Counting The Stars in The Sky

Roxanne Ryan

Philosophical Models

• 1750 Thomas Wright proposed that the universe was contained between two concentric spheres.

• 1755 Immanuel Kant he envisioned the universe to be similar to our solar system but on a much larger scale.

1610- Galileo resolves The Milky Way

William Herschel

1780’s he made the first attempt to map the Milky Way, by counting the stars in the sky.

Kapteyn Universe

He concluded that the sun was located 38 pc north of the galactic midplane and 650 pc from

the galactic center.

Harlow Shapley • Estimated the size of the Galaxy but

determining the distance to 93 globular clusters • Estimated the diameter of the Galaxy to be on the order 100 kpc (almost 10 times larger than Kapteyn’s)

Why They Were Wrong

• We now know that Kapteyn’s Universe was too small and Shapley’s was too large.

• Errors were both due the effects of gas and dust, called interstellar extinction on their distance estimates.

Actual ~ 50 kpc

Kapteyn’ s Galaxy ~ 15 kpc

Actual ~ 50 kpc

Kapteyn’ s Galaxy ~ 15 kpc

Actual ~ 50 kpc

Shapley’s Galaxy ~ 100 kpc

Why is the sky dark at night?Olber’s paradox

Solutions to Olber’s Paradox

“The only mode, therefore, in which, under such a state of affairs, we could

comprehend the voids which our telescopes find in innumerable directions, would be by

supposing the distance of the invisible background so immense that no ray from it

has yet been able to reach us at all.“~ Edgar Allan Poe(1848)

Solutions to Olber’s Paradox

Finite Speed of LightThe farther we look into the space the

farther we see into the pastFinite age of universe

Earth does not receive starlight from beyond a certain distance

Expanding spaceCause a redshift in light causing a drop in

energy

Counting The Stars in The Sky

Roxanne Ryan