May 22, 2007, jeb

Outline

• Star Formation
• Stellar Evolution
• Stellar Explosions
• Two Types of Supernovae
• Close Binaries
• Neutrinos
• Hertzsprung-Russell Diagram

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

Stellar Explosions

Heavyweight stars (more than 3 M_SUN)

Type II supernova - light increased by 10⁷ - 10⁸


1987 - in Large Magellanic Cloud (not in Milky Way Galaxy)

first visible to naked eye since 1604 (last in Milky Way Galaxy)
occurred 160,000 years ago since LMC is 160,000 light-years away




Many observed in telescopes every year - distant galaxies





Mass at time of collapse exceeds Chandrasekhar limit

• Collapses, heats, fuses heavier and heavier elements
• hydrogen -> helium -> carbon -> ... -> iron
  (iron cannot release energy on fusion)
• core collapse to extremely high density, and rebounds in violent explosion
• Heaviest elements of the universe created in the explosion




Neutron star often left at center of explosion



Most of the supernova energy carried off by neutrinos

• this was confirmed in 1987 when the neutrinos were detected by two underground detectors,
• Short burst of neutrinos arrived at Earth about 20 hours before visible light
  • observed in Japan
  • and in the US (near Lake Erie




Type I supernova

White dwarf and companion

Companion exchanges matter with the white dwarf, building eventually beyond the Chandrasekhar Limit




Crab Nebula

Supernova remnant from 1054

Most of the mass was ejected

Some mass left over as a neutron star at the core
....observed as a pulsar today

Two types of Supernovae

• Type I - binary system
  • Carbon-detonation supernova
  • White dwarf grows from accretion of material from companion, eventually exceeding the Chandrasekhar mass limit
  
  
• Type II - single massive star
  • Core-collapse supernova
  • Massive star expends its fuel supply, ending with iron core, and collapses and explodes

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