Galaxies
Galaxies are vast assemblies of stars, gas, dust, and other cosmic materials bound together by gravity, with billions existing in the universe. They are primarily classified into two types based on shape: spiral and elliptical galaxies. Spiral galaxies, like the Milky Way, feature a flat disk structure with prominent spiral arms, while elliptical galaxies are more three-dimensional and oval-shaped. Some galaxies, known as irregular galaxies, lack a defined structure and display unique forms. The universe is continually expanding, causing galaxies to move away from each other, and scientists estimate there are between 100 and 200 billion galaxies throughout space.
The formation of galaxies remains a subject of research, with theories suggesting they originated from smaller cosmic masses that merged over time. Significant contributions to our understanding of galaxies were made by astronomer Edwin Hubble in the 20th century, who developed a classification system still in use today and established that the universe is expanding. Some galaxies, labeled as active galaxies, emit vast amounts of energy, often due to supermassive black holes at their centers. The study of galaxies combines elements of astronomy and astrophysics, continuously revealing insights about the structure and evolution of the universe.
Galaxies
A galaxy is a massive cluster of stars, gas, dust, and other particles held together by gravity in space. Billions of galaxies of varying size and shape exist in the universe. Galaxies are usually divided into two basic types based on their shape: spiral galaxies and elliptical galaxies. Galaxies are gathered together in groups, with each group ranging from a just a few to tens of thousands.
Each galaxy can contain millions to trillions of stars. The Milky Way, the galaxy that contains Earth, has about two hundred billion stars. Galaxies also encompass many other interstellar objects, such as atomic hydrogen gas, cosmic rays, dark matter, molecular hydrogen, and molecules of carbon, hydrogen, and silicon, among others. Since the universe is always expanding, galaxies move away from each other at rapid rates. Scientists believe that between 100 and 200 billion galaxies exist in space. The oldest and most distant observed galaxy, discovered by the Hubble Telescope in 2016, is a galaxy designated GN-z11. It is approximately 32 billion light-years from Earth and is at least 13.4 billion years old, meaning that it came into existence no more than 400 million years after the Big Bang.
Galaxy formation remains a mystery to scientists. Most astronomers believe that smaller galaxy-like masses formed before the creation of galaxies. These masses eventually collided to form larger galaxies. As the universe expanded, the larger galaxies clustered together to form the present-day masses. Scientists have since observed small galaxies merging into larger ones, supporting this theory of galaxy formation.
Galactic Science History
The word galaxy derives from the Greek word galaxias, meaning "milky circle." Astronomers did not study galaxies outside of the Milky Way until the twentieth century. After scientists made improvements to the telescope, astronomers were able to better observe and compare different galaxies. Scientists first observed galaxies outside the Milky Way in the 1920s. American astronomer Edwin Hubble proved that the Andromeda Galaxy, the nearest galaxy to the Milky Way, was a separate galaxy. Hubble developed a method to measure the distance of outer galaxies. His work provided the basis for the science of extragalactic astronomy, which focuses on objects outside of the Milky Way. Through his observations, Hubble discovered that galaxies were moving away from each other, leading him to conclude that the universe was expanding. This observation was later known as Hubble's law.
Types of Galaxies
Hubble developed a classification system to identify galaxies that astronomers still use today. He designated each galaxy by their basic shape: elliptical or spiral. Elliptical galaxies are three-dimensional masses usually shaped like an oval. Hubble further categorized elliptical galaxies by their length using the numbers zero to seven. The most condensed and spherical elliptical galaxies were classified at zero, and the longest elliptical galaxies were classified at seven. Scientists later distinguished elliptical galaxies by their size, calling smaller ones dwarf ellipticals and larger ones giant ellipticals.
Spiral galaxies are shaped like flat disks and have a nucleus at the center protruding from both sides. Clusters of stars float around this disk. These galaxies are labeled spiral because of the spiral arms surrounding the nucleus. Spiral galaxies usually have two spiral arms, but some have more. These spiral arms wind around the center of the galaxy forming a coil shape. Bright, hot stars and dusty gas make up the spiral arms. Two types of spiral galaxies exist: normal and barred. Normal spiral galaxies have arms that expand outward from the center. Barred spiral galaxies have a bar structure in the center. The spiral arms twist out from the edges of this bar. Normal spiral galaxies are denoted with the letter "S." Barred spiral galaxies are denoted with the letters "SB." Spiral galaxies are further classified by how tightly wound they are using the notations "a," "b," or "c." The tightest-coiled spiral galaxies are labeled "a" and the loosest "c." For normal spiral galaxies, these notations would be "Sa," "Sb," and "Sc." For barred, notations would be "SBa," "SBb," and "SBc."
Elliptical and spiral galaxies differ in size and shape. The size of elliptical galaxies varies. Dwarf ellipticals are small and range from ten thousand to thirty thousand light-years in diameter. They are usually between one hundred thousand and ten million times the mass of the sun and have just as many stars. Giant ellipticals can be between more than one hundred thousand to a few million light-years in diameter. These galaxies can be ten trillion times the mass of the sun and have about the same number of stars. Spiral galaxies are usually very large. These galaxies are normally about one hundred thousand light-years in diameter and have a mass between one hundred billion and one hundred trillion times the mass of the Sun. The Milky Way, which is about one hundred thousand light-years in diameter, is considered a spiral galaxy.
Some galaxies do not fit into the two basic categories. Galaxies with different formations are called irregular galaxies. These galaxies do not have an organized structure or symmetry. Each irregular galaxy has its own unique shape and does not look like any other galaxy. Examples include dwarf spheroidal galaxies and starburst galaxies.
Active Galaxies
Galaxies emit energy from a central source. Some galaxies emit much more energy than others do. Galaxies that emit large amounts of energy are called active galaxies. These galaxies look strange in photographs and usually appear to have materials spurting out of their centers. Active galaxies produce one hundred times more energy than the Milky Way galaxy. The energy within active galaxies is often emitted in radio waves. Examples of active galaxies include compact radio galaxies, extended radio galaxies, Seyfert galaxies, and quasars. Each of these examples display one or more of the characteristics of active galaxies. Quasars are the most distant galaxies in the universe and produce the most energy. Though they are only a few light-years in diameter, they can be up to one thousand times more luminous than a giant galaxy, which is why they can be observed through a telescope. Astronomers believe active galaxies get their energy from supermassive black holes. Many believe a supermassive black hole lies at the center of the Milky Way.
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