observer. Light emitted beyond the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again. A black hole is surrounded by an event horizon, for example. This means that an outside observer cannot be affected by anything inside the black hole.

Warm Up1. Describe the origin of a neutron star (hint: talk about

degeneracy).2. What is a pulsar? Do we see all the pulsars that are

out there?3. Are space and time related?4. How do black holes form?5. What are the three ways that scientists think black

holes may form?6. Black holes are classified by what three

characteristics?7. What is a singularity?8. What is an event horizon?9. If I’m riding on a photon (seriously) and I shine a

flashlight in front of me, how fast are the photons traveling that are coming from the flashlight?

10. Theoretically, can matter travel through a black hole? Why or why not?

Reissner-Nordstrøm Black Holes A step up is the Reissner-Nordstrøm black hole. It has the singularity and two event horizons. The outer event horizon is a boundary where time and space flip. This means that the singularity is no longer a point in space, but one in time. The inner event horizon flips space-time back to normal.

Kerr Black Holes One that has both a magnetic field and spin is called a Kerr black hole. A Kerr black hole adds another feature to the anatomy - an ergosphere. The ergosphere resides in an ellipsoidal region outside the outer event horizon.

Kerr Black Holes

The ergosphere represents the last stable orbit, and the outer boundary is called the static limit. Outside of it, a hypothetical spaceship could maneuver freely. Inside, space-time is warped in such a way that a spaceship would be drawn along by its rotation.

Kerr Black Holes: The Naked Singularity

An interesting point that comes up in the case of a spinning black hole is that of the naked singularity. The faster the black hole rotates, the larger the inner event horizon becomes, while the outer event horizon remains the same size. They become the same size when the rotational energy equals the mass energy of the black hole. If the rotational energy were to become more than the mass energy, the event horizons would vanish and what would be left is a "naked singularity" - a black hole whose only part is the singularity.

Kerr Black Holes

• Yet another distinguishing feature of the Kerr black hole is that, since it rotates, the 0-D point that is the singularity in the Schwarzschild and Reissner-Nordstrøm black hole is spun into a ring of 0 thickness. Interesting theoretical physics can take place around this ring singularity. One consequence is that nothing can actually fall into it unless it approaches along a trajectory along the ring's side. Any other angle and the ring actually produces an antigravity field that repels matter.

Features of Black HolesTwo other features can characterize a black hole - the accretion disk and jets.

An accretion disk is matter that is drawn to the black hole. In rotating black holes and/or ones with a magnetic field, the matter forms a disk due to the mechanical forces present. In a Schwarzschild black hole, the matter would be drawn in equally from all directions, and thus would form an omni-directional accretion cloud rather than disk.

Features of Black Holes

• The matter in accretion disks is gradually pulled into the black hole. As it gets closer, its speed increases, and it also gains energy. Accretion disks can be heated due to internal friction to temperatures as high as 3 billion K, and emit energetic radiation such as gamma rays. This radiation can be used to "weigh" the black hole. By using the doppler effect, astronomers can determine how fast the material is revolving around the black hole, and thus can infer its mass.

Features of Black HolesJets form in Kerr black holes that have an accretion disk. The matter is funneled into a disk-shaped torus by the hole's spin and magnetic fields, but in the very narrow regions over the black hole's poles, matter can be energized to extremely high temperatures and speeds, escaping the black hole in the form of high-speed jets.

Finding Black Holes

• No black hole has actually been imaged in a telescope. Actually, this is in itself impossible because, simply by definition, one cannot see "nothing." A black hole can only be spotted by observing how the material around it acts. Through this method, astronomers have observed many black holes; they usually are found in the center of galaxies, and some believe that every galaxy harbors a black hole in its center.

Hypothetical Journey Through a Black Hole

• What would happen if you were to fall into a black hole? As the you approach the black hole, your watch would begin to run slower than the watch of your colleagues on the spaceship. Also, your comrades notice that you begin to take on a reddish color. This is due to the warping of space in the vicinity of the hole. Then, just before you "enter" the hole (pass through the outer event horizon), your friends would see you apparently "frozen" there, just outside the event horizon and to them, your watch would have stopped (if they could observe it). They would never see you enter the hole, because at that distance from the singularity, an object must travel at the speed of light to maintain its distance. Thus your dim, red image would stay frozen in their eyes for as long as the hole exists.

• However, from your vantage point, as you enter the black hole, nothing has changed. As you look "out" of the hole, the universe still looks relatively normal. However, you are drawn towards the singularity, and cannot escape its grasp. At this point, modern physics does not know what would happen. The most likely outcome is that you are compacted into a miniscule size upon the singularity.

• However, you would not actually survive the fall into the hole. The immense warping of space around the hole would cause a “spaghetti” effect - you would be pulled apart because your feet (assuming they went feet first) would be far greater than the force on your head, and they you would be pulled as one pulls dough into a rope. This would be rather unpleasant, as well as fatal.

White Holes?The idea of a white hole is the opposite of a black hole, and is entertained more in science fiction than in actual science journals. Some believe it is the "other side" of a black hole. It is theorized to spew matter and energy out. A flaw in this theory, as many scientists have noted, is that the matter ejected from the white hole would accumulate in the vicinity of the hole, and then collapse upon itself, forming a black hole.

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