The small northern constellation of Canes Venatici represents the hunting dogs – Chara and Asterion – of Bootes. While it basically consists of only two bright stars, it is still fairly easy to locate and is bordered by Ursa Major, Boötes and Coma Berenices. Canes Venatici dates back to the 17th century when it was introduced by celestial cartographer, Johannes Hevelius. It is also one of three constellations that represent dogs, along with Canis Major and Canis Minor. The constellation’s brightest star is Cor Caroli, named by Edmund Halley in memory of the king Charles I, King of England, or his son, Charles II. It also has 21 Bayer/Flamsteed designated stars.
While there is no real mythology associated with Cane Venetici, there is some nice history. The name of the constellation of Bootes means “herdsman”. Some of its component stars were traditionally described as representing his cudgel. When the Almagest was translated from Greek to Arabic, the translator did not know the Greek word, but rendered it as the nearest-looking Arabic word, meaning “having a hook”, probably thinking of a shepherd’s crook. When the Arabic text was translated into a Western European language, the translator mistook the Arabic word as “dogs” and again in Latin as “spearshaft having dogs”, which expression floated about the astronomical literature until Hevelius decided to find those dogs in the sky. The northern of the two hunting dogs was named Asterion, and was sometimes regarded as an independent constellation or at least an asterism. The southern dog was named Chara -from the Greek for ‘joy’,. The northern dog, called Asterion (‘little star’), is marked only by a scattering of faint stars. Johann Bode fancied the dogs with their names engraved on their collars in his Uranographia atlas.
For observers, Canes Venetici holds many surprises. Let’s start first with binoculars and small telescopes for the Heart of Charles – Alpha CVn.
Cor Caroli is perhaps one of the most splendid of all colorful double stars and has a delightful history that can be used during astronomy outreach programs. The name Cor Caroli means Charles’ heart, and was named by Sir Charles Scarborough in honour of Charles I, who was executed in the aftermath of the English Civil War, and otherwise associated to Charles II of England, his son, who was restored to the throne after the interregnum following his father’s death. Cor Caroli is a binary star with a combined apparent magnitude of 2.81. The two stars are 19.6 arc seconds apart in the sky and are easily resolved in small telescopes and steady binoculars. The system lies approximately 110 light years from Earth. The main star, a² Canum Venaticorum, is the prototype of a class of variable stars, the so-called a² Canum Venaticorum stars. These stars have a strong stellar magnetic field, which is believed to produce starspots of enormous extent. Due to these starspots the brightness of a² Canum Venaticorum stars varies considerably during their rotation. The brightness of a² Canum Venaticorum varies between magnitude +2.84 and +2.98 with a period of 5.47 days. Its spectral type is A0. The companion, a¹ Canum Venaticorum, of spectral type F0, is considerably fainter at +5.5 magnitude. What’s more, Cor Caroli marks the northern vertex of the Diamond of Virgo asterism.
Now, keep your binoculars and small telescopes at hand and let’s move on to tremendous globular cluster, M3. Messier 3 resolved into stars by William Herschel around 1784. This cluster is one of the largest and brightest, and is made up of around 500,000 stars. It is located at a distance of about 33,900 light-years away from our solar system. M3 has an apparent magnitude of 6.2, making it visible to the naked eye under dark conditions. From a moderate-sized telescope, the cluster is fully defined.
For all telescopes, let’s move on to M51! The Whirlpool Galaxy (also known as Messier 51 or NGC 5194) is an interacting grand-design spiral galaxy located at a distance of approximately 23 million light-years away. It is one of the most famous spiral galaxies in the night sky, for both its grace and beauty. The galaxy and its companion (NGC 5195) are easily observed by amateur telescopes, and the two galaxies may even be seen with larger binoculars. The Whirlpool Galaxy is also a study source for professional astronomers, who desire to further understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions. M51 is easy to find by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southeast. Its declination is +47°, so it is circumpolar for observers located above 43°N latitude. M51 is visible through binoculars on a dark night, but with modern amateur telescopes this galaxy is truly a sight to behold.
Now, let’s go for M63! The Sunflower Galaxy (also known as Messier 63 or NGC 5055) is an unbarred spiral galaxy consisting of a central galactic disc surrounded by many short spiral arm segments. It is part of the M51 galaxy group which also includes the Whirlpool Galaxy (M51). In the mid-1800s, Lord Rosse identified spiral structure within the galaxy, making this one of the first galaxies in which “spiral nebulae” was identified. Now hop to barred spiral galaxy M94 for some comparison. It was discovered by Pierre Méchain in 1781 and catalogued by Charles Messier two days later. Although some references describe M94 as a barred spiral galaxy, the “bar” structure appears to be more oval-shaped. The galaxy is also notable in that it has two ring structures. Can you see them? M94 contains both an inner ring with a diameter of 70″ and an outer ring with a diameter of 600″. These rings appear to form at resonance locations within the disk of the galaxy. The inner ring is the site of strong star formation activity and is sometimes referred to as a starburst ring. This star formation is fueled by gas that is dynamically driven into the ring by the inner oval-shaped bar-like structure.
For a completely different galaxy, try M106. Messier 106 (NGC 4258) is a spiral galaxy about 22 to 25 million light-years away from Earth. It is also a Seyfert II galaxy, which means that due to x-rays and unusual emission lines detected, it is suspected that part of the galaxy is falling into a supermassive black hole in the center. Nearby NGC 4217 is a possible companion galaxy.
While there are many more objects in Canes Venetici, one you really must visit is Y Canum Venaticorum (RA 12 45 07 Dec +45 26 24). Named as “La Superba” for its uncommonly beautiful red color by 19th century astronomer Angelo Secchi, what you are looking at is one of the reddest stars in the sky, and one the brightest of the giant red “carbon stars”. La Superba is the brightest J-star in the sky, a very rare category of carbon stars that have contain large amounts of carbon-13 (carbon atoms with 7 neutrons instead of the usual 6). Stellar temperature is believed to be about 2800 K, making it one of the coldest true stars known. Y CVn is almost never visible to the naked eye since most of its output is outside the visible spectrum. Yet, when infrared radiation is considered, Y CVn has a luminosity 4400 times that of the Sun, and its radius is approximately 2 AU. If it were placed at the position of our sun, the star’s surface would extend beyond the orbit of Mars.
To explain its remarkable coloration, it is necessary to understand that mid-sized stars, once they have finished fusing hydrogen to helium in their core, begin to fuse helium to carbon. During this so called red giant stage, the outer layers expand and cool, causing the star’s radiation output to move towards the red end of the electromagnetic spectrum. Near the end of the star’s life cycle, fusion products are moved outwards from the core by convection, thus creating a carbon abundance in the outer atmosphere where carbon monoxide and other compounds are formed. These molecules tend to absorb radiation at shorter wavelengths, resulting in a remarkable spectrum with even less blue and violet compared to ordinary red giants, giving the star its distinguished red color. La Superba is most likely in the final stages of fusing its remaining secondary fuel (helium) into carbon and shedding its mass at the rate of about a million times that of the Sun’s solar wind. It is also surrounded by a 2.5 light year-wide shell of previously ejected material, implying that at one point it must have been losing mass even 50 times faster than it is now. La Superba thus appears almost ready to eject its outer layers to form a planetary nebula, leaving behind its core in the form of a vanishing white dwarf star. Don’t miss it….