Astronomy Picture of the Day's
Glossary

Cepheid: A pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy. The existence of this period-luminosity relationship was a point of contention during the 1920 Curtis-Shapley debate.

coma: A spherical cloud of material surrounding the head of a comet. This material is mostly gas that the Sun has caused to boil off the comet's icy nucleus. This gas shines both by reflected sunlight and light emitted by excited molecules. A cometary coma can extend up to a million miles from the nucleus.

comet: A chunk of frozen gasses, ice, and rocky debris that orbits the Sun. A comet nucleus is about the size of a mountain on earth. When a comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet tails can be millions of miles long. There are thought to be literally trillions of comets in our solar system out past Neptune and Pluto, but only once per decade or so does one become near and bright enough to see easily without binoculars or a telescope.

dust grains: Not the dust one finds around the house, which is typically fine bits of fabric, dirt, or dead skin cells. Rather interstellar dust grains are much smaller clumps, on the order of a fraction of a micron across, irregularly shaped, and composed of carbon and/or silicates. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies. The exact nature and origin of interstellar dust grains is unknown, but they are clearly associated with young stars.

emission nebula: A type of nebula that shines by emitting light when electrons recombine with protons to form hydrogen atoms. The electron frequently approaches the proton in steps emitting energy as light as it gets pulled in. In one of the most common "steps," the recombining electron emits a photon of red light. Since many atoms in the nebula do this all at once, the nebula appears red in color. This type of nebula is created when energetic ultraviolet light from a hot star shines on a cloud of hydrogen gas, stripping away electrons from the atoms (ionization). The free electrons can then begin the process of recombination. Plural of nebula: nebulae.

fusion: A process where nuclei collide so fast they stick together and emit a great deal of energy. In the center of most stars, hydrogen fuses together to form helium. Fusion is so powerful it supports the star's enormous mass from collapsing in on itself, and heats the star so high it glows as the bright object we see today. Scientists here on earth are trying to make nuclear fusion in the laboratory a useful energy source.

galaxy: A system of about 100 billion stars. Our Sun is a member of the Milky Way Galaxy, which is sometimes just designated by capitalization: Galaxy. There are billions of galaxies in the observable universe. Exactly when and how galaxies formed in the Universe is a topic of current astronomical research.

H II region: A region of hot gas surrounding a young star or stars that is mostly ionized. The energetic light from these young stars ionizes the existing gas. This region typically appears red as it glows with the photons emitted when elections recombine with hydrogen protons.

helium: The second lightest and second most abundant element. The typical helium atom consists of a nucleus of two protons and two neutrons surrounded by two electrons. Helium was first discovered in our Sun. Roughly 25 percent of our Sun is helium.

hydrogen: The lightest and most abundant element. A hydrogen atom consists of one proton and one electron. A hydrogen nucleus is just a single proton. Hydrogen composes about 75 percent of the Sun but only a tiny fraction of the Earth.

infrared: Light that is so red humans cannot see it. A band of the electromagnetic spectrum between the visible and the microwave. Photons of infrared light are less energetic than photons of visible light.

Messier, Charles: While hunting for comets in the skies above France, 18th century astronomer Charles Messier made a list of the positions of about 100 fuzzy, diffuse looking objects which appeared at fixed positions in the sky. Although these objects looked like comets, Messier knew that since they did not move with respect to the background stars they could not be the undiscovered comets he was searching for. These objects are now well known to modern astronomers to be among the brightest and most striking gaseous nebulae, star clusters, and galaxies. Objects on Messier's list are still referred to by their "Messier number". For example the Andromeda Galaxy, the 31st object on the list, is known as M31.

neutrino: A small particle that has no charge and is thought to have very little mass. Neutrinos are created in energetic collisions between nuclear particles. The universe is filled with them but they rarely collide with anything.

neutron star: The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugarcube of neutron-star stuff on Earth would weigh as much as all of humanity! This illustrates again how much of humanity is empty space." Neutron stars can be observed as pulsars.

planet: A spherical ball of rock and/or gas that orbits a star. The Earth is a planet. Our solar system has nine planets. These planets are, in order of increasing average distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

QSO - Quasi Stellar Object, also Quasar: QSOs are objects that, at first glance, appear as normal stars. Upon closer inspection, however, QSOs have very large redshifts (i.e. the light they emit is strongly displaced toward the red end of the spectrum). Although their exact nature is controversial, they are commonly considered to be extremely distant, unusually bright nuclei of galaxies. If so, then the light we see from them would have been emitted when the universe was a fraction of its present age.

redshift: When the light an object emits is displaced toward the red end of the spectrum it is said to be redshifted. In general, photons of light that are emitted at a source at one energy and detected by an observer at a lower energy are redshifted. Often, the redshift of an object can be measured by examining atomic absorption or emission lines in its spectrum. Redshifts can be caused by the motion of a source away from an observer. For distant objects, redshifts can be caused by the expansion of the Universe.

reflection nebula: A type of nebula that shines by reflected light. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color. Plural of nebula: nebulae.

solar wind: The wind from the Sun. More specifically, particles, usually electrons and protons, continually streaming away from the corona of the Sun. The solar wind is extremely sparse, containing only a few fast moving particles per cubic centimeter at the Earth. The exact geometry and extent of the solar wind is not well known.

star: A ball of mostly hydrogen and helium gas that shines extremely brightly. Our Sun is a star. A star is so massive that its core is extremely dense and hot. At the high stellar core temperatures, atoms move so fast that they sometimes stick to other atoms when they collide with them, forming more massive atoms and releasing a great amount of energy. This process is known as nuclear fusion. Scientists have not yet been able to use nuclear fusion as a power source here on earth, but they are trying!

supernova: The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant .

ultraviolet: Light that is so blue humans cannot see it. A band of the electromagnetic spectrum between the visible and the X-ray. Photons of ultraviolet light are more energetic than photons of visible light.

white dwarf: A star that is the remnant core of a star that has completed fusion in its core. The sun will become a white dwarf. White dwarfs are typically composed primarily of carbon, have about the radius of the earth, and do not significantly evolve further.

X-ray: Light that is so blue humans cannot see it. A band of the spectrum between the ultraviolet and the gamma-ray. Photons of X-ray light are more energetic than photons in the ultraviolet but less energetic than photons in the gamma-ray. X-radiation can go through human skin tissue but is stopped by dense bones. This property thus makes X-rays valuable in medicine.

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