The Night Sky This Week (February 4 - 10)

The Planets Mercury Mercury is currently washed out in the solar glow, but makes its return to the dawn skies within a week. Venus For the past several months, dazzling Venus has been prominent in our morning sky. During this month, however, the planet will slowly fall back toward the Sun and will sink deeper into the twilight each consecutive morning.

If you watch the sky around 6:30 A.M. local time every morning, you can still notice Venus low above the southeast horizon. You might want to check your horizon in advance, to make sure that there are no tall trees or buildings that might obstruct your view of the planet. At magnitude -4, Venus will be the brightest star-like object in the sky, and will appear to be "gibbous phase" in even the smallest telescopes.

One good way to tell stars from planets is that looking at them with the unaided eye, stars twinkle and planets do not. The twinkling of stars, technically known as stellar scintillation, is caused by the Earth's atmosphere. Because stars are so incredibly distant from us, any disturbances in the atmosphere will bounce around the light from a star in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle".

On the other hand, planets are much nearer to the Earth and they look like small disks of light rather than point sources. The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, and the total effect is one of steady light.

Finder map - 30 minutes before sunrise, looking southeast.

Mars

Phobos, the larger and innermost of Mars' two moons, is heavily cratered and resembles a potato in shape. G. Neukum/ DLR/ESA [larger image]

Mars is moving eastward in direct motion (as it does most of the time) through the stars of Taurus and will cross over into Gemini next month, on March 5th. This brilliant yellow-orange world shines very high in the southeast sky after dusk, high above Orion and near the fairly bright star Beta Tauri, also known as Elnath. Seen in a telescope, Mars is 11" wide this week. At moderately high magnification it appears as a small, distinct reddish ball displaying subtle dark markings and a bright white South Polar Cap. The red color comes from rust - iron oxide - that makes up about 10-percent of Martian

soil. It is thought the oxide was created far back in the planet's history, when surface water was abundant and reacted with iron in the rocks on Mars' surface.

Mars is a small planet, closer in size to our Moon than to the Earth. It has two natural satellites, colorfully named Phobos (Fear) and Deimos (Panic), after the horses that drew the chariot of the Roman war god. They are rather unimpressive as moons go, resembling large asteroids. Phobos, for example, is 17 miles across its longest dimension, but it is 13 miles and 11 miles across its other dimensions. Thus, it is shaped like a potato. Deimos is even smaller and is similarly shaped.

Finder map - 7 P.M. local time, looking southeast.

Jupiter

Gas giant Jupiter, imaged by the Voyager spacecraft in 1979. NASA/JPL [larger image]

Giant Jupiter returns to view in the morning sky low above the southeast horizon, and gets more conspicuous each morning. The planet spans 33" this week and grows to 47" by the time it reaches opposition in July. The telescopic view of Jupiter will improve markedly as it climbs higher in the coming months, but you can still practice observing this planet and training your eye to see detail. Jupiter is the King of the Planets, and not just because of its enormous size - this monster planet is 88,700 miles in diameter. Jupiter also reigns over the other members of the solar system in the affections of amateur astronomers, because this great ball of gas is just so consistently interesting.

There is always a tremendous variety of interesting phenomena to see when Jupiter is in the sky. The planet shows detail in backyard telescopes and even the smallest optical aid will show its four bright moons. In small telescopes you can make out two or three of the darkest cloud bands, and as the scope gets bigger, the more you will see.

Four-inch telescopes can show multiple bands and the Great Red Spot. Larger scopes can see details in the bands such as texture, loops, and ovals, often in vivid color. Also visible in larger telescopes are transits of the moons across Jupiter's surface as well as the inky black dots of the moons' shadows as they transit the planet.

Finder map - 30 minutes before sunrise, looking southeast.

Saturn

This illustration shows a close-up of Saturn's ring system. The rings are believed to have formed after an icy moon was broken up by an impact with a comet or asteroid. NASA/CXC/M. Weiss [larger image]

The ringed planet rises in the east around 7 P.M. local time and is highest in the south in the early-morning hours. It shines at magnitude +0.3 and remains in Leo throughout February, lower left of 1st- magnitude Regulus (Alpha Leonis) in the late evening. Saturn's disk measures 20" across this week, while the rings span 45" and tilt only 8° to our line of sight, allowing us to see more of the planet's globe. Saturn is the most distant of the five planets known to ancient stargazers, and with an equatorial diameter of about 75,000 miles, it is the second largest planet circling the Sun. Appearing as a bright yellowish object in our night sky, Saturn presents a

maximum apparent diameter of 20" and reaches magnitude -0.4 at favorable oppositions, when it lies closest to the Earth. This is about ten times fainter than Jupiter.

The planet's most attractive feature is, of course, the magnificent ring system. The rings consist of countless particles of water ice, ranging in size from small grains to irregularly shaped pieces generally a few feet across. It is interesting to note that if all of the material of the rings of Saturn were formed into a single moon, the moon would be about the mass of Janus (one of the smallest of Saturn's moons) and only 1/20,000 the mass of the Earth's Moon.

Finder map - 9 P.M. local time, looking east.

Uranus

Uranus is lost in evening twilight in the west-southwest sky. The planet will be visible again in early May, only now in the morning sky.

Neptune

Neptune is hidden in the glow of the Sun. It will return to view in late March, low in the morning sky.

Pluto

The dwarf planet is too deep in the solar glare and cannot be observed until early March, when it will reappear in the morning sky.

The Deep Sky

Stars that do something are fun to watch. In late 1936, a 16th-magnitude star erupted in the constellation Orion, and by early the next year it had attained 10th magnitude - that is a brightening by a factor of over 250! Accompanying the mysterious star was a bright nebulosity, which glowed as a reflection of light from the luminous star. The 1936 uprising of FU Orionis was first thought to be a nova event. Novae (like the famed example Nova Cygni 1975, which appeared on August 29, 1975, and reached magnitude +2) are hardly unusual; many are found each year. They are surface

This artist's concept shows a young star and the whirling accretion disk surrounding it. NASA/JPL-Caltech [larger image]

explosions that take place on white dwarfs when they accumulate too much fresh hydrogen from a companion star. When the fresh fuel is burned, novae quickly fade. FU Orionis, however, challenged expectations by staying lit up for over 70 years. The star still shines at 10th magnitude today.

Since no other star was known to behave like FU Orionis and since little was known about the star itself, astronomers waited for further clues from FU Orionis or more examples to emerge. In 1969, a star embedded in the "East Texas" region of the North America Nebula (NGC 7000) jumped from 16th to 10th magnitude. The star, V1057 Cygni, attained maximum luminosity in mid-1970, and afterwards it slowly faded; it now lies three magnitudes below peak brightness.

Since then, another eight similarly behaving stars have been discovered and FU Orionis, the first, is the privileged prototype. FU Orionis stars are in fact pre-main sequence stars in the early stages of stellar development. They have only just formed from clouds of dust and gas in interstellar space, which occur in active star- forming regions.

At this early stage of evolution, a star is surrounded by an accretion disk, and matter is falling onto the outer regions of the disk from the surrounding interstellar cloud. Thermal instabilities, most likely in the inner portions of the accretion disk, initiate an outburst and the young star increases its luminosity. Our Sun probably went through similar events as it was developing.

FU Orionis is easily visible in a small backyard telescope. It lies in northern Orion near the imaginary shoulder of the great hunter, about 3° northwest of Betelgeuse and less than 2° southeast of Collinder 69. This is a wonderful and yet woefully neglected open star cluster for small telescopes, consisting of the stars that form Orion's "head".

Finder map - field width 10°, stars to magnitude +10.

Asteroids

This H chondrite meteorite is probably a sample of the crust of the asteroid 6 Hebe. R. Pelisson [larger image]

The Beehive star cluster (M44) is always a favorite of beginning deep sky observers. This month, the cluster also serves as a handy guide to novice asteroid hunters. The main-belt asteroid 6 Hebe lies about 10° southeast of M44, at the border between the constellations Leo and Cancer. Glowing at magnitude +8.9, Hebe stands out from most background stars. Although too faint for binoculars to pull in under city lights, it is well within their reach from a dark sky when the Moon is out of the way.

Wait until about midnight, when Hebe is highest above the southern horizon, then use the finder map below to hop over to its rough position with your binoculars or telescope. The tried-and-true method is to detect the asteroid's motion from one night to the next. Make a quick sketch of the star field through which Hebe is passing, and afterwards simply come back a night or two later and compare the field with your sketch to see which dot moved. That is Hebe.

The German astronomer Karl Ludwig Hencke discovered Hebe on July 1, 1847, from his private observatory in Kietz, Driesen. The asteroid measures about 130 miles across and is the thirteenth largest by mass, containing 0.5-percent of the mass of the entire asteroid belt. Hebe is probably the parent body of the H chondrite meteorites, which account for a remarkable 40-percent of all meteorites striking the Earth.

Finder map - field width 10°, stars to magnitude +9.5.

Comets

Comet 17P/Holmes and the famous eclipsing binary star Algol (lower left). Toni Scarmato [larger image]

By their very nature comets are unpredictable, but they are not supposed to be this unpredictable. On October 24, 2007, a "dirty snowball" known as periodic comet 17P/Holmes brightened by nearly a million times, literally overnight. For no apparent reason, it erupted from a very dim magnitude +17 to about magnitude +3, becoming bright enough to see with the unaided eye. At first, comet Holmes was so small that you needed a telescope to see it as anything more than a star- like point. It expanded daily, glowing pale yellow and looking in binoculars and telescopes like a circular "gas bubble" surrounded by a green halo. Although the comet has faded a little in recent days, it is still visible with the naked eye from suburban sites. Holmes currently resides in the far northern sky and does not set until the wee morning hours, unless

you live south of the United States or Europe. Look for it among the stars of the constellation Perseus, which is directly overhead as darkness falls. Shining nicely at about 4th magnitude, 17P/Holmes is easiest to find under a dark sky. If you fail to spot it with the naked eye, try sweeping with binoculars near the 2nd-magnitude star Algol (Beta Persei).

17P/Holmes is one of the so-called "short period" comets, meaning it orbits the Sun in less than two hundred years or has been observed at more than one perihelion passage. It actually takes only 6.9 years to go once around the Sun, ranging from just inside the main part of the asteroid belt to the orbit of Jupiter. The comet is presently 264 million miles from the Sun and 222 million miles from the Earth, having passed perihelion on May 4, 2007.

Finder map - field width 15°, stars to magnitude +8.

If you own a small telescope or a good binocular, you have a chance to see comet 46P/Wirtanen as it streaks through our inner solar system. The comet can be found among the background stars of Pisces the Fishes, and according to recent reports, it should glow at 8th magnitude.

Comet Wirtanen appears as a bright, round fuzz ball roughly 6' across, with no hint of a tail. The key to finding 46P/Wirtanen is to start one hour after sunset from a site that has an unobstructed view of the southwestern horizon. Do not let the magnitude fool you into thinking this will be an easy target from the city. Urban sky glow will easily make the comet disappear.

Comet 46P/Wirtanen, like all other comets, is a chunk of ice and dust only a few miles across. The ice is mostly frozen water and carbon dioxide, and the dust is simple silicates. When far from the Sun, the ice is frozen and the comet is too small and too faint to be seen. But as the comet approaches the Sun, it warms. Sunlight thaws the ice, which evaporates (it does not melt), carrying with it the dust that was embedded in it.

The gas molecules absorb solar radiation, and then reradiate it at another wavelength while the dust acts to scatter the sunlight. The effect of this is the creation of a coma - a spherical envelope of gas and dust (perhaps 60,000 miles across) surrounding the nucleus - and a long tail consisting of gases and more dust particles.

Finder map - field width 15°, stars to magnitude +8.5.

Meteors

The Virginids are a vast complex of a dozen or so radiants that become active in late January and persist until mid-April, without reaching a definite peak. Meteors from this stream appear at a slow speed (about 20 miles per second) from a large radiant that measures 15° by 10° in size. Currently the radiant is located in central Leo, midway between the stars Regulus (Alpha Leonis) and Chertan (Theta Leonis). It rises around 7 P.M. local time, and is well placed for observing after midnight, when activity should pick up. Several meteors per hour may be seen and the later you wait to begin observing, the more meteors you will see.

A bright meteor over the Mediterranean coast southeast of Antalya, Turkey. Tunc Tezel [larger image]

Meteors are about twice as common after midnight because the Earth's rotation is carrying us forward, in the same direction as the Earth's motion in its orbit. In other words, before midnight the meteoroids that the speeding Earth runs into must catch up to observers, for the world is turning us back away from them. Consequently we see fewer meteors (many fail to catch us), and those we do see appear to be moving slower.

After midnight the Earth's rotational velocity is added too, not subtracted from, the orbital velocity and in the early morning we are at the front edge of our planet, meeting the meteors as head-on as possible. Those morning meteors therefore are faster and, as a result, brighter on the average.

Map - Virginids radiant position throughout February.

Some meteors do not belong to any known shower. These are the sporadic meteors, caused by random bits of comet debris spread throughout the inner solar system. They appear randomly across the sky all year long.

On this week's night sky careful observers can expect around twelve sporadics per hour during the morning hours and two during the dark evening.

Observing Aids

Northern Hemisphere's Sky - This map portrays the sky as seen near 40° north latitude at 8 P.M. local time in early February and 7 P.M. in late February.

Southern Hemisphere's Sky - This map is plotted for 35° south latitude. It shows the sky at 10 P.M. local time in early February and 9 P.M. in late February.

Visibility of the Planets - The table provides general information about the visibility of the planets during the current week.

Phases of the Moon - This Moon Phase Calendar shows the Moon's phase for every day in February.

Jupiter's Moons - The diagram shows the positions of Galilean satellites on each day in February at midnight.

Sky Events February 4 - Jupiter is 4° north of the Moon around 1 A.M. EST. Venus is 4° north of the Moon around 7 A.M. EST. February 6 - Mercury is in inferior conjunction with the Sun. New Moon at 10:44 P.M. EST. February 7 - Asteroid 6 Hebe (magnitude +8.9) is at opposition. February 9 - Uranus is 3° south of the Moon around 5 A.M. EST.

The information provided on this page is accurate for the world's mid-northern latitudes. Finder maps for the five naked eye planets are plotted for 40° north latitude, but can also be used from other latitudes close to 40° north. Except the two all-sky maps, all other maps can be used no matter the latitude. Local time (local daylight time during summer) represents the time of the reader.

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