key terms

• Absolute Magnitude (M)


  The apparent magnitude that an object would have if it were placed at a distance of 10 parsecs away from Earth
• Achromatic Doublet


  A convex lens made of crown glass and a concave lens made of flint glass cemented together in order to focus all light rays in the same position. An achromatic doublet is a solution for spherical and chromatic aberration
• Apparent Magnitude (m)


  How bright an object appears in the sky. This depends on the object's brightness and its distance from Earth
• Arcsecond


  A unit used to measure small angles. An arcsecond is equal to 1/3600th of a degree
• Astronomical Unit (AU)


  The average distance between the centre of the Earth and the centre of the Sun
• Big Bang Theory


  The theory that the universe originated as a small, dense and hot region that expanded and cooled forming the structures in the universe we see today
• Binary Star System


  Two stars orbiting a common centre of mass
• Black Body Radiator


  A perfect emitter and absorber of all possible wavelengths of radiation
• Black Hole


  A region which has an escape velocity greater than the speed of light. Black holes are formed when the core of a giant star collapses
• 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
• Charge-Coupled Device (CCD)


  An array of light-sensitive pixels which become charged when they are exposed to light (by the photoelectric effect)
• Chromatic Aberration


  An effect caused by the different focal lengths of different wavelengths of light that leads to different colours being focused at different points. This can cause a white object to appear as if it has coloured edges
• 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/Diverging Lens


  A lens which spreads out incident light – the light rays diverge
• Convex/Converging Lens


  A lens which focuses incident light – the light rays converge
• Cosmological Microwave Background Radiation (CMBR)


  After the big bang the hot dense state of the universe was full of photons which interacted with the matter in the universe. At a certain time this interaction stopped due to the lower temperature of the universe and these photons were allowed to propagate freely, at this point these photons were gamma rays. At present the universe has expanded, redshifting these photons so that they are microwaves
• Doppler Effect


  The apparent change in the wavelength of a wave as the source moves relative to an observer. For a source moving away the wavelength increases (red shift), for a source moving towards the observer the wavelength decreases (blue shift)
• Eclipsing Binaries


  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
• Event Horizon


  The boundary of a black hole, along which the escape velocity is equal to the speed of light
• Exoplanet


  Planets that are not part of our solar system and orbit other stars. They are often difficult to detect due to the light of their host star obscuring them
• Eyepiece Lens


  The lens in a telescope that magnifies the image produced by the objective lens. It produces a virtual image at infinity in order to reduce eye strain for the user
• Focal Length (f)


  The distance between the centre of the lens and the principal focus
• Hipparcos Scale


  A way of classifying astronomical objects by their apparent magnitude. 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 so the scale is logarithmic
• Hubble's Law


  The speed of a galaxy moving away from ours is proportional to its distance away from us. The constant of proportionality is Hubble's constant
• Hydrogen Balmer Spectrum


  A spectrum formed from the excitation of hydrogen atoms from the n=2 level. The prominence of the Balmer lines from a star can give an indication of the star's temperature and state of the hydrogen within it
• Intensity


  The power received from a star per unit area
• Lens Power


  A measure of how closely a lens can focus a beam that is parallel to the principal axis (i.e. how long the focal length is). The shorter the focal length, the more powerful the lens
• Light Year (ly)


  The distance that an electromagnetic wave travels in a year in a vacuum
• Long-Lived Gamma Ray Burst

  Bursts of gamma radiation that last anywhere between 10 and 1000 seconds. These are thought to be associated with a type II supernova (the death of a massive star)
• Luminosity


  The rate of light energy released by a star. This is the same as the power output of a star
• Magnifying Power/Angular Magnification (M)


  The ratio of the angle made by the image from the eyepiece to the angle made by the object with the unaided eye
• Main Sequence Star


  The equilibrium stage of a star's life cycle, where the inward gravitational forces balance the outward forces caused by fusion. In this stage, hydrogen nuclei fuse to form helium
• Neutron Star


  An incredibly dense star that is formed when the core of a large star collapses. Protons and electrons are forced together under gravity to form neutrons
• Normal Adjustment


  When the distance between the objective and eyepiece lenses in a refracting telescope is equal to the sum of their focal lengths
• Objective Lens


  The lens in a telescope that collects light and creates a real image. Objective lenses should have long focal lengths and be large in order to collect as much light as possible
• 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
• Parsec (pc)


  The distance at which the angle of parallax is 1 arcsecond
• 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
• Principal Axis


  The line passing through the centre of the lens, perpendicular to its surface
• Principal Focus (F)


  In a converging lens: the point where incident rays travelling parallel to the principal axis will converge. In a diverging lens: the point from which the light rays appear to come