A galaxy is a gravitationally bound system of matter consisting of stars and remnants of stars, interstellar gas and dust, and dark matter as well. The word galaxy comes from the Greek word galaxias, which translated means milky in appearance.
There is a wide range in the size of galaxies from dwarf galaxies with just a few hundred million stars to supergiant galaxies which can have as many as 100 trillion stars; in all cases, these stars and other matter revolve around a central core known as the galactic center or galactic core. The 3 basic size categories of galaxies are 1) dwarf galaxies, 2) medium size spiral galaxies, and 3) gigantic elliptical galaxies.
Galaxies are categorized into 3 main shapes as they are seen visually – spiral, elliptical, and irregular. What follows is a synopsis of these 3 main categories of galaxies.
The vast majority of spiral galaxies consist of a flat disk containing stars and their remnants, interstellar gas and dust, and also dark matter which rotates around the center, also known as the galactic center, forming a sort of pinwheel shape. There is a large bulge of stars and other matter in this galactic center which may be indicative of a supermassive black hole pulling matter into it. This is thought to be the case in our own Milky Way Galaxy; Sagittarius A is a supermassive black hole at the center of the galaxy with about 4 million times the mass of the Sun. Supermassive black holes are the largest type of black hole and they contain quantities of matter anywhere from hundreds of thousands to billions of times the mass of our Sun. These bulges at the center of galaxies often have a faint halo of stars and other matter surrounding them – globular clusters of stars which are orbiting the galactic center.
Spiral galaxies get their name from the appearance of spiral structures, or arms, which emanate away from the galactic center and further into the galactic disk. It should be noted here that these spiral structures are often brighter than the bulge of the galactic center. This is due to the fact that more active star formation is generally occurring here and since these stars are earlier in their evolution they tend to ‘burn’ hotter and brighter.
Somewhere around two-thirds of all spiral galaxies also have a bar-like structure which extends from the galactic center to the beginning of the spiral structures, or arms. It is now believed that around 8 billion years ago only about 10 percent of spiral galaxies had this bar structure, at roughly 2.5 billion years ago approximately 25 percent had the bar structure, and in the present time, as previously stated, about 67 percent have this structure. Our own Milky Way Galaxy does, in fact, have this bar-like structure coming from its center, but it is somewhat difficult to see because of the Earths position in the galaxy. An infrared space telescope called the Spitzer Space Telescope has helped to confirm this fact.
Our Milky Way Galaxy is one of the medium-sized spiral galaxies, consisting of anywhere from 100 billion to 400 billion stars, depending on which estimate you look at and is about 100000 light-years across. We just don’t have the ability to get a really accurate estimate of the number of stars yet. The Milky Way Galaxy has 5 distinct arms – 2 major arms, Perseus and Sagitarrius, and 3 minor arms, Orion, Scutum-Cruz, and Norma-Centaurus. The Sun is a star located close to the Orion arm – it is very hard to accurately identify the arms of the Milky Way Galaxy since we are located within the plane and can’t photograph or observe them directly – we have to rely on observations and measurements of the positions of stars.
Spiral galaxies are the most common type of galaxy in the observable Universe with estimates ranging from 60 percent to about 77 percent – the Hubble Space Telescope has greatly increased our knowledge in this area. Our close neighbor galaxy, the Andromeda Galaxy, is the largest galaxy in the Local Group of galaxies(this includes the Milky Way) at about 220000 light-years across and roughly one trillion stars, about twice the highest estimate for the Milky Way. The largest known spiral galaxy known at the present time is a galaxy known by its catalog designation as NGC 6872 which is five times larger than the Milky Way Galaxy and located 212 million light-years away from Earth in the constellation of Pavo(the peacock), which is only visible in the southern hemisphere. NGC 6872 is a bar type of spiral galaxy about 522000 light-years across with roughly 1 to 2 trillion stars and is estimated to be approximately 5 billion years old.
Galaxies with an ellipsoidal and mostly smooth featureless shape are known as elliptical galaxies. Elliptical galaxies are believed to be somewhere around 10 to 15 percent of the galaxies in the Virgo Supercluster of galaxies – this is a huge mass of galaxies which in turn contains the Virgo cluster of galaxies and the Local Group cluster of galaxies, of which the Andromeda Galaxy and the Milky Way Galaxy are members. It should be noted here that elliptical galaxies are not the most common type of galaxy in the Universe; spiral galaxies are the most common and dominant type of galaxy.
Elliptical galaxies are now thought to be generally older galaxies with minimal star formation; they are composed mostly of older and lower mass stars with an interstellar medium more devoid of any type of matter than that found in spiral galaxies. In general, the stars in an elliptical galaxy are much older than those found in spiral galaxies. Also, elliptical galaxies can range from dwarf galaxies consisting of less than 100 million stars to huge supergiant galaxies of 100 trillion stars and perhaps even more.
Elliptical galaxies have several characteristics distinguishing them from other types, or categories, or galaxies. First, they are masses of stars which have a spherical or ovoid shape, and second, these stars tend to have mostly radial motions, or orbits, around their galactic center rather than the more planer and rotational types of motion seen in spiral galaxies. Third, they tend to have significantly less interstellar matter, in the form of gas and dust, than spiral galaxies. This, in turn, results in less new star formation – the stars in elliptical galaxies, as previously mentioned, are mostly older and lower mass stars which results in a more reddish color for these types of galaxies.
It is believed that all massive elliptical galaxies have a supermassive black hole in their center, the mass of which is directly proportional to the mass of the galaxy. This is evidenced by the velocity dispersion of stars surrounding this supermassive black hole in the center. These elliptical galaxies are more 3-dimensional than their spiral galaxy counterparts and stars tend to orbit their galactic centers in more random types of motion with higher degrees of eccentricity than the stars in the flatter rotational planes of spiral galaxies. These galaxies are more likely to be found in galaxy clusters. There is a wide range in the size and the mass of elliptical galaxies. Sizes can vary from several thousand light-years across(diameter) to more than 700000 light-years across, while the mass can vary from 100000 to 100 trillion times that of the Sun, a much greater range than for any other type of galaxy.
Irregular galaxies do not have the distinct shape that a spiral or elliptical galaxy has, such as a central bulge or spiral arms, and are thought to make up about 25 percent of all the galaxies in the Universe. These irregular galaxies tend to have a large amount of interstellar gas and dust and many were at one time spiral or elliptical galaxies themselves, but some external gravitational force may have caused them to become irregular in shape. They tend to be smaller than the other types of galaxies, many of them one-tenth the size of the Milky Way Galaxy, or even less. Some irregular galaxies are in the process of colliding with other galaxies; the gravitational force of the other, often larger, galaxy may be the cause of their irregularity.
The Magellanic Clouds are 2 smaller galaxies which orbit our Milky Way Galaxy – the Small Magellanic Cloud is a small irregular type of galaxy while the Large Magellanic Cloud is actually classified as a smaller spiral galaxy, although in years past it was classified as an irregular galaxy.
The Small Magellanic Cloud is a dwarf galaxy only about 7000 light-years across containing several hundred million stars. It is a distance of about 200000 light-years from Earth and is visible mainly in the southern hemisphere although it can be seen at times below a latitude of 15 degrees in the Northern Hemisphere.
The Large Magellanic Cloud is somewhat larger than the Small Magellanic Cloud but still a dwarf spiral galaxy only about 14000 light-years across containing about 10 billion stars which are about one-tenth the number in the Milky Way Galaxy. It is located at an approximate distance of 163000 light-years from Earth and is the fourth largest galaxy in the Local Group of galaxies behind Andromeda, the Milky Way, and Triangulum. The Large Magellanic Cloud is visible mainly in the Southern Hemisphere, although it can be seen south of 20 degrees latitude in the Northern Hemisphere, between the constellations of Dorado and Mensa. When viewed in dark night skies away from any light pollution it appears as a faint cloud-like object about 20 times as long as the Moon.
How Many Galaxies Are In The Universe?
We’ll end or discussion of galaxies with the question – how many galaxies are in the Universe? Over the decades and centuries, our knowledge of the Universe has steadily evolved. At one time, hundreds of years ago, it was thought that the Sun revolved around the Earth; some even believed the Earth was flat. Then, as we gradually attained more and more knowledge we realized that the Earth was a round globe, a sphere, which actually revolved around the Sun. Eventually, we realized that our entire Solar System was only a tiny speck in a huge mass of stars called a galaxy. As we kept learning even more we realized that this galaxy we live in, known as the Milky Way, was only one of billions of galaxies in the Universe. And today, we now know that there are at least 2 trillion galaxies in the observable Universe. This figure may even go up greatly in the future – is there a multiverse? And if so, how many universes would there be, each full of trillions of galaxies? At the present time, no one can really know, with our current state of technology this is an unanswerable question. But one thing that is known right now is that our Milky Way Galaxy is only a tiny speck in the Universe, and that speck may keep getting smaller and smaller.