Chapter 1
From the Big Bang to Galaxies
Earth's tilt relative to orbit around the Sun
Earth's orbit around the Sun (top view)
Light moves fast, but not infinitely fast: speed = c = 300,000 km/s
Light from Sun to Earth takes 8 minutes
Light from nearest star (Proxima Centauri) to Earth takes about 4 years
Light from Orion star forming region to Earth takes about 1500 years
So, we see Orion S.F.R. as it was 1500 years ago and 'it sees' Earth as
Definition of
Light Year
Even more distant objects:
Andromeda
is 2.5 million ly away
We've seen galaxies as far away as 12 billion ly
Observable part of universe= portion near enough for light to have Very distant objects appear as they
were long ago, when they were younger
Scale of the Milky Way
Diameter ~ 100,000 light years
Number of stars ~ 100 billion stars
Known Universe: ~ 100 billion galaxies
Timeline
Dinosaur extinction: 65 million years ago
Continents smashed together -> Appalachian mountains pushed up:
Compare: Galaxy rotates once every ~230 million years!
Solar system formed 4.6 billion years ago
Milky Way Galaxy formed ~12 to 13 billion years ago
Universe started ~14 billion years ago
On the cosmic timescale, human lives and even human civilization Big Bang and the expansion of the UniverseSpace, Time and Distance
we were 1500 years ago.
reached us since beginning of the universe
i.e. radius = speed of light x age of universe = c x 14 billion yrs
= 14 billion ly
Scale as a way to understand things
~220 million years ago
occurs during the blink of an eye.
How did the universe get to be like this?
Gravity
Nuclear Fusion -> Chemical change
