Thursday, September 13 , 2012
AST01H1 – Astronomy
Essential Concepts for Astronomy
Textbook (pg. 1)
Chapter 1 – Our Place in the Universe
1.1 Our Modern View of the Universe
It is very natural to assume that we inhabit a geocentric universe; however Earth is a planet orbiting a rather
average star in a vast universe
What is our place in the universe?
Solar System = The Sun and all the material that orbits it, including planets, dwarf planets, asteroids, comets,
and small solar system bodies. Although this term typically refers to our own system, it can apply to others as
well. The inner solar system includes the four planets closest to the Sun and the outer solar system includes the
four the farthest from it.
- Stars are balls of plasma that produce energy through nuclear fusion.
- Planets are rocky, icy, or gassy and don’t produce much energy (terrestrial planets vs. gas giants). In 2006,
the International Astronomical Union accepted a new definition of the term “planet”, adding one new
1. Planets orbit stars directly, not other planets.
2. Planets must be massive enough for their own gravity to compress them into spherical shapes.
3. Planets must have cleared their orbits. That means there can’t be a lot of other stuff orbiting the parent
star in orbits very similar to the planet.
*This is why Pluto has been demoted to the status of dwarf planet. Other dwarf planets include Eris, Ceres,
Makemake, and Haumea.
- A moon is any large body that orbits a planet. They can be larger than planets and non-spherical.
- A nebula is a cloud of gas (bright) and dust (dark).
Galaxy = A great island of star in space, containing from a few hundred million to a trillion or more stars, all held
together by gravity and orbiting a common centre.
- We inhabit Milky Way galaxy which is disk-shaped and is relatively large, containing over 100 billion stars
- Galaxy Clusters = A collection of galaxies bound together by gravity. Small collections (up to a few dozen
galaxies) are generally called groups, while larger collections are called clusters. The Milky Way is part of a
cluster known as the Local Group which contains about 40 or so galaxies. The Local Group is one part of the
Supercluster = A gigantic region of space where many individual galaxies and many groups and clusters of
galaxies are packed more closely together than elsewhere in the universe.
Universe (cosmos) = The sum total of all matter and energy—that is all galaxies and everything between them.
How did we come to be?
The entire universe is expanding, meaning that average distances between galaxies are increasing with time,
which implies that at one point in the past, they must have been closer together
- Big Bang = The name given to the event thought to mark the birth of the universe
- Based on the observed rate of expansion, we can conclude that this occurred about 14 billion years ago
On a smaller scale, the force of gravity has drawn matter together, which is how superclusters are formed, thus
individual galaxies and clusters, and that which is in them, do not expand
Within galaxies, gravity forces the collapse of dust and gas clouds to form stars and planets. Stars also go
through life cycles and are born when gravity compresses clouds until the centre becomes hot and dense
enough to generate its own energy through nuclear fusion. A dies when it exhausts all its usable fuel from this
- Nuclear Fusion = The process in which a larger two or more smaller nuclei slam together and make one
larger nucleus. 2
When a large star dies, it does so in an explosion called a supernova, in which their matter mixes with other
matter floating between stars in the galaxy and forms new gas and dust clouds from which new stars and
planets can be born, creating a process of cosmic recycling. Our galaxy is a product of such cosmic recycling.
Most of material from which we and Earth are made was created inside stars that lived before we were born
- The early universe contained only hydrogen, helium, and a bit of lithium, but Earth is made up of many other
elements that were also made by stars (e.g. carbon, nitrogen, oxygen, iron), thus we are “star struff”
Our solar system formed about 4.5 billion years ago
How can we know what the universe was like in the past?
Light travels extremely fast, about 300,000 km/s, but takes a long time to travel in space (e.g. Sun to Earth is 8
mins, Moon to Earth is 1 sec). Light from the stars takes years to reach us, so we measure distances in a unit
called light-years (this is because light travels at a finite speed).
- Light-Years = The distance that light can travel in one year, which is about 9.46 trillion km. Stars are only a
few light-years apart, but galaxies are millions of light-years away from one another.
e.g. The next largest galaxy, the Andromed