Astronomy Chapter 11 Vocabulary

interstellar medium

the matter between stars, composed of two components, gas and dust, intermixed throughout all of space

dust grain

an interstellar dust particle, roughly 10 to the -7 power meters in size, comparable to the wavelength of visible light

reddening

dimming of starlight by interstellar matter, which tends to scatter higher-frequency (blue) components of the radiation more efficiently than the lower-frequency (red) components

nebula

general term used for any “fuzzy” patch on the sky, either light or dark

emission nebula

a glowing cloud of hot interstellar gas. The gas glows as a result of one or more nearby young stars that ionize the gas. Since the gas is mostly hydrogen, the emitted radiation falls predominantly in the red region of the spectrum, because of the hydrogen-alpha emission line

dust lane

a lane of dark, obscuring inerstellar dust in an emission nebula or galaxy

dark dust cloud

a large cloud, often many parsecs across, which contains gas and dust in a ratio of about 1012 gas atoms for every dust particle. Typical densities are a few tens or hundreds of millions of particles per cubic meter

21-centimeter line

spectral line in the radio region of the electromagnetic spectrum associated with a change of spin of the electron in a hydrogen atom

molecular clouds

a cold, dense interstellar cloud that contains a high fraction of molecules. It is widely believed that the relatively high density of dust particles in these clouds plays an important role in the formation and preservation of the molecules

molecular cloud complex

collection of molecular clouds that spans as much as 50 parsecs and may contain enough material to make millions of Sun-size stars

protostar

stage in star formation when the interior of a collapsing fragment of gas is sufficiently hot and dense that it becomes opague to its own radiation. The protostar is the dense region at the center of the fragment

evolutionary track

a graphical representation of a star’s life as a path on the H-R diagram

T-Tauri phase star

protostar in the late stages of formation, often exhibiting violent surface activity. T Tauri phase stars have been observed to brighten noticeable in a short period of time, consistent with the idea of rapid evolution during this final phase of stellar formation

zero-age main sequence

the region on the H-R diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores

brown dwarf

remnants of fragments of collapsing gas and dust that did not contain enough mass to initiate corte nuclear fusion. Such objects are then frozen somewhere along their pre-main-sequence contraction phase, continually cooling into compact dark objects. Because of their small size and low temperature, they are extremely difficult to detect observationally

star cluster

a grouping of anywhere from a dozen to a million stars that formed at the same time from the same cloud of interstellar gas. Stars in clusters are useful to aid our understanding of stellar evolution because, within a given cluster, stars are all roughly the same distance from Earth

open cluster

loosely bound collection of tens to hundreds of stars, a few parsecs across, generally found in the plane of the Milky Way

associations

small grouping of (typically 100 or less) bright stars, spanning up to a few tens of parsecs across, usually rich in very young stars

globular cluster

tightly bound, roughly spherical collection of hundreds of thousands, and sometimes millions, of stars spanning about 50 parsecs. Globular clusters are distributed in the halos around the Milky Way and other galaxies