Introduction To Galaxies

When we look up at a night sky, what do we see? With just our eyes we probably
see a few thousand objects of varying intensity. They are mostly stars but there
are a few planets , comets, and meteors occasionally too. This is just our
galaxy. Our galaxy is a very small portion of the universe. 

Ours and other galaxies are made up of stars, planets, dust, gas and quite possibly a black hole for the most part. There are reportedly billion of other galaxies just like ours. Ours is fairly typical though, from what we can tell. Astronomers are still studying
what our overall shape is.

When we look at the night sky, we often see an extra bright band sweeping across
the sky. This is made up of a multitude of stars that are very far away. They
are so concentrated the band appears as its own light source in the sky. It's
truly incredible.

Structure of Galaxies

Galaxies can be several different shapes. In fact, the shape can be an indicator
of how that galaxy was formed. This in turn will reveal many of the processes
that happened during the formation.

Stars within any particular galaxy will usually be pretty far apart. They will
also not move far from the galactic plane until they get older. Older stars will
interact with other objects to a greater degree and start to wander slightly.   

There is a lot of dust that impedes our view. Because of this, radio astronomy
has been a key factor in discovering the shape of our own and other galaxies.
Our galaxy appears to be football shaped. I remember when that fact came out
that scientists were surprised as they expected it to be another shape.

The Milky Way Sky

On a clear and dark night, you can see a long cloudy band of stars. This is a
portion of our Milky Way galaxy. Why does it appear as a cloudy band of light?
This is a common question. It is because we are inside one of the spiral arms
of our galaxy. That band is us looking out from one of the arms. The spiral arms
appear patchy because of numerous gas clouds that cloud our vision.

Variable Stars

There fast became a need to measure everything around us. We did not know how
many stars were around us, how far away they were, or even what they were made
of. Astronomers even made some pretty bad guesses too. Finally, we discovered
variable stars. Many galaxies seem to have them and they helped Astronomers
greatly.

Variable stars are helpful because their brightness changes as a function of
time. With this information we can infer a great deal. That is why they are
helpful. The Cepheid variables are probably the most famous.

Mapping Our Galaxy With Variables

A cepheid is a pulsating variable star. The pulses happen because they are in a
later stage of life. The pulses are the stars contractions as they try to feed
off hydrogen. These pulses happen on a regular basis. Using this regularity, we
can tell the distance to them and relate to other astronomical objects. The
relationship between their periods and luminosities lets us derive the distance
to them. Since this is so useful, there is an ongoign search for pulsating
variable stars. Finding them lets us map out our galaxy. This is incredibly
useful and fun.

Motion of the Stars

The stars move and they do so constantly. This may be surprising to some since we
also know our planet moves in a constant orbit. In fact, our entire galactic
disk is rotating. The galactic disk includes all the stars, dust, and gas that
it contains.

Motion, of course, depends on how close you are to the galactic center. The
closer you are to the center the faster you would be moving. If you are very
far away then you will be moving slower than objects closer to the center.
There is a term that refers to this kind of motion and it is called differential
motion.

The Halo and Disk

Our galaxy consists of a disk and an outer halo of stars. These are different
components. Stars in the main disk have substantially different properties than
stars in the halo. This is due to age and how they were formed. The main disk
has stars of all ages and is tightly packed. The outer halo has very old stars
and is very sparsely populated. This halo is where we can find some globular
clusters also.

Formation of Galaxies

Ours and other galaxies start as a contracting cloud of gas. This is gas that
is becoming more cold and dense over time. This is what causes the contraction.
The gas was in big globs. There was no overall shape yet because the motion
between everything was pretty random at this point. this is the beginning of how stars form. 

As objects started interacting with each other, rotational motion started to
occur. This motion eventually flattened the gas and dust into a relatively
stable disk.

Over time, all the matter in the disk cooled even further. This is when star
formation ceased in our galaxy. Matter collected further into the plane of the
disk. There was a lot of heat loss and motion became less random.

Mass of a Galaxy

The mass of a galaxy is difficult to calculate accurately. However, we can get
decent estimates  by studying the motion of objects that are inside that
particular galaxy.
\( mass of galaxy = \frac{size^3}{period^2} \)
This is a real rough estimate, but it is a start. As can be seen, galaxies are
beyond large. This is why they exert such gravitational pull on objects close
to them.

Dark Matter

There has been lots of talk about dark matter in the last several years.
Journals have frequent and regular articles that discuss it. Most people are
just confused by it. At its face value, it does not make a lot of sense. I have
read it as being described as invisible. Others have said that we are walking
in a soup of it but we can't feel or see it. There have been many theories!

Another theory is that dark matter is all the hidden subatomic particles we
haven't discovered yet. This is at least interesting but I'm doubtful of it too.
The reason I am doubtful is that right now we don't even have a reason to
suspect there are that many particles floating around freely that violate the
laws of physics and go undetectable. Anyway, that is what I'm thinking, we will
see how things turn out.

So that leads us to the question: what is dark matter? I don't know any more
than the rest of the crackpots out there, including myself in that group, but I
am guessing it is just regular matter. This matter is probably very small and
there are probably dust clouds impeding view and blocking reflections from
light. Since this matter is also in deep space, it is very cold which is why we
can't detect radiation either. That is just my guess so take it with a grain of
salt. Everyone has a different guess. Some make a lot of sense such as brown
dwarfs and some are a bit more outlandish. Hopefully, we will find out more in
the coming years.