The universe is full of amazing and beautiful bodies. Some bodies are planets like the one we live on — Earth. Some objects are stars, burning balls of gas like our sun. There are moons, asteroids, comets, gas clouds and black holes. Gravity pulls the stars and the other space bodies into groups. These collections of stars, planets, moons, asteroids and other space bodies make up what is called a galaxy.
Galaxies are huge. They can contain millions and some even have trillions of stars. Most of the matter found in the universe is contained within one of the many galaxies. Scientists have no idea how many actual galaxies exist. There are so many that it is nearly impossible to count them all. Some are so far away or hidden behind dust clouds that scientists continue to work at identifying the many galaxies that exist.
Our sun is a rather small-sized star and just one of perhaps 400 billion stars in the Milky Way galaxy. The name 'Milky Way' comes from the Greek word Galaxios, which means milky circle. We live in the very edge of the Milky Way and so we can see our galaxy in the sky on a clear night. It indeed looks like a milky cluster of stars in a path above our heads.
The Mayans thought that path was the route to the underworld. Many other astronomers throughout time have observed the Milky Way and tried to explain its origin, its size, and its properties. Galileo studied the Milky Way with his early telescope. William Herschel attempted to map the Milky Way in 1785. It was Edwin Powell Hubble who in 1925 used a huge telescope to study the Milky Way and propose the existence of other galaxies. As scientists continue to study the Milky Way, they are discovering that it is far bigger than it was first thought.
Many astronomers believe that at the heart of most galaxies lies a strange object called a black hole. A black hole is a dense collection of matter. Its gravity is so intense that even light cannot escape. It is believed that the Milky Way has a black hole at its center.
The theory of black holes has existed for nearly 200 years, but scientists had not been able to investigate one until the Hubble Telescope. The Hubble has allowed scientists to observe the gravitational attraction of black holes on surrounding matter even though the black hole itself cannot be seen. To learn more, visit Black Holes - Gravity's Relentless Pull.
On June 13, 2012, NASA launched the Nuclear Spectroscopic Telescope Array — NuSTAR — to study black holes and other objects in space.
Take a peek at what scientists think might be black holes here.
Edwin Powell Hubble created a means of classifying galaxies based on their shape. There are four basic classifications with subgroups within those basic shapes.
Spiral Galaxies — Spiral galaxies are identified by their center nucleus and generally flat shape. Many have arms that spiral out from the nucleus. Our Milky Way is a spiral galaxy.
Barred Spiral Galaxies — Barred spiral galaxies are similar to spiral galaxies except that the arms lead out from a series of stars and not from the center.
Elliptical Galaxies — Elliptical galaxies can range from round to oval, but have no center bulge like spiral galaxies do.
Irregular galaxies — Irregular galaxies tend to be very irregular in shape and can look like a cloud or a gassy mass.
How do galaxies get their names?
Many galaxies have been given names based on their discoverer or their shape. The Magellan Clouds were named by Magellan's crew who saw them during their first trip around the world in 1519. The Sombrero galaxy was called such because it resembles a sombrero. Other galaxies have been designated by a catalogue of numbers. Catalogues have been created by scientists and observatories as new galaxies are discovered.