June 6, 2020

Cor Caroli: A Fine Springtime Double Star.

How about that eclipse, huh? Such shows are some of the best the sky has to offer. But as the Moon remains at slender crescent phase this week and Venus makes its dive sunward for its last transit of the 21st century, we thought we’d take a look at two fascinating double stars for small telescopes this week and next week. Our big transit of Venus post is coming up in early June, with facts and exclusive info that you’ve come to expect from this site, trust us!

Spring evenings see a rather obscure constellation by the name of Canes Venatici or the Hunting Dogs almost directly overhead. Created by Hevelius in the 16th century to fill in some of the blank spots in the sky, Canes Venatici is sandwiched between Bootes and Ursa Major and houses a fine globular (M3), a noted carbon star (Y Canum Venaticorum, or “La Superba”), several noted galaxies including M51, and a fine show piece of a double star.

Alpha Canum Venaticorum is the brightest star in the constellation, shining at about magnitude +2.8. This is one of the very few stars that possesses a modern name designation, often referred to as Cor Caroli or Charles’ Heart. Credit for the name goes to Sir Richard Scarborough, and the ‘Charles’ referred to is variously stated as either the martyred King Charles the 1st or his son Charles the 2nd. Cor Caroli is also sometimes referred to as “Chara” one of the mythological dogs that make up the constellation’s name sake along with nearby Beta Canum Venaticorum, or Asterion. Cor Caroli is also the northern apex of what is sometimes referred to as the “Springtime Diamond of Virgo” the other stars making up the asterism being Spica, Arcturus, and Denebola.

Cor Caroli: A finder chart. (Photo by Author).

Visually, the pair is easily resolved even at low power with binocs or a telescope with a separation of about 19” arc seconds. Cor Caroli is about 110 light years distant and the pair has a 650 astronomical unit separation and an orbital period of close to 8,000 years. In contrast, Neptune orbits our own Sun at an average distance of about 30 A.U.s! In other words, we haven’t observed the system through even 5% of its orbit since the invention of the telescope!

It’s a curious convention with this particular system that the brightest star is actually designated Alpha-2. Shining at a visual magnitude of +2.9, this A0 “dwarf” is nearly 3 times the mass of our Sun and has a luminosity output over 80 times as intense as Sol. Alpha-2 is an astrophysically interesting object, heavy in exotic elements such as mercury and europium and possessing a whopper of a magnetic field over 5,000 times that of Earth’s. Alpha-2 is also a slight variable fluctuating over a range of 0.2 of a magnitude over 5.5 days, most likely due to massive starspot activity on its surface. Alpha-1 is a relatively sedate sixth magnitude F0 type star just a little more massive than our Sun.

The position of Cor Caroli is;

Declination: 38° 18’ 54”

Right Ascension: 12h 56’ 0.5”

The pair has a very similar color in visual appearance, although sometimes a faint yellowish hue is noted. The pair shares a common proper motion of about 0.24” per year and rival such classic doubles as Mizar & Alcor and Albireo for their appeal. And unlike faint nebula and galaxies that vanish under the slightest hint of light pollution, double stars hold up well even under bright urban skies.

Next week, we’ll journey northward for a look a Xi Ursae Majoris!

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