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.Continue Reading
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 about Stellar Astronomy
A protostellar disk is created as the gas and dust of a stellar nursery collapse gravitationally in a process that eventually creates stars and solar systems. As the gas clumps together at the center of the cloud, physical forces pull it into a spinning disk shape.Full Answer >
Usually several light years across, a nebula is an extremely large cluster of clouds and dust often found between stars. Nebulae are made of many elements including carbon, hydrogen and helium.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 >
The primary difference between lenticular and spiral galaxies lies in the presence or absence of spiral arms around the galaxy's central bulge. According to the Sloan Digital Sky Survey, spiral galaxies have two distinct regions, a large central bulge and the outlying spiral arms. Lenticular galaxies also have a central bulge, but their outlying stars are less organized and more diffuse.Full Answer >