A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the closest of celestial neighbours and among others, the Local Interstellar Cloud, the neighbouring G-Cloud, the Ursa Major moving group and the Hyades. It is estimated to be at least 1000 light years in size, and is defined by its neutral-hydrogen density of about 0.05 atoms/cm3, or approximately one tenth of the average for the ISM in the Milky Way (0.5 atoms/cm3), and one sixth that of the Local Interstellar Cloud (0.3 atoms/cm3).
The Loop I Bubble is a cavity in the interstellar medium (ISM) of the Orion Arm of the Milky Way. From our Sun's point of view, it is situated towards the Galactic Center of the Milky Way galaxy. Two conspicuous tunnels connect the Local Bubble with the Loop I Bubble cavity. The Loop I Bubble is a supershell.
The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning 3,500 light-years in width and extending roughly 10,000 light-years in length. This galactic structure encompasses the Solar System, including Earth. It is sometimes referred to by alternate names such as the Local Arm or Orion Bridge, and it was previously identified as the Local Spur or the Orion Spur. It should not be confused with the outer terminus of the Norma Arm, known as the Cygnus Arm.
The Carina–Sagittarius Arm is generally thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates from the Galactic Center. These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as many HII regions, young stars and giant molecular clouds are concentrated in it.
The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 light-years away, it is now thought to lie 6,400 light years from the Solar System.
The Scutum–Centaurus Arm, also known as Scutum-Crux arm, is a long, diffuse curving streamer of stars, gas and dust that spirals outward from the proximate end of the Milky Way's central bar. The Milky Way has been posited since the 1950s to have four spiral arms — numerous studies contest or nuance this number. In 2008, observations using the Spitzer Space Telescope failed to show the expected density of red clump giants in the direction of the Sagittarius and Norma arms. In January 2014, a 12-year study into the distribution and lifespan of massive stars and a 2013-reporting study of the distribution of masers and open clusters both found corroboratory, though would not state irrefutable, evidence for four principal spiral arms.
The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.
The Serpens South star cluster is a relatively dense group of more than 600 young stars, dozens of which are protostars just beginning to form. The cluster is situated in the southern portion of the Serpens cloud. The stars are embedded in a dense filament of interstellar gas, which is part of the giant molecular cloud that has given rise to the cluster of young stars in W40. This entire complex is located at a distance of 1420 light-years from the Earth, and is approximately the same distance as the Serpens Main cluster.
The Gould Belt Survey is an astronomical research project led by the Center for Astrophysics | Harvard & Smithsonian, with the participation of several other institutions.
In astronomy, the Great Rift is a dark band caused by interstellar clouds of cosmic dust that significantly obscure (extinguish) the center and most radial sectors of the Milky Way galaxy from Earth's perspective.
The Taurus molecular cloud (TMC-1) is an interstellar molecular cloud in the constellations Taurus and Auriga. This cloud hosts a stellar nursery containing hundreds of newly formed stars. The Taurus molecular cloud is only 140 pc away from Earth, making it possibly the nearest large star formation region. It has been important in star formation studies at all wavelengths.
Cygnus-X is a massive star formation region located in the constellation of Cygnus at a distance from the Sun of 1.4 kiloparsecs.
Thomas M. Dame is Director of the Radio Telescope Data Center at the Center for Astrophysics | Harvard & Smithsonian, a Senior Radio Astronomer at the Smithsonian Astrophysical Observatory, and a Lecturer on Astronomy at Harvard University. He is best known for mapping the Milky Way galaxy in Carbon Monoxide and for the discovery of both the Far 3 kpc Arm and the Outer Scutum–Centaurus Arm of the Milky Way.
Westerhout 40 or W40 is a star-forming region in the Milky Way located in the constellation Serpens. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars. The distance to W40 is 436 ± 9 pc, making it one of the closest sites of formation of high-mass O-type and B-type stars. The ionizing radiation from the massive OB stars has created an H II region, which has an hour-glass morphology.
The Serpens–Aquila Rift (also known as the Aquila Rift) is a region of the sky in the constellations Aquila, Serpens Cauda, and eastern Ophiuchus containing dark interstellar clouds. The region forms part of the Great Rift, the nearby dark cloud of cosmic dust that obscures the middle of the galactic plane of the Milky Way, looking inwards and towards its other radial sectors. The clouds that form this structure are called "molecular clouds", constituting a phase of the interstellar medium which is cold and dense enough for molecules to form, particularly molecular hydrogen (H2). These clouds are opaque to light in the optical part of the spectrum due to the presence of interstellar dust grains mixed with the gaseous component of the clouds. Therefore, the clouds in the Serpens-Aquila Rift block light from background stars in the disk of the Galaxy, forming the dark rift. The complex is located in a direction towards the inner Galaxy, where molecular clouds are common, so it is possible that not all components of the rift are at the same distance and physically associated with each other.
The Radcliffe wave is a neighbouring coherent gaseous structure in the Milky Way, dotted with a related high concentration of interconnected stellar nurseries. It stretches about 8,800 light years. This structure runs with the trajectory of the Milky Way arms.
