Glossary of Astronomical and Cosmological Terms

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absolute magnitude: the magnitude an object would have at the standard distance of 10 parsecs.

absorption line: a more or less narrow range of wavelengths in a spectrum that is darker than neighboring wavelengths. Absorption lines are seen in stars.

angles: are measured in degrees or arcminutes (denoted by a single quote) or arcseconds (denoted by a double quote) or radians. 1 radian = 180/pi = 57.2958 degrees, 1 degree = 1o = 60 arcminutes = 60' = 3600 arcseconds = 3600".

baryon acoustic oscillation: the baryons (ordinary matter) and the photons in the CMB form a plasma with a high sound speed in the 400,000 years before the protons and electrons combine into hydrogen atoms. Sound waves carried by this plasma produce bumps that are seen in the anisotropy of the CMB and in the spatial distribution of galaxies.

baryon: a massive elementary particle made up of three quarks. Neutrons and protons are baryons.

blackbody: an object with a constant temperature that absorbs all radiation that hits it.

Cepheid: a type of pulsating variable star with a luminosity that can be determined from the period of its variation: Cepheids with long pulsation periods are bigger and thus more luminous than short period Cepheids.

cold dark matter: a type of dark matter that was moving at much less than the speed of light 10,000 years after the Big Bang. (see dark matter)

continuum: a smooth spectrum without emission or absorption lines. Some sources like tungsten lamps or blackbodies are purely continuum sources, while in sources with lines the continuum is a smooth spectrum drawn through the points between the lines.

CMB: Cosmic Microwave Background radiation, also CMBR, CBR and the "3 K blackbody radiation". Radiation left over from the hot Big Bang which has cooled by expansion to a temperature slightly less than 3 degrees above absolute zero.

cosmological constant: a term in Einstein's general relativity equations that leads to an acceleration of the expansion of the Universe. Usually denoted by Λ, the capital Greek letter Lambda when expressed with units of inverse length squared, or by ΩΛ or the lower-case Greek lambda (λ) when normalized to the critical density like Ω.

critical density: the density of the Universe necessary so the expansion rate of the Universe is just barely sufficient to prevent a recollapse. Numerically the critical density is 3Ho2/[8*pi*G] or 19*[Ho/100]2*10-30 gm/cc.

dark energy: a more general form of the vacuum energy density than the cosmological constant.

dark matter: a form of matter that does not emit light, absorb light, or scatter light. Its only interactions are gravitational.

density: the amount of matter in a volume divided by the volume, so the units are grams per cubic centimeter. Water has a density of 1 gram per cubic centimeter. The lower case Greek letter rho (ρ) is usually used to symbolize density in equations.

distance modulus: the difference between the magnitude and the absolute magnitude, so DM = 5 log10(D/[10 pc]).

dipole: a pattern with one hot side of the sky and one cold side of the sky.

Doppler: 19th century physicist who discovered the variation in the wavelength of waves caused by motion of the source.

electromagnetic force: one of the four forces of nature. Electromagnetic interactions hold electrons in atoms, hold atoms in molecules, and are used in all electronic devices.

electroweak: a unified force that combines the electromagnetic and weak nuclear interactions. Predicted by Weinberg and Salam, experimentally verified by Rubbia and van der Meer.

emission line: a more or less narrow range of wavelengths in a spectrum that is brighter than neighboring wavelengths. Emission lines are seen in quasars.

energy: the ability to do work, with units of ergs or Joules. One Joule is 10,000,000 ergs. One erg is the kinetic energy of a 2 gram mass moving at one cm/sec. Energy per unit time is power, and 1 Watt of power is 1 Joule per second.

equation of state: the ratio of the pressure to the energy density in the dark energy or vacuum energy. Usually denoted by w. For the cosmological constant w = -1.

escape velocity: the minimum velocity that will allow an object to escape from a gravitational field.

fine structure constant: usually denoted as α, the lower case Greek alpha, is the dimensionless ratio e2/(hbar*c) = 1/137.03599976... [in cgs units, or e2/(4*pi*epsilono*hbar*c) in MKS units], which gives the strength of the electromagnetic interaction. Here e is the electron charge, hbar is Planck's constant divided by 2*pi, and c is the speed of light.

flux: power per unit area. The flux from the Sun at the Earth is 1367 Watts per square meter. This total power is often divided up into different frequency or wavelength bands, giving for example Watts per square meter per Hertz or ergs per square cm per second per micron. 1 Jansky is 10-26 Watts per square meter per Hertz.

fluence: energy per unit area. Fluence is time times flux.

gamma ray: a very high energy photon, more energetic than an X-ray.

Gyr: gigayear, or one billion years. See for a table of all the metric prefixes from yocto (10-24) to yotta (1024).

grand unified theory: a model for unifying the strong nuclear force, the weak nuclear force, and the electromagnetic force into a single interaction. Several such GUTs have been proposed, but not yet experimentally verified.

gravitational potential: the gravitational energy per unit mass of a particle, equivalent to the acceleration of gravity times the height in ordinary circumstances near the surface of the Earth.

homogeneous: the same at all locations. Homogenized milk is not separated into cream and milk.

horizon: the edge of the visible Universe, but not the edge of the Universe since the Universe has no edge.

Hot Big Bang: A model of the Universe beginning at very high density and temperature, which expands and cools to become like the Universe we observe now.

hot dark matter: a type of dark matter that was moving at close to the speed of light 10,000 years after the Big Bang. (see dark matter)

Hubble constant: or Ho, the ratio of velocity to distance in the expansion of the Universe, so v = HD. The "o" [pronounced "naught"] on Ho means the current value, since the Hubble "constant" changes with time (but it is the same everywhere in the Universe at a given time). The measured value of Ho has also changed dramatically since even before Hubble's work, as shown in Huchra's Ho history.

inflationary scenario: a modification of the Big Bang model in which a large cosmological constant exists temporarily early in the history of the Big Bang, leading to a rapid accelerating expansion of the Universe, which is then followed by the normal Big Bang model with a decelerating expansion.

isotropic: the same in all directions. Anisotropic - not isotropic. Anisotropy - difference between different directions. In the standard color scheme for CMB anisotropy maps measured by the COBE DMR, red shows areas of the sky that are warmer, while blue shows the cooler regions.

Kelvin: a temperature scale that uses absolute zero as the zero point. The degree Kelvin, or K, is the same size as the Celsius degree, so the Kelvin temperature is just 273.15 degrees bigger than the Celsius temperature.

Lambda: the upper case Greek Lambda is usually used to denote Einstein's cosmological constant. A non-zero Lambda indicates a non-zero vacuum energy density and causes a long-range repulsive effect which leads to the accelerating expansion of the Universe.

Lyman alpha line: the strongest line in the spectrum of the most common atom in the Universe, hydrogen. It is emitted at a wavelength of 122 nm. The general formula for the wavelength of hydrogen lines is

1/wavelength = [n-2 - m-2]/[91.2 nm]
where n = 1, 2, 3, ... is the lower state quantum number and m = n+1, n+2, ... is the upper state quantum number. When n = 1, these lines with m = 2, 3, ... are called Lyman alpha, Lyman beta, ... and form the Lyman series. When n = 2, these lines with m = 3, 4, ... are called H-alpha, H-beta, ... and form the Balmer series.

MACHO: MAssive Compact Halo Object, and also one of projects searching for MACHOs by looking for gravitational microlensing: the other projects are EROS and OGLE. A MACHO is an object with a mass from about 10 billion tons to solar masses. If it is made out neutrons and protons then it is baryonic dark matter, but primordial black holes are a non-baryonic dark matter version of MACHOs.

magnitude: a scale used by astronomers to measure flux. Each 5 units on the magnitude scale corresponds to a 100-fold decrease in the flux. The Sun has magnitude -26.5. Sirius, the brightest star in the night sky, has magnitude -1.6. The faintest stars visible with the naked eye have magnitude 6.

micron: one micrometer, 0.000001 meters. Visible light has wavelengths between 0.4 and 0.7 microns. 1 micron is 10,000 Angstroms, or 1000 nanometers (nm). 1 nanometer is 0.000000001 meters.

nanowatt or nW: one billion'th of a watt.

neutralino: a particle predicted by supersymmetry models of the forces of nature. These models predict that each type of known particle will have a supersymmetric partner. The neutralino is the lightest electrically neutral supersymmetric partner, and it is a candidate for cold dark matter. As of 1999, no supersymmetric partner particles of any kind have been observed experimentally. A neutralino is one type of WIMP.

non-baryonic: not made up of neutrons, protons and electrons, and thus not like any of the known chemical elements.

Nucleon: a neutron or a proton - one of the particles inside an atomic nucleus.

Omega or Ω: the ratio of the density of the Universe to the critical density.

omega: the lower case Greek ω is often used to denote Ωh2 (or Omega*h^2), where h=Ho/[100 km/sec/Mpc]. Since Ω is the density divided by the critical density, and the critical density is 3Ho2/[8πG], this combination is just proportional to the physical density. ω=1 corresponds to 19*10-30 gm/cc, or in SI: 19 yoctograms per cubic meter.

parsec: a unit of distance used by astronomers, corresponding to a parallax of one arc-second. Equal to 3.085678x1013 kilometers, or 3.26 light-years. kpc: 1000 parsecs; Mpc: 1 million parsecs.

QSO: quasi-stellar object. The first discovered QSO's were radio sources, leading to the name quasi-stellar radio sources, or QSRS, or quasars. These objects look like stars on an image of the sky, but their spectra show strong emission lines at high redshift. The redshift means that quasars are very far away, and are thus the most luminous objects in the Universe.

quadrupole: a type of pattern on the sky which generally has two high spots and two low spots.

quantum fluctuations: the uncertainty principle in quantum mechanics leads to all allowed interactions having some probability of occurrence.

quark: An elementary, strongly interacting constituent of matter. Quarks come in six flavors: up, down, charm, strange, top and bottom. The up, charm and top quarks have electric charges of +(2/3)e, while the down, strange and bottom quarks have charges of -(1/3)e. The proton which has a charge of +e is constructed of two up quarks and one down quark: (uud), while the neutron is (udd).

redshift: the Doppler shift for objects receding from the Earth causes the wavelengths of light to get longer, and hence shift into the red part of the spectrum. Because of the expansion of the Universe, objects with high redshift are far away, and we see them as they were a long time ago.

spectrum: result of spreading out light by wavelengths. A rainbow is a natural spectrum. The eye is sensitive to waves from violet at 380 nm wavelength to red at 700 nm wavelength, but astronomers now study electromagnetic radiation from gamma rays through X-rays, ultraviolet, violet, blue, green, yellow, orange, red, infrared and radio waves.

Steady State: a model of the expanding Universe with constant density and physical properties. Matter must be continually created to maintain the constant density.

steradian: the unit of solid angle. There are 4*pi steradians in the entire celestial sphere. One square degree is (1/57.3)2 steradians because one degree is (1/57.3) radians.

strong nuclear force: one of the four forces of nature. The strong nuclear force holds the particles in the nucleus of atoms together.

time dilation: in special relativity, moving clocks appear to run slowly when compared to stationary clocks. This clock slowing is called time dilation.

vacuum energy density: Quantum theory requires empty space to be filled with particles and anti-particles being continually created and annihilated. This could lead to a net density of the vacuum, which if present, would behave like a cosmological constant.

weak nuclear force: one of the four forces of nature. The weak nuclear force is responsible for radioactive decay as well as the fusion reactions in the Sun that provide heat and light for the Earth.

WIMP: a Weakly Interacting Massive Particle, a possible form for cold dark matter.

z: z is the symbol used for the redshift. The formula is

1+z = λobsem
    = (Observed wavelength)/(Emitted wavelength)
where a line is emitted at one wavelength but observed at a different wavelength.

zero point energy: the uncertainty principle does not allow a quantum mechanical system to have a definite position and definite velocity at the same time. Thus a harmonic oscillator like a pendulum or a mass on a spring has a minimum energy that is larger than zero, since zero energy would require a definite position to zero the potential energy and a definite (zero) velocity to zero the kinetic energy. This minimum energy is 0.5*h*f, where h is Planck's constant and f is the frequency of the oscillator.

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© 1996-2012 Edward L. Wright. Last modified 27 Dec 2012