ESO Astronomical Glossary - G
The central region of a spiral galaxy is called the galactic nucleus. Galactic nuclei usually contain a high density of stars and gas. A supermassive black hole is thought to lie at the centre of most galaxies. The nucleus of a spiral galaxy is also known as the galactic bulge.
Galaxies are large structures of gas and dust, held together by gravity, where stars reside. They are further classified according to their size (see dwarf galaxy) and shape (see elliptical galaxy, spiral galaxy, irregular galaxy). They contain varying amounts of gas and dust, from which new stars can be born.
Galaxy clusters are large assemblies of galaxies that are bound together by gravity. They typically contain 50 to 1000 galaxies of all shapes and sizes and a large amount of dark matter according to models. The nearest galaxy cluster to our Local Group is the Virgo cluster, at a distance of approximately 60 million light years.
Gamma rays are a high-energy form of electromagnetic radiation produced only in the most violent events in the Universe. They are at the shortest-wavelength end of the electromagnetic spectrum with energies higher than X-rays. They can only be detected from space.
Gamma-ray burst (GRB)
Gamma ray bursts are short and intense flashes of gamma rays, lasting from milliseconds up to several minutes, followed by a longer lasting afterglow. As gamma rays can only be detected from space, dedicated satellites are used to track the sky for GRBs; ground-based telescopes then provide further observations at other wavelengths. GRBs are thought to be signs of a hypernova explosion or of a collision of two neutron stars, or of a neutron stars with a black hole.
Giant molecular cloud
Giant molecular clouds are interstellar clouds of cold gas (mainly molecular hydrogen) and dust that weigh as much as tens or hundreds of thousands of Suns. Mainly found in spiral galaxies, they are the sites for the majority of star formation. Once stars have formed inside the clouds, the power of their radiation will slowly heat and eventually dissipate the cold gas around them. Giant molecular clouds are also referred to as 'dark nebulae' or 'Bok globules', after Dutch astronomer B. Bok who first observed them in the 1940s, because of their dark appearance against bright backgrounds.
Giant stars are stars that are nearing the end of their life. No more hydrogen fuel remains in the core, and his causes them to become unstable and swell in size. Every star goes through one or more giant stages before ending its life as a white dwarf or undergoing a supernova explosion.
A globular star cluster is a spherical group of up to a million stars held together by gravity. These remote objects orbit the central regions of galaxies out to great distances and consist of old stars. Around 150 globular clusters are known in the halo of the Milky Way galaxy.
A gravitational lens is a massive object in space, such as a galaxy or galaxy cluster, that warps space and bends light that passes by it, due to its gravitational forces. Lensing studies are extremely valuable in and cosmology in particular, as their study can yield information on the nature and distribution of dark matter and dark energy. Gravitational lenses also allow astronomers to detect distant objects that would otherwise be too faint to detect. When the lens is very massive, such as a cluster of galaxies, warping or even multiple images of the background source are clearly seen in the image, astronomers speak of macrolensing. The study of very weak lensing, or microlensing, where the warp is not strong enough to be distinguished, has become very important in the search for exoplanets and MACHOs.