The science of spectroscopy is one of the most important fields of astronomy. Spectroscopy is the study of the composition of an object by examining the light it emits. The light emitted by an object such as a star contains information about what that star is made of. To understand how light contains such information, we need to understand the behavior of electrons and atoms.
Energy levels of electrons
Spectrum of neon: The dark lines are where electrons absorb photons
Most of us are probably familiar with the basic model of the atom. You have a central nucleus containing protons and neutrons, with electrons orbiting the nucleus. A more accurate model, however, is to think of the electrons not as objects orbiting the nucleus in a manner similar to the planets around the sun, but rather as an electron cloud. Electrons do not go around the nucleus in a single path. Rather, electrons exist in different energy levels. Electrons can jump from one energy level to another, either moving to a higher energy level or dropping to a lower energy level. In order for an electron to move to a higher energy level, it must be hit by a particle of light called a photon. When a photon hits an electron, that electron will absorb the photon and its energy, allowing the electron to jump to a higher energy level. To drop to a lower energy level, that electron will emit the same photon back into space.
Interestingly, different atoms will have different electron energy levels. For example, hydrogen has different energy levels than oxygen. The different energy levels between atoms mean that all photons emitted by their electrons will have a different amount of energy. Thus, depending on the energy of the photon, we can determine which atoms emitted those photons. Using what’s called a spectroscope, scientists can split a beam of light into its component colors and analyze those colors for any dark bands. The dark bands are where electrons absorb photons, allowing scientists to determine what kinds of atoms absorb those photons.
The composition of what objects can we see?
A spectrum of the sun used to determine its composition
Stars are the main sources of light in the universe, and so spectroscopy usually involves starlight in one way or another. When a star emits light, that light must pass through the star’s atmosphere. Atoms in the atmosphere will absorb and emit photons of the star, allowing astronomers to determine the star’s composition. In addition, some types of nebulae emit their own light, which allows scientists to determine their composition as well. Perhaps one of the most interesting applications of spectroscopy is the analysis of planetary atmospheres. When planets pass in front of their star, light from that star must pass through the planet’s atmosphere. The star’s photons will be absorbed by atoms in the planet’s atmosphere and then beamed back into space. Here on Earth, we can capture these photons in a spectroscope and determine the composition of the planet’s atmosphere. As this technology develops, we may soon have the ability to search for signs of life in the atmospheres of other planets.
Aidan Remple July 2, 2022 in Science
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