Our Place in the Universe:Our Place in the Universe:Sizing up the HeavensSizing up the Heavens

Reading: Marshak, Ch. 1Reading: Marshak, Ch. 1

Stellar Observations – Stellar Observations – What can you see in the stars?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.

ParallaxParallax – apparent shift in position of object – apparent shift in position of object relative to background due to change in relative to background due to change in perspective of viewer.perspective of viewer.

Stellar parallaxStellar parallax allows distance to the stars allows distance to the stars to be determined.to be determined.

Parallax only measurable at distances < Parallax only measurable at distances < several thousand light years. several thousand light years.

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

Taking the Temperature of a StarTaking 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 (O3) 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 CO2 and other “greenhouse gases” in atmosphere, thus retaining heat.

Do you know this sound?Do you know this sound?

What is it? How does it change?What is it? How does it change?

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

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

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

Our Deepest View into Space and TimeOur Deepest View into Space and Time

Expanding UniverseExpanding UniverseFirst recognized by Edwin Hubble in 1920’s.First recognized by Edwin Hubble in 1920’s.

Projected backward, the expanding universe Projected backward, the expanding universe implies that ~12-15 billion years ago all the implies that ~12-15 billion years ago all the matter of the universe exploded outward from matter of the universe exploded outward from a single point – a single point – “The Big Bang”“The Big Bang”

Stellar Evolution – Variations with MassStellar 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 Star Birth in Eagle NebulaEagle Nebula

Birth of A Solar System – Birth of A Solar System – Protoplanetary Dust RingsProtoplanetary Dust Rings

Prelude to Star DeathPrelude to Star Death

Relicts of a Supernova, 1054 A.D.Relicts of a Supernova, 1054 A.D.

How to “see” a Black HoleHow to “see” a Black Hole