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Lecture 2, Physics and the Universe Gravity, heliocentric system, atomic spectra, composition of the universe

Astronomy & Astrophysics
Course Code
Harald Pfeiffer

of 2
-Copernicus: heliocentric system
-Kepler: ellipses
-Galileo: telescope confirms Copernicus
-Newton: theory of gravity explains all planetary motions
-Einstein: concept of spacetime
-Wave-particle duality of light
Atomic spectra
-electromagnetic radiation allows us to characterize each element and molecule
-find out what the sun is made of
-speed of light c=300 000 km/s, highest possible velocity
The Universe
Solar System
-Oort cloud, Kuiper belt are found beyond the nine planets
-extrasolar planets are difficult to find!
Earth – Moon distance: 1.3 light seconds
Earth – Sun distance: 8 light minutes.
Neptune, the last (8th) planet: 4 light hours.
The Oort comet cloud: extends to about a light week.
The nearest star: 4 light years
Parallax method of Distance Determination
-tiny angle θ and 1 AU as the distance from Sun to Earth
D = 1 AU/θ
1 arc-second is 1/3600 of a degree
1 parsec is the distance corresponding to a parallax angle of 1 arc-second
Eg. Angular diameter of moon is 30 arc-minutes
-Sun is 0.01 AU in diameter; Neptune’s orbit is 30 AU
-the nearest stars are >1 parsec or 4 ly away
-Andromeda galaxy is 100 000 ly away
-visible light images of the milky way show black blotches
-this is caused by dust obscuring light
Milky way, a spiral galaxy
-a disk and a bulge
-1000 ly thick by 100 000 ly diameter
-sun is in the disk, 28 000 ly out from the centre
-at least 100 billion stars in this galaxy
-stars in the disk orbit the Galactic centre, bobbing up and down the same direction and plane
-stars in the halo and the bulge, orbit the Galactic center in random directions
Disk stars
-gas and dust where new stars are formed
-young and semi-young generations of stars
Halo stars
-no gas or dust
-old stars on extended orbits
Bulge stars: both young and old stars on tight orbits
Dark matter: 10x as massive as visible matter; measured by gravitational pull
Interstellar Matter (ISM)
-mostly gas and a bit of dust between stars
-almost a vacuum: 1 atom per cubic cm
-still much denser than intergalactic medium
-mostly H and He gas, with a little admixture (1% by wt) of relatively large grains of heavier elements
-hydrogen dominates: H2, H+ and H (neutral)
-dust grains absorb and scatter viz light, producing local pockets of `darkness`where new stars can form
-radio and infrared waves pass thru dust, which explains the red sunset
-infrared photo of milky way is not obscured by dust
Nebulae: heated ISM gas
-planetary nebulae are ejected matter from evolved stars, in their last stages of life
-parcels of hot gas due to thermonuclear reactions in their cores
-the Sun is a typical star
-properties determined primarily by mass
-massive stars are hot and luminous, and they are involved in creation of heavy elements
-most stars are less massive than sun
-observed range of stellar masses is 0.08 to 60 solar masses
-“brown dwarfs” are almost stars: below 0.08 solar masses, too small to generate thermonuclear energy
-planets are below 0.01 solar mass
-stellar spectra tell us about brightness and surface temp as well as composition of a star