D e s e r t E x p o s u r e
May
2009
Constellation of the Month: Sextans, the Sextant
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Click the image for a larger map
The southern sky contains many more of the non-classical constellations than the northern sky, and one of these is Sextans, the Sextant. This constellation has no really bright stars, but many galaxies, mostly faint. One of the more interesting is NGC 3115, the Spindle Galaxy. Many of the galaxies in this constellation are part of the Leo-Sextans Supercluster. A supercluster is a group of galaxy clusters that seem associated with each other, and is one of the largest structures in the universe. |
Sextans is another of the non-classical constellations, named by Johannes Hevelius. This Polish astronomer spent most of his life in Danzig, where he was involved in managing the town as a town councilor and as mayor. But Hevelius also had a "night life" as an active astronomer, beginning in 1639. Two years later, he built an observatory with excellent instruments, including a 105-foot focal length telescope.
His early observing was done not with a telescope, but with two measuring instruments — a quadrant and an alidade. He used these to chart the sky and created 10 new constellations, eight of which are still recognized by the International Astronomical Union. One of these, Sextans, is named in memory of his astronomical instruments, books and observatory that were destroyed in an arson fire on Sept. 26, 1679. Hevelius immediately rebuilt the observatory and was ready for the great comet of December 1680. His health suffered from the shock of the loss, however, and he died seven years later. His remains are buried in St. Catherine's Church in Danzig.
Hevelius was the last astronomer to do major astronomical work without a telescope. Even so, he used his telescope for four years to chart the lunar surface. This earned him the reputation as the founder of lunar topography. He also discovered four comets, in 1652, 1661 (also known as C/2002 C1 Ikeya-Zhang, rediscovered in 2002), 1672 and 1677. Observing these comets led him to the thesis that they follow a parabolic path around the Sun, as we now know to be true.
In terms of area, Sextans is 47th among the 88 constellations. Sextans is located just below the bright star Regulus in the constellation Leo. This rather small constellation has few interesting objects, mostly faint galaxies. One galaxy that is rather interesting is called the Spindle Galaxy, NGC 3115 (not to be confused with the other Spindle Galaxy, NGC 5866, in Draco).
NGC 3115 is a lenticular-type galaxy, which is like a spiral galaxy, but does not have any spiral arms in its disc. It is almost edge-on to us, giving it a long, narrow shape in our sky. NGC 3115 was discovered by William Herschel on Feb. 22, 1787. It is located 32 million light-years from us and is several times bigger than our Milky Way galaxy. This galaxy appears to be composed mostly of old stars, with little or no new star formation.
At the core of the Spindle Galaxy, like many others, there is a super-massive black hole, this one with a mass equal to some 2 billion of our Suns. It was discovered in 1992 by John Kormendy of the University of Hawaii and Douglas Richstone of the University of Michigan. They also determined that the black hole is no longer growing, indicating that this galaxy's gas and dust has all been converted into stars.
Telescopically, NGC 3115 looks like a needle in the sky. It is magnitude 9.9, and is 7.2 by 2.5 minutes of arc in size. These edge-on galaxies are often among the more spectacular telescopic views, and NGC 3115 is one of the brighter ones. Many have a dark dust lane running along the edge, but that is not the case for this Sextans' Spindle Galaxy.
The Planets for May 2009
Last month, Mercury popped up into our evening sky. It is still there as May begins, some 12 degrees up in the west-northwest sky shining at magnitude 1.2. Mercury's disc is 9.2 seconds-of-arc across, and is a 22-percent sunlit crescent that is becoming larger and more of a crescent every day. By May 12, it will have moved so close to the Sun that it will be lost in the solar glare.
High in the south-southeastern sky, Saturn is still in eastern Leo. Just 15 degrees from Regulus, the Ringed Planet has a yellowish glow at magnitude +0.8. It is still moving eastward among the stars, with a disc that is 18.3 seconds-of-arc across and shrinking as the Earth moves away from it. The Rings are 41.6 seconds-of-arc across, tilted up 4.1 degrees with the southern face showing. Saturn sets around 3 a.m.
Watch the Skies (all times MDT)
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Jupiter rises an hour before Saturn sets, so both will be in the sky at the same time, except near opposite horizons. Jupiter is magnitude -2.4, moving slowly eastward in eastern Capricornus. The disc is 39.5 seconds-of-arc across. Jupiter is getting closer to the Earth, so the disc is getting larger as we approach opposition in August.
Venus appeared in our morning sky last month and is already above Mars. It is now traveling eastward among the stars of Pisces, shining brilliantly at magnitude -4.5. Venus comes up around 4 a.m. and is visible until the sky gets too bright to discern it. At midmonth, the Goddess of Love is a 36-percent sunlit crescent, 31.4 seconds-of-arc across. Venus is getting smaller and becoming less of a crescent as the month goes on.
Mars rises 20 minutes after Venus. It is a reddish object glowing at magnitude 1.2 in the eastern sky. Mars' disc is still a tiny 4.6 seconds-of-arc across, not really worth observing yet. Mars starts the month traversing the northeastern corner of Cetus and then continues its rapid eastward movement back into Pisces. As the month ends, it slips over the border into Aries. Mars will be in opposition in January 2010, so get your telescope aligned 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.