ABSOLUTE BEGINNERS
This article featured in
the April 2001 Beginners Magazine
GALAXIES
Galaxies are the largest
formations or groups of individual stars that we know. All the stars we see in the night sky are
part of a huge family of stars which form our galaxy called the Milky Way or
the Galaxy (with a capital ‘G’). We see
the nearest stars to us as individual stars but as we look at those further
away they tend to merge into the fuzzy glow of the Milky Way. This effect is rather like standing in a
pine wood, the trees nearest to us are individuals but in the distance they
merge into a solid mass of trees.
Looking out into
space beyond our own galaxy we can see other galaxies of many different shapes
and sizes. As amazing as it seems there
are clusters of galaxies and even super clusters of clusters of galaxies. Galaxies can be seen stretching out into the
universe as far as our most powerful telescopes can see.
Our galaxy forms
part of what is known as the ‘local group’ of galaxies comprised of about 30
members. The local group is dominated
by two large spiral type galaxies, ours and the Great Galaxy in the
Constellation of Andromeda which can be seen with the naked eye on a very clear
night. The Milky Way has more than 100
billion stars and the Andromeda galaxy is about twice the size with about 200
billion stars. All the other members of
the local group are smaller and many are located like satellites around the
large spirals.
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Edwin Hubble's classification
of of galaxies
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Galaxies are
classified into four types, these are Spiral, Barred Spiral, Elliptical and Irregular. Spiral and Barred Spiral galaxies are
further divided into three subdivisions a, b and c depending on how tightly the
arms are wound. They are therefore
referred to as Sa, Sb and Sc or SBa, SBb and SBc. Elliptical galaxies have seven subdivisions ranging from E0 for
those that appear round to E7 for those that are nearly cigar shaped.
GALAXIES
All the stars we can see in the sky even using a telescope are in our galaxy which we call the Milky Way. Our Sun is a single star which is one of about 100 billion other stars making up our galaxy. The Milky Way is a spiral galaxy which if we could see it from above would look like a Catherine Wheel or a whirlpool. Our position in the Milky way is on the edge of one of the spiral arms about two thirds of the way out from the centre of the galaxy, see the diagram below. There is a central ball of stars from which curved arms comprised of millions of stars extend. Galaxies are rather like islands of stars in the immense emptiness of space.
Our galaxy is one of about thirty galaxies in our
local group. This group is made up of
our Milky Way and another similar spiral galaxy to ours called the Great
Andromeda Spiral Galaxy and a number of smaller galaxies. There are a number of small galaxies rather
like islands associated with our galaxy, two of these are called the Megellanic
Clouds. These are satellite galaxies
and can be seen from the southern hemisphere as small patches which appear to
have broken off from the Milky Way. The
Great Andromeda Spiral Galaxy can be seen with the naked eye on a clear night
as a smudge of light and is easily visible in binoculars or a telescope. Some other galaxies can be seen with small
telescopes but with large powerful telescopes countless millions can be seen
stretching out to the limits of our largest instruments in all directions. There are four basic types of galaxies these
are :
SPIRAL GALAXIES
Spirals are usually large and have a bright central
ball of stars with curved arms spreading out from the ball to form a flat disc. Like our own galaxy ‘The Milky Way’, many
galaxies other galaxies have spiral arms.
Some have arms like curved spokes in a wheel some gently curved some
tightly wrapped around the central ball.
Others have what looks like a straight bar of stars extending out from
the central ball with the spiral arms attached to ends of the bar, these are
known as Barred Spiral Galaxies. Spiral
galaxies are classified as a, b or c depending on how tight the arms are wound
preceded by S for Spiral and SB for barred.
e.g. Sa or SBc.
The Great Andromeda Spiral Galaxy is our closest spiral neighbour and
can even be seen with the naked eye on a very clear night and from a dark
location.
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M31 The Great Spiral Galaxy in Andromeda |
M87 A giant elliptical galaxy |
ELLIPTICAL GALAXIES
These are huge balls of thousands of millions of
stars. They do not have spiral arms but
are elliptical (egg shaped). Many of
these Elliptical Galaxies are the largest of all star groups, some having many
hundreds of billions of stars.
Elliptical Galaxies are classified according to how flattened they are,
nearly round ones are known as E0 and sausage shaped ones E7. Most Elliptical Galaxies are very far away
and therefore very faint and need a telescope to see them. There are some indications that the giant
elliptical galaxies grew from the collision of two or more smaller
galaxies. There are indeed some
galaxies which can be seen in the process of colliding and possibly combining.
IRREGULAR GALAXIES
These galaxies are as the name implies large groups
of stars but with no classifiable shape, in other words they may be any
shape. Our Milky Way Galaxy is one of
the two main galaxies in what is known as the Local Group comprised of about 30
associated galaxies. Our spiral and the
other spiral known as M31 or The Great Andromeda Spiral, have smaller irregular
galaxies associated with them as satellite galaxies. Ours can be seen from the southern hemisphere as islands broken
off the Milky Way and are known as the Magellanic Clouds. We cannot see the Megellanic Clouds from Britain
because they are too far south but are visible with the naked eye further
south. Some large elliptical galaxies
have huge numbers of satellite galaxies comprised of all the other types,
forming a large family. It is thought
that our galaxy may have as many as ten small irregular galaxies in attendance.
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