D e s e r t E x p o s u r e
December 2009
Constellation of the Month: Phoenix, the Phoenix
Phoenix is a constellation low in our southern sky during December evenings.
 |
Click the image for a larger map
The constellation Phoenix does not quite clear the southern horizon on December nights here in the desert Southwest. There are no deep sky objects in this constellation, except for faint galaxies. The constellation is saturated with these faint "fuzzballs." |
Like last month's constellation, Grus, Phoenix was introduced by Petrus Plancius in late 1597, or early 1598, on a 14-inch celestial globe prepared in collaboration with Amsterdam cartographer Jodocus Hondius the Elder. Phoenix first appeared in a sky atlas when Uranometria was published in 1603 by Johann Bayer.
Even though the ancients did not consider this group of southern stars a phoenix, this mythical bird still has a rich mythology. The phoenix had a colorful plumage and a tail of gold and scarlet. When its 500-year lifespan was coming to an end, it would build a nest of branches and herbs, set the nest on fire and go out in a fierce ball of flame. From the ashes, a new phoenix would rise to begin another 500-year life.
Greek sophist Flavius Philostratus referred to the phoenix as living in India, but migrating to Egypt every 500 years; some think the Egyptian phoenix is actually the flamingo of East Africa. Another possibility is that the phoenix is a stork or a heron-like bird called benu, known from the Egyptian Book of the Dead. The ancient Lebanese descended from the Phoenicians, who claim to be sons of the phoenix. In China, the mythical Fenghuang is superficially similar to the phoenix, and is the leader of the birds.
This constellation is far to the south, barely coming above our southern horizon, and most of its stars are fainter than fifth magnitude. The brightest is Ankaa, from an Arabic word meaning "the boat." Phoenix is also the radiant of a minor meteor shower, the Phoenicids, which peaks on Dec. 5.
Phoenix is filled with galaxies, but all are faint and most are far away. One galaxy in this constellation that is actually fairly close is known as ESO245-G 007 or, more informally, the Phoenix Galaxy. ESO stands for "European Southern Observatory" and this object is from its catalog of celestial objects. This galaxy is close by, a satellite of our Milky Way galaxy.
First discovered in 1976 by Hans-Emil Schuster and Richard Martin West, the Phoenix Galaxy was mistaken for a distant globular cluster, since its stars are so distinct. Further observation determined that this is actually a dwarf galaxy, only 1.44 million light-years away. This makes it part of the Local Group of Galaxies, which includes our own Milky Way, the Andromeda Galaxy (M31) and the Triangulum Galaxy (M33). These three are the only spiral galaxies in the Local Group; all the rest, including the Phoenix Galaxy, are dwarf galaxies.
Watch the Skies
(all times MST)
Dec. 2, 12:30 a.m. — Full Moon Dec. 8, 5:13 p.m. — Last Quarter Moon Dec. 16, 5:02 a.m. — New Moon Dec. 18, 10 a.m. — Mercury greatest elongation east (20 degrees) Dec. 19, 10 p.m. — Jupiter 0.6 degrees south of Neptune Dec. 21, 10:47 a.m. — December Solstice, winter begins Dec. 24, 10:36 a.m. — First Quarter Moon Dec. 31, 12:13 a.m. — Full Moon (Blue Moon) |
The Phoenix Galaxy is 4.9 by 4.1 minutes-of-arc across and shines at a faint magnitude 13.1, but what makes it interesting is the age of the stars in its core. When astronomers started to observe the spectra of stars, they divided them into two groups, Population I and Population II, based on the amount of metals in their spectrum. In astronomy, "metals" means any element heavier than helium. Stars with more metals were assigned to Population I, while stars with little metal were assigned to Population II. As astronomers began to better understand how stars form and age, they found that Population I were the younger stars. Their metals were formed when the older Population II stars exploded as supernovas, creating heavier elements that spread out into the galaxy. There they mixed with the gas clouds that eventually formed the younger stars.
The Phoenix Galaxy was given that name not only because it is in the constellation Phoenix, but because the core of this dwarf galaxy is made up of younger Population I stars that formed about 50 million years ago. Its outer regions are composed of older Population II stars. This makes it appear that stars are mysteriously forming in the core, while none are forming in the outer regions.
Another feature is a hydrogen cloud 6 minutes-of-arc southwest of the galaxy. Since the core stars are younger at the west end of the Phoenix's core, astronomers think the gas cloud was ejected during star formation. The gas cloud is moving too slowly to escape the Phoenix Galaxy and will eventually turn around, falling back into the Phoenix, making it appear as an irregular galaxy and triggering a new round of star formation. The galaxy will be reborn out of its own ashes just like the mythical bird.
The Planets for December 2009
Mercury puts in an appearance in the evening sky during December. Coming out from the back side of the Sun, Mercury's disc will be mostly full in the early part of the month. On Dec. 18, Mercury will reach its maximum distance, east of the Sun at 20.3 degrees. It will be magnitude -0.5, its disc 61.5% illuminated and 6.7 seconds-of-arc across. Mercury will then turn around and head back toward the Sun, disappearing into the evening twilight by the New Year.
Moving eastward through eastern Capricornus, Jupiter is the other planet in our evening sky. At mid-month, Jupiter sets around 8:45 p.m. and its disc is 34.9 seconds-of-arc. As it gets dark, Jupiter will be shining at magnitude -2.1, about a third of the way up in the southwest.
As the night progresses, Mars comes up around 9 p.m. in western Leo. Mars will be moving eastward in the beginning of the month, but on Dec. 21, Mars will become stationary and then turn back westward. From Mars, the Earth has reached greatest western elongation and is starting back toward the Sun. At mid-month Mars is magnitude -0.4 and its disc is 11.2 seconds-of-arc across. Now is the time to start observing Mars, as it will reach opposition at the end of next month.
Saturn rises at 12:45 a.m., shining at magnitude +0.9. The Ringed Planet is moving slowly eastward in central Virgo. Saturn's disc is 17.3 seconds-of-arc across. The rings are 39.2 seconds-of-arc, tipped down 4.7 degrees with the northern face showing.
Venus is too close to the Sun to be visible.
Winter begins in the northern hemisphere at 10:47 a.m. MST on Dec. 21, as the Sun reaches its southernmost point for the year, the December Solstice.
December has two full moons this year. The second full moon in a month is traditionally called a "Blue Moon." It happens every two to three years, usually during one of the longer months. February is too short (even with a Leap Day) to ever have a Blue Moon.
Enjoy the holidays, and "keep watching the sky"!