Chapter 31: Astronomy and Cosmology

Students explore how luminosity, defined as the total radiant energy emitted by a star per unit time, serves as a foundation for distance calculations through the inverse square law of radiant flux intensity, which describes how radiation diminishes with distance from its source. The concept of standard candles, including Cepheid variable stars and Type Ia supernovae, enables astronomers to determine distances to remote galaxies by comparing their known intrinsic brightness with their observed brightness from Earth. The chapter then addresses methods for determining stellar characteristics by treating stars as ideal black body radiators. Wien's displacement law connects a star's surface temperature to the wavelength at which it emits maximum radiation, explaining why hotter stars appear blue while cooler stars appear red. The Stefan-Boltzmann law relates stellar luminosity to both radius and temperature, allowing astronomers to calculate the physical size of distant stars once their temperature is known. The chapter culminates with exploration of cosmic expansion and the Big Bang theory. Doppler redshift observations reveal that spectral lines from distant galaxies are shifted toward longer wavelengths, indicating recession motion. Hubble's law quantifies this relationship, showing that recession velocity increases linearly with distance, a discovery that fundamentally changed understanding of the universe's nature. Working backward through this relationship and accounting for the cosmic microwave background radiation permeating space, scientists estimate the universe's age at approximately 14 billion years, establishing empirical support for the Big Bang model of universal origin and evolution.