The Orion Nebula is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion,[b] and is known as the middle "star" in the "sword" of Orion. It is one of the brightest nebulae and is visible to the naked eye in the night sky with an apparent magnitude of 4.0. It is 1,344 ± 20 light-years (412.1 ± 6.1 pc) away and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light-years across. It has a mass of about 2,000 times that of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk, while the two are similar. While they are similar, an accretion disk is hotter, and spins much faster. It is also found on black holes, not stars. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.
A proplyd, short for ionized protoplanetary disk, is an externally illuminated photoevaporating protoplanetary disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to Earth.
A rogue planet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf.
Photoevaporation is the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. The term is typically used in an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.
Sigma Orionis or Sigma Ori is a multiple star system in the constellation Orion, consisting of the brightest members of a young open cluster. It is found at the eastern end of the belt, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The combined brightness of the component stars is magnitude 3.80.
The Flame Nebula, designated as NGC 2024 and Sh2-277, is an emission nebula in the constellation Orion. It is about 1350 light-years away. At that distance, the Flame Nebula lies within the Orion B cloud of the larger Orion Molecular Cloud Complex.
NGC 2244 is an open cluster in the Rosette Nebula, which is located in the constellation Monoceros. This cluster has several O-type stars, super hot stars that generate large amounts of radiation and stellar wind.
HD 100546, also known as KR Muscae, is a pre-main sequence star of spectral type B8 to A0 located 353 light-years from Earth in the southern constellation of Musca. The star is surrounded by a circumstellar disk from a distance of 0.2 to 4 AU, and again from 13 AU out to a few hundred AU, with evidence for a protoplanet forming at a distance of around 47 AU.
FU Orionis is a variable and binary star system in the constellation of Orion, that in 1937 rose in apparent visual magnitude from 16.5 to 9.6, and has since been around magnitude 9. The name FU Orionis is a variable star designation in the Argelander system, which are assigned sequentially as new variables are discovered. FU Orionis is about 1,360 light years distant and is associated with the molecular cloud Barnard 35.
The Becklin–Neugebauer Object(BN) is an object visible only in the infrared in the Orion molecular cloud 1 (OMC1). It was discovered in 1967 by Eric Becklin and Gerry Neugebauer during their near-infrared survey of the Orion Nebula. A faint glow around the center-most stars can be observed in the visible light spectrum, especially with the aid of a telescope.
AB Aurigae is a young Herbig Ae star in the Auriga constellation. It is located at a distance of approximately 531 light years from the Sun based on stellar parallax. This pre-main-sequence star has a stellar classification of A0Ve, matching an A-type main-sequence star with emission lines in the spectrum. It has 2.4 times the mass of the Sun and is radiating 38 times the Sun's luminosity from its photosphere at an effective temperature of 9,772 K. The radio emission from the system suggests the presence of a thermal jet originating from the star with a velocity of 300 km s−1. This is causing an estimated mass loss of 1.7×10−8 M☉ yr−1.
The Lambda Orionis Cluster is an open star cluster located north-west of the star Betelgeuse in the constellation of Orion. It is about five million years old and roughly 1,300 ly (400 pc) away from the Sun. Included within the cluster is a double star named Meissa. With the rest of Orion, it is visible from the middle of August in the morning sky, to late April before Orion becomes too close to the Sun to be seen well. It can be seen from both the northern hemisphere and the southern hemisphere.
Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula, the Ring Nebula, the Dumbbell Nebula, the Eskimo Nebula, and the Retina Nebula. They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System, with masses of around 0.00001 times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula.
In planetary science a streaming instability is a hypothetical mechanism for the formation of planetesimals in which the drag felt by solid particles orbiting in a gas disk leads to their spontaneous concentration into clumps which can gravitationally collapse. Small initial clumps increase the orbital velocity of the gas, slowing radial drift locally, leading to their growth as they are joined by faster drifting isolated particles. Massive filaments form that reach densities sufficient for the gravitational collapse into planetesimals the size of large asteroids, bypassing a number of barriers to the traditional formation mechanisms. The formation of streaming instabilities requires solids that are moderately coupled to the gas and a local solid to gas ratio of one or greater. The growth of solids large enough to become moderately coupled to the gas is more likely outside the ice line and in regions with limited turbulence. An initial concentration of solids with respect to the gas is necessary to suppress turbulence sufficiently to allow the solid to gas ratio to reach greater than one at the mid-plane. A wide variety of mechanisms to selectively remove gas or to concentrate solids have been proposed. In the inner Solar System the formation of streaming instabilities requires a greater initial concentration of solids or the growth of solid beyond the size of chondrules.
PDS 70 is a very young T Tauri star in the constellation Centaurus. Located 370 light-years from Earth, it has a mass of 0.76 M☉ and is approximately 5.4 million years old. The star has a protoplanetary disk containing two nascent exoplanets, named PDS 70b and PDS 70c, which have been directly imaged by the European Southern Observatory's Very Large Telescope. PDS 70b was the first confirmed protoplanet to be directly imaged.
A circumplanetary disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. They are reservoirs of material out of which moons may form. Such a disk can manifest itself in various ways.
AK Scorpii is a Herbig Ae/Be star and spectroscopic binary star about 459 light-years distant in the constellation Scorpius. The star belongs to the nearby Upper Centaurus–Lupus star-forming region and the star is actively accreting material. The binary is surrounded by a circumbinary disk that was imaged with VLT/SPHERE in scattered light and with ALMA.
HK Tauri is a young binary star system in the constellation of Taurus about 434 light-years away, belonging to the Taurus Molecular Cloud.
