Constellation of the Month: Triangulum, the Triangle
Triangulum is one of the old constellations in the night sky. Its three brightest stars form a right triangle just east of Pegasus and south of Andromeda. Even though these stars are not very bright (third and fourth magnitude), they are bright enough to have been recognized as a constellation long before the Hellenic age. The Greeks thought the constellation looked like their letter delta, and so they called it Deltoton. It was eventually renamed Triangulum, the Triangle.
Since this grouping does not carry the name of a great hero or damsel in distress, you can guess that the mythology behind it is equally ordinary. Some see it as representing a river delta, especially that of the Nile. Others associate it with the island of Sicily, which has roughly the same shape. Persephone was taken to Hades from Sicily, so it is sacred to her mother, Demeter. Demeter begged Zeus to put it in the sky, and so today you can find "Sicily" just southwest of the Italian mainland and in our evening sky.
The most interesting (and only) object in Triangulum is the spiral galaxy M33, the Pinwheel Galaxy. At magnitude 5.7, this is a reasonably bright galaxy, and in a really dark area, it can be seen with the naked eye, making it the most distant object you can see without magnification—some 18 million trillion miles away. Binoculars will enhance the experience, but M33 measures 73 by 45 arc minutes—over twice the size of the full Moon, making it one of the largest objects in our sky. The Pinwheel's large size causes its light to be spread over a very large area, making the surface of the galaxy appear fainter than usual in binoculars or a telescope.
M33 was probably first found by Hodierna before 1654, but the first record of its appearance was made after its independent discovery by the famous comet hunter Charles Messier in 1764. He put it down as the 33rd object in his catalog of objects in the sky that are not comets; hence it is called M33. A picture of M33 will show you that we are viewing it almost face-on, so we can see the whole galaxy at once. Its spiral arms make it look like a pinwheel.
M33 is part of the Local Group of galaxies, which includes the Andromeda Galaxy and our own Milky Way as well as a number of smaller ones. Gravity keeps them all together and is pulling them closer. M33 is an average-sized galaxy, but is small compared to either our Milky Way or the Andromeda Galaxy. Being relatively close by, M33 was one of the first "spiral nebulae" that was identified as a galaxy. The key to its identification was the discovery of a certain type of variable star, called a Cephied. These variables brighten and dim at a regular rate, which is related to the total amount of light the star puts out, no matter where it is in the universe. By measuring the brightness of one of these stars as viewed from Earth, and knowing how fast it oscillates, you can compute how much light the star is actually putting out and thus how far away it is from us. When astronomers measured the Cephieds in M33, they discovered that it was not in our Milky Way galaxy, but far, far outside, making it a galaxy in its own right.
The Planets for January 2006
Venus passed its brightest point for this apparition last month, as it continues to move between the Earth and the Sun. As the month begins, Venus is heading back toward the Sun and by the end of the first week of the year it will be gone from the evening sky. At the beginning of the month it will be a thin crescent shining at magnitude -4.4, 58.4 seconds-of-arc across.
Mars remains in Aries all month, moving eastward among the stars as the Earth continues to outrun it in their respective orbits. At midmonth the God of War will be magnitude -0.2. It is shrinking from its maximum size of last year and is now only 10.3 seconds of arc across. Mars is high in the northeast at sunset and sets around 2 a.m.
Cancer comes up around 6:30 p.m. at midmonth, bringing with it the planet Saturn. The Ringed Planet will reach opposition on Jan. 27, when it is on the opposite side of the Earth from the Sun. This is the time it is nearest the Earth this year, also making it the biggest for the year. At opposition, Saturn's ball will be 20.4 seconds-of-arc across, while the rings are 46.2 seconds-of-arc across. The southern face of the rings are showing, tilted up 18.9 degrees. Saturn is shining at magnitude -0.5. It's a great time to view it through a telescope.
Jupiter comes up around 2 a.m., just as Mars is setting on the other side of the sky. Moving slowly eastward in Libra, Jupiter is the bright "star" in the eastern sky until sunrise. The King of the Planets is well south of the equator this apparition, so it will never be really well placed for observing, but it reaches the highest point in the sky it can reach this month just before sunrise. At mid-month Jupiter will be at magnitude -1.9, stretching 34.6 seconds-of-arc across.
Mercury is low in the eastern sky just before sunrise for the first half of the month. Its best view was back in December, but it is still hanging around in our morning sky. Each day will see it slowly slip back toward the Sun until it gets lost in the twilight. The almost full Messenger of the Gods will be magnitude -0.8 at midmonth and a tiny 4.7 seconds-of-arc across as it slips around the far side of the Sun. If you miss Mercury, you will not have to wait long to see it again. It will be visible in the evening during the second half of February, so don't fret and "keep watching the sky"!