Authors: Kitty Ferguson
cosmic distance ladder:
Set of overlapping distance-measurement techniques by which astronomers over the centuries have bootstrapped their way to measuring farther and farther distances in the universe.
cosmic microwave background radiation (CMBR):
Radiation detectable in the microwave range of the spectrum, composed of photons released from primordial cosmic material as it thinned out owing to the expansion of the universe, at the epoch of photon decoupling, about 300,000 years after the Big Bang. Discovered by Penzias and Wilson in the mid-1960s.
cosmological constant:
The mathematical device Einstein added to his equations to allow the universe to remain static.
cosmology:
The study of the origin, structure, and evolution of the universe. The word usually implies study of the universe on the large scale.
cosmos:
A word that has become synonymous with “universe on the large scale,” though it traditionally implies an orderly, harmonious universe.
critical density:
The density that would allow the universe to balance eternally on the knife edge between expansion and collapse, expanding at precisely the right rate to avoid recollapse. Omega for a critical density universe equals 1.
curvature of space-time:
Einsteinʼs general theory of relativity explains the force of gravity as the way the distribution of mass in space-time causes something that resembles the warping, denting, and dimpling in an elastic surface by balls of different weights and sizes lying on it.
dark matter:
Matter that canʼt be observed to radiate energy in any part of the electromagnetic spectrum.
deceleration parameter:
Quantity designating the rate at which the expansion of the universe is slowing down.
decoupling:
In this book, the separation of photons from particles of matter that made possible the cosmic microwave background radiation.
deferent:
One of the devices used in Ptolemaic astronomy. It is a circle that carries another circle. The second circle is called an epicycle.
Doppler shift:
A change in the apparent wavelength of radiation (such as light or sound) emitted by a moving body.
dynamics:
The area of science that deals with how bodies respond and move as the result of the action of forces.
eccentric:
In Ptolemaic astronomy, a circle on which a planet moves that is centered near but not precisely on the Earth.
electromagnetic spectrum:
The complete range of electromagnetic radiation. Visible light is one form of electromagnetic radiation but represents only a very small part of the complete electromagnetic spectrum, which also includes gamma rays, X-rays, ultraviolet rays, infrared rays, microwaves, and radio waves.
emission spectrum:
A spectrum produced by an incandescent gas, consisting of only a few isolated colors. Each kind of gas has its own pattern.
epicycle:
In Ptolemaic astronomy, a circle in which a planet moves, which in turn moves on a larger circle, the deferent, which is centered on the Earth.
equant:
In Ptolemaic astronomy, a location interior to the deferent from whose point of view the angular velocity of the epicycle traveling on the deferent is constant.
flat universe:
Cosmological model in which the universe expands at precisely the right rate to avoid recollapse. A universe in which omega equals 1.
flatness problem
: The puzzle of why the universe appears to be balanced between being an open or a closed universe.
fractal:
An object or situation that has the characteristic feature of self-similarity – an unending series of motifs within motifs repeated on every scale. The Mandelbrot Set is the most familiar example of the fractal quality.
Galactic disk:
If we picture the Galaxy as a giant fried egg, the disk is the white of the egg. This is the region in which the spiral arms are found.
Galactic halo:
An aggregation of stars, globular star clusters, and thin gas clouds, centered on the core of the Galaxy and extending beyond the known borders of the Galactic disk.
gamma rays:
Electromagnetic radiation of very short wavelength and very high energy.
globular clusters:
Dense, jewel-like, spherical clusters of stars.
gnomon:
A protrusion like the raised part of a sundial.
gravitational lensing:
Multiple images and other distortions in the light from quasars and other distant objects produced by the warping of space-time by objects such as galaxy clusters in the foreground.
gravitational redshift:
The shift in wavelengths toward the red end of the spectrum caused by a gravitational field.
gravity:
One of the four basic forces of nature. Gravity always attracts, and it works on all levels, from the tiniest fundamental particles to the largest objects in the universe.
Heisenberg uncertainty principle:
The principle in quantum physics according to which the position and momentum of a particle can never both be known with exactitude at the same time. It implies that empty space can never really be empty, but that instead there are always and everywhere in the universe tiny energy fluctuations.
horizon problem:
The puzzle having to do with how few of the particles in the early universe would have had time to be in contact with one another as cosmic expansion began.
Hubble constant:
The constant that denotes the expansion rate of the universe.
Hubble flow:
Movements of objects in the universe that is directly attributable to the expansion of the universe.
imaginary numbers
: Numbers that when squared yield a negative number.
imaginary time:
Time measured using imaginary numbers.
impossible object:
An object that canʼt really exist because it contradicts itself.
inflation theory (inflationary Big Bang theory):
A theory that has the universe going through a very brief phase of extremely rapid expansion early in its existence.
inverse square:
Technically speaking, “proportional to the power -2 (minus 2).” For example, if you have two light bulbs of equal wattage and place light bulb B twice as far away from you as light bulb A, light bulb B will look only a fourth as bright as light bulb A. The strength of the gravitational force also varies as the inverse square of the distance.
ionized:
The state of an atom in which it has fewer or more electrons than is normal.
isotrophy:
The quality of being the same in all directions.
Keplerʼs laws:
Three laws discovered by Johannes Kepler. The first states that a planet travels in an elliptical orbit around the Sun, with the Sun as one of the foci of the ellipse. The second states that the orbital velocity of a planet is directly related to where the planet is in its orbit, and the nearer the Sun the greater the velocity. The third shows the relationship between the lengths of time the planets take to complete their orbits and their distances from the Sun.
least squares:
A statistical method of estimating values from a set of observations.
light-year:
The distance traveled by light in one year in a vacuum. Equal to about 5.88 trillion miles or 9.4607 kilometers.
Local Group:
The group of galaxies of which the Milky Way Galaxy is a member.
mass:
How much matter there is in a body, or how much a body resists any change in its speed or direction.
megaparsec
: One million parses. About 3.26 million light-years.
micrometer:
An instrument for measuring very small dimensions. Placed inside a telescope, and having cross-wires that can be moved across the image with the turning of a screw, it serves as a ruler to measure the apparent size of the object being viewed.
moving cluster method:
A technique that studies the way paths of stars in a cluster seem to converge or diverge, as an aid to measuring the distance of the cluster.
nebulae:
Formerly described as distant hazy, fuzzy celestial objects. Some are now known to be galaxies, whereas others are clouds of gas, dust, or debris from events such as supernovae.
neutrinos:
Particles with very small mass that very rarely interact with any kind of matter and are extremely difficult to detect. First suggested in 1930 by Wolfgang Pauli as a way of explaining a loss of energy in some nuclear reactions.
nova:
As modern astronomy since the 1930s makes the distinction, a nova is a less drastic flare-up of a star – usually a white dwarf star that is part of a binary system – as opposed to a supernova, which is an explosion that demolishes the entire star once and for all. Earlier astronomy used the term
nova
for any sudden appearance (flare-up) of a star where none had been observed before.
O and B stars:
Massive, brilliant, short-lived stars, less than about 10 million years old.
omega:
The mass density of the universe.
open universe:
Cosmological model in which the universe expands forever, thinning out eternally. A universe in which omega is less than 1.
opposition:
A planet is said to be in opposition when it is on the opposite side of the Earth from the Sun.
optical astronomy:
Study of the universe by observing radiation in the optical (visible) range of the spectrum.
parallax shift:
The apparent change in an objectʼs position against the background when viewed from two different places.
parsec:
A unit of distance equaling 3.26 light years; a little more than 30 trillion kilometers.
pencil beam survey:
A survey that is limited to a tiny circular area of the sky. The small circle under observation becomes a cone as it goes deeper into space, and the survey produces a cone-shaped three-dimensional map that keeps getting larger as it reaches farther distances.
perspicillum:
An early name for the telescope.
photon:
The “messenger particle” that carries the electromagnetic force. The photon is the particle of light and all other forms of electromagnetic radiation (gamma rays, X-rays, radio waves, microwaves, etc.).
primeval atom:
Abbé Georges-Henri Lemaître theorized that there was a time when everything that makes up the present universe was compressed into a space only about thirty times the size of our Sun. He called that the “primeval atom.”
proper motion:
Change in position of a star, relative to other stars, over a period of years. More generally, the individual drifting of stars through space.
Ptolemaic astronomy:
The combination, recombination, and reworking of Ptolemyʼs ideas as they were used by astronomers over the centuries. Earth-centered astronomy.
quantum physics (quantum mechanics
): The study of the universe on the level of the very small – molecules, atoms, and elementary particles.
quasars:
Pointlike sources of light whose redshifts show that they are billions of light-years distant. Thought to be the nuclei of young galaxies.
radio astronomy:
Study of the universe by observing radiation at radio wave-lengths.
radius:
Shortest distance from the center of a circle or sphere to the circumference or surface.
redshift:
Displacement of the spectral lines in light coming from distant stars and galaxies that are moving away from Earth. Redshift can also be caused by a gravitational field.
reflecting telescope (or reflector):
A telescope in which incoming light is reflected from a primary mirror back to an eyepiece.
refracting telescope (or refractor):
A telescope in which light is redirected (refracted) by a main lens to a specific point from where a second lens presents the image to the eye.
refraction of light problem:
The way the Earthʼs atmosphere bends and smears light rays passing through it.
relativity, general theory of:
Einsteinʼs theory in which the gravitational force is explained as a curvature of space-time caused by the presence of mass, and in which the curvature of space-time dictates the movement of mass. As John Archibald Wheeler expressed it, “Space-time grips mass, telling it how to move; and mass grips space-time, telling it how to curve.”
retrograde motion (or retrogression):
When a planet seems to stop in its eastward motion and travel westward for a while.
singularity:
The dimensionless point that may exist at the center of a black hole, where all the mass of the collapsing star has shrunk to infinite density and infinite space-time curvature. Singularity may also be used to name any point in space-time where the curvature of space-time becomes infinite, such as the Big Bang singularity.
solar mass:
The mass of the Sun.
space-time:
The combination of the dimensions of space and time. The space-time we experience has three dimensions of space and one dimension of time.
spectral lines:
Bright and dark lines seen in the spectra of stars and other luminous objects.
spectroscope:
An instrument for observing and studying spectra.
speed of light:
The speed of light in a vacuum is approximately 186,300 miles or 299,800 kilometers per second, often rounded off to 186,000 miles or 300,000 kilometers per second.