An Introductory Tour of the Universe

As this may be the first time you have taken an interest in Astronomy, I thought it would be a good idea to give you an introductory tour of the Universe. We will start with objects that are close to home and most familiar to us, and steadily move outward. This tour should not be taken until you have first read through the Units of Measurement page.

We start with the Earth ... our home planet. What distinguishes this planet from all others is that we are the only place in the Solar System where water exists simultaneously in three states of gas, liquid, and solid. This unusual circumstance is necessary for life as we know it, and is one of the reasons why we are here. Additionally, you might think that there is a lot of water on the Earth. This is quite true, but when you consider the entirety of the Earth's volume, water is very rare. It may be on the surface, but there is not as much in the Earth.

A short average distance of 380,000 km brings us to our moon. By definition, a moon is any object that orbits a planet. Another term is "satellite," but to distinguish those which we launch and those which are here on their own, we use terms like "natural satellites," and "artificial satellites." This is not to be confused with the Satellites which you frequently find at construction sites. Those are not at all related to Astronomy, although you might find a connection with Uranus.


Our Sun is actually an average, middle-age star, but since we are so close to it, it is the star which give us light and life. At a distance of 149,600,000 km, it is quite a bit farther away than the Moon is from the Earth. Yet both the Moon and the Sun appear to be the same size in the sky. The Sun may look the same size as the Moon, but it is actually 1.3 million times larger than Earth. Its great distance from us just makes it look smaller. The Sun is a ball of seething hot gases, burning at a surface temperature of 6000K, and an internal temperature of 15,000,000K.


Favorite planets like Jupiter and Saturn are gas giants and might be star wannabes were it not for the fact that they are simply too light on mass. They are beautiful to look at, and will be very interesting to study later. Jupiter lies 714,000,000 km from the Sun, and Saturn is 1,400,000,000 km from the Sun. Both are huge by planetary comparison. Jupiter is almost 1000 times bigger than Earth, and Saturn, with its rings, would barely fit between Earth and the Moon.

Moving out of the Solar System, we find that there are a lot of stars out there. The closest star to us is Proxima Centauri. At a distance of 4.3 light years, we call it a near neighbor. Students really struggle to grasp star distances, so here is something to give you a better picture. Light gets to the Earth 8 minutes after it leaves the Sun. It takes that same sunlight 4 hours to reach Pluto. Now, consider that 4.3 YEARS are required for that light to reach our nearest star, and you get the idea that things are far apart in space.

The stars in our home galaxy reside in pairs, widespread groups, or compact groups. Some stars are being born from giant gas and dust clouds called "nebulae," while other stars are in their final stages of their lives. We have many pictures of stars which used to shine brightly, but have long since dies gentle or violent deaths. The picture to the left is the little constellation called the Pleiades. Japanese call this group Subaru, and the symbol for this make of car is this group stars. It is a small collection of hot and young stars just a few 100s of light years away.

This famous picture, taken by the Hubble Space Telescope is a close-up of the Eagle Nebula. These pillars are huge clouds of gas and dust with new stars forming from small knots of material which are presently undergoing gravitational collapse into new stars.

Moving away from our local galaxy, we travel a distance of 2.9 million light years and arrive at the Andromeda galaxy. This is a huge collection of stars just like our Milky Way. With over 200,000,000,000 stars, this is a large galaxy, but because the object is so far away, the starlight blends together to give this beautiful picture. This object is the most distant thing you can see with your naked eye, but you will need quite a dark and moonless sky to see it. Of interest to you may be the relationship between distance and time. The light from this galaxy left its location 2.9 million years ago and is just arriving today. You are not looking at Andromeda the way it is, but the way it used to be 2.9 million years ago. As you look out into space, you are also looking back into time.

You will be interested to know that there are lost of galaxies in the Universe ... perhaps more than even you can imagine.

The final photo is of a small section of the night sky, and it reveals countless galaxies. It turns out that galaxies are grouped into Local Groups and Clusters of galaxies. Clusters are grouped into Superclusters, which in turn are grouped into stringy features called Filaments. These filaments are the largest structures known to man. You cannot see the filaments in this image, but you can see over 2500 individual galaxies!

Space may seem quite cluttered, but remember that there are vast distances of emptiness between stars and galaxies. And among all the immensity of the Universe, here we are ... on this little planet, orbiting an insignificant star, in a huge galaxy of stars, among countless galaxies. One could easily become despairing at how small we really are in comparison to the Universe, and feel hopeless in any feeble attempt to understand it. Einstein was quoted however, "The most incomprehensible thing about the Universe is that we can comprehend it." In this course, I hope to introduce you to as many interesting aspects of Astronomy as possible and leave you feeling awestruck but encouraged to learn more.

Thanks for taking the tour and expressing your comments of awe and wonder. Now, it is time to learn about finding things that are in the sky over your heads at night, but just before you do that, you will need to become familiar with some of the Units of Measurement, that astronomers use to describe the size of stars and planets, as well as the distance to them. You could also return to the Introduction to the Starry Sky .

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