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SkiesIdaho
Skies
February 2007
Vol. 4 No. 2
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, Sirius. Sirius is the lucida of the constellation of Canis Major, the Big Dog. It’s half way up in the sky when you face towards the southwest in February. Sirius is the brightest star in the heavens, and only the planets Venus, Mars, and Jupiter can surpass its brilliance. Sirius is bright white and throws off shards of color when it’s low to the horizon (there’s more atmosphere lower to the horizon to refract its starlight). If you were born in 1998 then Sirius is your birthday star this year because the light you see tonight left Sirius 9 years ago. The name Sirius comes from the Greek word for scorching. During the Dog Days of summer (early August), the sun and Sirius are close together in the sky. The Greeks believed the additional heat of Sirius added to the sun’s heat, making these days especially hot.
Sirius has a strange companion. In the early 19th century, astronomers discovered that Sirius was shifting back and forth as it slowly drifted across the sky. It was as if something massive was tugging on the star. But try as they may, no astronomer could discover what that object was. It wasn’t until 1854 when Alvan Clark turned his new 18 inch refracting telescope to the star that he discovered a tiny spark of a star next to Sirius. The star was not all that faint, but its closeness to Sirius made it impossible to see with previously inferior telescopes.
The companion to Sirius, called the Pup Star, orbits Sirius with a period of fifty years. From the amount of tugging Sirius experiences from this star, we know the pup star has a mass equal to our sun. But our sun viewed from only 8.7 light years away would easily be visible to the eye. But the Pup isn’t, so what gives? Stars like the pup star have a spectrum that indicates they’re very hot. Therefore every square foot of these stars are brighter than an equal area of our sun. This implies that stars like the pup star are so faint because they’re very tiny (some where around the size of our planet). As best as 19th century astronomers knew, stars hotter than the sun should also be larger the sun.
This makes the pup star the first white dwarf star to be seen in a telescope. Once white dwarfs were like our sun, moderately bright and moderately large. But their nuclear fuel ran out and the stars collapsed on themselves. No longer is fusion supporting white dwarfs. Only the dislike of electrons for each other keeps them from collapsing any smaller. The compression of matter to white dwarf levels makes them incredibly dense. A billiard ball of white dwarf would weigh as much as a tank (around the weight of 100 family cars). In several billion years our sun will undergo the same fate and collapse to become a white dwarf.
A nice sight for your binoculars or small telescope can be found near Sirius. It’s the open cluster M-41. The cluster is scattered across an area as large as the full moon, so use low power optics. In a small telescope you can see up to 50 stars in this cluster.
● The moon passes close to Regulus and Saturn on the night of the 3rd
● The Zodiacal Light becomes visible after sunset during the first week of the month
● There’s a close alignment of three planets on the 7th.
● There’s a close alignment of three planets on the 7th.
The moon is full on the 1st. The full moon occurs when the moon is as opposite the sun in the sky as it can get. There’s only one point that’s exactly opposite the sun, so technically, the moon is full for only an instant of time (11:45 PM this month, which is 10:45 for Oregon and 12:45 AM the next day for the Midwest). But as far as the human eye is concerned, the moon is full nearly all day. The full moon in February is often called the Snow moon because this month usually has the most snow for the year.
On the morning of the 3rd the moon passes very close to the brightest star of Leo. Look for the moon at 6:00 AM and you’ll find Regulus one degree to the left of the moon. Over twice as far away on the moon’s left is another star, a yellow-white star that doesn’t blink. That’s Saturn.
Beginning the evening of the 4th, the Zodiacal Light is visible in the west after sunset. Wait until it gets dark and look for a pillar of faintly glowing light. The faint pillar is sunlight reflecting off of interplanetary dust. You have about two weeks to watch the Zodiacal Light before moonlight interferes with its visibility.
The moon is at apogee on the 7th at 7:00 PM. Its distance, which is its greatest distance from earth this month, is 251,650 miles.
On the evening of the 7th, you can see Venus, Mercury, and Uranus within the field of view of your binoculars. Venus will be the bright star nine degrees above the west-southwest horizon at 7:00 PM. Your binoculars probably have a field of view of seven degrees, so the view in them should look a lot like the drawing below.
Forty years ago the United States was in preparation for the upcoming manned moon landing. In order to successfully land, we needed to identify safe landing sites. And finding them was the job of the Lunar Orbiters. The Lunar Orbiter series of spacecraft were unmanned spacecraft developed by the Jet Propulsion Laboratory. The third one, Lunar Orbiter 3, was launched on February 4th, 1967. Lunar Orbiter 3 took over 600 pictures of the lunar surface in eight days. Seven months later it was commanded to impact the lunar surface. The intentional destruction of all five Lunar Orbiters was to insure that they would not create a hazard for the Apollo crews. The Lunar Orbiters were as tall and wide as an adult (minus its solar panels). Along with its pictures, the Lunar Orbiters showed that radiation levels on the moon were mild and that the incidence of micrometeoroids was a bit lower than in earth orbit. If you ever visit Washington DC, be sure to visit the National Air and Space Museum. The last time I was there, they had a test model of the Lunar Orbiters hanging above the cafeteria.
Sixty years ago on the 12th, a large meteor was observed over Russia. Eyewitnesses were attracted to its great brightness and the large boom it created when it broke up. The Sikhote-Alin meteorite left a dust trail that was 20 miles long and persisted for hours. It’s believed that 900 tons of meteorites fell over a nearly one square mile strewn field. Because of the great quantity of material, Sikhote-Alin meteorites are inexpensive for meteorites. The original body of Sikhote-Alin was a nickel-iron (93% iron and 6% nickel) asteroid that got too close to earth. Thankfully, it was small body that broke a part in the air.
On the 14th 35 years ago, the Soviet Union launched Luna 20. The unmanned spacecraft successfully returned over an ounce of lunar material. At the same time, the Apollo missions were returning around 100 pounds of rocks and dust from the moon. Luna 20 spent one day on the moon taking photographs and drilling its sample. On the other hand, Apollo astronauts were spending three days on the moon and driving for miles.
How many readers remember the first launch of John Glenn? It occurred 45 years ago on the 20th (February 20, 1962). John Glenn was the first American astronaut to orbit the earth. The previous two astronauts (Shepard and Grissom) didn’t orbit the earth, they made 15 minute suborbital hops (their Redstone boosters didn’t have the power to put their Mercury capsules into orbit).
John Glenn’s mission was to make three orbits around the earth. But if you watch The Right Stuff, you’ll get the impression that his mission was to last for seven orbits. The message, “go for at least seven orbits” just meant his Atlas booster had put him into a high enough orbit that the Mercury wouldn’t reenter the atmosphere for at least seven orbits. Glenn named his spacecraft, Friendship 7. The seven after each Mercury capsule’s name was to indicate that it took all seven astronauts to make the Mercury project a success.
Before firing his retro rocket to return to earth, ground control received a radio signal indicating that the heat shield of Friendship 7 was loose. The Mercury heat shield was not solidly mounted to the base of the Mercury, it was held against the base of the capsule with clamps. This allowed the heat shield to drop away from the Mercury shortly before it splashed down. The heat shield, still attached to the Mercury with a cloth tube, formed a simple air bag to cushion the water impact of the Mercury. However, in John Glenn’s case, a faulty switch between the heat shield and capsule indicated that a clamp holding the heat shield against the capsule had released the heat shield. No one was confident that signal was correct, but just in case, Glenn was asked not to jettison the retro rocket (called the retro pack) from Friendship 7. Normally, after the retro pack fires all three of its solid rocket engines, it’s released from the Mercury. Retaining the retro pack was insurance against the heat shield being loose because its three straps should keep the heat shield in place until the retro pack burned up on reentry. By that time, aerodynamic forces would keep the heat shield in place. As it turned out, ground control was right and the switch was faulty. So john Glenn safely recovered in the Atlantic after a five hour flight. Since Glenn had become an American hero, he was not allowed to fly in space again. So Glenn left NASA and went on to become a senator for Ohio. But in 1998 he arranged for a trip on the Space Shuttle and brought back memories for many older Americans.
It’s been twenty years since the supernova SN1987A was discovered. On February 23rd, 1987, a new star in the Large Magellanic Cloud (LMC) was discovered. SN1987A is the closest supernova to earth since the invention of the telescope. Since the LMC is 168,000 light years away, the supernova really exploded in 166,000 BC.
It wasn’t just the light of SN1987A that was studied. Neutrino detectors buried underground (buried for shielding against cosmic rays) detected 25 neutrinos from the explosion. This was the birth of neutrino astronomy.
SN1987A formed when the core of a very massive star could no longer generate enough energy to support its weight. The core’s collapse squeezed electrons and protons together to form neutrons and a burst of neutrino radiation. Neutrinos are among the lightest subatomic particles. They carry no charge and hardly ever interact with matter. In fact, the average neutrino could travel through more than 50 light years of lead without being stopped. The only reason our neutrino detectors could see 25 of them is that countless trillions of neutrinos were created in the explosion of SN1987A.
Today the Hubble Space Telescope (HST) shows a growing ring centered on the explosion. The ring is not material from the explosion, but is the reflection of light from the explosion off of nearby dust.
The moon is close to the Pleiades on the evening of the 24th. Look for the nearly first quarter moon high in the southwest. The Pleiades are four lunar diameters below the moon.
The Rosetta spacecraft flies past Mars on the 25th. Rosetta, a European spacecraft, will be the first to orbit a comet. Its target is comet 67 P/Churyumov- Gerasimenko. But to get there, Rosetta will fly past earth three times and Mars once. Still, with all those gravitational boosts in speed, it will take Rosetta until Spring of 2014 to reach the comet. Once there, Rosetta will go into orbit and begin studying the comet. As the comet approaches the sun, Rosetta will watch as the nucleus becomes active. Rosetta also carries a lander to take a close look at a comet from its surface.
On February 28th, the New Horizons spacecraft will fly close to Jupiter. Its Jovian flyby will accelerate New Horizons so it can reach Pluto in only nine years. Without the gravitational kick of Jupiter, it would take New Horizons 12 years to reach Pluto.
This Month’s Topic
It’s not easy to tell the true color of a star. Now that statement may have sounded silly. Of course you can tell the color of a star, you just need to look at it. But in reality, it’s more complicated than that.
Astronomers are not interested in the color that a star appears to the eye. They’re really interested in the brightest color of a star. That’s because as a star’s temperature changes, so does its brightest color. By understanding this principle, astronomers can determine the temperature of the star’s surface, and therefore its mass and age.
The term you need to be familiar with is “Blackbody Radiation” (BBR). BBR is the range of colors and intensities that a perfectly black object emits as it gets hot. While the perfect black body doesn’t exist, stars act approximately like ones. How does the color of a perfect black body change with temperature? Think back for a moment to the color of a slab of iron as a blacksmith forges it. When the iron is warm, it emits a dull red color. As the blacksmith’s forge warms the slab of iron, the iron’s color changes to a bright red, then dull orange, to bright orange, to a yellow, and finally white. This change in color is similar to what astronomers are looking for in stars. The exception is that astronomers are looking for the intensity of each color while our eyes (and brain) are combining the colors into one). So we may say a hot slab of iron is yellow, but really the iron is still giving off some red and orange light along with the yellow. In fact it’s even giving off a little green light that’s too faint for our eyes to detect as a particular color.
To really see the colors of a star, astronomers use a spectrograph. A spectrograph splits the colors of star light apart so that their individual intensities can be measured. If we plot the colors of a hot body in the horizontal direction and each color’s intensity in the vertical direction, we’d end up with a chart looking like this.
Notice that as you move across the chart the color reaches a peak in intensity and then grows dimmer. A star’s color at peak intensity tells astronomers the temperature of the star.
Our eyes and brain are bad at doing what a spectrograph does. Our eyes and brain combine all the colors they see into a single color. Not only that, but our eyes are not equally sensitive to all colors. Our eyes respond best to light that’s yellow-green and not nearly as sensitive to the extremes of red and blue. And of course we’re blind to infrared and ultraviolet. Since we can’t see infrared and ultraviolet, we can’t see some of the light emitted by the hottest or the coolest stars.
Since our eyes and brain combines colors and we’re blind to some colors, we see cool red stars as red (since they have very little green and even less blue light) but the hottest stars as white (since the hottest stars emit all colors from red to blue). We’ve been taught that cool stars are red, and that’s true to our eyes. We’ve also been taught that the hottest stars are blue, but that’s not what our eyes see. Once a star gets to the point of being yellow-green hot, we’re going to start seeing it as white hot.
Here’s another complicating factor. The color receptors in our retina (its light sensitive part) are the cones. Cones are not as sensitive to faint light as are the rods, or the black and white light receptors in our retina. Most stars are faint to the eye, therefore, even when they have a distinct color, they’re usually too faint for our cones to detect (and our brain to make out) their color). This is where a telescope brings out the colors of stars too faint for our unaided eye to see.
Elements have their own colors that they emit when they’re hot. This is their spectrum, or specific set of colors they emit when hot. Remember, black body radiation emits all colors in a smooth continuum. The spectrum of an element (and therefore of an atom) is discrete, and not continuous. There are some stars that strongly emit light from atoms in their atmosphere. When this occurs, the smooth continuous band of colors that a star emits as a black body is overlaid with discrete colors of the elements in its atmosphere. When the star is bright enough, we can detect this combination of colors. In those cases, the star doesn’t have an overall color that’s related to its temperature.
Finally there’s a contrast effect. A binary of two stars with colors that only slightly different from each other take on an exaggerated differences in color when they can be compared side by side. A good example is the star Albireo (Beta Cygni). The two stars appear to be golden yellow and blue-green. But if you could view the stars a part from one another, you’d see less of a contrast in their colors.
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/
http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1967-008A
http://en.wikipedia.org/wiki/Sikhote-Alin_Meteorite
Luna 20, http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1972-007A
http://www-pao.ksc.nasa.gov/kscpao/history/mercury/ma-6/ma-6.htm
Rosetta, http://rosetta.esa.int/science-e/www/object/index.cfm?fobjectid=2279
Dark Skies and Bright Stars,
Your Interstellar Guide