D e s e r t E x p o s u r e
June 2010
Constellation of the Month: Crux, the Cross
There are two crosses in the sky. The Northern Cross is actually the constellation Cygnus, which is visible only in the morning sky during June. The other is the constellation Crux, the Cross, which is so far south that only one of its five major stars edges over the southern horizon on June evenings. The northernmost star of Crux, the second-magnitude Gacrux, is half a degree above the southern horizon around 8 p.m. Observing it is virtually impossible, since the Sun is still above the horizon at that time.
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Crux, the (Southern) Cross, just barely comes above our southern horizon, shown here as a white line. This chart shows stars that are actually below our horizon and not visible from the desert southwest. From the southern hemisphere, Crux is a very obvious constellation that was carved out of Centaurus. A dark nebula called the Coalsack calls this constellation home. It is a cloud of dust and gas that is not illuminated by a nearby star, so it appears dark and it strongly obscures the stars behind it. |
Crux is the smallest of the 88 official constellations recognized by the IAU, only 68 square degrees in area. In antiquity (some 4,000 years ago), Crux was visible in the Mediterranean area and was known to the Greeks. Over the centuries, precession took Crux farther south and the Greeks forgot its existence. Having been lost to the ancient civilizations, none of the stars in this obvious constellation received either Latin or Arabic names, so modern (and unimaginative) names were invented instead.
Four main stars form the cross, with a fifth stuck in the southwest quadrant. The brightest star is Acrux (Alpha Crux), the southernmost star of the cross, which shines at magnitude 0.8. It is actually a multiple-star system, with the two visible components 4 seconds of arc apart. These two are magnitude 1.4 and 2.1 and both are blue B-class stars. They take at least 1,500 years to orbit each other, but since they have barely moved since the invention of the telescope, that figure might be bigger. The brighter star is itself a binary star, but the companion is so close we know it is there only by its spectroscopic signature. The companion takes only 76 days to orbit the primary and is some 12 times more massive than our Sun.
Going clockwise from Acrux, Mimosa, also know as Becrux (Beta Crux), is the next brightest star at magnitude 1.3. The name Mimosa is a recent one drawn from the color of the star. This is also a multiple star, but like the brightest star in the Acrux system, its components are too close to split with a telescope. The secondary takes five years to orbit the primary star.
The next star is the one that comes above our horizon, Gacrux (Gamma Crux), a red giant that shines at magnitude 1.6. Some 88 light-years away, it is a semi-regular variable star that pulses with a period of about 90 days. There is also a white star along the same line of sight that appears some two minutes of arc away from Gacrux, but it's unrelated.
The last star in the cross is Delta Crucis, also a B-class star that has stared to evolve into a red giant. It is magnitude 2.8. The fifth star is Eta Crucis, hard to see at magnitude 4.1.
This small constellation boasts the most prominent dark nebula in the sky, the Coalsack, which overlaps neighboring constellations Centaurus and Musca. The Coalsack appears as a dark patch about five by seven degrees. It is a large cloud of dust and gas about 600 light-years away. Unlike bright nebula, the Coalsack has no bright stars near it, so there is no starlight to shine off the dust particles, like blue-colored reflection nebulae. Nor is there ultraviolet light to energize the gas and make it glow like red-colored emission nebulae. The Coalsack was given its name by British sailors who saw it as they sailed into the southern hemisphere.
The Coalsack also absorbs the light coming from the stars behind it, making the area appear bereft of stars. Even though it looks totally black, the dust and gas are still illuminated by the distant stars of our galaxy, so it is glowing, but at only 10% of the brightness of the surrounding sky.
This dark nebula is so obvious in the sky that it was known in prehistoric times, but only in the southern hemisphere. It was first recorded by Vincente Yanez Pinzon in 1499. It was named "il Canopo fosco" (the dark Canopus) by Amerigo Vespucci and was also called the "Black Magellanic Cloud," as opposed to the bright Magellanic Clouds.
The Planets for June 2010
Venus appears in our western sky as it gets dark. Moving from central Gemini through Cancer and just barely into Leo during June, Venus is still 76% illuminated as it continues to swing around the Sun toward us. At midmonth, Venus' disc is 14.8 seconds-of-arc across as its distance to us decreases. Venus is a brilliant magnitude -4.0 and finally drops below the western horizon by 10:50 p.m.
Mars is moving eastward in Leo, passing 0.9 degrees north of the first-magnitude star Regulus on June 6. It appears a third of the way up in the western sky as it gets dark and sets by 12:30 a.m. Mars is falling behind the Earth, making Mars appear smaller (5.6 seconds-of-arc across at midmonth) and fainter (magnitude 1.3).
Saturn is just over the border from Leo in Virgo. It moves very slowly eastward during June, having reached its stationary point at the end of last month. Glowing at magnitude 1.0, the Ringed Planet's disc is 17.6 seconds-of-arc across. The rings are 39.9 seconds-of-arc across, tipped down 1.8 degrees with the northern face showing. Saturn sets at 1:40 a.m.
As Saturn goes down in the western sky, Jupiter will rise in the eastern sky. With a disc that measures 36.9 seconds-of-arc across, Jupiter glows at magnitude -2.4. During the month Jupiter will continue to move eastward along the ecliptic in Pisces.
Watch the Skies (all times MDT)
June 4, 4:13 p.m. — Last Quarter Moon June 6, 8 a.m. — Mars 0.9 degrees north of Regulus Noon — Jupiter 0.5 degrees south of Uranus June 12, 5:15 a.m. — New Moon June14, evening — Venus near the Moon June 18, 10:29 p.m. — First Quarter Moon June 21, 5:28 a.m. — June Solstice, summer begins June 26, 5:30 a.m. — Full Moon, partial lunar
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The Messenger of the Gods will be visible low in the east-northeast shortly before sunrise. Mercury passed its farthest point from the Sun at the end of May, but can still be glimpsed for the first week or two of June. This is not a favorable apparition for northern-hemisphere observers. On June 1, Mercury will be magnitude 0.1 and its disc 7.2 seconds-of-arc across and shrinking. The 51% illuminated disc will grow fuller as it moves away from us to pass behind the Sun on June 28.
The June Solstice occurs Monday, June 21, at 5:28 a.m., marking the beginning of summer in the northern hemisphere and maximum daylight. Five days later, the full Moon will partially pass through the Earth's shadow for a partial lunar eclipse. The Moon enters the penumbral edge of the Earth's shadow at 2:57:21 a.m., but the subtle shading will probably not be visible for another 20 or 30 minutes. The Moon will continue into the shadow, entering the umbra where no direct sunlight can reach its surface at 4:16:57 a.m. Maximum eclipse occurs at 5:31:34 a.m. when 54% of the Moon will be in the umbra. For us, the eclipse will end at 6:03 a.m. when the Moon sets. While there is no high science to be done, it is always fun to watch a lunar eclipse. So get up early June 26 and "keep watching the sky"!