Constellation of the Month: Gemini, the Twins
High in our sky on these windy April nights, two bright stars of similar brightness stand not far from each other. Looking at the fainter stars moving southwest from them, you can make out what appear to be two stick figures forming the constellation Gemini, the Twins. The two bright stars at the head of the stick figures represent brothers Castor and Pollux.
In Greek mythology, they are the offspring of Leda of Sparta, who was married to King Tyndarus. One night, Zeus came to her in the guise of a swan and seduced her. The same night, her husband had also enjoyed her company; the result was two sons, one immortal and one mortal. Pollux, the immortal one, was very strong, while his mortal brother, Castor, could exert tremendous control over horses. They were both brought up by Chiron, now known as the constellation Sagittarius.
As adults, they joined Jason and the Argonauts in the search for the Golden Fleece, and joined in the Trojan War to return their sister Helen to her husband, Menelaus. When the twins decided to settle down, they carried off the wives of two of their cousins, Idas and Lynceus, who did not seem to be upset by this. After some time, the twins and their cousins decided to raid some cattle, and trouble developed when they tried to split the herd. Idas divided a cow into quarters and proposed that the two who could eat the fastest would keep the cattle. The twins, taken aback by this, lost to the cousins who left with the cattle. Castor and Pollux, infuriated by losing their booty, went after the cousins to reclaim their share.
A tremendous battle ensued and Idas killed the mortal Castor with a spear, while Pollux killed Lynceus with a single blow. Idas picked up a tombstone and was about to bash it over Pollux's head when Zeus struck him dead with a lightning bolt. Pollux, shattered by the death of his brother, begged Zeus to kill him, too. Instead, Zeus took both brothers and placed them together in the sky for all eternity.
The main telescopic object in Gemini is M35, an open cluster like the Pleiades and Hyades. M35 is a bright cluster that from a dark location would be visible as a faint patch near the feet of Castor. Telescopically, it is about the size of the full Moon, rich in stars from magnitudes 9 to 16. Some 15,000 trillion miles away, this cluster is 100 million years old and 134 trillion miles across.
Not far away from M35 is IC 443, sometimes called the Jellyfish Nebula. (IC stands for Index Catalog of nebulae found in the years 1888 to 1894 and the second Index Catalog of nebulae found in the years 1895 to 1907.) This nebula is the remnant of a supernova that exploded 35,000 years ago. Astronomers think the explosion was off-center, giving the supernova a kick. Thousands of years later, the supernova-turned-neutron-star slammed into a neighboring gas cloud. As the neutron star plowed through the gas, it generated X-rays that have been detected by X-ray astronomy satellites plus radio waves detected by the Very Large Array (VLA) radio telescope near Socorro here in New Mexico. IC 443 contains one of the only six percent of supernovae that have become pulsars. IC 443 is difficult to see even in a large telescope.
The pulsar in IC 443 was discovered in 2000 by three high school students
from North Carolina School for Science and Mathematics, using data from
NASA's Chandra X-ray Observatory and the VLA. Their analysis of the Chandra
X-ray data led them to request time on the VLA to confirm their observations.
Those observations did not show pulses from the pulsar, but did show
high-energy electrons, typical of pulsars.
The Planets for April 2006
Mars starts the month in Taurus, but quickly moves into Gemini. It passes M35 on April 17 as it heads deeper into Gemini. The Earth continues to pull away from the Red Planet, making it appear only 5.3 seconds-of-arc across and fading at magnitude 1.4. Mars is a little over halfway up in the west at sunset, and sets around 12:45 a.m.
Making its home in the constellation Cancer again this month, Saturn is high in the east at sunset, setting around 2:45 a.m. The Ringed Planet shines at magnitude -0.1, as the Earth pulls away from it as well. The ball of the planet is 18.6 seconds-of-arc across, and the rings are 42.1 seconds-of-arc across with the southern face showing. They are tilted up 20 degrees to our line-of-sight.
The Earth is catching up to Jupiter, which is getting brighter and larger. Still stuck in Libra, the King of the Planets comes up around 9:30 p.m., and is visible the rest of the night. Jupiter will be at magnitude -2.5, stretching 44.0 seconds-of-arc across at mid-month.
Venus rises about 4:30 a.m., and shines in our morning sky until the Sun drowns it out. It starts the month in Capricornus, sails through Aquarius and, just as the month ends, slides into Pisces. At magnitude -4.2, Venus is now more than half-full and getting fuller. On April 15, Venus is 19.7 seconds-of-arc across.
Mercury is in our morning sky in April, but this
is not a good opportunity for viewing since it will never get far above
the eastern horizon before the Sun rises. Starting in Aquarius, Mercury
moves into Pisces, clips the edge of Cetus, and continues back into
Pisces. Mercury gets smaller and fuller during the month. At mid-month,
it will be magnitude -0.1, but only 6.9 seconds-of-arc across. So try
to take a look for it in the early morning, and "keep watching the sky"!