Hunger at Home
Living on the Edge
Every Picture Tells a Story
The Scorpion King
Giving a Lift
Quest for Fire
Columns & Departments
Constellation of the Month: Corona Borealis,
the Northern Crown.
Our Constellation of the Month is Corona Borealis, the Northern Crown. It was originally just called Corona, but with the addition of the southern constellation Corona Australius to the roster of constellations, "Borealis" was added to differentiate the two.
The Crown originally belonged to the beautiful Ariadne, daughter of King Minos of Crete. The King had a problem. His wife had borne a monster that was half man and half bull, the Minotaur. King Minos could not kill his own son, so he had the famous architect Daedalus design a labyrinth so complex that the Minotaur could never escape.
The Minotaur still needed to be fed. So the King forced Athens to send him seven of its handsomest young men and seven of its most beautiful women each year. The Athenians went into the maze, where the Minotaur would eventually find and consume them. This worked well (at least for the Cretans) for two years.
By the third year, the hero Theseus decided he would be one of those to go to Crete and stop this slaughter of Athenians. His father, Aegeus, agreed and gave him a set of black sails for the trip out and white sails for his victorious return. In Crete, Minos' daughter Ariadne fell in love with Theseus and offered to help him if he would take her away. Ariadne gave him a small sword and a ball of string, and Theseus led the other Athenians into the labyrinth, unwinding the string as they went.
When the hungry Minotaur fell upon Theseus, the hero pulled out the sword and slew the monster. Then the Athenians followed the string out of the maze. With Ariadne, they sailed for Athens. When they stopped at the island of Dia to take on fresh water and rest, however, Theseus received a message from a goddess as he slept that Ariadne was promised to a god. Not daring to interfere with the gods, Theseus sailed away and left the sleeping Ariadne behind.
When he sighted Athens, Theseus forgot to lower the black sails and raise the white ones. Aegeus, watching from the cliffs, saw the black sails and, his son had been killed, threw himself off the cliffs into what we now call the Aegean Sea.
Back on Dia, Ariadne awoke to find Theseus had abandoned her. The god Bacchus came upon her and, smitten by her beauty, embraced her and stopped her tears. Taking the crown from her head, he flung it into the sky. Bacchus took Ariadne to Olympus and married her; her cast-off crown became the constellation Corona Borealis.
Astronomically, Corona Borealis has little to offer. Its brightest star, Alphecca, is only magnitude 2.2 and is slightly variable. But the most famous variable star in Corona Borealis, discovered in 1796, is R Coronae Borealis ("R" or "R CrB" for short). R CrB is a supergiant carbon star, an unusual type of variable star. Most variables are usually faint and then brighten up, whereas RCrB is normally bright, then suddenly fades by more than six magnitudes.
Here's why: R Coronae Borealis' atmosphere is rich in carbon, the same stuff that makes up both graphite (pencil lead) and diamonds. R CrB occasionally blows off part of its outer atmosphere, sometimes in our direction. The carbon in this puff of gas cools to form a soot that blocks the light from the star coming toward us, reducing the star's brightness over 250 times for several months to a year. The process by which it throws off the gas cloud is unknown and is the subject of much research.
All the variable stars that fade by ejecting carbon clouds are called R CrB stars, after this prototype. The whole sky has only about 20 of these rare stars.
The Planets for July 2005
Saturn is too close to the Sun to view this month, but Mercury is making a brief appearance in the evening sky. It will already be visible as the month opens, a little south of Venus. July 7 will see Mercury make its final close approach to Venus for this appearance, only 1.6 degrees apart. The next night, the Moon will join the pair, forming a line from the Moon to Mercury, with Venus in the middle. By July 9, the Moon will have moved on eastward, and Mercury will turn around and head back toward the Sun, disappearing into its glow within two weeks. Venus, meanwhile, will continue to climb higher into the evening sky. At mid-month Venus will be magnitude -3.9 and 11.6 seconds of arc across, appearing as a slightly less than full moon in a telescope. It has been on the far side of the Sun, but is racing along in its orbit and is starting to swing around the Sun. It will continue to chase us for the rest of the year.
Jupiter is starting to sink into the west, only some 34 degrees up in the southwest. The King of Planets remains in Virgo all month. At magnitude -2.0, Jupiter is only 35.3 seconds of arc across. On July 13, the Moon will be 0.8 degrees from Jupiter from the desert southwest, while South Americans will see the Moon go in front of Jupiter. The Moon will go in front of the star Antares for us on July 17, disappearing around 9:52 p.m. and reappearing at 10:27 p.m. This event can be seen with the naked eye, but binoculars will make it easier to view.
Mars rises about 12:45 a.m. in July, staying in Pisces all month. The Red Planet is becoming better placed for observing. Brightening at magnitude -0.2, Mars is growing larger each day, reaching 10.2 seconds-of-arc across at mid-month. With Mars reaching opposition in November, the next fleet of Mars-exploring spacecraft will be launched in the next few months, starting with NASA's Mars Reconnaissance Orbiter in August. We are learning many new things about the planets, as will you if you "keep watching the sky"!