February 16, 2009, jeb

Outline

• Interstellar Medium
• Star Formation
• Stellar Evolution
• Close Binaries
• Neutrinos
• Hertzsprung-Russell Diagram

Interstellar Medium


Total mass about equivalent to stars in Milky Way
99% of the mass is gas
1% of the mass is dust
average temperature about 100 K

Gas
• about 1 atom per cubic centimeter
  • compared to Earth's atmosphere of 3 x 10¹⁹ per cubic cm
• 90% (by number) hydrogen
• 10% (by number) helium
• small amounts of other atoms (C,O,N,Ca,Na.....)
Dust
• 1000 dust particles / km³
• 1 dust particle / 10¹² atoms
Interstellar reddening
Extinction
Polarization
HI
HII
21 cm line
. . . . spin-flip in hydrogen

Nebulae
• Dark (or Absorption Nebulae)
  • Horsehead Nebula
• Reflection Nebulae
  • Pleiades
• Emission Nebulae
  • Typically O- and B-type stars heating their surroundings
  • Orion Nebula
Molecular Clouds
• cold regions -> molecules can form as atoms stick together
• densities are often high, as great as 10⁶/cm³
• dominant molecule H₂
• But other molecules are useful due to their distinctive lines
  • like OH, H₂O, NH₃, H₂CO, CO, HCN, etc.

Temperature of gas determines whether it will be:

molecular cloud (coldest): about 20 K

HI region (warmer): about 100 K

HII region (warmest): about 10,000 K

The Births of Stars

Birth - cold, dark clouds of interstellar gas and dust collapse - PROTOSTAR - IR radiation

At this stage of evolution (IR emission) the protostar is known as a T Tauri star


Clouds are about 90% hydrogen, 9% helium, less than 1% other (by number of atoms)

Collapse is initiated by SHOCK WAVES from:
• stellar wind of nearby heavy stars (O, B)
• supernova explosion

During collapse, gravitational energy heats globules from a few degrees K to millions

Eventually, THERMONUCLEAR FUSION begins (at about 10 million K), and hydrostatic and thermal equilibrium are established

Heaviest stars collapse fastest
• 15 M_SUN --> 10⁵ years
• 2 M_SUN --> 10⁷ years

New star formation is associated with
• Open or galactic clusters
• OB associations
• HII regions (HII is ionized hydrogen)
Example: Orion Nebula and Orion Molecular Cloud



Stellar association

Loosely bound group of new-born stars


Nebulae:
• Absorption nebula
  • Cloud of gas and dust which obscures more distant light
    • Horsehead nebula
• Reflection nebula
  • Cloud of dust illuminated by stars (bluish)
    • Pleiades
• Emission nebula
  • Glowing from excitation caused by nearby stars
    • Orion Nebula



Main sequence stars begin. after the formation stage, on the ZAMS line

ZAMS = zero-age main sequence




Brown Dwarf

If the collapsed mass is less than 0.08 M_SUN, it never gets hot enough for thermonuclear fusion --> failed star

Evolution of stars

Different thermonuclear processes are possible as temperature increases



temperature		process
8 x 106 K		proton-proton chain
20 x 106 K		CNO cycle
100 x 106 K		triple alpha
600 x 106 K		carbon-helium fusion
109 K		carbon burning




Stars spend most of their lives on the main sequence

CORE HYDROGEN BURNING




Main sequence phase ends when the CORE HYDROGEN BURNING ends

All core hydrogen fused into helium




Core cools, shrinks, heats

SHELL HYDROGEN BURNING begins

Star heats and expands -> Red Giant

• subgiant branch of HR diagram
• red-giant branch of HR diagram

Core is hotter, surface cooler, surface larger --> brighter




HELIUM BURNING

When core reaches 100,000,000 K -> Helium fuses to carbon

triple alpha process




HELIUM FLASH

Runaway helium fusion initially for stars with mass less than about 3 M_SUN




HORIZONTAL BRANCH

Post-helium flash stars on the H-R diagram




Ages of Star Clusters

Assume that the stars in a cluster are born at the same time

H-R diagram "turn-off point" tells age

Young clusters - metal rich (Population I)

• Open or galactic cluster - recently born stars

Old clusters - metal poor (Population II)

• Globular clusters - thousands of very old stars




---

Death of Lightweight stars (less than 3 M_SUN)

Hydrogen Core ----> Helium Core ------> Carbon
(15,000,000 K) .......... (10⁸ K) .....



Red giant

Very bright, enlarged star

• Betelgeuse ("Beetle juice" in Orion)




Helium flash

Runaway helium fusion in the core of a low mass star

red supergiant -

• radius of Mars' orbit
• bright as 10,000 suns




Planetary nebula (25-60% of the star is ejected)

white dwarf may soon appear at the center




White dwarf -

• spent low-mass star
• cools slowly
• no longer generates energy




Chandrasekhar Limit - less than 1.4 M_SUN

Degenerate electron pressure

Density = 10⁹ kg/m³

• reference: water = 1000 kg/m³

Radius(1 M_SUN) = 4200 km (a little smaller than the Earth)

Very hot, very faint

Example: Sirius B

Hubble Space Telescope has studied white dwarfs in M4




Black dwarf

After the white dwarf cools and dims, it becomes a black dwarf

• A cold, dense, burned-out ember in space

Close Binaries

STARS SEPARATED BY FEW STELLAR DIAMETERS

CRITICAL SURFACE - figure eight-shaped boundary around star defining gravity domain. Two lobes called Roche lobes

As a Star grows old, it may expand to become a red giant
If matter escapes the Roche lobe, it may fall onto the companion star, resulting in Mass Transfer
• Nova
  • Mild Stellar Explosion
  • Star with sudden outburst of energy
  • degenerate partner collects hydrogen from red giant, eventually ignites
  • Brightness increases as much as 10,000 fold or more
  • Same star may flare up into Nova many times during its lifetime
• Type I Supernova

Example: Algol
• Mysterious age-mass relationship
  • heaviest star is "least evolved" - still on the main sequence
  • explained by mass transfer

Neutrinos

Properties

• No charge
• Very little particle mass
• Hardly interacts with anything (passes through light years of matter)
• Very difficult to detect
• Travels at nearly the speed of light

Neutrinos provide astronomers with information from within the stars

• Emission from SN1987A observed by two underground detectors
  • Short burst of neutrinos arrived at Earth about 20 hours before visible light
  • Kamiokande in Japan
  • Detector near Lake Erie in US
• Solar neutrinos are also routinely detected on Earth
  • Large underground detectors

Hertzsprung-Russell Diagram

recall the H-R regions
red giants
blue giants
main sequence
white dwarfs
red dwarfs

pre-main sequence evolution of a solar mass star

pre-main sequence evolution depends on mass

post-main sequence evolution of a solar mass star

solar mass star ejects the planetary nebula,
and eventually collapses to a white dwarf

heavier stars follow different post-main sequence paths,
and avoid the helium flash