PHYSICS 20B Lecture Notes - Lecture 14: Vesto Slipher, Edwin Hubble, Cosmic Distance Ladder
Lecture 14
A Universe of Galaxies
● Similar to how planets are the building blocks to our solar system, galaxies are the
building blocks to the Cosmos
● Because light travels in a finite speed, the galaxies we are viewing now are actually
galaxies viewed from the past. We can then see how galaxies have evolved.
Olbers’ Paradox:
Why is the night sky dark?
● The apparent brightness (b) of a star decreases by 1/d^2.
● The area of shells of stars surround the earth increases by d^2. Thus, the effects cancel
each other out.
Solution to Olbers Paradox
● The Universe has a finite age, 14 billion years. Recalling that light also has a finite
speed, and recalling how what we see are are objects in the past; we can only see light
that has travelled for 14 billion years. Thus, not all of the light from all of the stars has
had time to reach us.
Edwin Hubble
● Spiral nebulae are “island universes”
● Evidence of a Universe of other galaxies, more galaxies than thought.
Vesto Slipher
● Galaxies almost always show a redshift, increasing in wavelength
Hubble’s Law
● Hubble found distances to Slipher’s galaxies using Cepheid variable stars
● Speed (v) is proportional to its current distance from us “d”
○ V = Ho d
● This means the the Universe IS expanding
Galaxies as Building Blocks
● Hubble showed that through the “standard candle” , the ANdromeda Nebula is a distant
galaxies, just like our Milky Way
● Galaxies are the building blocks of the Cosmos
● Remember that light takes a while to travel, so the projections we see in the sky are
actually older than they seem, from the past. Through this, we can study how these
building blocks have evolved.
● Galaxies cluster by forming groups of varying scale, filaments, etc.
Document Summary
Similar to how planets are the building blocks to our solar system, galaxies are the building blocks to the cosmos. Because light travels in a finite speed, the galaxies we are viewing now are actually galaxies viewed from the past. We can then see how galaxies have evolved. The apparent brightness (b) of a star decreases by 1/d^2. The area of shells of stars surround the earth increases by d^2. The universe has a finite age, 14 billion years. Recalling that light also has a finite speed, and recalling how what we see are are objects in the past; we can only see light that has travelled for 14 billion years. Thus, not all of the light from all of the stars has had time to reach us. Evidence of a universe of other galaxies, more galaxies than thought. Galaxies almost always show a redshift, increasing in wavelength. Hubble found distances to slipher"s galaxies using cepheid variable stars.
