6. Glossary -- Explanation of Technical Terms
- Accretion disk
- A disk of gas surrounding a star. The star may be a white dwarf, a neutron star, or a stellar black hole in a close stellar binary system. In these cases, the material of the accretion disk comes from the nearby companion star of the binary system. Accretion disks may also surround newborn stars or the massive black holes found in the centers of many galaxies.
- The smallest part of a chemical element, such as hydrogen, helium, or carbon. Atoms possess a compact nucleus surrounded by one or more electrons.
- Atomic nucleus (pl. nuclei)
- The core of an atom, consisting of at least one proton (i.e., the nucleus of the hydrogen atom) or of protons and neutrons. The number of protons determines the nature of the atom: The nucleus of hydrogen has one proton, that of helium has two protons, that of carbon has six protons, etc.
- Black hole
- An object whose gravity is so strong that nothing -- neither particles nor light -- can escape from it. To escape would require a speed greater than that of light, which is not allowed.
- Chemical burning
- A chemical reaction among atoms or molecules that releases energy, such as heat or light. An example is the burning of wood: Molecules of wood (mainly cellulose) react with oxygen to produce carbon dioxide and water, and give off energy.
There are also chemical reactions that require the input of energy. An example is photosynthesis, in which carbon dioxide and water are combined to form plant material and molecules of oxygen. The energy for this chemical reaction is provided by sunlight.
Chemical reactions do not change the nature of atoms, in contrast to nuclear reactions.
- A state of matter characterized by extreme compactness or density (e.g., matter that is a million times or more as dense as water). The pressure of degenerate matter is dominated by quantum mechanical effects and is largely independent of temperature (unlike the pressure of air in a tire, the terrestrial atmosphere, or the gases in normal stars, such as the Sun).
- Einstein's equation, E = mc 2
- The equation that expresses the equivalence of energy (E) and mass (m) via the square of the speed of light (c).
- Elementary particle of negative electric charge and a mass that is only about 0.0005 that of the proton. Electrons are basic constituents of atoms.
- A measure of being able to do work. There are many forms of energy, such as heat, mechanical, electrical, radiant, chemical, and nuclear energies. Energy is measured in such units as the joule (J), erg, kilowatt-hour (kW-hr), kilocalorie (kcal), foot-pound (ft-lb.), electron-volt (ev), and British thermal unit (BTU).
- Event horizon
- The boundary or "point of no return" of a black hole. Nothing -- neither particles nor photons (i.e., electromagnetic radiation) -- can escape from inside the event horizon. To escape would require a speed greater than that of light, which is not allowed.
- The temperature scale popular in the US. Degrees Fahrenheit (TF) relate to kelvins (TK), the temperature scale used in science, by the formula
TF = 9/5 TK - 459.7.
- Gamma rays
- The most energetic form of electromagnetic radiation [followed, with progressively lower energies, by X-rays, ultraviolet rays, optical radiation (light), infrared radiation, and radio waves].
- Gravity or gravitational force
- An attractive force between matter. Gravity is one of the four fundamental forces of nature (the others are the electromagnetic, nuclear, and weak forces). For instance, gravity holds the planets together, keeps us on the surface of the Earth, and keeps the planets in orbit around the Sun.
- Hertzsprung-Russell diagram
- A two-dimensional graph of stellar brightness versus surface temperature. The diagram reveals a number of distinct groupings of stars, the most prominent of which is the main sequence. Others are the supergiants, red giants, and white dwarfs.
- Hydrogen burning
- The conversion of nuclei of hydrogen into nuclei of helium and the release of large amounts of energy. Hydrogen burning is the process by which the Sun and other main sequence stars obtain their energy. Hydrogen burning, like other kinds of nuclear burning, requires extremes in temperature and density.
- Interstellar medium
- The gaseous and dusty matter present in the space between a galaxy's stars.
- An atom that has lost one or more electrons. Ions have a positive electric charge. The simplest ion is the nucleus of a hydrogen atom, which is a proton.
- Long-period variable
- A highly evolved, very luminous red giant star whose atmosphere expands and contracts in repeating cycles (i.e., it pulsates) with periods from several months to several years.
- Main sequence star
- A star that derives its energy from the conversion of hydrogen into helium in its core. The Sun is a main sequence star. (See also Hertzsprung-Russell diagram.)
- The quantity of matter in an object, usually expressed in kilograms or pounds. An object's mass is responsible for its inertia (i.e., its resistance to being accelerated) and the gravitational force it exerts on other matter.
- An elementary particle with zero electric charge and very little mass. Neutrinos are produced copiously by many of the nuclear reactions powering stars. Neutrinos interact only very weakly with other matter.
- A particle with zero electric charge and a mass only slightly greater than that of the proton. Along with protons, neutrons are present in atomic nuclei (the only exception is the nucleus of hydrogen-1, which consists of a single proton).
- Neutron star
- A compact star consisting predominantly of neutrons. Neutron stars have masses in the range of about one to three solar masses and sizes of around 12 miles. Their density is comparable to that of atomic nuclei (i.e., about 100 to 1,000 trillion times the density of water).
- Nova (pl. novae)
- The sudden brightening of a white dwarf that is accreting hydrogen-rich matter from a nearby companion star. The brightening may be due to explosive hydrogen burning on the white dwarf's surface (called a classical nova) or the gravitational energy released during the crashing of matter from an accretion disk onto the white dwarf (dwarf nova).
- Nuclear burning
- Reactions in which nuclei of atoms combine to form nuclei of heavier atoms and release energy. Also called nuclear fusion. (See also, Nuclear energy)
- Nuclear energy
- The energy released during nuclear burning. The term also refers to the energy released by the breakup (fission) of very heavy atomic nuclei, such as uranium-235 and plutonium-239.
- See Atomic nucleus.
- Planetary nebula
- An expanding shell of gas ejected by a red giant star late in its life. (The term has nothing to do with planets.)
- The force per unit area in a gas or liquid, typically expressed as pounds per square inch in the US. The scientific unit is newtons per m2 (also known as the pascal). For instance, atmospheric pressure at sea level on Earth equals 14.7 lbs./in2 = 1.01 x 105 newtons per m2. The pressure at the center of the Sun is approximately 300 billion Earth atmospheres, or 3 x 1016 newtons per m2.
- A particle with positive electric charge that, along with neutrons, is present in atomic nuclei.
- A rotating neutron star whose radiation is observed as regular pulses.
- Red giant
- A post-main sequence star of modest mass (a few solar masses or less) with an extended, relatively cool atmosphere.
- Solar mass
- A mass equal to that of the Sun -- 2 x 1030 kg or about 330,000 Earth masses.
- Solar wind
- A flow of charged particles (mainly protons and electrons) streaming from the Sun's outermost layer, the corona, into interplanetary space. The solar wind is extremely weak, carrying away only about 0.0001 solar masses in the course of the Sun's main sequence lifetime of about 10 billion years.
- Speed of light
- The speed of light in vacuum, usually represented by the letter c. c = 300,000 kilometers/second = 186,000 miles/second.
- A self-luminous gaseous body that typically generates energy by nuclear reactions in its interior. Note that white dwarfs and neutron stars that no longer possess nuclear reactions, but shine by radiating stored-up heat that originally was derived from nuclear reactions, are also referred to as stars.
- Stellar mass transfer
- The transfer of matter from one star to another in a close stellar binary system.
- The star at the center of our planetary system that gives us warmth and light, and thus sustains life on Earth. The Sun is a main sequence star with a radius of 700,000 kilometers (435,000 miles), mass of 330,000 Earth masses, and a surface temperature of 6,000 kelvins (10,000 degrees Fahrenheit).
- A massive, very luminous post-main sequence star, often with an extended atmosphere. Depending on the supergiant's surface temperature, one distinguishes blue, yellow, or red supergiants.
- The explosive end of stars that either disrupts the precursor star (supernova of type I; precursor star is a white dwarf of about 1.4 solar masses) or ejects a large part of the precursor star's outer layers, leaving behind a neutron star or black hole (supernova of type II; precursor star has more than about eight solar masses). Supernovae are among the brightest events in today's universe and sometimes shine as brightly as the galaxy in which they reside.
- Supernova remnant
- The expanding shell of gas from a supernova and the swept-up interstellar gas and dust that the shell pushes along and with which it intermingles.
- White dwarf
- A compact star, typically with a mass between about one half and slightly more than one solar mass (but less than 1.4 solar masses) and a radius comparable to that of the Earth. The density of white dwarf matter is about a million to 10 million times that of water. White dwarfs are the burned-out stellar remnants of formerly normal stars of modest mass (less than eight solar masses) that have exhausted their nuclear fuel.
- Energetic electromagnetic radiation. Energy-wise, X-rays fall between gamma rays (more energetic) and ultraviolet radiation.
- X-ray binary system
- Stellar binary system that emits X-rays. Such systems consist of a neutron star or black hole that accretes matter from a nearby, mass-losing stellar companion.
- X-ray burster
- An X-ray binary system that emits bursts of X-rays, often in rapid, irregular succession.
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