Interstellar Medium

1
Interstellar Medium

• Gas, Dust Nebulae

2
Clouds of Gas and Dust

• Composition hydrogen (75, helium 25) small
  amounts of dust (solid) and molecules
• Atoms of elements, sodium, calcium, carbon
• Molecules CN, CH, CO, H2
• Methylamine CH3NH2, formic acid HCOOH
• (can form gylcine NH2CH2COOH, an amino acid)
• Interstellar grains - smoke size particles
• 0.01 to 0.1 um
• 1 of mass, 10-12 in numbers
• carbon compounds, silicates, ices
• may be surface reactions on grains that can form
  more complex molecules

3
Cloud Properties

• Size from a fraction of a pc (0.1) to a few
  hundred pc, a few pc more typical
• Density (/cc) 10 to 1015 (100 to 1000 typical)
• Temperature 10 to 20 K but some may be 106 K or
  higher
• Mass 0.1 to 100,000 Ms, (100 to 1000 typical)

4
Types of Interstellar regions

• Molecular Clouds (10 K), mostly molecules
• HI regions 200 K, neutral hydrogen
• HII regions 10,000 K ionized hydrogen
• Hot Interstellar Medium 106 K, heated by shock
  waves from Super Nova

5
Interactions of Clouds of Dust and Gas with
Radiation

• Absorption and Re-radiation, Scattering
• Absorption
• Cool Clouds
• Ionization
• Excited electron levels
• Absorption lines
• Re-radiation in all directions and may be at
  longer wavelengths than absorbed photons
  emission lines
• Hot clouds emission lines

6

• Molecular Clouds complex spectra in infrared
  and microwaves
• Vibration and rotation
• 21 cm hydrogen line (absorption and emission)
  tool to map clouds

7
Scattering

• Scattering effect of dust grains
• Small size a fraction of wavelength
• Particles will scatter short wavelengths more
  and long ones less (Rayleigh scattering)
• Interstellar Reddening
• Obscuration corrections to brightness of stars
• Dust grain also emit infrared characteristic of
  cloud temperature

8
Nebula as star forming regions

• Stars that form in clouds (O and B stars) heat
  the surrounding gas producing HII regions
• As O B explode they compress gas that leads to
  further star formation
• See figure 20.5

9
Multiple Star Systems

• Two or more stars gravitationally bound
• Types
• optical double stars
• Physical binaries
• Visual binary
• spectroscopic binary
• eclipsing binary
• astrometric binary
• Semi major axis .1 to 100 AU
• Mass 0.5 to 20 Ms

10

• Co-orbiting stars orbit about center of mass
• Relationships
• a a1 a2
• Keplers 3rd Law m1 m2 a3/p2
• m2/m1 a1/a2

11
Period and Semi-major Axis

• Visual or Eclipsing Binary
• Period repetition of position or eclipse
• Spectroscopic Binary
• Period from repetition of Doppler shift
• Orbital velocity form Doppler shift
• Semi-major axis
• Visually
• Calculate from period and velocity

12
Doppler Effect

• Shift of Wavelength (frequency) due to motion
  toward or away from observer
• Blue Shift (shorter wavelength)
• Motion toward observer
• Red Shift (longer wavelength)
• Motion away from observer
• Velocity is a fraction of speed of light
• v C ( ??/?)

13
Example

• The normal wavelength of Ha 656.3 nm
• Suppose it is observed in the spectrum of a star
  as 656.17 nm.
• This is a blue shift object has a velocity
  toward the observer
• V c ((656.3-656.17)/656.3)
• V 3x105 (0.13/656.3) 59.4 km/s

14
Example

• Suppose p0.9 y, a10.3 a21.8
• M1 M2 (2.1)3/(0.9)2
• M1/M2 1.8/0.3 6
• 6 M2 M2 11.4 Ms
• M2 (11.4/7) Ms 1.62 Ms
• M1 6 Ms 9.77 Ms

15
Percentage of Multiple Star Systems
16
Formation of Multiple Star Systems

• Fission splitting of fast rotation protostar
• Capture gravitation attraction of one star for
  another passing by
• Sub-fragmentation during gravitation collapse

17
Binary Systems with Mass Transfer

• Roche surface
• Evolution of one star to giant stage
• Outer atmosphere of star fills lobe
• May transfer mass into lobe of second star
• Nova stars addition of mass from one star to
  another that may be a WD
• WD gains mass and ignites Hydrogen burning and a
  nova type explosion

18
Contact binaries

• 0.8 to 5 Ms
• Roche limits touch
• WD partner receives mass from other partner
• Form through fission of rotating protostar
• Very short periods
• Old pulsars binary systems with mass transfer
  that speeds up rotation of neutron star

19
Star Clusters and Associations

• 3 Types
• Open (Galactic) clusters
• Star Associations
• Globular Clusters

20
Typical Characteristics
21
Milky Way Galaxy

• Shape
• Size
• Sun Location
• Halo

22
(No Transcript)
23
(No Transcript)
24
Open (Galactic) Clusters

• 100 to 1000 stars
• 5 to 50 pc size
• Young stars, star forming clouds of gas
• Break apart with time due to individual motions
• Separate in about 100 million years
• 2 stars (5 and 5.5 km/s)
• Relative velocity
• 0.5 x 0.2 108 107 AU 4 pc
• Quickly dissipate over time

25
(No Transcript)
26
(No Transcript)
27
Star Associations

• More open and fewer stars
• 10 to 100 stars
• 10 to 100 pc
• contain O and B or T-Tauri stars
• star forming region
• OB associations
• T associations

28
Globular Clusters

• Massive and stars are very close together
• (average distance is 0.5 ly for 106 cluster)
• distance of proxima centauri is 1.29 pc (4.2 ly)
• Exist in halo of the galaxy
• Very old 12 14 billion years old
• formed early in the history of the galaxy

29
(No Transcript)
30
(No Transcript)
31
Distance to Stars via Clusters

• Methods
• Parallax up to 50 pc
• Extended to a few 100 by satellite methods
• Main-Sequence Fitting
• (brightness plot apparent brightness and then
  adjust to get the brightness ratio and hence the
  distance)

32
Variable Stars

• Stars in the instability strip
• Bright high luminosity
• Relationship between brightness period and
  luminosity or absolute magnitude
• Cepheid Variables
• Periods of 3 to 50 d
• Luminosity 200 Ls for type II, 1000 Ls for type
  I
• RR Lyrae stars
• Periods 0.3 to 1 d
• Luminosity 100 Ls

33
Example

• Suppose we observe a variable star with a period
  of 10 days in star cluster of unknown distance
• The apparent magnitude is 13.5
• Check Cepheid Variable Properties
• the absolute magnitude of the Cepheid star is
  -0.5 (550 Ls)

34
Example

• m-M 13.5 (-.5) 14
• BR 2.51214
• DR 2.51214/2 2.5127 630
• Dist 10DR 6300 pc

35
Corrections for Interstellar Reddening

• Dust in nebula
• Calculate from the amount of reddening of spectra
• Clusters are closer that they appear

36
Using Clusters to Understand Stellar Evolution

• Plot cluster on HR diagram
• all stars are at about the same distance
• can assume that apparent magnitudes are in the
  same relationship as absolute magnitudes or
  luminosities
• If we know distance to cluster we know luminosity
• Observe the cluster on HR diagram , ms, giants,
  etc.
• Assume that there is little or no second
  generation star formation

37

• MS turn off point indicates age of cluster
• Figure 22.5

38
Globular clusters

• Very old turn off at the age of Sun
• Little or no O, B and A stars
• Smaller stars so clusters have a reddish color
• Age 12 14 billion years
• Implications for halo of our galaxy
• Spread in ages of 2 to 3 billion years
• Globular shapes
• More spherical (slightly flattened) not
  dominated in evolution by rotation
• Slow rotation

39

• Globular clusters are a source of strong x-ray
  radiation
• Closeness leads to possible mass exchanges and
  hence x-ray emissions
• Heavy element fractions near zero