Astronomy Flashcards
This set of cards was created by the Physics Subject Ambassadors however, as there are no definite definitions in the textbook, these are a guide taken from the Astronomy booklet and online research; though they may be a good representation of each term's definition, these need supplementary research for better understanding
Principal Axis
The straight line through the centre of the lens perpendicular to the lens
Converging Lens
Convex lens which makes parallel rays converge to a focal point/principal lens
Diverging Lens
Concave lens which makes parallel rays diverge. The point where the rays appear to come from is the principle focus/focal point
Focal Length
Distance between centre of the lens to the focal point
Focal Plane
The plane of each side of the lens perpendicular to the principal axis containing the principal focus
Real Image
With the object at different distances beyond the principal focus of the lens, the position of the screen is adjusted until a clear image of the object is seen on the screen. The image is described as a Real Image because it is formed on the screen where the light rays converge
Virtual Image
With the object nearer to the lens than the principal focus, a magnified image is formed. The lens acts a magnifying glass. But the image can only be seen when you look into the lens from the other side of the object. The image is called a Virtual Image because it is formed where the light rays appear to come from.
Magnifying Glass
The image is virtual, upright and larger than the object; only seen by viewing it through the lens
Normal Adjustment
When a telescope is adjusted so the virtual image seen by the viewer is at infinity;
Normal adjustment is when the distance between the objective and the eyepiece is equal to their focal lengths added together.
Angular Magnification
The ratio of the angle subtended at the eye by the image formed by an optical instrument to that subtended at the eye by the object when not viewed through the instrument.
Angular magnification = focal length of objective/focal length of eyepiece
Collecting Power
The amount of light a telescope collects (which is proportional to the square of the objective diameter)
Spherical Aberration
Optical effect that occurs when incoming light rays focus at different points due to the surface geometry of the mirror
Chromatic Aberration
Optical effect that occurs when the refraction of different wavelengths of light through a lens fails to converge at a single point
Angular Separation
The angular difference between two objects
Rayleigh Criterion
Resolution of the images of two point objects is not possible if any part of the central spot of either image lies inside the first dark ring of the other image.
Minimum Angular Resolution
θ = λ/D
Charge Coupled Device (CCD)
Integrated circuit etched onto a silicon surface forming light-sensitive elements called pixels
Parallax
The apparent displacement of an observed object due to a change in the position of the observer
Astronomical Unit (AU)
Mean distance between the centre of the Sun to the Earth
1 AU = 1.496x10^11 m
Parallax Angle
The angle subtended to the star by the line between the Sun and the Earth
Parsec
1 Parsec (pc) is the distance to a star at which 1 astronomical unit subtends an angle of 1 arcsecond (3.08x10^16m)
Intensity
Light energy per second per unit surface area received from a star at normal incidence on a surface
Hipparcos Scale
First classification of stars in six magnitudes of brightness; a star with magnitude 1 being one of the brightest in the sky, and a star of magnitude 6 being just visible to the human eye
Apparent Magnitude (m)
A number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower it’s magnitude value. (The Sun, at apparent magnitude of −26.7, is the brightest object in the sky)
Absolute Magnitude (M)
The magnitude the star would have if it was placed at a distance of 10 parsecs from Earth (The Sun has an absolute magnitude of 4.83)
Inverse Square Law
I ∝ 1/d^2
Intensity ∝ 1/(distance from Earth)^2
Black Body Radiation
A Black Body is a body that is a perfect absorber of radiation (absorbs 100% of incident radiation of all wavelengths). The thermal electromagnetic radiation emitted from a Black Body is referred to as Black Body Radiation
Wien’s Displacement Law
λ(max)*T = constant = 0.0029 mK
Photosphere
The light-emitting outer layer of a given star (One uses Wien’s Law to work out the absolute temperature T of this layer). Often referred to as the surface of the star
Stefan’s Law
P = σAT^4
Power output of the star = (Stefan-Boltzmann Constant)(Surface Area)(Absolute Temperature)^4
Luminosity
Power Output of a star
Balmer Lines
The name given to the hydrogen absorption lines corresponding to the excitation of hydrogen atoms from the n=2 state to higher energy levels. These can be used in addition to T to determine the spectral class of a star
Dwarf Star
A star that is much smaller in diameter than the Sun
Giant Star
A star that is much larger in diameter than the Sun
Supernova
Rapid expulsion of mass due to a collapsing star that releases so much energy as to outshine its host galaxy
Standard Candles
A class of objects whose distance can be found due to their known peak luminosity/absolute magnitude (approx M = -19) and presence of strong silicon absorption line
Dark Energy
Hypothesis used to explain the breakdown of Hubble expansion laws at large distances
Neutron Star
Core of a supernova after all the surrounding matter has been thrown off into space. It is extremely small in size compared with a star such as the Sun
Pulsars
Rapidly rotating neutron star which emits radio waves in two beams (like a lighthouse)
Black Hole
An object so dense that not even light can escape it due to the immense gravitational pull at its centre (an infinitely dense point called a singularity). This is because the escape velocity is greater than the speed of light (c)
Event Horizon
Sphere surrounding the black hole from which nothing can emerge (the radius of which is the Schwarzschild radius; r = 2GM/c^2)
Red Shift
The characteristic spectral lines of the elements are all at a longer wavelength than you would expect. Frequency falls and the wavelength increases as the object is moving away
Blue Shift
The characteristic spectral lines of the elements are all at a shorter wavelength than you would expect. Frequency increases and the wavelength decreases if the object is moving nearer
Doppler Shift
The change in wavelength and frequency as an object moves towards/away from the viewer
Spectroscopic Binary
Two or more stars that orbit each other but are too far away to be individually resolved
Hubble’s Law
v = Hd
Big Bang Theory
The theory that the universe was created in a massive primordial explosion and has been expanding ever since
Quasar
A supermassive black hole surrounded by orbiting disks of gas; more like a massive star than a galaxy in terms of size, but its light output is on a galactic scale or greater
Supermassive Black Hole
Largest type of black hole, on the order of hundreds of thousands to billions of solar masses, and is found in the centre of almost all currently known massive galaxies (including the Milky Way)
Exoplanets
A planet that orbits a star outside our solar system
