Astronomy Chapters 17-24 Key Terms and Concepts

Comprehensive vocabulary flashcards covering the key terms and definitions across Chapters 17 through 24, including stellar properties, binary systems, distance measurement, interstellar matter, star formation, stellar evolution, stellar death, and general relativity.

apparent brightness

How bright an object appears in the sky, as contrasted with its actual luminosity.

brown dwarf

An object intermediate in size between a planet and a star; sometimes called a "failed star".

color index

Difference between the magnitudes of a star in two different spectral regions.

giant

A star of exaggerated size with a large, extended photosphere.

luminosity

The rate at which a star emits electromagnetic energy into space; the total power output of an object.

magnitude

An older system of measuring the amount of light we receive from a star.

proper motion

The angular change per year in the direction of a star as seen from the Sun.

radial velocity

Motion toward or away from the observer; the component of velocity that lies in the line of sight.

space velocity

The three-dimensional speed and direction with which an object is moving through space.

spectral class

Classification of stars according to their temperatures and spectra; O, B, A, F, G, K, M with L, T, and Y.

binary stars

Two stars that revolve about each other.

eclipsing binary

Binary star where the plane of revolution is nearly edge-on to our line of sight.

H–R diagram

A plot of luminosity vs surface temperature (or spectral type) for a group of stars.

main sequence

A line on the Hertzsprung–Russell diagram containing the majority of stars.

mass-luminosity relation

For many ($90\%$ of all) stars, more massive stars are more luminous.

selection effect

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

spectroscopic binary

A binary star whose nature is indicated by periodic variations in radial velocity.

visual binary

A binary star in which the two components are telescopically resolved.

white dwarf

A low-mass star that has exhausted its nuclear fuel and has collapsed to a very small size.

Cepheid

A yellow supergiant pulsating star that varies periodically in brightness; useful in deriving distances.

light curve

A graph that displays the time variation of the light from a variable or eclipsing binary star.

luminosity class

A classification of a star within a given spectral class; e.g. our Sun, a G2V star, has luminosity class V.

parallax

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

parsec

A unit of distance equal to $3.26\text{ light-years}$; at a distance of $1\text{ parsec}$, a star has a parallax of $1\text{ arcsecond}$.

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 $1\text{ day}$, useful for finding distances.

baryon cycle

The cycling of mass in and out of the interstellar medium including gas from intergalactic space.

cosmic rays

Atomic nuclei (mostly protons) and electrons that strike Earth’s atmosphere with exceedingly high energies.

H II region

A region of ionized hydrogen in interstellar space.

interstellar dust

Tiny solid grains in interstellar space thought to consist of a core of rocklike material.

interstellar extinction

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

interstellar medium (ISM)

(or interstellar matter) The gas and dust between the stars in a galaxy.

Local Bubble

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; blocks ultraviolet radiation so molecules are able to form.

nebula

A cloud of interstellar gas or dust; 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; scatters blue more 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; where stars form.

Herbig-Haro (HH) object

Luminous knots of gas 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 gas planet.

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 rocky 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.

association

A group of young stars whose spectral types, motions, and positions in the sky indicate a common origin.

globular cluster

Spherical star cluster (each with hundreds of thousands of stars) that form a system around our Galaxy.

helium flash

Ignition of helium in the triple-alpha process in the dense core of a red giant star.

main-sequence turnoff

Location in the H–R diagram where stars begin to leave the main sequence.

nucleosynthesis

The building up of heavy elements from lighter ones by nuclear fusion.

open cluster

Loose cluster of stars, containing dozens to thousands of members; located in the spiral arms of our Galaxy.

planetary nebula

Shell of gas ejected by a hot, dying, low-mass star (glows because of UV energy from the central star).

triple-alpha process

A nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus.

zero-age main sequence

A line denoting the main sequence on the H–R diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy from nuclear reactions, but whose chemical composition has not yet been altered substantially by nuclear reactions.

Chandrasekhar limit

The upper limit to the mass of a white dwarf (equals $1.4$ times the mass of the Sun).

degenerate gas

Gas that resists compression because no two electrons can be in the same place at the same time.

millisecond pulsar

Pulsar that rotates hundreds of times per second (and its pulses are thus measured in milliseconds).

neutron star

A compact object of extremely high density composed almost entirely of neutrons.

nova

Explosion in a binary system, temporarily increasing its luminosity by hundreds to thousands of times.

pulsar

Rotating, magnetic neutron star that is energetic enough to produce a detectable beam of radiation and particles.

type I supernova

Explosion of a white dwarf that has accumulated extra material from an orbiting star.

type II supernova

Enormous explosion produced when a star with mass $> 8\,M_{\text{Sun}}$ dies.

accretion disk

The disk of gas and dust found orbiting newborn stars, as well as compact stellar remnants.

black hole

A region in spacetime where gravity is so strong that nothing—not even light—can escape.

equivalence principle

Concept that a gravitational force and a suitable acceleration are indistinguishable.

event horizon

A boundary in spacetime such that events inside the boundary can have no effect on the universe.

general theory of relativity

Einstein’s theory relating gravity and the structure (geometry) of space and time.

gravitational redshift

An increase in wavelength of light when propagating from or near a massive object.

gravitational wave

Disturbance in the curvature of spacetime caused by changes in the distribution of matter.

singularity

Point of zero volume and infinite density to which a black hole must collapse.

spacetime

System of one time and three space coordinates.