Posts Tagged ‘star’

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.


The corona is a huge envelope of gases that form the outer portion of the Sun. It is seen as faint extensions of the Sun’s outer atmosphere during total eclipses and can be observed using a corongraph, a device that is used to create an artificial total solar eclipse.

The shape of the corona varies with the sunspot cycle. At maximum, it is much more even. At minimum, it is seen to be much more irregular with polar tufts, equatorial streamers and plumes being visible.

It has a temperature of between 1 and 2 million K and naturally at that temperature, is a plasma, for example, iron has been identified but with half of its electrons stripped away. It is indeed a very high energy environment. The corona of the Sun is the origin of the solar wind. An estimated 3 million tonnes of material is ejected from the Sun each second.

This material is experienced as the solar wind with ‘gusts’ of up to several hundred km per sec. The Earth is protected from this bombardment of charged particles by its magnetic field which shunts them into the Van Allen Belts. When these overflow at the poles, they create the aurora borealis and aurora australis.

The solar wind appears to be strongest where it flows from coronal holes. These are areas of lower temperature punched through the corona by the powerful magnetic fields of the Sun.

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.



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.


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.

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.

Carbon Star

Red stars of spectral types R and N, containing an unusual amount of carbon in their atmosphere.

Carbon stars were discovered by Father Pietro Angelo Secchi in the 1860s.

Blue Shift

If an astronomical body is moving towards the observer, the light will seem to be shifted to the blue end of the spectrum. The faster the movement, the greater the blue shift. It occurs because the wavelength of light is slightly compressed by the Doppler effect as the body moves towards the observer. Blue shift is measured by looking at the key spectral lines. For an object moving towards the Solar System, they will appear closer to the blue end than normal. The faster the object is approaching, the greater the blue shift will be.

Black Hole

I guess no-one will probably see this page …

A localised region of space from which not even light can escape, due to a super massive star collapsing in on itself. This is because the gravitational field of a black hole is so strong that the escape velocity is greater than the speed of light. Astronomers believe that the location of large black holes can be observed because of radiation emitted from the accretion disc as matter is pulled into the black hole. The event horizon marks the outer limits of these objects.

Black Dwarf

A dead star, which has used up all its reserves of energy. The ultimate fate of a White Dwarf. It is therefore a small, cold star.

Binary Star

double star in which the components orbit one another. Some binary systems orbit so close to one-another the two stars are distorted by each other’s gravity.

If the two stars eclipse one-another when seen from the Earth, the light changes regularly and predictably over a period of time. This type of binary is an eclipsing binary and is a type of variable star.


The branch of astronomy dealing with the movements and positions of celestial bodies.

Astrometry dates back to the earliest days of astronomy when the first star catalogues were being produced, e.g. that of Hipparchus in 190BC. Early measurements were probably made using cross-staffs to measure the relative positions of stars from one-another and from features on the horizon. As time progressed, more sophisticated instruments were used, such as the astrolabe.

The same principles are still used in modern astronomy but have become very precise and can measure the wobbles in the movements of stars that could indicate the presence of extrasolar planets or to find astrometric binary star systems.

Astrometry also includes the measurement of parallax. If you observe an object from two widely spaced locations, you will measure a slight difference in position. From the annular differences in position and knowing the distance between the two locations of observation, the distance of stars can be determined. That was taken to a new level with the Hipparcos satellite launched by the ESA. Early parallax measurements were limited to relatively close stars, however, the distances of several cepheid variable stars was measured. Thes can then be used as ‘standard candles’ when seen in distant galaxies to give a reasonable estimate of their distances. Further refinement to astrometric determination of distant objects came with the advent of interferometry.

As with all measurements of the extremely large and extremely small, astrometric measurements require careful error correction. As knowledge and instrumentation improves, distances and speeds of star movements are constantly being refined and it is believed that it is now possible to see the peturbations in stellar motion caused by planets not much more massive than the Earth.

Accretion Disc

An accretion disc is a disc of material formed under the influence of gravity. It forms from smaller particles which are drawn towards the central body by the influence of Gravity.

The central body is generally a star although it can also be the nucleus of a galaxy or a black hole.

Also the rotating disc of material formed around a Black Hole.

The force of gravity is also reponsible for the emission of electromagnetic radiation from the disc. Gravity compresses the particles of the disc, heating them and giving rise to the radiation. The radiation wavelength depends on the strength of the gravitational force. Accretion discs around a black hole for instance can emit X-rays as the gravitational forces are immense. Accretion discs around forming or newly formed stars will only emit infrared as the gravitational forces are less, leading to a lower level of energy and therfore longer wavelengths of radiation. The Hubble Space Telescope is alleged to have seen and measured an accretion disc around a black hole using gravitational lensing to work out the colour profile.

The accretion discs around black holes and quasars are probably the most efficient way of generating energy from matter known, with about 10% matter to energy conversion. It is thought that accretion discs could be the source of the Gamma Ray Bursts seen from time to time in the universe. The Eddington Luminosity of Eddington Limit defines the point at which the outflow of energy from a star exactly balances the inward pull of gravity i.e. it is stable. Super eddington accretion discs are thought to be the source of the gamma ray bursters. Turbulence can cause material to fall inwards which is then converted into high energy radiation. These are very short lived events

Accretion discs round young stars supply the material for planets to form.

Accretion discs are also found in binary star systems. It happens when the two stars are of unequal ages and sizes and close together – the younger one will have lived out its life and become a white dwarf or even neutron star, then, when the older one reaches the end of its main sequence life, it expands. It it expands sufficiently, the outer envelope of gases can be more strongly attracted by the gravity of the smaller star and forms an accretion disc around the other.

The physics of an accretion disc is way beyond what we intend to explain here, however, if you want a detailed explanation, click here.

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.


Aberration of light is the apparent displacement of a star from it’s true position in the sky. It is caused by a combination of the motion of the Earth in orbit round the Sun (about 30 km per sec) and the finite velocity of light (299,792.5 km per sec or , if you prefer imperial units, 186,252.5 miles per second). The rotation of the Earth also gives rise to the aberration of starlight.

To understand aberration, we need to start off with a simple easy to understand example from the familiar world around us. Imagine you are in a parked car and you look out of the window and the falling rain. Imagine that there is no wind so the rain is falling vertically. As the driver pulls away and picks up speed to say 30mph, you notice that the rain is no longer falling vertically. Actually it is, but you are moving forwards, past the raindrops, thus greating the illusion that the rain is falling diagonally, slanting towards the back of the car.

OK, so back to the starlight. The Earth is moving forwards through space and, despite the high speed at which light travels, the starlight we see effectively is slanting backwards compared to the direction of movement of the Earth in its orbit. But the Earth moves in an ellipse round the sun so the direction of ‘slant’ of the light changes too. The net result is that if the precise position of a star is recorded throughout the year, it will be seen to describe a small ellipse around its ‘true’ position … the ‘true’ position being where the star would have been seen had the Earth been stationary.

There is also a very much smaller daily effect caused by the rotation of the Earth. This is called diurnal aberration.

The maximum displacement is 20.5 seconds of arc. This number is called the constant of aberration.  For a much more thorough treatment, including a discussion of relativity and aberation, click here.

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