Double Stars

Double Stars

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Look at Mizar, the second star in the tail of the Great Bear, and you will see a  much fainter star, Alcor, close beside it. Use a telescope, and Mizar itself is seen to be made up of two components, one rather brighter than the other. 

The two Mizars are genuinely associated, and make up a physically connected or binary system, while Alcor is also a member of the group even though it is a long way from the bright pair.

 Binary systems are very common in the Galaxy; surprisingly, they seem to be more plentiful than single stars such as the Sun. 

Many double stars are within the range of small telescopes, and some pairs are even separable with the naked eye; Alcor is by no means a difficult naked-eye object when the sky is reasonably clear and dark.

 Yet not all doubles are true binaries. In some cases one component is simply seen more or less in front of the other, so that we are dealing with nothing more significant than a line of sight effect. Alpha Capricorni, in the Sea-Goat, is a good case of this.

The two components are of magnitudes 3.6 and 4.2 respectively, and any normalsighted person can see them separately without optical aid. The fainter member of the pair is 1600 light-years away, and over 5000 times as luminous as the Sun; the brighter component is only 117 light-years away, and a mere 75 Sun-power. 

There is absolutely no connection between the two. As so often happens, appearances are deceptive. The components of a binary system move together round their common centre of gravity, much as the two bells of a dumbbell will do when twisted by the bar joining them.

 If the two members are equal in mass, the centre of gravity will be midway between them; if not, the centre of gravity will be displaced towards the ‘heavier’ star. However, the stars do not show nearly so wide a range in mass as they do in size or luminosity, so that in general the centre of gravity is not very far from the mid position. 

With very widely separated pairs, the orbital periods may be millions of years, so that all we can really say is that the components share a common motion in space. This is true of Alcor with respect to the Mizar pair, while the estimated period of the two bright Mizars round their common centre of gravity is of the order of 10,000 years. 

The real separation seems to be about 60,000 million kilometres (over 37,000 million miles). With the Mizar group there is another complication. In 1889 E. C. Pickering, at the Harvard Observatory, examined the spectrum of the brighter component (Mizar A), and found that the spectral lines were periodically doubled. At once he realized that he was dealing with a binary whose two components were much too close to be seen separately. 

The revolution period is 20.5 days, and the two stars are about equal in brightness. There are times when one component is approaching us, and will show a blue shift, while the other is receding and will show a red shift; therefore the lines will be doubled. When the orbital motion is transverse, the lines will be single.

 Mizar A was the first-known spectroscopic binary; later it was found that both Mizar B and Alcor are also spectroscopic binaries. The eighth-magnitude star between Alcor and the bright pair is more remote, and not one of the group. 

The position angle or P.A. of a double star – either a binary or an optical pair – is measured according to the angular direction of the secondary (B) from the primary (A), reckoned from 000 degrees at north round by 090 degrees at east, 180 degrees at south, and 270 degrees at west back to north. 

In general, it may be said that a 3-inch (7.6-cm) telescope will separate a pair 1.8 seconds of arc apart provided that the two components are equal; a 6-inch (15.2-cm) will reach down to 0.8 second of arc, and a 12-inch (30.5-cm) will reach to 0.4 second of arc. Arich or Gamma Virginis is a good example of a binary which has changed its appearance over the years.

 The components are exactly equal at magnitude 3.5, and the orbital period is 171.4 years. Several decades ago it was very wide and easy, but it is now closing up, and by 2016 the star will appear single except with giant telescopes. This does not mean that the components are actually approaching each other, but only that we are seeing them from a less favourable angle. 

With Zeta Herculis  the period is only 34 years, so that both the separation and the position angle alter quite quickly; so also with Alpha Centauri, the brighter of the two Pointers to the Southern Cross, where the period is 79.9 years. 

In 1995 the separation is 17.3 seconds and the P.A. is 218 degrees; by 2005 the separation will have decreased to 10.5 seconds and the P.A. will have increased to 230 degrees. (Alpha Centauri is the nearest bright star beyond the Sun. 

The dim red dwarf Proxima, more than a degree away from Alpha, is slightly closer to us; it has always been regarded as a member of the group, though there are suggestions that it is merely ‘passing by’.) 

In many cases the two components of a binary are very unequal. Sirius has its dwarf companion, only 1/10,000 as bright as the primary – though it must once have passed through the red giant stage and been much more luminous than it is now. 

Then there are pairs with beautiful contrasting colours; Albireo or Beta Cygni is a yellow star with a companion which is vivid blue, while some red supergiants, notably Antares  and Alpha Herculis (Map 9) have companions which look greenish by contrast. Multiple stars are also found. A famous case is that of Epsilon Lyrae, near Vega 

The two main components are of magnitude 4.7 and 5.1 respectively, and can be separated with the naked eye; a telescope shows that each is again double, so that we have a quadruple system. Theta Orionis, in the Great Nebula , has its four main components arranged in the pattern which has led to the nickname of the Trapezium. 

Castor in Gemini, the senior though fainter member of the Twins , is an easy telescopic pair; each component is aspectroscopic binary, and there is a much fainter member of the group which also is a spectroscopic binary. 

It used to be thought that a binary system was formed when a rapidly spinning star broke up, but this attractive theory has now fallen from favour, and it seems much more likely that the components of a binary were formed from the same cloud of material in the same region of space, so that they have always remained gravitationally linked. 

If their initial masses are different they will evolve at different rates, and in some cases there may even be exchange of material between the two members of the pair.

Source: Atlas of the Universe - Patrick Moore