NGC 4414, located 60 million light years from Earth, is a typical spiral galaxy in the constellation of Coma Berenices, and is about 55,000 light years in diameter.
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Astronomers predict that at the center of each galaxy there should be a supermassive black hole (in the Milky Way's case, Saggitarius A*). This black hole might be the reason why the galaxy holds on to its gravity and stars and planets don't just fly off to intergalactic space. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky
Way. When the greeks named the Milky Way, they supposedly called it 'Milky' because they believed te godess Hera sprayed milk on the sky. Galaxies range in size from dwarfs which only have a few hundred million (108) stars, to supergiant galaxies with more than one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.
Recent estimates of the number of galaxies in the observable universe range from 200 billion (2 x 10 11) to 2 trillion (2 x 10 12) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs (or megaparsecs). Galaxies are also the reason we know the Big Bang happened. When Edwin Hubble was studying the motions of galaxies, he discovered that they were moving away from Earth, and then speculated that they have been, at some point in time, all together in one single point, foming the Big Bang theory. Nowadays we can measure how much a galaxy is moving away from us using the Doppler Shift. The Doppler Shift in light is very consistent and accurate. The redder the galaxy appears, the further away it is. There are only a handfull of exceptions to this rule. The Andromeda Galaxy, which is moving towards us, appears bluer, due to its movement towards the Milky Way. Every other galaxy is moving away, proving that the space-time fabric is expanding with time.
The constitution of Galaxies[]
Galaxies are made out of three main components: dark matter, dark energy and normal matter. Dark matter and dark energy are types of matter and energy that don't interact with regular particles are are thought to make up nearly 95% of the Universe. These are also a try to explain the formation of galaxies and their current state. In a normal occasion, a supermassive black hole isn't enough to keep an entire galaxy together, so there has to ba another explanation that our telescopes can't detect: Dark Matter. This is thought to be the glue of the Universe, the reason why it doesn't fall apart. The other 5% percent is just normal matter like atoms, photons, stars, planets, me, you, the Sun, dust, everything we have yet discovered.
The Milky Way Galaxy, a standard spiral galaxy.
Types of Galaxies[]
Here is shown a photo of the Milky Way from our perspective. This is a standard spiral Galaxy, with a radius of nearly 100,000 light-years. The Milky Way is an example of a spiral galaxies, but there are more types of galaxies like irregular and elliptical. Irregular galaxies are self explanatory. They are just irregular patches of stars and dust that come up together for one of two reasons. They can be formed by the collision of two galaxies, or just form naturally, getting that irregular shape due to a lack of 'stellar organisation'. Elliptical galaxies are just a smooth and easily recognisable type of galaxy. They rarely have any features, making them appear with an ellipsoidal shape. Irregular and elliptical galaxies tend to be the youngest and less organized galaxies. Spiral galaxies on the other hand, get their name because of their spiral arms, also easily recognizable. a spiral galaxy has many arms where stars, planets and dust gather together.