Absorption line: A dark band or decrease in light intensity in a spectrum caused by atoms absorbing certain wavelengths from passing light. Each element or molecular compound produces a unique pattern of absorption lines in a spectrum, allowing astronomers to identify the presence and quantity of each element in the object being observed.
Active galaxy: A galaxy that has an unusually bright center, possibly because it harbors a giant nursery of hatching stars, or because it may contain a central "engine," possibly a massive black hole, that produces enormous energy as it swallows surrounding matter.
Angstrom: A unit of measure equal to one ten-billionth of a meter. As many as one million Angstroms would fit within the diameter of a human hair. It is often used as a measure of the wavelengths of light, especially in the optical and ultraviolet regions of the light spectrum. There are 10 Angstroms in one nanometer (nm), another unit often used for measuring very small sizes.
Arc minute: 1/60 of a degree. The full moon is about 30 arc minutes, or half a degree, across.
Arc second: A unit of measurement for very small angular sizes; equal to 1/3,600 of a degree, or 1/60 of an arc minute. An arc second is about the smallest angular size typically measurable from a good ground-based observing site.
Deuterium: An isotope of hydrogen, also known as "heavy hydrogen." A deuterium atom's nucleus contains one proton and one neutron, rather than just a single proton like regular hydrogen. Deuterium was created shortly after the Big Bang, and has been slowly destroyed in stars since that time, in a process known as "astration."
Diffraction grating: A device etched with many fine parallel lines or grooves that is used to spread light into its component wavelengths, or spectrum.
Electromagnetic spectrum: A term used to describe the entire range of wavelengths of electromagnetic radiation, including gamma rays, X-rays, ultraviolet, visible (optical), infrared and radio radiation. Any part of the electromagnetic spectrum can be categorized according to the size of its wavelengths. The shorter its wavelength, the higher the light's energy. Visible light is just a small piece of the electromagnetic spectrum. Its wavelengths are from about 4,000 to 7,000 Angstroms (400 to 700 nanometers).
Emission line: A bright band or increase in light intensity in a spectrum caused by atoms emitting light at specific wavelengths as the electrons transition to lower energy levels in the atom. Emission lines arise from gas that has been heated or energized by a power source, such as a bright young star or a supernova blast wave.
Intergalactic medium (IGM): An extremely tenous distribution of matter (primarily hydrogen and helium) in the extensive regions between galaxies and groups of galaxies in the distant universe. Despite its low density, the IGM may harbor much more matter than all of the known stars and galaxies in the Universe, and yet it is extremely difficult to study directly. The IGM may hold keys to understanding how structures such as galaxies formed out of the diffuse "soup" of material created in the Big Bang. The far ultraviolet spectral region offers great promise for studying this elusive component of the Universe.
Interstellar medium (ISM): The vast regions of space between the stars in a galaxy. Although we think of space as "empty," the ISM contains regions of gas and dust from which new stars are forming and to which dying stars return their processed material, in a process known as galaxy evolution. The ISM is a dynamic place, with everything from super-cold clouds of gas near absolute zero (-459.4 degrees Fahrenheit) to regions heated by stellar winds or supernova explosions to millions of degrees. Interestingly, even the densest parts of the ISM would be considered an excellent vacuum on Earth.
Ionization: The process of removing electrons from atoms. An atom with equal numbers of electrons and protons is said to be neutral. Removing one or more electrons creates an atom that is "ionized" and has an electrical charge. Ionization can occur when atoms are exposed to powerful electromagnetic radiation (called "photoionization"), or when atoms collide with other atoms or particles (called "collisional ionization").
Isotope: An atom is classified as belonging to a particular element based on the number of protons in its nucleus. However, two atoms of the same element may have differing numbers of neutrons, which gives them different atomic weights and makes them different "isotopes." A specific isotope is identified by the combined mass of the protons and neutrons; for example, the common form of carbon has an atomic mass of 12, meaning it has 6 protons and 6 neutrons, but a radioactive carbon isotope, carbon-14, has 6 protons and 8 neutrons. Chemically, it is still carbon. Deuterium is an isotope of hydrogen.
Light year: A unit of distance used in astronomy, equivalent to the distance light travels in one year, or about 5.9 trillion miles. The nearest star to the sun is 4.3 light years, or about 25 trillion miles away.
Main sequence stars: A term used to describe stars of differing mass and surface temperature that all shine by fusing hydrogen into helium. This is the most stable period of a star's evolutionary history. Our sun is a main sequence star.
Nebula: A general term describing a diffuse cloud of dust and gas in interstellar space.
Nucleosynthesis: A process whereby lighter elements are fused together to create heavier elements. This process occurs primarily in the centers of stars, but can also occur during the explosions of stars at the end of their lifetimes. Nucleosynthesis of simple elements like hydrogen (and its isotope deuterium) and helium occurred in the early universe just minutes after the Big Bang.
Parsec: A unit of distance used by astronomers. It is equivalent to 3.26 light years, or 19.2 trillion miles. A kiloparsec is 1,000 parsecs and a megaparsec is one million parsecs.
Photon: A packet of electromagnetic energy that can be thought of as a "light particle" traveling at the speed of light (about 186,000 miles per second).
Planetary nebula: A halo of gas and dust ejected by low mass stars near the end of their evolution. Through processes that are not entirely understood, stars less massive than a few times the sun expel their outer layers, leaving behind the "core" of the star as a white dwarf star. The nebula ultimately disperses back into the interstellar gas, enriching this gas with material from the star.
Quasars: Extremely distant objects that look like stars when viewed through a conventional telescope, but actually emit much more energy than an entire galaxy. Many astronomers believe quasars are a more energetic version of nearby active galaxies.
Recombination: The process whereby ionized atoms capture free electrons, thus reducing their state of ionization. The opposite of ionization.
Red shift: The systematic shifting of spectral features (emission lines or absorption lines) toward longer wavelengths (or toward the "red" in optical light), usually caused by an object's motion away from the observer (the "Doppler effect"). Motion of an object toward the observer would cause the opposite effect, or a "blue shift" of the spectral features.
Resolution: This term can have different meanings depending on context. With regard to light analysis, "spectral resolution" refers to a spectrograph's ability to see fine detail in an observed spectrum. A resolution of 30,000 means that the spectrograph can "see" details that are only 1/30,000th the wavelength of the light being observed. For example, at light wavelengths of 1000 Angstroms and a resolution of 30,000, FUSE can observe details in a spectrum as small as 0.033 Angstroms. If two spectral lines were separated by only this amount, FUSE could separate them enough to see them as two separate lines; this would be like seeing two adjoining sheets of paper in a stack of paper 10 feet (3 meters) tall.
Spectrograph: An instrument that separates radiation into a spectrum and records the spectrum in one way or another for later analysis. Early ground-based spectrographs used photographic plates to record spectra, but modern instruments use sophisticated electronic detectors to record the light for quantitative analysis in a computer. Such detectors can be made to "see" light, such as ultraviolet light, that would otherwise be invisible to our eyes.
Spectroscopy (or Spectral analysis): The technique of light analysis that involves interpretation of data from a spectrograph. In astronomy, spectroscopy amounts to a technique of "remote sensing"; we can learn about temperatures, densities, chemical compositions, and motions of distant objects using only the different kinds of light energy they send to us. Spectroscopy is responsible for much of the quantitative information we know about the Universe around us.
Supernova: A titanic explosion of a single star that, after millions or even billions of years, has come to the end of its life cycle. Supernova explosions return much of a star's processed material back into space where it mixes with other gas and eventually forms into new stars and planets. Only stars that begin life more than several times the mass of our sun are thought to die in this fashion.
White dwarf: A star that has burned all of its nuclear fuel and shrunk under the effect of gravity to about the size of a planet. These small stars have retained much of their mass, however, resulting in matter so dense that a piece the size of a golf ball would weigh 35 metric tons. Our sun will eventually collapse to a star of this type.
A product of the FUSE Project at Johns Hopkins University. (8/98)
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