The Properties of Stars

What about stars other than the Sun?

History of Star Names

• knowledge of Ptolemy's catalog of stars passed on to Europe by Arabian astronomers about 1000 yrs ago

• al- is Arabic article ---> Aldebaran, Altair, Alcor (for that matter, algebra, alchemy, alkali, almanac ...)

• only about 6000 stars visible with the naked eye (and only the brightest are named)

Distances to Stars

• light-years and parsecs (1 pc = 3.26 ly)
  • light-year is distance that light travels in year = 10¹⁶ m = about 6 million million miles

  • parsec is distance that produces parallax shift of 1 arcsec as seen from Earth = 40 million times Sun's diameter = about average separation of stars in Milky Way galaxy

  • more parsecs ---> less parallax shift

  • review parallax (applet)
    

• nearest stars (here or here), stellar ``forest'' (to scale, redwood forest trees would be separated by about 3 Earth radii!!!)

• cluster main sequence fitting (if know distance to nearby cluster, compare offsets between main sequences)

  

• RR Lyrae, Cepheids (period-luminosity relation) (demo)

• proper motion

  

Brightness of Stars

• apparent brightness
  • number of photons collected at Earth from star

  • depends on star's intrinsic brightness and distance

  • defined in relation to Sun or to another prominent star

• relative brightnesses to Vega

  • Sun ---> 4 x 10¹⁰ units of brightness

  • Sirius (apparently brightest star in the night sky) ---> 3.6 units of brightness

  • inverse square law of light and assumption that Sirius and the Sun have equal total luminosities ---> Sirius is sqrt(4 x 10¹⁰/3.6) or 100,000 times farther away than the Sun (but, this assumption is flawed -- Sirius is intrinsically brighter than the Sun -- so Sirius is even further away)

• luminosity (intrinsic or absolute brightness)

• dust (microscopic grains blown off carbon-rich stars, also consist of silicates, ices, even organic molecules)
  • extinction
    • dust grains absorb optical and UV light, re-emit in far infrared ---> dimming of starlight at most wavelengths

      

  • reddening

    • grains absorb light and re-emit it in different direction (scattering)

    • scattering more efficient at small wavelengths ---> red light passes through clouds of dust, while blue light is scattered

    • light from star behind dust cloud is reddened

    • observer looking at dust cloud from side would see bluish color from blue light scattered out of beam

    • blue light from reflecting nebulae caused partly by dust scattering and partly by blue light emitted by hot star

      

      

    • reddening and scattering responsible for sunset

      

      

      

What can we measure about stars that helps us understand them?

• luminosity

• color

  

• spectra

  

• classify and plot (and hope to see some patterns...)

Spectral Classes of Stars

• 100 yrs ago women ``computers'' at Harvard classified hundreds of thousands of stars by looking at photos of their spectra

• the women were paid 25 cents/hr, less than 1/2 the rate for men, and they could not be staff members, take classes, earn a degree

• wavelengths of prominent spectral lines ---> what elements made up each star

• Annie Jump Cannon (born in 1863) worked out a classification system based on surface temperature of stars

  

Description	Spectral class	Surface Temperature	Source of Absorption Lines
hottest, bluest	O	30,000 K	ionized helium atoms
Bluish	B	18,000 K	neutral helium atoms
bluish-white	A	10,000 K	neutral hydrogen atoms
white	F	7000 K	neutral hydrogen
yellowish-white	G	5500 K	neutral hydrogen, ionized calcium
orangeish	K	4000 K	neutral "metal" atoms
red	M	3000 K	molecules and neutral metals

The Hertzprung-Russell Diagram

• dependences on temperature and luminosity

  

• temperature, luminosity ---> diameter if you use the Stefan-Boltzmann law:

  

  where sigma is a constant and equals 5.67 x 10^-8.

  

• what happens when we plot the temperatures and luminosities of many stars?

  

  

• main sequence
  • smooth relationship between surface temperature and luminosity

  • stars, like Sun, converting hydrogen to helium in their core by nuclear fusion

  • stable, due to hydrostatic equilibrium

  • massive main sequence stars are larger, more luminous, and hotter than lower mass main sequence stars

  • massive ms stars are also shorter-lived than less massive ones (a 10x solar mass star lives 2.0 x 10⁷ yrs on ms)

  • most main sequence stars are between 50 times and 1/10th as massive as the Sun

• luminous stars are big and may be hot (blue) or colder (red giants and supergiants)

• dim stars are small and may be hot (white dwarfs) or cool (red dwarfs)

• giant stars and supergiant stars must be bigger than Sun because they are more luminous but have about same temperature

• lists of nearest stars and of apparently brightest stars are different!
  • giants and supergiants are the least common, but the easiest to see!

  • white dwarfs (which lie off of ms) and cool, small, dim red dwarfs on ms are most of stars in our Galaxy

Can we learn anything else about the stars?

many stars are in binaries --> mass, using Kepler's and Newton's laws

spectroscopic binary applet
(most figures on this page come from http://zebu.uoregon.edu)