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Cepheid Variable Stars:John Goodrick (1784) discovered Delta-Cephei.Henrietta Leavitt (1908)--Period Luminosity Relation:(overlay). Stars varying in brightness with period 1-50 days have linear relation between period and luminosity. This gives luminosity and thus distance to stars even in other galaxies.
The Interstellar Medium:1. Dust--recycled star material. Light undergoes to changes ingoing through it (Trumpler 1930): A. reddening--preferential scattering-blue light (why sky is blue). B. absorption--this affects flux and measured distance.2. Molecular Clouds--H2 molecules--dense MC are star formation regions (stellar nurseries like Orion Nebula).
Cepheid Variable Stars:
John Goodrick (1764-1786), deaf person, discovered variable star Delta-Cephei at age 17. He died at age 22!
Henrietta Leavitt (1908)--Period Luminosity Relation:Stars varying in brightness with period 1-50 days have linear relation (in logarithmic plot) between period and luminosity. This gives luminosity and thus distance to stars, even in other galaxies.
The distance ladder—Cepheids take us out to about 25 Mpc
We now want to study the ‘interstellar medium’!
The Interstellar (between the stars) Medium:
1. Dust--recycled star material.
Dust grains in space are very complex!
1930: Trumpler observes open clusters. Finds dust is dimming and reddening spectrum.He is able to calculate influence of dust on apparent brightness. This changed distance to stars radically!
A.reddening--preferential scattering-blue light (why sky is blue).
Why the sky is blue.
Reflection nebulae are blue for the same reason.
Nebula in Cepheus
B. absorption--this affects flux and inferred distance.
2. Dense Molecular Clouds--H2 molecules--star formation regions.
Possible origin: Monatomic H atoms meet in chat room (on dust grain)
to make H2
Dense molecular clouds form stellar nurseries like Orion Nebula
Proper Motion (angular movement of star)
The star with the largest proper motion is called Barnard's Star. It moves 10.3 seconds of arc per year
Comparison of two photographic plates taken 22 years apart shows evidence of it’s real space motion
(denoted by an arrow).
Proper motion gives transverse velocityDoppler shift gives radial velocityPythagorean theorem then yields space velocity.
Radial velocity is in the line of sight (a line from the observer to the star).Transverse velocity is across the line of sight.
