Plus the planets for June.
Just underneath the tail of the Big Bear (Ursa Major) is a medium-sized field of faint stars that form the constellation of Canes Venatici, the Hunting Dogs. This constellation started out in ancient times as the Club of Boötes, the neighboring constellation representing a herdsman. In translating Ptolemy's Almagest to Arabic, it was mistranslated as a "spear shaft." Much later, it was again translated into Latin, and once again it was mistranslated into "dogs." Polish astronomer Johannes Hevelius solidified the mistranslation when he named the constellation Canes Venatici, the Hunting Dogs, in his star atlas Firmamentum Sobiescianum, published posthumously in 1687.
One of the reddest stars in the sky is located in this constellation. It is a variable star, so it has a letter designation, Y Canum Venaticorum (for short, Y CVn, typically pronounced "Y Can Ven"). It varies in brightness from magnitude +4.8 down to +6.3 over a 160-day cycle. Nineteenth century Italian astronomer Angelo Secchi, impressed with its beauty, gave the star its common name, La Superba.
In addition to varying in brightness, this star is a carbon star, the brightest such star in our sky. Carbon stars are older giant stars that have more carbon than oxygen in their upper atmosphere. The carbon combines with all the available oxygen to form carbon monoxide. The remaining carbon atoms form other carbon compounds that float around in the star's atmosphere. These tend to absorb the bluer colors in the starlight, leaving the redder colors to escape into space. We see these reddish colors when we look at these stars, giving them an intense red appearance.
These older stars are huge: If Y CnV were in our solar system, its outer surface would be out around the orbit of Mars. Y CnV is not very bright in the visible part of the spectrum, but if you include the infrared part of the spectrum, Y CnV would be over 4,000 times brighter than our Sun.
In this stage of their life, these stars are still fusing hydrogen into helium, but also creating heavier elements like oxygen and carbon. The heavier elements get dredged up from the core into the outer atmosphere. Because these stars are so huge, the outer atmosphere is cooler, only some 4,500 degrees Fahrenheit (compared to our Sun at almost 10,000 degrees Fahrenheit). This coolness allows the formation of molecules like carbon monoxide and other carbon compounds that give the star its reddish color.
The huge size of the star also lets its outer layers easily get lost into space. Y CnV is surrounded by a cloud of gas two and a half light-years across, indicating that in the past it must have been losing mass some 50 times faster than it is now. If only people could lose mass as they get older like carbon stars do….
The Planets for June 2012
Mercury makes an appearance in our evening sky in June. Appearing out of the twilight glow during the first week of the month, Mercury continues to move away from the Sun until June 30 when it is 26 degrees from the Sun. The Messenger of the Gods starts the month in Taurus and moves rapidly into Gemini and finally ends the month in Cancer. On June 30, Mercury sets at almost 10 p.m. and shines at magnitude +0.6. Its disc is 40% illuminated, 8.1 seconds-of-arc across and increasing as it swings out from around the Sun.
Mars starts the month in southeastern Leo, moving eastward into Virgo on June 20 to end the month there. You can find Mars 50 degrees up in the southwest as it gets dark; it sets around 1 a.m. At midmonth, the God of War's disc is 7.2 seconds-of-arc across and it is magnitude +0.7.
By the end of June, Saturn shares the constellation Virgo with Mars. At midmonth, the Rings are tilted down 12.5 degrees with the northern face showing. They are 40.7 seconds-of-arc across while the disc is 17.9 seconds-of-arc across. Saturn is 50 degrees up in the south as it gets dark and sets around 3 a.m.
Jupiter and Venus are too close to the Sun to be visible, but Venus is going to be visible for a brief time on the evening of June 5, when it crosses the Sun's surface as seen from Earth. This transit of Venus is a rare event. Transits of Venus occur in pairs, eight years apart. The last one was June 8, 2004, the first of this pair. The current one will be followed by a period of 105.5 years before the next one in December 2117, followed by another in December 2125. There will be another gap of 121.5 years before the next one, and the pattern will repeat again.
Watch the Skies
For this year, Venus will start to enter the Sun's disc at 4:06 p.m. It will take 17 minutes for Venus' entire disc to slide onto the Sun. The middle of the transit occurs at 7:27 p.m. While the transit goes on for another three hours, for us the Sun will set around 8:11 p.m., with Venus still on its disc.
If you plan to view the transit, you will need a solar filter. This filter will block most of the Sun's light, allowing you to safely view the transit. Do not look at the Sun directly or you could damage your eyes.
In another noteworthy event, the Moon travels through the northern portion of the Earth's shadow on the morning of June 4. The penumbral phase starts at 2:48 a.m., followed by the umbral phase beginning at 4 a.m. The greatest eclipse will be at 5:04 a.m., when 37% of the Moon will be in the umbral shadow. The Moon leaves the umbral shadow at 6:06 a.m., just three minutes before it sets.
The June Solstice is June 20 at 5:09 p.m., marking the end of spring and the beginning of summer in the northern hemisphere. In the southern hemisphere, autumn ends and winter begins. For us, this is the shortest night of the year, with the longest day. The Sun will be in far eastern Taurus, just about to move into Gemini.
With all this activity, this will be an exciting month for astronomers, beginner, amateur and professional, to "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.