D e s e r t E x p o s u r e
May 2010
Constellation of the Month: Antlia, the Air Pump
Abbè Nicholas Louis de Lacaille strikes again this month with another scientific device, Antlia, the Air Pump. This constellation of faint stars has as its brightest member a magnitude 4.25 orange giant, not a very brilliant example of the stars in our sky. De Lacaille created this constellation to commemorate the air pump invented by the French physicist Denis Papin. The original name, Antlia Pneumatica, was shortened when it became one of the 88 official constellations of the International Astronomical Union.
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Click the image for a larger map
Antlia is a small constellation at the edge of the Milky Way. It is just east of the three constellations that used to be the ship sailed by Jason and the Argonauts, Argo. The stars in this constellation are all faint and there are only a few deep sky objects, including the spiral galaxy NGC 2997. |
Antlia is the 62nd-ranked constellation in terms of size, at only 239 square degrees. Even though it is on the edge of the Milky Way, it holds only a few deep sky objects, among them the spiral galaxy NGC 2997, first observed by William Herschel in 1793. This spiral galaxy has two arms, tilted 45 degrees to our line of sight. Like a round plate held at an angle, the tilt gives it an oval appearance. This galaxy's oval is 6.8 minutes-of-arc across by 8.6 minutes-of-arc high and shines at magnitude 10.1. NGC 2997 is 24.8 million light-years away, moving away from us at 680 miles per second. At this distance, the galaxy would be 62,000 light years across, a little smaller than our Milky Way's 100,000-light-year diameter.
NGC 2997 is a classical spiral galaxy. This type of galaxy has two arms that wrap around the nucleus, giving it the look of a pinwheel. The arms are areas where an unusually high number of stars are being formed. Many of the stars are small and faint, but some are large and very bright. The light of the more-luminous stars makes the arms glow brighter than the other parts of the galaxy that have fewer of these stars. The arms also seem to have an unusual number of hot gas clouds scattered along their length. These appear reddish in color photos of galaxies like NGC 2997 because they are glowing in the red light peculiar to energized hydrogen atoms, a major constituent of an interstellar gas cloud.
After astronomers established that spiral nebulae were actually outside our own galaxy, the next problem was the existence of the spiral arms themselves. The speed at which individual particles travel around a galaxy's nucleus varies with their distance from the center of the galaxy. The particles (stars, planets, dust, gas, etc.) nearer the center complete their journey around the nucleus more quickly than those farther out. So if the arms started out as radial arms (straight lines out from the nucleus), over time they would become more and more tightly wound around the nucleus, as the inner segments go around faster than the outer ones.
Since the spiral galaxies we can see are not very tightly wound, the arms cannot be locked in place along with the stars and gas of the galaxy. In 1964, C.C. Lin and Frank Shu proposed that the spiral arms were actually density waves that traveled around the galaxy independent of the stars and gas. According to the theory, the arms are areas of higher density that travel around the galaxy at a different speed than the stars and gas.
As interstellar gas clouds enter the density wave, they are compressed, increasing the likelihood the gas cloud will collapse to form stars. Among those that form are massive bright blue stars that light up the spiral arm. These have a short (astronomically speaking) lifespan. As the newly formed stars move out of the density wave, the massive, bright stars are the first to burn out, and with them fades the trademark glow of the spiral arm. Dust clouds that did not collapse to form stars become thinner and cooler as they exit the density wave; their glow fades as well, sinking back to the level of the rest of the galaxy. The spiral arm has moved on and is being lit up by the newly formed bright stars from other now-hot gas clouds that have entered the ever-moving density wave forming the arm.
The Planets for May 2010
Venus is first to grace the evening sky as it gets dark. This brilliant planet starts the month in central Taurus and moves eastward into central Gemini by month's end. At midmonth, Venus's disc is still 85% illuminated and has grown to 12.1 seconds-of-arc across. The Goddess of Love will become less full but larger as the month goes on. Venus will shine at magnitude -4.0 and sets by 10:15 p.m.
A short while after Venus appears, Mars will be visible high in the southwestern sky, glowing at magnitude +0.9. The shrinking disk is only 7.0 seconds-of-arc across at midmonth, and sets around 1:45 a.m. During the month, Mars will drift eastward from Cancer into Leo.
Watch the Skies
(all times MDT)
May 5, 10:15 p.m. — Last Quarter Moon |
Saturn is in the next constellation eastward from Mars, Virgo. The Ringed Planet sets around 3:45 a.m. after starting the night a little more than halfway up in the south-southeastern sky. At midmonth, Saturn's disc is 18.5 seconds-of-arc across while the rings are 42.0 seconds-of-arc across and tilted down 1.7 degrees with the northern face showing. Yellowish Saturn shines at magnitude +0.9.
Jupiter rises around 3:30 a.m. and will be visible for the rest of the night. It starts the month in Aquarius; its slow east-northeastern motion carries it northward over the border into Pisces, where it spends the rest of the month. Its disc is 36.2 seconds-of-arc across, and it shines at magnitude -2.2.
Mercury pops up into the morning sky for the last half of May. This apparition will not be a particularly good one for northern-hemisphere observers because the path that all the planets follow, the ecliptic, makes a very shallow angle with the eastern horizon in May. So even though Mercury is 25 degrees from the Sun at greatest western elongation on May 26, it is only 8 degrees above the horizon as civil twilight begins. On that day, the Messenger of the Gods will be magnitude +0.5. The disc will be 40% illuminated and 8.1 seconds-of-arc across. It will become fuller as the month progresses.
The winter constellations with their brilliant first-magnitude stars are leaving our evening sky, to be replaced with the fainter spring constellations. While there are fewer bright stars, there are many more galaxies, including those of the Virgo Cluster. So look at these distant galaxies through your telescope and "keep watching the sky"!