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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, H 2), and the formation of H II regions.
[4] [7] Horizontal filaments appear to emit thermal radiation, accelerating thermal material in a molecular cloud. [3] They have been proposed to be caused by the outflow from Sagitarius A*, the Milky Way's central black hole, impacting vertical filaments and H II regions of ionized gas around hot stars. [4]
About half the total mass of the Milky Way's galactic ISM is found in molecular clouds [9] and the galaxy includes an estimated 6,000 molecular clouds, each with more than 100,000 M ☉. [10] The nebula nearest to the Sun where massive stars are being formed is the Orion Nebula , 1,300 light-years (1.2 × 10 16 km) away. [ 11 ]
These filaments of cloud have dense "cores" of gas embedded within them—many of which are likely to gravitationally collapse and form stars. The Herschel results for this region, and subsequently reported results for other star-forming regions, imply that fragmentation of molecular-cloud filaments are fundamental to the star-formation process.
RCW 36 (also designated Gum 20) [5] is an emission nebula containing an open cluster in the constellation Vela.This H II region is part of a larger-scale star-forming complex known as the Vela Molecular Ridge (VMR), a collection of molecular clouds in the Milky Way that contain multiple sites of ongoing star-formation activity. [1]
The molecular cloud at these wavelengths is traced by emission from warm dust in the clouds, allowing the structure of the clouds to be probed. Wavelet analysis of the molecular clouds in the approximately 11 square degree Herschel field of view breaks up the clouds into numerous filaments, mostly in and around the Westerhout 40 region. [20]
The B cloud of the Vela Molecular Ridge is a structure independent of the others, located at about 2000 parsecs in a remote and peripheral region of the Orion Arm. In it are contained 7 of the infrared sources associated with objects of Class I, none of which has been studied in detail. [ 2 ]
This cloud covers an angular area of 4.5° × 6.5° on the celestial sphere.It consists of two major regions of dense gas and dust. The first contains a star-forming cloud (L1688) and two filaments (L1709 and L1755), while the second has a star-forming region (L1689) and a filament (L1712–L1729).