SM

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Measuring Stellar Luminosities

  • Luminosity: Total power (energy per second) a star radiates into space.

  • Apparent Brightness: Amount of starlight reaching Earth, measured as energy per second per square meter.

Relationship Between Brightness and Luminosity

  • Brightness depends on both distance and luminosity.

  • Luminosity (in watts) and apparent brightness relate by the formula:

    • Brightness = Luminosity / (4 * π * distance²)

  • Determining luminosity requires measuring distance and apparent brightness.

Stellar Distance Measurement

  • Parallax: Apparent shift in position of a nearby star against distant stars due to Earth's orbit.

  • Parallax angle relates to distance (in parsecs and arcseconds).

Stellar Temperatures

  • Thermal Radiation: Emitted spectrum depends on temperature; hotter stars emit more light and higher-energy photons.

  • Temperature range:

    • Hottest stars: 50,000 K

    • Coolest stars: 3000 K

    • Sun's surface temperature: 5800 K.

Spectral Type

  • Absorption lines indicate ionization level and temperature.

  • Spectral classification (hottest to coolest): O, B, A, F, G, K, M.

Binary Star Systems

  • Types:

    • Visual Binary: Observed orbital motion.

    • Spectroscopic Binary: Measured via Doppler shifts.

    • Eclipsing Binary: Light varies due to eclipses.

  • Mass measurement possible through orbital dynamics in binary systems.

Hertzsprung-Russell Diagram (H-R Diagram)

  • Plots stars based on luminosity and temperature.

  • Main Sequence: Most stars reside here.

  • Giants/Supergiants: Larger radii and greater luminosity compared to main-sequence stars.

  • White Dwarfs: Smaller radii and lower luminosity.

Stellar Classes

  • Classifications include spectral type and luminosity class: I to V.

  • Examples:

    • Proxima Centauri (M5.5 V)

    • Betelgeuse (M2 I)

Mass and Lifetimes

  • Higher mass stars have shorter lifetimes; smaller stars are longer-lived.

  • High-Mass Star: Blue, larger, high luminosity, short-lived.

  • Low-Mass Star: Red, smaller, low luminosity, long-lived.

Variable Stars

  • Stars that vary significantly in brightness are called variable stars.

  • Pulsating Variable Stars: Change brightness in a regular cycle.

  • Cepheid Variables: A class of pulsating variables with defined periods.

Stellar Clusters

  • Open Clusters: Few thousand loosely packed stars.

  • Globular Clusters: Dense packs with millions of stars.

  • Age determined by the main-sequence turnoff point.