From the February 2016 issue

Play ball!

It's the season for diamonds.
By | Published: February 22, 2016 | Last updated on May 18, 2023
It’s April, and that means another baseball season is underway. To celebrate, let’s take a field trip to a pair of diamonds. No, we won’t be running the base paths at Wrigley Field or Yankee Stadium. Our diamonds are of the cosmic variety, and both lie in the constellation Virgo.

The first is a huge naked-eye asterism (a recognized star pattern that’s not a constellation) first suggested by astronomy popularizer (and Curious George author) Hans A. Rey. In his 1952 book The Stars: A New Way to See Them, Rey introduced readers to the “Virgin’s Diamond.” He created this four-sided figure out of the stars Arcturus (Alpha [α] Boötis) as the eastern point, Spica (Alpha Virginis) as the southern point, Denebola (Beta [β] Leonis) to the west, and Cor Caroli (Alpha Canum Venat-icorum) to the north. Like the Summer Triangle, autumn’s Great Square, and the Winter Hexagon, the Virgin’s Diamond (more commonly known as the Virgo Diamond or the Great Diamond) is a convenient guidepost to identify the constellations of spring.

We’ll revisit this stellar quartet next month, looking at their distances and those of other prominent spring stars.

Virgo Diamond
The Virgo Diamond lies at approximate right ascension 12h33m and declination –0°39′. It is visible through a 10-inch telescope if your seeing (atmospheric steadiness) is good. This view shows a field 2°40′ wide.
Both images: Software Bisque: TheSky
For now, let’s step outside for a telescopic look at Cor Caroli, a superb double star. We won’t need a big, high-power telescope. A 2.4-inch scope at just 50x will split the 19.3″ that separate its components, which shine at magnitudes 2.9 and 5.5. Sources disagree as to their colors. The first time I saw Cor Caroli, the stars seemed white and bluish. On a more recent occasion, the brighter star looked a bit off-white. What colors do you see?

The other Virgo Diamond isn’t as readily seen as Rey’s. It’s faint and surprisingly small, composed of five stars of magnitudes 10.9 to 13.7, and squeezed into an area small enough to be covered by Jupiter when it lies at opposition. To capture it, you’ll need the finder chart on this page and a medium-size telescope (6 inches or larger) coupled with an eyepiece that magnifies at least 150x.

This diamond appears to have been discovered by Noah Brosch of the Tel Aviv Obs-ervatory. Brosch spotted it on a Palomar Observatory Sky Survey plate. He reported it in the December 1991 issue of Monthly Notices of the Royal Astronomical Society, describing it as “a system of five stars, containing objects of 12–13 mag and arranged in the form of a diamond.” He surmised the system was probably an evaporating small cluster. Because Brosch was the first on the scene, some sources refer to the stellar group as Brosch’s Diamond.

Virgo Diamond
This magnified view of the Virgo Diamond shows a field of view 52′ wide. The faintest stars shown are approximately magnitude 15.
News of the discovery caught the attention of North Carolina amateur Roger Ivester, who viewed Brosch’s Diamond on the night of April 14, 1993. Through a 10-inch f/4.5 reflector at 190x, he saw a faint grouping of four stars but could not spot the fifth. Nine years later, on an evening of exceptional seeing conditions, he finally caught a fleeting glimpse of the elusive star with the 10-inch scope and a magnification of 266x.

One thing that really im-pressed Ivester about Brosch’s Diamond was its symmetry. Each side is 33″ to 37″ long, giving it the square appearance of a baseball diamond. Home plate is the magnitude 10.7 star TYC 4948–53–1. Stars of magnitudes 13.2, 13.2, and 12.3 represent first through third base, respectively. Most challenging is the magnitude 13.7 fifth star, just 7″ from third base at a spot where the third baseman would play. Ivester has so vigorously promoted Brosch’s Diamond that some amateur astronomers call it Ivester’s Diamond.

Last spring, I viewed the Diamond with a 10-inch f/5 reflector. I star-hopped from Zaniah (Eta [η] Virginis) using 60x and came upon something tiny and fuzzy, not unlike the four-star asterism that comprises M73. A boost to 208x revealed the Diamond. Home plate and third base were readily visible; first and second required averted vision. In a rich Milky Way field, the Virgo Diamond would have been lost, but in this star-poor region of Virgo, it stood out dramatically. I couldn’t see the fifth star, but I was observing under turbulent skies with a 5th-magnitude limit.

Questions or comments? Email me at In my next column, we’ll time travel to the stars of spring. Clear skies!