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Vulpecula, the Fox

Plus the planets for September.


The obvious grouping of three bright stars almost overhead in late summer and early fall is called the Summer Triangle. Made up of three first-magnitude stars from three different constellations, this asterism is hard to miss. In the middle of the Summer Triangle is the faint constellation Vulpecula, the Fox. Composed of stars no brighter than fourth magnitude, this constellation was created by Johannes Hevelius in his 1687 star atlas Firmamentum Sobiescianum sive Uranographia.


(Click the image for a larger map.)


Facing south and looking overhead, the three stars of the Summer Triangle surround the faint constellation Vulpecula, the Fox. The Milky Way runs through this constellation and you can find many deep-sky objects. The Dumbbell Nebula (M27), a planetary nebula, and NGC 7052, an edge-on galaxy, are two of the major objects. You can also find Brocchi’s Cluster (Collinder 399), commonly called the Coathanger star cluster.

Vulpecula was originally called Vulpecula cum Ansere (The Little Fox with the Goose) and was depicted as a small fox that had caught a goose. Even though the constellation is now just called Vulpecula (the Fox), the goose is still remembered in its brightest star, Anser (the Goose). This constellation has the Milky Way running through it, so there are many interesting deep-sky objects here. It is also the home of the first pulsar ever discovered.

Pulsar PSR B1919+21 (referring to the coordinates of its approximate position in Vulpecula) was discovered in 1967 using a radio telescope. Like an optical telescope, a radio telescope collects energy in a portion of the electromagnetic spectrum, but instead of concentrating light, it concentrates radio waves.

A steerable dish-type radio telescope, such as in use at the Very Large Array (VLA) west of Soccoro, has a bowl-shaped reflector that focuses radio waves very much like a mirror in an optical telescope focuses light. But as you work with lower and lower frequencies, the dish has to get bigger and bigger to be effective, until it becomes too big to steer around the sky. At these lower frequencies, radio astronomers use arrays of dipole antennas connected together. Each antenna picks up the very weak signal from the sky; adding them together increases the strength of the signal.

At Cambridge, England, in the 1960s, Anthony Hewish wanted to study the scintillation (like the twinkling of stars) in the radio spectrum to help determine if a radio source is small or spread out over a wide area of the sky. He worked out a radio telescope composed of 2,000 dipole antennas spread out over four acres that observed the sky at 81.5 megahertz (part of US VHF television channel five). Jocelyn Bell was working on her PhD under Hewish and spent two years building the telescope. In 1967, she started operating the completed telescope and taking data.

Each night the telescope generated 96 feet of chart recorder paper, which Bell (now Bell-Burnell) would analyze. In July 1967, she uncovered a bit of "scruff" on one of those 1,152 inches of chart paper. She determined that this signal pulsed every 1.3373 seconds. It was extremely regular, and at first it was thought to be some kind of Earth-based interference, since the telescope would pick up the signal at almost the same time every day.

After a few days, it became clear that the signal came into the view of the radio telescope every sidereal day (23 hours, 56 minutes, 4.1 seconds), instead of a solar day (24 hours). Whatever was causing the signal, it was in the sky and not on the Earth. Since no one had ever detected this kind of signal, they called it "LGM-1" for Little Green Men, a reference to a proposed extraterrestrial civilization that might be sending out a regular signal. They soon discovered more of these pulsating signals in different parts of the sky. There could not be multiple ET civilizations transmitting on this frequency, so they decided to change the name to "pulsar," for pulsating star.

In 1968, two astronomers, Thomas Gold and Franco Pacini, were independently able to explain these pulsars as rapidly rotating neutron stars with a strong magnetic field. Each pole of the magnetic field emits a beam of electromagnetic radiation straight out from the pole. As the beam crosses the Earth, we get a burst of light and radio waves. At the time, this was so radical an idea that Gold was not allowed to present his theory at the first international conference on pulsars. The discovery of the pulsar in the Crab Nebula late in 1968, with its 0.033-second period, ruled out all the other theories, leaving Gold and Pacini's theory as the only possible explanation. Now, it is a well-accepted theory that no one disputes.



The Planets for September


Mercury starts the month in Leo, west of the Sun. It travels eastward, passing the Sun on Sept. 10, moving into the evening sky, and later in the month moves into Virgo. This will be a very poor evening view of the Messenger of the Gods. The ecliptic is tilted down toward the horizon in September and, to make things worse, Mercury itself is moving southeastward. The result is that Mercury barely gets above the horizon near the end of the month, but it can get no higher, sliding along the horizon, moving a little farther south each day. On Sept. 30, Mercury will be near Spica, with a disc that is 5.0 seconds-of-arc across and is 92% illuminated. Mercury sets at 7:30 p.m. and glows at magnitude -0.4.

Watch the Skies
(times MDT)

Sept. 8, 5 a.m. —
       Jupiter 37 minutes-of-arc north of the Moon's edge
       7:15 a.m. — Last Quarter Moon
Sept. 15, 8:11 p.m. — New Moon
Sept. 22, 8:49 a.m. —
        September Equinox, autumn begins
        1:41 p.m. — First Quarter Moon
Sept. 29, 9:19 p.m. — Full Moon

Saturn is also in Virgo this month, moving slowly eastward away from Spica. Just barely 14 degrees above the west-southwestern horizon as it gets dark, the Ringed Planet is not a good observing target this month. At midmonth, the rings are tilted up 14.8 degrees with the northern face showing and are 35.5 seconds-of-arc across. Saturn's disc is 15.6 seconds-of-arc across and it glows at magnitude +0.8. Saturn sets at 9 p.m.

The God of War starts the month in eastern Virgo. Moving eastward into Libra, Mars sets around 9:30 p.m.; it is also not a good observing target. At midmonth, Mars is magnitude +1.2 with a disc that is 5.0 seconds-of-arc across. As it gets dark, Mars is 21 degrees up in the southwest.

Jupiter is moving eastward in Taurus between the horns of the Bull. It shines at magnitude -2.5 and its disc is 41.0 seconds-of-arc across. The King of the Gods rises at 11:30 p.m.

Venus rises around 3:30 a.m. and by the time it starts to get light, it is 36 degrees up in the eastern sky. The Goddess of Love's disc is 17.7 seconds-of-arc across and 65% illuminated. Venus is a brilliant beacon shining at magnitude -4.2. Starting the month in Gemini, Venus travels eastward through Cancer and moves into western Leo as the month ends.

Traveling south, the Sun reaches the equator on Sept. 22 at 8:49 a.m., the September Equinox. On this date the length of the daylight hours will be the same as the length of the nighttime hours. So start digging out those winter coats for cool autumn evening observing and "keep watching the sky"!



An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.




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