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Week 7 - Stars and Constellations What is the most important celestial body in the solar system? Did you answer Earth? There is no question that our planet is special, but it would completely lifeless without the sun. The most important object in our solar system is not a planet, but a star. Without the sun's energy, life on Earth could not exist. We owe our existence to a star. In this lesson, you will learn about stars and constellations. Definition Almost everyone knows that the sun is a star, but what exactly is a star? A star is a ball of gas held together by its own gravity. This gravitational force continually tries to collapse the star by pulling gas molecules toward its center. Gravity is counteracted by the pressure of the hot gas inside the center of the star. Gravity pulls inward, and the pressure pushes outward. The two forces balance each other and the star maintains a spherical shape. This balance is called hydrostatic support. Many planets, like Jupiter and Saturn, are made of gas, so what makes a star different? Unlike planets, stars generate their own energy through nuclear reactions deep within its core. The incredible mass of a star creates the perfect condition for the fusion of hydrogen into helium. The energy from this reaction is released by the star as light and heat. When you look into the night sky and see a star, you are seeing self-generated light. When you look at the moon or a planet, the light you see is reflected light from the sun. A star is like a flashlight, while the other objects are simply mirrors. Stars are made mostly of hydrogen and helium. Since they generate energy by fusing hydrogen into helium, the percentage of each element changes throughout the stars' life cycles. Magnitude Which star do you think is the brightest in the Milky Way galaxy? The sun? Would it surprise you to learn that the brightest star, Canopus, is almost 14,000 times brighter than the sun? Yet, this star is only the second brightest object in the night sky. Why does the sun seem so bright in comparison? Brightness is determined by both distance and luminosity. A star which radiates a lot of light, but is far away from the Earth, can appear less bright than a less luminous star closer to Earth. Imagine sitting outside at night with a candle. The light from the candle will seem much brighter than the light from a streetlight a mile down the road. However, if the two light sources were placed next to one another, the candle's luminosity would be much less than the streetlight's. 1 The brightness of a star as called its magnitude. Magnitude is measured on a numerical scale, with a lower number indicating a higher magnitude. A star with a magnitude of 5.5 is less bright than a star with a magnitude of -2.7. There are two types of magnitude - apparent and actual. The brightness of a star it appears from Earth is called apparent magnitude. Apparent magnitude measures a star's brightness based on luminosity and distance. Close stars usually have higher apparent magnitudes than more distant ones. The sun, for example, has the largest apparent magnitude (-26.73) of any other star in the universe. The actual brightness of a star is called its absolute magnitude. Unlike apparent magnitude, absolute magnitude does not consider distance. It measures a star's brightness based on its luminosity alone. Apparent and absolute magnitude. The magnitude of a star depends on two factors. The first factor is the intensity of the radiation emitted from the star. The second factor is the star's distance from earth. A star that is far away may seem dimmer than a star that is close. For example, the light from a lamp on your desk appears very bright. The same light appears dimmer when seen from across the street. Although the lamp seems dimmer, its actual brightness has not changed. Astronomers use the term apparent magnitude to describe how bright a star appears from earth. Apparent magnitudes are written with a lower case "m." The term absolute magnitude is used to describe how bright a star would appear one A.U. from earth. Absolute magnitudes are written with an upper case "M." With absolute magnitude, the brightness of a star is measured independent of distance. The sun is extremely close to the earth and has an apparent magnitude of -26.7. However, the absolute magnitude of the sun is 4.8. In contrast, the supergiant Betelgeuse is very far away. It has an apparent magnitude of 0.41. However, this star is actually much brighter than the sun, and has an absolute magnitude of -5.6. Classification Have you ever been outside for too long without sunscreen? Did you get sunburned? The heat from the sun is extremely hot, but the sun itself is only a medium temperature star. Stars are classified by temperature and color into seven main categories - O, B, A, F, G, K, and M. O stars are the hottest and M stars are the coolest. O and B stars are extremely bright, and very rare. M stars are very common, but fairly dim. Each letter is followed by a number from 0 to 9. This number represents a sub-classification of temperature, from hottest (0) to coldest (9). For example, the sun is classified as a G2 star. The color of a star is connected to its temperature. Although it seems backwards, the hottest stars are blue in color, while the coolest are red. In between, from hottest to coolest, are white, yellow, and orange stars. Since the sun has only a medium temperature, it is a yellow star. Average Luminosity Star Type Color Example (Sun = 1) O Blue 1,400,000 10 Lacertra B Blue 20,000 Rigel Spica A Blue 80 Sirius 2 F G K M Blue -white White - yellow Orange - red Red 6 1.2 0.4 0.04 Canopus Sun Aldebaran Betelgeuse Constellations Orion. Ursa Minor. Scorpius. Do you recognize these names? They are the names of major constellations in the night sky. A constellation is a group of stars which form a pattern when seen from Earth. The stars are divided into 88 constellations, ranging from the large Centaurus constellation (which includes 101 stars) to the small Crux, or even Southern Cross, a constellation made of only four stars. Constellations have been passed down by ancient civilizations, most prominently the Greeks and Romans. Many constellations have myths and legends associated with them. The Pegasus constellation, for example, represents the winged horse that flew out of Medusa's head after she was killed by Perseus in Greek mythology. Constellations only make sense from Earth. Stars which appear close together in a constellation are not often close to each other in space. The relative position of the stars depends on a person's point of view. In other words, a person looking up at the stars in New York City will not see the same constellations as a person stargazing in Santiago, Chile. This is because the Earth is a sphere. The Southern Hemisphere looks into space at a different angle than the Northern Hemisphere. The Earth's rotation also causes constellations to "move." Constellations which appear to rise and set are called seasonal constellations and can only be seen during certain times of the year. Constellations which never seem to move are called circumpolar. Let's Review Stars are massive balls of gas held together by gravity. They generate their own light and heat through nuclear fusion reactions. Apparent magnitude describes a star's brightness as seen from Earth. Absolute magnitude describes a star's actual brightness. Stars are classified according to temperature (O, B, A, F, G, K, M) and color (blue, white, yellow, orange, red). The sun is a medium sized, medium temperature, yellow star (G2). Constellations are groups of stars that form patterns when viewed from Earth. Different constellations are seen in the Northern Hemisphere than in the Southern Hemisphere. HISTORICAL OBSERVATIONS Before electric lights obscured the sky, people could easily see the night sky. In ancient times, man would gaze at the magnificent night sky. In his mind he imagined the outlines of people 3 and animals. The early Greeks gave names to these star-shaped figures from their religious and cultural stories. These stories are known today as myths. For centuries men have looked up at the night sky and wondered whether those numberless specks could affect their lives. People who studied the stars were called astrologers. Early astrologers may have started as true scientists. However, astrology quickly became a religious practice. Astrology became part of the Babylonian religion. To them the zodiac was a sacred pathway for the sun and the planets. Stars became familiar signposts in the sky to sailors and travelers. Some of the first inventions were used by sailors to use the stars as a means to find their way in the vast oceans. Ancient peoples told time by the sky. The sun and stars marked the time of day and night. In other words, celestial clocks! The passage of time may be more accurately recorded by the stars than could even be expected from the precision of our modern atomic clocks. The moon measured the month. Stars marked the year and its seasons. Observers noticed that stars set about four minutes earlier each night and concluded that the star day is four minutes shorter than the sun day. As people studied the sky, they observed at least four different motions: 1. Most stars rise in the east and set in the west; 2. Both the sun and moon rise in the east and set in the west; 3. The sun appears over the horizon farther north in summer and farther south in winter; 4. The moon does not always rise at the same time. CONSTANT MOTION Do you think you are sitting still in your chair right now? If you could look at yourself from outer space, you would see that you are turning a giant somersault once every 24 hours as the earth turns on its axis at about 1,600 kilometers per hour. You are also making a circle around the sun at the speed of 107,200 kilometers per hour. Also, our solar system revolves around the center of our galaxy, the Milky Way, at the rate of 69,200 kilometers an hour. The whole galaxy is traveling toward the star Vega at about twenty kilometers per second. So you see, you are definitely not sitting still in your chair. 4 The Milky Way is the galaxy which contains our solar system. On a summer evening you can see a dusty trail of stars stretching across the sky. This is our Milky Way galaxy consisting of hundreds of billions of stars. Instead of seeing each star individually, the combined light appears as a faded band if the sky is very dark. With a telescope you can see many more stars. People in ancient times thought that all stars were part of the Milky Way. Today, we know of many other galaxies similar to the Milky Way. To study the Milky Way as a whole is difficult for scientists on the earth because we are located within it. We cannot look at the Milky Way galaxy from the outside to observe its size and shape. Outer space is so staggering that a unit of measurement was invented to express the great distances. A light year represents the distance that light travels in one year. One light year is almost 6 trillion miles. This distance is based on the apparent speed of light. Measurements by sensitive instruments indicate that the current speed of light is about 186,000 miles a second. The distances to stars therefore appear to be very great. The nearest star, other than the sun, is Proxima Centauri, which is one of three stars in the Alpha Centauri system. It is 4.3 light years away. Imagine that if the earth was only the size of a small marble, the nearest star would be about 24,000 miles away! Because light travels so fast, 4.3 light years is a vast distance. Many stars appear to be even further from the earth. For example, 300 or 1,000 light years away or greater. It is important to understand that a "light year" is a measure of distance and not time. It is the distance that light would travel in one year. STAR CHARTS The constellations, or imaginary figures, that the ancients observed are star groups that seem to travel together in space. Star charts have been developed to help follow the everchanging picture formed by the heavenly bodies. Each season has its own pattern of stars. Once you learn to recognize the principal constellations of each season, you can use them as a guide along with a star chart to find other stars. The Winter Sky. Of all the months of the year, February is best for stargazing. It has an exciting parade of stars. 5 Orion, the hunter, is one of the best-known constellations in the sky. Three stars make up Orion's belt. Betelgeuse, a giant red star, marks his right shoulder. Betelgeuse is one of the largest stars man has found. Its diameter is 400 million miles. Rigel, a brilliant blue star, is positioned at Orion's left knee. He appears to be holding his right arm over his head. If you imagine a line drawn along Orion's belt toward the southeast, you will find the star, Sirius, the dog star. Sirius is the second brightest star in the sky. Sirius is the nose of one of Orion's hunting dogs, Canis Major. The imaginary line made by Orion's belt toward the northwest passes just under the horn of Taurus, the bull. The horns of Taurus form a V-shape in the sky and contain the star, Aldebaran. Pleiades, the seven sisters, are located on the shoulder of Taurus. Usually only six stars can readily be seen; but when viewed with a telescope, many more stars can be seen. The horns of Taurus point toward two stars located above Orion's second dog, Canis Minor. These stars are the Gemini twins, Pollux and Castor. The Spring Sky. The spring sky contains the best-known of all the constellations: the Big Dipper, also known as Ursa Major, the big bear. It can be seen all night because it does not rise or set. The spring sky is not as spectacular as the winter sky. Locate the Big Dipper. How many stars are in the Big Dipper? The two end stars in the bowl of the Dipper are called the pointer stars because they point to Polaris, the North Star. Polaris is not one of the brightest stars, but it is one of the most useful to man. The North Star can be used to find direction. It is located almost directly over the North Pole. All other stars appear to rotate around it during the night. Polaris is the end star in the handle of the Little Dipper. The Little Dipper is also called Ursa Minor, the little bear. Notice the gentle arc in the handle of the Big Dipper. Follow that arc toward the southeast to a bright star called Arcturus. Arcturus is in the constellation Boötes. To the east on the horizon is the constellation Libra, the scales. This constellation is the symbol for justice. The most prominent constellation in the spring sky is Leo, the lion. Leo is almost overhead at the zenith. Leo's head resembles a backward question mark. The star Regulus is the period. The rear end of the lion is shaped like a small triangle. The Summer Sky. Darkness comes much later in the summer, so your observation of the stars will be at a later hour than during other seasons. The summer sky has three bright stars that form what is known as the summer triangle. The constellation Lyra, the harp, will be found near the zenith. This group of stars is easy to locate because it contains the bright star Vega. 6 East of Lyra is the constellation Cygnus. Cygnus means swan; but because of its shape, this constellation is often called the Northern Cross. The brightest star in Cygnus, Deneb, is located in the tail of the swan. Below Cygnus to the southeast is Aquila, the eagle. Aquila's brightest star is called Altair. Altair, Vega, and Deneb make up the summer triangle. West of Lyra is found the constellation that immortalized Hercules, a most popular hero of ancient mythology. With his foot on the head of Draco, the dragon, he is about to club him. In his other hand Hercules holds the golden apples from the garden of Hesperides which Draco guarded. To the south of Lyra is Sagittarius, the archer. Sagittarius was a mythological centaur who lived on a mountain in Thessaly. On the southwestern horizon is Scorpius, the scorpion. Orion appears in the winter sky and Scorpius in the summer sky. The Autumn Sky. The parade of stars across the autumn sky is the least spectacular of the four seasons. The summer triangle has moved off to the west, and Orion is just rising on the eastern horizon. The Perennial Sky. The polar stars are the stars found around Polaris, the North Star. In the northern hemisphere, these stars belong to every season because they never set. They are called circumpolar stars because they seem to revolve around the North Star, and because they do not go below the horizon. The constellations are these: the Little Dipper; the Big Dipper; Draco, the dragon; Cassiopeia, the queen; Cepheus, the king; and Camelopardalis, the giraffe. Cepheus resembles a house; Cassiopeia, his queen, looks like the letter W. Early evening darkness is the best time for stargazing. Hold the star chart over your head. Make sure that north on your chart is lined up with true north. Use a flashlight covered with red cellophane to read your chart and compass. White light deadens the ability of your eyes to see the stars. Do not be in a hurry to find all the constellations at once. Locate one or two at a time and watch them for several nights. Then look for another group of stars. The constellations on your star map are extremely small when compared to the constellations in the sky. The spaces between the real stars are much larger than you imagined, and they will cover huge areas of the sky. GEOCENTRIC THEORY 7 The geocentric theory of the universe originated with Aristotle and was later modified by Ptolemy. According to these men, the earth was the center of the universe. Study these drawings. On the right is a drawing of the universe as Aristotle described it. The other represents Ptolemy's view of the universe. Examine how they are alike and how they differ. Aristotle. For almost 2,000 years after his death, Greek philosopher Aristotle reigned as the supreme authority in scientific matters. Among these ideas was the geocentric theory, which held that the earth was the center of the universe, and that all planets revolved around it. According to the theory, the planets are fastened to a series of crystalline spheres concentric with the earth (see illustration above). The nearest sphere is the atmosphere. The next four spheres are the four "elements:" earth, water, air, and fire. Beyond the sphere of fire is a region containing ether. The planets occupy other spheres stretching away from Earth in this order: the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, and the fixed stars. At the outermost limits of the universe is the sphere of the Prime Mover, the First Cause. The Prime Mover is the source and origin of all motion and life. The Prime Mover is the force that keeps each of the spheres in motion. Aristotle's written work, On the Heavens, presents his cosmology. He argues entirely from theoretical principles. The geocentric theory, however, contradicts observations. Several planets periodically show a slower, or even retrograde, motion. The apparent movement of each planet was ingeniously explained as the result of the combined motions of a number of spheres. Altogether, over 50 separate circular motions were required to explain the puzzling phenomenon. 8 The simplest explanation is generally the most accurate one. Everything that moves is moved from outside itself, but where do celestial bodies derive their motion? A problem in Aristotle's concept of the universe was obvious from the first. Several stars did not move in the same direction and with the same regularity as did all the others. These few mavericks appeared to wander through the constellations--sometimes moving forward, sometimes standing still, sometimes even moving backward against the backdrop of the regular stars. Aristotle's spheres could not explain these wanderers and their sometimes retrograde travel. Ptolemy. In the third century AD, Ptolemy modified Aristotle's model and developed a series of mathematical explanations for the unusual movements of the wanderers. Like Aristotle, Ptolemy taught that the earth was fixed at the center of the heavens. The sun, moon, and known planets revolved around the earth in circles. Beyond them the stars were attached to a crystal-clear sphere. Beyond all was the Prime Mover. Ptolemy's modification contained the explanation that each planet moves in a small circle, the center of which is carried on the circumference of a larger circle. The center of this larger circle is the earth. By this system of epicycles, Ptolemy explained the apparently erratic planetary movements. WANDERERS If you watch the night sky for some time, you will find five bright objects that do not move the same way that the fixed stars move. They do not belong to constellations. They move among the constellations at different speeds. They even change directions. The Greeks called these bodies wanderers. We call them planets. Like the sun, the planets appear to travel eastward through the fixed stars. Periodically, they appear to slow down. They seem to stop briefly and then move backward. They slow down again, stop, and then move eastward again. This different pattern of movement makes the term wanderer appropriate. Mars, Jupiter, and Saturn always appear brightest when they are halfway along their backward paths. 9 Most of the sky-watchers long ago thought that the earth was the center of the universe. Their conclusion was based on appearances. The sun, moon, stars, and planets were thought to be fastened to spheres that spun around the earth. This explanation did not satisfy Aristarchus, a teacher at Alexandria, Egypt, in 290 BC. If the planets were fastened to spheres, he reasoned, they would always be the same distance from the earth. Then why did they change in brightness, he asked? Aristarchus suggested that the sun was at the center of the universe. He said that the earth was a planet, and that the planets moved around the sun. Very few scholars agreed with him, so in time his idea was forgotten. METEORS Most people have seen streaks of light in the sky and called them "shooting stars," or "falling stars." A better name for the light is meteor. Space is full of metal and rock that move at high speed. When they are in space, the small pieces of metal and rock are called meteoroids. When they enter our earth's atmosphere, they heat up and give off light. If the piece manages to get completely through the atmosphere without being consumed by heat, the portion of a meteor that hits the ground is called a meteorite. 10 The average speed of a meteoroid in space is about 85,000 miles per hour. When a meteoroid enters the atmosphere, friction with the atmosphere causes it to heat up. Most meteoroids vaporize and disappear well above the earth's surface. Meteoroids range in size from grains of sand to great boulders. Approximately two tons of meteoric dust is estimated to fall on the earth every 24 hours. Most of the dust is composed of iron and nickel. Since both iron and nickel-iron are magnetic, small meteoritic particles can be recovered by dragging a small magnet over the ground. About 10% of the material clinging to the magnet will be dust from outer space. Examine some of it under a low-powered microscope if one is available. Meteor "showers" are named according to the constellations from which they appear to fall. Meteors radiating from the constellation Leo are called Leonids; those from Orion are called Orionids. The following table lists the main annual meteor showers and the dates of maximum shower activity. 11 Table of the Main Annual Meteor Showers Number of Name of Days It Will Shower Last Quadrantids 4 Lyrids 4 Eta Aquarids 8 Pons? Winnecke Delta Aqaurius 3 Perseids 25 Oraconids 1 Orionids 14 Leonids 7 Andromedes ? Geminids 14 Date of Maximum Activity January 3 April 22 May 5 Hours of Maximum Activity 12-4 a.m. 12-4 a.m. 12-4 a.m. Approximate Number of Meteors Per Hour 28 7 7 June 29 12-4 a.m. ? July 29 August 12 October 10 October 20 November 17 November 21 December 13 12-4 a.m. 12-4 a.m. 12-4 a.m. 12-4 a.m. 12-4 a.m. 12-4 a.m. 12-4 a.m. 27 69 ? 21 21 ? 23 HUBBLE CONSTANT The distances between celestial bodies require unique methods of measurement. These methods are used to measure tremendous distances, sometimes even in the billions of lightyears. One such method is triangulation. This technique uses parallax angles to measure distances, but it is difficult to calculate and limited in range. Another technique is called the red shift method. It was developed by the scientist Edwin Hubble in the 1920s. Hubble discovered that observed galaxies were heading away from the earth in all directions. He used spectral analysis of the light emitted from stars in the galaxies to support his finding. Hubble showed that the more distant galaxies were proceeding away from us at faster rates than closer galaxies. The red shift was greater for distant galaxies. Hubble established a relationship between distance and velocity called the Hubble Constant. The Hubble Constant is expressed in kilometers per second per megaparsec, or km/sec/Mps. Radial outward velocity (line-of-sight motion from us) is represented by v, and the distance of the galaxy from the earth is represented by d. 12 Great care is needed in calculating this constant since the distances being considered are so vast that any slight variation would lead to errors of great proportions. CEPHEID VARIABLES During the development of different measuring methods, astronomers discovered a unique type of star. Cepheid Variables, also called Cepheids, are stars which brighten and dim periodically. The time they take to brighten, dim, and brighten again is called a period. Cepheid periods are regular and can be used to determine distance. The longer the period, the greater the star's absolute magnitude, or true brightness. Once absolute magnitude is determined, it is compared with apparent magnitude, or what we see from earth. Distance is calculated from the difference. Cepheids are considered to be stellar "yardsticks." By comparing distances of objects against the assumed distances of Cepheids, distances of far-away objects can be calculated. There have been several Cepheids found in the Large Magellanic Cloud and the Small Magellanic Cloud galaxies. These two small galaxies are companion galaxies whose distances have been measured by triangulation. Triangulation is thought to be reliable to about 300 light-years. If this is the case, distance measurements to the Magellanic Clouds (150,000 and 173,000 lightyears) would be subject to an error factor of roughly 500. With an error factor this large, reliable results would not be attainable. 13