With the bright full moon lighting up the sky this week, it's a good time to turn your telescope on objects unaffected by this natural "light pollution" — twin-like double stars.
A century ago, double stars were a favorite target for amateur astronomers with small telescopes. Then, for some reason, they fell out of fashion. This is hard to fathom as they are perfect objects for small telescopes to observe.
Many double stars are very pretty to look at because of contrasts in color and brightness, while some are beautiful because both components are almost identical.
Some are true binary stars — locked in orbit around each other — while others are mere "optical doubles," stars not gravitationally linked that coincidentally line up (as seen from Earth) so that they look like twins when in fact they are not.
Springtime offers some fine examples of all of these, and they are bright enough to be easily visible. Some are easy in any telescope, but some are a challenge.
In this handy chart, the springtime double stars are shown in yellow, while other bright stars are shown in green.
Spring's double stars
Let's start with the most famous double star of all: Mizar, the middle star in the handle of the Big Dipper, also known as the constellation Ursa Major.
This was well-known as a twin star long before the invention of the telescope. Its companion star Alcor was used as a vision test in ancient times. It's not surprising that this was one of the first stars that the newly invented telescope was pointed at, and it was discovered that Mizar itself is a nice double: two stars of magnitude 2.3 and 4.0 separated by 14 arcseconds.
Distances in the sky are measured by the angles between them. A full circle contains 360 degrees, each of which is divided into 60 arcminutes, each of which in turn is divided into 60 arcseconds.
On this scale the sun and moon measure about 30 arcminutes in diameter, and the planet Jupiter is 45 arcseconds wide.
The two stars in Mizar are separated by 14 arcseconds, about a third of the diameter of the planet Jupiter. Mars, at its largest a few months ago, would just fit between these two stars, as seen from Earth.
Mizar is actually an optical quadruple star. The two components of Mizar itself are separated by 14 arcseconds. These in turn are separated by 709 arcseconds (that's about 11.8 arcminutes) from Alcor. In between Mizar and Alcor, and a bit off to the side, is a fourth star with no name.
The twin star tour
Tucked just under the handle of Big Dipper is an obscure little constellation called Canes Venatici, the Hunting Dogs. This constellation is better known for its many galaxies than for its sparse stars.
The one star that stands out is Cor Caroli, which is Latin for "Charles' Heart." This refers to the two King Charles' of England: the one before and the other after the English Civil War (1641–1651).
Cor Caroli, like the Mizar target, is a wide double star easily seen with any telescope. Its two components, magnitudes 2.9 and 5.5, are very different in brightness and separated by 19 arcseconds. Both Mizar and Cor Caroli are easily split with an 80mm refractor at 30 power.
The other three double stars of note are more of a challenge.
Algieba, Gamma Leonis, is at the far left side of the "backwards question mark" that makes up on half of the constellation Leo. Its two stars, magnitudes 2.2 and 3.5, are only 4 arcseconds apart, half the current diameter of Mars. This star can also be split by an 80mm refractor, but you will need quite a bit of magnification, at least 100 power.
Izar, also known as Epsilon Boötes, is just to the left of the bright springtime star Arcturus. This star is doubly difficult because its stars are quite close, just under 3 arcseconds, and of very different brightness, magnitudes 2.4 and 4.9.
The final star, Porrima or Gamma Virginis, is just above the bright star Spica and below the planet Saturn. Five years ago, the two components of this star were so close together, 0.35 arcseconds, that there wasn't a telescope on Earth that could see them as double.
This year, they are separated by 1.5 arcseconds, and are still a challenge in even quite a large telescope. The two stars are equally bright, both magnitude 3.5, and require at least a 250mm telescope and 200 power, as well as steady atmospheric conditions.
But the effort is worthwhile, when you spot these two tiny pinpoints of light right next to each other.
When you're out with your telescope in the bright moonlight this week, be sure to check out the three bright planets currently in the sky: Venus in the west, Saturn in the southeast, and Mars high overhead.