The Life and Death of Stars

What is a Star?

• A star is a sphere of plasma gas that fuses atomic nuclei in its core and so emits light

• The name star can also be tagged onto a body that is somewhere on the star evolution process e.g. proto-star, neutron star.

Stellar Evolution• Stars begin as a nebula of gas and dust.• Protostar- gravity pulls it together, temperature rises.• Main Sequence- Temperature, color, luminosity, & size

are directly proportional to the mass.• Red Giant or Supergiant- large, luminous, low temp.• Ending is based on initial mass and can be either:

– A Planetary nebula- outer layers of star blown off causing a:

White Dwarf- small, dim, white star with high temperature. OR/

– A Supernova- star explodes leaving a:

Neutron Star (Pulsar is one type)- only has neutrons, or a

Black Hole- so small and dense that even light cannot escape it.

Star Birth

Stages of Star Birth

• Our goals for learning

• Where and Why do stars form?

• What slows the contraction of a star-forming cloud?

• How does nuclear fusion begin in a newborn star?

Where and Why do stars form?

Star-Forming Clouds• Stars form in dark

clouds of dusty gas in interstellar space

• The gas between the stars is called the interstellar medium

Molecular Clouds

• Most of the matter in star-forming clouds is in the form of molecules (H2, CO,…)

• These molecular clouds have a temperature of 10-30 K and a density of about 300 molecules per cubic cm

Fragmentation of a Cloud

random motions of different sections of the cloud cause it to become lumpy

Each lump in which gravity can overcome pressure can become a star

Gravity within a contracting gas cloud becomes stronger as the gas becomes denser

What slows the contraction of a star-forming cloud?

Trapping of Thermal Energy

• As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infrared and radio photons to escape

• Thermal energy then begins to build up inside, increasing the internal pressure

• Contraction slows down, and the center of the cloud fragment becomes a protostar

Conservation of Angular Momentum

• Behaves just like solar system formation,

• The rotation speed of the cloud from which a star forms increases as the cloud contracts

• Heating, spinning and flattening occur

Examples of Proplyds in the Orion Nebula

In the animation's visible images, how do the protostars appear?

A.They are bright points of light.

B.They are bright fuzzy features.

C.They are faint fuzzy features.

D.They do not appear in the image.

How does nuclear fusion begin in a newborn star?

From Protostar to Main Sequence

• Protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion

• Contraction must continue until the core becomes hot enough for nuclear fusion

• Contraction stops when the energy released by core fusion balances energy radiated from the surface—the star is now a main-sequence star

Birth Stages on a Life Track

• Life track illustrates star’s surface temperature and luminosity at different moments in time

Assembly of a Protostar

• Luminosity and temperature grow as matter collects into a protostar

2. Convective Contraction

• Surface temperature remains near 3,000 K while convection is main energy transport mechanism

3. Radiative Contraction

• Luminosity remains nearly constant during late stages of contraction, while radiation is transporting energy through star

4. Self-Sustaining Fusion

• Core temperature continues to rise until star arrives on the main sequence

Life Tracks for Different Masses• Models show that

Sun required about 30 million years to go from protostar to main sequence

• Higher-mass stars form faster

• Lower-mass stars form more slowly

Open cluster: A few thousand loosely packed stars

What is the initial energy source for a contracting protostar?

A.pressure

B.gravitational potential energy

C.fusion

D.kinetic energy

What have we learned?

• Where and why do stars form?– Stars form in dark, dusty clouds of molecular

gas with temperatures of 10-30 K– Stars form in clouds that are massive enough

for gravity to overcome thermal pressure (and any other forms of resistance)

– Such a cloud contracts and breaks up into pieces called proplyds that go on to form stars

What have we learned?

• What slows the contraction of a star-forming cloud?– The contraction of a cloud fragment slows when

thermal pressure builds up because infrared and radio photons can no longer escape

• How does nuclear fusion begin in a newborn star?

� Nuclear fusion begins when contraction causes the star’s core to grow hot enough for fusion

Forces Controlling Stars

• Our goals for learning• How does nuclear fusion occur in stars?

• What is thermal pressure?

• What is degeneracy pressure?

• How do these control the mass of a star?

How does nuclear fusion occur in stars?

High temperatures cause particles to smash together. Enables nuclear fusion to happen in the core

Sun releases energy by fusing four hydrogen nuclei into one helium nucleus

Total mass is 0.7% lower

Stable Star• Star’s core fuses hydrogen into helium,

releasing energy.

• The energy causes gas expansion pushing outward against the gravity pulling inward. This is called thermal pressure.

• When the pressures balance, the star is in equilibrium and burns steadily.

Stellar Thermostat

• The balancing act between thermal expansion pushing out and gravity contracting inward, forces the temperature to stay constant.

Stellar Thermostat

Thermal pressure drops, gravity gains =>core contracts and heats back up

Thermal pressure increases over gravity.=>core expands, which causes core to cool down

If temperature falls because fusion rate drops :

If temperature rises because fusion rate increases:

Pauli Exclusion Principal:

Laws of quantum mechanics prohibit two electrons from occupying same state in same place

When star run out of fuel other atomic forces becomes important

•Electrons can never occupy the same state at the same time.

•As a result, they cannot be completely crushed without being forced into each other, so they exert a force outward acting against gravity.

•This is called degeneracy pressure.

Electron Degeneracy Pressure

Thermal Pressure:

Depends on heat content

The main form of pressure in most stars

Degeneracy Pressure:

Particles can’t be in same state in same place

Doesn’t depend on heat content

How do these Forces Control the Mass of a Star?

– Low mass means low gravitational force so electrons are not crushed and degeneracy pressure is effective.

– Fusion will not begin if contraction stops before the core temperature rises above 107 K.

– At very high mass, high gravitational force causes extreme amounts of fusion and very high thermal and radiation pressure.

– If radiation pressure is high, then star will be large. Too high and the star will explode

Limiting the Mass of Stars

> 100 MSun

Stable stars

< 0.08 MSun

(MSun is the mass of the Sun)

• Radiation pressure so great, star blows apart

• Gravity and heat overcome degeneracy pressure in the core, core fuses - stars shine!

• Degeneracy pressure supports the core of the star, no fusion!

Brown Dwarfs• Degeneracy pressure

halts the contraction of objects with <0.08MSun before core temperature become hot enough for fusion

• Starlike objects not massive enough to start fusion are brown dwarfs

What have we learned?

• How does nuclear fusion occur in stars?– The core’s extreme temperature and density are

just right for nuclear fusion of hydrogen to helium through the proton-proton chain

– Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

What have we learned?

• What is thermal pressure?– hot gases moving more, cause expansion, which

exerts an outward pressure inside a star

• What is degeneracy pressure?– electrons cannot be at the same state at the

same time and exert a repulsion force to prevent it from happening.

A star's life is a struggle between ________ wanting to crush it, and ________ wanting to expand it.

A.gravity, nuclear energy

B.gravity, convection

C.gas pressure, radiation

D.nuclear energy, radiation

Lives in the Balance

• Our goals for learning

• How does a star’s mass affect its life and death?

Stellar Mass and Life Path

• High mass stars (> 8 MSun) end with an explosion

• Lower mass stars < 4 MSun) end by material loss

Life as a Low-Mass Star

• Our goals for learning• What are the final life stages of a low-mass

star?

• How does a low-mass star die?

Stellar Old-Age

• A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.

• It starts to leave the main sequence once the core is mostly made of helium

• Observations of star clusters show that a star becomes larger, redder, and brighter after its time on the main sequence is over.

The Changing Star• He is inert, so is generating no

thermal pressure. => Core contracts

• H begins fusing in a shell around the core.

• Shell fusion is even more fast burning than before. => brightness increases

• Thermal pressure increases. => Star expands into red giant

Feedback Loop• degeneracy pressure not

thermal pressure now operates in the core.

• The increasing fusion rate in the shell does not stop core contraction => Causes even more shell fusion, brightness and star expansion.

• This is a runaway process until the core reaches a critical mass of helium.

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion.

Hot enough for Helium fusion : three He nuclei combine to make carbon

Helium Flash

• Core temperature rises rapidly when helium fusion begins

• Helium fusion rate skyrockets until thermal pressure takes over and expands core again.

• The core has switched back on and the star contracts and burns evenly.

Helium burning stars neither shrink nor grow because core thermostat is temporarily fixed.

Life Track after Main Sequence

• Changes in brightness or temperature show up on the H-R Diagram

• Dying stars form their own branch away from the main sequence line

Life Track after Helium Flash

• Models show that a red giant should shrink and become less luminous after helium fusion begins in the core

When a low-mass star can no longer fuse hydrogen into helium in its core_______

A. hydrogen fusion will begin in a shell around the core.

B. helium will begin to fuse into carbon in the core.

C. all fusion reactions stop and the star becomes a white dwarf.

D. the outer layers of the star blow off in a slow but massive stellar wind.

As the helium core collapses; the luminosity _________,

and the temperature ________ .

A.increases, increases

B.increases, decreases

C.decreases, stays the same

D.stays the same, increases

How does a low-mass star die?

Double Shell Burning• The core fuses until a

critical mass of inert carbon is in the core and helium fusion stops.

• He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer.

• fusion rate spikes in a series of thermal pulses.

Planetary Nebulae

• A final pulse ejects the H and He into space as a planetary nebula

• The core left behind becomes a white dwarf

Planetary Nebulae

The apparent shape of a planetary nebula depends on the angle at which we see it.

Side view Angled view End view

End of Fusion

• Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements (some He fuses to C to make oxygen)

• Degeneracy pressure supports the white dwarf against gravity

Low-Mass Star Summary

1. Protostar

2. Main Sequence:H fuses to He in core

3. Red Giant: H fuses to He in shell around He core

4. Helium Core Burning: He fuses to C in core while H fuses

to He in shell

5. Double Shell Burning: H and He both fuse in shells

6. Planetary Nebula leaves white dwarf behindNot to scale!

Life Track of a Sun-Like Star

• Helium burning stars form a horizontal branch

• Double shell burning stars are called Assymtotic Branch Giants (ABGs)

What have we learned?

• How does a star’s mass affect its life?– Mass affects life length and how explosive the stars

death becomes.

• What are the life stages of a low-mass star?– H fusion in core (main sequence)– H fusion in shell around contracting core (red giant)– He fusion in core (horizontal branch)– Double-shell burning (red giant)

• How does a low-mass star die?– Ejection of H and He in a planetary nebula leaves

behind an inert white dwarf

Life as a High-Mass Star

• Our goals for learning• What are the final life stages of a high-mass star?

• How does a high-mass star die?

What are the life stages of a high-mass star?

High-Mass Old Age

• Late life stages of high-mass stars are similar to those of low-mass stars:– Hydrogen core fusion (main sequence)– Hydrogen shell burning (giant)– Helium core fusion (supergiant)

– Feed-back loop continues with carbon core fusion

Helium Capture

• stars with >4MSun have core temperatures that allow helium to fuse to make heavier elements

Advanced Nuclear Burning

• Core temperatures in stars with >8MSun

allow fusion of elements as heavy as iron

Multiple Shell Burning• Advanced nuclear

burning proceeds in a series of nested shells

• This is a Red Supergiant

Life Tracks of High Mass Stars

How does a high-mass star die?

• Iron is a dead end for fusion because nuclear reactions involving iron do not release energy

• (Fe has lowest mass per nuclear particle)

• In an >8MSun star, iron builds up in core until degeneracy pressure is overwhelmed.

• The core suddenly collapses, then core bounce causes a supernova explosion

Type II Supernova Explosion

• Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos

• Neutrons collapse to the center, forming a neutron star

Supernova Remnant

• Energy released by collapse of core drives outer layers into space

• Heavier elements are created in the explosion.

Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au and U

Supernova Remnant

• Energy released by collapse of core drives outer layers into space

• The Crab Nebula is the remnant of the supernova seen in A.D. 1054

Neutron stars are barely as large as a small town

Life Stages of High-Mass Star

1. Protostar

2. Main Sequence:H fuses to He in core

3. Red Supergiant: H fuses to He in shell around He core

4. Helium Core Burning: He fuses to C in core while H fuses

to He in shell

5. Multiple Shell Burning: Many elements fuse in shells

6. Supernova leaves neutron star behind

Not to scale!

What have we learned?

• What are the life stages of a high-mass star?– They are similar to the life stages of a low-mass

star, but go further– Higher masses produce higher core temperatures

that enable fusion of heavier elements

• How does a high-mass star die?– Iron core collapses, leading to a supernova

How does a star’s mass determine its life story?

Role of Mass• A star’s mass determines its entire life story

because it determines its core temperature

• High-mass stars with >8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions

• Low-mass stars with <2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs

• Intermediate mass stars can make elements heavier than carbon but end as white dwarfs

The Mass of Stellar Corpses• <1.44 Solar Masses, Degeneracy pressure

maintains the corpse => White Dwarf

• 1.44 - 3 Solar Masses, Degeneracy pressure overwhelmed, Plasma becomes neutrons =>Neutron Star

• >3 Solar Masses, Neutron pressure overwhelmed, star collapses to a point =>Black Hole!

What have we learned?• How does a star’s mass determine its life story?

– A star’s mass determines its entire life story because it determines its core temperature and how fast a star uses its fuel.

– Low-mass stars have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs

– High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.

– Intermediate mass stars can make elements heavier than carbon but mostly end as white dwarfs.