The Birth, Life and Death of Stars

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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.

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Stellar “Forest”

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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

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Star Birth• Cold gas clouds

contract and form groups of stars.

• When O and B stars begin to shine, surrounding gas is ionized

• The stars in a cluster are all about the same age.Prasad 5U6_StarLife

Cloud Collapses to Form Stars

Radiation from protostars arises from the conversion of gravitational energy to heat.

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Pre-Main Sequence Contraction• Protostars

contract until core reaches HHe fusion temperature.

• Low mass protostars contract more slowly.

• Nature makes more low-mass stars than high-mass stars.

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Hydrogenfuel

Hydrogenburning core

Helium“ash”

Anatomy of a Main Sequence Star

shell

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

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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)

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H, He, C burningH, He, C burningSince 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 processPrasad 12U6_StarLife

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).

Indistinguishable particles are not allowed to stay in the same quantum state.

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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.Prasad 16U6_StarLife

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 17U6_StarLife

Sun’s life

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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.

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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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Life of a 15 solar mass star

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Stellar Evolution in a Nutshell

Mass controls the evolution of a star!

0.5 MSun < M < 8 MSun M > 8 MSun

Mcore < 3MSunMcore > 3MSun

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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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The evolution of 10,000 stars

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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?• It was lost in a supernova explosion• It flows outward in a planetary nebula• It is converted into energy by nuclear

fusion• The core of the Sun gravitationally

collapses, absorbing the massPrasad 30U6_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 starsPrasad 31U6_StarLife