Constellation of the Month: Delphinus, the Dolphin
Delphinus, the Dolphin, is a small constellation just northeast of Aquila. As a sea creature, Delphinus is associated with Poseidon, god of the sea.
The myth is that Poseidon decided he should get married, despite his many dalliances with mortal women. As a sea god, he was concerned that his bride be able to tolerate the sea. Poseidon had 50 sea nymphs in his court, and decided he would marry one named Thetis. Before the wedding, he discovered that any son of Thetis would be greater than his father, something that Poseidon could not tolerate. Poseidon had seen Thetis' sister, Amphitrite, dancing on the island of Naxos and fell in love with her instead.
Amphitrite was horrified by the thought, and fled into the Atlas Mountains. Poseidon sent many messengers to try to convince her to marry him, but the one who finally succeeded was Delphinus. Amphitrite returned to the sea and married Poseidon. They had many children.
Delphinus is composed of relatively faint stars, around magnitude 4 or 5, that form a diamond shape just northeast of Aquila. The nose of the dolphin in the northeast end of the diamond shape is Gamma Delphinus, one of the few interesting objects in this constellation. It is a double star whose components are now 9.2 seconds of arc apart. The westernmost one, Gamma-1 Delphinus, is a spectral type F star shining at magnitude 5.14, and Gamma-2 is a spectral class K shining at magnitude 4.27. The resulting colors are a white Gamma-1 and a pale yellow-orange Gamma-2, a nicely contrasting pair even in a small telescope.
With a spectral class of K (remember, the main sequence from hottest to coolest is O-B-A-F-G-K-M), Gamma-2 might be expected to be fainter than our Sun (a G star), but the reality is that the spectral class tells you more about the surface temperature of the star than its luminosity. Gamma-2 has recently stopped burning hydrogen and is expanding, causing its surface to be cooler than expected for its mass. The extra surface area makes the star 26 times brighter than the Sun; it is 1.7 times more massive. Eventually, the helium "ash" from the hydrogen burning will ignite and Gamma-2 will brighten again. Gamma-1 is a little younger than Gamma-2m since it is less massive (1.5 Suns) than Gamma-2 and is still burning hydrogen.
The pair have been observed by astronomers over the past 200 years. Astronomers have computed a highly elliptical orbit for the pair that brings them as close as 3.7 billion miles, and sends them as far apart as 55 billion miles. They repeat this cycle every 3,200 years.
The Planets for November
As November begins, little Mercury is still low in the west-southwest shortly after sunset. Moving from Scorpius into Ophiuchus, moving eastward among the stars, Mercury will pass Antares on Nov. 9 and then slow down as it begins to drop back towards the horizon. By mid-month it will have disappeared in the bright evening twilight. Mercury starts November as a crescent at magnitude -0.2, 6.4 seconds-of-arc across. By its disappearance at midmonth, it will have faded to magnitude 1.5, become more of a crescent, and grown by 2.8 seconds-of-arc.
Venus is in Sagittarius all month, drifting from the western edge to the eastern edge of that constellation. In doing so, it passes the second-magnitude star Nunki on Nov. 18. Venus will not change position much from day to day, until the end of the month when it starts to move northward. The crescent Venus is becoming a thinner crescent as it brightens. At mid-month it will be magnitude -4.5 and 29.1 seconds-of-arc across.
The crown jewel of the November sky is the god of war, Mars. It rises around sunset and is in the sky all night. Mars passed its closest point to us last month. Mars' orbit is taking it away from the Sun (and Earth) even as it reaches the point directly opposite the Sun in our sky (hence the term "opposition"). Mars remains in Aries all month. Look for the bright orange star shining at magnitude -1.12. Through a telescope, Mars is 14.4 seconds-of-arc across and looks full.
Cancer is home to Saturn this month. It comes up around 11 p.m. and stays in view the rest of the night. Saturn is moving very slowly eastward among the stars. Telescopically, the rings are 42.5 seconds-of-arc across, with the southern face showing, while Saturn's ball is 18.7 seconds-of-arc across. The rings are tilted away from us at an angle of 19.6 degrees.
The Leonid meteor shower peaks on the morning of Nov. 17. While this shower is usually a good one, the almost full moon will spoil the show for this year. Like most meteor showers, the Leonids are named for the constellation the meteors appear to radiate from—in this case, Leo, the Lion. If you do try to observe the Leonids, take a look low in the east-southeast just before sunrise and you will see the first magnitude star Spica above the bright Jupiter, which is coming back into the night sky after being passed by the Sun. Nights get cold in November, so dress warmly for the Leonids and "keep watching the sky"!