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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.