definitions and concepts

• absolute magnitude, M

  the apparent magnitude that an object would have if it were placed at a distance of 10 parsecs away from the earth
• achromatic doublet

  a convex lens made of crown glass and a concave lens made of flint glass, cemented together to focus all light rays in the same position

  • solution for spherical and chromatic aberration
• apparent magnitude, m

  how bright an object appears in the sky

  • depends on the object's brightness and its distance from earth
• arcsecond

  a unit used to measure small angles

  • equal to 1/3600th of a degree
• astronomical unity, 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 seen today
• binary star system

  two stars orbiting a common centre of mass
• black body radiator

  a perfect emitter and absorber of all possible wavelengths fo radiation
• black hole

  a region which has an escape velocity greater than the speed of light

  • 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 the 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

  • 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 apparant change in the wavelength of a wave as the source moves relative to an observer

  • red shift - for a source moving away, the wavelength increases
  • blue shift - for a source moving towards the observer, the wavelength decreases
• eclipsing binaries

  a binary stary system in which the stars' plane of orbit is in the line of sight of the earth

  • 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

  • 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

  • produces a virtual image at infinity 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 one
  • 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'sconstant
• 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
• lens power

  a measure of how closely a lens can focus a beam that is parallel to the principal axis

  • the shorter the focal length, the more powerful the lens
• light year, ly

  the distance that an electromagnetic wave travels sin a year in a vaccum
• long-lived gamma ray burst

  bursts of gamma radiation that last anywhere between 10 and 1000 seconds

  • though to be associated with a type 2 supernova - the death of a massive star
• luminosity

  the rate of light energy released by a star

  • 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 state, 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

  • should have long focal lengths and be large 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

  • 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 fo an objective lens

  • collects light and focuses it onto a secondary mirror in a reflecting telescope
• principle axis

  the line passing through the centre of the lens, perpendicular to its surface
• principle 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 from
• protostar

  a young star fromed when clouds of gas and dust are pulled together under gravity

  • protostars are surrounded by a circumstellar disc, and when the centre becomes hot enough the star will begin to fuse elements