Whilst binoculars, particularly the image-stabilised variety (see Chapter 3), can show literally hundreds of double stars, the use of a small telescope will considerably increase the number of pairs of stars that can be seen. It also allows the user to see stellar colours more
Figure 1.2. a The proper motion of 5 Herculis. Measurements of the position angle and separation of star B with respect to A over many years shows the relative motion between the two. b shows the real situation with star A moving towards PA 187° at a rate of 0.159" per year whilst B moves towards PA 275° by 0.117" per year.
b a b a easily. In a 90-mm telescope, most of the closest pairs that can be seen are binary pairs - the two stars are physically connected by a mutual gravitational bond -and they rotate around the common centre of gravity in periods ranging from a few tens to a few millions of years.
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