Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
39 Cards in this Set
- Front
- Back
|
aberration
|
An effect that causes the apparent direction of a star (as observed from the Earth) to differ from its true direction in a way that changes systematically throughout the year. The effect arises from the combination of the finite speed of light and the movement of the a Earth as it orbits the Sun, and requires that a telescope being used to observe a star must be pointed slightly away from the true direction of the star.
|
|
|
absolute visual magnitude
|
A numerical measure of the intrinsic brightness of a star, equal to the apparent visual magnitude the star would have if observed from a standard distance of 10 parsecs, in the absence of interstellar absorption.
|
|
|
absorption
|
A process that leads to a decrease in the intensity (of flux density) of radiation as it passes through a medium (e.g. the interstellar medium).
|
|
|
absorption lines
|
Narrow wavelength or frequency ranges in a spectrum where the spectral flux density is less than at adjacent wavelengths (or frequencies).
|
|
|
absorption lines
|
Narrow wavelength or frequency ranges in a spectrum where the spectral flux density is less than at adjacent wavelengths (or frequencies).
|
|
|
absorption spectrum
|
A spectrum in which absorption lines are prominent.
|
|
|
accretion
|
Any process by which materialism added to an astronomical body. This may,for example, occur during the formation of a protostar or during the transfer of matter in an interacting binary system.
|
|
|
accretion disc
|
A disc of gas which forms around massive objects such as an accreting star in an interacting binary system. Material spirals inwards within the disc and falls onto the central object from the inner edge of the disc.
|
|
|
active regions
|
Areas on the photosphere of the Sun where magnetic field lines are concentrated. These generate a number of different phenomena such as sunspots and flares.
|
|
|
angular momentum
|
A measure of the momentum associated with rotation. The magnitude (L) of a body's angular momentum at any time is found by multiplying its movement of inertia by its angular speed at that time. The SI unit of angular momentum is the kg m2 s-1.
|
|
|
angular speed (ψ)
|
The rate at which an object (e.g. a wheel) turns, or at which a body (e.g. a satellite) orbits another body. Defined as the angle turned through in unit time. The SI unit of angular speed is the radian per second (rad s-1)
|
|
|
apparent visual magnitude
|
A numerical measure of the apparent brightness of a body. For a star, it is determined by the flux density received in the V band, i.e. a band that approximates the response of human vision versus wavelength.
|
|
|
astroseismology
|
The study of stellar interiors from observations of global oscillations of their photospheres. Analogous to helioseisnology but using only the combined light from the whole surface of the star.
|
|
|
astronomical unit (AU)
|
The average distance from the Earth to the Sun. More precisely, the semimajor axis of the Earth's elliptical orbit around the Sun. (The size of the Earth's orbit varies slightly, so for precise work the informal definitions given above have been superseded by a more technical and exact definition.)
|
|
|
astronomical unit (AU)
|
The average distance from the Earth to the Sun. More precisely, the semimajor axis of the Earth's elliptical orbit around the Sun. (The size of the Earth's orbit varies slightly, so for precise work the informal definitions given above have been superseded by a more technical and exact definition.)
|
|
|
asymptotic giant branch (AGB)
|
A region on the Hertzsprung-Russell diagram occupied by large cool stars after helium core burning has been completed. On the H-R diagram, stars of different masses approach this region from the left and evolve upwards so that the evolutionary tracks approach each other.
|
|
|
astronomical unit (AU)
|
The average distance from the Earth to the Sun. More precisely, the semimajor axis of the Earth's elliptical orbit around the Sun. (The size of the Earth's orbit varies slightly, so for precise work the informal definitions given above have been superseded by a more technical and exact definition.)
|
|
|
asymptotic giant branch (AGB)
|
A region on the Hertzsprung-Russell diagram occupied by large cool stars after helium core burning has been completed. On the H-R diagram, stars of different masses approach this region from the left and evolve upwards so that the evolutionary tracks approach each other.
|
|
|
aurora
|
A luminous atmospheric phenomenon observed in high latitudes. It is due to visible light emitted from atoms and molecules in the Earth's atmosphere that have been excited by collisions with electrons from the magnetosphere.
|
|
|
Balmer absorption lines
|
A set of absorption lines due to photoexcitation of electronic transitions in hydrogen atoms, in which the electron makes a transition from a state corresponding to the energy level with n = 2, to a higher energy level.
|
|
|
baryon
|
An elementary particle that comprises three quarks. Protons and neutrons are baryons.
|
|
|
big bang model
|
The generally accepted scientific account of the evolution of the Universe from the earliest phases of its expansion. A feature of this model is that early in the history of the Universe, all matter was in a very hot and dense state, and the temperature of the matter dropped as time progressed.
|
|
|
binary star
|
A system of two stars in orbit around each other.
|
|
|
bipolar outflow
|
The phenomenon in which material ejected from a stellar system takes the form of two opposing jets. Some such jets appear to be associated with the early stages of stellar evolution.
|
|
|
black hole
|
A region of space from which electromagnetic radiation and matter are unable to escape due to the action of gravity. Such regions are expected to be created by the catastrophic collapse of some massive stars.
|
|
|
black hole candidate
|
An object believed, through its observed properties, to be a possible black hole.
|
|
|
black-body spectrum
|
The spectrum of a black-body source (or ideal thermal source). This is a continuous spectrum with a characteristic shape, the peak wavelength depending on the temperature of the source, in accord with Wien's displacement law. A characteristic of sources that produce spectra that are close to the black-body form is that there is a degree of interaction between electromagnetic radiation and the material that makes up the source. (This leads to the formal definition of a black-body source as one which has the property that it absorbs perfectly any electromagnetic radiation that is incident on it and emits a black-body spectrum.)
|
|
|
blue-shift
|
The decrease in the observed wavelength of radiation relative to the wavelength at which the radiation was emitted (the frequency is correspondingly increased). A common cause of blue-shift is the relative movement of the source to the observer (an example of the Doppler effect).
|
|
|
Boltzmann constant
|
The constant that relates kinetic energy and temperature in a gas (or pressure and temperature in a fixed volume and quantity of gas). It has the value k = 1.38 x 10-23 J K-1.
|
|
|
bow shock
|
The boundary of the Earth's magnetosphere where the solar wind is deflected. More generally it refers to any boundary between the flow of fast moving and stationary gas or plasma.
|
|
|
broadband spectrun
|
A spectrum covering a wide range of wavelengths or frequencies, which represents the energy distribution of a source. It does not generally show narrow features such as absorption lines.
|
|
|
brown dwarf
|
A stellar object of mass less than about 0.08M., in which the core temperature is too low for hydrogen burning to be initiated.
|
|
|
carbon burning
|
Nuclear fusion reactions in which two carbon nuclei fuse to form a heavier nucleus with a mass number of 20 (e.g. sodium, neon or magnesium). Carbon burning occurs in massive post main sequence stars. The term is sometimes also used to refer to any nuclear fusion reaction where carbon is a reactant, for example when a carbon nucleus combines with a helium nucleus to produce an oxygen nucleus.
|
|
|
cataclysmic variable
|
A form of interacting binary star in which material is transferred from which fills its Roche Lobe onto a white dwarf. They exhibit dramatic changes in luminosity that are most prominent in the optical, ultraviolet and X-ray parts of the spectrum.
|
|
|
centre of mass
|
A point in a body, or amidst a set of bodies, that moves through space in the same way that the whole mass of the system would move were it concentrated at that point.
|
|
|
Cepheid
|
A type of regular variable star, which pulsates with a period in the range from about a day to about 1000 days. The changes in radius, temperature and hence luminosity, arise from instabilities in the envelope of such evolved giant or supergiant stars.
|
|
|
Chandrasekhar limit
|
The theoretical upper limit to the mass of a white dwarf; about 1.4M . Above this limit, electron degeneracy pressure cannot support the star and it collapses further.
|
|
|
chromosphere
|
The region of the Sun just above the photosphere. It is the lowest layer of the Sun's atmosphere, characterised by a reddish hue, and by an increase in temperature with altitude in all but its lower reaches.
|
|
|
circumstellar disc
|
A disc (or torus) of material surrounding a protostar. Possibly because of its rotational motion, it has not collapsed into the protostar.
|
