Archive for the ‘Stellar’ Category

Celestial Equator

A great circle on the celestial sphere that is midway between the two poles of rotation. It really amounts to the projection of the Earth’s equator onto the sky. It can be thought of as an imaginary ring, 90 degrees from either pole in the sky.

Celestial Latitude

The angle north or south of the ecliptic to an object. Used to help to describe the location of a star on the imaginary celestial sphere.

Celestial Longitude

The angle that is measured eastward along the ecliptic, from the vernal equinox, to the foot of a circle that is perpendicular to the ecliptic and passing through the object.

Celestial Sphere

This does not actually exist. Early humans believed that stars were fixed to a crystal sphere in the sky, at a great distance from Earth. This is because there is no sense of distance in the night sky and the stars always seemed to be fixed in position relative to each other. The idea is a handy one when dealing with positioning and angles between objects in the sky. See also celestial latitude, longitude, azimuth.

Cepheid Variable

An important type of variable star. Stars of this type have short periods, from a few days to a few weeks; these stars are perfectly regular. The period of a Cepheid depends on its absolute magnitude. So, when the period of variability is known, its absolute magnitude can be deduced. Comparing this to its apparent magnitude (how bright it is when seen from Earth) gives the distance. They are sometimes referred to as “standard candles” because of their usefulness in determining the distances of galaxies. They are named after the star Delta Cephei, which was the first one of this type of star to be discovered.

Circumpolar Stars

Cirumpolar stars lie within a region of the sky that is always visible round the celestial pole closest to the observer. An object in this area will therefore never set, at any time of the night (or day of course) and can be observed at any time of the year e.g. the Plough asterism is in the circumpolar region from the UK and can be seen in all four seasons, Orion is not circumpolar and so can only be observed for part of the year.

If you want to know if a star will be circumpolar, you need to know your latitude. Subtract that from 90 and all stars within that distance from the pole will be circumpolar.

On the other hand, all stars within that distance from the opposite pole will never rise at your latitude.


A cluster is a group of stars whose members are genuinely associated. A cluster of stars is formed from the same gas/dust cloud. There are two main types: open and globular.

Cluster is also the name given to a group of four spacecraft that have been placed in Earth orbit in order to study the interactions between particles ejected from the Sun and the Earth’s magnetic field. The first attempt to put Cluster into space in 1996 failed when the Ariane rocket exploded. Oops.


The end product of a very massive star, which has collapsed to form a very high density object. It is a general term that refers to any sort of collapsed star – white dwarf, Neutron star black hole for example. It is also referred to in one of the models used to explain a hypernova – a super large supernova.

Colour Index

A measure of a star’s colour, which helps astronomers to tell its surface temperature. It is the difference between the magnitude of a star measured in two different areas of the spectrum. The areas are B (blue), V (violet) and U (ultraviolet) regions. The B-V is the most common index used and is close to zero for a white star. It is extremely useful in the classification of stars, it can tell astronomers if the star is a main sequence star, a giant star or a supergiant star.

The intrinsic colour index is a modified form that has been corrected for interstellar extinction.



There are several astronomical meanings. It can be used to describe the hazy looking patch that surrounds the nucleus of a comet or the blurred effect surrounding the images of stars on a photographic plate, or in the observers field of view in a telescope (or binoculars) due to defects in the lenses.


This is pronounced ‘Koh-meez’ and means the fainter companion of a double or binary star. The plural is comites.

Double Star

Double star – when you observe a star through a telescope or binoculars, it appears to be two stars as opposed to one when seen with the naked eye.

Sometimes this is a line of sight effect where the two stars are in reality totally unconnected with each other. In other cases the two stars are a genuine pair, orbiting one another, in which case they are referred to as a binary star.

Stars that are only connected by line of site are referred to as optical doubles or visual doubles.

Double Star Programme is also the designation of the fist ever collaboration between the Chinese and the European space agencies. It ran from early 2004 to late 2007. Two Chinese satellites were launched into orbits at 90 degrees to one another in order to study ‘global physical processes in Earth’s magnetic environment and its response to solar disturbances’ (esa). Working in conjunction with the existing Cluster satellites, Double Star Programme has msde several new discoveries, notably ‘Space is fizzy’ (density holes in the Solar wind in the region of the bow shock), Chorus emissions found at a greater distance (areas where high energy particles that can damage electronic equipment) and something called neutral sheet oscillations in the magnetotail.

Dwarf Star

Originally put forward by Ejnar Hertzsprung in the early years of the 20th century, the term dwarf star has come to be applied to any star on the main sequence, for example, the Sun is regarded as a yellow dwarf star. The term has been extended to other stars that are not on the main sequence such as white dwarfs and brown dwarfs.A brown dwarf is an gaseous object that is larger than jupiter but less than 1% of the mass of the Sun. It emits energy feebly as it can convert hydrogen into duterium but can go no further as it’s mass is too low.

The term ‘Red Dwarf’ was used as the title to the highly successful and immensely funny TV comedy by Naylor and Grant; the eponymous Red Dwarf was a space mining ship.


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