Our Place in the Universe

Our Cosmic Address -- a thumbnail sketch:


The Earth
 The Earth is one of nine eight planets which orbit the Sun in the Solar System .
The Sun




The Sun is a very typical star, like many of the thousands of stars we see at night.

There are stars more massive and brighter than the sun, and stars less massive and fainter than the sun. The sun is so bright because it is so close to us.
The Night Sky




The stars we see at night make up a small part of the Milky Way galaxy, a vast assembly of hundreds of billions of stars.

The Sun is located about 2/3 of the way out from the center of the Milky Way galaxy.
the Andromeda Galaxy




Galaxies often come in groups or clusters. The Milky Way is part of a group of three big galaxies and lots of dwarf galaxies (~ 30 total) known as the Local Group .
the Virgo Cluster




Clusters of galaxies come grouped together in loose associations known as superclusters. The Local Group is part of the Local Supercluster.

The entire Universe is filled with billions and billions of galaxies and galaxy clusters.
The Hubble Deep Field

Our Address:

Astronomy 201
Sears 354
Case Western Reserve University
10900 Euclid Ave
Cleveland, OH 44106
The United States of America
The Earth
The Solar System
The Milky Way Galaxy
The Local Group
The Local Supercluster
The Universe

Our Origins

When we look at the Universe around us, we find that on the largest scales (not on the scales of desks, people, and planets!) the Universe is expanding. If we imagine the clock running backwards, at one point all of our Universe must have been compressed in a small space. We can estimate when this expansion started -- roughly 13-14 billion years ago, the beginning of our Universe called the Big Bang. Galaxies started forming a billion years or so after the Big Bang.


Star Forming Gas Clouds
 Within galaxies like ours, stars have formed continuously over the age of the universe, some dying quickly (in millions of years), some living for a very long time (billions of years).

Before stars existed, the universe was almost entirely made of hydrogen and helium, the most basic elements. Once stars form, the intense heat at their centers fuses hydrogen and helium into heavier elements such as carbon, oxygen, nitrogen, iron, etc.

When a star dies, it expels most of these elements back into space, to be incorporated into the next generation of stars. (Let's think about this...)



Planetary Nebula:
Death of a Low Mass Star

Supernova:
Death of a Massive Star


Our Timescale:

How do we "know" this? Can we see these changes in the Universe? In many cases, Yes!

Images of Space and Time

We see light from distant objects. Light travels at 300,000 km/s. Fast, but not infinitely fast -- it takes time for light to get to us:
 We define a light-year as the distance travelled by light in 1 year: 9,460,000,000,000 km, or nearly 10 trillion km. So we say that the nearest star is 4.3 light years away.
 Astronomy is like a time machine -- you look out in space, and back in time.

The Scale of the Universe

The Sizes of Planets:

(in order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto)



Important: This is not an accurate representation of their relative distances from the Sun, only their relative sizes!

A scale model for size: