D e s e r t E x p o s u r e
February 2010
Constellation of the Month: Caelum, the Chisel
With no star brighter than magnitude 4.5, Caelum, the Sculptor's Chisel, is another in the group of constellations created by Abb Nicolas Louis de Lacaille in 1756. He originally labeled it with the French les Burins (the Chisels), but it was Latinized to Caelum Scalptorium. Astronomers later shortened it to just Caelum. It was described by de Lacaille as a pair of crossed burins (sharp engraving tools) connected by a ribbon. Since this is a recent constellation, there is no mythology associated with it.
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Click the map for a larger view.
Caelum appears just above our southern horizon on February evenings. Another recent constellation, this one represents a sculptor's chisel. The only deep-sky objects visible in Caelum are galaxies. Every constellation in the sky has at least a few galaxies that are bright enough to be seen with small or medium telescopes. With very large telescopes, the sky is littered with galaxies, over 200,000 per square degree. |
Caelum is a small constellation, the eighth smallest in the sky. There are no Messier objects located here, and the only deep-sky objects are faint galaxies. Alpha Caeli is the brightest star, at magnitude 4.46; it is a double star with the primary a class F2 star, about 1.5 times larger than our Sun. Most of the light from system comes from this primary star, which puts out 5.2 times more light than our Sun. The secondary is a red class M star, magnitude 12.5, that's very hard to see because it is only 6 seconds-of-arc from the much brighter primary.
This star is only 66 light-years away, and Alpha Caeli A spins once in 1.4 days. Compare this to the equatorial region of our Sun, which rotates once in 25 days. Alpha Caeli B is only 30% of the mass of our Sun, but it is also a flare star that can brighten by more than a magnitude when it is "flaring." This extra brightness comes from the release of pent-up magnetic energy in the star.
Astronomers measured the separation of these two stars at 3 seconds-of-arc in 1896 and 6.3 seconds-of-arc apart in 1933, but it has not been measured since. The two stars are at least a thousand astronomical units (a thousand times the average distance from the Earth to the Sun) apart and take about 130 years to orbit each other, with B doing most of the traveling.
When you have a small planet orbiting a vastly larger star, the planet will travel in an ellipse around the star's center; the star sits at one of the foci of the ellipse and does not move at all, like a child racing around a maypole. As the mass of the planet gets larger, the star is no longer fixed and the arrangement is more like two people facing each other, each grasping the other's arms and both swinging around the space between them.
With a heavy planet (or a light star), it is like an adult and a child twirling around each other. The heaver adult will swing the child around much more than the lighter child can swing the adult. In our solar system, Jupiter and the Sun both swing around their common center of gravity, a point just outside the Sun's surface. Looking back at the Sun from outside the solar system, the Sun appears to wobble slightly back and forth in a 12-year period, the same as it takes Jupiter to orbit the Sun. The common center of gravity is called the barycenter (from the Greek barus for heavy and center).
When two equal-mass stars orbit each other, it is like two adults swinging around their common center point where their hands link. In space, the two equal-mass stars both orbit around the point that is midway between them (their barycenter).
The situation becomes much more complex in a star system with three, four or more stars. All the stars in these systems orbit around their common barycenter. This point will move around as the different stars travel in their orbits. It can be very complex to compute these orbits and astronomers must often resort to using computers. They first predict the motion of these stars over a short period of time as the gravity of the other stars tugs on them. The computer then uses their new positions to compute the motion over another short period. This process repeats until the astronomer has predicted each star's path far enough into the future for their purposes. This process is called numerical integration and is very often the only way to solve these kinds of problems.
The Planets for February 2010
Jupiter takes its last bow early this month as it sinks into the evening twilight. As it does, Venus enters the evening sky from "stage west," the western horizon.
In central Aquarius all this month, the King of the Planets will be visible from the time it gets dark until it sets. By mid-month, it will be too low to see in the Sun's glare. On Feb. 1, Jupiter's disc is only 33.3 seconds-of-arc across, shining at magnitude -2.0.
Watch the Skies
(all times MST)
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As Jupiter departs, Venus will be coming up. They pass each other on Feb. 16, but the passing will be too low to be visible. By the end of this short month, Venus will be almost full at magnitude -3.9. Its disc is a tiny 10 seconds-of-arc across. Venus spends the last two-thirds of the month in Aquarius, moving eastward among the stars. At the end of the month, Venus sets before 7 p.m.
Mars moves slowly westward among the stars of Cancer. Having just passed opposition, it is visible almost all night, low in the east as it gets dark and setting around 6 a.m. Mars is still a bright magnitude -1.0 with a disc 13.3 seconds-of-arc across.
The Ringed Planet spends the month in Virgo, sliding slowly westward against the stars. Saturn's disc is 19.1 seconds-of-arc across at midmonth. The Rings are still showing their northern face, tipped down 4.4 degrees, 43.2 seconds-of-arc across, and the whole system shines at magnitude +0.7.
Mercury starts the month in the eastern sky as it gets light. It passed greatest elongation late last month, and is moving back toward the Sun. On Feb. 1, it rises at 5:36 a.m. and glows at magnitude -0.1 in Sagittarius. Mercury is very low with an altitude of only 7 degrees in the southeast as it gets dark. The ecliptic is at a shallow angle to the horizon, since this is one of the southernmost parts of the ecliptic. This means that Mercury will appear at almost the same altitude during the month even though it is moving closer to the Sun. By Feb. 24, Mercury has moved into Capricornus and rises just as it starts getting light, ending its appearance in our morning sky.
As the weather warms, spend more time under the starry dome and "keep watching the sky"!