Galaxies are made of stars, dust and dark matter, all held together by gravity. They come in a variety of shapes, sizes and ages, and many have black holes at their centers.Know More
Galaxies contain a different number of planets, star systems, star clusters and types of interstellar clouds. In between them is a sparse interstellar medium of gas, dust and cosmic rays. The black holes at the center of most galaxies are considered to be the primary driver of active galactic nuclei found at the core, and their surroundings produce enormous amounts of energy that astronomers can see over great distances. Material surrounding the black hole is accelerated outwards by its jets. Other galaxies contain objects like quasars, the most energetic bodies in the universe, at their cores.
Galaxies are categorized according to their apparent shape, referred to as their visual morphology. A common form is the elliptical galaxy which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms, and those with irregular shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Interactions between neighboring galaxies, which can result in a merger, sometimes induce significantly increased incidents of star formation leading to starburst galaxies.Learn more in Stellar Astronomy
The four types of galaxies are spiral, barred spiral, elliptical and irregular. Each group spans large variations in size, number of stars, star density and general configuration.Full Answer >
Spiral galaxies are the most common type of galaxy in the universe, making up 77 percent of all known galaxies. These galaxies have long arms that wrap around a bright bulging center.Full Answer >
Astronomers classify galaxies by their shape into elliptical, spiral and irregular categories. Each group is further subdivided into more specific types, such as dwarf ellipticals or barred spirals.Full Answer >
There are between 30 and 54 known galaxies in the local group to which the Milky Way belongs. Some astronomers include dwarf galaxies in this number.Full Answer >