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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?

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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.


the stellar cycle

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

The Stellar Cycle

New (dirty) molecularclouds are leftbehind by thesupernova debris.

Cool molecular cloudsgravitationally collapseto form clusters of stars

Molecular cloud

Stars generatehelium, carbonand iron throughstellar nucleosynthesis


star birth
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.



Cloud Collapses to Form Stars

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

pre main sequence contraction
Pre-Main Sequence Contraction
  • Protostars contract until core reaches HHe fusion temperature.
  • Low mass protostars contract more slowly.
  • Nature makes more low-mass stars than high-mass stars.




Anatomy of a Main Sequence Star

Hydrogenburning core




up the red giant branch
Up the red giant branch

As 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 in ~5 Gyr

Sun today


helium fusion at the center of a giant
Helium fusion at the center of a giant
  • While the exterior layers expand, the helium core continues to contract, while growing in mass, and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon
  • Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion.
  • For a star with M< 1 Msun, the carbon core never gets hot enough to ignite nuclear fusion.
  • 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)


h he c burning

Carbon-triple alpha process

H, He, C burning

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

  • H, He, C
  • C, He, H
  • H, C, He
  • He, C, H


the sun s path
The Sun’s Path


planetary nebula formation
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


what holds the white dwarf from collapsing
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.


what holds the white dwarf from collapsing16
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.



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”.

1 teaspoon = 1 elephant


sun s life
Sun’s life


what is a planetary nebula
What is a planetary nebula?
  • A large swarm of planets surrounding a star.
  • A disk of gas and dust around a young star.
  • Glowing gas in Earth’s upper atmosphere.
  • Ionized gas around a white dwarf star.


the lead up to disaster
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.
supernova type ii core collapse
Supernova! Type II (Core-Collapse)
  • 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.


production of heavy elements
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!


stellar evolution in a nutshell

0.5 MSun < M < 8 MSun

M > 8 MSun

Mcore > 3MSun

Mcore < 3MSun

Mass controls the evolution of a star!

Stellar Evolution in a Nutshell


the h r diagram


The H-R diagram

Which of these star is the hottest?

What are Sun-like stars (0.5 Msun < M < 8 Msun) in common?

What about red dwarfs (0.08 Msun < M < 0.5 Msun) ?

Where do stars spend most of their time?

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

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


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 mass


a star cluster containing would be most likely to be a few billion years old
A star cluster containing _____ would be MOST likely to be a few billion years old.
  • luminous red stars
  • hot ionized gas
  • infrared sources inside dark clouds
  • luminous blue stars