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50 Cards in this Set

  • Front
  • Back

Parsec

The distance at which a star must lie in order for its measured parallax to be exactly 1 arc second; 1 parsec equals 206,000 AU.

Magnitude scale

A system of ranking stars by apparent brightness, developed by the Greek astronomer Hipparchus. Originally, the brightest stars in the sky were categorized as being of first magnitude, while the faintest stars visible to the naked eye were classified as sixth magnitude. The scheme has since been extended to cover stars and galaxies too faint to be seen by the unaided eye. Increasing magnitude means fainter stars, and a difference of five magnitudes corresponds to a factor of 100 in apparent brightness.

Photometry

Branch of observational in which the brightness of a source is measured through each of a set of standard filters.

Giants

Stars with a radius between 10 & 100 times that of the Sun.

Red Supergiants

Extremely luminous red stars. Often found on the asymptotic-giant branch of the Hertzsprung-Russell diagram.

Color-magnitude diagram

A way of plotting stellar properties, in which absolute magnitude is plotted against color index.

Blue Giants

Large, hot, bright stars at the upper-left end of the main sequence on the hertzsprung-Russell diagram. Its name comes from its color & size.

Red-Giant Region

The upper-right corner of the Hertzsprung-Russell diagram, where red-giant stars are found.

Binary star systems

A system that consists of two stars in orbit about their common center of mass, held together by their mutual gravitation attraction. Most stars are found in the binary-star systems.

Eclipsing binary

Rare binary-star system that is aligned in such a way that from earth we observe on star pass in front of the other, eclipsing the other star.

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.

Emission nebulae –
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.
Molecular clouds –
A cold, dense interstellar cloud that contains a high fraction of molecules. It is thought that the relatively high density of dust particles in these clouds plays an important role in the formation and preservation of the molecules.
Evolutionary track –
A graphical representation of a star’s life as a path on the Hertzsprung-Russell diagram.
Brown Dwarfs –
Fragments of collapsing gas and dust that didn’t contain enough mass to initiate core nuclear fusion. Such an object is then frozen somewhere along its pre-main-sequence contraction phase, continually cooling into a compact dark object. Because of their small size and low temperature, brown dwarfs 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.
Proper motion –
The angular movement of a star across the sky, as seen from Earth, measured in seconds of arc per year. This movement is a result of the star’s actual motion through space.

Apparent magnitude

The apparent brightness of a star, expressed using the magnitude scale.

Spectral classes

Classification schemes, based on the strength of stellar spectral lines, which is an indication of the temperature of a star.

Supergiants

Stars with a radius between 100 and 1000 times that of the Sun.

White Dwarfs

A dwarf star with sufficiently high surface temperature that it glows white.

Main sequence

Well-defined band on the Hertzsprung-Russell diagram on which most stars are found, running from the top left of the diagram to the bottom right.

Red Dwarfs

Small, cool faint stars at the lower-right end of the main sequence on the Hertzsprung-Russell diagram.

Spectroscopic parallax

Method of determining the distance to a star by measuring its temperature and then determining its absolute brightness by comparing with a standard Hertzsprung-Russell diagram. The absolute and apparent brightness of the star give the star’s distance from Earth.

Visual binary

A binary-star system in which both members are resolvable from Earth.

Radius-luminosity-temperature relationship

A mathematical proportionality, arising from Stefan’s law, which allows astronomers to indirectly determine the radius of a star once its luminosity and temperature are known.

Dust grains

interstellar dust particles, roughly 10-7 m in size, comparable to the wavelength of visible light.

Polarization

The alignment of the electric fields of emitted photons, which are generally emitted with random orientations.

Dust lanes

Lanes of dark, obscuring interstellar dust in an emission nebula or galaxy.

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 opaque to its own radiation. The protostar is the dense region at the center of the fragment.

Bipolar flows

Jets of material expelled from a protostar perpendicular to the surrounding protostellar disk.

Open clusters

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

Apparent brightness

The brightness that a star appears to have, as measured by an observer on Earth.

Absolute magnitude

The apparent magnitude a star would have if it were placed at a standard distance of 10 parsecs from Earth.

Dwarfs

any stars with radius comparable to, or smaller than, that of the Sun (including the Sun itself).

Red giants

Giant stars whose surface temperature is relatively low so that it glows red.

H-R diagram

A plot of luminosity against temperature (or spectral class) for a group of stars.

Blue supergiants

The very largest of the large, hot, bright stars at the uppermost-left end of the main sequence on the Hertzsprung-Russell diagram.

White-dwarf region

The bottom-left corner of the Hertzsprung-Russell diagram, where white-dwarf stars are found.

Luminosity class

A classification scheme that groups stars according to the width of their spectral lines. For a group of stars with the same temperature, luminosity class differentiates between supergiants, giants, main-sequence stars, and subdwarfs.

Spectroscopic binary

A binary-star system which appears as a single star from Earth, but whose spectral lines show back-and-forth Doppler shifts as two stars orbit on another.

Extinction

The dimming of starlight as it passes through the interstellar medium.

Nebula

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

Dark dust clouds

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 radiation

Radio radiation emitted when an electron in the ground state of a hydrogen atom flips its spin to become anti-parallel to the spin of the proton in the nucleus.

Zero-age main sequence

The region on the Hertzsprung-Russell diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores.

Shock waves

Waves of matter, which may be generated by a newborn star or supernova, that pushes material outward into the surrounding molecular cloud. The material tends to pile up, forming a rapidly moving shell of dense gas.

Globular clusters

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.