Irregular Galaxies

Irregular Galaxies are simply all the galaxies which are not spiral or elliptical.
They can look like anything and have many different characteristics. Irregular galaxies have no rotational symmetry.

Many irregular galaxies probably used to be spiral, or elliptical until they had some kind of accident which changed them such as crashing with another galaxy.
Many other irregular galaxies probably were never spiral or elliptical; they simply didn't evolve that way.

Irregular galaxies get their odd shapes in many ways. One way irregular galaxies are formed is when galaxies collide or come close to one another, and their gravitational forces interact. Another source of irregular galaxies may be very young galaxies that have not yet reached a symmetrical state. Also, in some irregular galaxies, like M82, young stars eject energetic bubbles of gas, giving the galaxy a blobby look.

The Large Magellanic Cloud (LMC) is an irregular-shaped galaxy in the Local Group. The irregular shape may be the result of a disturbance, perhaps a collision of two galaxies. The Large Magellanic Cloud (LMC) is near the constellation Dorado, and is 163,000 light-years away.

Large Magellanic Cloud

Large Magellanic Cloud

The Cartwheel Galaxy is an irregular galaxy that has a ring-like structure that is the result of a head-on collision of two galaxies! It started out as a regular spiral galaxy that was hit by a smaller galaxy. The ring-like band of stars formed much like ripples form in water when a rock is tossed into it. The Cartwheel Galaxy is about 500 million light-years from Earth in the constellation Sculptor.

Cartwheel Galaxy

Cartwheel Galaxy

Elliptical Galaxies

An elliptical galaxy is a galaxy with a smooth, elliptical shape. It is also called an "E" or "E-type" galaxy. The stars found in Elliptical Galaxies are often very old. This is because elliptical galaxies don't actively create new stars. The only stars found with in them were created along time ago.
Although they are usually smaller, they can vary in size. Most have only a few thousand stars, but some can have billions of stars. The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters

The stars in an elliptical galaxy are often very close together making the center look like one giant star. If the Earth were inside an elliptical galaxy it would be bright both day and night.
Unlike spiral galaxies, elliptical galaxies are generally yellow-red in color, do not have spiral arms, and contain little interstellar dust or gas. They are generally found in rich clusters of galaxies.

M87 and M32 are examples of elliptical galaxies

M87

M32

Spiral Galaxies

The most beautiful type of galaxies are Spiral Galaxies. Spiral galaxies are galaxies with a central, dense area and spiraling arms which are often sites of star formation. These common galaxies have two major parts: a central, flat disk containing a dense cloud of interstellar matter and young star clusters (mostly on the arms) and a central bulge (or nucleus) containing older stars

So where do the spirals come from? Like ripples in a pond, the spiral arms seen in this kind of galaxy are circling waves. These waves cause new stars to form. That's right, they are like star farmers, planting star seeds where ever they go.
Some of the new stars created in the wave are very large. Because of their size these large stars glow brighter than their smaller cousins, causing the nearby dust clouds to glow brightly. Thus any area near one of these waves glows like a fluorescent light.

In other words you can't actually see the waves, the spirals that we see are the glowing clouds illuminated by large, hot stars. As the waves move on the clouds behind them dim down, no longer glowing until another wave passes through.

The large bright stars created in the waves don't live very long. Their large size makes them burn all their fuel quickly. Usually they die before they ever leave the wave. Only the smaller stars which do not glow brightly survive to leave the waves they formed in.
The Milky Way and the Andromeda Galaxy (M31) are two of a multitude of known spiral galaxies. The Milky Way Galaxy is a spiral galaxy; our sun and solar system are a small part of it. Most of the stars that we can see are in the Milky Way Galaxy. The main plane of the Milky Way looks like a faint band of white in the night sky. The Milky Way is about 100,000 light-years in diameter and 1,000 light-years thick. This spiral galaxy formed about 14 billion years ago. It takes the sun roughly 250 million years to orbit once around the Milky Way. The Earth is about 26,000 light-years from the center of the Milky Way Galaxy. The center of the Milky Way galaxy is towards the constellation Sagittarius. The Local Arm is the arm of the Milky Way Galaxy where our solar system is located. It is also called the Orion Arm.

The Andromeda Galaxy (also known as M31) is the closest major galaxy. It is a spiral galaxy (like our galaxy) and is in the Local Group. It is flanked by two dwarf elliptical companion galaxies (M32 and M110). It is part of the Local Group, a cluster of galaxies to which we (in the Milky Way) belong. The Andromeda Galaxy can just be seen with the naked eye in the constellation Andromeda. Andromeda is the farthest object that can be seen with the naked eye. It is about 2,400,000 light-years from Earth. It is 150,000 light-years wide. Recently, the Hubble Space Telescope found that Andromeda has a double nucleus. This second nucleus is probably from an ancient collision with a smaller galaxy.

The Andromeda Galaxy

Galaxies

A galaxy is a huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces. There are billions of Galaxies in the Universe. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting the galaxy's center of mass. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds.
The Earth, Sun and the rest of our solar system are a tiny part of the Milky Way Galaxy, a spiral galaxy. The Milky way Galaxy is just one galaxy in a group of galaxies called the Local Group. Within the Local Group, the Milky Way Galaxy is moving about 300 km/sec (towards the constellation Virgo).
The galaxy that is nearest to our galaxy is the Sagittarius Dwarf galaxy, which is about 24 kiloparsecs or 80,000 light years from us. The Large Magellanic Cloud is another close galaxy; it is about 50 kiloparsecs from us.
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.
Galaxies often crash into one another. Even our own galaxy has had others pass right through it. Don't worry though, galaxies can pass through each other quite safely. Stars are so far apart that the chances of two colliding is very unlikely
There are three major types of galaxies: spiral (with arms), elliptical (no arm), and irregular (without rotational symmetry). The only difference between the three is what shape they are.

Dark Nebulae

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. They are very similar to reflection nebulae in composition and look different primarily because of the placement of the light source. Dark nebulae are not seen by their emitted or reflected light. Instead, they are seen as dark clouds in front of more distant stars or in front of emission nebulae.
The Horsehead Nebula in Orion is probably the most famous example of a dark nebula. It is a dark region of dust in the shape of a horse's head that blocks the light from a much larger emission nebula behind it.

The Horsehead Nebula

Supernova Remnants

Supernovae occur when a massive star ends its life in an amazing blaze of glory. A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas forms a supernova remnant, a special type of diffuse nebula.

A typical supernova remnant is at most a few light-years across. One of the best examples of a supernova remnant is the Crab Nebula (M1) in Taurus. It is illuminated by a pulsar which was created by the supernova. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant.

The Crab Nebula (M1)

Planetary Nebulae

Although it is called a planetary nebulae, it actually has nothing to do with planets. These nebulae were given this name because they often look like planets in small telescopes due to their round shape. A planetary nebulae is formed when a dying sun sized star begins to shed its outer layers. These nebulae are emission nebulae with a spectral emission that is similar to the emission nebulae found in star formation regions. However, planetary nebulae are denser and more compact than the emission nebulae in star formation regions.
A planetary nebula is a shell of gas produced by a star as it nears the end of its life cycle. The outer shell of gas is usually illuminated by the remains of the star at its center. Our Sun will probably become a planetary nebulae. When the Sun begins to die it will expand, and become what is called a giant star. It will grow so large that it may engulf the Earth. After millions of years as a giant star the sun will again shrink down to its normal size. As it shrinks much of its surface layers will be shed leaving behind a beautiful ring. This ring is only visible for about 50,000 years. Over time, the nebulae mixes into surrounding space eventually becoming too thin to see. A typical planetary nebula is less than one light-year across.
The Ring Nebula (M57) in Lyra is one of the best examples of a planetary nebula. The Cat's Eye Nebula is another example of a planetary nebula.

The Ring Nebula (M57)

The Cat's Eye Nebula