Gamma-ray astronomyGamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy. In most known cases, gamma rays from solar flares and Earth's atmosphere are generated in the MeV range, but it is now known that gamma rays in the GeV range can also be generated by solar flares. It had been believed that gamma rays in the GeV range do not originate in the Solar System.
Galactic discA galactic disc (or galactic disk) is a component of disc galaxies, such as spiral galaxies, lenticular galaxies, and the Milky Way. Galactic discs consist of a stellar component (composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust). The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center.
Astronomical radio sourceAn astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe. In 1932, American physicist and radio engineer Karl Jansky detected radio waves coming from an unknown source in the center of the Milky Way galaxy. Jansky was studying the origins of radio frequency interference for Bell Laboratories. He found "...
Cosmic rayCosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own galaxy, and from distant galaxies. Upon impact with Earth's atmosphere, cosmic rays produce showers of secondary particles, some of which reach the surface, although the bulk is deflected off into space by the magnetosphere or the heliosphere.
Galactic CenterThe Galactic Center is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center. The Galactic Center is approximately away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.
Galactic planeThe galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms galactic plane and galactic poles usually refer specifically to the plane and poles of the Milky Way, in which Planet Earth is located. Some galaxies are irregular and do not have any well-defined disk. Even in the case of a barred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar.
Galactic bulgeIn astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy.
GalaxyA galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass.
MagnetarA magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays. The existence of magnetars was proposed in 1992 by Robert Duncan and Christopher Thompson. Their proposal sought to explain the properties of transient sources of gamma rays, now known as soft gamma repeaters (SGRs).
Galaxy formation and evolutionThe study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang.
