61 Cygni
A nearby optical double star in the constellation of Cygnus. It was the first star whose distance was measured using the parallax method.

absolute magnitude
A measure of the inherent brightness of a star independent of any observer. Equivalent to the apparent magnitude of a star seen from a distance of 10 parsecs.

A variable star in the constellation of Perseus.

apparent magnitude
A measure of the brightness of a star as seen by the observer. Based on a system set up by the Greek astronomer Hipparchus.

Bessel, F. W.
A German astronomer who was the first to measure stellar parallax and determine the distance of a star.

An observing instrument consisting of two small telescopes joined together to allow stereoscopic vision.

diffraction grating
A mirror with very fine grooves that separates light into its different colours on reflection.

Doppler Effect
The apparent change in the wavelength of waves due to the relative movement of the source of the waves relative to the observer.

double star
A group of two (or more) stars that appear to the naked eye to be one, but can be separated into two with a powerful enough instrument.

European Space Agency
The major space organisation in the world after America (and possibly Russia). It comprises many European countries.

A huge conglomeration of billions of stars. Our Sun is part of the Milky Way galaxy.

An Italian astronomer living in the early 17th century. The first person to make and use an astronomical telescope. With it, he discovered Jupiter's moons and the phases of Venus.

Hertzsprung-Russell diagram
A graph plotting absolute magnitude against spectral type. Shows the correspondence between temperature and luminosity.

Greek astronomer living around 500 BC. Invented the system of magnitudes upon which we now measure brightness and made the first advances in astrometry (measuring astronomical dimensions).

Hooke, Robert
English scientist of the 17th century who made advances in fields of physics, biology and astronomy.

Hubble, Edwin
Early 20th century American astronomer who discovered the expansion of the Universe by studying the motions of many galaxies.

Hubble's Law
Relationship between a galaxy's distance and its speed of recession. First formulated by Edwin Hubble.
v = H * d
where v is the speed
d is the distance
H is Hubble's constant

Hubble graph
A graph plotting a galaxy's recession speed against its distance. This graph displays Hubble's Law.

Inverse Square Law
The relationship between a intensity of light coming from a source and the distance of the observer from that source. It states that the intensity is inversely proportional to the square of the distance (ie If you are twice as far from a light, it will appear four times as dim).

light curve
A graph plotting the apparent magnitude of a variable star against time.

light year
The distance travelled by a beam of light in the vacuum in one year. It is approximately 10 million million km.

Applied to measures that increase by 1 whenever the factor they depend on is multiplied by a certain number. For example, if a value increased from 10 to 100 to 1000, then a logarithmic measure of that value could increase, say, from 1 to 2 to 3.

Gives the total amount of radiation being given off by a star in 1 second.

main sequence
The stage in a star's life during which it is shining normally by nuclear reactions in its centre. During this time, stars lie on a diagonal line on the Hertzsprung-Russell diagram.

Newton, Isaac
English mathematician/astronomer/physicist who first formulated a Law of Universal Gravitation. He also made important contributions to optics and invented the Newtonian reflecting telescope.

parallax angle - P
The angle formed by the Earth's radius as measured from a star. Half the apparent angle moved by the star against the background stars when seen from opposite sides of the Earth's orbit.

The distance at which a star would have a parallax angle of 1". Approximately 3.26 light years or 32 million million km.

The length of time between successive peaks in the brightness of a variable star.

Glass shape with triangular cross-section that causes light to be spread out into its separate colours when shined through it.

red giant
After a star has finished burning the hydrogen in its core, it enters this stage before dying. The star swells up and turns red.

The brightest star in the night sky with an apparent magnitude of -1.4. It is fairly close to the solar system at about 8 light years.

sound waves
Sound is transmitted as a wave of compressed air. Disturbed air particles in turn disturb more particles allowing energy to be transferred. These disturbances are picked up by our ears and we 'hear' the sound.

spectral type
A letter (O,B,A,F,G,K,M) that is characteristic of a star's colour. O stars are blue, then through white, yellow and orange to M stars which are red.

The pattern of colours produced when the light from an object is passed through a prism or diffraction grating.

Stefan's Law
The relationship between luminosity, surface area and surface temperature of a star.