The Birth, Life and Death of Stars. How can we learn about the lives of stars when little changes...

The Birth, Life and Death of Stars

How can we learn about the lives of stars when little changes except on timescales much longer than all of human history?

Suppose you had never seen a tree before, and you were given one minute in a forest to determine the life cycle of trees. Could you piece together the story without ever seeing a tree grow?

This is about the equivalent of a human lifetime to the lifetime of the Sun.

Stellar “Forest”

Imagine an enormous cloud of gas and dust many light-years across. Gravity, as it always does, tries to pull the materials together. A few grains of dust collect a few more, then a few more, then more still. Eventually, enough gas and dust has been collected into a giant ball that, at the center of the ball, the temperature (from all the gas and dust bumping into each other under the great pressure of the surrounding material) reaches 15 million degrees or so.

A wondrous event occurs.... nuclear fusion begins and the ball of gas and dust starts to glow. A new star has begun its life in our Universe.

So what is this magical thing called “nuclear fusion” and why does it start happening inside the ball of gas and

dust? It happens like this.....

As the contraction of the gas and dust progresses and the temperature reaches 15 million degrees or so, the pressure at the center of the ball becomes enormous. The electrons are stripped off of their parent atoms, creating a plasma.

Eventually, they approach each other so fast that they overcome the electrical repulsion that exists between their protons. The nuclei crash into each other so hard that they stick together, or fuse. In doing so, they give off a great deal of energy.

This energy from fusion pours out from the core, setting up an outward pressure in the gas around it that balances the inward pull of gravity. When the released energy reaches the outer layers of the ball of gas and dust, it moves off into space in the form of electromagnetic radiation. The ball, now a star, begins to shine.

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Watching stars being born

The Bubble Nebula

Here you can see the old dust and gas being blown away by the heat of the new star.

Molecular cloud

Molecular cloud

Cool molecular cloudsgravitationally collapseto form clusters of stars

Stars generatehelium, carbonand iron throughstellar nucleosynthesis

The hottest, mostmassive stars in thecluster supernova –heavier elements areformed in the explosion.

New (dirty) molecularclouds are leftbehind by thesupernova debris.

The Stellar Cycle

12

Hydrogenfuel

Hydrogenburning core

Helium“ash”

Anatomy of a Main Sequence Star

shell

Throughout their lives, stars fight the Throughout their lives, stars fight the inward pull of the force of gravity. It is inward pull of the force of gravity. It is only the outward pressure created by only the outward pressure created by the nuclear reactions pushing away the nuclear reactions pushing away from the star's core that keeps the star from the star's core that keeps the star “intact”. But these nuclear reactions “intact”. But these nuclear reactions require fuel, in particular hydrogen. require fuel, in particular hydrogen. Eventually the supply of hydrogen runs Eventually the supply of hydrogen runs out and the star begins its demise.out and the star begins its demise.

Up the red giant branchUp the red giant branchAs hydrogen in the core is being used up, it starts to contract, raising temperature in the surrounding. Eventually, hydrogen will burn only in a shell. There is less gravity from above to balance this pressure. The Sun will then swell to enormous size and luminosity, and its surface temperature will drop, a red giant.

Sun todaySun in ~5 Gyr

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Helium fusion at the center of a Helium fusion at the center of a giantgiant

While the exterior layers expand, the helium core While the exterior layers expand, the helium core continues to contract, while growing in mass, and continues to contract, while growing in mass, and eventually becomes hot enough (100 million Kelvin) for eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbonhelium to begin to fuse into carbon

Carbon ash is deposited in core and eventually a helium-Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion.shell of hydrogen undergoing nuclear fusion.

For a star with M< 1 Msun, the carbon core never gets hot For a star with M< 1 Msun, the carbon core never gets hot enough to ignite nuclear fusion.enough to ignite nuclear fusion.

In very massive stars, elements can be fused into Fe.In very massive stars, elements can be fused into Fe.

The Sun will expand and cool again, becoming a red giant. Earth will be engulfed and vaporized within the Sun. The Sun’s core will consist mostly of carbon.•Red Giants create most of the Carbon in the universe (from which organic molecules—and life—are made)

Death of a StarDeath of a Star When all fuel runs out, the core collapsesWhen all fuel runs out, the core collapses Outer regions of star explode outwards: SupernovaOuter regions of star explode outwards: Supernova SN shine more brightly than a galaxy for a few hours/daysSN shine more brightly than a galaxy for a few hours/days

H, He, C burningH, He, C burning

Since fusing atomic nuclei repel each Since fusing atomic nuclei repel each other because of their electric other because of their electric charge, the order of easiest to charge, the order of easiest to hardest to fuse must be hardest to fuse must be

(1)(1) H, He, CH, He, C

(2)(2) C, He, HC, He, H

(3)(3) H, C, HeH, C, He

(4)(4) He, C, HHe, C, H Carbon-triple alpha process

The Sun’s Path

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Planetary Nebula Formation

• When the Red Giant exhausts its He fuel– the C core collapses

white dwarf– No fusion going on inside …

this is a dead star.

• He & H burning shells overcome gravity– the outer envelope of the

star is blown outward a planetary nebula

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What holds the white dwarf from collapsing?

• As matter compresses, it becomes denser.• 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).

What holds the white dwarf from collapsing?

• The resulting outward pressure which keeps the electrons apart is called electron degeneracy pressure – this is what balances the weight.

• Only if more energy drives the electrons into higher energy states, can the density increase.

• Adding mass can drive electrons to higher energies so star shrinks.

• At 1.4 solar masses—the Chandrasekhar Limit—a star with no other support will collapse, which will rapidly heat carbon to fusion temperature.

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On-line Lessons: The Birth and Death of Stars

The end of a Sun-like star

• The outer parts of the star (that formed the Red Giant) then drift off into space and cool down making a Planetary Nebula.

• Planetary nebulae have nothing to do with planets, of course, they just look a bit like them in small telescopes!

• Here you can see a planetary nebula called M57 with its White Dwarf in the middle.

Image from the Liverpool Telescope

1 teaspoon = 1 elephant

WD has a size slightly less than that of the earth. It is so dense, one teaspoon weights 15 tons! WD from an isolated star will simply cool, temperature dropping until it is no longer visible and becomes a “black dwarf”.Prasad 27U6_StarLife

What is a planetary nebula?

(1) A large swarm of planets surrounding a star.

(2) A disk of gas and dust around a young star.

(3) Glowing gas in Earth’s upper atmosphere.

(4) Ionized gas around a white dwarf star.

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What is a planetary nebula?

(1) A large swarm of planets surrounding a star.

(2) A disk of gas and dust around a young star.

(3) Glowing gas in Earth’s upper atmosphere.

(4) Ionized gas around a white dwarf star.

What is left after a Supernova?• Because the star was so big, the collapse

does not stop even with a White Dwarf, but an even more dense object called a Neutron Star is made.

• The density of a Neutron star is about 1x1018 kg/m3 (that is 1,000,000,000,000,000,000!)

• Sometimes the collapse cannot stop at all and a Black Hole is made, from which not even light can escape!

• The debris of the explosion is blown away and forms a glowing cloud called a Supernova Remnant.

The Crab Supernova

Remnant

The lead-up to disaster

• In massive stars (M > 8 Msun), elements can be fused into Fe.

• 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.

• Now the electron degeneracy pressure disappears.

• What comes next … is core collapse.

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• The core implodes, but no fuel there, so it collapses until neutron degeneracy pressure kicks in.

• Core “bounces” when it hits neutron limit; huge neutrino release; unspent fuel outside core fuses…

• Outer parts of star are blasted outward.• A tiny “neutron star” or a black hole remains at the

center.

Supernova! Type II (Core-Collapse)

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Supernova 1987a before/after

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Production of Heavy Elements

(There is evidence that the universe began with nothing but hydrogen and helium.)

• To make elements heavier than iron extra energy must be provided.

• Supernova temperatures drive nuclei into each other at such high speeds that heavy elements can be made.

• Gold, Silver, etc., -- any element heavier than iron, were all made during a supernova.

We were all once fuel for a stellar furnace. Parts of us were formed in a supernova!

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May 2006April 2004

Belinda Wilkes

We are made of stardust!

Stellar Evolution in a Nutshell

Mass controls the evolution of a star!

0.5 MSun < M < 8 MSun M > 8 MSun

Mcore < 3MSunMcore > 3MSun

The H-R diagram1. Which of these star is the

hottest?2. What are Sun-like stars

(0.5 Msun < M < 8 Msun) in

common?3. What about red dwarfs

(0.08 Msun < M < 0.5 Msun) ?

4. Where do stars spend most of their time?

5. Which is the faintest? the sun, an O star, a white dwarf, or a red giant?

O

Stars with M < 0.08 Msun Brown dwarf (fusion never starts) Answers: 1. O star, 2. end as a WD, 3. no RG phase,

lifetime longer than the age of the Universe, 4. MS, 5. WD

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If we came back in 10 billion years, the Sun will have a remaining mass about half of its current

mass. Where did the other half go?

1. It was lost in a supernova explosion

2. It flows outward in a planetary nebula

3. It is converted into energy by nuclear

fusion

4. The core of the Sun gravitationally

collapses, absorbing the massPrasad 40U6_StarLife

If we came back in 10 billion years, the Sun will have a remaining mass about half of its current

mass. Where did the other half go?

1. It was lost in a supernova explosion

2. It flows outward in a planetary nebula

3. It is converted into energy by nuclear

fusion

4. The core of the Sun gravitationally

collapses, absorbing the massPrasad 41U6_StarLife

A star cluster containing _____ would be MOST likely to be a few

billion years old.

(1) luminous red stars

(2) hot ionized gas

(3) infrared sources inside dark clouds

(4) luminous blue stars

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A star cluster containing _____ would be MOST likely to be a few

billion years old.

(1) luminous red stars

(2) hot ionized gas

(3) infrared sources inside dark clouds

(4) luminous blue stars

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Birth and Death of Stars - Summary

Collapsing cloud

A new star

Red Giant Massive Massive

starsstars

White Dwarf and Planetary

Nebula

Supernova Supernova Remnant and Remnant and Neutron StarNeutron Star

Sun-like Sun-like starsstars