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SkiesIdaho
Skies
June 2007
Vol. 4 No. 6
Idaho Skies is a column for beginning amateur astronomers and those interested in astronomy. Suggestions about the column are gladly accepted by the columnist, at paul.verhage@boiseschools.org
This month look for Coma Berenices and the open star cluster Melotte-111. Coma Berenices is Latin for Berenice’s Hair. Berenice II was the queen of Egypt and wife to King Ptolemy III (he ruled Alexandria from 246 to 221 BC or roughly 100 years after Alexander the Great founded the city). Berenice was known for her long amber-colored hair. When her husband went to war, Berenice promised her tresses to the goddess Aphrodite if her husband would return safely. Ptolemy III did return and his queen kept her word and cut her tresses. They were then placed in a temple of Aphrodite as an offering. Later the tresses were found to have disappeared. Their royal astronomer, Conon, convinced the couple that the tresses had been placed in the heavens and were now visible as what we call Coma Berenices.
Originally the cluster in Coma Berenices was the tuft on the end of Leo’s tail. The cluster was never cataloged as a Messier or NGC object, even though it’s very visible. Instead it was first cataloged by astronomer Melotte as his 111th catalog entry. MEL-111 is 5 degrees across and 280 light years away. At least 37 stars have been identified as members of this cluster. Its brightest stars are 50 times brighter than our sun and its faintest are 1/3rd the sun’s brightness. Our sun, if viewed from MEL-111, would hardly be visible in a pair of 7X50 binoculars. The volumes of the Pleiades and MEL-111 are about the same, but MEL-111 has far fewer stars. Since MEL-111 is a bit older than the Pleiades, some of its original stars have probably escaped the cluster, leaving it star poor.
MEL-111 is too large for a telescope. So use a pair of binoculars to observe this cluster. In binoculars the stars of this cluster will fill most of the field of view. Coma Berenices and MEL-111 can be found high in the west on June evenings. Find Leo and Saturn first and then point your binoculars at MEL-111.
The moon, Jupiter, and Antares form a compact group on the 1st. Look low in the southeast at 1:00 AM for the moon. Antares is the orange star four degrees (eight lunar diameters) west of the moon and Jupiter is the bright yellow-white star six degrees above the moon. They almost fit within your binoculars’ field of view.
Tiny Mercury reaches its greatest distance from the sun on the 2nd. This is a difficult to see appearance, as Mercury will never be very high above the horizon. To find Mercury, first locate the Evening Star in the west at 10:30 PM. Now look down and to the right of Venus, towards the west northwest horizon. The only noticeable star close to the horizon is Mercury. Mercury’s distance above the horizon will only be the width of your palm as seen from your outstretched arm. In two weeks Mercury will be lost in sunlight as it drops back to the horizon, so look for this world soon.
While you’re observing Mercury and Venus this week, there’s a spacecraft doing the same thing. The MESSENGER spacecraft makes a flyby of Venus on the 5th. The flyby is designed to take energy away from MESSENGER. That way the spacecraft can fall into a lower (smaller) orbit that matches speed and position with Mercury. Without today’s Venusian flyby, MESSENGER would need to carry a lot more fuel (weight) in order to slow down enough. Blame it all on earth’s higher orbital velocity. You can read more about the MESSENGER website, http://messenger.jhuapl.edu/
Jupiter’s at opposition on the 5th. Opposition means the planet is opposite the sun in the sky. While astrologers make a big deal of a planet’s location in the sky, astronomers know better. At opposition, a superior planet (that’s a planet that orbits the sun at a greater distance than the earth) is at its closest distance from earth. That means a superior planet is best seen through a telescope when it’s at opposition. At opposition, planets rise around sunset, set around sunrise, and reach their greatest angle above the horizon around midnight. Jupiter remains well placed, but low, for telescopic viewing all summer.
The moon reaches last quarter phase at 5:43 AM (4:43 for Oregon and 6:43 for the Midwest) on the 8th. The last quarter moon appears as a nice half circle covered in lots of maria (lava plains). Since it’s at last quarter, the moon is a morning object for the next two weeks.
Tonight on the 8th, Venus reaches its greatest western elongation. Checking my planetarium program, I see that Venus will be 15 degrees above the horizon at 11:00 PM tonight. That’s roughly the width of your palm and extended thumb (think hitch hiking) when viewed from your outstretched arm.
If you have a telescope you can look for a double shadow on Jupiter. On the 9th beginning at 3:18 AM (2:18 in Oregon and 4:18 in the Midwest), the moons Ganymede and Io will cast simultaneous shadows on Jupiter for the next 50 minutes. This is not visible in binoculars so you’ll need a telescope to see them. Your telescope’s image of Jupiter must be well-focused and of reasonably high magnification. In a telescope that inverts images, Ganymede’s shadow is on Jupiter’s lower left and Io’s on Jupiter’s lower right.
On the morning of the 10th the moon guides you to Mars. At 5:00 AM Mars is the yellowish star below and slightly to the left of the moon. You can’t miss it; Mars is the brightest star in the area. Their distance apart is 5-1/2 degrees, or eleven lunar diameters. That sounds like a large angle, but the moon is smaller than it appears to our eyes.
The moon is at perigee or its closest point to earth on the 12th at 11:00 AM (10:00 in Oregon and noon in the Midwest). The distance between the centers of the earth and the moon is 226,041 miles. Look at your car’s odometer. Has it been driven that far?
The 12th is the 40th anniversary of the launch of Venera 4, a Soviet Venusian lander. The spacecraft entered the atmosphere of Venus directly from space, without first going into orbit around the planet. This way the spacecraft didn’t require fuel to slow down. The Soviets didn’t realize at the time how hostile the planet’s atmosphere would be to the spacecraft. The capsule parachuted down to an altitude of 15.5 miles before it stopped transmitting telemetry due to the high air temperature and pressure.
Not long after it gets dark on the 12th you’ll be able to see the Beehive star cluster one degree below Venus. Look in the west for the Evening Star, the brightest object you’ll see. In binoculars you’ll see a scattering of stars, the Beehive star cluster, just below Venus.
The moon is new on the 14th at 9:13 PM (8:13 in Oregon and 10:13 in the Midwest). So you won’t see the moon for the next couple of days.
Two days after the Soviets launched a spacecraft to Venus, the United States did the same. On June 14, 1967, the United States launched Mariner 5, a spacecraft originally designed for a flight to Mars. Unlike the Soviet spacecraft, Mariner 5 was designed to study the atmosphere of Venus during a flyby of the planet. On October 19th Mariner 5 flew past Venus at a distance of 2,400 miles. The spacecraft returned data on the magnetic fields of interplanetary space and magnetic field, radiation, and ultraviolet emission of Venus.
Recall that Venus passes close to the Beehive on the 12th. Well on the 17th the moon joins them. In binoculars the three of them will look similar to what I have drawn below.
On the 18th the first dwarf planet to be discovered is it’s closest to earth. What is this dwarf planet? Why it’s Pluto. Pluto will only be thirty times farther away from the sun than the earth. So while it only takes sunlight eight minutes and 20 seconds to reach earth, it takes sunlight four hours and 12 minutes to reach Pluto. Tomorrow night Pluto will be at opposition, or exactly opposite the sun in our sky. That means Pluto is at its brightest for the year. However, since Pluto is small and distant, it’s roughly 1,300 times too faint to be seen by the human eye. A telescope of at least eight inches diameter, and probably larger, is needed to see this distant cold dwarf planet.
The moon is near Saturn on the night of the 18th. The farther west you live, the closer together you’ll see these two astronomical bodies. In Idaho, Saturn and the moon will only be 1 degree, or two lunar diameters apart from each other at moonset.
Our moon is two degrees away from Regulus on the night of the 19th. For your best view of this alignment, look for it shortly after it gets dark. Regulus is the brightest star in Leo the Lion and it’s just to the right of the moon.
If weather and technology cooperate, the Dawn spacecraft will be launched for the asteroid belt on the 21st. Dawn is an ion engine powered spacecraft that will spend years traveling to Ceres and Vesta. Ceres is 580 miles across, or about the size of Texas. It’s the largest asteroid, water rich, and uniform in composition. Ceres has bright patches that appear to be frost. So possibly Ceres has a hydrological cycle like earth, where water moves to its surface and forms polar caps. Vesta is often visible from earth with the naked eye. It’s 330 miles across and the only asteroid known to have erupted lava. This makes Vesta more like the moon than the other asteroids.
Summer begins on the 21st at 12:06 PM (11:06 AM in Oregon and 1:06 PM in the Midwest). Called summer solstice, it’s the day with the greatest number of hours of sunlight. We experience our greatest number of daylight hours today because the sun passes its closest to overhead. The earliest sunrise occurred a week ago and the latest sunset will occur a week from now.
The moon is at first quarter on the 22nd at 7:15 AM (6:15 in Oregon and 8:15 in the Midwest). So get your telescope or binoculars out. Here’s a simple map of some lunar maria that you can identify with binoculars.
Two days later (the 24th) the moon is at apogee, or its greatest distance from earth. The distance between earth and its satellite is 251,369 miles. That’s 25,328 miles farther away than it was at perigee on the 12th.
On the night of the 27th and morning of the 28th you’ll find the moon only one degree south of Antares. Antares is the lucida, or brightest star, of Scorpius. It’s a red giant star that is close to the end of its life. In human years, it’s still a long time until Antares blows, but in star years, it’s just around the corner. Say, what’s that even brighter star 7 degrees above the moon? Why that’s not a star, it’s the planet Jupiter.
The moon is full on the night of the 30th at 7:49 AM (6:49 in Oregon and 8:49 in the Midwest). Its brilliance washes out many of the faint nebulas and clusters along the Milky Way, so don’t expect to see much in the way of faint fuzzies tonight.
This Month’s Topic
Emmanuel Kant essentially got it right. In 1755 he believed that the universe was filled with a thin gas. The gas should be unstable and the slightest change in its density would cause it to break it up into collapsing volumes. Random motions within the cloud add up to give it a slight rotation. And just like an ice skater, that rotation gets faster as the cloud shrinks smaller. The rotation within the cloud produces a rotating disk around the equator of the shrinking sphere. In time that equatorial disk could produce a family of planets around the newly formed star. Spiral nebula seen through telescopes looked a lot like Kant’s model of forming stars. So before the Great Andromeda Nebula was understood to be a galaxy, it was believed to be a solar system in the process of forming. Kant’s nebular hypothesis was just a concept in 1755; he had neither the math nor the tools to further develop his idea. Modern astrophysicists today have a more complete view of star formation due to their superior theory and instruments. Here’s the picture they have developed.
Giant Molecular Clouds (GMC) are cool clouds of dust and gas that are light-years across. Normally the dust and gas doesn’t collapse because it moves around fast enough, is hot enough to create supporting pressure, and has a powerful magnetic field that prevents its collapse. But under some conditions, pockets of in the dust and gas begin to collapse. What causes the GMC to become unstable and begin collapsing? The movement of the GMC above or below the plain of the galaxy or even shockwaves from a near by supernova explosion can create pockets of denser dust and gas. There is some evidence that a supernova induced the collapse of our solar system. Found in some meteorites are tiny inclusions of elements with unusual isotopes that just predate the formation of our solar system.
So once a pocket of gas and dust begins collapsing, it’s all over, right? Not quite. Compression heats a gas and hot gas wants to stop collapsing and begin expanding. According to the Ideal Gas Law which you may remember from high school and college chemistry, collapsing a volume of gas to half its initial volume doubles its temperature*. So it appears that a collapsing cloud should eventually get too hot to shrink any further and therefore stars will never form. Since there are stars, how is the collapsing cloud shedding its excess heat and collapsing further? It appears that dust is absorbing the excess energy. However, we know the first stars to form in the universe 13.7 billion years ago didn’t have dust and so they were probably much more massive than stars we see today. It would take their immense weight to let gravity to overcome a hot gas’ hesitance to collapse.
One property of matter that must be conserved during collapse is angular momentum. The angular momentum of a body is calculated by multiplying its radius by its speed of rotation (in reality it can be more difficult because you have to calculate and add up the angular momenta of every piece of the cloud). To be conserved means its value (or amount) never changes. Therefore, if the radius of the cloud shrinks, its rate of spin must increase. The perfect example of this is when a spinning ice skater brings her extended arms close to her sides. As she does so her rate of spin increases.
The infall of material at the poles of a spinning cloud and the resulting increase in its spin rate changes its sphere shape into a disk. The gas and dust falling into the growing star’s equator in called an accretion disk. For the growing star to grow larger, the large angular momentum of the inside edge of the accretion disk must be moved into its outer edges. As it loses its angular momentum, material in the inner edge of the accretion disk can fall into the forming star, or proto star.
Along with spin rate, the strength of the clouds magnetic field also increases as it gets smaller. The proto star’s accretion disk and magnetic field may be responsible for the bipolar outflow or jets seen in forming stars. Bipolar jets are fountains of dust and gas shooting out of the poles of a proto star.
If it wasn’t for the bipolar flows, the proto star’s radiation pressure and solar wind would eventually halt the infall of gas from the accretion disk, choking off the proto star’s supply of gas. This choking off would limit the mass of stars to around eight solar masses. But we know that stars of much greater mass are forming today.
At a temperature of around six million degrees, heavy hydrogen, or deuterium, begins to fuse. If the cloud is limited to a mass 13 to 75 as great as planet Jupiter, then the crush of dust and gas can’t compress the proto star any further and it gets no hotter. A star that can only fuse deuterium is called a brown dwarf. Brown dwarfs finish fusing their deuterium in a few tens of millions of years. After that, they slowly cool down. Astronomers can identify brown dwarfs because they still contain lithium, an element that’s destroyed by the fusion temperatures of real stars.
A more massive cloud can shrink into a smaller and hotter sphere. And at a core temperature of around 15 million degrees the new star will begin fusing hydrogen into helium. The fusion process contains multiple steps that involve the fusion of protons into heavier elements and two light helium nuclei fusing into a common helium nuclei with the release of two protons which are used at a later time. Once hydrogen fusion is established, the crushing weight of the star is balanced by the expanding pressure generated by the heat of fusion inside the core. At this point the star has entered the Main Sequence and slowly ages.
The birth of a new star is invisible to our telescopes. That’s because a baby star is swaddled in a dusty cocoon that visible and ultraviolet light cannot penetrate. However, through infrared astronomy astronomers have seen the light of these new stars. At some point the emission of light (photons) blows the rest of the dust and gas away from the star, stopping its growth. Now the new star is visible in our telescopes.
* The doubling of temperature only works for an absolute temperature scale. Absolute temperature scales are those with no negative values and that begin at absolute zero, or the coldest temperature possible. There are two of these temperature scales, Rankin and Kelvin. The Rankin scale uses the Fahrenheit degree, but shifts the scale downwards over 500 degrees so that it begins at absolute zero. The Kelvin scale uses the Celsius degree and shifts the Celsius scale down by 273 degrees.
Observer’s Handbook 2007, The Royal Astronomical Society of Canada
Space Calendar, http://www.jpl.nasa.gov/calendar/
Night Sky Explorer (software)
Kaler, Stars, <http://www.astro.uiuc.edu/~kaler/sow/>
Johns Hopkins University Applied Physics Lab, MESSENGER, <http://messenger.jhuapl.edu/>
Goddard Spaceflight Center, <http://nssdc.gsfc.nasa.gov/nmc/tmp/1967-060A.html>
http://nssdc.gsfc.nasa.gov/nmc/tmp/1967-058A.html
Jet Propulsion Laboratory, Dawn, <http://dawn.jpl.nasa.gov/>
Burnham’s Celestial Handbook, 1978
University of Chicago, Star Formation, 11 Nov 1995, <http://archive.ncsa.uiuc.edu/Cyberia/Bima/>
Wikipedia, Star Formation, <http://en.wikipedia.org/wiki/Star_Formation>
Dark Skies and Bright Stars,
Your Interstellar Guide