Exam 2- AST 121

Planetary Nebula

circular nebula of nearly uniform brightness, sometimes with a visible star at the center.

Globular Cluster

a spherical cluster containing thousands of stars

Spiral Nebula

a nebula with visible spiral structure

Spectral Line

a dark or bright line in an otherwise uniform and continuous spectrum, resulting from the emission or absorption of light in a narrow frequency range, compared with the nearby frequencies

Spectrum

the set of colors, and their intensities, emitted by a particular light source. A continuous spectrum shows a full rainbow of colors. An absorption (or dark line) spectrum is missing a few specific colors. An emission (or bright line) spectrum contains only a few specific colors

Spectroscopy

the study of the colors that make up light from a particular source, including the absence of certain colors (dark lines)

Photometry

the quantitative measurement of the brightness of a light source (i.e. a star, in stellar photometry). In photographic photometry brightness is measured by the size of the spot produced on the photographic negative

Wavelength

the peak-to-peak distance in a electromagnetic wave. Related to the color and refrangibility of the light as well as the frequency of the wave.

Radial Velocity

relative velocity of an object along an observer’s line of sight. May be measured by the Doppler shift of the light emitted by the object

Doppler Shift

a shift in the wavelength of light resulting from the relative motion of the observer and the light source. A shift toward the red indicates that the observer and source are moving away from each other, a shift towards the blue indicates motion towards each other

Spectral Type/Class

a group of stars with similar spectra. The features of a star’s spectrum are determined primarily by the surface temperature of the star

Luminosity

the amount of energy emitted by a light source per unit time (i.e. wattage, is in a light bulb)

Absolute Magnitude

a measurement (on a logarithmic scale) of the luminosity of a light source. More luminous objects have smaller absolute magnitudes

Apparent Magnitude

a measurement (on a logarithmic scale) of the intensity of a light source. Depends on the luminosity of the light source, as well as the distance from the observer and possible absorption. Brighter objects have smaller apparent magnitudes

Color Index

the apparent magnitude of an object measured photographically minus the apparent magnitude measured visually. Redder objects have larger color indexes

Light Year

a unit of distance equal to the distance that light travels (through vacuum in one year)

Parsec

a unit of distance equal to the distance at which a star would have an annual parallax of 1'“

Main Sequence

a feature of the H-R diagram running diagonally from top left to bottom right. Stars on the main sequence are known as “dwarfs” with hotter (bluer, earlier spectral type) stars being much brighter than cooler (redder, later spectral type) stars

Giant Branch

a feature of H-R diagram running horizontally across the top. Stars in the giant branch have absolute magnitudes that are roughly independent of spectral class, and are known to be very large

Spectroscopic Parallax

a method of estimating the distance to a star using its spectral type and apparent magnitude

Proper Motion

the apparent linear (not oscillatory) motion of a star across the celestial sphere

Parallactic Motion

the potion of a star’s proper motion that is due to the motion of our solar system through space

Peculiar Motion

the portion of a star’s proper motion that is due to that star’s actual motion through space

Star Streams

two groups of stars near the sun, which tend to move in opposite directions (as discovered by Kapteyn)

Reflection Nebula

a cloud of dust in space that shines because it reflects the light from nearby stars

Island Universe

an isolated system of stars. In modern terminology, a galaxy

Stereocomparator

a device that allows the user to compare two photographs, rapidly switching back and forth between the two in order to detect small differences

Kapetyn Universe

a model of our galaxy developed by Jacobus Kapetyn on the basis of statistical studies of stars in the early 20th century