Year 13 - Astrophysics Flashcards
1 Arc Second
A unit used to measure small angles. An arcsecond is equal to 1/3600th of a degree. A degree contains 60 minutes, A minute contains 60 seconds.
Advantages of CCDs
Greater quantum efficiency. A greater proportion of (incident) photons are detected. Can expose for long periods. More light is collected. Idea that it can detect (more) wavelengths beyond the visible. More energy is collected from the star.
Angle Subtended
This is the angle between two points as seen by the viewer.
Astronomical Unit, AU
The mean distance between the centre of the Earth and the centre of the Sun.
Cassegrain Reflecting Telescope
A reflecting telescope with a concave primary mirror and a small convex secondary mirror in the centre, with the eyepiece lens just behind the centre of the primary mirror.
Chromatic Aberration
Light of different wavelengths refracted to different foci, blue focal length closest to lens. Can give images with a spectrum at the edges. Can be overcome by using a mirrored telescope.
Collecting Power
A measure of the ability of a lens or mirror to collect incident EM radiation. Collecting power is directly proportional to the area of the objective lens/primary mirror.
Concave Lens/Mirror
A lens which spreads out incident light. This causes the light rays to diverge.
Convex Lens/Mirror
A lens which focuses incident light to a focal point. This causes the light rays to converge.
Eyepiece Lens
The lens nearest to the eye. It magnifies the image that is formed by the objective lens.
Focal Length
The distance between the focal point and the lens/curved mirror.
Focal Point
The point at which parallel light rays passing through a lens or reflected from a curved mirror converge.
Magnification
The ratio of the image height and object height or the Ratio of the angle subtended by the image and the angle subtended by the object.
Normal Adjustment
The distance between the Objective and Eye piece lens is equal to the sum of the focal points.
Objective Lens
The lens nearest to the object. It collects and focuses the light from the object to form a real image
Parabolic Mirror
A non-spherical mirror in the shape of a parabola. Using a Parabolic Mirror eliminates Spherical Aberration.
Parallax
The apparent change of position of a nearer star in comparison to distant stars in the background. This happens as a result of the Earth’s orbit around the sun. Parallax can be used to define the Parsec.
Primary Mirror
The mirror equivalent of an objective lens. The primary mirror collects light and focuses it onto a secondary mirror in a reflecting telescope.
Principle Axis
The line passing through the centre of the lens/mirror, perpendicular to its surface.
Quantum Efficiency
The percentage of photons incident, which are detected by the receptor/receiver. A CCD has a Quantum Efficiency of over 90%. The Eye has a Quantum efficiency of 1% to 4%.
Rayleigh Criterion
Minimum subtended angle between two objects whose images can be resolved. Minimum angle is when the central maximum of the diffraction pattern of light from one object coincides with the first minimum of the diffraction pattern of the second object.
Real Image
Real images can be projected onto a screen, are inverted and are on the opposite side of the lens to the object. The object needs to be further away from the lens than the focal point.
Reflecting Telescope
A telescope which uses mirrors to focus incident light onto an eyepiece lens. Normally a Primary and Secondary Mirror.
Refracting Telescope
A telescope which uses lenses to focus incident light. Normally an Objective and Eyepiece Lens.
Resolving Power
The ability of a telescope to produce separate images of close-together objects.
Secondary Mirror
The secondary mirror is convex and reflects the light through a hole in the middle of the primary mirror, where the image is then magnified by a lens.
Spherical Aberration
When there are different focal points for rays at different distances from axis; shortest focal length for paraxial rays. This can be reduced by having a parabolic mirror.
Virtual Image
Formed on the same side of the lens as the object. The light rays do not cross after refraction, so the image cannot be projected onto a screen. The object needs to be less that the focal length from the lens for this to occur.
A - Class Star
1) Colour: Blue/White
2) Temperature: 7,500K – 11,000K
3) Absorption Lines: H (strongest), ionized metals
Absolute Magnitude
The apparent magnitude that an object would have if it were placed at a distance of 10 parsecs away from Earth.
Apparent Magnitude
How bright an object appears in the sky. This depends on the object’s brightness and its distance from Earth.
B - Class Star
1) Colour: Blue
2) Temperature: 11,000K – 25,000K
3) Absorption Lines: He, H
Black Hole
An object that has an escape velocity greater than the speed of light.
OR
An object that has a gravitational field strength that is so great that light cannot escape.
Event Horizon
The boundary of a black hole, along which the escape velocity is equal to the speed of light. The Gravitational attraction is so great that not even light can escape.
F - Class Star
1) Colour: White
2) Temperature: 6,000K – 7,500K
3) Absorption Lines: ionized metals
G - Class Star
1) Colour: Yellow/White
2) Temperature: 5,000K – 6,000K
3) Absorption Lines: ionized & neutral metals
Hipparcos Scale
On the scale the brightest stars have an apparent magnitude of 1 and the faintest visible stars have an apparent magnitude of 6. The intensity of a magnitude 1 star is 100 times greater than a magnitude 6 star.
Hydrogen Balmer Lines
Formed from the excitation of hydrogen atoms from the n=2 level. G-Class stars have the strongest Hydrogen Balmer Lines.
K - Class Star
1) Colour: Orange
2) Temperature: 3,500K – 5,000K
3) Absorption Lines: neutral metals
Light Intensity
The power (Energy per Second) received from a star per unit area.
Light Year, Ly
The distance that an electromagnetic wave travels in a year in a vacuum.
Luminosity
The amount of EM energy (light) per unit time. It is the same as Power.
M - Class Star
1) Colour: Red
2) Temperature: < 3,500K
3) Absorption Lines: neutral atoms, TiO
Main Sequence Star
When the inward gravitational forces inside a star balance the outward pressure forces caused by fusion. In this stage, hydrogen nuclei fuse to form helium. Most stars spend the majority of their life-cycle as a Main Sequence Star.
Neutron Star
Star made up of neutrons, extremely dense. Held together by the balance of Gravity inwards and Strong Force outwards (less than 0.5fm).
O - Class Star
1) Colour: Blue
2) Temperature: 25,000K – 50,000K
3) Absorption Lines: He+, He, H
Parsec
Distance at which 1AU subtends an angle of 1/3600th degree or 1 arcsec.
Protostar
A young star formed when clouds of gas and dust are pulled together under gravity. When the centre becomes hot enough, the star will begin to fuse elements together.
Red Giant
A red giant forms after a star has run out of hydrogen for nuclear fusion. The star begins to fuse heavier elements in the core such as Helium and Carbon. The inner core becomes hotter and denser. Outer layers begin to expand and the star becomes bigger.
Schwarzschild Radius
The name given to the radius of a black hole’s Event Horizon.
Stefan’s Law
The power output of a star is directly proportional to its surface area and its absolute temperature to the power of 4.
Super Red Giant
In a red super giant, a much higher number of nuclear fusion reactions occur, producing greater amounts of energy much faster. This causes rapid contraction and expansion of the star which becomes so unstable that a supernova occurs.
Supernova
An object/star that produces a rapid increase in brightness, due to it exploding.
Type I Supernova
When a star in a binary system accumulates matter from its companion star. When the star reaches critical mass, it will explode.
Type Ia Supernova
When a white dwarf star explodes. They can be used as a standard candle since they always reach the same maximum absolute magnitude (-19.3).
Type II Supernova
The consequence of a high mass star dying when it runs out of fuel. The outer layers implode and then rebound from the dense core causing an explosion.
White Dwarf
A white dwarf is a stellar core remnant (dead star). It is very dense: its mass is comparable to the Sun’s, while its volume is comparable to Earth’s. The low luminosity comes from the emission of residual thermal energy; no fusion takes place.
Wien’s Displacement Law
The peak wavelength of emitted radiation from a Black Body is inversely proportional to its absolute temperature in Kelvin.
Big Bang Theory
The theory suggests that, over the past 14 billion years, the Universe has expanded from an extremely hot and dense point and is still expanding. Evidence comes from observations of the red shift of distant galaxies and the CMB Radiation.
Big Bang: Cosmic Microwave Background Radiation
1) Em radiation from all parts of the Universe, ✔
2) The spectrum has a peak in the microwave region, temp 2.7 K ✔
3) It can be interpreted as the radiation left over from the Big Bang / the photons are red shifted to longer wavelengths and lower energies ✔
Big Bang: Relative Abundance of Hydrogen and Helium
1) Fusion occurred when the Universe was very young, helium produced from fusing hydrogen. ✔
2) Fusion stopped as the Universe expanded/cooled ✔
3) Relative abundance of hydrogen and helium, ratio of 3:1, as cooled too rapidly for fusion of larger nuclei ✔
Binary Star System
A two star system with the stars orbiting a common centre of mass.
Doppler Effect
Change in (apparent) frequency [or wavelength] (1) due to relative motion between source and observer (1).
Doppler Shift
The Doppler shift is the ratio of the change in frequency/wavelength of the moving source and the unshifted frequency of the wave emitted, z = Δf/f.
Eclipsing Binary Stars
A binary star system in which the stars’ plane of orbit is in the line of sight of the Earth. This means that the stars will appear to cross over each other as they orbit.
Exo-Planet
Any planet beyond our solar system.
Exo-Planets: Radial Velocity Method
Periodic shift in spectra of star due to star’s movement around common centre of mass with planet. Sometimes referred to as Stellar Wobble. Earth-like planet mass much less than mass of Sun-like star so effect slight.
Exo-Planets: Transit Method
Dip in brightness of star as planet crosses in front. Planet must be orbiting in the same plane as the observer. Alignment must be correct to eclipse, so many possible candidates not observed. Earth-like planet could be observed provided not too far away.
Hubble’s Law
Hubble’s law relates how fast a galaxy is moving away from us, v (recessional velocity), to its distance from Earth, d. The velocity is directly proportional to the distance assuming acceleration at a constant rate.
Modifications to Expanding Universe
Rate of expansion of Universe is increasing, (due to) dark energy.
Quasars
High power/powerful radio emitter (Highly Red shifted). Nucleus of an active galaxy; a supermassive black hole surrounded by a disc of matter.
