Studying for EXAM III On Sun and Stars Many chapters in book, which has way too many details lecture notes Focus on the general, repeating features Emphasis.

Studying for EXAM III

• On Sun and Stars

• Many chapters in book, which has way too many details lecture notes

• Focus on the general, repeating features

• Emphasis on concepts and reasoning (Why are more massive stars hotter, etc.)

Distance Measurements with variable stars• Extends the cosmic

distance ladder out as far as we can see Cepheids – about 50 million ly

• In 1920 Hubble used this technique to measure the distance to Andromeda (about 2 million ly)

• Works best for periodic variables

Cepheids and RR Lyrae: Yard-Sticks

• Normal stars undergoing a phase of instability

• Cepheids are more massive and brighter than RR Lyrae

• Note: all RR Lyrae have the same luminosity

• Apparent brightness thus tells us the distance to them!– Recall: B L/d2

Galaxies – Island Universes

• A historic tour of the discovery of the dwindling significance of humans in the universe:

• From the center of the universe towards the edge of an average galaxy amongst 100 billion others

How do we know where we are?

• “Obviously” we are living on a flat Earth at the center of the universe, as a quick look tells us:– The stars, Sun, Moon and planets rotate us– There is no apparent curvature of the ground– The Milky Way is a band that surrounds us – There are no signs for any movement of the

Earth (like wind, or forces throwing us off)

Logic to the Rescue

• How do we avoid these wrong conclusions?– Sound data – Flawed interpretation/reasoningFurther observations are necessary to decide!

• Do we have to question everything?– Yes, in principle.– The signature of genius is to ask the right

question, not necessarily to answer them.

Exploring our own Island Universe: The Milky Way

• A galaxy is a huge collection of stars, gas, dust, neutron stars, and black holes, isolated from others and held together by gravity

Our view of the Milky Way

• Appears as a milky band of light across the sky

• A small telescope reveals that it is composed of many stars (Galileo again!)

• Our knowledge of the Milky Way comes from a combination of observation and comparison to other galaxies

How do we know?

• Question: How can we say anything about our Milky Way, if we cannot see it from outside?

Obviously a bogus picture of our milky way!

Enter: the Genius

• William Herschel (XVIII century)• Simple model:

– Assumed all stars have the same absolute brightness

– Counts stars as a function of apparent magnitude

– Brighter stars closer to us; fainter stars further away

– Cut off in brightness corresponds to a cut off at a certain distance.

• Conclusion: there are no stars beyond a certain distance

Herschel’s Findings• Stars thinned out very fast at right angles to Milky Way• In the plane of the Milky Way the thinning was slower

and depended upon the direction in which he looked • Flaws:

– Observations made only in visible spectrum– Did not take into account absorption by interstellar gas and dust

Discovering other Island Universes

• Data: Lots of nebulous spots known in the nightsky

• Questions: What are they? All the same? Different things?

• Need more observations!

Build bigger telescopes

The first nebula discovered to have spiral structure: M51

M99 is a spiral, too!

• Q: do we live in a spiral?

• Q: Are we in the center of the spiral?

• Most probable answer: No!

Enter: next genius

• Harlow Shapley used variable stars, e.g. RR Lyrae stars, to map the distribution of globular clusters in the galaxy

• Found a spherical distribution about 30 kpc (30,000 pc) across– This is the true size of the

galaxy

• Sun is (naturally!) not at the center – it’s about 26,000 ly out

Standing on the shoulders of Giants

• Shapley used methods developed by others to measure the distance to globulars

• Cepheid variables show luminosity-period correlations discovered by Henrietta Leavitt

• Shapley single-handedly increase the size of the universe tenfold!

Structure of the Galaxy

Intra-galactic Dynamics

• Three main parts of a galaxy:– Bulge (center of

galaxy)– Disk (rotating

around center)– Halo (orbiting

around bulge with randomly inclined orbits)

Properties of Bulge, Disk and Halo

Disk Halo Bulge

Highly flattened spherical football-shaped

young and old stars only old stars young and old stars

has Gas and dust none lots in center

Star formation none since 10 billion yrs in inner regions

White colored, reddish yellow-white blue spiral arms

An up-to-date “Reconstruction”