Using special equipment like a spectrograph or a spectroscope , astronomers can split light from space into a spectrum and examine its spectral lines to infer what compounds are emitted or absorbed. Additionally, astronomers can learn about the density and temperature of the emitting or absorbing compounds and what the magnetic field strength was like in the environment where the light was emitted or absorbed.
From spectroscopy, we have learned that stars are mostly made of hydrogen, that Saturn's moon Titan has methane in its atmosphere, that comets contain a large amount of water, and much, much more. It was by using spectroscopy that we discovered the first extrasolar planets. We cannot yet do spectroscopy on the planets themselves because we cannot see them against the overwhelming glare of their stars, but we can easily do spectroscopy on the planets' stars.
To understand why this is important, it is necessary to understand the Doppler effect of light. In the bottom spectrum you can see a number dips. Absorbing material could be the extended layers of a star or interstellar clouds of gas or dust. The absorption lines close to each other below 5, Angstroms in the bottom spectrum are the calcium H and K lines and can be used to determine how quickly stars are zooming around the galaxy.
A basic piece of information derived from a spectrum is the distance to the galaxy, or specifically, how much the light has stretched during its journey to Earth. Because the universe is expanding, the light emitted by the galaxy is stretched toward redder wavelengths as it innocently moves across space. We measure this as redshift.
To determine the exact distance of a galaxy, astronomers measure the well-studied pattern of absorption and emission lines in the observed spectrum and compare it to the laboratory wavelengths of these features on Earth.
The difference tells how much the light was stretched, and therefore how long the light was travelling through space, and consequently how far away the galaxy is. In the top galaxy spectrum mentioned earlier, we measure the strong red emission line of hydrogen-alpha to be at a wavelength of roughly 7, Angstroms. Since we know that line has a rest wavelength of 6, Angstroms, we calculate a redshift of 0.
The galaxy emitted that light when the universe was roughly These days, modern spectroscopy uses diffraction gratings to disperse the light, which is then projected onto CCD s Charge Coupled Devices similar to those used in digital cameras. The 2-dimensional spectra are easily extracted from this digital format and manipulated to produce 1-dimensional spectra like the galaxy spectrum shown below. The resultant rainbow is really a continuous spectrum that shows us the different energies of light from red to blue present in visible light.
But the electromagnetic spectrum encompasses more than just optical light. It covers all energies of light, extending from low-energy radio waves, to microwaves, to infrared, to optical light, to ultraviolet, to very high-energy X-rays and gamma rays. Three types of spectra: continuous, emission line and absorption. Each element in the periodic table can appear in gaseous form and will produce a series of bright lines unique to that element.
Hydrogen will not look like helium which will not look like carbon which will not look like iron Thus, astronomers can identify what kinds of stuff are in stars from the lines they find in the star's spectrum.
This type of study is called spectroscopy. The science of spectroscopy is quite sophisticated. From spectral lines astronomers can determine not only the element, but the temperature and density of that element in the star. The spectral line also can tell us about any magnetic field of the star.
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