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

Reflection Nebulae

A reflection nebulae is completely different from an emission nebulae. It is a cloud of dust and gas that reflects the light energy from a nearby star or group of stars rather than emitting their own light. It glows as the dust in it reflects the light of these nearby stars. These nebulae are frequently bluish in color because blue light is more efficiently reflected than red light.
When light passes by a particle of dust, the blue color in that light is scattered, while the rest of the colors in the light are allowed to travel undisturbed. This blue light travels around the cloud bouncing off of dust particle after dust particle until it eventually escapes the cloud and reaches our eyes.
A reflection nebula surrounds the Pleiades Cluster. The Trifid Nebula (M20) in Sagittarius is another good example of a reflection nebula.

Pleiades Cluster

The Trifid Nebula (M20)

Emission Nebulae

One type of nebulae we are going to explore are called Emission Nebulae, because they emit their own light. The cloud itself is actually glowing. Within this type of nebula, bright stars inside the clouds energize the atoms in the cloud with ultraviolet radiation. As these atoms fall back to lower energy states, they get rid of the extra energy by emitting it in the form of red light. The process is similar to that of a neon light. This causes the nebula to glow. Emission nebulae tend to be red in color because of the abundance of hydrogen. Additional colors, such as blue and green, can be produced by the atoms of other elements, but hydrogen is almost always the most abundant. A fine example of an emission nebula is the Orion Nebula (M42).

Orion Nebula (M42)

Nebulae

A nebulae is a cloud of gas and dust in outer space. These clouds are often very large, spanning across many light years. There are many different kind of nebulas in the sky.
Nebulae are often the sites of star formation. In fact, all stars, planets, and solar systems are formed from nebulae. A nebula may lie undisturbed for many millions or billions of years as it waits for just the right conditions. Eventually, the gravity from a passing star or the shock wave from a nearby supernova explosion may cause swirls and ripples within the cloud. Matter begins to coalesce into clumps and grow in size. As these clumps get larger, their gravity increases. Gravity continues to pull in matter from the nebula until one or more of the clumps reach critical mass. The clumps are forming protostars. As gravity squeezes even tighter, the core temperature eventually reaches 18 million degrees. At this point, nuclear fusion begins and a star is born. The solar wind from the star will eventually blow away all of the excess dust and gas. Sometimes other smaller clumps of matter around the star may form planets. This is the beginning of a new solar system. Several nebulae have been found to be stellar nurseries. The Eagle Nebula, and the Orion Nebula are both sites of active star formation.

The Eagle Nebula

The Orion Nebula

There are a few nebulae that can be seen with the naked eye and many more that can be detected with a good pair of binoculars. A telescope is required to bring our fine details. Unfortunately, the human eye is not sensitive enough to bring out the rich colors of most nebulae. It is the photograph that does the most justice to these incredible objects. Until recently, time exposures on film were the best way to bring a nebula's true colors. Today, digital photography has simplified the process. New tools like the Hubble space telescope are giving us views of nebulae that have never been seen before. Areas of active star formation have been identified in many galaxies that were once thought to be inert.

Neutron Stars

If a star is a high mass star with a mass above 5 solar masses, it doesn't form a white dwarf when it dies. These stars end their lives much more violently. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. In the final stages of their lives, they proceed to fuse increasingly heavier elements until they have exhausted all possible fusion sources. When fusion ceases, gravity drives the core to implode, resulting in a titanic supernova explosion, leaving behind a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons.
Examples of supernova remnants are Vela, Crab Nebula, Veil Nebula and Supernova 1987A. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.

White Dwarf Stars

When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity. The outcome of a star's struggle between gravity and pressure depends entirely on its birth mass. Stars with masses below about 5 solar masses swell into red giants near the ends of their lives, after which the envelope is ejected as a planetary nebula, while the core becomes a white dwarf. Examples of planetary nebulae are the Ring Nebula, Eskimo Nebula, Helix Nebula and the Cat's Eye Nebula.
A white dwarf shines because it was a hot star once, but there's no fusion reactions happening any more. A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.

Supergiant Stars

If our Sun is an average sized star, there are some true monsters out there. They're the supergiant stars, and they come in two flavors: red and blue. The supergiants are the most massive stars out there, ranging between 10 to 70 solar masses, and can range in brightness from 30,000 to hundreds of thousands of times the output of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years down to just a few hundred thousand years. Supergiant stars live fast and die young, detonating as supernova; completely disintegrating themselves in the process. Supernova explosions can be brighter than an entire galaxy, and can be seen from very far away.
First, let's take a look at a red supergiant star. These are stars with many times the mass of the Sun, and one of the best known examples is Betelgeuse, in the constellation of Orion. Betelgeuse has 20 times the mass of the Sun, and puts out about 135,000 times as much energy as the Sun. It's one of the few stars that have ever had their disk imaged; astronomers estimate that it's 1,000 times the radius of the Sun. With that size, Betelgeuse would engulf the orbits of Mars and Jupiter in our Solar System. Astronomers guess that Betelgeuse is only 8.5 million years old, and they expect that it will detonate as a supernova within the next 1000 years or so. When it does finally go off, the supernova explosion will be as bright as the Moon in the night sky. Another example of a red supergiant is the star Antares.
Blue supergiants are much hotter than their red counterparts. A good example of a blue supergiant is Rigel, also in the Orion constellation. Rigel has a 17 times the mass of the Sun, and 66,000 times the luminosity of the Sun – it's the most luminous star in the neighborhood. It's not as large as a red supergiant, with only 62 times the radius of the Sun.

Red Giant Stars

On the other end of the spectrum are the red giant stars. While blue is the hottest color of stars, red is the coolest color they can have. A red giant is born when a star like our Sun reaches the end of its life and runs out of hydrogen fuel in its core. This forces the star to begin nuclear fusion with helium, increase in luminosity and bloat up many times its original size. When our Sun becomes a red giant, it will expand to consume the orbits of the inner planets, including Mercury, Venus and Earth.
When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star's life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf. Prominent bright red giants in the night sky include Aldebaran, Arcturus, and Gamma Crucis.
So, regular stars become regular red giants. But there are even larger red giants out there; the red supergiants. These are massive stars with more than 20 times the mass of the Sun. They enter the red giant phase of stellar evolution, but instead of merely expanding to the orbit of the Earth, they can expand to more than 1,500 times the radius of the Sun. Imagine a star that extended out past the orbit of Saturn.

Blue Giant Stars

Stars come in many shapes and sizes and they come in many colors. The color of a star depends on its temperature. The coolest stars are red, while the hottest stars are blue. And the temperature of a star depends entirely on its mass. The more massive a star, the hotter it's going to be. Stars don't get more massive or hot than blue giant stars.
If a star has enough mass, it will have a surface temperature greater than about 10,000 Kelvin and shine with a blue color. Some blue giants blaze with a surface temperature of 20,000 Kelvin or more, and are extremely luminous. Just for comparison, a star like our Sun only has a surface temperature of about 6,000 Kelvin. A blue giant star can put out 10,000 times as much energy as the Sun. The largest and hottest stars in the Universe are these blue giant stars.
A familiar example is the blue giant star Spica, located in the constellation Virgo. It is the 15th brightest star in the nighttime sky and is 260 light years distant from Earth. Another Blue Giant is Bellatrix. Although its magnitude varies slightly, this blue giant is always prominent. It is found in the constellation of Orion the Hunter.
The true monsters of the Universe are blue supergiant stars, like Rigel. These have surface temperatures of 20,000 – 50,000 Kelvin and can be 25 times larger than the Sun. Because they're so large, and burn so hot, they use up their fuel very quickly. A middle-sized star like our Sun might last for 12 billion years, while a blue supergiant will detonate with a few hundred million years. The smaller stars will leave neutron stars or black holes behind, while the largest will just vaporize themselves completely.
An even more extreme example is the blue hypergiant Eta Carinae, located about 8,000 light years away. Eta Carinae is a monster, estimated to have more than 100 times the mass of the Sun. It's burning fuel at such a tremendous rate that it puts out 4 million times as much energy as the Sun, with a surface temperature of 40,000 Kelvin. Astronomers expect Eta Carinae to detonate as a supernova in a few hundred thousand years.

Red Dwarf Stars

Our Sun is such a familiar sight in the sky that you might think stars like our Sun are common across the Universe. But the most common stars in the Universe are actually much smaller and less massive than the Sun. The Universe is filled with red dwarf stars.
Red dwarf stars are the most common kind of stars in the Universe. These are main sequence stars but they have such low mass that they're much cooler than stars like our Sun.
Astronomers categorize a red dwarf as any star less than half the mass of the Sun, down to about 7.5% the mass of the Sun. Red dwarfs can't get less massive than 0.075 times the mass of the Sun because then they'd be too small to sustain nuclear fusion in their cores.
Red dwarfs do everything at a slower rate. Since they're a fraction of the mass of the Sun, red dwarfs generate as little as 1/10,000th the energy of the Sun. This means they consume their stores of hydrogen fuel at a fraction of the rate that a star like the Sun goes through. The largest known red dwarf has only 10% the luminosity of the Sun.
With such an efficient use of hydrogen, red dwarf stars with 10% the mass of the Sun are thought to live 10 trillion years. Our own Sun will only last about 10 to 12 billion or so.
You might be interested to know that the closest star to Earth, Proxima Centauri, is a red dwarf star. Unfortunately, these stars are so small and dim that they can't be seen without a telescope.

Yellow Dwarf Stars

These medium sized stars are yellow because they have a medium temperature. Stars in this classification have a surface temperature between 5,300 and 6,000 K, and fuse hydrogen into helium to generate their light. They generally last for 10 billion years. Alpha Centauri is an example of a yellow dwarf star. Our sun is also a yellow dwarf. Near the end of their lives, these medium sized stars swell up becoming very large. When this happens to the Sun it will grow to engulf even the Earth. Eventually they shrink again, leaving behind most of their gas. This gas forms a beautiful cloud around the star called a Planetary Nebula.
When will the Sun expand into a giant, and then shrink leaving behind a planetary nebula? Don't worry; the sun is only about 5 billion years old. It still has another 5 billion years before it will expand, and then turn into a planetary nebula.
The Sun is so hot that when it dies, it will take a long time to cool off. The sun will die in about 5 billion years, but it will still glow for many billions of years after that. As it cools, it will be what is called a white dwarf star. Eventually, after billions maybe even trillions of years, it will stop glowing, at that point it will be what we call a black dwarf star. There are still no black dwarf stars in the Universe.

Main Sequence Star

The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they're all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.
A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.
The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun, or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun.

Stars

When you look outside at night you can see many beautiful stars. If you are in a dark country area you might see as many as 3000 of them. In the city you can't see nearly as many, but they are still pretty. There are several different kinds of stars in the sky. Some are very big. A couple have been found that are 100 to 200 times bigger than the sun. At the end of their lives these large stars can stretch themselves out past the orbit of the planet Uranus. Some very old stars are smaller than the Earth. Scientists study stars, and place them in groups based on how they are alike, and how they are different.