Archive for the ‘Stellar’ Category


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.

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.

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.

Airy Disc

Airy Disc – limiting what you can see

Larger telescopes can see smaller objects. They can also resolve closer binary stars and finer details. Why? Magnification? No, it is all to do with the Airy disc …

The apparent size of a star’s disc produced by diffraction effects in an optical telescope. No matter how well made a telescope is, it will never be perfect. The light passing through a telescope will be diffracted. The diffraction creates a sequence of rings of decreasing brightness. The central ring is the brightest and it is called the Airy disc.

The Airy disc has an inverse relationship to the aperture of the telescope. The larger the telescope, the smaller the Airy disc. The size of the Airy disc limits the resolution of a telescope.

In a refracting telescope, about 84% of the starlight reaching the telescope goes into the airy disc, the other 16% forms diffraction rings around the disc, degrading the image and limiting the resolution. Less light makes it into the Airy disc in a reflecting telescope because of the presence of the secondary mirror. It is for that reason that a reflecting telescope of a given aperture will always out perform a reflector of the same quality.

George Airy was the 7th Astronomer Royal of the UK and it is named for him. Why? Because he was the chap who worked all this out of course! But he wasn’t the first to observe the phenomenon, that honour falls to John Herschel. Or at least it was Herschel who first described it.

But how does the Airy disc arise. See the Wikipedia article for a thorough mathematical explanation.

For a simple non-mathematical treatment this will have to do. Most readers of this article will appreciate that diffraction occurs when light passes through holes of a comparable size to its wavelength. It’s not just holes that create diffraction effects. Edges can do that too. So because a lens or a mirror has a finite size, it also has an edge.  It is because of the presence of the edge that diffraction takes place.


Calculating the altitude of an astronomical body

The altitude is the angle of elevation (height in degrees) of a star or other astronomical object above the observer’s horizon. The altitude of an astronomical object changes throughout the observing session because of the Earth’s rotation. It is not an absolute measurement of position.

Diagram illustraring the altitude of a star

Diagram illustraring the altitude of a star

It’s not vastly useful as a means of measuring sky positions but you might use it as a convenience to shout over to your mate to give them an idea of the position of something interesting in the sky.

The maximum altitude of any astronomical object occurs when the object passes the observer’s southern meridian.

It could also be useful if you can figure out the maximum altitude of an object above the horizon from somewhere eles e.g.  if you are heading abroad for a holiday. That way, if it is faint and only gets 20 degrees above the horizon, it probably wouldn’t be worth looking at as it would suffer from too much extinction.

To work out the approximate maximum altitude of a star is done as follows:

The maximum elevation of the celestial equator for a particular spot is:
90 – LAT.

So for an observer at the latitude of London (51.5 N), the maximum elevation will be 90 – 51.5 = 38.5 over the southern horizon.

Then add (for north declination) or subtract (for south declination) the tabulated declination of the object you want to view.

Say you want to know the maximum altitude that the Andromeda Galaxy M31 will reach. Look up the declination (more or less 41degrees N) so it will be 41 degrees above the celestial equator i.e. 79.5 degrees altitude at most.

I say approximate because odds are you will be higher or lower than the real horizon (90 degrees from the zenith). There is a correction that can be applied using alebraic stuff but unless you need to use it for navigation, the approximate altitude is close enough.


Angular Distance

There are many ways to measure angular distance using your body …

The apparent distance between two celestial objects. It is measured in degrees, arcminutes (an arcminute is a 60th of a degree) and arcseconds (an arcsecond is a 60th of an arcminute). On average, the distance from your thumb tip to the tip of your little finger of your outstretched hand at arms length is 20 degrees. The width of your palm will be about 12 degrees and the width of the tip of your little finger is about 1 degree. The angular diameter of the Moon (and the Sun) is more or less 1/2 degree.

Angular Distance

Angular distance between two astronomical objects

An observer looks at two different objects. The angle between them can be measured e.g. by using a cross staff. This angle is the angular distance of the two objects in the sky. It is expressed in degrees, arcminutes and arcseconds.


The apparent close approach of one celestial body to another. This is a line of sight effect.

Diagram showing an appulse

Diagram showing an appulse

Explanation of the diagram: The image on the right is what an observer sees. The diagram shows what is really going on. The orange circle represents a planet, the blue circle represents the Earth. As the planet travels round its orbit, to an observer on Earth, it will be seen to pass close to some stars. In reality, the two objects are separated by many light years. This is referred to as a ‘line of sight effect’.


A distinctive group of stars that is not one of the recognised constellations.

The Plough is a cracking good example, it is not a constellation, it is an asterism that is part of the constellation Ursa Major. Another well known asterism, brought to popularity by Patrick Moore during the early days of his long-running TV astronomy programme ‘The Sky at Night’ is the Summer triangle of Altair, Deneb and Vega. I think that southern hemisphere astronomers refer to it as the ‘Northern Triangle’.

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’.


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.


The smallest unit of a chemical element which retains its own characteristics. An excellent book on the atoms and Einstein’s theory of relativity is “Mr Tompkins in paperback” by George Gamow.

When atoms combine in nuclear reactions, such as the nuclear fusion at the heart of stars, a small amount of mass is ‘lost’. It is not in fact truly lost, the missing matter is converted into energy as described by Einstein’s fameous equation.

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.

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.

Bok Globules

The Dutch astronomer Bart Bok first drew attention to these small black objects. They appear in gaseous emission nebulae and are thought to be protostars that are still forming but have not yet become hot enough to shine.

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.

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