Galactic Thin DiskEN C Y C LO PEDIA OF ASTRONOMY AND ASTROPHYSICSGalactic Thin DiskOf the components of our MILKY WAY GALAXY, the thin diskis the most prominent part to our eyes. It manifests itselfas the band of faint light that we see encircling the wholesky. Except for the bulge in the direction of the center ofour Galaxy, the stars that make up the Milky Way as wesee it are part of the thin disk, just as our Sun is part of thatsame population of stars. A remarkable fact is that whenwe look at the Milky Way from the southern hemispherein winter—when the center of the Galaxy is almost at thezenith at midnight—the appearance is strikingly similarto somewhat later Hubble class, edge-on galaxies such asNGC 891 (of Hubble type Sb).The young diskAs we will see below, the thin disk is made up of starsof a large range of ages. The youngest have only justformed out of the interstellar medium; the gas and dustbetween the stars out of which stars are being formed isgenerally considered to be part of the young disk. Thegas can be mapped easily using the 21 cm line of neutralhydrogen (H I) and this has been done extensively forour Galaxy sinceRADIOASTRONOMY started to be practisedin a systematic manner in the 1950s. It took untilthe 1970s, with the advent of aperture synthesis radiotelescopes, until it could be done with sufficient angularresolution in external galaxies. The neutral hydrogenextends usually much further than the stellar disk, oftenexhibiting a deviation from the plane of the inner galaxy.This can be seen in edge-on galaxies as a curving upwardon one side and a symmetric curving downward atthe opposite. In moderately inclined galaxies there arekinematic signatures of these so-called warps.The measurements of the neutral gas have to besupplemented by observations of the distribution of themolecular gas, most abundant in molecular hydrogenH2. This, however, cannot be observed directly andits distribution is inferred from radio measurements ofspectral lines of carbon monoxide, CO. This is transferredinto distributions of H2by a still rather dubious ‘Xfactor’ which descibes the ratio of H2versus CO. This Xfactor is predicted on theoretical grounds and the dubiousassumption is made that it is the same from galaxy togalaxy and with galactocentric radius in spite of knownvariations in heavy-element abundance and radiationfield (the latter being important for photodissociation (seePHOTODISSOCIATION REGIONS)). The molecular gas is found tobe concentrated to a large extent in giant molecular clouds(GMCs) that contain masses up to 106M(see INTERSTELLARMOLECULAR CLOUDS). In these GMCs star formation appearsto be occurring.The interstellar gas then is distributed in a verythin layer—much thinner than the older disk stars—with a thickness of order 100 pc. The molecular gasis more concentrated toward the galaxy’s center. Thevelocity distribution at any position in the disk is largelyisotropic with (one-dimensional) velocity dispersions oforder7kms−1.STAR FORMATION occurs in groups. This is because forthe conditions in the interstellar medium a gas cloud canonly be unstable with respect to its own gravity if its massexceeds about 105or so M. This so-called Jeans’ masscan be derived crudely as follows. Consider a sphericalregion of radius R and density ρ, or mass M =43πR3ρ.The gravitational (potential) energy can be calculated tobe =−3GM25R.The kinetic energy due to the random motions withdispersion V21/2is roughlyT =12MV2.The virial theorem then tells us that this region will notcontract under its own gravity as long as +2T is positive,or in other words as long as there is sufficient kinetic energyto overcome the gravitational force. This then translatesinto a maximum radius for stability that is called the Jeans’length,RJeans=5V24πGρ1/2and the mass contained within this radius is the Jeans’mass.In the interstellar medium near the Sun the gasdensity is of order 1 H atom cm3and the velocitydispersion is about 7 km−1. Then RJeans∼ 400 pc. Thisis more than the thickness of the gas layer. A circular areaof the disk with this radius contains about 106Min theform of gas.Clusters of newly formed stars are called associations.There are two types, depending on the kind of stars seenin them. The most prominent are theOB ASSOCIATIONS; theirbrightest stars are heavy O and B stars and they are usuallyaccompanied byH II REGIONS, where the interstellar gas isionized by the UV radiation from these hot stars. Thesecond type is called T associations; these are made up ofT TAURI STARS, which are stars of solar mass or somewhatless contracting on their way to the main sequence. Theexistence of these two types of associations is one basis fora proposed concept of bimodal star formation, in whichmassive and less massive stars are formed independently.The distribution of OB associations can be studied inmuch detail. In agreement with the fact that associationsare formed out of the interstellar medium recently, theyare also found in our Galaxy in a thin layer, comparable inthickness with that of the gas. The local system of OB starsdeviates from that of the plane of the Galaxy; it is confinedto a narrow, tilted layer; this is calledGOULD’S BELT, since itis reflected in the distribution of OB stars on the sky andthis is also generally along a great circle inclined to thatof the Milky Way. The velocity distribution of OB starsalso deviates from being isotropic, although the velocitydispersion is comparable with that of the gas. The longCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Galactic Thin DiskENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSaxis of the velocity distribution does not point toward theGalactic center (has a so-called vertex deviation); this isprobably the result of local disturbances in the smoothdensity distribution in the disk due to the spiral structureof the Galaxy.Open clustersStars are being born in groups or clusters, eachrepresenting of the order of 105M. Most clusters dissolvein the course of time as a result of the gravitationalinteractions with the surroundings. Irregularities, suchas those associated with spiral structure, are believed toplay a major role in this, but also condensations in theinterstellar medium, such as the GMCs, are important. Itis