A galaxy, also known as a planetary system, is a highly compact, gravitating mass of stars, gas, debris, and other dark matter. The word galaxy came from the Greek word galaxia, which means “Milky Way”. When astronomers talk about a galaxy, they are referring to a group of hundreds or thousands of galaxies that together form the vast majority of the Universe.
The most common type of galaxy in the Universe is the bulge, which is made up of numerous compact spiral galaxies. These have satellite galaxies that, like our own, are constantly spinning in an elliptical orbit around the central black hole. Because of their close togetherness, they form a network of huge elliptical bulges. These have received more recent study than other types of globular clusters, such as elliptical spiral galaxies.
Another type of galaxy consists of millions of spiral discs that are extremely fast rotating. These discs are much further apart than the bulge and tend to rotate more slowly. Two of the largest elliptical galaxies are called Messier and Swift-VLF, which each contain over a hundred billion stars. Both of these feature elliptical spiral arms with a mass of nearly ten thousand solar masses. They also feature numerous satellite galaxies, which are much like our own. These satellites are much less distant, and are only a few hundred million light-years across.
The largest of all spiral galaxies, however, is the Milky Way, which is estimated to be only ten to twenty thousand light-years in diameter. Its spiral arm consists of about five hundred million stars, making it one of the most massive elliptical galaxies in the Universe. Because of its massive size, the Milky Way is very heavy, weighing in at almost two hundred billion solar masses. Even so, it only accounts for about half of the entire Milky Way’s mass, making it one of the most irregular galaxies known to science. One of the biggest reasons for this is that the elliptical shape of its spiral arm is irregular, which makes it difficult to study using conventional telescopes.
One of the major ways astronomers study a galaxy is by looking at it using a telescope called an optical/electron telescope. This kind of telescope is designed to look at stars at a distance of about five light-minutes, or about one light-hour, giving us a clear view of the milky way through a telescope’s optical lens. By taking into account the effects of parallactic light, which occurs when a star passes in front of a host galaxy, we can calculate the distance of a star from the nucleus of a galaxy, or the Milky Way, using this method. Estimates have range between one to five million years for the age of the Milky Way, making it the most old-known great galaxy.
The supermassive black hole itself is actually much further away than astronomers first thought. In fact, it is estimated that it holds together a hundred billion stars in its clutched arm, making it the most massive object in the entire Milky Way galaxy. Its presence has also been announced with the confirmation of a telescope that was built to detect solar systems around other galaxies, including ours. This means that it is slightly larger than our own, making it feel really big, and giving astronomers a better idea of how our galaxy and universe are pieced together. Using a supercomputer to design a model of the Milky Way that astronomers can follow, they were able to examine many other models to find out exactly where the holes and clusters were in the model, confirming the accuracy of the calculations.
Astronomers estimate that there are at least twenty five thousand galaxies within our own Milky Way, and that most of these lies between ten and fifty million light-years across. Although it is difficult to study individual galaxies, like some of the other structure in the universe, like quasars, they do form groups that tend to look similar to each other, or to a smaller background galaxy. The Milky Way is probably best studied in a group, because it has thousands of very small satellite galaxies (icycles) surrounding itself, all of which contribute to a massive, densely-settled structure that astronomers can study with telescopes.
Although astronomers have discovered many extrasolar planets orbiting close to their own, they still have only mapped the centers of several major galaxies. It is believed that every galaxy holds ten times more dark matter than the sun, although scientists don’t know how many normal galaxies there are, or even whether they are made of gas or dark matter. Estimates range from a few hundred thousand to millions of billions of galaxies, making the number much larger than the sun.