Star Characteristics:

How far (R in parsecs)?

Distance to nearby star determined from stellar parallax, (see Power Point) which is ½ the maximum angular difference in position:

R (in parsecs) = 1/ (in arc seconds)

1 parsec is the distance at which the parallax of a star is 1 arc second. Parallax method works for stars closer than about 100 parsecs. (1 parsec = 3.26 LY.)


How bright (L in watts)?

Luminosity at the source is determined from apparent brightness and distance (R).

Apparent magnitude (old way). We can see about 1,000 stars in Northern Hemisphere with naked eye. Hipparchus rated them from 1 to 6. A ‘1’ is 2.52 x brighter than a ‘2’, etc. We see things ranging in brightness from the sun at ‘-26′ magnitude to the faintest objects seen at about ’26’ magnitude.

Flux (new ‘apparent brightness’):

f (watts/m2) = L/4R2 = Power/unit area of sphere.

From R, the distance, we get L, the luminosity (watts of source).


How Big (r in meters)?


The Stephan-Boltzman Law gives the surface flux from surface temperature, T.

f(surface) = constant x T4 for a black body.

(Recall the black body spectrum provides T.)

Thus, the radius of a star r comes from,

f(surface) = L/4r2.

Flow of ideas: parallax–>R, R & f –>L, L & T–>r.


Spectral Class (Color-Temperature Class):


Annie Jump Cannon (1912) classified dark lines for 200,000 stars!






Subclasses: Sun is a G2V star.

  1. 1-10 Spectral Subclass.
  2. Luminosity Class I-V (I–supergiant, V–main sequence).


The Evolution of a Star:


Hertzsprung-Russell Diagram (1910): A plot of Luminosity vs Surface Temperature, T (see overlay). A one solar mass star like the sun goes through the stages of: proto-star, main sequence, red giant, planetary nebula, white dwarf.


Multiple Star Systems:


  1. Optical double–a false binary–two stars not bound together.
  2. Spectroscopic–don’t see separate stars, just separate spectra.
  3. Eclipsing–one star eclipses another. Two dips–light curve.
  4. Visual binary–actually seen as separate.


Mass Luminosity Relation: Mass determines evolution (Eddington): L = M3.5 (overlay). Mass yields Luminosity, and thus distance by r2 law of light.

Cepheid Variable Stars:

John Goodrick (1784) discovered Delta-Cephei.

Henrietta Leavit (1908)–Period Luminosity Relation:

(PPT). 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 in

going through it (Trumpler 1930):

  1. reddening–preferential scattering-blue light (why sky is blue).
  2. absorption–this affects flux and measured distance.
  3. Molecular Clouds–H2 molecules–dense MC are star formation regions (stellar nurseries like Orion Nebula).


Other Variable Stars:

  1. Mira Variables–period over 50 days. 3 mos. to several years. Mira (Ceti Omricon) in Cetus, the Whale, varies by 6 magnitudes (2.5 each magn.) In 11 months. Egg-shaped distorted pulsation. M stars in giant phase 700 x solar diameter. Long Period Variables.
  2. RR Lyrae variables–Short Period Variables–Cluster Variables–P=less than one day. Many in globular clusters with same mass and average brightness. 0.6 absolute magnitude (magnitude at 10 parsecs). Light Curve has distinctive shape with spikes. Core Helium burning–instability strip (Cepheids too).


Stellar Motion:

Proper motion: angular velocity

Transverse velocity: from distance

and proper motion.

Radial velocity: from Doppler shift–

v/c = /.

Space velocity: from transverse and

radial by Pythagorean Theorem.

C2 = A2 + B2.

Proper motion of most stars is very small. Exception: Barnard’s Star


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