Our Place in the Universe: Sizing up the Heavens Reading: Marshak, Ch. 1

Stellar Observations – What can you see in the stars? Position and apparent changes in position through time (I.E., motion)  mass (from orbital motions), distance. Brightness and apparent variations through time  size, temperature, distance. Color (spectrum) and apparent variations through time  composition, temperature, direction of motion and velocity relative to observer. Apparent Size and shape and variations through time  stars, galaxies, nebula (clouds of dust and gas), comets, planets, etc.

Parallax – apparent shift in position of object relative to background due to change in perspective of viewer. Stellar parallax allows distance to the stars to be determined. Parallax only measurable at distances < several thousand light years.

*Stellar Brightness (“magnitude”) – depends on distance and intrinsic brightness due to size and temperature. * By identifying class of star, can estimate absolute magnitude and solve for distance.

Taking the Temperature of a Star * Spectrum – the distribution of wavelengths (colors) of light. * The hotter the body, the more energy emitted and the shorter the wavelengths (violet, ultraviolet, gamma rays, x-rays) Composition As light passes through the outer gases of the star, certain wavelengths are absorbed “fingerprinting” its composition. Same ideas apply on Earth: Ozone (O 3 ) in upper atmosphere absorbs UV light, protecting surface; sunlight strikes Earth, converts to heat, and radiates as long wl infrared radiation (heat); infrared is absorbed by CO 2 and other “greenhouse gases” in atmosphere, thus retaining heat.

Do you know this sound? What is it? How does it change?

The Doppler Effect Approaching sound source compresses sound waves, shortening wavelength and increasing pitch. Receding sound source stretches wavelengths and lowering pitch.

Red Shift The Doppler Effect applied to light – Spectrum of approaching stars is shifted to shorter wave-length (blue shift) whereas receding stars experience red shift. The faster the relative motion, the greater the shift. Amazingly, almost all stars in the Universe are red shifted.

Amazingly, the Greater the Distance, the Greater the Red-Shift

Our Deepest View into Space and Time

Expanding Universe First recognized by Edwin Hubble in 1920’s. Projected backward, the expanding universe implies that ~12-15 billion years ago all the matter of the universe exploded outward from a single point – “The Big Bang”

Stellar Evolution – Variations with Mass White dwarfs – “degenerate matter”: a teaspoonful weighs several tons Neutron Stars – protons and electrons forced together to form neutrons – a pea-sized piece would weigh 100,000,000 tons Black Holes – so massive and dense not even light can escape its gravitational pull.

Star Birth in Eagle Nebula

Birth of A Solar System – Protoplanetary Dust Rings

Prelude to Star Death

Relicts of a Supernova, 1054 A.D.

How to “see” a Black Hole