Stellar Properties

Stars: Stellar Properties

Stars are balls of gas held together by force of gravity and generate energy and light by nuclear fusion.

A star’s “color,” or wavelength gives information on:

  1. Temperature
  2. Composition
  3. Conditions
  4. Motion (Doppler Shift)
  5. Classification Scheme

What to Measure and How to Classify Stars:

  1. Spectroscopy
    • To determine composition
      • Absorption line produced when an electron absorbs a photon; emission line produced when an electron emits a photon
      • Dual nature of light: light behaves as waves (electromagnetic waves) or as particles (photons)
      • High energy electromagnetic waves are high energy photons, low energy electromagnetic waves are low energy photons
      • Energy of a photon defined by: E = hf, where h is Planck’s constant (h = 6.63 x 10 ^ – 34 joules sec) and f is the frequency of the electromagnetic wave
      • Three Types of Spectra:
        • Continuous Spectrum: appears as a rainbow spectrum
        • Emission Spectrum: appears as distinct color lines, characteristic of chemical elements
        • Absorption Spectrum: appears as black lines on a rainbow background, reverse of emission spectrum
    • To determine temperature
      • All objects give off thermal radiation
      • Peak wavelength corresponds to maximum intensity of radiation
      • Peak wavelength of electromagnetic radiation is related to temperature
      • Wien’s Law: W = 0.00290/T
      • As temperature increases, wavelength decreases
      • The hotter an object, the bluer the radiation
    • To determine density
      • The thicker the spectral line, the greater the abundance of the chemical element present
    • To determine motion
      • Doppler shift of spectral lines
      • Red Shift = moving away
      • Blue Shift = moving closer
    • To determine distance
      • Measured in light years (ly) – distance light travels in one year and parsecs (pc) – one parsec is 3.26 light years
      • Parallax: the only direct measure of stellar distance, the angle across the sky that a star seems to move with respect to a background of distant stars) between two observation points at the ends of a baseline of one astronomical unit (A.U.); a star one parsec from Earth has a parallax of one arc second
  2. Brightness
    • Apparent Brightness
      • Affected By: absolute (true) brightness, distance, intervening space, Earth’s atmosphere, and eyes’ visual response
      • Measured by apparent magnitude “m,” relative brightness as seen on Earth; brightest star (m=1) to faintest (m=6); a 1st magnitude star is 100 times brighter than a 6th magnitude star
    • Absolute Brightness
      • Measured by absolute magnitude “M”
      • The magnitude of a star observed from a distance of 10 parsecs (1 parsec = 3.26 light years)
      • Stars further than 10 parsecs would “appear” brighter; M increases
      • Stars closer than 10 parsecs would “appear” dimmer; M decreases
    • m-M = 5 log (r/10)
  3. Distances
    • Distance as the primary factor in the decrease of stellar brightness as perceived on Earth, used to determine absolute brightness
    • Inverse Square Law: the intensity of light varies inversely with the square of the star’s distance from the Earth
  4. Mass and Size
    • MASS: For binary stars, both the period of revolution of one star orbiting the other and the distance between the two stars can be measured
    • SIZE: Diameters of stars can be determined from temperature and luminosity (calculated from absolute brightness) =>  L  =   σ  T^4  A, where L = luminosity, σ = distance, T = temperature, and A = absolute brightness
  5. Classification Scheme
    • Spectral Types: O, B, A, F, G, K, M; subtypes 0 to 9 (e.g. B1, A4, G2, and M0)
    • O stars are more than 10 times hotter than M stars
    • Developed by Annie Jump Cannon in the late 1800’s
  6. H-R Diagram: to study evolutionary tracks

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