Best Double Stars for Small Telescopes: 15 UK Targets

The best double stars for small telescopes are the one class of target that a British back garden does not spoil. Streetlights wash out galaxies and nebulae, but a pair of stars is a pair of bright points, and light pollution barely touches them. They need no dark adaptation, they are fine with the Moon up, and a 70mm refractor or a 130mm Dobsonian will split dozens of them. What they do need is a list of which ones are worth the effort, and an honest idea of which your telescope can manage.

This list gives 15, grouped by the season they are best placed from the UK, with the magnitudes and separations that decide whether a given scope will split them. Separations are in arcseconds (a sixtieth of an arcminute), and for slow-moving binaries they drift by a few tenths over the years, so treat the figures as current approximations rather than fixed values.

What your aperture can split

The rule of thumb is Dawes’ limit: divide 116 by the telescope’s aperture in millimetres and you get the closest separation, in arcseconds, at which two equal stars just begin to show as two.

Two warnings stop that table being misleading. First, Dawes worked it out for pairs of equal, fairly faint stars. When one star is much brighter than the other, the glare of the primary swamps the companion, and a pair can be several times harder than its separation suggests. Izar, Rigel and Polaris below all fall into that trap. Second, the British atmosphere usually limits you before the optics do. On a night of poor seeing, stars boil and a 2-arcsecond pair will not split in any aperture. Our guide to astronomical seeing explains how to spot a steady night before you set up. For the resolution side of choosing a scope, see telescope aperture explained.

The 15 at a glance

September to November: the evening sky right now

Albireo (Beta Cygni). The showpiece and the right first target. At the head of Cygnus, which sits almost overhead after dark in September, a golden magnitude 3.1 star and a blue magnitude 5.1 companion lie 34 arcseconds apart. Any telescope splits it at 25x to 30x. Keep the power low: the colour contrast is strongest when both stars are small and sharp, and it is the one double where nearly everyone sees colour at once.

61 Cygni. Two orange dwarfs of roughly magnitudes 5.2 and 6.0, about 31 arcseconds apart and slowly widening. The view is modest; the story is not. It was the first star to have its distance measured, by Friedrich Bessel in 1838, and at about 11 light years it is one of our nearest neighbours. Find it by sweeping from Deneb towards the corner of the Cygnus cross.

Epsilon Lyrae, the Double Double. Close to Vega, high in the west on September evenings. Sharp eyes, or any binoculars, show two stars 208 arcseconds apart. A telescope then splits each of those into a further pair, and published separations for the two close pairs range from about 2.2 to 2.8 arcseconds depending on the source and date. Magnitudes run from 5.1 to 6.0. An 80mm refractor at around 120x can show all four on a steady night; BBC Sky at Night Magazine says a well-collimated 6-inch scope reveals them reliably. If your 130mm reflector shows two fuzzy ovals instead of four points, check the collimation before blaming the air: our collimation guide takes ten minutes.

Gamma Delphini. Delphinus is a small kite-shaped constellation east of Altair, and Gamma sits at the tip of the dolphin’s nose. Magnitudes 4.3 and 5.1, 9.2 arcseconds apart, so any scope splits it at 50x. Observers disagree about the colours, calling them yellow and emerald or orange and lime, which is part of the fun.

Almach (Gamma Andromedae). Often ranked second only to Albireo for colour. An orange primary around magnitude 2.3 and a blue companion around magnitude 5, about 10 arcseconds apart. It rises through autumn evenings and is well placed from October. Use 60x to 80x.

Gamma Arietis (Mesarthim). Two near-identical white stars of magnitudes 4.5 and 4.6, 7.6 arcseconds apart, looking like the headlights of a distant car. Split at 50x to 75x. Aries is low in the east in September and far better placed by November.

Polaris. The Pole Star has a magnitude 9.1 companion 18 arcseconds away. On paper that is easy; in practice the 2.0 magnitude primary is roughly 700 times brighter, and the companion hides in its glare. A 70mm scope at 100x will show it on a good night. Because Polaris never moves from the same spot, it is the one double you can try on any clear night of the year, and a good test of how steady the air is.

December to February: winter pairs

Castor (Alpha Geminorum). Castor carries the Alpha label although Pollux, the other twin’s head, is brighter. In a telescope it is two white stars of magnitudes 1.9 and 3.0, now around 6 arcseconds apart. The gap is slowly widening on an orbit of roughly 450 years; in 1970 the pair was under 2 arcseconds, so older books describing it as difficult are out of date. Split at 100x.

Rigel (Beta Orionis). A hard pair despite the 9.5-arcsecond gap, because the companion is magnitude 6.8 against a primary of about 0.1. The brightness ratio is the whole challenge. As the Webb Deep-Sky Society’s Rigel notes put it, it needs a good night to see Rigel B clearly in the glare of the primary. Most observers find 100mm or more and very steady air make the difference, and in anything smaller it is a genuine test. Try it only when Orion is high in the south, around January evenings, since low altitude from Britain means poorer seeing.

The Trapezium (Theta-1 Orionis). Not a double but a small multiple star at the heart of the Orion Nebula. A small scope at moderate power shows four stars in a lopsided box, embedded in the brightest part of the nebula. It is the easiest multiple star to find in the sky, and the view of stars sitting inside glowing gas is one of the best in the hobby.

March to June: spring doubles

Mizar (Zeta Ursae Majoris). In the handle of the Plough. The naked eye shows fainter Alcor 11.8 arcminutes away; a telescope then splits Mizar itself into white stars of magnitudes 2.2 and 3.9, 14 arcseconds apart. An 80mm refractor does it at 30x. From Britain the Plough never sets, so you can find Mizar every clear night of the year, though it is highest in spring.

Cor Caroli (Alpha Canum Venaticorum). Below the handle of the Plough. Magnitudes 2.9 and 5.5, 19 arcseconds apart: another 30x split in any telescope, with a faint tint difference between the two.

Algieba (Gamma Leonis). In the Sickle of Leo. Two golden stars, magnitudes 2.2 and 3.5, with published separations between about 4 and 5 arcseconds. An 80mm refractor splits it at 100x or more. It is the best close pair of the spring for a first-time observer.

Izar (Epsilon Bootis). The spring test piece. The separation is just under 3 arcseconds, but the magnitudes are 2.4 and 4.9, so the fainter blue-green companion sits on the edge of the bright orange primary’s glare. A 100mm or larger scope at 150x on a steady night is what most people need, and even then it may show as a notch on the side of the star rather than a clean gap. The 19th-century name Pulcherrima, “the most beautiful”, is earned if you get it.

Porrima (Gamma Virginis). Two near-equal white stars of magnitudes 3.6 and 3.7 on a 169-year orbit. The pair closed to an unsplittable minimum in 2005 and is now opening again, currently about 3 arcseconds apart. Each year it gets a little easier, and it is worth noting the date you first split it.

How to get the best out of a double

• Start low, then go up. Centre the star at your lowest power, then step up through your eyepieces. Most wide pairs look best at 30x to 60x, pairs of 5 to 10 arcseconds at 75x to 100x, and pairs under 3 arcseconds need 150x or more. The eyepieces guide explains how to work out the magnification each of yours gives.
• Let the telescope cool. A tube brought out of a warm house produces heat currents that blur close pairs. Give a reflector 30 to 45 minutes outside.
• Pick the highest target. The higher a star, the less air you look through. From the UK, a star is best when it crosses the meridian, due south.
• Look for the diffraction pattern. At high power a well-focused star is a small disc with a faint ring around it. A companion sitting on that first ring is the classic appearance of a pair right at your telescope’s limit.
• Skip what is too low. Some famous doubles are poor from Britain. Antares, whose green companion lies about 2.5 arcseconds away, never climbs more than about 12 degrees above the southern horizon from southern England, and less further north, where the air is too turbulent to split it.

If you need help locating any of these, how to find objects with a telescope covers star-hopping from bright stars. For the rest of the season’s highlights, our night sky month by month page and the wider list of the best things to see with a telescope pick up where this list leaves off.

Frequently asked questions

What is the easiest double star to see with a small telescope? Albireo in Cygnus, for most of the year, and Mizar in the Plough in spring. Albireo’s stars are 34 arcseconds apart and Mizar’s 14, so any telescope splits them at 25x to 30x. Albireo also shows strong gold and blue colour.

Can I see double stars from a light-polluted town? Yes. Double stars are bright points of light, so they are barely affected by light pollution or moonlight. They are one of the best targets for a garden in a town or city, and a good use of nights when the Moon rules out galaxies.

What magnification do I need to split a double star? Wide pairs over 15 arcseconds split at 30x to 60x. Pairs of 5 to 10 arcseconds need around 75x to 100x. Close pairs under 3 arcseconds, such as Izar or the Double Double, usually need 120x to 150x or more, plus steady air.

Why can’t my telescope split a double star it should be able to? The three usual causes are poor seeing, a telescope that has not cooled down, and bad collimation in a reflector. A big brightness difference between the two stars also makes a pair much harder than its separation suggests, which is why Izar and Rigel defeat scopes that easily manage equal pairs.

What is Dawes’ limit? An empirical rule for the closest equal pair a telescope can just resolve: 116 divided by the aperture in millimetres gives the answer in arcseconds. A 130mm telescope’s Dawes’ limit is about 0.9 arcseconds. Real-world results are usually worse, because of the atmosphere.

Are double stars really two stars orbiting each other? Most bright doubles are true binaries bound by gravity, such as Castor, 61 Cygni and Porrima, which have measurable orbits. A few are optical doubles, two unrelated stars that only line up from Earth. The eyepiece view looks the same either way.