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