Absorption spectrum. A pattern of dark lines produced when the light from a continuous spectrum passes through a thin gas whose atoms or molecules absorb certain wavelengths of light. Thus an absorption spectrum can be used to identify the elements in the absorbing gas.
Accretion. The process whereby planets grew by accumulating dust grains and gases from the clouds that surrounded the primeval Sun.
Achromatic lens. A lens made by cementing together two lenses made of different types of glass, each with a different index of refraction. Thus one half of the lens counteracts the aberration caused by the other half, and the resulting image is free of chromatic aberration.
Adaptive optics. The use of computer-directed actuators to bend a telescope’s mirror to counter the effects of atmospheric turbulence and produce a clearer image.
Additive color mixing. Adding one colored light to another to produce a third color.
Anaerobic bacteria. Bacteria that are killed by the presence of oxygen, and therefore must exist in oxygen-free environments such as fetid water or dead tissue. They probably represent some of the oldest forms of life, dating back to the time before there was oxygen in Earth’s atmosphere.
Aqueous humor. The watery fluid that fills the space in the eye between the cornea and the lens.
Astigmatism. An aberration in a lens (including the human eye) caused when the curvature of the lens is such that the lens cannot focus both horizontally and vertically at the same time.
Astrometric technique. The technique of detecting extrasolar planets by measuring the gravitational pull that the planet exerts on its star’s proper motion.
Atom. The smallest unit of matter that can take part in chemical reactions. Each chemical element (i.e., oxygen, gold, iodine) consists of atoms unique to that element. Atoms are composed of a nucleus that contains protons and neutrons (except for hydrogen, whose nucleus is a single proton), and electrons that orbit around the nucleus.
Atomic weight. The mass of an atom, determined by the number of protons and neutrons it contains. (The atom’s electrons have negligible mass.)
Autotroph. Organisms that can live without needing to ingest organic compounds. For example, green plants, through photosynthesis, use the energy of sunlight to create foodstuffs for themselves out of water and chemicals in the soil in which they grow.
Bandwidth. In communications, the range of frequencies needed to transmit or receive a signal.
Big bang theory. The cosmogenic theory that is based on the concept that the universe began in a singular event in which all the matter and energy we now observe was created. The big bang theory, in one variant or another, is favored by most cosmologists over the steady state theory.
Bioluminescence. The production of light by living organisms such as deep-sea fishes, fireflies, etc.
Black hole. The collapse of a very massive star can proceed until the object becomes so dense that not even light can escape its powerful gravitational field and all the mass of the collapsing star shrinks into a dimensionless point.
Brain stem. The part of the brain at the top end of the spinal column; it projects into the base of the skull. Apparently the oldest part of the brain, it is often referred to as the reptilian brain, since it looks rather like the brain of a reptile. Very basic functions such as heartbeat and breathing are controlled by the brain stem.
Camera obscura. Latin for “dark room.” A tightly-closed chamber that is completely darkened inside, except for light allowed into it from a pinhole in one side.
Carbon dating. Also called radiocarbon dating. Living organisms ingest both ordinary carbon-12 and its radioactive isotope carbon-14 from the carbon dioxide in the air and/or eating other organisms. When the organism dies, the carbon-14 slowly decays. By measuring the ratio of carbon-12 to carbon-14 in a fossil, it is possible to estimate the time when it was alive. This technique is good to an age of about 120,000 years.
Cataract. A clouding of the eye’s lens, caused by aging, disease, or genetic disposition, that degrades the transparency of the lens. Untreated, it can lead to blindness. Modern treatments include total replacement of the defective lens with a plastic lens.
Cerebellum. The part of the brain that is responsible for motor activities such as balance and movement. The cerebellum appears to be the depository for some kinds of memory, particularly memory of learned responses.
Cerebral cortex. A layer of intricately folded gray cells that lies atop the cerebrum, the cerebral cortex contains more cells than any other part of the human brain. It is responsible for learning and other “higher” mental faculties.
Cerebrum. The largest part of the human brain, the cerebrum is a mass of whitish fibers. It is overlaid by an intricately folded layer of gray cells no more than an eighth of an inch thick called the cerebral cortex. The cerebrum coordinates sensory information.
Chemical evolution. The process in which long-chain carbon molecules grew more and more complex, more than three billion years ago, until they developed the ability to reproduce themselves exactly out of simpler molecules. Chemical evolution led to the first living molecular creatures.
Chemiluminescence. Emission of light from a substance caused by a chemical reaction other than heating it.
Chromatic aberration. Literally, “colored distortion.” A distortion of lenses that produces rings of color in the image, which can be solved by using achromatic lenses.
Circadian rhythms. Daily variations (circadian: about a day) in bodily chemistry that regulates growth, body temperature, sleeping, eating, work habits, and other daily routines.
Coherence. The property of waves of electromagnetic energy (especially light) in which the waves have the same frequency and spacing between them.
Color constancy. The fact that the colors the eye perceives remain fairly constant despite the spectral power distribution of the light illuminating them.
Complimentary colors. Two colors that produce white when mixed together.
Conservation of angular momentum. In any spinning body, the energy invested in the spinning remains constant. If the body contracts, the spin rate increases. If the body grows larger, the rate of spin decreases.
Conservation of mass/energy. The amount of mass and energy in any closed system will always remain constant, whether that system is a sealed box or the universe as a whole. Until 1905, it was thought that energy and matter were two separate entities. Einstein’s theory of special relativity showed that energy and matter are two facets of the same thing; this is the meaning of his famous equation, E = mc2.
Constellation. A pattern of stars that suggests a shape, such as an animal or a figure from mythology. The International Astronomical Union has divided the sky into eighty-eight such patterns, which astronomers use to help identify and name celestial objects.
Continuous spectrum. A blend of many colors, usually obtained when a dense object reaches a temperature at which it emits light.
Cornea. The tough, transparent outermost layer of the eye. The cornea is curved and helps to focus light entering the eye before the light reaches the lens.
Corpus callosum. The thick band of neural tissue that connects the left and right hemispheres of the cerebrum.
Cosmic inflation. The concept that the universe grew very rapidly in the first few moments after the big bang.
Cosmic rays. Not rays at all, but rather energetic particles produced in deep space, presumably by supernovas.
Cosmological constant. A mathematical term that Einstein inserted into his general theory of relativity when he realized that the theory predicted the universe must be either expanding or contracting. Einstein believed the universe to be static. When latter evidence showed that the universe does appear to be expanding, he called the cosmological constant “the greatest mistake of my career.”
Critical day length. Most plants will not flower until the day reaches a certain length. Autumn bloomers, for example, will not flower until the daylight hours become short enough, while spring bloomers will not flower until the daylight hours become long enough. (See photoperiodism.)
Deuteranopia. The form of color blindness in which the person cannot distinguish green.
Diabetic retinopathy. The most common cause of blindness among adults, affecting about one-quarter of all diabetics, in which the blood vessels that supply the retina leak fluid and blood into the eyeball. Laser surgery can reduce the loss of vision.
Diffraction grating. A plate of metal or glass scored with fine lines, used as a prism to spread light into a spectrum.
Diffraction. The slight spreading or bending of a light beam when it passes through a narrow slit or goes past the edge of an obstruction. The light beam forms a pattern of light and dark bands.
Doppler effect. More properly, the Doppler-Fizeau effect. Austrian physicist Christian Doppler showed that sound waves from a moving source will shift toward higher frequencies as the source approaches, and toward the lower frequencies as it recedes. French physicist Armand Fizeau showed that light behaves the same way, shifting toward the blue end of the spectrum as the light source approaches and toward the red as it recedes.
Dyes. Color materials that are usually dissolved in a solvent, such as gelatin or clear plastic.
Dynamo. A device for producing an electrical current, now usually called a “generator.” It transforms the mechanical energy of a spinning electrical conductor (usually an “armature” or copper wire) into electrical energy. Invented in 1830 by Michael Faraday.
Electromagnetic spectrum. The entire range of electromagnetic energy, from radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
Electron. Negatively-charged subatomic particles that orbit the nuclei of all atoms, electrons are the basic particles of electricity. They are about 1836 times lighter than protons.
Emission spectrum. A pattern of bright lines produced by glowing gases. The emission lines can be used to identify the elements in the gas.
Endogenous pacemaker. An internal clock, presumably within the brain, that controls the body’s circadian rhythms.
Estivate. Going dormant through the hottest (and often driest) part of the year.
Ether. Any of a series of chemical compounds used medically for anesthesia. (See luminiferous ether.)
Excimer laser. A laser in which the working medium is a mixture of a noble gas (for example, xenon) and a halogen (such as fluorine). The mixture is called an excimer, which is an abbreviation of excited dimer.
Extrasolar planets. Planets orbiting stars other than the Sun.
Farsightedness. See hypermetropia or presbyopia.
Fluorescence. Short-lived luminescence.
Focal point. The spot where light is concentrated by a lens.
Galaxy. An assemblage of billions of stars. Our Sun and solar system are part of the Milky Way galaxy, which contains at least 100 billion stars and enough loose gas and dust to build billions more. Galaxies may be spiral, elliptical, or irregular in shape. More than a billion galaxies have been observed.
Gamma rays. The shortest wavelength electromagnetic radiation.
General relativity. See relativity.
Geocentric theory. The concept that the Earth is at the center of the solar system. In ancient times it was believed that the Earth was at the center of the entire universe. Also called the Ptolemaic theory, after Claudius Ptolemaeus, the second-century A.D. Greek/Egyptian astronomer.
Glaucoma. Usually caused by an increase in the fluid pressure of the aqueous humor that fills the space between the cornea and the iris. Untreated, the increasing pressure can damage the optic nerve, causing blindness. Although glaucoma cannot be cured, it can be controlled through medication (eye drops), surgery, or laser treatment; often two or more treatments are used in combination.
Half-life. A measure of the time it takes for the atoms of a radioactive element to spontaneously break down, or decay, into another element or isotope. A radioactive element’s half-life is the time it takes for half of its atoms to decay into lighter, so-called “daughter” elements. For example, uranium-238, the most common type of uranium, has a half-life of 4.51 billion years. That is, in any given quantity of uranium-238, half of it will have decayed into lead in 4.51 billion years.
Halogen. A group of five nonmetallic elements: fluorine, chlorine, bromine, iodine, and astatine, in order of their chemical activity.
Heliocentric theory. The concept that the Sun is at the center of the solar system. Also called the Copernican theory, after Nicholas Copernicus, who proposed the idea in 1543.
Hertzian waves. Radio waves, so-called after the German physicist who discovered them in 1888, Heinrich Hertz.
Heterotroph. Organisms that ingest organic compounds (such as other creatures) to live. Animals are heterotrophs.
Hibernate. Going dormant through the coldest part of the year.
Hippocampus. Rooted in the Greek word for “seahorse,” which this tiny structure resembles slightly in shape, the hippocampus is a memory-storage area in the brain involved in vision.
Hologram. A three-dimensional image produced by a laser.
Holography. The method of producing three-dimensional images by using laser light.
Hypermetropia. Farsightedness (also known as hyperopia and presbyopia). The inability to focus on objects close to the eye.
Infrared light. Light (i.e., electromagnetic waves) of wavelengths longer than the visible red, but shorter than microwaves. Invisible to the eye, infrared radiation can be sensed by the skin.
Interference patterns. Patterns of light and dark created when two light waves interact with each other.
Intraocular lens (IOL). A plastic replacement for the eye’s natural lens.
Inverse square law. The brightness of a light source decreases as the square of its distance. Thus doubling the distance cuts the brightness to one-quarter. All forms of electromagnetic energy—and gravity—follow the inverse square law.
Ion. An atom that has lost one or more of its orbital electrons.
Isotope. Different forms of the same chemical element, which have the same number of orbital electrons and protons in the nucleus but a different number of neutrons in the nucleus. Because the number of electrons determines the atom’s chemical behavior, the various isotopes of an element are all chemically the same element, despite their varying atomic weights.
Lakes. Pigments that consist of particles that have been dyed.
Laser. A device that produces beams of intense, monochromatic, coherent light. The word laser is an acronym from light amplification by stimulated emission of radiation.
LASIK surgery. LASIK is an acronym for Laser in-Situ Keratomileusis, a surgical procedure in which an excimer laser is used to remove corneal tissue. The procedure treats nearsightedness, farsightendness, and astigmatism.
Latent image. The image that forms on exposed film which cannot be seen until the film is treated chemically (or “developed”) to produce visible negative and positive images.
Lateral brightness adaptation. The sensitivity of one area of the retina is modified to some extent by the intensity of light falling on the retinal areas around it. For example, an area of gray will appear lighter when surrounded by white than it will when surrounded by black.
Lateral inhibition. The way the eye adjusts to increases in illumination by closing down the iris, reducing the amount of light allowed into the eye by up to a factor of sixteen, and regulating the retina’s sensitivity to it by chemical reactions of the pigments in the rods and, in the case of bright light, particularly in the cones.
Leonardo box. An apparatus for physically showing how an object should be drawn from various points of view, developed in the Renaissance to allow artists to learn how to draw an object that is foreshortened, that is, distorted because of the angle at which it is viewed.
Light-year. The distance (in a vacuum) that a photon will travel in one year, just under six trillion miles, or about 9.45 trillion kilometers.
Limbic system. Situated atop the brain stem, deep inside the core of the brain, the limbic system is often referred to as the mammalian brain since it is highly developed in mammals. The limbic system is intimately involved in emotional responses, particularly those that are directly connected to survival, such as eating, fighting, and sex.
Luminescence. The emission of light from a substance caused by some mechanism other than heating it. Short-lived luminescence is called fluorescence; longer-lived luminescence is called phosphorescence.
Luminiferous ether. A substance hypothesized to pervade outer space which was thought to serve as the medium to propagate light waves. It was never detected. Once physicists realized that electromagnetic energy can propagate through the vacuum of space, the concept of a luminiferous ether was discarded.
Macular degeneration. The progressive breakdown of the cone cells in the center of the retina, an area known as the macula, which slowly blurs the eye’s central vision and ruins color vision, although side vision is not affected. While laser treatment can seal leaking blood vessels that often accompany macular degeneration, there is no effective therapy for the disease itself. Victims of macular degeneration can be helped by magnifying glasses of various types, but nothing now known can halt the progress of the disease or reverse its effects.
Main sequence star. The stable phase of a star after it has condensed out of an interstellar gas cloud and achieved nuclear fusion of hydrogen into helium. The Sun is in its main sequence phase.
Maser. Acronym for “microwave amplification by stimulated emission of radiation.”
Monochromatic light. Light of one color only. Sunlight, often called white light, consists of a broad mixture of wavelengths from 400 to 700 nanometers (nm). Monochromatic light consists of one individual color, in a wavelength band of no more than 50 to 100 nm.
Myopia. Nearsightedness, the inability to focus clearly on objects at a distance.
Nearsightedness. See myopia.
Nebula. A catch-all word used by astronomers to describe anything faint and fuzzy. Before they were known to be galaxies in their own right, many such objects were referred to as nebulas (or nebulae).
Neonatal jaundice. Caused by the inability of a baby’s incomplete liver to break down the substance bilirubin and excrete it. Almost half of all newborn infants, especially those born prematurely, are susceptible to neonatal jaundice. Untreated, it can cause brain damage, mental retardation, and death. Neonatal jaundice can be treated by exposure to intense light.
Neutrino. A subatomic particle of no electric charge and little or no mass, neutrinos are produced in nuclear reactions. They are so unreactive that a neutrino could penetrate fifty light-years of lead without being stopped.
Neutron star. The collapsed core of a star that has suffered a supernova explosion. The matter of the star is packed so densely that protons and electrons are squeezed together into neutrons. Neutron stars are only a few miles in diameter, although they may contain as much material as several Suns.
Neutron. A subatomic particle with no electrical charge and a mass almost the same as that of the proton. Neutrons are found in the nuclei of all atoms except hydrogen.
Noble element. Six elements which do not enter into chemical reactions, except under very special conditions. They are, in order of their atomic weight, helium, neon, argon, krypton, xenon, and radon. All are gaseous under ordinary condition, hence they are often referred to as the noble gases.
Nova. An exploding star, so called because in pretelescopic times a bright new (“nova” in Latin) star would appear where none was visible before the explosion.
Nuclear fission. The process in which the nuclei of atoms split apart to produce lighter nuclei and release energy.
Nuclear fusion. The process in which the nuclei of atoms combine to produce heavier nuclei and release energy.
Optic chiasm. A system of nerves leading from the retinas at the rear of the eyeballs into the brain, which mixes the nerve impulses from each retina so that each half of the brain receives sensory information from both eyes.
Parallactic shift. The amount of change in position that a star undergoes when viewed from two different positions. Astronomers usually measure the star’s position from opposite ends of the Earth’s orbit. Observed parallactic shifts are all less than one second of arc.
Parallax. The change in apparent position of an object when viewed from two different positions. Astronomers use the parallactic shift of a star against the background of fainter stars to measure the star’s distance.
Parsec. A measure of astronomical distance equal to 3.26 lightyears. The word is a combination of “parallactic shift” and “second of arc.” If a star showed a parallactic shift of one second of arc (1/3600th of a degree), its distance would be 3.26 lightyears away. This is approximately the angular size that a 25-cent piece would show at a distance of three miles. There are no known stars as close to our solar system as one parsec.
Phosphorescence. Long-lived luminescence.
Photoelectric effect. The act of photoionization, most commonly seen when a beam of light is used to complete an electric circuit, as in an “electric eye” door.
Photoionization. The ability of photons to knock electrons off atoms, thereby ionizing them.
Photon. The fundamental unit of light; a quantum of electromagnetic energy.
Photoperiodism. In biology, the mechanism that determines the timing of certain activities based on changes in the length of the day. (See critical day length.)
Photosynthesis. From Greek roots that mean “putting together with light.” The process used by green plants to produce food out of water and nutrients in the soil, using the energy of sunlight.
Pigments. Colored powders that are suspended in a medium such as oil.
Planetesimal. A body that approaches the size of a planet. As the planets of our solar system grew out of primeval gas and dust grains, many planetesimals must have been formed. (See accretion.) Earth’s Moon is probably the result of a collision early in the history of the solar system between our planet and a planetesimal that was almost the size of present-day Mars.
Plasma. A gas that is ionized: that is, some or all of its constituent atoms have been stripped of orbital electrons. A plasma consists, then, of free electrons, ions (atoms that have lost one or more electrons), and un-ionized atoms. While the free electrons can carry electrical currents and be affected by magnetic fields, the plasma as a whole is electrically neutral, since the negative charges of the electrons are balanced by the positive charges of the ions.
Population inversion. A condition in which atoms or molecules are maintained in an excited state and prevented from undergoing spontaneous emission.
Presbyopia. See farsightedness or hypermetropia.
Primary color. The three colors which, when mixed, can produce all the other colors of visible light. They are red, green, and blue. These are the colors to which the cone cells in our retinas are sensitive.
Proper motion. The motion of a celestial object across the observer’s field of view.
Protanopia. The form of color blindness in which the person cannot distinguish red.
Proton. A positively charged subatomic particle found in the nuclei of all atoms. Its mass is 1.67 x 10-24 grams, or nearly 1836 times the mass of the electron.
Pulsar. Neutron stars that emit pulses of radio, light, and other electromagnetic energy. They spin on their axes in seconds.
Pumping method. The method used to impart energy into the medium used to produce laser action. The most common pumping methods are optical, electrical and chemical.
Quantum. (Plural quanta) The fundamental unit of energy. For electromagnetic energy, quanta are also called photons.
Quantum mechanics. Also called quantum theory. Based on the concept that energy comes in discrete packages, or quanta, quantum mechanics explains the behavior of energy and subatomic particles.
Quintessence. A postulated force that is the opposite of gravity, suggested as the force that propelled the cosmic inflation of the universe in the first few moments after the big bang.
Radioactive dating. Techniques for estimating the age of rocks or fossils by measuring the amount of radioactive elements in a sample. For example, uranium gradually decays into lead.
Knowing the rate at which uranium decays (see half-life) allows the age of the rock to be calculated.
Red giant star. A star that has aged beyond its main sequence phase and has swelled to much greater size than it was originally. Its surface temperature cools as the star expands. When the Sun reaches its red giant phase, its outer envelope will encompass the orbit of Earth.
Redshift. When a source of light moves away from an observer, its light is shifted toward the red end of the spectrum, due to the Doppler effect. Most of the galaxies are redshifted, indicating that the universe is expanding.
Reflection. The bouncing back of light (or sound) when it hits a surface.
Refraction. The bending of light when it passes from one medium (such as air) into another (such as water).
Relativistic velocity. A velocity of more than 90 percent of the speed of light.
Relativity. Special relativity (1905) shows that energy and matter can be converted from one to the other, and that no object in the universe can travel faster than the speed of light in a vacuum. General relativity (1915) enlarged on Newton’s concept of gravitation, treating gravity as a geometric property of spacetime in which massive bodies warp the geometry of spacetime.
Retina. The light-sensitive area in the rear of the eye. It is connected to the brain by the optic nerve.
Retinex theory. An explanation of human color vision developed by Edwin Land, inventor of the Polaroid camera. The word retinex is coined by amalgamating retina and visual cortex.
Secondary color. A color which, when added to a primary color, produces white. Also called a complementary color.
Solar wind. The stream of particles emitted by the Sun, consisting mostly of bare nuclei of hydrogen atoms (i.e., protons) and electrons. Other stars emit stellar winds, thereby enriching the interstellar medium with potential building elements for new stars and planets.
Spacetime. The concept that time is an integral feature of space, a fourth dimension along with the three dimensions of space (length, width, and depth).
Spatial coherence. Light waves that are in phase, all of the same size and shape.
Special relativity. See relativity.
Spectral power distribution. The energy content of various wavelengths of light (i.e., colors).
Spectroscope. A device for studying the spectra of light sources.
Spectrum. (Plural spectra.) The array of colors when a light source is spread by passing it through a prism or a diffraction grating. A rainbow is a natural spectrum. Visible light, however, is only a small part of the much broader electromagnetic spectrum, which includes radio waves, microwaves, infrared, visible light (in all its colors), ultraviolet, X-rays, and gamma rays.
Spontaneous emission. The process in which an electron in an excited state relaxes to a lower-energy state and emits a photon.
Steady state theory. The cosmogenic theory that is based on the concept that the universe had no beginning and will have no end, and that matter and energy are being continuously created.
The steady state theory is not generally accepted by cosmologists.
Stimulated emission. The process in which an electron in an excited state is struck by a photon; the electron relaxes to a lower-energy state and emits a photon (as it does in spontaneous emission), but the incoming photon continues out of the atom, together with the photon emitted by the electron.
Stromatolites. Among the oldest living creatures of Earth, stromatolites exist among rocks dated at approximately 3 billion years old. They are fossilized remains of filaments and mats of photosynthetic algae. Such blue-green algae still exist in Australia and elsewhere.
Subtractive color mixing. Mixing colors by absorption and selective transmission or reflection of light, as when pigments or dies are mixed, or when colored filters are placed in a beam of light.
Superluminal. Faster than the speed of light in a vacuum, which is 300,000 kilometers per second (186,000 miles per second).
Supernova. The final catastrophic explosion of a massive star, so powerful that it can release as much energy in a day as the Sun does in a billion years. Supernovas (also called Supernovae) can be observed in other galaxies, millions of light-years away.
Symbiosis. A cooperative arrangement by organisms of different species that were once independent of each other.
Temporal coherence. Light waves that have the same spacing between them, so that the time between wave crests is the same.
Time dilation. When traveling at relativistic velocity (a velocity close to the speed of light), time elapses more slowly than it does at lower velocities.
Total internal reflection. The reflection of light that is moving through a dense medium when it strikes the boundary between that medium and a medium that is less dense. If the angle at which the light strikes the boundary is small, the light is totally reflected back inside the dense medium. At steeper angles the light passes through the boundary as if it were a window, rather than a mirror.
Tritanopia. The form of color blindness in which the person cannot distinguish blue.
Twin paradox. Because of time dilation a twin who travels at relativistic velocity will age more slowly than the twin who remains on Earth.
Ultraviolet light. Light (i.e., electromagnetic waves) of wavelengths shorter than visible violet but longer than X-rays. Certain wavelengths of ultraviolet light stimulate melanin in human skin to darken, thereby causing tanning.
Uncertainty principle. A basic tenant of physics which states that it is impossible to know both the position and velocity of a subatomic particle at the same time. The more accurately one factor is known, the less accurate will be the knowledge of the other factor.
Virtual image. The image seen in a mirror.
Virtual reality. Stimulation of the senses, particularly vision and touch, to produce the effect of actually seeing, touching, and interacting with an environment that exists only in a computer program. Sometimes called “alternate reality” and “cyberspace.”
Vitreous body. The transparent jelly that fills the eyeball between the lens at the front and retina in the rear.
White dwarf star. A star that has collapsed to about the size of Earth. Its surface is quite hot, and its density is so high that a spoonful of its matter would weigh thousands of tons.
White light. A mixture of all the wavelengths of visible light, from 400 to 700 nanometers (nm). Sunlight is considered white light.
Wormhole. The speculation that the collapsed matter of a black hole may somehow tunnel through space and time to emerge at another time and place in the universe: in effect, a spacetime warp.
X-rays. Light (i.e., electromagnetic waves) of wavelengths longer than ultraviolet light but shorter than gamma rays. Because they can penetrate material opaque to visible light, X-rays are used for medical examinations.