AST 112 Chapters 18, 19, 20, 21 Notes & Definitions

binary stars

two stars that revolve about each other

brown dwarf

an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 1/12 the mass of the Sun

eclipsing binary

a binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it

H–R diagram

(Hertzsprung–Russell diagram) a plot of luminosity against surface temperature (or spectral type) for a group of stars

main sequence

a sequence of stars on the Hertzsprung–Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right

mass-luminosity relation

the observed relation between the masses and luminosities of many (90% of all) stars

selection effect

the selection of sample data in a nonrandom way, causing the sample data to be unrepresentative of the entire data set

spectroscopic binary

a binary star in which the components are not resolved but whose binary nature is indicated by periodic variations in radial velocity, indicating orbital motion

visual binary

a binary star in which the two components are telescopically resolved

white dwarf

a low-mass star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final state of life

cepheid

a star that belongs to a class of yellow supergiant pulsating stars; these stars vary periodically in brightness, and the relationship between their periods and luminosities is useful in deriving distances to them

light curve

a graph that displays the time variation of the light from a variable or eclipsing binary star or, more generally, from any other object whose radiation output changes with time

luminosity class

a classification of a star according to its luminosity within a given spectral class; our Sun, a G2V star, has luminosity class V, for example

parallax

an apparent displacement of a nearby star that results from the motion of Earth around the Sun

parsec

a unit of distance in astronomy, equal to 3.26 light-years; at a distance of 1 parsec, a star has a parallax of 1 arcsecond

period-luminosity relation

an empirical relation between the periods and luminosities of certain variable stars

pulsating variable star

a variable star that pulsates in size and luminosity

RR Lyrae

one of a class of giant pulsating stars with periods shorter than 1 day, useful for finding distances

baryon cycle

the cycling of mass in and out of the interstellar medium, including accretion of gas from intergalactic space, loss of gas back into intergalactic space, and conversion of interstellar gas into stars

cosmic rays

atomic nuclei (mostly protons) and electrons that are observed to strike Earth’s atmosphere with exceedingly high energies.

H II region

the region of ionized hydrogen in interstellar space

interstellar dust

tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices

interstellar extinction

the attenuation or absorption of light by dust in the interstellar medium

interstellar medium (ISM)

(or Local Hot Bubble) a region of low-density, million degree gas in which the Sun and solar system are currently located

Local Fluff

a slightly denser cloud inside the Local Bubble, inside which the Sun also lies

molecular cloud

a large, dense, cold interstellar cloud; because of its size and density, this type of cloud can keep ultraviolet radiation from reaching its interior, where molecules are able to form

nebula

a cloud of interstellar gas or dust; the term is most often used for clouds that are seen to glow with visible light or infrared

reddening (interstellar)

the reddening of starlight passing through interstellar dust because dust scatters blue light more effectively than red

exoplanet

a planet orbiting a star other than our Sun

giant molecular clouds

large, cold interstellar clouds with diameters of dozens of light-years and typical masses of 10⁵ solar masses; found in the spiral arms of galaxies, these clouds are where stars form

Herbig-Haro (HH) object

luminous knots of gas in an area of star formation that are set to glow by jets of material from a protostar

mini-Neptune

a planet that is intermediate between the largest terrestrial planet in our solar system (Earth) and the smallest jovian planet (Neptune); generally, mini-Neptunes have sizes between 2.8 and 4 times Earth’s size

protostar

a very young star still in the process of formation, before nuclear fusion begins

stellar wind

the outflow of gas, sometimes at speeds as high as hundreds of kilometers per second, from a star

super-Earth

a planet larger than Earth, generally between 1.4 and 2.8 times the size of our planet

transit

when one astronomical object moves in front of another