A supermassive black hole is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.
A stellar black hole is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a gamma ray burst. These black holes are also referred to as collapsars.
A rogue planet is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from planetary systems in which they are formed and later ejected, or they can also form on their own, outside a planetary system. The Milky Way alone may have billions to trillions of rogue planets, a range the upcoming Nancy Grace Roman Space Telescope will likely be able to narrow down.
Sagittarius A*, abbreviated Sgr A*, is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Lambda Scorpii.
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light. These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light. Gravitational microlensing was first theorised by Refstal (1964) and first discovered by Irwin et al (1988). The first object in the sky where it was discovered was the Einstein cross or Huchra lens 2237 +0305. The initial lightcurve of the object was published by Corrigan et al (1991). In Corrigan et al (1991) they calculated that the object causing the microlensing was a Jupiter sized object. This was the first discovery of a planet in another galaxy.
The Optical Gravitational Lensing Experiment (OGLE) is a Polish astronomical project based at the University of Warsaw that runs a long-term variability sky survey (1992–present). The main goals are the detection and classification of variable stars, discovery of microlensing events, dwarf novae, and studies of the structure of the Galaxy and the Magellanic Clouds. Since the project began in 1992, it has discovered a multitude of extrasolar planets, together with the first planet discovered using the transit method (OGLE-TR-56b) and gravitational microlensing. The project has been led by professor Andrzej Udalski since its inception.
OGLE-2003-BLG-235L (MOA-2003-BLG-53L) is a star in the constellation of Sagittarius. The first gravitational microlensing event for which a planet orbiting the lens was detected around this star. The event occurred in during July 2003. Two groups observed and independently detected the event: the Optical Gravitational Lensing Experiment (OGLE) and the Microlensing Observations in Astrophysics (MOA), hence, the double designation. It is an orange dwarf star of spectral type K, which is accompanied by a giant planet.
OGLE-2005-BLG-071L is a distant, magnitude 19.5 galactic bulge star located in the constellation Scorpius, approximately 11,000 light years away from the Solar System. The star is probably a red dwarf with a mass 43% of that of the Sun.
MOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b, is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of the lowest-mass extrasolar planets at the time of discovery. It was found when it caused a gravitational microlensing event on May 24, 2007, which was detected as part of the MOA-II microlensing survey at the Mount John University Observatory in New Zealand.
MOA-2007-BLG-192L is a low-mass red dwarf star or brown dwarf, approximately 3,000 light-years away in the constellation of Sagittarius. It is estimated to have a mass approximately 6% of the Sun's. In 2008, an Earth-sized extrasolar planet was announced to be orbiting this object.
MOA-2007-BLG-400L is a star located 22472.1 light-years away in the constellation of Sagittarius. This star is presumed to be a red dwarf with a spectral type of M3V, based on its mass of 0.35 MS.
An intergalactic star, also known as an intracluster star or a rogue star, is a star not gravitationally bound to any galaxy. Although a source of much discussion in the scientific community during the late 1990s, intergalactic stars are now generally thought to have originated in galaxies, like other stars, before being expelled as the result of either galaxies colliding or of a multiple-star system traveling too close to a supermassive black hole, which are found at the center of many galaxies.
HD 217786 is a binary star system in the equatorial constellation of Pisces. With an apparent visual magnitude of 7.78, it requires binoculars or a small telescope to view. The system is located at a distance of 181 light-years from the Sun based on parallax, and is drifting further away with a radial velocity of +10 km/s. Kinematically, the star system belongs to the thin disk population of the Milky Way.
A rogue black hole is an interstellar object without a host galactic group. They are caused by collisions between two galaxies or when the merging of two black holes is disrupted. It has been estimated that there could be 12 rogue black holes in the Milky Way galaxy.
OGLE-2011-BLG-0462, also known as MOA-2011-BLG-191, is a stellar-mass black hole isolated in interstellar space. OGLE-2011-BLG-0462 lies at a distance of 5,000 light years in the direction of the galactic bulge in the constellation Sagittarius. The black hole has a mass between 1.6 and 4.4 M☉, but might weigh as much as 7.1±1.3 M☉ The discovery of OGLE-2011-BLG-0462 makes this object the first truly isolated black hole found. OGLE-2011-BLG-0462 was discovered through microlensing when it passed in front of a background star that was 20,000 light years away from Earth. The black hole's gravity bent the star's light, causing a sharp spike in brightness that was detected by the Hubble Space Telescope. It took six years to confirm the existence of OGLE-2011-BLG-0462. Its initial kick velocity has been estimated to have an upper limit of 100 km/s.
