Study of the Milky Way Galaxy.docx - Study of the Milky Way...

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Study of theMilky Way GalaxyStarsare not distributed randomly throughout space. Many stars are insystems consisting of two or three members separated by less than 1,000AU. On a larger scale,star clustersmay contain many thousands ofstars.Galaxiesare much larger systems of stars and usually include cloudsof gas and dust.Milky Way GalaxyThe Milky Way Galaxy as seen from Earth.© Dirk HoppeThe solar system is located within theMilky Way Galaxy, close to itsequatorial plane and about 8kiloparsecsfrom the galactic centre. Thegalacticdiameteris about 30 kiloparsecs, as indicated by luminous matter.There is evidence, however, for nonluminous matter—so-calleddark matter—extending out nearly twice this distance. The entire system is rotatingsuch that, at the position of theSun, the orbital speed is about 220 km persecond (almost 500,000 miles per hour) and a complete circuit takesroughly 240 million years. Application ofKepler’s third lawleads to anestimate for the galactic mass of about 100 billion solar masses. Therotational velocity can be measured from theDoppler shiftsobserved inthe21-cm emissionline of neutral hydrogen and the lines of millimetrewavelengths from variousmolecules, especiallycarbon monoxide. At greatdistances from the galactic centre, the rotational velocity does not drop offas expected but rather increases slightly. This behaviour appears to requirea much larger galactic mass than can be accounted for by the known(luminous) matter. Additional evidence for the presence of dark matter
comes from a variety of other observations. The nature and extent of thedark matter (or missing mass)constitutesone of today’s major astronomicalpuzzles.There are about 100 billion stars in the Milky Way Galaxy. Starconcentrations within the galaxy fall into three types: openclusters,globular clusters, and associations (seestar cluster).Open clusterslieprimarily in the disk of the galaxy; most contain between 50 and 1,000 starswithin a region no more than 10 parsecs in diameter.Stellarassociationstend to have somewhat fewer stars; moreover,theconstituentstars are not as closely grouped as those in the clusters andare for the most part hotter.Globular clusters, which are widely scatteredaround the galaxy, may extend up to about 100 parsecs in diameter and mayhave as many as a million stars. The importance to astronomers of globularclusters lies in their use as indicators of the age of the galaxy. Becausemassive stars evolve more rapidly than do smaller stars, the age of a clustercan be estimated from itsH-R diagram. In a young cluster the mainsequence will be well populated, but in an old cluster the heavier stars willhave evolved away from the main sequence. The extent of the depopulationof the main sequence provides an index of age. In this way, the oldestglobularclustershave been found to be about 12.5 billion years old, whichshould therefore be the minimum age for the galaxy.

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Donahue
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