Types of Spectra – Learn

Information about stars and galaxies is collected by observing the radiation that they emit through spectral analysis. Spectroscopy is the process of splitting the different wavelengths of radiation apart. The spectra are a result of electrons absorbing or emitting energy as they change energy states. There are several types of spectra which can be studied when observing celestial bodies:

  • Emission – nebulae and quasars
  • Absorption – the atmosphere of stars
  • Continuous blackbody – used to observe galaxies and the inner layers of stars

Emission Spectra

Electrons can exist in discrete energy levels called shells. When a gas is heated, electrons absorb energy and move to higher energy states. The electrons don’t last long in these higher energy states before falling back down to their original state. Energy will be released when this electron drops down to a lower energy state. The energy that is released will correspond to light of a specific frequency. Electrons can exist in, and move between multiple energy states and this can result in a large number of light bands being observed on an emission spectrum.

Absorption Spectra

When light with a continuous spectrum passes through a gas, most of the light will pass straight through undisturbed. Electrons that are in the gas can absorb energy that is of a specific value, wavelength and frequency. The particular energies that are absorbed are unique to each element. The energy is absorbed as electrons move to higher energy states. The electron will soon move back to a lower energy state and release the same amount of energy they absorbed. However, when this occurs, the energy is re-emitted in all directions. This results in only a fraction of the original energy being emitted in the direction of the observer. These smaller amounts of energy appear as dark bands on the absorption spectra. The energies that have been absorbed indicate the elements that the light has passed though.

Continuous Blackbody Spectrum

Hot bodies like lava, heated metals, tungsten filaments and stars all emit a continuous spectrum of radiation – they emit radiation at all wavelengths or frequencies. The intensity of the radiation varies for different wavelengths. A graph of intensity vs wavelength is known as a Planck curve. The shape of the graph depends on the temperature of the body and not the material it is made from. Each Planck graph has a characteristic ‘peak’ intensity that corresponds to a certain wavelength. This wavelength can be applied with Wien’s law to determine the temperature of that body.

Often it is difficult to distinguish between galaxies, stars, or clouds of gas or nebulae. Observing the spectrum of these objects can help us identify what it is that we are looking at.

  • If the spectrum is an absorption spectrum, we know that it is produced by stars.
  • If it is an emission spectrum, we know that it is produced by diffuse clouds of gas.