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SkiesIdaho
Skies
March 2007
Vol. 4 No. 3
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 the star Adhara in the constellation of the Big Dog, Canis Major. Canis Major has two fairly bright stars; with its brightest being the most brilliant star in the sky. The next brightest star of the constellation is Adhara. If it wasn’t for nearby brighter Sirius, Adhara would stand out as a reasonably bright star on its own. Astronomers call Adhara Epsilon Canis Majoris, but since we’re on informal terms with the stars, we can just call it Adhara.
Adhara is 432 light years away from earth. That means the light we see from it tonight left in the year 1582, the year the western world began making the transition from the older Julian calendar to the newer Gregorian calendar. Since Sirius is just under nine light years away, Adhara is 50 times further away from the sun than Sirius. If Adhara was as close to us as Sirius, Adhara would appear seven times brighter to us than it does now. So when viewed from the same distance as Sirius, Adhara is more than twice as bright as Sirius.
Its spectrum tells astronomers that Adhara has a surface temperature of 36,000 degrees. That makes the star nearly four times hotter than our sun. Including ultraviolet radiation with its visible light, Adhara emits 15,000 times more light than the sun. It’s this luminous because Adhara is 12 times heavier than the sun (the star’s greater mass causes hydrogen, its fuel, to fuse faster). If Adhara replaced our sun, we’d have a new sun that’s 13 times larger and over 3,000 times brighter (our atmosphere would block much of Adhara’s ultraviolet radiation, so we wouldn’t see it). Needless to say, we couldn’t live on the earth if it orbited Adhara.
Look for Adhara tonight. It’s the lower right star in Canis Major. You’ll find the Big Dog almost due south at 10:00 PM in early March.
You’ll find Saturn close to the moon on the evening of the 1st. At 8:00 PM (that’s when it gets dark) look for the waxing gibbous moon in the east. To the moon’s upper right and one degree (two lunar diameters) away is a moderately bright pale yellow star. That’s Saturn. In a small telescope Saturn’s largest satellite Titan is located to the planet’s upper right.
Twenty-five years ago on the 1st, Venera 13 landed on the planet Venus. Venera 13 was essentially a spherical pressure vessel mounted on top of a doughnut shaped landing pad. Venera 13 detected lightning in the Venusian atmosphere during its descent and provided our first glimpse of the planet’s surface after its landing. An analysis of material drilled from the planet’s surface indicated that the region around the lander was a lava field. Venera 13 continued to relay data to its orbiter for over two hours before failing due to the 850 degree air temperature. That’s all folks!
March 2nd is the 35th anniversary of the launch of Pioneer 10, the first spacecraft to fly beyond the asteroid belt and explore mighty Jupiter. The successful passage of Pioneer 10 through the asteroid belt proved there’s too little material in the belt to pose a risk to spacecraft (our asteroid belt looks nothing like the asteroid belt in the movie, The Empire Strikes Back). As it passed 80,000 miles above the Jovian cloud tops, Pioneer 10 returned images of Jupiter that were three times better than the best images earth-bound telescopes could take at the time. Passing this close to Jupiter exposed Pioneer 10 to more than enough radiation to kill a human passenger, had there had been one.
The gravity of Jupiter flung the hardy Pioneer hard enough for it to escape the solar system. In two million years Pioneer 10 will coast past the star Aldebaran, the brightest star in Taurus the Bull. If anyone is there looking for it, they’ll find a gold colored plaque bolted to the spacecraft. Etched in the plaque are drawings that indicate something about the designers and home of Pioneer 10. It’s been four years now that Pioneer 10 stopped communicating with earth. Most likely its plutonium power source has decayed too much to adequately power the spacecraft.
As the moon rises on the 3rd (6:30 PM), it’s leaving the earth’s shadow. The further east you watch moon rise, the more of this lunar eclipse you’ll see. Unfortunately this is a poor lunar eclipse for the US since we’ll only see the last bit of it. A much nicer lunar eclipse takes place in August.
Four days after its twin Venera 13, the Venera 14 spacecraft also landed on Venus. On March 5th, 1982, the Soviet Venera 14 dropped in for a nearly one hour exploration of the planet’s surface. Like its twin, Venera 14 sampled the ground near the lander and returned images of the surface. The region around Venera 14 was volcanic, just like the surface around Venera 13. At the end of nearly an hour on the surface, the Venera 14 lander, now barbeque, failed.
Beginning on the evening of the 6th, the Zodiacal Light becomes visible in the evening. March is a particularly good month to look for the Zodiacal Light because it rises so steeply. So shortly after it gets dark, look for a faint glowing pillar of light rising in the west. From a dark location the Zodiacal Light can be seen rising half way up to the zenith (overhead).
The moon is at apogee on the 6th at 9:00 PM (8:00 for Oregon and 10:00 for the Midwest). Its distance from earth (its greatest for March) is 252,185 miles.
Until the late 1970’s, the only planet known to have rings was Saturn. That changed 30 years ago on March 8th, 1977. A telescope mounted inside an aircraft, where it flew above most of the atmosphere, was monitoring the brightness of a star as it was occulted by Uranus. Occultations occur when a planet (or the moon) passes between the earth and a distant star, cutting off its light. Planetary astronomers like occultations because monitoring the star’s brightness during the occultation gives clues to the planet’s atmospheric structure. To their surprise astronomers detected several small decreases in the star’s brightness before and after it passed behind Uranus. Since the dips in the star’s brightness were symmetric with respect to Uranus, the dips were most likely created by a ring around the planet. Had it been due one moon or several moons, the dips would not have been symmetric around the planet.
Uranus is now known to have eleven dark and narrow rings. Instead of bright ice like Saturn’s, the rings of Uranus are made of rocky material and dust. The rings are between 1 and 60 miles wide. Stacked together, they span a width of 8,000 miles. Some of Saturn’s rings on the other hand span a width close to 200,000 miles.
Don’t forget to set your clock ahead one hour on the night of the 10th. Daylight Saving Time begins at 2:00 AM on the 11th, four weeks earlier this year than last year. Daylight Saving Time allows some of our activities to occur later in the day, when there is still adequate sunlight. The earlier start of Day light Saving Time this year is an attempt to reduce our reliance on energy and therefore foreign oil. Of course, we could also stop driving Hummers and other SUVs to pick up groceries.
At 10:00 PM (9:00 for Oregon and 11:00 for the Midwest) on the 11th the moon reaches the third quarter phase. The third quarter moon looks like a half moon like first quarter. But unlike first quarter, the third quarter moon is visible after midnight (rather than before) and the moon’s western hemisphere (rather than eastern) is in sunlight.
At 1:00 PM (noon for Oregon and 2:00 for the Midwest) on the 18th the moon reaches perigee, or its closest approach to earth. Since perigee occurs on the same day as new moon, the increased gravity of the moon lines up with the sun’s gravitational pull to give us larger tides today.
The moon is new on the 18th at 10:43 PM (9:43 for Oregon and 11:43 for the Midwest). If we were living on the other side of the world, we’d be able to watch a solar eclipse today. Solar and lunar eclipses often happen two weeks a part. But in our case this year, both eclipses this month occurred when the sun or moon were not well placed for the US.
Hip hip hooray, Spring beings on the 20th at 6:07 PM (5:07 for Oregon and 7:07 for the Midwest). The point at which the sun crosses the equator from the southern hemisphere (going north) marks the beginning of Spring and is called the Vernal Equinox. In a world without an atmosphere the day and night are equal in length (12 hours long) on the day of the equinox. From this day on (at least until the bringing of Autumn), the days grow longer and the nights shorter.
Late on the 22nd, shortly before moonset, you’ll find the Pleiades close to the moon. According to my planetarium program, at 11:00 PM, the Pleiades (M-45) is ½ degree to the crescent moon’s upper left. Since the moon is a crescent and the Pleiades star cluster is on the dark part of the moon, the cluster’s light is less affected by moonlight. So this should be a nice view for your binoculars.
Wernher von Braun was born 95 years ago on the 23rd. Von Braun (1912-1977) so desired to begin the space age that he was willing to work with an evil dictator to make it happen. Although no doubt, he would have preferred to pursue his dream in the United States. Science fiction writers and the science writer Hermann Oberth created his intense interest in space flight when he was young. Initially von Braun worked with a group of amateur rocket enthusiasts called the VfR (Verein für Raumschiffahrt or Spaceflight Society). Due to the Versailles Treaty, Germany was prevented from developing large artillery during this time (late 1930s). So the German military acquired an interest in rocketry as a “legal” way to get around the limitations of the treaty. The German military provided funding to the VfR in the hopes that the civilian club would develop successful rockets with military capability. Eventually there was enough success that the German military changed the VfR a military research program and put the young von Braun, who demonstrated knowledge, leadership, and management skill, in charge of the missile program. During tests of the von Braun’s V-2, it achieved a peak altitude of 117 miles, making it the first vehicle to reach the boundary of space. But Hitler, thankfully, didn’t adequately support the missile program until late in the war. As a result, Germany spent a lot of money very late developing a weapon that didn’t make much a difference in the war’s final outcome. In fact, the money spent developing the V2 would have better been spent (from Germany’s point of view) on conventional weapons. Had the V2 been fully funded early in the war, things could have turned out different. That’s because von Braun’s team was designing the A9 and A10, missiles that would eventually lead to a two stage missile capable of reaching the shores of the United States.
After World War II, V-2 parts, von Braun, and many of his engineers were brought to the United States (before the Soviets could find them) to test and eventually develop our missile program. Von Braun hoped the US would begin a space program, but we were shortsighted and only wanted to learn how to build and fly missiles. Von Braun knew how to reach the American public through; he wrote articles for popular magazines like Colliers and developed entertaining educational and promotional television programs for Walt Disney. It wasn’t until the launch of Sputnik 1 that von Braun was given a real chance to launch satellites. After the beginning of the moon race, von Braun lead the United States, his new home, on the greatest adventure of the 20th century. It’s sad that he and his V-2 missile have such a mixed history.
The moon is at first quarter on the 25th at 11:00 AM (10:00 for Oregon and noon for the Midwest). Tonight would be a great night to look at it with your telescope or pair of binoculars.
Look to the moon’s lower left on the night of the 27th (the best time is around 11:00 PM). There you’ll see a large sprinkle of stars called the Beehive Star Cluster (known to astronomers at M-44). This will be nice through your binoculars.
It’s sort of anticlimactic, but Venera 8 landed on Venus 35 years ago. On March 27, 1972, the Venera 8 spacecraft went into orbit around Venus after releasing a lander. The lander’s parachute opened at an altitude of 36 miles and the capsule began refrigerating itself in preparation for the harsh conditions it would experience upon landing. During the parachute descent, the lander detected a cloud bank at an altitude of 22 miles. Winds below six miles altitude were blowing at less than ½ mph. For 50 minutes the lander reported on the conditions at the surface, including an atmospheric pressure 90 times greater than that of earth and air temperatures greater than 850 degrees (that’s hotter than your oven). Even though Venus is cloud shrouded, enough sunlight filters through the clouds that from its surface it looks like a typical overcast day here on earth.
Saturn is close to the moon for a second time this month. Look for the ringed planet less than one degree south of the moon on the 28th. Closest approach takes place at 10:00 PM (9:00 for Oregon and 11:00 for the Midwest).
Alpha Leonis is just over a degree from the moon’s right on the evening of the 29th. Don’t know who alpha Leonis is? Why that’s the star Regulus, the brightest star in Leo the Lion. This is another sight for your binoculars.
This Month’s Topic
It may sound like something atoms do, but carbon dating is really about using a well understood property of atoms to determine how long ago organic materials stopped absorbing carbon from the atmosphere (that’s generally when they die). And in most cases that’s close to the age of objects. Any living object that’s exposed to air, either through breathing it like plants or eating the plants like animals, can be carbon dated. So while carbon dating works for dating plants and animals, it does not work for dating metal or rocks. The oldest objects that can be dated with carbon dating is around 60,000 years. How can we date objects based on their carbon content and why can we only date them back to 60,000 years?
First we need to take a close look at the carbon atoms. An atom of carbon has a nucleus with six protons and six neutrons. Each proton weighs nearly as much as each neutron, so a carbon atom has a weight of twelve atomic mass units. Note that the six electrons in orbit around the carbon nucleus are relatively weightless, so we don’t include their weight.
It was discovered that there are other carbon atoms with slightly greater weight. A small percentage of carbon atoms have an additional neutron in their nucleus. Since the neutron is neutral (has no charge) there are no additional electrons in orbit around this heavier carbon atom. The weight of this atom of carbon is 13 atomic mass units. We call this type of carbon, carbon-13 for short. It shouldn’t surprise you then that the original carbon is called carbon-12 by physicists and chemists. Carbon-13 (or C-13) with its extra neutron is stable, so it will never decay. Since the number of protons in each atom is the same (remember, it’s only the number of neutrons that changed), there’s no need to add more electrons. And since it still has the same number of electrons around it, it doesn’t behave any differently chemically. So in almost any chemical compound, you can replace a C-12 atom with a C-13 atom and never notice the difference in its chemical behavior (there can be a small change in how fast the atom chemically reacts because of its additional weight).
When an element, like carbon, has more than one form of the atom, we call the new forms isotopes. So C-12 and C-13 are carbon isotopes. Remember, the only difference between the isotopes of an atom is the number of neutrons in the nucleus. If the number of protons is changed, then we have created a new element and would no longer be dealing with isotopes of the same element.
Carbon, it turns out, has one more isotope. This one has six protons (because it’s carbon) and eight neutrons. This isotope is called carbon 14, or C-14. With the additional neutron crowding inside the nucleus, C-14 is unstable. Eventually one of the extra neutrons inside the nucleus of C-14 will split into a proton and electron. The proton will remain inside the nucleus and the electron will be emitted as a beta particle (so beta radiation is just high energy electrons emitted by an atomic nucleus). Since a neutron has become an additional proton, the carbon atom becomes a stable nitrogen atom with seven protons and seven neutrons.
The time and date that a radioactive atom will decay is not something you can calculate. Since it’s a quantum event, the decay of a neutron inside an unstable C-14 nucleus is a random event. This doesn’t mean radioactive decay is can’t be predicted. The chance that a neutron inside a C-14 nucleus will decay within a given time can be calculated. For C-14, half the atoms inside a sample will decay in 5,740 years. Which ones and exactly when they will decay is not known and cannot be determined before hand. The time required for half the atoms in any sample to decay is characteristic of the isotope and is called the isotope’s half life.
If half the C-14 in the world decayed in less than 6,000 years and the earth is 4-1/2 billion years old, then why is there still C-14 left around? That’s because some cosmic rays are high energy neutrons. When a cosmic ray neutron collides with a nitrogen-14 atom in the atmosphere, the nitrogen-14 nucleus converts into a C-14 nucleus. The rate of creation of C-14 is slow, but balanced by its radioactive decay. So the amount of C-14 in the atmosphere remains constant (not quite, there’s more on that later). Carbon-14 created by cosmic rays eventually combines with oxygen to form a carbon dioxide molecule. Every plant will inhale and accumulate some of this C-14 in their bodies. When animals eat those plants they uptake the C-14 into their bodies. The ratio of C-14 to C-12 in plants and animals remains constant as long as they live. So right now, about one of every trillion carbon atoms in your body is a radioactive C-14 atom.
It is the half life of a radioactive element that makes it so useful for dating events. But to use C-14, first the object has to die. For as long as it breathes air or eats plants that do, the amount of C-14 absorbed is equal to the amount of C-14 that decays. But once an object, like a tree or animal dies, no more C-14 is absorbed into its body. From that point on, the amount of slowly C-14 decreases as C-14 atoms radioactively decay. Remember, C-12 and C-13 are not radioactive. So the amount of original C-12 and C-13 remains constant (if they’re not replaced with mineral during fossilization). Therefore the ratio of C-14 to C-12 and C-13 decreases as time passes. So at the moment of death, one of every trillion carbon atoms will be a C-14. In 5,740 years one of every two trillion carbon atoms is now C-14. Wait another 5,740 years (almost 12,000 years after death) and there’s only one C-14 atom for every four trillion carbon atoms. That sounds like a tiny number of atoms to deal with, doesn’t it?
However, if only 10% of your mass is in carbon, then every four ounces of your flesh would contain in the neighborhood of 600,000,000,000,000,000,000,000 carbon atoms. That’s 600 billion trillion atoms. Therefore right now there are 600 billion C-14 atoms for every four ounces of your body weight. Six thousand years from now that will drop to 300 billion C-14 atoms. In 60,000 years, the greatest date that carbon-14 can be used to date, there will still be around 600 million C-14 atoms in every four ounces of your body.
Now scientists do not counts trillions of atoms to find how many are C-14. In the old days (1950s), a radiation detecting device (these were more sensitive than Geiger counters) was used to detect the radiation emitted by a decaying C-14 atom. The more decays per second, the more C-14 in the object, and therefore the younger it was. But since C-14 has a half life near 6,000 years, radiation detectors can only be used to date large objects. Today, particle accelerators and mass spectrometers are used to isolate C-14 from the rest of the carbon in a sample. This method lets researchers date smaller samples with more precise dates.
One of the discoveries made with carbon dating is that earth’s cosmic ray flux has not been constant over time. Solar activity can shield earth from cosmic rays. When the sun is more active, it reduces the number of cosmic rays entering into our atmosphere. That also means it reduces the creation of C-14. This was discovered by counting tree rings. Each ring in a tree is grown during a one year period. By counting tree rings from Bristle Cone Pine trees and measuring their C-14 content, the discrepancy between the carbon date and the actual age was discovered. Thanks to ancient trees, we know how much the cosmic ray flux has changed over time. This has also let researchers create correction tables for measuring C-14 dates that go back to around 10,000 years ago.
There are limitations to carbon dating. First it can only be used on living objects that were well exposed to the atmosphere during their life time. Also, they can’t be older than about 60,000 years or else there will be too little C-14 to measure (instruments have limitations to their accuracy and precision). There are many other objects that we’d like to date that are either too old or are inorganic. For these objects (like rock layers), we need new radioactive isotopes. There are over a dozen radioisotopes used by geologists today and they point to a solar system that’s 4-1/2 billion years old. It’s amazing what you can do with a handful of atoms.
Observer’s Handbook 2007, The Royal Astronomical Society of Canada
Space Calendar, http://www.jpl.nasa.gov/calendar/
Night Sky Explorer (software)
The One Hundred Greatest Stars, James Kaler
Venera 13, http://nssdc.gsfc.nasa.gov/nmc/tmp/1981-106D.html
Voyage to Jupiter, NASA
http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhome.html
http://cfa-www.harvard.edu/iauc/03000/03051.html#Item1
http://www.enchantedlearning.com/subjects/astronomy/planets/uranus/uranusrings.shtml
http://geography.about.com/cs/daylightsavings/a/dst.htm
http://www.hq.nasa.gov/office/pao/History/sputnik/braun.html
http://nssdc.gsfc.nasa.gov/nmc/tmp/1972-021A.html
http://en.wikipedia.org/wiki/Radiocarbon_dating
Dark Skies and Bright Stars,
Your Interstellar Guide