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Part IV: Galaxies and theUniverse
Chapter 9The Milky Way Galaxy
9.1Measuring the Milky Way
The Milky Way we see in the night sky is actually agalaxyAnd we are IN this Galaxy (Capitalized = MW Galaxy)赻看成嶺謃成峰蚫近高蠛各不同不識廬山面目襻蕑葌蠇觧山襦蘇軾
To know how the Milky Way looks like, we needto know thedistanceof different stars from usOne of the first attempts was done by Herschelusing visible stars, assuming all stars having thesame luminosityUnfortunately, he was not aware that most of thegalaxy, particularly the center, is blocked fromview by vast clouds of gas and dust.
This infrared view of our Galaxy shows much moredetail of the galactic center than the visible-light viewdoes, as infrared is not as much absorbed by gas anddust.
multiwavelength_milky_way.swf
Variable StarsVariable stars are stars whose luminosities varywith timeWe may measure distance by variable starsWe have already encountered variable starsʹnovae, supernovae, and related phenomenaʹwhich are calledcataclysmic variables.There are other stars whose luminosity varies in aregular way, but much more subtly. These arecalledintrinsic variables.Two types of intrinsic variables have been found:RR Lyraestars andCepheids.
The upper plot is an RRLyrae star. All such starshave essentially the sameluminosity curve, withperiods from 0.5 to 1 day.The lower plot is aCepheid variable; Cepheidperiods range from about1 to 100 days.
These variable stars arelocated on the instabilitystrip in the H-R diagramThe variability of thesestars comes from adynamic balance betweengravity and pressureʹthey have largeoscillations aroundstability.Most importantly, thesevariable stars are muchbrighter than the mainsequence stars, and can bedetected far away
Stellar models and observations of nearby variablestars consistently show that these stars obey someperiod-luminosity relationships
This allows us to measure the distances tothese stars.RR Lyrae stars all have about the same luminosity;knowing their apparent magnitude allows us tocalculate the distance.Cepheids have a luminosity that is stronglycorrelated with the period of their oscillations;once the period is measured, the luminosity isknown and we can proceed as above.
cepheid_as_standard_candle.swfCepheidStarInDistGalaxy.mov
We have nowexpanded our cosmicdistance ladder onemore step.RR Lyrae stars aremuch more commonthan Cepheids, butless luminousWith the moreluminous Cepheids,we can estimatedistances out to about25 million parsecs
Globular Clusters(1885-1972)During the second decade ofthe 20th century, HarlowShapley studied the globularclusters and constructed amore accurate picture of theextent of our Galaxy and ourplace within it.

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