I first bumped into the “Diamond of Virgo” while perusing the 1912 book A Beginner’s Star-Book by Kelvin McKready. It’s a fun little treatise that brought the sky down to Earth for an earlier generation.
While the diamond’s other three facets are easy to spot, 3rd-magnitude Cor Caroli can be tough to pick out with today’s light pollution. Look for it all alone under the crook of the Big Dipper’s handle. Cor Caroli is one of spring’s showpiece binary stars, although its two suns are difficult to distinguish through most binoculars. Even though they are separated by 19″, their widely disparate magnitudes of 3 and 6 make the stars tough to resolve at low powers. I’ve never had much luck splitting them through my 10×50 binoculars, but I can with the extra oomph from my 16x70s. Give it a try, and let me know how you make out at firstname.lastname@example.org.
Cor Caroli’s constellation, Canes Venatici, contains two dogs owned by nearby Boötes the Herdsman. Only two stars are bright enough for naked-eye viewing: Cor Caroli and Chara, to its northwest. Personally, I can’t see two dogs using just two stars. The best I can imagine is a hot dog!
What Canes Venatici lacks in naked-eye appeal, it makes up for by hosting some great hidden treasures. To see for yourself, start by finding Cor Caroli and Chara with your binoculars, and then look exactly halfway in between. Shift just to the northeast (upper left) of that center point, and take a careful look for a small, grayish smudge of light. See it? That’s the spiral galaxy M94. At 8th magnitude, M94 is going to be tough to spot at first, but don’t give up if you can’t find it right away. Be patient. To help zero in, look for a small diamond of faint stars to the galaxy’s north-northwest (upper right). I saw M94 recently through my 10×50 binoculars as a tiny, round disk highlighted by a faint, starlike center.
Return to Cor Caroli, and let your attention drift about half a field east, to a pretty double star formed by the 6th-magnitude suns 15 and 17 Canum Venaticorum. Striking as they are through binoculars, 15 and 17 form only a chance alliance from our earthly perspective. It turns out that 17, the easternmost of the two, lies just over 200 light-years away. You’d have to travel another 900 light-years to get to 15.
Move another binocular field to the northeast, and you should spot an obvious asterism of four stars that form a backward number 7. Although the stars — 19, 20, and 23 Canum Venaticorum, and SAO 44519 — are not physically related to one another, they create a unique pointer to help us find M63. Over 37 million light-years away, this challenging spiral galaxy floats just 1°, or the distance covered by two stacked Full Moons, north of the 7. Through my 10×50 binoculars, M63 looks like a tiny, cigar-shaped blur close to a very faint star. M63 is a little fainter than M94, so you’ll need to support your binoculars very steadily to locate it.
Head back to the backward 7, and follow its aim toward the northwest for about one field. There, amongst all of the white stars, stands a deep-red ruby known as La Superba, a name bestowed by the 19th-century Italian astronomer Father Angelo Secchi (1818–1878). Cataloged more formally as Y Canum Venaticorum, this beautiful variable star flickers between magnitudes 5 and 6.5 over a period of about 160 days. When we look its way, we are seeing a rare example of a red-giant star that is overly rich in carbon. Nearing the end of its life, La Superba is releasing carbon into space. These carbon molecules absorb the blue end of the spectrum to give the star its vivid color.