Spectroscopy

Spectroscopy is the study of how light interacts with matter. It allows astronomers to uncover detailed information about distant celestial objects, such as their composition, temperature, density, mass, distance, luminosity, and motion.

How Spectroscopy Works

When light interacts with atoms or molecules, it can be absorbed, emitted, or scattered. These interactions create spectra—patterns of light across different wavelengths—that serve as unique fingerprints for each element or molecule.

Atomic Transitions: The Source of Spectral Lines

Atoms consist of a nucleus surrounded by electrons in discrete energy levels. When electrons absorb energy, they jump to higher levels. When they fall back down, they release energy as light.

• The energy of the photon emitted or absorbed corresponds exactly to the difference between two energy levels.

• Because each element has a unique set of energy levels, it produces a distinct set of spectral lines.

Types of Spectra

1. Continuous Spectrum

• Produced by a hot, dense object like the interior of a star or molten rock.

• Appears as a full, unbroken rainbow of colors with no gaps.

• The spectrum’s shape follows Planck’s Law and peaks at a wavelength that depends on the object's temperature.

2. Emission Spectrum

• Produced by a hot, low-density gas.

• Shows up as bright lines of specific colors against a dark background.

• Each bright line corresponds to a specific atomic transition.

• Common in glowing gas clouds (nebulae) and excited atmospheres.

3. Absorption Spectrum

• Occurs when light from a continuous source passes through a cooler gas.

• Appears as a continuous spectrum with dark lines where specific wavelengths are absorbed.

• These dark lines tell us which elements are present in the intervening material, such as a star’s atmosphere.

Applications in Astronomy

• Chemical Composition: Spectral lines identify the elements present in stars, nebulae, planets, and galaxies.

• Temperature and Density: The strength and shape of spectral lines can reveal physical conditions like temperature, pressure, and magnetic fields.

• Motion and Velocity: The Doppler effect causes spectral lines to shift in wavelength if an object is moving toward or away from us—this allows astronomers to measure speed and direction.

• Star Classification: Different types of stars show distinct spectral features, which are used in stellar classification systems (like OBAFGKM).

• Redshift and Cosmology: For very distant galaxies, redshifted spectral lines help determine their distance and the expansion of the universe.