The distance that light travels through space in 1 year, equal to 9.5 x 10^15 m.
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Galaxies
Assemblies of stars prevented from moving away from each other by their gravitational attraction.
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Parallax
When a relatively near star shifts in position against the background of more distant stars as the Earth moves around its orbit. Occurs because the line of sight to a nearby star changes direction over 6 months.
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Astronomical Unit
The mean distance from the centre of the Sun to the Earth, equal to 1.496 x 10^11m
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Parallax Angle
The angle subtended by the star to the line between the Sun and the Earth. The angle is half the angular shift of the star's line of sight over 6 months.
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Parsec
The distance to a star which subtends an angle of 1 arc second to the line from the centre of the Earth to the centre of the Sun.
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Intensity of a Star's Light
The light energy per second per unit surface area received from the star at normal incidence on a surface.
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Apparent Magnitude (m)
A measure of its brightness which depends on the intensity of light received from the star.
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Absolute Magnitude (M)
The star's apparent magnitude if it was at a distance of 10 parsecs away from Earth.
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Black Body
A body that is a perfect absorber of radiation and therefore emits a continuous spectrum of wavelengths.
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Photosphere
Light emitting outer layer of a star. (surface of the star)
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Luminosity P
The power output of the star.
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Balmer Lines
Hydrogen absorption lines (correspond to excitation of hydrogen atoms from the n=2 state to higher energy levels)
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Dwarf Star
A star that is much smaller in diameter than the Sun.
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Giant Star
A star that is much larger than the diameter of the Sun.
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HR Diagram: Main Sequence
A heavily populated diagonal belt of stars ranging from cool low-power stars of absolute magnitude +15 to very hot high-power stars of absolute magnitude about -5. Greater the mass, the higher up the main sequence it lies.
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HR Diagram: Giant Stars
Absolute magnitudes in the range of about +2 to -2 so they emit more power than the Sun and are 10 to 100 times bigger. Red giants are cooler than the sun.
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HR Diagram: Supergiant Stars
Absolute magnitudes of -5 to -10 and are much brighter and larger than giant stars. Relatively rare compared to giant stars.
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HR Diagram: White Dwarf Stars
Absolute magnitudes +15 to +10 and are hotter than the Sun but emit much less power. Smaller in diameter to the Sun.
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Supernova
A star exploding which releases so much energy it can outshine its host galaxy.
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Neutron Star
The core of a supernove after all the surrounding matter has been thrown off into space. Extremely small in size compared with a star.
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Pulsars
Pulsating radio stars. High speed rotating neutron stars that produce beams of radio waves.
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Black Hole
An object so dense that not even light can escape from it.
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Event Horizon
A sphere surrounding a black hole from which nothing can ever emerge. The radius is called the Schwarzchild radius.
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Other cards in this set
Card 2
Front
Assemblies of stars prevented from moving away from each other by their gravitational attraction.
Back
Galaxies
Card 3
Front
When a relatively near star shifts in position against the background of more distant stars as the Earth moves around its orbit. Occurs because the line of sight to a nearby star changes direction over 6 months.
Back
Card 4
Front
The mean distance from the centre of the Sun to the Earth, equal to 1.496 x 10^11m
Back
Card 5
Front
The angle subtended by the star to the line between the Sun and the Earth. The angle is half the angular shift of the star's line of sight over 6 months.
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