Have you ever considered the possibility that there is a planet, which is similar to ours, or that there are beings which live outside the solar system?
Have you ever tried counting the stars during the night?
People have had questions like these for centuries. Although there is no proof yet that there are extraterrestrial beings or that there is a planet out there fit for human life, humankind has made great leaps in the field of astronomy. For instance, we now know many things about galaxies, including the fact that our solar system is just a tiny part of a single galaxy called the Milky Way.
What are galaxies?
Galaxies refer to groups of stars, gas, dust and dark matter. Each galaxy has interstellar clouds and star systems or clusters. The term galaxy is derived from the Greek word “galaxias” which means milky. Therefore, the name Milky Way Galaxy is somewhat redundant. When referring to our galaxy, it is better to use simply the Milky Way.
What are the shapes of galaxies?
There are several kinds of galaxies based on shape.
- Spiral galaxy. The first kind is the spiral galaxy. It has the shape of a disk, with a central bulge. A dramatic description of what a spiral galaxy looks like is that of a whirlpool. Another representation would be that of a pinwheel. Imagine how the pinwheel looks, with spiral arms coiling out of the bulge in the center. Similar to a pinwheel, the arms of the spiral galaxy rotate, but very slowly. Nearly all spiral galaxies are older than two billion years. Examples of spiral galaxies include our own galaxy and the Whirlpool Galaxy.
- Elliptical galaxy. The second kind based on shape is the elliptical galaxy. The configuration of elliptical galaxies varies from nearly perfect spheres to relatively flattened globes. The brightest part of an elliptical galaxy is its center. According to many astronomers, elliptical galaxies show little or no movement. Compared to spiral galaxies, elliptical galaxies have less dust and gas and have fewer new stars. However, it is also said that the largest galaxies are of this shape. Many scientists believe that elliptical galaxies form when two or more galaxies collide and merge. The galaxies resulting from such collisions and mergers are called starburst galaxies.
- Irregular galaxy. The third type of galaxy is the irregular galaxy. Galaxies of this kind lack a definite shape and consist mostly of blue stars and gas cloud but only a little dust. Examples of irregular galaxies include the Sextans A and the Magellanic Clouds.
- Ring or lenticular galaxy. Other galaxies can be ring-like or lenticular. Ring galaxies have a ring structure with a bare core surrounded by stars and interstellar dust and gas. It is believed that these type of galaxies form when a smaller galaxy goes through the core of a spiral galaxy. An example of a ring galaxy is the Hoag’s Object. On the other hand, a lenticular galaxy is a combination of elliptical and spiral shapes. Its spiral arms are not well defined and are surrounded by a halo of stars arranged in an ellipse. An example of a lenticular galaxy is called NGC 5866.
How did galaxies form?
There are two main principles that scientists use to explain the formation of galaxies. The first principle is called “bottom-up”. Theories that use this principle mainly state that galaxies were derived from individual globular clusters, which merged or aggregated to form galaxies. The second principle is called “top-down”. Theories using the top-down principle state that large galaxies and clusters formed initially. Afterwards, the smaller stars formed within these galaxies.
The starting point of all these theories is the same – the Big Bang. According to the Big Bang Theory, the universe started to exist about 10 to 20 billion years ago. The first hydrogen and helium atoms started to form three hundred thousand years after the initial event. Afterwards, through a series of aggregation made possible by the force of gravity, the galaxies, stars and other materials in the universe appeared.
What is the Milky Way Galaxy?
Our own Milky Way is a spiral galaxy, which contains approximately two billion stars. Scientists predict that its mass totals to about six hundred billion times that of the Sun’s mass. Our galaxy is only able to complete a revolution after about 250 million years. Given that a light-year is the distance traveled by light in a vacuum within a year, at a speed of 300,00 kilometers per second, the diameter of the Milky Way is approximately 100,000 light years. Our galaxy contains approximately 100 billion stars and our Sun is one of them. Our own solar system is about 25,000 light years away from the center of our galaxy.
Most scientists estimate that the actual number of galaxies in the universe ranges from 100 billion to 200 billion. With their powerful telescopes and cameras, only galaxies that are 10 to 13 billion light-years away from the earth can be photographed. Therefore, there is still a lot to discover about galaxies.
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