It's a coincidence of the night sky that each season has a characteristic geometric pattern.
Summer sports a triangle, fall a square, winter a hexagon and spring a sparkling diamond. The cluster appears in the eastern sky in March evenings to herald the arrival of spring in the Northern Hemisphere.
First, let's take a closer look at each of the Spring Diamond's four stellar sentries.
Arcturus is the unquestioned leader of both the Spring Diamond and the large, kite-shaped constellation Boötes (the Herdsman). This giant sun is more than a hundred times as luminous as our home star and blazes with a golden-yellow-to-orange cast. At a magnitude of -0.05, it ranks as the fourth brightest of all the stars in the sky, thanks to its proximity: just 36.7 light-years away.
To find Arcturus and beyond, here is a simple mnemonic phrase — "Follow the arc to Arcturus." That means just this: extend the curve of the Big Dipper's handle southward to find Arcturus, the eastern point of the Diamond. The same arc, continued past Arcturus for about the same distance, will bring a skywatcher to the second brightest gem in the Diamond.
That star is Spica, the jewel of the Y-shaped zodiacal constellation Virgo (the Virgin) and the southern tip of the Spring Diamond. (The mnemonic continues "And speed to Spica.") The warm, fiery hue of Arcturus contrasts strikingly with Spica's icy electric-blue glow. Ranking as the 16th brightest among the heavenly host, at visual magnitude +0.97, it is intrinsically more than 20 thousand times as luminous as our sun. It is an eclipsing binary system with an orbital period of four days; the stellar pair stay close enough that they cannot be resolved as two stars through a telescope. Refined determinations of its distance place this dynamic duo about 250 light-years away from us.
Denebola, the western corner of the Spring Diamond, lies at the eastern end of the distinctive zodiacal figure of Leo (the Lion), marking the tip of the lion's tail. A bluish-white sun, it glows softly at magnitude +2.14 from its distance of 36 light-years from Earth. It has two faint companions — one 6th magnitude and the other 8th magnitude, both just to the south — that are visible in a good pair of binoculars or small telescope. However, neither of the two are associated with Denebola; they are merely bystanders in the same field of view as seen from Earth.
Finally, the northern tip, Cor Caroli, lies in the constellation Canes Venatici (the Hunting Dogs) and, at magnitude +2.81, is the faintest of the diamond markers. It is a strange, silvery magnetic variable sun, about 115 light-years away from Earth. This object is also an attractive double star for high-powered binoculars and small telescopes.
Remember that the lower the figure of magnitude, the brighter the star. The Spring Diamond offers a helpful visual orientation to star magnitudes, since this four-star pattern spans that part of the magnitude scale running roughly from 0 to 3.
Not so empty after all
The Diamond itself appears about as wide as the Big Dipper and about twice as long. And at first glance, the Spring Diamond appears to encompass a rather dull region of the sky that is dark and empty to the eye.
However, closer inspection, especially on a dark (moonless), clear night, reveals a large, hazy patch of light in the upper-right part of the Diamond. This faint fuzz of stars belongs to the constellation Coma Berenices (Berenice's Hair). Averted vision shows this to be a loose swarm of individual stars scattered over an area 5 degrees across. Your clenched fist held at arm's length measures 10 degrees in width, so this open star cluster measures roughly half a fist wide.
Unfortunately, until Thursday (March 12), the moon will sweep right through the Spring Diamond and light up this region of the sky. In the nights that follow, however, the moon will move away to the east, allowing us to see the Diamond and Coma Berenices.
A domain of island universes
Also located within the Diamond is one of the most remarkable areas of the heavens. Sometimes called the Coma-Virgo Cloud of Galaxies and often referred to in older astronomy texts as "The Field of the Nebulae," it's now known colloquially as "The Realm of the Galaxies."
Here, an estimated 1,200 to 2,000 galaxies throng together in a huge supercluster. If you own a moderately large reflecting telescope of 6-inch aperture or greater, a sweep of this region will reveal literally dozens of these galaxies, which appear as myriad faint and fuzzy patches of light. Every one of these dim blobs is in itself a star city, likely containing tens of billions of stars.
The Coma-Virgo Supercluster, also known as Abell 1656, dominates our intergalactic neighborhood. It represents the physical center of our Local Supercluster and influences all the surrounding galaxies and galaxy groups by the gravitational attraction of its enormous mass.
And this cluster of galaxies is just the nearest of the large aggregations of galaxies relative to us, somewhere between 40 and 70 million light-years away. Plenty of other equally stunning gems dot the universe.
- Night sky, March 2020: What you can see this month [maps]
- The 10 must-see skywatching events to look for in 2020
- The brightest planets in March's night sky: How to see them (and when)
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.
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Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.