ISP205 Lecture #22, March 29, 2001
- Announcements:
- Interstellar Gas
- Stars ~95% of visible matter in Galaxy but lots of
space inbetween
- Space between stars filled with gas and dust
(billions of solar masses in our Galaxy)
99% gas
1% dust
- Gas density: 1e-3 to 1e5 atoms per cubic centimeter
in clouds
- Temperatures: ~40K to Millions K (parts heated by
supernova
explosion shock waves)
- Composition: similar to sun
- Observation of gas:
- HII regions
- Ionized H recombines emitting various photons,
including
the red 646nm Halpha photons (3rd to 2nd excited state)
- Energy source: UV radiation from a closeby star
- 21cm radiation from cool HI regions
Proton and electron spins misalign - transition in lower
energy state that emits 21cm photons (radio photons)
- Additional absorption lines in background stars
- Interstellar Dust
- Image of Milky Way visible, IR,
Radio, .....
- Other images of nebulae with lots of dust;
- Composition: Water. Methane and Ammonia ices around
silicate or graphite or iron core.
- Grain size: 10-100 nm
- Observation of dust:
- Blocking star light (Dark Nebulae)
- IR observations
- Interstellar reddening (DEMO)
blue light (~ f4) scatters much more than red
light
- Clouds
- Gas and dust are not evenly distributed but
concentrated
in many clouds
- Typical cloud size: 10's of Ly
- Biggest clounds: Giant molecular clouds
- Densest clouds
- 100-1,000,000 solar masses
- Interior shielded from radiation by outside dust
layers
therefore cold (10's of K) and molecules can form
(alcohol, ....)
- Cosmic Rays
- High speed atomic nuclei, electrons and positrons
(near the speed of light)
- Composition similar to solar system, but some light
elements
enhanced - breakup products from collisions
- Charged particles follow magnetic field lines in the
Galaxy and
Earth
- Probably made in Supernova explosions in our Galaxy
(with the excetion of ultra hig henergy cosmic rays
that might come from other galaxies)
- Birth of a Star
- Most stars are born in giant molecular clouds
(remember formation of the solar system)
- Protostar: newly forming star before hydrogen ignition
- Protostars start producing radiation (grav. energy)
that
creates a wind blowing away remaining dust
- Jets: often wind can only blow in certain directions
and creates therefore a stream of dust.
- Herbig-Haro Objects: Jets hitting nearby material
producing visible glow
- Protostars on the HR diagram
- Extrasolar Planets
- Observations of planetary disks around newly formed
stars
(picture)
- Disks have "doughnut" shape for older proto
stars
Possible explanation: formation of a giant planet took place
(picture)
- Detection of extrasolar planets: detect orbital motion
of the star !
- Method 1: Detect "wiggle" directly fom
changes in stars position
- Method 2: Detect motion with doppler effect
- First extrasolar planet discovered by M. Mayor and D.
Queloz 1995
orbiting 51 Pegasi (40 Ly away)
- So far: (book) 20 planets found around other stars
- Surprise: Hot Jupiters
Giant planets very close to the star (most easy to detect !)
Maybe they formed further away and moved then closer
(breaking due to dust)