Astronomy 122Mid-term Exam 3 Review
The Interstellar Medium
and Formation and Evolution of Stars
(Chapters 18, 19, 20)
February 16, 2009, jeb
Outline
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)
- Reflection Nebulae
- 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
- 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 MSUN --> 105 years
- 2 MSUN --> 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
- Reflection nebula
- Cloud of dust illuminated by stars (bluish)
- Emission nebula
- Glowing from excitation caused by nearby stars
- 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 MSUN, it never gets hot enough for thermonuclear fusion --> failed star
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
- HELIUM FLASH
- Runaway helium fusion initially for stars with mass less than about 3 MSUN
-
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)
- Old clusters - metal poor (Population II)
Death of Lightweight stars (less than 3 MSUN)
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 MSUN
- Degenerate electron pressure
- Density = 109 kg/m3
- reference: water = 1000 kg/m3
- Radius(1 MSUN) = 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
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
- 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
