November 9, 2016
Astronomy 122
Prof. J. Brau

• Interstellar Medium
• Star Formation
• Stellar Evolution

Interstellar Medium


Total mass of interstellar medium of the Milky Way Galaxy
about equivalent to mass of the stars in the galaxy

99% of the mass is gas
1% of the mass is dust
average temperature about 100 K

temperature varies from few degrees to hundreds (or even thousands) of degrees Kelvin, depending on heating from nearby stars
Gas
• about 1 atom per cubic centimeter
  • compared to best "vacuum" achieved in lab on Earth = 10⁴/cm³
  • Earth's atmosphere is 3 x 10¹⁹ per cubic cm
• about 90% (by number) hydrogen
• about 9% (by number) helium
• small amounts of other atoms (C,O,N,Ca,Na.....)
Dust
• 1000 dust particles / km³
• 1 dust particle / 10¹² atoms

This interstellar medium provides material for the formation of stars in the future

Important definitions
• Interstellar reddening
• Extinction
• Polarization
• HI - hydrogen atom
• HII - ionized hydrogen
• 21 cm line . . . . spin-flip in hydrogen

Nebulae
• Dark (or Absorption Nebulae)
  • Horsehead Nebula
• Reflection Nebulae
  • Pleiades
  • Reflection nebulae are bluish
• Emission Nebulae
  • Typically O- and B-type stars heating their surroundings
  • Orion Nebula
  • Emission nebulae are reddish

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 observables 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


Local Bubble
• about 100 pc wide containing 200,000 stars
• region of interstellar space much thinner (5000 atoms/m³) and hotter (500,000 K) than expected
• thought to have been carved out by many supernova explosions within the past million years

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


Disk of nebular gas forms, jets form, and disk is eventually blown away

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

Collapse may be initiated by SHOCK WAVES from:
• stellar wind of nearby heavy stars (O, B)
• supernova explosion
• can initiate chain-reaction of star formation

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

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
• Stars of different masses follow different evolutionary paths

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
  • often found with newborn star clusters



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



End result of the collapse of the star is loosely associated stars in Star Clusters
• Newborn cluster NGC 3603
  
• Pleiades or Seven Sisters
  • example of an open cluster
  • H-R diagram reveals this is young cluster, less than 100 million years old


Over time, these clustering stars will separate and become isolated stars, like the Sun, or isolated small groups, such as binary star systems

Evolution of stars

There are several types of stars appearing on the H-R diagram:
• Blue giants (main sequence)
• Red dwarfs (main sequence)
• Red giants
• White dwarfs
And "stars" not appearing on the H-R diagram:
• Protostars
• T Tauri star
• Brown dwarfs
• Black dwarfs

Different thermonuclear processes are possible as temperature increases



Temperature	Process
10 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




Neutrinos are produced in the core of main-sequence stars by the proton-proton chain.

Provide astronomers with information from deep within stars
• Properties of neutrinos
  • No charge.
  • Very light (little mass).
  • Travel through material without hardly interacting (therefore, hard to detect).
  • Travel at nearly the speed of light.
• Large underground detectors "see" neutrinos.
• Solar neutrinos are also routinely detected on Earth





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

Due to the death of the more massive stars

the interstellar medium becomes more metal rich with time.

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 hot, very bright, enlarged star

• e.g. Sun will expand to engulf Earth and probably even Mars.
  • Betelgeuse ("Beetle juice" in Orion)
  • Sun will become a red giant in about 5 billion years




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 Binary Stars

Stars separated by a few stellar diameters.
Heaviest star can be least evolved.

Critical Surface - figure eight-shaped boundary around star defining gravity domain.
• Two lobes called Roche lobes.
  • Each lobe defines region of gravitational dominance of each star.


As most massive Star grows old, it may first expand to become a Red Giant .
• If matter escapes the Roche lobe of the red giant, it may fall onto the companion star, resulting in Mass Transfer
  • This can change the evolution of the stars (determined by mass)
  • Resulting in a mysterious age-mass relationship where the heaviest star is least evolved.


Example: Algol





Summary of the evolution of stars





Estimated Stellar Lifetimes(in units of 106 years)

MASS (solar masses) SPECTRAL TYPE ON THE MAIN SEQUENCE PERIOD OF CONTRACTION TO MAIN SEQUENCE (106 yrs) ESTIMATED LIFETIME ON THE MAIN SEQUENCE (106 yrs) PERIOD FOR MAIN SEQUENCE TO RED GIANT (106 yrs) RED GIANT DURATION (106 yrs) 30 O5 0.02 4.9 0.55 0.3 15 B0 0.06 10 1.7 2 9 B2 0.2 22 0.2 5 5 B5 0.6 68 2 20 3 A0 3 240 9 80 1.5 F2 20 2,000 280   1.0 G2 50 10,000 680   0.5 M0 200 30,000     0.1 M7 500 107     Source: Fundamental Astronomy, edited by H. Karttunen et al., 1994