Star Color and Temperature

  • Interchangeability of Color and Temperature

    • Star color is directly related to star temperature.
    • Cool stars appear red (e.g., Betelgeuse), while hot stars appear blue (e.g., Rigel).
    • Example from everyday life (restroom sink) exhibits a different relation of color to temperature (blue = cold, red = hot).
    • In the context of stars, blue indicates higher temperature, similar to blue flames of a blowtorch.

  • Black Body Radiation

    • Measurement of stars involves observing at different wavelengths and comparing them to black body radiation curves.
    • "Black body" refers to a theoretical object that absorbs all incident light without reflecting any.
    • A black body curve illustrates the peak wavelength of radiation emitted at various temperatures.
    • The Sun, at about 6,000 K, peaks in the visible spectrum of light.
    • If a star's emission does not peak in the visible range, it could mean it is too cool to be seen with the naked eye.

  • Determining Star Temperature

    • The B filter (Blue Violet) and V filter (Green Yellow) can determine a star’s unique blackbody spectrum.
    • These two filters provide insights into the temperature without needing to measure all wavelengths.
    • Example of black body spectra:
    • Star A peaks at 330,000 K (blue, hotter).
    • Star C peaks at 3,000 K (yellow, cooler).

  • Star Temperature Ranges

    • Star temperatures range from about 3,000 K (red) to 30,000 K (electric blue).
    • Our Sun is categorized as a G2 star (in the yellow range).

  • Absorption and Emission Lines

    • Absorption and emission lines in a star's spectrum indicate both chemical composition and temperature.
    • Stars above 25,000 K exhibit strong helium absorption lines.
    • Stars around 10,000 K show strong hydrogen lines.
    • Stars cooler than 4,000 K have many absorption lines due to the presence of more molecules; these lead to the complexity of their spectra.

  • Composition and Classification of Stars

    • Stars are predominantly composed of hydrogen and helium, with minor elements comprising 1% or less.
    • Variation in absorption lines arises from temperature differences, not elemental differences.

  • Spectral Classification of Stars

    • Stars are classified from O to M, where O is the hottest and M is the coolest (mnemonic: "O Be A Fine Girl, Kiss Me").
    • Each class subdivides into 10 divisions (0-9), where a lower number indicates a hotter star.
    • Example: An F9 star is warmer than a G2 star, despite the latter number being lower than 9.
    • The Sun is classified as a G2 star, representing its mid-range temperature compared to other stars.