D e s e r t E x p o s u r e
February 2012
Pictor, the Painter
Plus the planets for February.
Pictor, the Painter, is located in the southeastern sky on February evenings. This constellation is another creation of Abbé Nicolas Louis de Lacaille. Lacaille put together a catalog of 10,000 southern stars called Coelum australe stelliferum, which was published in 1763. It included 14 new constellations, among them Equuleus Pictoris, the Painter's Easel. Even though the name was shortened to Pictor when it became an official constellation, it is still usually represented as an easel.
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Pictor is located between the bright star Canopus to the east and the Great Magellanic Cloud to the southwest. The brightest star in this constellation is Alpha Pictoris, which is only magnitude 3.2, not very bright as stars go. The next brightest star, Beta Pictoris, is magnitude 3.9 — also not very bright, but much more interesting.
Beta Pictoris is 1.75 times as massive as our Sun, and puts out 8.7 times as much light. It is very young, only 8 million to 20 million years old. Because of its youth, the dust and gas left over after the formation of Beta has not yet been blown away by the light and solar wind that Beta itself produces. The dust and gas, warmed by Beta, emit infrared radiation, which was first picked up by the Infrared Astronomy Satellite (IRAS) in 1983.
Astronomers know that a spectral class A6 star like Beta emits most of its energy toward the blue end of the spectrum, so this infrared radiation had to come not from the star, but from cooler matter orbiting it. In 1984, astronomers carefully took pictures of Beta Pictoris, blocking out the light from the star itself. The resulting images showed a disc of material orbiting around the star. The disc is very flattened, much like our early solar system must have been. Because of this similarity, it is believed that there are planets still forming within the disc.
The Hipparcos satellite, which was launched in 1989 and operated until 1993, made very precise measurement of the positions of many stars, including Beta Pictoris. It observed Beta when the Earth (and Hipparcos) were on one side of the Sun and again when we were on the opposite side of the Sun. Astronomers took the measured positions from both sides and used trigonometry to compute the distance to Beta Pictoris as just 63.4 light-years away, using the diameter of Earth's orbit as a baseline.
Since it is so close, we are able to see more detail in this system. Looking closely at the disc, we can see belts of small rocky bodies, called planetesimals, at approximately 1.3, 2.6, 4.8 and 7.6 billion miles from the star. Perhaps these rings represent bands where the planetesimals are banging together to form larger and larger bodies. These bands may represent orbits that will someday be home to planets.
In addition to the planet formation in progress, a massive planet is already orbiting Beta, about 930 million miles from the star. Evidence for this planet is the lack of dust in this region of Beta's disc, which would have been cleared out by the planet. We have actually observed this planet, which is not quite as far from Beta as Saturn is from our Sun.
Observations of meteoroids (meteors observed before they enter our atmosphere) have shown that there is a stream of these particles coming from Beta's dust disc entering our solar system. This stream seems to be the main source of interstellar meteoroids in our solar system. While most of the meteors that we see are from comets orbiting our Sun, a few come from Beta Pictoris. So next time you see a sporadic meteor streak across the sky, you can think to yourself, "That might have come from Beta Pictoris"!
The Planets for February
Mercury appears out of the evening twilight during the last half of the month. Starting the month in Capricornus, it zips through Aquarius and ends the month in Pisces. At month's end, the Messenger of the Gods has a disc that is 6.5 seconds-of-arc across and 65% illuminated, moving toward the crescent phase. It will set at 7:15 p.m.
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(times MST)
Feb. 7, 2:54 p.m. — Full Moon Feb. 8, 5 a.m. — Saturn stationary Feb. 14, 10:04 a.m. — Last Quarter Moon Feb. 15 — Mars farthest from the Sun Feb. 21, 3:35 p.m. — New Moon |
Still gracing our evening sky, Venus is 35 degrees above the west-southwestern horizon as it gets dark. At midmonth Venus' disc is 16.6 seconds-of-arc across and 69% illuminated, becoming less full as the days go on. Venus starts the month in Aquarius; moving eastward, it almost immediately slips over the border into Pisces, where it ends the month. Venus sets around 9:10 p.m.
Moving slowly eastward in southwestern Aries, Jupiter shines brightly at magnitude -2.3 at midmonth. With a disc that is 37.4 seconds-of-arc across, the King of the Gods is about halfway up in the west-northwest as it gets dark and sets around 11 p.m. It is past its best viewing during this apparition, but will be back again later in the year.
Mars is sliding over the border from Virgo into Leo as the month begins. Moving westward, Mars will end the month under Leo's hindquarters where it glows at magnitude -0.6. The God of War's disc is 11.9 seconds-of-arc across and 96% illuminated. Mars will continue to become fuller as it moves toward opposition early next month. This is a great time to observe Mars since its disc is almost as large as it will get this year.
Saturn is moving westward in eastern Virgo. Rising at 11:30 p.m., Saturn is visible the rest of the night, being highest around 5 a.m. The Ringed Planet is magnitude +0.5 at midmonth when its Rings will be 40.8 seconds-of-arc across and tipped down 15.0 degrees with the northern face showing. Saturn's disc is 18.0 seconds-of-arc across.
In order to adjust for the fact that there are 365.2425 days in a year, we add a leap day once every four years to compensate for the extra day that accumulates during that time. If we did not make this adjustment, the equinox would slowly drift away from March 21. Enjoy the extra day you get to "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.