page_title: Bulge (astronomy)
section_title: Central compact mass
hierarchical_section_title: Bulge (astronomy) / Central compact mass
caption_attribution_description: English: Nestled within this field of bright foreground stars lies ESO 495-21, a tiny galaxy with a big heart. ESO 495-21 is just 3000 light-years across, a fraction of the size of the Milky Way, but that is not stopping the galaxy from furiously forming huge numbers of stars. There are also indicators for a supermassive black hole in its centre - an unusual component for a galaxy of its size.
context_page_description: In astronomy, a galactic 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. 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. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.
context_section_description: Most bulges and pseudo-bulges are thought to host a central relativistic compact mass,
  which is traditionally assumed to be a supermassive black hole. Such black holes by definition can not be observed directly (light cannot escape them), but various pieces of evidence suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly with bulge properties.
  The M–sigma relation relates black hole mass to the velocity dispersion of bulge stars, 
  while other correlations involve the total stellar mass or luminosity of the bulge, 
  the central concentration of stars in the bulge, 
  the richness of the globular cluster system orbiting in the galaxy's far outskirts,
  and the winding angle of the spiral arms.
  Until recently it was thought that one could not have a supermassive black hole without a surrounding bulge.
  Galaxies hosting supermassive black holes without accompanying bulges have now been observed.
  The implication is that the bulge environment is not strictly essential to the initial seeding and growth of massive black holes.
caption: ESO 495-21 may host a supermassive black hole, an unusual feature for a galaxy of its size.[10]
exif: {"Image ImageDescription": "Nestled within this field of bright foreground stars lies ESO 495-21, a tiny galaxy with a big heart. ESO 495-21 is just 3000 light-years across, a fraction of the size of the Milky Way, but that is not stopping the galaxy from furiously forming huge numbers of stars. There are also indicators for a supermassive black hole in its centre - an unusual component for a galaxy of its size."}