Galaxies are the foundations of the universe and we have identified millions
of them. In this chapter I want to mention and talk about the most defining
A spiral galaxy has a flattened disk and multiple spiral arms. Our galaxy is a good example. There are more stars in the center of this type of galaxy. Its density is greater there. The way we classify galaxies originates with Edwin Hubble, who made many great discoveries in astronomy. He had a naming convention that used small and large letters. The letters indicated bulges and shapes for example. There were more letters used as well.
Bulges of these galaxies can contain old stars. These older stars do not
contribute as much light as newer stars.
The galactic planes of most galaxies contain large amounts of dust and gas.
Thick areas of gas and dust block our attempts at discovering what is going on.
This is a good example of astronomers having to use radio astronomy to "see"
into them. Visibility is too poor for telescopes to penetrate.
When studying galaxies, it is important to remember that astronomers will not have perfect views. They are inferring a lot from various data. It is pretty accurate though considering we can't actually see it with our eyes.
Elliptical galaxies also have a dense nucleus of stars. They do not have spiral arms. There is also not a galactic disk. All these characteristics make ellipticals very different than spiral galaxies.
There is great variety in size and shape among ellipticals. Some are very large! Their shape can vary a lot. A few types are almost circular and others are extremely elliptical. There are many differences in these types of galaxies. This means they formed under widely varying conditions.
There are a few things in common among elliptical galaxies. One of these is very little cold gas. You would expect a galaxy to have massive quantities of it. Instead, there are large quantities of hot gas. There is also very little star formation going on. The large quantities of hot gas would be more plausible if there was a lot of new stars being born. So this is very unusual.
There are countless exceptions to the above categories. I can understand why this may be confusing. It is confusing to me also. Why have categories for a few types but have nothing for a million others? I guess it was just the best that could be done at the time. Remember, you can't actually see these far away galaxies to any detail with a telescope currently.
These exceptions can have all different shapes, sizes, and characteristics. They tend to be rich in resources and have many young stars. This means star formation is still going strong in those galaxies.
They are usually smaller than spiral or ellipticals. This means they contain fewer stars and have less volume in space.
Differences Between Galaxies
So far, we have covered a few different types of galaxies and their main attributes. This includes our own galaxy. So are we like the other galaxies as a whole or are we different? Well, while not entirely unique, we are not really very common either.
Spiral galaxies have several spiral arms. These arms come from the type of star formation that they underwent. Our galaxy and other spirals have large amounts
of cool gas throughout.
Elliptical galaxies are very similar in those characteristics too. Minus the spiral arms, they are very similar to spiral galaxies. Large amounts of cool gas and a dense nucleus are both common attributes.
That makes up just a small fraction of galaxies though. Everything else is different. There are varying degrees of differences but the average galaxy that is not nearby is just wildly different.
With galaxies further away, better methods had to be discovered to calculate distances. At first we used plain geometry. Then the great cepheid discoveries really helped. Now, we use objects deep in space that we know about. A star, nebulae, or another galaxy that is always the same really helps. We can calculate its brightness based on its distance. That makes it easier to compare to other stars or galaxies.
Groups Of Galaxies
Everything in our universe is in groups or clusters. The atomic nucleus, organisms, planets around our sun, stars around the black whole, and then clusters of galaxies. It is just the way it is. Most matter in the universe is in clumps of similar types of matter.
Galaxies are certainly no exception. They group together because of gravity and other forces. Small numbers are usually called groups. Large numbers of galaxies are referred to as clusters.
Galaxies move along with everything else in the universe. Most are moving away from us and from each other too. This is called dispersion. We know this because astronomers analyze the light of everything they see in space. This is spectrum analysis and it allows us to see the different wavelengths of the light that is being broadcast by any particular object. When the light is reddish it is known as being red-shifted. Red-shifted light occurs when an object is moving away from us.
This is the universal expansion of the universe that you will hear about sometimes. Everything is moving farther apart. To what and to where, we do not know. We just know that it is all moving somewhere.
We first tried to classify galaxies according to their shape. This worked out fair but not great. We can discuss galaxies with respect to their energy too though. Our home galaxy is a normal one. It is pretty average as far as size and brightness. There are other types of galaxies though. Active galaxies are far more energetic. Because of this, they are of great interest to astronomers.
The funny thing is that we can't really tell by looking at them in optical wavelengths. They start to get exciting when seen at wavelengths above and below optical frequencies.
Among active galaxies, there are still more subtypes. All of these are highly energetic galaxies. There are not many of these. It is rare to find a new one.
These active galaxies spew out a lot of energy. They do so in mostly the radio wavelengths of the spectrum. They look a lot different than our own normal galaxy. Size is another difference that we share. Radio galaxies are much larger. The energy emitted can be several times larger than our own galaxy. That makes the energy enormous. Radio galaxies will usually have visible jets and lobes.
The jets are where matter has been violently ejected perpendicular to the core. Matter that is ejected like this can be a long way from its parent galaxy. Lobes are areas of radio emissions that can be extremely large.
The energy production in active galaxies is astounding. This is what is meant by saying a galaxy is energetic. There are a few reasons for this. One of the main ones is that the central black hole is thought to be much larger. It is so large because more material falls into it. More material means more friction and release of energy in forms of radiation.
Accretion disks power the displays of energy we know as jets. Matter drawn in increases the disk's mass. Material that leaves causes friction in the disk and more energy is released and used to power this process to an even greater degree. It's simply amazing! Jets can appear far away in most cases. These jets are made up of electrons and protons that are blasted out into space perpendicular from the disk.