Chapter 11 Flashcards
Interstellar medium
The matter between stars, composed of two components, gas and dust, intermixed throughout all of space
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 nebulae
A glowing gas 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 lanes
A lane of dark, obscuring interstellar dust in an emission nebula or galaxy.
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 cloud complexes
Collection of molecular clouds that spans as much as 50 parsecs and may contain enough material to make millions of Sun-size stars.
Evolutionary track
A theory that explains observations in a series of gradual steps, explainable in terms of well-established physical principles.
Zero-age main sequence
The region on the Hertzsprung-Russel diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores.
Brown dwarfs
Remnants of fragmentation of collapsing gas and dust that did not contain enough mass to initiate core 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 age and chemical composition and lie at 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.
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.
Dark dust cloud
A large cloud, often many parasecs 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.
Molecular gas
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.
T-Tauri phase
Protostar in the late stages of formation, often exhibiting violent surface activity. T Tauri phase stars have been observed to brighten noticeably in a short period of time, consistent with the idea of rapid evolution during this final phase of stellar formation.
Association
Small grouping of (typically 100 or less) bright stars, spanning up to a few parsecs across, usually rich in very young stars
