4.1 Dissecting the Universe

Introduction

The Universe and the Solar System

The universe is a very big place! It's made up of matter, energy, and a whole lot of empty space. This section is an introduction to the universe and everything in it. We will look at some scientific theories about how the universe, and our solar system, came to be. We will examine the parts of the universe that we know about: the galaxies, stars, planets, moons, comets, and asteroids.

We know quite a bit about space, but there's even more we don't know. It is important to think of our knowledge as a growing and changing body of information. In order to do this, it is often helpful to look at the history of our knowledge. Why did people want to investigate the skies in the first place? What were some scientific theories about space in the past? Through this investigation, we will see where our knowledge has come from, and think about where it might lead us in the future.

Planets, Stars, Nebula, Galaxies

Organizing the Universe

Hubble deep field photos. Reproduced with permission of NASA and Space Telescope Science Institute.

The universe is made up of all matter and space. All of the stars, planets, gases, dust, and even all energy fits within the term “universe.” It can be hard to imagine the universe and how big it is.

Galaxies

Within the universe, there are galaxies that contain billions of stars. Around some stars there are planets, asteroids and comets.

A galaxy contains stars, gas, and dust, all held together by gravity. Even the smallest galaxies contain hundreds of thousands of stars. The Milky Way galaxy, which is where we live, contains more than 100 billion stars.

Galaxies can be classified, or sorted, by their shape. Elliptical galaxies are either round or oval shaped. Spiral galaxies look like pinwheels with curved arms that reach out from the centre. Galaxies that don't fit into these two groups are called irregular galaxies.

The Milky Way

Have you ever looked up in the night sky and seen the Milky Way? What you are seeing is actually just one of the spirals of our galaxy, the Milky Way.

So how do galaxies fit into the big picture? Let's say that each galaxy contains about 100 billion stars. The universe contains about 100 billion galaxies. That means that the universe contains about ten billion trillion stars! That's 10 000 000 000 000 000 000 000 stars!

This starts to give us an idea about how big the universe is. Remember also that between all of those stars is a huge amount of empty space. In fact, most of the universe consists of empty space.

Another way to think about it is this: there are more stars in the universe than there are grains of sand on all the beaches on Earth!

Stars

The stars you see in the night sky are actually huge balls of gas, held together by gravity. Stars generate energy in the form of light, which is why you see them glow in the sky. Stars can be many different sizes, colours and temperatures. At the centre of our solar system, our Sun is an average-sized yellow star.

Star clusters are groups of stars that formed at the same time and in the same place. Star clusters remain together for a long time because they are held together by gravity.

When you look at the stars in the night sky, you may be able to see patterns and shapes. Ancient observers also saw patterns and gave them names. We call these patters of stars constellations.

Solar System

Our solar system contains the Sun and the eight planets that orbit the Sun. Also in our solar system are more than 60 moons, millions of asteroids, and billions of comets.

A planet is currently defined as a body that orbits the Sun, has a large enough mass to retain a “round” shape, and dominates its neighbourhood. The planets in our solar system are, in order from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Until 2006, Pluto was the ninth planet. There was always some debate about Pluto's classification as a planet. In 2006, astronomers demoted Pluto to the class of dwarf planet. The debate continues, as some astronomers don't agree with the decision that was made.

A moon can be defined as a large body that orbits a planet. Asteroids are small rocky bodies that revolve around the sun, and are sometimes called minor planets or planetoids. A comet is a ball of rock and ice that orbits the sun. Comets have a characteristic glowing tail formed when the sun heats the centre of the comet, and vapours are released.

Measuring the Universe

Now that we have a sense of what the universe is made up of, we will look at how big it is. You probably have an idea of the vastness of space, but what can we use to measure distances in space?

A Light-Year—The Stellar Yardstick

In space, light-years are often used as the main unit of measurement. The light-year is NOT a measurement of time; it's the distance that a beam of light travels in one year. The speed of light is 300 000 kilometres per second. In one year, light travels 9.46 trillion kilometres! Instead of saying 9 460 800 000 000 kilometres, astronomers say 1 light-year.

You can see that 1 light-year is a huge distance. But, in the scope of the universe, 1 light-year is quite small. For example, the Milky Way galaxy has a diameter of about 100 000 light-years. Try writing that number out—that's a lot of zeroes!

Stellar Distances

Because some distances in space are so huge, astronomers use three terms to describe the kinds of distances they are talking about.

• Interplanetary distances are the distances between planets. These are measured in kilometres (km), or in Astronomical Units (AU). One AU is equal to the distance between the Earth and the Sun—about 150 000 000 km.
• Interstellar distances are distances between the stars in a galaxy. These distances are measured in light-years.
• Intergalactic distances are the distances between galaxies, and are also measured in light-years.

Labelling the Universe