Constellation of the Month: Dorado, the Swordfish
Deep in the southern sky, just on our southern horizon, lies the constellation Dorado, the Swordfish. While Dorado is usually depicted as a swordfish, Dorado, in Spanish, actually refers to the dolphinfish.
This is another constellation created by Petrus Plancius, based on observations of Pieter Dirkszoon Keyser and Frederick de Houtman in voyages to the southern hemisphere. This constellation first appeared on a 14-inch-diameter celestial globe released in 1597 by Plancius with Jodocus Hondius. Johann Bayer's Uranometria (1603) contains the first depiction of Dorado by that name in a celestial atlas. Other 17th and 18th century atlases showed this constellation as Xiphias, the Swordfish, a name that first appeared in Johannes Kepler's edition of Tycho Brahe's star list in the Rudolphine Tables of 1627. Even so, more astronomers used the name Dorado, which was ultimately adopted by the International Astronomical Union.
Dorado has many interesting objects, but the most spectacular is visible to the naked eye: The Large Magellanic Cloud (LMC) appears as a dim glowing patch that looks somewhat like a small cloud on the southern border of Dorado, adjoining the constellation Mensa. The first written record of the LMC was from the Persians around 964 AD, followed around 1503 by Amerigo Vespucci, the Italian cartographer whom the Americas are named after. But it was Ferdinand Magellan, after his 1519 voyage, who popularized the Large and Small Magellanic Clouds that would be named after him.
The galaxies in our area of the universe are bound together by their mutual gravity into the Local Group. The LMC is the fourth largest galaxy in the Local Group, after M31 (the Andromeda Galaxy), our own Milky Way and M33 (the Triangulum Galaxy). Only 170,000 light years away from us, the LMC is very close considering that our Milky Way galaxy is 100,000 light-years across.
But the Magellanic Clouds, long considered to be part of the Local Group, are actually visitors to our region of space. They arrived here within the last 3 billion years. The Large and Small Magellanic Clouds are traveling together through the universe, but they are moving too fast to be part of the Local Group, according to recent observations. They will pass through our area of space on a parabolic path, their track deflected by the gravity of the other galaxies in the Local Group.
The two Magellanic Clouds make close passes with each other as they travel. When they do, their mutual gravity disturbs the gas clouds in each galaxy. The disturbed clouds start to coalesce to form stars, making each close pass a temporary period of intense star formation (called a "star burst"). As the stars form, their solar winds blow the remaining gas out of their area. Since so many stars are forming all at once, the gas gets blown completely out of the Cloud, to where our Milky Way's gravity pulls it in toward us in the Magellanic Stream. The Magellanic Stream started about 2.5 billion years ago; too faint to be seen, it consists of gas that is only visible by the radio waves it emits.
The LMC is almost 11 degrees across (about 21 full moons) and about 9 degrees high. It looks somewhat like a detached section of the Milky Way. The central five degrees are somewhat brighter and more easily seen, so the LMC may appear to be smaller than its actual size. The total magnitude of the LMC is 0.9.
We are looking down on the disc of the Large Magellanic Cloud, but not straight down. The northeast end is closer to us than the southwest end, with the LMC tilted at an angle of about 35 degrees to our line of sight. The LMC was probably a barred-spiral galaxy originally, but its encounters with the Small Magellanic Cloud have distorted it so that the east and west ends of the central bar are bowed closer to us than the middle.
Next month, we will continue to investigate the LMC, its brilliant starburst region (the Tarantula Nebula), and the nearest supernova to us in recent years, which also occurred in the LMC, Supernova 1987a.
The Planets for February 2011
Dominating our western sky, the King of the Planets can be found 30 degrees up just south of west as it gets dark. With Jupiter lower than it was last month, you have to look through more of our atmosphere to see it — increasing the atmospheric distortion. This means the prime observing season for Jupiter is coming to an end. Starting the month in Pisces, Jupiter moves eastward into Cetus just five days before the end of the month. At midmonth, the Giant Planet's disc is 34.6 seconds-of-arc across and magnitude -2.2. Jupiter sets at 8:45 p.m.
Watch the Skies
(all times MST)
Feb. 2, 7:31 p.m. — New Moon
Feb. 11, 12:18 a.m. — First Quarter Moon
Feb. 18, 1:36 p.m. — Full Moon
Feb. 20, Evening — Look for Zodiacal Light (next 2 weeks) in the west
Feb. 24, 4:26 p.m. — Last Quarter Moon
Saturn rises around 10 p.m. The Ringed Planet is in Virgo, moving slowly westward. The Rings are 42.0 seconds-of-arc across and tipped down 10 degrees with the northern face showing. Saturn's disc is 18.5 seconds-of-arc across. This is a good time to observe Saturn, as it is high in the sky during the early morning hours.
Like a tiny gem on the east-southeastern horizon, Venus rises at 4:15 a.m. At a brilliant magnitude -4.2, Venus' disc is 67% illuminated and 17.3 seconds-of-arc across. Venus spends the entire month in Sagittarius, moving from the far western border to the eastern border of the constellation. Venus is starting to work its way around toward the back side of the Sun, becoming smaller and fuller each day.
Mercury and Mars are both too near the Sun for observation this month.
Starting around Feb. 20, take a look in the western sky as it gets dark to see if you can find the Zodiacal Light. You will need to be in a dark area away from city lights. Look for a ghostly cone of light coming up from just south of west. For two weeks after this date, sunlight reflecting off the dust in our Solar System's plane (roughly the ecliptic) is well placed for northern hemisphere observers to see as the Zodiacal Light. So look west and "keep watching the sky"!