D e s e r t E x p o s u r e
July 2010
Constellation of the Month: Circinus, the Compasses
For our constellation of the month, we have another creation of Nicolas Louis de Lacaille, Circinus, the Drafting Compasses. Its northern third just peeks over our southern horizon on July evenings, but no bright stars mark its presence; Circinus then just as quietly slides back below the horizon a few hours later. At 85th of the 88 official constellations in size, it's only 93 square degrees. Its brightest star is magnitude 3.2, and the other eight named stars are even fainter. None is nearby.
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Click the image for a larger map
Circinus barely manages to make itself known in our southern sky just after it gets dark. This tiny constellation contains only a few deep-sky objects in it, most faint and tiny. One is the Circinus Galaxy, a Seyfert class galaxy that would be much brighter if it were not hiding behind the dust and gas in our own galaxy. |
The only interesting galaxy here is simply called the Circinus Galaxy (also known as ESO 97-G13). A 12th-magnitude galaxy, it is not very bright, but it is part of the class called Seyfert Galaxies, which typically have a very bright nucleus. Since this galaxy is only 13 million light-years away, it should have been noticed long ago. Unfortunately, Circinus is right in the heart of the Milky Way, not far from the center of our galaxy in Scorpius. We look at the Circinus Galaxy through an obscuring thickness of dust and gas that hides much of its light.
The Circinus Galaxy, like all Seyfert galaxies, has a bright nucleus at the heart of a spiral-type galaxy. It is believed that there are black holes at the center of most galaxies — in the case of Seyferts, supermassive black holes. Since the galaxy is rotating, material pulled into the black hole doesn't fall directly inward, but spirals down into an accretion disc circling the black hole.
The supermassive black hole is tiny by astronomical standards, and at these distances even the accretion disk can't be resolved with the largest telescopes. So we can't tell if the light shining from the nucleus comes from the accretion disc itself, or from gas clouds around the nucleus energized by ionizing radiation from the black hole.
In the Circinus Galaxy's nucleus, the supermassive black hole beams out material in two cones: one we can see and one in the opposite direction, hidden by dust and gas. These cones stretch from the black hole out into the halo of the galaxy, transporting material away from the black hole.
This material has formed into two rings centered on the black hole. The outer ring is 1,300 light-years from the black hole and the inner ring is only 260 light-years away. Both rings have large amounts of dust and gas that are interacting to form new stars on timescales of only 40 to 150 million years, much shorter than the age of the galaxy.
Seyfert galaxies, first identified by Carl Keenan Seyfert in 1943, are a subclass that have an active galactic nucleus (AGN). While all AGN galaxies have activity at their core, Seyferts have some of the brightest nuclei of nearby galaxies. Seyferts have been heavily studied because they're believed to be lower-energy versions of what goes on in the far distant quasars.
The Planets for July 2010
Mercury makes an appearance in the evening sky this month, visible low in the west for the last three weeks of the month. Mercury starts July near the Sun in Gemini. It moves rapidly eastward through Cancer, and ends the month in Leo. It passes Regulus on July 27. At midmonth the Messenger of the Gods is magnitude -0.5, 79% illuminated and becoming less gibbous. At that time, it is 5.6 seconds-of-arc across. As with all western apparitions, Mercury has just come from the far side of the Sun, looking small and full. It moves quickly around in its orbit until it's as far from the Sun as it can get, then swings back between the Earth and Sun, becoming very large with a thin crescent disc. Mercury sets around 9:15 p.m.
Venus is next up in the evening sky. It moves from the west to the east side of Leo during July. On July 15, it is 17.3 seconds-of-arc across and 65% illuminated. The Goddess of Love is magnitude -4.1 and sets around 10:30 p.m. Venus will be near Regulus on Friday evening, July 9. Venus slides eastward along the ecliptic, staying about the same distance from the Sun. But the ecliptic itself swings farther south as planets travel eastward along this section, so Venus will also be lower in the sky as it gets dark later in the month.
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July will see Mars travel from the middle of Leo into eastern Virgo near Saturn. Mars is 30 degrees up in the west-southwest as it gets dark and sets by 11:15 p.m. Mars is 92% illuminated and 4.9 seconds-of-arc across, glowing at magnitude 1.4.
Saturn moves slowly eastward among the stars of western Virgo and sets around 11:45 p.m. The Ringed Planet's disc is 16.7 seconds-of-arc across at midmonth. The Rings are 37.9 seconds-of-arc across, tipped down 2.6 degrees with the northern face showing.
Jupiter is also moving slowly eastward in central Pisces, until it becomes stationary on July 23. It then turns around and heads westward for the rest of the month. The King of the Planets shines at magnitude -2.6 and rises just before midnight. Jupiter's disc is 40.6 seconds-of-arc across.
July 11 is New Moon, and for those in the South Pacific there will be a total solar eclipse as the Moon moves over the face of the Sun, completely blocking the Sun's light along a narrow band that passes over the Cook Islands, Tahiti, Chile and on to Argentina. The maximum duration is 5 minutes, 20.2 seconds of darkness. For us, the Moon will be well south of the Sun, so it will be just another New Moon. So take a look two days later for a very thin crescent in the early evening and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens
is co-director of Desert Moon Observatory in Las Cruces.