Stars come in a dazzling array of colors, from cool reds to scorching blues. These hues aren't just for show – they reveal a star's surface temperature. Cooler stars glow red or orange, while hotter ones blaze blue or white.

Astronomers use special filters to measure star colors precisely. By comparing the brightness through different filters, they can determine a star's temperature, composition, and evolutionary stage. This color information helps paint a vivid picture of stellar life cycles.

Star Color and Temperature

Star color and temperature relationship

• The color of a star directly relates to its surface temperature
  • Cooler stars appear red or orange (Betelgeuse, Antares)
    • Have surface temperatures around 3,000-4,000 K
  • Hotter stars appear blue or white (Rigel, Sirius)
    • Have surface temperatures around 10,000-40,000 K
• Wien's law describes the relationship between color and temperature
  • $\lambda_{max} = \frac{2.898 \times 10^{-3}}{T}$, where $\lambda_{max}$ is the peak wavelength in meters and $T$ is the temperature in Kelvin
  • As temperature increases, the peak wavelength shifts to shorter wavelengths corresponding to bluer colors
• The color-temperature relationship results from black body radiation
  • Stars emit radiation across a continuous spectrum
  • The peak wavelength of the emitted radiation is determined by the star's effective temperature

Filters for measuring star colors

• Astronomers measure star colors using photometric filters
  • Filters allow only specific wavelength ranges of light to pass through
  • Common filter systems include Johnson-Cousins (UBVRI) and Sloan Digital Sky Survey (ugriz)
• Magnitude differences between filters provide color information
  • The B-V color index measures the difference between magnitudes in the B (blue) and V (visible) filters
    • A larger B-V value indicates a redder star
    • A smaller B-V value indicates a bluer star
• Filters enable standardized, quantitative measurements of star colors
  • Consistent use of filters allows for comparison of colors between different stars and star systems (Milky Way, Andromeda)

Color index and star characteristics

• Color indices, such as B-V, can be used to estimate star temperatures
  • Bluer stars with lower B-V values have higher surface temperatures
  • Redder stars with higher B-V values have lower surface temperatures
• Color indices provide information about star composition and evolution
  • Main sequence stars follow a clear color-temperature relationship
    • B-V values range from -0.4 for the hottest, most massive stars to +1.5 for the coolest, least massive stars
  • Giants and supergiants deviate from the main sequence color-temperature relationship
    • Their B-V values are higher (redder) than main sequence stars of the same temperature
  • Peculiar stars, such as carbon stars or stars with unusual chemical abundances, may have unique color index values
• Combining color indices with other observations like luminosity and spectral lines allows for a more comprehensive understanding of star properties and evolutionary stages (main sequence, red giant, white dwarf)

Stellar Classification and Evolution

• Stellar spectroscopy is used to analyze the light emitted by stars and determine their properties
• The stellar classification system categorizes stars based on their spectral characteristics and temperature
• The Hertzsprung-Russell diagram is a powerful tool for understanding stellar evolution, plotting stars' luminosity against their temperature or spectral class
• Stellar evolution describes the changes in a star's properties over its lifetime, influenced by factors such as initial mass and composition