When time is tight and the weather turns cold, I turn to a class of celestial objects that are very forgiving of observing conditions, light pollution, and telescope aperture: double and multiple stars. There are thousands of these objects visible during the year. Many are run-of-the-mill sights. But many more present a lovely appearance in a small telescope, revealing color, brightness contrast, and a jewel-like appearance that appeal to the artistically inclined while also packing plenty of physics and sheer challenge to the observer.
In this instalment of Cosmic Pursuits, I share with you three double and multiple stars in and around the throne of the legendary Ethiopian queen (along with a couple of extraordinary ‘bonus objects’). So grab a small telescope and head out to see them for yourself. The map above shows you where to find these relatively bright star systems and ‘bonus objects’….
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Let’s begin with an easy one, the fine double star η (eta) Cassiopeiae. Also called Achird, it’s one of the most colorful and appealing doubles anywhere in the sky. The primary star shines at magnitude 3.5 with a yellow-gold color. The 7th-magnitude secondary is officially red-orange, but the visual contrast with the primary makes it appear reddish-purple, or perhaps garnet. Because of its coloring, some call this star the “Easter Egg” double.
The two stars of the η Cas system are just 19 light year away, right next door in cosmic terms, and they make a turn around each other every 480 years. Since their discovery by William Herschel in 1779, the stars have executed a considerable portion of their full revolution. In 1890, the stars were just 5″ (arcseconds) apart. They’re now spaced by about 13″ and getting wider, and they will continue to move apart until 2150 when they will be separated by 16″. Any telescope at 50x or more will split this pair.
Onward, now, to σ (sigma) Cassiopeiae. This pair is fainter and much tighter than eta. The components are magnitude 5.0 and 7.1 and their separation is 3″. You will need all the magnification you can muster to split this star. Steady sky is essential. If the air is moving and the stars are shimmering, it may not be possible to resolve this star at all. The colors of each component are blue to blue-white. Both stars are massive B-type dwarfs, and they lie at a considerable distance of 4,200 light years, more than 200 times farther than eta Cass.
While you’re in the area… just a degree to the north of σ Cass lies the very fine and large open star cluster NGC 7789. This cluster is old (more than one billion years), far (more than 8,000 light years), and faint (most stars are magnitude 11 or 12). Because it’s so old, many blue-white stars have turned into red giants, so the cluster is quite colorful. It was discovered by Caroline Herschel in 1783, and her nephew John Herschel called it a “most superb” cluster. You’ll need at least a 3-inch scope to resolve some of the stars in this cluster. More aperture is better.
Also while you’re in this region have a look at the star ρ (rho) Cassiopeiae. It’s not a double star, but it’s an astounding star nonetheless, so large and luminous that the word “supergiant” doesn’t fit. Astronomers classify rho Cass as a yellow hypergiant, one of only a few known in the galaxy. The star is unstable, bouncing back and forth between red and blue as it burns through its nuclear fuel at a profligate clip. Rho lies nearly 10,000 light years away. With an absolute magnitude of -9.5, some 500,000 times as bright as our Sun, it still appears visible without optical aid. The star is likely ejecting mass at a rapid rate. But it will retain more than enough to detonate as a supernova in the next several hundred thousand years.
Now to the other end of Cassiopeia to see the resplendent triple star ι (iota) Cassiopeiae. This star consists of a primary of magnitude 4.6 and two companions of magnitude 6.9 and 8.4. The two brighter components are just 2.5″ apart. That’s very tight, so the star is a challenge in a 3″ scope at 100x. If your sky is steady enough, power up to get the best view. The primary and its faintest companion are separated by about 7″, but the difference in brightness still makes for a challenge. The star is worth a look for the technical challenge of splitting it and for the beautiful color contrast of yellow (for the faintest star), pale lilac, and blue. The primary (the brightest component) is physically very similar to the star Cor Caroli in the constellation Canes Venatici. Both have strong magnetic fields, huge starspots, and high concentrations of some heavy elements in the atmosphere.
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