Posts Tagged ‘brightness’

Black Body

A body that absorbs all the radiation that it receives, giving it an albedo (reflecting power) of zero i.e. it is 100% efficient at absorbing radiation.  A black body also radiates with 100% efficiency, it is therefore a theoretical object although many stars come very close to this theoretical position.

Astrometric Binary

A binary star system in which the fainter component can not be seen

In an astrometric binary, the secondary component is invisible to the eye but is observable from the gravitational effects on the proper motion of the brighter companion. The components of a binary system revolve around a common centre of mass. As the pair move through space, in an astrometric binary, the visible star will be seen to move with a wobble. The path through space will be a ‘wave’ shape instead of a straight path.

There can be various reasons why the secondary component might not be seen. It may be too far away for the light to be detected, it could be a very cold star; the two stars could be too close and too far away to be resolved by telescopes or the primary may be much brighter and simply drown out the light from the secondary.

This method of looking at the movements of stars has been applied to the search for extrasolar planets. As instruments become ever more sensitive, smaller and smaller perturbations can be detected meaning smaller and smaller orbiting masses can be ‘seen’.


Albedo of an astronomical body

A measure of how reflective a body is. Albedo is expressed as a percentage, the higher the percentage, the higher the albedo and therefore the more reflective the object is.

On the face of it, albedo is a straightforward thing, however, the albedo of a planet varies from place to place. Dark surfaces absorb more light than light surfaces. Rough surfaces scatter light in all directions and therfore reflect less back to the observer. Thus when talking about the albedo of an astronomical body, one generally means the average albedo.

Cloudy planets like the gas giants and Venus have high albedos because clods are good reflectors whilst the rocky planets have lower albedos.

The light reaching your eye from an object is reduced thr further it has to travel so the albedo at the edge of a planet is less than at the centre, assuming the same surface composition. Accurate measurements of albedo need to take this into account and so there are two types of albedo, spherical and geometrical. The latter assumes the planet, asteroid, moon or whatever is a uniform sphere whilst the latter compares the reflecting power of the object with that of a flat white disc of the same diameter and distance as the object.

Absolute Magnitude

The absolute magnitude of a star is the brightness that a star would appear if it was at a distance of 10 parsecs from the Earth. It is a very convenient way of comparing brightness of different stars as it is a standardised measure.

The Sun, as our nearest star, is also the visually brightest, however, when compared to other stars by using the absolute magnitude scale, it is fairly faint – it has an absolute magnitude of 4.8, faintly visible to the naked eye buch a lot dimmer than we see the stars of the Plugh constellation.

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