daho
SkiesIdaho
Skies
September 2007
Vol. 4 No. 9
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, Deneb, the lucida of the constellation of Cygnus, the Swan. The word Deneb is Arabic for the word tail, and the name refers to the fact that the star represents the tail of the swan. Deneb is the 19th brightest star in the sky. It, along with the stars Vega and Altair, form what is called the Summer Triangle. The Summer Triangle is an asterism and not a constellation. The Summer Triangle passes directly overhead at 9:00 PM in early September.
Deneb is between 1,600 and 2,600 light years away. That makes Deneb the most distant star you can see with your naked eye. The light of Deneb that you see tonight left the star either just before the fall of Rome or just as Greece was getting ready to lay the foundation of mathematics.
Deneb is a huge star. It has 20 times the mass of the Sun, a diameter 200 times greater than our Sun’s diameter, and is 250,000 times brighter. If it were to replace our Sun, Deneb would fill the orbit of Earth. For Earth to maintain its mild climate while in orbit around Deneb, Our planet would have to orbit Deneb at a distance ten times greater than Pluto currently orbits the Sun. In the classification of stars, Deneb is an “A”, the 3rd hottest category (O and B type stars are hotter than A stars and our Sun is a G type which is the 5th hottest). Deneb has a surface temperature of 16,000 degrees Fahrenheit, which makes it white hot. The supergiant Deneb is so hot that it blows material off of its surface at a rate a thousand times greater than the Sun. This large mass lose is not enough, however, to reduce Deneb to safe levels. Within a few million years Deneb will end its life in a supernova explosion. The core of Deneb is no longer fusing hydrogen into helium. Its core is instead fusing helium into carbon and oxygen.
Deneb is located directly overhead at 9:30 PM in early September and at 8:30 PM later in the month. Look for Deneb as the faintest and most northeast member of the Summer Triangle.
There’s a possibility that Idaho and Oregon will be treated to a meteor storm on the morning of the 1st. Predictions place the storm’s peak at 5:37 AM (4:37 for Oregon), but be prepared to begin observing 20 minutes earlier. Since you need time to dark adapt your eyes, you really ought to get outside no later than 5:00 AM. The odds favor Oregon since the skies will be darker there. For Idaho the dawn will be breaking. So Idahoans may want to head southeast into northwest Nevada or northern California. The meteor shower is the Aurigids and they normally produce a weak shower. But this year earth may slam into the dust left behind the tail of comet Kiess 2,000 years ago. Keep your fingers crossed.
The nearly third quarter moon appears next to the Pleiades on the morning of the 3rd. This will be a pretty sight for your binoculars or rich field telescope. You may want to try using a spotting scope of you have one.
The moon is at third quarter at 10:30 PM (9:30 for Oregon and 11:30 for the Midwest) on the 3rd. That’s a good time, if you’re willing to get up early or stay up late, to go moon watching.
Here’s help finding Mars. The solar system’s fourth planet is to the lower right of the moon on the morning of the 4th. Their distance apart is 5 degrees, or roughly the width of your palm when viewed at the end of your extended arm.
Thirty years ago on the 5th the Voyager 1 spacecraft was launched into space. Its target was Jupiter and beyond. Its twin, Voyager 2 was launched a few weeks earlier, but it was taking the slow road. So Voyager 1, with its late but faster start, got to Jupiter and the outer solar system first. After flying through the Jovian system two years later, Voyager 1 was targeted for the Saturnian satellite, Titan. Its close passage to Titan bent its trajectory upwards and out of the plain of the solar system, ensuring it wouldn’t pass close to another planet. But Voyager 1’s successful reconnoiter of Saturn and its satellites let Voyager 2 fly on to Uranus and Neptune. Today Voyager 1 is the most distant man-made object at over 10 billion miles from the sun.
The Beehive star cluster is three degrees to the upper right of the thin crescent moon on the morning of the 8th. To see this you’ll need to go outside at around 5:00 AM. By then the moon will have risen high enough to be seen with the Beehive in your binoculars. You can wait until later to see them, but the dawn is beginning to break and its light will reduce the number of stars visible in your binoculars. In binoculars you’ll see that the Beehive spans an area slightly greater than the moon. The light of the moon left less than 1-1/2 seconds ago but the light of the Beehive left 500 years ago.
Before there were manned Apollo landings on the moon, there were unmanned Surveyor landings. Forty years ago on the 8th Surveyor 5 was launched to the moon. Helium, used to pressurize the fuel in the Surveyor’s propellant tanks, began leaking early in the mission. So the Jet Propulsion Laboratory modified its descent profile and it successfully landed the spacecraft in the Sea of Tranquility on September 11, 1967. Since two of the first five Surveyors never made it to the moon, Surveyor 5 was the third successful American moon landing. Surveyor 5 landed within 18 miles of its original target and took over 19,000 images over a two month period. One of the scientific instruments Surveyor 5 carried was an alpha backscatter experiment that measured the composition of the lunar surface. The results from this experiment indicated that in the maria the moon’s surface consisted of basalt lava, similar to what we see in Idaho. Later small engines on Surveyor were briefly fired to determine how much of the lunar surface consisted of loose material.
Do you remember Conestoga 1? Well back on September 9, 1982 it was the first attempt to launch a private rocket for profit. Conestoga was to be a low cost launch vehicle. All four of its stages were solid fueled and came from surplus Minuteman missile components. Although the 50 foot tall rocket successfully placed its payload into low earth orbit, there was only one launch of the Conestoga.
The first four satellites of Jupiter were discovered by Galileo in 1610. It would be another 282 years before the next satellite of Jupiter was discovered by astronomer Edward Barnard on September 9, 1892 (115 years ago). Amalthea is a small satellite that orbits closer to Jupiter than the Galilean satellites (which accounts for the difficulty in discovering it). The Barnard who discovered Amalthea has a red dwarf star named after him that you may be familiar with, Barnard’s star. Bernard’s Star is the second closest star to our sun and is moving very fast. In 8,000 years Bernard’s Star will be closer to earth than Alpha Centauri. And it will still be too faint to be seen without a pair of binoculars.
The moon is new at 6:44 AM (5:44 for Oregon and 7:44 for the Midwest) on the 11th. Enjoy the dark skies for the next couple of nights.
The moon is at apogee, or its farthest distance to earth on the 15th. The moon’s farthest distance this month is 252,054 miles.
Just as it is getting dark on the 17th go out and look for the waxing crescent moon. You’ll see the lucida of Scorpius, Antares, three degrees to the upper left of the moon. Jupiter is the brighter star higher still above the moon. You’ll only have a good view of this event between 9:00 and 10:00 PM. The moon will be low in the southwest sky.
Konstantin Tsiolkovsky was born 150 years ago on the 17th. Tsiolkovsky (pronounced like see-ol-kof-ski) was a Russian school teacher who dreamed of space flight. His designs and calculations were decades ahead of their time. Tsiolkovsky’s work inspired others to make his dreams a reality. On the far side of the moon there is a large crater named after him. One of Tsiolkovsky’s most famous statements is, “The Earth is the cradle of the mind, but we cannot live forever in a cradle”. Tsiolkovsky died in 1935.
The moon reaches the first quarter phase on the morning of the 19th. So tonight would be a great time to view the moon through your telescope or binoculars.
Now that the moon is out of the way of the morning skies, you’ll have a good view of the Zodiacal Light from the 21st until early October. To see this band of dust in orbit around the sun, go outside no later than about 5:30 AM (any later and the dawn will begin to interfere). You’re looking for a faint triangular glow of light rising from the east. The glow in dark skies looks like the light of dawn or dimmer portions of the Milky Way. In dark skies I’ve seen it reach half way up to the zenith.
Autumn begins on the 23rd at 3:51 AM (2:51 for Oregon and 4:51 for the Midwest). The Autumnal Equinox is the name given to the point on earth’s orbit where the sun appears to rise exactly in the east and set exactly in the west for everybody, and not just for people living on the equator. Our day is exactly 12 hours long on the equinox, if you ignore refraction by the earth’s atmosphere. For the southern hemisphere spring is just beginning today.
Venus will appear its brightest on the morning of the 24th. In astronomical terminology, its brightness is magnitude -4.5. That makes it the brightest object in the sky after the sun and moon. Venus is 2-1/2 magnitudes brighter than the next brightest object, Jupiter and 3 magnitudes brighter than Sirius, the brightest true star. Since the magnitude scale is a logarithmic scale based on how our eyes perceive brightness, every magnitude is 2-1/2 times brighter then the next. Also, the magnitude scale gets brighter the smaller the number.
The moon is full on the 26th. Since this is the first full moon after the Autumnal equinox, this month’s full moon is the Harvest Moon. Shine on, shine on harvest moon.
The moon is at perigee the next day, the 27th. The distance between the centers of the earth and moon is 223,332 miles.
Salyut 6, a Russian space station, was launched 30 years ago on the 29th. In its four year lifespan, 29 cosmonauts visited the station, with some staying as long as six months. Many of the cosmonauts visiting Salyut 6 were foreigners from the communist block of nations. At the same time, the United States didn’t have a manned space program at all, since the Space Shuttle was facing delays and cost overruns. But Salyut 6 didn’t start out on the right foot. The first spacecraft to attempt to dock with it, Soyuz 25, failed when its automatic systems failed (in four attempts). Because of its limited power supplies Soyuz 25 was forced to return to earth after two days in space. Salyut 6 allowed Soviet cosmonauts to surpass the American record for number of days in space (a record the Soviets hadn’t held in 12 years). Experiments performed on Salyut 6 included imaging the earth and its atmosphere, plant growth, a gamma ray telescope, human physiology, and materials processing in zero gee. On July 29, 1982, the abandoned Salyut 6 was destroyed through a controlled reentry into earth’s atmosphere.
Go outside shortly before morning dawn begins on the 30th (around 5:00 AM). Look overhead and you’ll see a gibbous moon near the Pleiades for the second time this month. Like last time, this is best seen in binoculars.
This Month’s Topic
Exoplanets II
Until 1991 there were only eight known planets in the entire universe (nine if you mistakenly count Pluto). Today astronomers know of another 250 of them. But these planets are extra-solar planets, or exoplanets.
The first exoplanets were not discovered orbiting another star. Astronomers monitoring the radio pluses of a pulsar named PSR 1257+12 detected a periodic variation in its timing. The pulses smoothly began arriving earlier and then arriving later than at their average rate. Something with mass was pulling on pulsar PSR 1257+12 and displacing it slightly. From the period and amount of these timing variations, astronomers were able to deduce the existence of a small solar system of three planets orbiting the pulsar. Either these exoplanets formed from the debris of the supernova explosion that created the pulsar or they are the cores of gas giant planets that survived the supernova explosion.
Exoplanets are detected by their effect on their sun. Currently there are three effects astronomers look for when searching for planets. Either the star’s spectral lines shift, the star periodically dims, or the star flashes brighter.
When a planet orbits a star, the planet’s gravity tugs on the star just ever so slightly. We often think of the planets in our solar system as orbiting the sun which is firmly nailed to the center of the solar system. In reality, the sun and planets orbit around a common center of gravity. That means that while the planets orbit in huge circles around the sun, the sun is orbiting a point in the solar system. Since the sun is so more massive than the planets, the radius of its orbit is so much tinier.
The tiny tugging of a star by a planet is detected, and its magnitude measured, by monitoring the spectrum of the star. The star’s tugging shows up as a tiny shift in the star’s spectral lines. High resolution spectrometry can measure the tiny variations necessary to detect the planet’s effect. By measuring small variations as tiny as one part in 100 million, the movement of a star as slow as one yard per second (or about two miles per hour) can be detected.
The trick to doing this is to make the spectroscope ultra-stable. Any variation within the spectroscope results in noise that will drown out any signs of planets. One way astronomers remove the variations in their spectroscope is to shine star light through a cell containing iodine vapor. The iodine creates a reference of spectral lines where variations in the spectroscope can be detected and subtracted out of the star’s spectrum. Any wobbling of the star’s spectral lines that remains after the spectroscope’s variations are removed, are then attributable to the star and not the spectroscope.
If the planet orbits close to a star, or if the planet is massive, or better yet, the planet is both close and massive, its effect on the star is stronger. The amount of wobble that a planet induces in a star’s spectrum indicates how fast the planet tugs around its sun. The period in which the star wobbles tells us the orbital period of the planet. By combining the mass of the star with information on the star’s wobble, the mass of the planet, its distance from the star can be calculated. But this is only true if the planet orbits the star edge on from our perspective.
The problem with this method of detecting exoplanets is that we don’t know if the orbit is edge-on from our perspective or tilted. The more tilted or inclined the orbit of an exoplanet, the more massive the exoplanet must be to create the observed wobbling. So astronomers need something more to help them learn about the properties of exoplanets. That’s where transits, the second method of detecting exoplanets, comes into play.
Transits occur when an exoplanet passes directly in front of a star from our perspective. When this occurs, the brightness of the star decreases by a small amount. For exoplanets we know today, the decrease in brightness is only around 1%. If astronomers can combine the Doppler detection method with a transit, they can discover not only its true mass and distance from its sun, but also its size and density (which indicates something about it composition).
An exoplanet’s distance from it star indicates the amount of solar radiation it is exposed to. This information allows astronomers to estimate the surface temperature of a planet (making an assumption about the planet’s color, or absorption of solar radiation). Recently the infrared space telescope SIRTF measured the infrared radiation of an exoplanet by comparing the infrared brightness of the star when the exoplanet was in front of the star with the brightness when the exoplanet was behind the star.
The final method of detecting exoplanets relies on the exoplanet’s gravity briefly focusing star light. That effect, predicted by Einstein, is called microlensing. Telescopes looking for planetary gravitational microlensing events monitor thousands of stars by staring into the Milky Way.
Because of the current level of sensitivity of detection methods, astronomers are more likely to detect massive planets in close orbits to their suns (the exception is gravitational lensing, but it hasn’t discovered many exoplanets yet). Astronomers assume there are solar systems in the galaxy that are similar to our solar system, but that they are currently unable to detect them. But the fact that there are so many large planets in such close orbits to their stars (called hot Jupiters) was a surprise to astronomers. Gas giants can only form where they can condense in the cooler outer region of the solar system. In these cooler regions are where atoms and molecules of stuff like water, hydrogen, and helium move slow enough that planets can gravitationally attract and hold on to them.
The existence of hot Jupiters tells astronomers that gas giant planets can migrate. They form more distantly from their sun and then move in closer during the formation of the solar system. Migrating hot Jupiters will either swallow more earth-like terrestrial planets or perturb their orbits. So there’s very little chance for another earth-like planet orbiting close to a star that has a hot Jupiter. However, the possibility of life in such a solar system is still possible on the moons of the hot Jupiter. Perhaps some hot Jupiters have satellites with liquid oceans and comfortable atmospheres.
Exoplanets have an astronomical designation and no formal names like our planets. Exoplanets are simply named after their sun followed with a lower case letter that indicates the order in which the planet was discovered. If this naming scheme was used in the solar system, the Sun would be referred to as Sun a, Mercury would become Sun b, Venus would become Sun c, etc.
Even though astronomers have not discovered earth-like planets yet (but they are getting close), they do have an interesting zoo of exoplanets. Here’s a small sample of what has been found to date.
In 1998 the first planet to orbit a red dwarf star (Gliese 876 b or GL 876 b) was discovered. Since then two additional exoplanets have been discovered in this solar system. The outermost two GJ 876 b and GJ876 c are gas giants that reside in the habitable zone of this red dwarf. While it’s unlikely the two exoplanets could support life, perhaps satellites in orbit around them may.
An exoplanet orbiting the red dwarf Gliese 436 was discovered to weigh slightly more than Neptune. It transits GJ 436 from earth’s perspective and from the amount of dimming the exoplanet causes, the exoplanet is now known to have a density of two grams per cubic centimeter. This implies it consists of half water and half rock. Therefore its structure is a hybrid between Neptune and the earth, with a rocky core and a hot water crust compressed into a solid (hot ice).
2M1207 b is the first planet discovered to be orbiting a brown dwarf, or failed star. However, the International Astronomical Union says this exoplanet may actually be another brown dwarf. Infrared images of this system taken by telescopes on earth show two objects, the brown dwarf and its possible exoplanet companion. Perhaps this is the first time we have seen an exoplanet.
Exoplanet OGLE-2005-BLG-390Lb was discovered in 2006 via microlensing. Instead of measuring the wobble an exoplanet exerts on its sun, microlensing measures the deflection of starlight by the exoplanet’s gravity. This is the only method that can detect planets in distant orbits. It appears OGLE-2005-BLG-390L b is 2.6 astronomical units from its star and has a mass of around six times earth’s mass. It’s so distant from its sun that the planet is very cold.
The first planet to be detected by the transit method is named HD 209458 b and it was discovered in 1999. Spectrometers have since watched this exoplanet transit its sun. The atmosphere of HD 209458 b (unofficially named Osiris) was recently found to contain silicate dust clouds and some water vapor. In addition its atmosphere contains the elements of oxygen, carbon, and sodium. Since it orbits so close to its sun, hydrogen is evaporating from its atmosphere like a comet’s tail. It’s a dark planet that only reflects 30% of the sunlight shining on it. The high temperature of this exoplanet expands its atmosphere until it’s 35% larger than Jupiter’s diameter, even though the planet only weighs 70% as much as Jupiter. Its temperature was measured at 1,300 degrees F by the Spitzer infrared space telescope.
Recently Gliese 581 c was discovered to reside within the habitable zone of its red dwarf. If so, there is a possibility that liquid water and life could exist on this exoplanet. But that’s still a big if. The planet would have to avoid a run away green house effect in order to support life.
Observer’s Handbook 2007, The Royal Astronomical Society of Canada
Space Calendar, http://www.jpl.nasa.gov/calendar/
Night Sky Explorer (software)
Stars, http://www.astro.uiuc.edu/~kaler/sow/
Sky and Telescope, New Track Media LLC, September 2007, vol. 114 No. 3
Voyager, the interstellar mission, 10 July 2007, <http://voyager.jpl.nasa.gov/>
Surveyor 5, 10 July 2007, <http://nssdc.gsfc.nasa.gov/nmc/tmp/1967-084A.html>
Conestoga, 10 July 2007, <http://www.astronautix.com/lvs/constoga.htm>
Mars Global Surveyor, 10 July 2007, <http://mars.jpl.nasa.gov/mgs/>
Malin SSS, 10 July 2007, <http://www.msss.com/msss_images/2007/04/13/>
The life of Konstantin Eduardovitch Tsiolkovsky, 10 July 2007, <http://www.informatics.org/museum/tsiol.html>
Mars Observer, 10 July 2007, <http://en.wikipedia.org/wiki/Mars_Observer>
Gliese 876, 28 July 2007, 26 July 2007, <http://en.wikipedia.org/wiki/Gliese_876>
HD 209458 b, 28 July 2007, 24 July 2007, <http://en.wikipedia.org/wiki/HD_209458_b>
2M1207 b, 28 July 2007, 24 July 2007, <http://en.wikipedia.org/wiki/2M1207b>
For more information on space, astronomy, and robotics in Idaho check out the NearSys webpage, http://nearsys.org.
Dark Skies and Bright Stars,
Your Interstellar Guide