What are they and what do they look like?
Galaxies are huge collections of stars, dust and gas and usually contain several million to over a trillion stars. Galaxies can range in size from a few thousand to several hundred thousand light-years accross. 90% of galaxies are actually composed of largely unknown substance called dark matter. Individual galaxies are separated by distances in excess of millions of light years. Every galaxy is controlled and held together by one central gravitational force. The Earth and its solar system are contained within the Milky Way galaxy, a large disk-shaped barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in width.
The Milky Way galaxy is believed to contain more than three hundred billion stars, and has a total mass of six hundred billion times the gross mass of the sun. Intergalactic space, or the space between the galaxies of the universe, is filled with an unsubstantiated plasma matter that has an average density less than one atom per cubic meter. There are believed to be more than one hundred billion galaxies within the total expanse of the universe. Galaxies with less than a billion stars are considered "small galaxies." In our own galaxy, the sun is just one of about 100 billion stars.
There are three distinct types of galaxies: ellipticals, irregulars and spirals .
Elliptical galaxies make up around 10 - 15% of all the galaxies in the Universe and mainly comprise of older low-mass stars. Typically, an Elliptical galaxy will be found close to the centre of a galaxy cluster. Elliptical galaxies are also named because of their shapes. Elliptical galaxies range from circular (remember, a circle is an ellipse!) to long, narrow, and cigar-shaped. Elliptical galaxies are denoted by the letter E. They are also given a number from 0 to 7. An E0 galaxy looks like a circle. An E7 galaxy is very long and thin.
The above information was sourced from Wikipedia
Most galaxies fit one of the three described types, but about 3% of the galaxies we observe are very different. These so-called "irregular galaxies" do not have a lot of common features. Many of them are the results of galaxy collisions or near misses. One type of irregular galaxy is called a "starburst galaxy." Starburst galaxies shine brightly as many new stars are born in a short period of time. Irregular galaxies are denoted by the letters Irr.
Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the centre into the disk. These arms are brighter in appearence than the centre of the galaxy and this is due to the continuing formation of stars. Our own Milky Way has long been considered to be a Spiral galaxy but it is very difficult to tell from our position. It is beleived that spiral galaxies are the most common galaxies found in the Universe.
The Milky Way:
For hundreds of years scientists beleived that the Milky Way was the entire Universe. This beleif was shattered in the 1920's by Edwin Hubble when he peered through his 100" telescope at Mount Wilson Observatory in California. He noticed that distant stars were contained in nebulae that were too distant to be part of our galaxy. The Milky Way is home to our Solar System and 400 billion other stars. These stars, like our sun, have the potential to supply the right conditions for orbiting planets to sustain life.
The size of the Milky Way has been a topic of debate for many years but astronomers estimate its size to be 100,000 light years accross and approximately 1,000 light years thick. To get a fair idea of the vast size of the Milky Way, take a look up at the sky at night - every object visible to the naked eye is located inside our galaxy.
Our solar system is made up of the sun and the nine planets that orbit it. The sun is situated about 26,000 light years from the centre of the galaxy on the outer edge of one of the spiral arms and it takes 250 million years for our solar system to complete one lap of the galaxy. In this lap, we (the nine planets and the sun) are travelling at 155 miles/sec. The arms of the Milky Way are named for the constellations that are seen in those directions. The major arms of the Milky Way galaxy are the Perseus Arm, Sagittarius Arm, Centaurus Arm, and Cygnus Arm; our Solar System is in a minor arm called the Orion Spur. The central hub (or central bulge) contains old stars and at least one black hole; younger stars are in the arms, along with dust and gas that form new stars. The Milky Way is located reletivly near other galaxies that make up the Local Group. Within this Local Group can be found the Milky Way's largest neighbour, the Andromeda Galaxy.
Andromeda is wider and brighter that our Milky Way and it is also a Spiral galaxy. Although Andromeda is the nearest galaxy to us, it is still 2.52 million light-years from our solar system. In 2004 astronomers beleived that they discovered the first planet outside our own galaxy and this planet is located inside Andromeda. The planet revealed itself thanks to a phenomenon called microlensing where the gravitational field of an object closer to Earth acts like a magnifying glass. The team at the time thought that they were observing two stars orbiting each other but calculations have persuaded them that they actually observed a star with a smaller, planet sized companion about six times bigger than Jupiter. In our own galaxy, more than 300 stars have been discovered orbiting stars similar to our sun, but this is the first discovery of it's kind outside the Milky Way.
The above information was sourced from Wikipedia
Fact about andromeda:
- The Andromeda galaxy is reported to have been mentioned as far back as 965 AD
- Andromeda can be seen with the naked eye appearing as a milky-blur
- If a star within Andromeda explodes (goes super nova), the event would not be seen for 2.5 million years - light travels 6 trillion miles (10 trillion Km) in one year!
- It takes 200,000 years for light to travel accross Andromeda
- For centuries astronomers regarded Andromeda as been part of our Milky Way. Only in the 1920's did Edwin Hubble discover that it was a separate Galaxy
Facts About Galaxies:
- There are 100 billion galaxies that are visible
- The nearest galaxy to our Milky Way is Andromeda
- The typical size of a galaxy is 3,260 light years to 326,000 light years across
- The average galaxy contains 40 billion stars
- The number of stars in a typical large galaxy (like our Milky Way) is 200 - 400 billion