Distances Near objects appear to move more than far objects against a distant horizon. Trigonometric parallax is used to measure distance to near stars. Parallax time A r y Earth’s orbit d distant stars time B near stars
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Trigonometric parallax is used to measure distance to near stars.Parallax
Earth’s radius fixed
Define distance by angle
The parsec (pc) is the distance that would result in one arc second of parallax.
1 pc = 3.086 1016 m
1 pc = 2.06 105 AUParsec
Arcturus 11.3 pc (ESA)
Use for absolute magnitudes
Precise HR diagram
Distant stars can be fit on the main sequence.
Measure luminosity and apparent magnitudeSpectroscopic Parallax
Better to average stars at the same distance
Globular clusters are dense with 100,000 stars in a 20-100 pc region with less than 0.3 pc separating the stars.
Open clusters tend to be smaller and younger.Star Clusters
Radial pressure waves move at the speed of sound.
The period of vibration is inversely proportional to the square root of the density.Vibrational Modes
~ 4 to 15 M
Color classification F to K
The period and apparent luminosity determine the distance.
Density ~ 10-3 kg/m3
Vibrational period ~ 106 sCepheid Variables
~ 1 M
Color classification A
These are type I stars.
Found in globular clusters
Useful for galactic distances
Density ~ 10 kg/m3
Vibrational period ~ 4 104 sRR Lyrae Variables
RR Lyrae stars in one night time lapse
Not on main sequence
As stars pass through band they oscillate.Instability Strip