Science Chapter 9

The galaxy containing the solar system, visible as a broad band of faint light in the night sky.

Any of numerous large-scale aggregates of stars, gas, and dust that constitute the universe, containing an average of 100 billion (10¹¹) solar masses and ranging in diameter from 1,500 to 300,000 light-years. Also called nebula.

A loose, irregular grouping of stars that originated from a single nebula in the arms of a spiral galaxy. Also called galactic cluster.

A system of stars, generally smaller in size than a galaxy, that is more or less globular in conformation.

the cluster of galaxies to which our galaxy and the Andromeda Galaxy belong

A group of neighboring clusters of galaxies.

Spiral galaxy, elliptical, and irregular

• the branch of astronomy concerned with the evolution and structure of the universe

A change in the observed frequency of a wave, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. The motion of the source causes a real shift in frequency of the wave, while the motion of the observer produces only an apparent shift in frequency. Also called Doppler shift.

a shift in the lines of the spectrum of an astronomical object towards a longer wavelength (the red end of an optical spectrum), relative to the wavelength of these lines in the terrestrial spectrum

a shift in the spectral lines of a stellar spectrum towards the blue end of the visible region relative to the wavelengths of these lines in the terrestrial spectrum: a result of the Doppler effect caused by stars approaching the solar system.

The cosmic explosion that marked the origin of the universe according to the big bang theory.

the cooled remnant of the hot big bang that fills the entire universe and can be observed today with an average temperature of about 2.725 kelvin

matter known to make up perhaps 90% of the mass of the universe, but not detectable by its absorption or emission of electromagnetic radiation

A form of energy hypothesized to reside in the structure of space itself, responsible for the accelerating expansion of the universe. Dark energy theoretically counterbalances the kinetic energy of the universe's expansion, entailing that that the universe has no inherent curvature, as astronomical observations currently suggest. Dark energy appears to account for 73 percent of all the energy and matter in the universe.