Diamond of Virgo

It has been said that diamonds are a girl’s best friend. Now any girl with her feet firmly planted on the ground knows for a fact that a girl’s best friends are Rocky Mountain Chocolate Factory truffles and control-top pantyhose.

But if I were to allow myself a brief indulgence in the alternate reality espoused by DeBeers and Tiffany’s, I would set my sights a little higher. Give me the Diamond of Virgo.

The Diamond of Virgo is a wonderful four-star, spring-into-summer asterism that, when spotted, reveals the location of four constellations. Please step into my showroom.

1) About an hour after sunset, face the western horizon, that is, toward the direction in which the sun set earlier.

Star maps created with Your Sky

2) Look high in the sky, a little towards the north, for the large, distinctive seven-star asterism (star pattern) known as the Big Dipper. In its current orientation, its handle will be sticking up above its bowl. Extend the curve of the handle in an imaginary line away from the bowl, until you reach a very bright star. This is Arcturus, the first point of our stellar diamond. Arcturus (ark-TOUR-uss) is an orange giant star, the brightest star in the constellation Bootes (boh-OH-teez) the Herdsman, and the fourth brightest star in the sky.

The Diamond of Virgo asterism

3) Now continue that same imaginary curving line toward the southwestern horizon until you reach the next bright star: Spica, brightest star in the constellation Virgo the Maiden and the second point of our diamond. Spica (SPY-kuh) is a blue-white dwarf star, and not as bright as Arcturus. Take a moment to compare Spica to Arcturus in order to see the subtle color difference. Can you discern the orange or golden color of Arcturus now, as compared to the blue to blue-white light streaming from Spica?

4) Below Arcturus and Spica, that is, closer to the western horizon, look for the shape of the constellation Leo the Lion. Our fearless feline dives after the setting sun. His head is marked by the Sickle asterism, which looks like a backwards question mark. His tail, higher in the sky than the head, is marked by a right-triangle asterism. The star in the right triangle that is highest above the western horizon is Denebola, the third star in our celestial gemstone. Denebola (denn-EBB-oh-luh) is a white dwarf star, and it’s dimmer than either Arcturus or Spica.

5) Mining our sky diamond has been fairly easy up to now. The fourth and final star is the most challenging, because it’s dimmer than the other three.

Our quarry is Cor Caroli (core CARE-oh-lye), brightest star in the constellation Canes Venatici (KAY-neez vee-NATT-uh-sigh). It lies to the right (or north) of the other three diamond points, and it lies between Spica and the handle of the Big Dipper, closer to the handle. To aid in finding it, draw an imaginary line between Denebola and the star at the end of the Big Dipper’s handle. That line will cross the white dwarf star Cor Caroli, which is a bit dimmer than Denebola.

If you can’t spot it, you may need to try again at a darker location, as an absence of light pollution is a big plus when hunting dimmer stars.

Cor Caroli is Latin for Heart of Charles and commemorates Charles II, King of England from 1630 to 1685. It was astronomer Edmond Halley (of Halley’s Comet fame) who honored the king—40 years after his death—with this designation, although it may have been Charles’s court physician who first suggested the heavenly namesake.

Speed on to Spica

Last year we learned how to arc to Arcturus using the handle of the Big Dipper. This year, let’s continue along that path and complete a well-known memory prompt for beginning stargazers that bids us “arc to Arcturus and speed on to Spica.”

1) About an hour after sunset, face east. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your back to the west, and you’ll be facing approximately east.

2) Tilt your head back and look at the zenith, the point in the sky that’s directly overhead. A little ways to the left (toward the northern horizon) and just east of the meridian, you’ll find the distinctive seven-star asterism (recognizable star pattern) known as the Big Dipper. The Big Dipper is in the constellation Ursa Major the Big Bear.

Now extend the curve of the Big Dipper’s handle— heading away from the Dipper’s bowl— to the next star along that arc that‘s brighter than the end star of the handle. You’ve arced to Arcturus! Arcturus is the brightest star in the constellation Bootes the Herdsman.

Star maps created with Your Sky

3) To speed on to Spica, simply continue the arc past Arcturus to the next bright star. This is the blue-white dwarf star Spica (SPY-kuh), which is a bit dimmer than Arcturus. Spica lies 260 light years away. A light year is the distance light travels in one Earth year, nearly six trillion miles.

Spica is the brightest star in the constellation Virgo the Maiden. The name Spica comes from the Greek word for ear of grain. The figure of Virgo is associated with the harvest, and she is often depicted carrying an ear of grain.

Virgo in Johann Bode’s 1782 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

The ancient star figure of Virgo is not easily apparent to the modern stargazer. Virgo is one of those constellations characterized by a loose, sprawling collection of nondescript stars with no showy asterisms. If it weren’t for luminous Spica punctuating the arc from Arcturus, most of us would be hard-pressed to locate the dowdy maiden at all.

But now you can arc to Arcturus and speed on to Spica. This is an easy asterism to teach others, and best of all, it covers ports of call in three constellations!

Astronomy Essential: A telescope gathers light; it does not magnify.

The function of any telescope— whether it uses mirrors, lenses, or both— is to gather light and focus it into a clear, bright image.

The eyepiece— the accessory that is inserted into the focuser and through which the observer peers— is the optical element that magnifies the image produced by the telescope. The magnification of the image, or power as it is also called, changes each time a different sized eyepiece is inserted into the telescope.