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
Stellar distances are inversely proportional to the parallax angle. 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 AU Parsec
Near Stars • The Hipparcos satellite measured parallax of 118,000 stars. • Resolution: 0.001 arc-second and 0.2% luminosity. • Gaia launches in 2011 to measure 109 stars in the galaxy Arcturus 11.3 pc (ESA)
The Hipparcos data provides very precise distances. Use for absolute magnitudes Precise HR diagram Distant stars can be fit on the main sequence. Measure luminosity and apparent magnitude Spectroscopic Parallax
Spectroscopic parallax assumes stars on the main sequence. 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
Type I Clusters Hot young stars Lots of gas and dust Abundant in heavy elements Active star formation Type II Clusters Old red stars No gas and dust Few heavy elements No star formation, just old stars Cluster Types
Standard Candle • Up to 30 pc distance is measured with parallax. • Less certainty to 300 pc • Longer distances by spectroscopic parallax • The best measure of large distances are variable stars. • Luminosity directly related to the period.
Thermal motion in a star relates the speed to potential energy. 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
Cepheid variables are massive relatively cool stars. ~ 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 s Cepheid Variables
RR Lyrae variables are short period white 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 s RR Lyrae Variables
Variables in M3 RR Lyrae stars in one night time lapse
Cepheid and RR Lyrae stars fall in a narrow band on the HR diagram. Instability strip Not on main sequence As stars pass through band they oscillate. Instability Strip
The sun is in a galaxy called the Milky Way. Observed as a diffuse band Millions of stars in a telescope The Milky Way is thicker in some directions. Appears as a band across the sky Band of Stars
Disk • The band of the Milky Way is the same view a viewer would have sitting inside a disk of stars. • This disk has type I stars with gas and dust. sun sun top view side view
Halo • Astronomers measure the distance to globular clusters. • Type II globular clusters are in a sphere around one point. • This sphere is the galactic halo. • The center of the sphere is the center of the galaxy. sun globular clusters
Size and Shape • To view the galaxy from inside, we • measure the distance to globular clusters • measure distributions of hydrogen gas in the disk. • The Milky Way is 50,000 pc across with a central bulge. • The stars group in arms.
Galactic Structure • The galactic nucleus is bright and massive. • It is obscured by the dust of the galactic disk. • The Milky Way is probably similar to M83.