DepartmentAstronomy & Astrophysics
AST 101 Week 2
•What’s in the Universe?
oThe universe has many, many stars
oA star is a huge sphere of super-hot gas (plasma) that produces energy by
nuclear fusion (smashing atoms together) the sun is a typical star. Difference
between planets and stars is that stars generate energy through nuclear
oA planet is a large body orbiting a star. Planets may be various mixtures of
rock (mars), gaseous (Saturn), or icy (Neptune)
•In 2006, the international astronomical union accepted a new definition of the term
“planet” adding one new criterion to the traditional definition:
o1. Planets orbit stars directly (not other planets)
o2. Planets must be massive enough for their own gravity to compress them
into spherical shapes.
o3. Planets must have cleared their orbits. That is, there can’t be a lot of other
stuff orbiting the parent star in basically the same orbit as the planet.
oA moon is any large body which orbits a planet.
•Comets are small, lumpy, icy bodies. Asteroids are small, lumpy, rocky bodies.
Together, they probably form a continuum of objects with compositions from icy to
•A nebula is a cloud of gas and dust. We see the light emmited by the gas (bright
parts) and the places where light is blocked by dust (dark parts)
•Nebulae are where stars are from. Stars form when parts of the nebula are crushed
by their own gravity.
•A galaxy is a “city of stars.” Large galaxies like the Milky Way, contain about 100
billion stars. As we have said, galaxies are grouped into galaxy clusters.
What’s all ofthis doing in the universe?
The universe is a very dynamic place. Everything is moving relative to everything
The earth rotates on its axis once every 24 hours. We call this motion rotation. Once
every 365.24 days, the earth goes around the sun once. We call this motion revolution.
The whole solar system orbits around the center of the millkey way once every 230
million years. The galaxies of the local group are all hizzing around, rather lke a jar full or
fireflies. All of the galaxy clusters are moving apart from one another.
In the early 1900’s a guy called Hubble noticed that all distant galalxies are moving away
from the local group.
We call this relationship Hubble’s Law: v vs d
It says that the speed with which a distant galaxy is moving away from us, v, is proportional
to the distance to that galaxy. Hubble’s law was the first piece of solid evidence that we live
in an expanding universe created in a Big Bang. However, don’t misunderstand what
hubble’s law means. It doesn’t mean that were at the centre of an expanding universe.