Astronomy

Types of planets

A closer look at the planets revolving around the Sun shows that there are two major types of planets. These are called Terrestrial (earth-like) and Jovian (jupiter-like). Below is a table that outlines the major characteristic differences: With these characteristics in mind, you can predict whether a planet is Terrestrial or Jovian. For example, if a new planet were discovered that has no moons, you could predict it to be Terrestial, because a characteristic of Terrestrial planets is that they have few or no satellites.

Astronomical Unit (AU)

A unit that equals the Earth- Sun distance. It is convient for measuring distances with the solar system, distances from the sun to the various planets can be conveniently stated using this unit.

Orion the Hunter

According to Greek mythology, Orion was a giant and a great hunter. The Celestial Equator passes just above Orion's belt, making this constellation visible throughout the entire world. Betelguese is a red giant located about 652 light-years away. Rigel is a blue supergiant about 815 light-years away. The name Rigel comes from Arabic for "foot." In the "sword" of Orion (use your imagination here—it is hanging from his "belt") you can see the Great Orion Nebula, sometimes referred to as M42. If you look there with a pair of binoculars or a telescope, you will see this gaseous cloud fairly well.

The dark ages

After the fall of the Roman Empire, Europe fell into what has been called the Dark Ages. During this period of time, there was very little advancement of knowledge and understanding about the world and the universe. It is important to note, however, that the so-called Dark Ages really only affected Europe. Cultures in other areas of the world thrived during this time. One of these cultures was the Arab culture in the Middle East. The Arab nations took Ptolemy's work and expanded upon it. In fact, Almagest is the Arabic translation of Ptolemy's original work. Most of the names of stars are Arabic. "Al" followed by some other name indicates the Arabic nature of that name. If it were not for the Arabs, we would not have known very much about ancient Greek astronomy. Most of what we do know comes from Arabic translations of Greek works. The Dark Ages came to an end around the fifteenth or sixteenth century, beginning a period of time called the Renaissance. Ptolemy's work had been the standard for over a thousand years, but eventually things started going wrong with his predictions. Tiny errors and inaccuracies that were not noticeable in Ptolemy's time became compounded and multiplied over the centuries. Fifteenth-century astronomers made some adjustments to Ptolemaic theory, but it was clear that this would not help, as the system became much too complex to explain away the errors and remain consistent with observations.

Einstein

Albert Einstein revolutionized the thinking of the scientific world by introducing a completely different way of looking at the universe. He proposed a theory called general relativity. In this theory, gravity was not a force, but rather a property of the geometric configuration of the universe. Put simply, gravity is merely "curved space." Objects that have a large mass curve space more than objects with a smaller mass. This is only a brief, oversimplified statement of his theory. A better understanding would require a background in advanced calculus. His theory has had a couple of interesting offshoots. One of these is the existence of black holes. Before Einstein, black holes were never predicted or observed. Now that we know what to look for, we have found black holes. Another interesting offshoot of Einstein's theory is gravitational lensing. This occurs as the curvature of space (gravity) actually bends the path of light. During an eclipse in 1919, astronomers were able to photograph and carefully measure the positions of stars, showing that they had apparently shifted position. Thus, it was observed that the gravity of the sun actually did bend light rays coming from these stars, and gravitational lensing was accepted.

Heat transfer

All forms of matter are composed of atoms or molecules which are in constant motion. Because of this vibratory motion, all matter is said to have thermal energy. Whenever a substance is heated it's atom move faster and faster. This results in an increase in thermal energy. It is the average motion of the atoms or molecules that we sense when we determine how hot of cold it is. We often describe this as the temperature. Temperature is really a measure of the average motion of atoms or molecules in a substance. Temperature is not the same thing as heat. Heat is the energy that flows because of differences of temperature. For example, when you touch a hot pan heat is transferred from the pan to your figures. Also, when you touch an ice cube, heat flows from your hand to the ice cube. This if two objects of different temperatures come into contact the warmer object will become cooler and the cooler object will become warmer until they reach the same temperature. Three mechanisms of heat transfer are conduction, convection and radiation.

Galactic distances

An important aide to understanding our own galaxy, as well as others, is the ability to determine the distance to other galaxies. This can be tricky since galaxies are extremely far away. However, scientists have come up with several ways of estimating the distances in 1912 American astronomer Henrietta leavitt published her discovery of what is called the period luminosity relationship, this deals with a special kind of star known as a cepheid variable star. These stars get brighter and dimmer in a regular repeating patter. Henrietta leavitt determined that the period of this pattern was related to the stars luminosity. This has since been named the period luminosity relation. Knowing the luminosity and measuring the apparent magnitude, she could calculate the distance to that star. So if you could find a cepheid variable star, you now had the means by which to calculate the distance to that star. There are other indicators of distance that astronomers use. Most of the rest of them are estimates. For example, if we assume that the largest red supergiants in other galaxies are the same size and brightness as the largest ones in our galaxy, we could use them to tell us distances up to 50 million light years. The other indicators are based on this principle as well and allow us to estimate distances up to 15 billion light years.

Motions of the sky

An observer on the earth would notice several kinds of motions in the sky. In the daily motion of the sky, the Sun, Moon, and stars appear to move from east to west in twenty-four-hour cycles. This is caused by the earth's rotation, or spinning, on an imaginary axis. The earth is spinning like a top in space. The axis is slightly tilted and points almost exactly at Polaris. For this reason Polaris has been called the "North Star." It may be used as a way to find North. Looking in the night sky, Polaris always seems to be in the same spot if the observing location is the same. Stars and constellations seem to rotate around Polaris. Another important celestial motion is the earth's motion around the Sun. The earth revolves around the Sun, completing one revolution every 365 ¼ days. We have defined this period of time as a year. Our planet rotates on its axis, which is tilted 23 ½ degrees from the plane of its orbit around the Sun. This means that the Sun will be directly overhead at different locations throughout the year. Direct, head-on sunlight allows the most energy per area to reach the surface. The greater the angle of incident light, the more spread out the energy must be. For this reason the earth has four seasons.

Phases of the moon

Ancient astronomers realized that the Moon reflected sunlight and that the Moon orbited the earth in approximately four weeks. They were able to deduce this because of observed lunar phases. New moon occurs when the unlit side of the Moon faces the earth. We cannot see the Moon at all during this phase because the Moon is in the same part of the sky as the Sun. As time passes, more of the Moon is illuminated (at least the part that we can see from Earth). This phase is called waxing crescent moon. Waxing means growing larger. At first quarter moon, the angle between the Moon, Earth, and Sun is 90°, and we are able to see exactly one-half of the Moon's face illuminated. During the next week, we see more and more of the Moon. This is called waxing gibbous moon. When the Moon and Sun are on the opposite sides of the earth we see a full moon, when the entire hemisphere is illuminated to our view. The full moon would rise as the Sun sets, and set as the Sun rises. Over the next couple of weeks, we begin to see less and less of the Moon. First there is waning gibbous moon (waning means growing smaller), followed by last quarter moon, waning crescent moon, and then new moon again. Half of the Moon is always illuminated by the Sun and half is always dark. As observers on the earth, we see different amounts of the Moon's illuminated half, depending on the position of the earth, Moon, and Sun. The Moon's shadow occasionally passes across part of the earth's surface. This phenomenon is called a solar eclipse. There are two parts to a shadow: the umbra and the penumbra. The umbra is the darkest part of the shadow. No light from the source passes directly through this region. The penumbra is a shadow where only some of the light is blocked, but not all. As a result, a solar eclipse may be a total eclipse (umbra) or a partial eclipse (penumbra), depending on the location of the observer. A solar eclipse can only occur during a new moon, when the Moon is between the earth and the Sun. Similar to a solar eclipse is a lunar eclipse. A lunar eclipse occurs when the Moon passes through the earth's shadow. This can occur only during a full moon, when Earth is between the Moon and the Sun. The Moon's face darkens for a time. Thus, a solar eclipse is when light from the Sun is blocked, and a lunar eclipse is when light from the Moon is blocked. So why don't we have eclipses twice a month? The answer to this question lies in the fact that the Moon's orbit around the earth is slightly tilted compared to the earth's orbit around the Sun. The earth, Moon, and Sun have to line up just right in order for there to be an eclipse.

Andromeda and Pegasus

Andromeda was the daughter of Cepheus and Cassiopeia. According to Greek mythology, Cassiopeia boasted that Andromeda was more beautiful than the daughters of Nereus. The angry sea god sent the sea monster Cetus to destroy the kingdom. The people of the city chained Andromeda to a rock by the sea in hopes that this would appease the sea monster. Perseus came to the rescue, flying upon the winged horse Pegasus. Notice that the "horse" appears upside-down here. The hind legs of Pegasus make up the constellation of Andromeda.

Radio telescopes and telescopes

Another method of gaining astronomical data is the use of radio telescopes. A radio telescope uses a large parabolic dish to reflect radio waves to a focus. This approach searches for radio waves instead of visible light, as with previously mentioned telescopes. There are two properties of the earth's atmosphere that affect how well telescopes are able to detect distant objects. The first is transparency. This refers to how clear the sky is. If there is a lot of haze, smog, or cloud cover, we say the transparency is bad. The second property is called seeing. Seeing refers to turbulence in the upper atmosphere, which can cause dim incoming light to refract slightly. This has the effect of blurring the image in a telescope. Light cannot be brought into a sharp focus when the seeing is bad. To achieve the best seeing, most professional telescopes are placed at the tops of mountains (to reduce the amount of atmosphere the light must go through). A combination of good transparency and good seeing is rare. Thus, we can see that the best way to obtain astronomical data is to do it from space. The earth's atmosphere absorbs certain wavelengths of radiation and also creates a distorting effect when attempting high resolution pictures. The Hubble Space Telescope (HST) has produced amazing results and has expanded our understanding of our universe. HST is about the size of a city bus. The heart of the telescope is a 2.4 m (94.5 inch) diameter mirror designed to capture visible light, infrared, and ultraviolet radiation before it reaches the earth's atmosphere. It may not be as large as Earth-based reflecting telescopes, which can be eight meters in diameter, but we get much higher resolution images from HST because it is outside of the atmosphere. Although Hubble was last serviced in 2009, it continues to surprise us with new discoveries. The following video describes the origins and contributions of the Hubble Space Telescope.

Aquila the Eagle

Aquila was named after a bird that belonged to Zeus. Aquila was send to kidnap Ganymede from his fields to become the cupbearer for the gods. Altair is a bright star that lies along the Milky Way.

Aries the ram

Aries is connected with one of the great epics of Greek myth, the story of Jason and the Quest for the Golden Fleece. Hermes gave Aries, the golden ram, to Nephele, who used the ram to carry her children away to safety. The ram was then sacrificed and the Golden Fleece given to the king, Aeetes.

Aristarchus

Aristarchus had a different perspective of retrograde motion. Imagine two runners on a circular track. Both runners may run the same direction, but as one runner overtakes and passes another, it may appear to the faster runner that the slower runner actually moves backward for a time. Aristarchus proposed a heliocentric theory. This means "sun-centered." He believed that the Sun was at the center of the universe and that the earth and other planets moved around the sun. As the earth passes another planet, it appears to move backward for a time. This sun-centered idea was not very well accepted in Ancient Greece. To them it was obvious that the earth was at the center of the universe and that everything went around the earth. (An earth-centered theory would be called geocentric.) Aristarchus also measured and calculated the moon's relative size by looking at the earth's shadow across the moon during a lunar eclipse.

Aristotle

Aristotle proposed a theory of the universe that entailed two types of motion: 1) towards/away from and 2) around. Any motion in three dimensions could be described using a combination of these two types of motion. Thus, an apple would move towards the earth if you dropped it. Since it would always fall towards the ground, no matter where you were, he concluded that the earth must be spherically shaped. The Sun, Moon, and stars moved around the earth. It was obvious. There were, however, seven wanderers which he called "planets". The word planet comes from the Greek word for wanderer. So, in order to incorporate this into his theory, he devised a system of celestial spheres to which the Sun, Moon, stars, and seven wanderers were permanently affixed. These spheres moved around at various speeds, allowing the theory to agree with observations. There were problems with Aristotle's theory, however. One of the major problems had to do with retrograde motion. This occurs when a planet appears to stop, move backwards, and then start moving forward again (when compared to the background stars).

Black holes

As we have discussed previously, Einstein predicted that gravity could bend light. Theoretically, then, it was possible for an object to have strong enough gravitational pull that not even light could escape. This theoretical object was given the name black hole. Black hole are not like giant vacuums that go around sucking everything in. As a matter of fact, from a distance, you couldn't tell the difference gravitationally between a black hole and anything else that had the same mass. If the sun were to become a black hole (which it couldn't because it is too small), then the earth could continue to orbit exactly the same as it does right now. Only when you are close to a black hole do you begin to notice strange things. The nearer you get to a black hole, the more distorted space and time become. I'm stronger gravity time runs slower. Why can't light escape from a black hole? One way to measure the strength of gravity is called the escape velocity. Escape velocity depends only on two things, the mass of the larger body and how far from the center you are. The farther you are from the center, the slower the escape velocity. Also, the more massive the object is, the harder it is to escape. Because of the suns large mass, the escape velocity is about 60 kilometers per second. If we could shrink the mass of the sun into an object the size of the earth, the escape velocity would become 500 kilometers per second this is about the size and density of a white dwarf. If we could shrink the sun further and further then eventually the escape velocity would equal the speed of light, and the. We have a black hole. (The sun would have to be shrunk to fit into a ball with a 3 kilometer radius in order to become a black hole). No one can know what goes on inside a black hole. Scientists have defined the event horizon of a black hole. This is the boundary of the black hole (point of no return). Outside of the event horizon escape is still possible but once inside there is no escape. What little we really known about black holes comes from the effects on matter outside the event horizon. The mass of a black hole determines the extent of the event horizon. The more massive the black hole the larger the event horizon. The centers of most galaxies are believed to contain supermassive black holes. So how do we see a black hole if light cannot escape? Astronomers have devoted black holes by looking at their companions. Most stars in our galaxy are binary, or double stars. If one star evolved into a black hole the other star simply continues to revolve around it. From earth, we can detect a wobble in this companion star as it orbits an invisible mass. This wobble provides astronomers with an idea of just how much mass the invisible companion has. From this it can be determined whether it is a black hole or not.

Life cycle of stars

At first there was a huge cloud of gas and dust called a nebula, which began to collapse under its own gravity. Most of the material collected in the center, which is called a protostar. As the gases continued to compress under the weight of gravity, temperatures continued to rise. More pressure yielded even higher temperatures, until it was hot enough for hydrogen burning (fusio) to occur. When the star was born as a star it expelled its most outermost layers violently in what is called the T tauri wind, which essentially stripped the solar system clean of any remaining gases, thus preventing any further accretion by outer planets. High mass protesters become blue stars, very hot and bright. Low mass stars become yellow stars somewhat cooler and a little dimmer. These are now main sequence stars and will spend the majority of their lives on the main sequence. Higher mass stars do have much more hydrogen but they burn it at a greatly increased rate as compared to low mass stars, so high mass stars tend to spend much shorter times on the main sequence. After ten billion years or so our sun would consume all of the hydrogen converting it to helium, we have about 5 billion years left. A star dies when there is not any more fuel for its central core to continue to produce heat. The core is then essentially all helium because all of the core hydrogen is converted to helium. Without fusion to generate core heat, the core begins to collapse. Converting gravitational energy into heat energy in an attempt to maintain thermal equalibrium. There is still some hydrogen left in the star, but not in the core. A thin shell of hydrogen burning begins of the heat generated by compressing gases as the core collapses. This thin shell moved outward as hydrogen near the core is exhausted this yields more helium which falls into the core. With an imbalance in the heat production the outer layers begin to cool. The hydrogen burning shell causes these layers to expand the star becomes a red giant. Up to this point, the life cycle is very similar for both high mass and low mass stars, but after this the life become very different. High mass stars have enough marred that as the core contracts enough heat is generated to include helium fusion, a similar life cycle is then followed by carbon fusion, oxygen fusion, and silicon fusion, if there is enough matter in the star. By the time there is no longer enough internal heat and pressure to generate fusion of any elements the huge mass begins to contract until it creates conditions for a massive explosion called a supernova, the stars outer layers are ejected outward into a huge explosion. The central core that remains is extremely massive. With so much mass involved the star becomes a neutron star, in fact it could be said that a supernova is the transition bewtewwn an ordinary and neutron star a neutron star is a star whose final stages consist of very closely packed neutrons. Scientists didn't really like this idea because the escape velocity at the surface of a neutron star would be one half the speed of light. In 1968 an astronomer discovered a series of very regular radio pulses coming from various objects in the universe. These pulsating radio sources were called pulsars. Scientists believe that pulses are rapidly rotating neutron stars. As it's outer layer contract the star spins faster. If a neutron star is part of a binary system it may become a pulsating x Ray source. This happens when matter from the ordinary star nearby is pulled into the neutron star. This matter collects on the surface of the neutron star until it becomes hot enough for hydrogen burning. The. There is a burst of x Ray radiation. This scenario is commonly called a burster. In a similar fashion, a white dwarf in a binary system could gather matter from its companion, forming a layer of hydrogen on its surface. Eventually it becomes hot enough to begin hydrogen burning and a nova is seen as a rapid increase in the brightness of a star in the visible range of light. A burster is an x Ray flash from neutron star binaries, and a nova is a visible flash from white dwarf binaries. Stars can become black holes.

Auriga the Charioteer

Auriga has been denoted in history as a charioteer, driving his chariot across the sky for centuries. The bright star Capella appears in this constellation.

Newton

By the mid-seventeenth century, mathematical astronomy was done exclusively based on observed data. Theories and ideas were then adjusted to fit the observations. Isaac Newton came up with a new approach. He began by making three assumptions about the nature of the universe and then tried to see what observational results would follow logically from those assumptions. These are now known as Newton's laws. They are general statements that apply to all forces and bodies. Newton's Laws of Motion Law Description Law of Inertia An object at rest or in constant motion will remain that way unless acted upon by an unbalanced force. Law of Acceleration The acceleration of an object is proportional to the force acting on it. (F = ma) Law of Reactions For every action force, there is an equal and opposite reaction force. Newton was able to show that he could use these three laws and a formula for the force of gravity (which he had also derived) to derive Kepler's laws. Thus, Newton was able to accurately predict and describe the observed orbits of the Moon, other planets, and comets. Newton's Law of Universal Gravitation Fg = (Gm1m2)/(d2) Two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. "Fg" is the gravitaion force, "m1" and "m2" are the masses of the two bodies, and "d" is the distance between the two bodies. "G" is a constant, the value of which we are not concerned with in this course. This equation is also known as the inverse square law. Newton's idea that the universe was governed by forces acting on bodies served very well until the twentieth century came along and a young patent clerk decided that he had a lot of free time on his hands in which to think about the nature of the universe.

Canis Major and Canis Minor the Great and Little Dog

Canis Major is marked by the brightest star visible, Sirius. Sirius (the Dog Star) is the fifth closest star to the Sun, at nine light-years. Sirius is said to be responsible for the hot, muggy "dog days" that occur in September. Legend says that because Sirius rises the same time as the Sun during the late summer, its brightness adds to the Sun's energy, producing additional warmth. Canis Minor is a small, but important, constellation. The bright star Procyon is a bright yellow star, somewhat similar to our sun.

Capricornus The Goat

Capricornus has been recognized as a goat since Babylonian and Chaldean times. Usually, it is depicted as a goat with a fish tail, which seems to relate to the Greek god Pan. Pan, fleeing a monster called Typhon, jumped into the river Nile. The part of him that was below water turned into a fish, while the rest of him remained as a goat. For this reason, the constellation is also known as the sea-goat for it is part goat and part fish.

Cassiopeia the Queen

Cassiopeia was the queen of ancient Ethiopia. She is located in the sky next to her husband, Cepheus and her daughter Andromeda. Legend has it that the queen was extrordinarily beautiful and vain. This caused both her and her family many problems.

Cepheus the King

Cepheus was the king of ancient Ethiopia. He was Cassiopeia's husband and Andromeda's father. Shaped like a simple house, the constellation Cepheus comes to a point where we see a special star, a Cepheid. Later in the course, you will learn more about Cepheid variable stars.

Comets

Comets can well be described as dirty snowballs. They are largely composed of ice and dust. Comets are usually in highly-elliptical orbits that are often tilted from the ecliptic. As a comet approaches the Sun, heat from the Sun begins to vaporize the ice, resulting in a fuzzy haze of gases surrounding the nucleus (solid part of the comet). This haze is called the coma and can extend for as far as a million kilometers. Solar wind and radiation pressure then blow these luminous gases outward into a long, flowing tail. There are, in fact, two different tails produced. The ion tail is made of ionized atoms that are swept directly away from the Sun by the solar wind. The dust tail is made of dust particles that are blown away from the comet's coma by radiation pressure. It is usually arched. Astronomers usually discover about a dozen comets each year, most of which are long-period comets. They have orbital periods of 1-30 million years. Now, because astronomers are discovering long-period comets at a rate of about one per month, it is logical that there is a vast reservoir of comets orbiting about 50,000 AU from the sun. This has been called the Oort cloud, after the Dutch astronomer Jan Oort who first proposed its existence. There are an estimated 12 billion comets that reside in the Oort cloud.

Venus

Continuing our journey outward from the Sun we find Venus. This planet closely resembles Earth in its size, physical composition, and density. In fact, the earth is only slightly larger than Venus. However, there are significant differences. Venus' atmosphere is 95 times as dense as Earth's and is made up mostly of carbon dioxide (CO2). Earth's atmosphere is mostly nitrogen. The dense CO2 in Venus' atmosphere acts like a giant greenhouse, allowing solar radiation to enter the planet but trapping the heat in. As a result, the surface temperature of Venus is hotter than any other planet—even hotter than Mercury! The average surface temperature is 482°C (900°F). That's hot enough to melt lead! Venus rotates in a direction opposite that of Earth. This means that if you were on Venus, the Sun would rise in the west and set in the east. It rotates very slowly. In fact, the Venus "day" (243 days) is longer than the Venus "year" (225 days). Venus exhibits weather patterns, mostly high-altitude, high-speed circulations of clouds that contain sulfuric acid. Venus was the first planet to be explored with remote spacecraft. Mariner 2 was launched in August, 1962. More recently, the Magellan spacecraft, launched in May 1989, has been able to map 98 percent of the surface of Venus using radar mapping techniques. Most of the surface is covered by volcanic rock from lava flows that form the planet's vast plains. The Messenger spacecraft left in August 2004 and was able to fly by Venus in 2006 and 2007. Messenger is scheduled to study Mercury for a year in 2011.

Light year

Distance=speed x time, multiply the speed of light times the number of seconds in a year, and you get the distance of one light year. a light-year is the distance that light travels in one year. Since there are 31,536,000 seconds in a year, then we see that one light-year equals: 1 light-year = (300,000 km/s) x (31,536,000 s) = 9,500,000,000,000 km.

Classifications of galaxies

Edwin Hubble was the first astronomer to use distance indicators to determine the distances to other galaxies. While studying what was called the andromeda nebula he realized that he had found a cepheid variable. In 1924 he presented his finding that the andromeda nebula was 2.2 million light years away. That meant that it was much larger than was ever thought. It has to be a galaxy. Continuing his study of galaxies he discovered that they could be classified into four broad categories spirals, barred spirals, ellipticals, and irregulars. As he studied further, Hubble noted that spirals could be divided into more specific groupings based on the tightness of the spiral arms and the size of the central bulge. Spirals with tightly wound spiral arms and a large central bulge were called Sa galaxies, moderately wound spiral galaxies are called Sb, and loosely wound spiral galaxies with very small central bulges were called Sc. Similar divisions were made within the barred spiral galaxies SBa, SBb, and SBc. Galaxies are not the largest entities in the universe. They are grouped in clusters. The Milky Way and andromeda galaxy belong to a Galactic cluster called the local group. There are only two dozen or so galaxies in the local group, and is considered to be a small cluster. The Virgo cluster on the other hand contains 1,000 galaxies. It is located around 50 million light years away. The observable mass of such clusters of galaxies is not enough to explain the observed motions, so scientists believe that there is a lot of matter in the universe that we are unable to observe from earth, which is referred to as dark matter.

Eratosthenes

Eratosthenes was another Greek astronomer who contributed to our understanding of astronomy. He noticed that on the longest day of the year (the summer solstice, or June 21) the sun cast no shadow down a well in Syene, Egypt (presently the city of Aswân). He realized that the sun was directly overhead on this day in Syene. On the same day in Alexandria, Greece, there was a shadow in a well. Using trigonometry with this information, he was able to calculate the angle of the sun at Alexandria. Knowing the distance from Alexandria to Syene, he calculated the circumference of the earth (the distance around the earth). His calculations were quite accurate, off by a very small percentage of the true circumference.

Refracting Telescopes

Even though light travels extremely fast in the vacuum of space, it must slow down considerably when traveling through a dense substance, such as glass. Besides slowing down, light actually changes direction as it passes from one medium to another (such as from air to glass). This phenomenon is known as refraction. Because of the refracting properties of glass, a convex lens (thicker in the middle, thinner at the edges) causes light rays coming in to converge to a point called the focus. Using two lenses, one could construct a telescope. The first lens (larger objective lens) focuses light from a distant object, which can then be magnified and examined by using a second lens (eyepiece lens). This arrangement is called a refracting telescope. The magnification or magnifying power of a refracting telescope is given by a simple formula: the ratio of the focal length of the objective lens divided by the focal length of the eyepiece lens. Each color of light bends a little differently when it goes through glass. This effect is known as chromatic aberration. This can be seen by using a prism to split white light into various colors of light. Opticians can get around a lot of chromatic aberration by using certain chemicals in the glass to help reduce the effects until they become hardly noticeable. Despite their great power to magnify distant objects, refracting telescopes also have many disadvantages. Some of the light from distant objects dims as it passes through the lenses. Lenses must be totally free of any imperfections such as debris or bubbles, or images will be distorted. It is impossible to completely wipe out the effects of chromatic aberration. To build a large refracting telescope would require very strong supports to hold sufficiently large lenses. These supports would tend to block out important light. So the refracting telescope is limited in its use. Today, most large telescopes are reflecting telescopes

Quasars

Ever since, the 1960s astronomers have been observed certain objects that are over a billion light years away. To be able to see such an object, it would have to be unbelievably luminous. These objects appeared to be stars, but they were found to be moving away from us at such high velocities that they had to be extremely far away. These objects were given the name quasi- stellar objects or quasars for short. Quasars emit huge amounts of energy from a very small volume. A typical quasar is 100 times brighter than an average galaxy, such as the Milky Way. In fact, it is believed that quasars are no larger than our solar system. Imagine the concentration of energy necessary for something to be as bright as 100 galaxies, yet be only the size of our solar system! The extremely high concentration of energy necessary for quasars leads astronomers to believe that the centers of quasars contain black holes.

Galileo

Galileo Galilei was a contemporary of Kepler. He was the first person ever to use the new technological device, the telescope, for celestial observation. He observed things that no one had imagined, such as sunspots, mountains on the moon, and phases of Venus (similar to lunar phases). Galileo discovered that the size of Venus as seen through his telescope was directly related to its phase. When Venus was nearly full (gibbous), it was much smaller than when it was just a sliver (crescent). This observation supported the idea that planets revolve around the sun. In 1610, Galileo discovered four moons orbiting Jupiter. He observed their motion to be back and forth, which implied that they were in fact going around Jupiter. Astronomers were able to show later that these moons obeyed Kepler's Third Law, thus providing further evidence for our understanding of the universe. Galileo was treated as a heretic for his scientific ideas about a heliocentric universe, but his ideas inspired an English boy who was born one year after Galileo's death—Isaac Newton.

Pisces the fish

Has been seen as either one or two fish for thousands of years. According to Greek mythology, when the giant Typhoeus came upon the gods, they all transformed into animals in panic. Aphrodite and her son Eros, who were bathing in the river when Typhoeus came, took on the shape of a pair of fish.

Aquarius the water bearer

He is placed in the sky near the fish, a dolphin, a river, and a sea serpent. According to Greek mythology, Aquarius was Ganymede, whom Zeus kidnapped to be the cupbearer for the gods.

Bootes the herdsman

He is said to have been rewarded with a place in the sky for inventing the plow. Arcturus is a yellow-orange star, 37 light-years away. It is the fourth-brightest star visible from the earth.

Hercules (the Warrior)

Hercules was the half-mortal son of Zeus, king of the Greek gods. He was very strong and was a great hero. Please note that Heracles is the Greek name for Hercules, and these names are often used interchangeably.

Radiation

Is the third and probably most important mechanism of heat transfer. The sun is the source of nearly all of the energy on the earth, this solar energy is transmitted through space by means of radiation. Radiation or electromagnetic radiation is how energy from the sun is transmitted to the earth. This is the only type of wage that requires no medium and can travel through the vacuum of space. In fact, these waves travel at nearly 300,000 km/s. Energy from the sun is more than just visible light. There is an entire spectrum of electromagnetic radiation. From lowest to highest energy, we find radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays, and cosmic rays.

Convection

Is the transfer of heat because of the movement of mass from one location to another. It takes place only in liquids or gases. Heat gained by the lowest layer of the atmosphere by either conduction or radiation is most often transferred by convection, the statement warm air rises and cool air sinks is an example of convection this is how heat is transferred from the core of the sun to the outer layers.

Jupiter

Jupiter is the largest of the planets and the fifth planet away from the Sun. Its name comes from the Roman king of the gods (Zeus to the Greeks). Jupiter is almost like a miniature solar system. It has seventy-nine known satellites orbiting it. Galileo was the first to detect objects moving around Jupiter. With his telescope, he discovered four moons orbiting Jupiter. Through careful observation, he determined that they did indeed obey Kepler's laws. Most of what we know about Jupiter, though, has come from remote spacecraft observation. Pioneer 10, launched in 1972, was the first spacecraft to visit the giant planet. The next year Pioneer 11 was launched. Voyager 1 and 2, launched in 1977, were perhaps the greatest of NASA's space probe missions. Voyager 1 visited both Jupiter and Saturn. Voyager 2 visited Jupiter, Saturn, Uranus, and Neptune. This feat is only possible once every 175 years because of the alignment of the planets! As a result of these spacecraft missions, we have learned much about Jupiter. Jupiter is made mostly of hydrogen and helium. Its atmosphere also consists mostly of hydrogen and helium, though compounds of sulfur and phosphorus create the brown and orange hues seen. Ammonia ice crystals form white clouds in Jupiter's atmosphere. Several distinct bands appear in the atmosphere. These are strong wind belts. Jupiter rotates once every 9 hours and 55 minutes and takes 11.86 years to revolve around the sun. A very thin ring was also discovered around Jupiter. One of Jupiter's striking characteristics is the Great Red Spot. This is really a huge storm that has been raging for over 300 years! Two Earths would fit side by side on the Great Red Spot. The Voyager spacecrafts also observed huge lightning storms in Jupiter's atmosphere. Jupiter's axis of rotation is tilted only 3° from the vertical, so there is apparently no seasonal variation on the planet. In 1994, a comet struck Jupiter. This comet was discovered in 1993 and was called Comet Shoemaker-Levy 9 (SL-9). It was calculated that SL-9 had been in orbit around Jupiter and that, in 1992, it had passed very near to Jupiter. This close run caused the comet to break into fragments. This was the first time that scientists were able to predict such a collision and then observe it directly. Collisions of this magnitude occur about once every thousand years. The satellites of Jupiter are interesting to study. Only through remote spacecraft were we able to learn any detailed information about the moons of Jupiter. The four largest moons — often called the Galilean Satellites — are Io, Europa, Ganymede, and Callisto. The Voyager mission managed to photograph an erupting volcano on Io. This was the first time we had ever observed a volcanic eruption anywhere other than Earth. Its surface is full of orange and yellow regions. These are believed to be caused by sulfur-rich compounds on the surface of Io. Europa is about the same size as our moon and is the brightest Galilean satellite. Its surface is marked with dark lines, which are cracks in the icy surface. It is made mostly of ice. There is a rocky core at the center. It is believed that beneath the icy surface of Europa lies a liquid water ocean, heated by changing gravitational pulls between Europa, Jupiter, and the other Galilean satellites. Ganymede is the largest of all satellites in the solar system. It is even larger than the planets Mercury and Pluto. It is made up of about 50 percent ice and 50 percent rock. There are regions of dark and light patches on the surface. Callisto is only slightly smaller than Ganymede and is the least dense of the Galilean satellites. Callisto is made mostly of ice. In August 2011, NASA is scheduled to launch Juno to study the atmosphere, magnetosphere, origin, and interior of Jupiter.

Electromagnetic spectrum

Just how are the different types of EM waves formed? Think back on how any EM wave is formed—from electrons jumping from a higher energy level to a lower one. Electrons that jump farther will generate photons with more energy. These energetic photons form EM waves of higher energy. All electromagnetic waves travel at the same speed in a vacuum—the speed of light (300,000 kilometers per second). The types of waves on the spectrum are radio, infrared, visible light, ultraviolet, x-rays, and gamma rays. This is the order of increasing energy as well as increasing frequency. Since the speed of all EM waves is the same, we can see that if frequency gets higher, then wavelength must get shorter. Thus, the order of EM waves is also in decreasing order of wavelength. Another unique property of EM waves is the fact that they require no medium. EM waves can travel through the vacuum of space.

Lyra the Harp

Known as the lyre or harp, Lyra was a gift given to Apollo's son Orpheus, who played it exquisitely. Vega is one of the brightest stars visible here on Earth.

Gemini the twins

Leda gave birth to twins castor and Pollux. Pollux was immortal and castor was mortal when castor died Pollux begged Zeus to allow castor Immortality. Zeus reunited these brothers in the sky. Notice that not only are the brothers shown in the constellation but as the two bright stars at their heads.

Leo the Lion

Leo was the Newman lion in Greek mythology. Leo was terrifying the people and killing sheep, but was so strong that no spear could kill him. Hercules strangled the beast until he died. Hera was so made at Hercules that she sent Leo's spirit to dwell in the sky. On November 17 the leonid meteor shower originated from the direction of Leo

Libra Themis

Libra, seen as a set of scales, is associated with Themis, the Greek goddess of justice. Themis gave the scales to her daughter Astraea, who lived with the mortals during the Bronze Age, using her scales to weight the good and bad in a persons heart, and thus assign their fate.

Mars

Mars received its name after the Roman god of war, and is sometimes referred to as "the red planet" because its surface contains large amounts of iron oxides, which have a reddish color. Although smaller than both the Earth and Venus, and only twice the size of the Moon, astronomers had long believed that Mars was the most likely candidate for another planet to have life. In the nineteenth century, Percival Lowell used a telescope and noticed what appeared to be straight lines criss-crossing the Martian surface. He proposed that these were canals dug by living, intelligent beings. He drew many sketches of these "canal systems." Another reason that scientists believed there might be life on Mars was the seasonal color changes on the surface. Scientists speculated that this was evidence for blooming Martian vegetation during the summer months and dormancy during winter months. In order to find out more, we have sent numerous unmanned spacecraft to Mars. Four Mariner spacecraft have studied Mars. None of them found any canals or other evidence of life. They did find a thin atmosphere consisting primarily of carbon dioxide. Mariner 9 discovered a huge planet-wide dust storm going on. After several weeks, when the storm had subsided, Mariner 9 was able to reveal evidence that there was once running water on the Martian surface. Dry riverbeds and flood plains were discovered. In 1975, the Viking 1 and 2 missions were sent to Mars to find out if there was any life. No one expected at this point to find civilizations, or even intelligent life, but they still believed in the possibility of simple life on Mars. Both Viking 1 and 2 contained landers, which made it successfully to the surface and gave us our first up-close photographs of another planet. The surface was barren, consisting of a rolling plain with rocks scattered all around. The landers conducted experiments to determine if there was life in the soil, but results were inconclusive. They did not find any evidence that organic molecules were there. However, different evidence came to light with NASA's Phoenix. In 2007 and 2008, the Mars lander Phoenix detected snow! About 4 km above the surface, snow clouds formed, but the snow was vaporized before reaching the surface. Phoenix also detected calcium carbonate, a definite indicator of liquid water in the past. The first indication of calcium carbonate was the soil releasing hot carbon dioxide. This leads scientists to again wonder about the possibility of life on Mars. Temperatures on Mars range from -120°C in winter to -14°C in summer. Mars has two satellites, or moons, Phobos and Deimos. Both are small and irregularly shaped. They were probably asteroids that were captured into orbit around Mars by gravity.

Meteors, Meteoroids, Meteorites

Meteoroids are chunks of rock in space, much the same as asteroids, but we usually refer to them as asteroids only if they are larger than a few hundred meters across. A meteor is a brief flash of light that is visible when a meteoroid strikes the earth's atmosphere. Meteors are commonly called shooting stars. If meteoroid fragments survive and strike the earth, then they are called meteorites.

Corona Borealis (the northern crown)

Named for its shape, Corona Borealis is Latin for "northern crown." Corona Borealis is believed to be Ariadne's wedding crown, made by the goldsmith Hephaestus and a gift from Ariadne's husband Dionysus.

Neptune

Neptune was the final planet to be visited by Voyager 2. Neptune was observed to be very similar to Uranus. It is nearly the same size, and also appears blue because of methane in its atmosphere. However, Neptune's atmosphere appears to be more active than Uranus's. A large dark spot was observed on Neptune. This is a huge storm, similar to the Great Red Spot of Jupiter. The highest wind speeds of any planet were observed here, reaching as high as 2400 kilometers per hour (1500 mph). Neptune has eight satellites, the largest of which is called Triton. Triton orbits Neptune every six days. The surface of Triton is the coldest temperature directly observed in the solar system at -235°C (-391°F). It is believed that Triton is similar to Pluto. The Voyager 2 discovered that Neptune had a very faint set of rings, thus making rings a feature common to all Jovian planets.

Copernicus

Nicolaus Copernicus studied astronomy and detailed the advantages of a heliocentric cosmology (sun-centered). It was much simpler than the cumbersome geocentric cosmology (earth-centered). He had read the work of Aristarchus and expanded on his ideas. He realized that in a heliocentric model, he could determine which planets were closer to the Sun than to the earth; he could even determine the order of planets around the Sun. Because Mercury and Venus are always observed fairly close to the Sun, he concluded that they must be closer to the Sun. The other visible planets—Mars, Jupiter, and Saturn—can be seen in the middle of the night when the Sun is far below the horizon. This could only occur if the earth were between the Sun and these planets. So the orbits of Mars, Jupiter, and Saturn must be larger than the earth's. At this point we may talk about geometrical arrangements, or configurations, of the planets (see figure below). When an inner planet (Venus or Mercury) is lined up between the earth and the Sun, this is called inferior conjunction. When it is on the opposite side of the Sun, it is called superior conjunction. The angle between the Sun and a planet as viewed from the earth is called the elongation. At greatest eastern elongation, a planet is as far east of the Sun as it can be. The planet appears above the western horizon after sunset and is referred to as an evening star. When we say a planet is east of the Sun, think of the path the Sun takes. It appears to move from east to west (rising in the east, setting in the west). A planet that rises after the Sun is said to be east of the Sun, because if you could see it at noon, when the Sun is highest, the planet would be to the east of the Sun. Also, a planet that rises just before the Sun would be west of the Sun, because at noon it would be to the west of the Sun. Greatest western elongation is the farthest west a planet reaches. It may be seen before sunrise in the eastern horizon and is called a morning star. Outer planets are simpler in their configurations. There are only two special configurations, opposition and conjunction. At opposition, the planet is on the opposite side of the earth (from the Sun). Such a planet could be seen directly overhead at midnight. Conjunction occurs when the planet is lined up directly behind the Sun. Copernicus realized that just studying the planets at various configurations would not really tell him anything about the orbits of these planets around the Sun because the earth is in constant motion as well. He was careful to distinguish between two kinds of periods. A period is the amount of time it takes to complete one revolution. There are the synodic period and the sidereal period. The synodic period refers to the time that elapses between two successive identical configurations, as seen from the earth (from one opposition to the next, from one conjunction to the next, etc.). The sidereal period is the true orbital period of a planet (the time it takes the planet to complete one orbit around the Sun). The synodic period can be readily measured by observing the sky. The sidereal period, however, would have to be calculated. So Copernicus did the calculations. With values for the sidereal periods of each planet, he calculated the distances of each planet from the Sun. His values were quite accurate. A Dutch astronomer by the name of Tycho Brahe decided to test Copernicus' theory of a heliocentric universe. Brahe spent his lifetime making accurate observations of stars and planets, achieving an unprecedented level of precision. We know that when we walk, nearby objects appear to shift position when compared to very distant objects. If Copernicus was correct, then the nearby stars should shift position relative to more distant stars as the earth orbits the Sun. In spite of Brahe's accurate measurements, he could not detect any such shifting, so he concluded that Copernicus was wrong. In reality, stars are much farther away than anyone at that time realized, so his naked-eye observations would have been unable to detect the shifting. (Modern telescopes have recorded this shifting.) Brahe's work did not prove beneficial to Copernicus' work in his lifetime, but when Brahe died, most of his charts and data fell into the hands of his assistant, Johannes Kepler.

Ursa Major and Ursa Minor the Big Dipper and the Little Dipper

Often called the Big Dipper and Little Dipper, their names mean Great Bear and Little Bear. According to Greek legend, Zeus and his lover Callisto had a son named Arcas. Hera, Zeus' jealous wife, turned Callisto into a bear. Arcas almost killed her while he was hunting. Zeus rescued Callisto and placed them both in the sky. Callisto is Ursa Major and Arcas is Ursa Minor. Polaris is known as the North Star because Earth's north pole points almost exactly at Polaris.

Cygnus the Swan

One myth claims that Orpheus was transported to the sky as the swan Cygnus so that he could be near his precious lyre. Deneb is 25 times more massive and 60,000 times brighter than our sun. It is distant, about 1,500 light-years away.

Comparison of the Terrestrial Planets

One of the striking characteristics of these planets is the atmosphere. Both Venus and Mars have atmospheres of mostly carbon dioxide (CO2). So why does the earth have a nitrogen-oxygen atmosphere? Vast amounts of liquid water on the surface of Earth are responsible for dissolving much of the CO2 that once existed in our atmosphere. Much CO2 is also chemically bound in rocks such as limestone and marble. Essentially, the reason we don't have much CO2 in our atmosphere is because of life. Living processes, such as photosynthesis, take the CO2 out and replace it with oxygen. On the earth's surface, the atmosphere weighs down with a pressure of 14.7 psi (pounds per square inch). This pressure is defined as one atmosphere (1 atm) and is equal to 29.92 inches of mercury, 760 mmHg, 101.3 kPa, or 1013 mb. (These are just different units used to measure pressure.) Surface temperatures on Venus are hot enough to melt lead. Mars has a very thin, dry atmosphere. It is marked by huge extinct volcanoes and gigantic canyons. The earth is distinctive in its liquid water oceans, life, active geologic processes—such a plate tectonics and volcanoes—and its folded mountains. So why did the earth end up so different from its neighbors? What happened to the water on Venus and Mars? You might suppose that if they were all formed from the same "stuff" they should all be very similar. Water most likely did exist on both Venus and Mars in quantities similar to that of the earth. Venus, however, was so close to the Sun that all of the water evaporated. In the upper atmosphere, harmful radiation from the Sun split the water molecules into their component hydrogen and oxygen, which were less dense than the CO2 atmosphere; so they floated to the extreme upper portion of Venus' atmosphere and were eventually lost to space. Mars, however, yields the opposite story. It is too cold on Mars to support liquid water, so scientists believe that most of the water is frozen in the polar caps or in permafrost across the surface of Mars. Earth is positioned in just the right place to have temperatures that allow liquid water to exist on the surface. Earth has a strong magnetic field, but neither Venus nor Mars has much of one. Scientists believe that the liquid part of our iron core generates a magnetic field as the earth rotates. Venus could theoretically have a magnetic field if it rotated faster, but its period of rotation is about 243 Earth days. Mars, on the other hand, has a very weak magnetic field; scientists believe this is because it lacks a molten iron core. The earth's strong magnetic field actually protects us from harmful particles that shoot out from the Sun. Our magnetic field creates what is termed the magnetosphere, or an envelope of magnetic protection.

Parsec

One parsec equals about 3.26 light-years. Both light-years and parsecs are convenient units for measuring distances to nearby stars. For example, the nearest star to our sun is about 4.3 light-years (1.3 parsecs) away. This means that light coming from this star takes 4.3 years to get here! When we look into the night sky we are looking into the past. Suppose a star that is 500 light-years away were to explode tonight. We would not know about it for 500 years! It would take that long for the light to arrive here.

Pluto

Pluto was considered a planet from the time it was discovered in 1930 until August 2006, when the International Astronomical Union formally defined what a planet is. Pluto did not meet the classifications of a planet, but was reclassified as a "dwarf planet" along with other bodies like Ceres and Eris. Thus there are officially only eight planets in our solar system.

Ptolemy

Ptolemy was able to solve this problem by inventing the idea of epicycles, or spheres within spheres. The planets were attached to a smaller sphere (epicycle) which would spin while the larger sphere (called the deferent) spins around the earth. The motion caused by the epicycle would sometimes add to the speed of the planet and sometimes subtract from its speed. Thus, occasionally, we see retrograde motion. Ptolemy was actually able to predict quite well the exact location of the planets by incorporating epicycles to Aristotelian theory. Ptolemy compiled much of the astronomical information available in his time. He calculated the sizes of each epicycle and deferent. He assembled all of his calculations into thirteen volumes called Almagest, in which the positions and paths of the Sun, Moon, and planets were described with unprecedented accuracy. It was so successful that for over a thousand years Ptolemy's work endured as a useful description of how the heavens worked

Sagittarius the archer

Sagittarius is the gentle centaur Chiron. He taught heroes like Achilles, Jason, and Hercules, and loved archery, music, and medicine. Hercules mistakenly shot Chiron with a poisoned arrow, causing Chiron such pain that he offered to give up immortality and substitute himself for Prometheus' punishment. (Prometheus had given fire to mortals, and been chained to a rock as punishment, where each day an eagle would eat his liver. Each night it grew back.)

Active galaxies

Some galaxies are much brighter than normal galaxies, but not as bright as quasars. These are called active galaxies. The first galaxies were discovered in 1943 during a survey of spiral galaxies by Carl seyfert. These galaxies have very bright starlike galactic nuclei and are called seyfert galaxies. Seyfert galaxies sometimes vary in brightness. One example, called NGC 1068, can change its luminosity in a few weeks by an amount equal to the total luminosity of the Milky Way! There are other types of active galaxies, but we will not go into details in this course.

Saturn

Saturn is the sixth planet from the Sun. The most distinguishing characteristic of Saturn is its beautiful set of rings. The rings have seven major divisions and are composed mostly of ice crystals. These ice crystals range in size from a few centimeters to several meters. The major rings have several hundred little ringlets. A possible explanation for the rings is that a moon or comet ventured too close to Saturn and was crushed by strong gravitational forces. Another possibility is that the object never fully formed and disintegrated before it could completely form. A third possibility is that the object was shattered by other large objects revolving around Saturn. These ringlets are held in place by the gravitational interaction between Saturn and its satellites. Thus, they are not able to drift away or coalesce into a single object. Saturn is composed mostly of hydrogen. Its atmosphere contains very fast moving jet streams. Near the equator, wind speeds of 1770 kilometers per hour (1100 mph) were measured. The planet rotates once every 10 hours and 40 minutes and takes 29.46 years to complete one revolution around the Sun. Its axis of rotation is tilted 26.7°, so Saturn does have seasons. Saturn has a strong magnetic field as well. It is 1000 times stronger than Earth's magnetic field. Saturn has eighty-two known satellites. Of these, Titan is the largest. It is slightly larger than Mercury and has a very thick atmosphere of nitrogen. This is much more similar to Earth's atmosphere than the carbon dioxide atmospheres of Venus and Mercury. Unfortunately, the Voyager probes were unable to peer through the thick atmosphere of Titan. A newer probe, the Cassini probe, has been dispatched to study Titan and the Saturn system more closely. So far, Cassini has sent back evidence that Titan is composed of both rock and ice, still thoroughly mixed. This has led scientists to wonder if there is an ocean beneath the surface of Titan. Cassini has also found liquid hydrocarbons, particularly ethane, on Titan's surface. This is the only known occurance of surface liquid in the solar system besides on Earth. It will be interresting to see what else Cassini can uncover about Saturn and her moons.

Magnitude scale

Scientists developed a magnitude scale to describe how bright an object appears to an observer. The system was actually used by the Ancient Greek astronomer Hipparchus. He called the brightest stars first magnitude stars then second magnitude stars (ect.) sixth magnitude stars were the dimmest that could be seen. In the 19th century astronomical techniques improved to the point that we could actually measure the amount of light arriving from a star. So we improved the magnitude scale in such a way that a difference in magnitude of 5 would correspond to a factor of 100, the actual difference in light arriving. It would take stars 100 stars of magnitude 6+ to equal the light coming from one star of magnitude +1. A shift in magnitude of 1 would be a factor of about 2.5, becuase 2.5x2.5x2.5x2.5x2.5= 100. So it takes about 2 magnitude +3 stars to provide as much light as we receive from one +2 magnitude star. The magnitude scale was expanded to include dimmer objects now visible through telescopes and brighter objects. Negative numbers were used to include bright objects . For example Venus at its brightest shines with a magnitude of -4.4. Sirius the brightest star has a magnitude of -1.5. The sun has a magnitude of -26.7. These magnitudes are actually what is called apparent magnitude because it is how bright the object appears to the observer. This scale says nothing about how bright an object actually is. Bright objects that are very far away appear to be dim, whereas even dim objects may appear relatively bright if they are close. To better understand stars, astronomers have developed a scale of absolute magnitude, which is a measure of the energy output of the star. Absolute magnitude is defined as the apparent magnitude an object would have if it were located at a distance of ten parsecs. If the sun were at that distance it would have an apparent magnitude of +4.8 so the absolute magnitude of the sun is +4.8. Absolute magnitudes for other stars range from -10 for the brightest to about +15 for the dimmest. So the sun is pretty average as far as abosulote magnitude is concerned. As light from a star moves away from the star, it spreads out. As it spreads out it appears dimmer. This is why the farther away from a light source you are the dimmer it appears. The inverse square law tells us that the apparent brightness of a light source is inversely proportional to the square of the distance between the source and the observer. If you double the distance then the apparent magnitude drops by a factor of four (22=4). Triple the distance and the light appears only 1/9 as bright. Using the inverse square law, scientists have developed an equation that relates absolute magnitude appears to magnitude and distance to a star, knowing two of these, you could find the third. Many scientists prefer to speak of a stars luminosity instead of absolute magnitude because luminosity is a direct measure of how much energy output a star is giving off. Recalls that Ls (the luminositymof the sun) is 3.90 x 10^26 watts. For confidence stellar luminosities are often expressed in terms of Ls. The brightest stars (mag -10) have luminosities of 10^6 Ls, or one million times the luminosity of the sun. Dim stars (mag +15) have luminosities of 10^-4 Ls. There is a direct correlation between the mass of a star and its luminosity. The more massive the star, the greater the luminosity.

Scorpius the scorpion

Scorpius is the scorpion who killed the legendary hunter Orion. Scorpius may have been sent by Apollo, who was concerned for his sister Artemis, or Scorpius may have stung Orion simply to disprove his boast that he could kill anything, man or beast. These two constellations are placed in opposite sides of the sky to avoid further trouble between them, according to legend. Antares is a red supergiant star.

The Jovian planets

The Jovian planets were formed far away from the Sun. As a result, temperatures were cool enough that volatile substances such as methane, water vapor, ammonia, and other gases could collect in large amounts. The inner planets were simply formed in a region that was too warm for these volatile substances to condense substantially. Scientists believe that the formation of the Jovian planets was a two-step process. First, large protoplanets formed out of ice-coated dust grains and other such material. Then, under the gravitational pull of these massive protoplanets, gases became trapped in substantial quantities. As a result, the Jovian planets are much larger than the terrestrial planets. The four Jovian planets are Jupiter, Saturn, Uranus, and Neptune. Each of them has a very thick atmosphere consisting mainly of hydrogen, with some helium and other gases.

The Milky Way

The Milky Way often refers to a band of many stars that can be seen on a clear dark night. It is really our edge on view of our galaxy. A Galaxy is a large group of stars. An average galaxy will contain approximately 100 billion stars and measures about 100,000 light years in diameter. The galaxy has a definite structure to it. Much of the matter is found near the center of the galaxy. A lot is found rotating around this center in a relatively flat disk. The sun is found in this disk, which is often called a galactic disk. There are some clusters of stars that are orbiting the galactic center at some angle to this disk. Because of their spherical shape, they are called globular clusters.

The moon

The Moon is Earth's only natural satellite. It is the only alien world upon which men have landed. Before the Apollo missions that actually sent men to the moon, NASA launched the automated Ranger, Surveyor, and Lunar Orbiter spacecraft to study the Moon between 1964 and 1968. The Apollo missions included six crews of two astronauts landing on the Moon and exploring its surface. These astronauts were able to bring back over 2000 samples of lunar rocks and soil. Most of these rocks were volcanic in origin. There appears to have been no volcanic activity on the Moon for over 3 billion years, according to scientists. The only surface activity is caused by micrometeorites, atomic particles from the sun, rare impacts of large meteorites, and by spacecraft and astronauts. The origin of the Moon is still a mystery to scientists. There are four possible theories: the Moon formed near the earth as a separate body; it was somehow torn from Earth; it formed elsewhere and was captured into orbit by Earth's gravity; or it was the result of a massive collision of the earth with an asteroid about the size of Mars.

Sun's motions

The Sun is directly above the equator at the vernal equinox (March 21). As the earth revolves around the Sun, its axis does not change direction. Thus, at the summer solstice (June 21), the Sun is directly above the tropic of Cancer (23 ½ degrees N latitude). This is the farthest north that the Sun can be found straight above the ground. On the autumnal equinox (September 23), the Sun is once again above the equator. Finally, on the winter solstice (December 21), it is above the tropic of Capricorn (23 ½ degrees S). The Nothern Hemisphere enjoys summer while the Sun's rays are hitting it most directly. Solar energy is more concentrated in the Northern Hemisphere between the vernal equinox and the autumnal equinox. Conversely, the Southern Hemisphere receives more concentrated solar energy between the autumnal equinox and the vernal equinox. For this reason, winter in the Northern Hemisphere is at the same time as summer in the Southern Hemisphere. The term period is often used in astronomy to describe the time required to complete a cycle of some sort. For the earth we have the period of rotation equal to twenty-four hours (one day), and the period of revolution equal to 365 ¼ days (one year).

Draco the dragon

This constellation can be seen at any time of year in the northern hemisphere. The stars that make up Draco are not very bright, so at times it may be difficult to pick out. The dragon's tail wraps around close to the handle of Ursa Minor and Polaris.

Perseus the Warrior

The brave Perseus came to the rescue of Andromeda, riding the winged horse Pegasus. The bright star Algol has been called the star that winks. About every three days, its brightness drops by a factor of 3 for about 10 hours. This is because Algol is really a double star, and when one passes in front of the other, it blocks some of the light.

Mercury

The closest planet to the Sun is Mercury. Because of its proximity to the Sun, it was difficult to obtain very good images of Mercury from Earth-based telescopes. Most of what we know about Mercury comes from space probes that we have sent there. The Mariner 10 spacecraft, launched in 1973, passed within 703 kilometers (437 miles) of Mercury's surface. Photographs sent back to Earth from Mariner 10 revealed a heavily-cratered surface, much like the surface of our own moon. We also found out that Mercury has a weak magnetic field and a trace of atmosphere (one-trillionth the density of Earth's atmosphere). Temperatures on Mercury range from 467°C (872°F) on the side facing the Sun to -183°C (-298°F) on the side facing away from the Sun. This range in surface temperature is the largest for any planet or moon in the solar system. Mercury completes an orbit around the Sun in eighty-eight days, and rotates once every fifty-nine days. This combination makes the Mercury day (from noon to noon on Mercury) 176 Earth days, two Mercury years long!

Spectra and HR diagrams

The color of a star reveals the surface temperature of that star. Hotter stars give off shorter wavelength radiation. On the visible color spectrum reds and organges are long wavelength, and blues and violets are short wavelength. So blue stars are hotter than red stars. A stars spectrum gives a clue to the temperature of the star. The spectrum of a star refers to the specific frequencies of. Electromagnetic East Jon that it gives off. Angelo secchi was the first to attach a spectroscope to his telescope and point it toward stars. He noticed that different stars exhibited different spectral Lin's. He was the first astronomer to classify stars by their spectral types. The spectral classification system went through some modifications and today stars are classified each with a designated letter O, B, A,F,G,K,M. Stellar spectra were shown to be directly related to stellar temperatures. So this list of spectral types is also a listing of temperature classes generally the hotter a star burns the more quickly it will exhaust its fuel and end its time in the main sequence o stars are the hottest in excess of 35,000 degrees c, m stars are the coolest around 3,000 degrees c. The seven classifications ads subdivided into ten subcategories assigned an integer from 0 to 9 our sun is a G2 star. As astronomical techniques improved astronomers were able to more accurately measure parallax and thus the distance to many stars (allowed more accurate measurements of absolute magnitude).This occurred at about the same time that spectral astronomy began to make progress. Hertzprung and Russell were two astronomers who independently discovered a relationship between spectral types of stars and absolute magnitude. Plots of this kind are known as H-R diagrams. When they plotted this information they found that stars weren't randomly distributed, and were in specific groups. The majority of stars fell onto a curved line and are called main sequence stars the sun is one of these. To the right and just above the main sequence are a group of stars that are brighter and cooler. Because cooler stars shine less, these stars must be very large and are called giants, and are called red giants because they appear red in the nighttime sky. Arcturus in the constellation bootes is a red giant. There are a few stars that are considerably bigger and brighter than normal red giants, and are called super giants. Betelgeuse in Orion and Antares in Scorpius are supergiants. The other distinct grouping of stars lies below and to the left of the main sequence. These stars are hot, dim, and small, and are called white dwarfs.

Parallax angles

The distance to nearby stars can be determined using a concept called parallax. Parallax is the apparent shifting of position of an object because of a change in the pound of view. The distance from the earth to a star can be measured using the stars parallax. If we measure the parallax angle in arc- seconds, then the distance is equal to the inverse of the parallax. In math language d=1/p (distance equal 1 over p) where d is the distance measured in parsecs (1pc= 3.26 ly) and p is the parallax angle measured in arc seconds

Cancer the crab

The envious Hera sent cancer to distract Hercules while he was fighting the monster hydra. Cancer was crushed under Hercules feet during the battle. But was rewarded for his efforts by being given a place in the sky.

Reflecting telescopes

The law of reflection states that the angle of incidence equals the angle of reflection. This means that if you were to draw a perpendicular line to the surface of a mirror, then a light ray coming in would be reflected out at the same angle from that line. Isaac Newton realized that a concave mirror would cause light rays to converge to a focus. (Concave means that it is thicker at the edges than in the middle.) Using this idea, he constructed the first reflecting telescope. Again, the focal length is the distance from the mirror to the focus. Four different focusing schemes have been developed for reflecting telescopes. Reflecting telescopes are also not without problems. Similar to chromatic aberration is something called spherical aberration. This is created when a concave mirror is cut into a spherical surface. Light rays from the outer edges of the mirror focus slightly closer than light rays from near the center of the mirror. This problem can be avoided by using a parabolic shaped mirror instead of a spherical mirror. Unfortunately, it is much more difficult to polish a mirror's surface to a parabola, so parabolic mirrors are more expensive.

Protoplanet Hypothesis

The most commonly believed theory for the formation of the solar systems involved the formation of the sun as well as all other components. The protoplanet hypothesis claims that the solar system began as a huge cloud of gas and dust called a nebula. This slowly rotating cloud begins to condense because of gravity forming a concentration of matieral in the center, which would eventually become the sun. The test of the material forms a flat disk as material begins to rotate faster. This would explain why the orbits of the planets now are very nearly in the same plane (ecliptic) temperatures near the center of the new sun, would be much warmer than in father regions. For this reason icy debris close to the sun would be vaporized, leaving only rocky material. This would explain why the terrestrial planets are composed largely of rocky material. Farther away, icy subdtances would remain solid, which explains why the Jovian planets are made mostly of less dense icy, and gaseous matieral. The inner planets therefore, were formed mostly from the fusing together of solid rocky material, and began simply as dust grains and pebbles collided and stuck together, and then over a few million years, these little accumulations coalesced into objects called planetesimals, with a diameter of about 100 km. Gravitational attraction between planetesimals then caused collisions, which coalesced into larger bodies called protoplanets. The process of accumulating material in this wat is called accretion. The rocky material was composed largely of mineral forming elements such as iron, silicon, magnesium, sulfur, aluminum, calcium, and nickel. Energy from the collisions as well as from radioactive decay caused these minerals to melt, so terrestrial planets began as spheres of molten rock. The formation of the outer planets was also marked by the accretion of rocky and gaseous material, but there was so much more gaseous and icy material out there that these substances dominated the accretion, the sun was in its formative stages during this time of planetary accretion.

Planet Order

The solar system is composed of our Sun, eight planets (most with moons), asteroids, comets, and so forth. The order of the planets from the Sun outward is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (Pluto has been considered a "dwarf planet" since 2006. For nostalgia's sake, we leave it in the following table.) In Figure 3.1 we see the planets shown in order of increasing distance from the Sun. Below is a table listing names of the planets, along with the distance from the sun, number of satellites, and average temperature for each planet.

Electromagnetic Spectrum

The source of light is the atom. Recall that atoms are made of a nucleus with electrons outside. These electrons can exist only in specific areas, known as energy levels. Electrons are only stable in their ground state, or lowest energy level. If an atom absorbs energy, an electron jumps to a higher energy level. Since this is unstable, the electron will return to a lower energy level. In order to do so, it must release the absorbed energy. It does this by emitting a photon. A photon is a tiny packet of energy. Light is made of a stream of photons. The amount of energy in the photon is equal to the amount of energy that the electron absorbed.

Fusion and heat transfer

The sun makes up 99.86 percent of the mass of the solar system. All of the planets, asteroids, comets, dust, moons, ect. make up the other 0.14 percent, 109 earths would fit across the sun's diameter, over 1.3 million earths would fit inside the sun. Even so, our sun is average sized for a star . Fusion: in the 19th century people believed that the sun was made of burning coal. This was the only way they knew how to explain the vast amount of energy that the sun continous Ky put forth. In 1905 Albert Einstein proposed the theory of special relativity, one of the implications of this theory was that energy and matter are interchangeable according to the famous equations E=MC2, more simply put, mass (m) can be changed into energy (e) in an amount equivalent to mc2, where c is the speed of light, now the speed of light is a huge number so c2 is extremely large. A very small amount of matter can produce a very large amount of energy. In the 1920s a British astronomer Arthur eddington suggested that temperatures at the core of the sun were hot enough to allow thermonuclear fusion. In thermonuclear fusion, four hydrogen atoms actually fuse together to form a single helium atom, a hydrogen atom consists of a single proton, and a helium atom consists of two protons and two neutrons. Two of the four protons from the hydrogen atoms change from protons into the neutrons. This release two positively charged electrons called positrons, to get rid of the positive electric charges. In addition, massless particles called neutrinos are also given off. The four hydrogen atoms have a slightly greater mass than one helium atom. When hydrogen is converted into helium, a slight amount of mass is lost, the mass that is lost is converted into energy. The sun mass (ms) is 2x10^30 kg and the suns total energy output called its luminosity (L s) is 3.9 x 10^26 watts. To produce this luminosity the sun needs to consume 600 million metric tons of hydrogen every second. This may seem like a lot but at that rate of consumption the sun still has about 5 billion years of fuel left, the process of fusion by which hydrogen is converted to helium is commonly called hydrogen burning, even though there is no real burning taking place. Hydrogen burning takes place at the suns core, where temperatures are hot enough to allow protons to move fast enough to overcome electrical repulsion and fuse together. Temperatures is in the suns core reach 15 millions degrees c. The sun is not undergoing any dramatic changes right now. It is in balance both mechanically and thermally. This means that it can support its own weight (hydrostatic equilibrium) and that it keeps putting out a constant amount of energy (thermal equalibrium). So how does the sin keep shining steadily. Energy flows from hot to cold areas by one of three mechanisms discussed below.

Earth

The third planet from the Sun is our own Earth. As seen from space, Earth's distinguishing characteristics are the large blue oceans, brown and green land masses, and white clouds. Our atmosphere consists of 78 percent nitrogen, 21 percent oxygen, and one percent other gases. This is a unique mixture of gases found nowhere else in the solar system. Earth is the only planetary body in the solar system known to harbor life. It orbits the Sun at an average distance of about 150 million kilometers (93 million miles). Earth is the fifth largest planet in the solar system and the largest of the Terrestrial planets. Active geologic processes have left very little evidence of meteoric impact craters on the earth's surface. Erosion from wind and water, combined with tectonic activity of volcanoes and earthquakes, have nearly completely erased any such evidence. Earth is unique in many ways. One example of this is the vast amount of liquid water found on the surface. Over 70 percent of the earth's surface is covered with liquid water. Water can exist as a liquid only in a narrow range of temperatures. Above 100°C, water boils, becoming a vapor. Below 0°C, water freezes, becoming a solid. Earth is located just the right distance from the sun to allow average temperatures to fall within this range. This is important because liquid water is essential to life as we know it. Our planet rotates rapidly, requiring only about twenty-four hours to complete a rotation. Because of this rapid rotation and a molten, nickel-iron-rich core at the earth's center, we have a strong magnetic field surrounding us. It is this magnetic field that allows a navigator to use a compass. The compass needle lines up with the magnetic field of the earth, pointing the navigator to the direction of north, from which other directions are known. This magnetic field of Earth serves another, more critical function as well. It deflects most of the harmful particles and radiation of the solar wind. (The solar wind is a stream of charged particles emanating from the sun.) Our atmosphere protects us from most meteors as well, most of which burn up before they can reach the surface. The first American satellite, Explorer 1, was launched in January, 1958. Explorer 1 discovered an intense radiation zone, now called the Van Allen radiation belts, surrounding the earth. Other satellites have helped us learn more about our planet. Satellites are used daily to help predict weather. These satellites have undoubtedly saved thousands of lives by giving warnings of approaching hurricanes or other violent weather. Satellites are also used to monitor our natural resources.

What to look for in a telescope

There are many different factors to take into account when shopping for a telescope. You may consider the price, length, diameter, brand name, type of focusing, type of mounting, reflecting vs. refracting, the purpose for buying, etc. A good rule of thumb is to decide how much you are willing to pay for a telescope and then buy the largest diameter that you can for that price. The larger the diameter, the more light it will be able to collect. The amount of light collected is proportional to the area of the telescope's opening (sometimes called aperture). Magnification is not an important factor at all. Magnification can be changed with a simple change of eyepiece. Every time magnification is doubled, the amount of light received is reduced by one-fourth. Thus, telescopes that boast 400x or 600x magnification are really not useful for studying most of the objects in the night sky, because very little usable light remains. The optimum viewing magnification is about 15x per inch diameter. For example, a six-inch diameter telescope yields an optimum magnification of 15 x 6, or 90x.

Charge coupled device

There are other ways of gathering astronomical data besides looking through an eyepiece of a refracting or reflecting telescope. Astronomers used to use cameras in conjunction with telescopes in order to record their views of the sky. However, now a device called a charge-coupled device (CCD) is available. It is a very sensitive light detector made of a thin silicon wafer about the size of a postage stamp. The wafer is divided into an array of tiny light sensitive compartments called pixels. When astronomers focus the light from a telescope onto a CCD, electric charges build up separately in each pixel in proportion to the amount of light on each pixel. This information is then transferred to a computer for more thorough analysis. CCDs have as many as four million pixels, so extreme detail is possible using this technology.

Asteriods

There is much debris still revolving around the sun that formed early in the solar system's history. Just as conditions produced two types of planets, there are also two types of debris left over. Rocky fragments are called asteroids, and dusty chunks of icy material are called comets, which we will discuss more later. In the late 1700s, a German astronomer by the name of Johann Elert Bode popularized a catchy formula for determining the distances of planets from the Sun. It was given the name Bode's law, even though it is not a physical law. Astronomers now regard this "law" as pure coincidence, but it did play an important role in discovering several objects that revolve around the Sun. Here is a summary of Bode's law: Write the sequence of numbers 0, 3, 6, 12, 24, 48, 96, etc. (Each number is twice the previous number). Add 4 to each number in the sequence. Divide each of the resulting numbers by 10. As you can see from the previous table, you end up with a remarkably close match to the actual distance from the Sun for nearly all of the planets. In 1781, William Herschel discovered Uranus, whose average distance from the Sun was very close to the distance predicted by Bode's Law. Because of this, astronomers began to search for the missing planet between Mars and Jupiter. In 1801, a Sicilian astronomer, Giuseppe Piazzi, discovered a faint object which was shown to orbit the Sun every 4.6 years at an average distance of 2.77 AU. This was very close to the distance predicted by Bode's Law for the missing planet. Ceres, however, was too small to be a planet, its diameter being only 914 km. Heinrich Olbers discovered another faint object similar to Ceres in 1802. He called it Pallas. It also orbits the Sun every 4.6 years at an average distance of 2.77 AU, but is even smaller and dimmer than Ceres. The discovery of two smaller objects at about the right location led astronomers to believe that perhaps the missing planet had broken apart or exploded. In 1891, Max Wolf began using photographic techniques to look for asteroids. He personally discovered 228 different asteroids. Today we know of over 25,000 asteroids, although we only know the orbits of about 3000 of them. Most of these asteroids revolve around the Sun at distances between 2 and 3½ AU. This region of the solar system , in between Mars and Jupiter, has been called the Asteroid Belt. Jupiter, being the largest planet, exerts gravitational forces on the asteroids. This results in certain gaps in the asteroid belt, known as Kirkwood gaps. Another effect of Jupiter's gravity is that asteroids at certain distances are trapped into Jupiter's orbit because of the combined effects of Jupiter's and the Sun's gravity. The French mathematician LaGrange calculated that Jupiter would trap asteroids 60° in front and behind it in its orbit. These have been called the Lagrangian points, as his hypothesis was proven correct. The asteroids discovered at Jupiter's Lagrangian points are called the Trojan Asteroids. Asteroids occasionally collide with each other and even with the inner planets. Earth experienced a near-miss in 1989 when an asteroid passed within 800,000 km. This is about twice the distance to the moon, extremely close on the interplanetary scale. Typical collision velocities are estimated to be around 1 to 5 km/s (2,000 to 11,000 mph), which is more than enough to shatter rocks. Interasteroid collisions often produce fragments of rock which shower down on the inner planets. Fortunately, most of these are so small that they have little effect. Occasionally, however, a large fragment does collide. The result is an impact crater. Asteroids whose orbits cross the earth's orbit are called Apollo Asteroids.

Uranus

Uranus is the seventh planet out from the Sun. It was once thought to be a comet, until William Herschel, an English astronomer, discovered it was a planet. Uranus is a little odd. Unlike most other planets, it is tipped on its side. Its north and south poles alternately point almost directly at the Sun as it revolves around it. This makes the temperatures at the north and south poles quite different from each other. Uranus takes 84 years to make the trip around the Sun once. Voyager 2 discovered that Uranus's magnetic field does not follow the normal north pole to south pole arrangement, but rather it is offset from the planet's center, and is inclined 60° from the poles. The atmosphere of Uranus is made mostly of hydrogen, with about 12 percent helium and trace amounts of ammonia, methane, and water vapor. It appears blue because methane absorbs all other colors of light. Wind speeds of up to 600 kilometers per hour have been observed on this planet. The average temperature on Uranus is -220°C. Uranus has eleven rings composed mostly of dark, boulder-sized chunks. Uranus has 15 satellites, ten of which were discovered by the Voyager 2 spacecraft (which discovered much about Uranus's moons and rings). Miranda is perhaps the most interesting satellite because it appears that early in its formation it was shattered, but somehow it re-formed with some of the earlier core material now exposed on its surface. Even before Voyager 2 passed by Uranus to confirm, astronomers discovered rings around it when they observed Uranus pass in front of a bright star, momentarily dimming the star's light.

Kepler

Using the detailed observations of Brahe, Johannes Kepler was the first scientist ever to propose that planetary orbits are not circles. He proposed that planets orbit the Sun in an ellipse. This has come to be known as Kepler's first law. An ellipse can be drawn by placing two thumbtacks on a piece of paper and tracing out the shape with a loop of string kept taut and hooked on both thumbtacks. The largest diameter is called the major axis; the smallest is called the minor axis. The locations of the thumbtacks are called foci (the plural of focus). According to Kepler, planets orbit the sun in an ellipse, with the Sun at one focus. Kepler realized that planets do not move at uniform speeds through their orbits. A planet moves more rapidly when it is nearest the sun (perihelion). It moves more slowly when it is farthest from the sun (aphelion). After a lot of trial and error, he discovered a relationship between a planet's speed and its distance from the Sun. This has been called the law of equal areas, or Kepler's second law. Imagine a line joining the planet and the sun. As the planet moves around the sun, this line "sweeps out" a certain area. Kepler calculated that in equal intervals of time, this line would sweep out equal areas. One of Kepler's later discoveries was the relationship between the sidereal period and the length of the semimajor axis of a planet. The semimajor axis can be approximated as the average distance from the planet to the sun. If a planet's sidereal period (P) is measured in years and the semimajor axis (a) is measured in astronomical units (defined as the earth-sun distance), then P2= a3. This is Kepler's Third Law.

Electromagnetic waves

Vibrating electrons in atoms create a special type of wave—an electromagnetic wave. Light is an example of an electromagnetic wave. (We will use the abbreviation "EM" for electromagnetic.) An EM wave is a wave made of continuously changing electric and magnetic fields. All electromagnetic waves are created the same way light is created. Electrons that jump from one energy level to another create photons.The electrons first absorb energy to jump to a higher energy level; when they go from this higher energy level back to their original (lower) energy level, they release energy as photons (the same amount they absorbed to jump to the higher energy level). A stream of photons makes up the EM wave. The specific type of wave created depends only on how much energy the photons have. When we arrange all of the electromagnetic waves in order of their energy, we get the electromagnetic spectrum. On the EM spectrum we can see that radio waves have the lowest energy and gamma rays have the highest energy.

Virgo the Virgin

Virgo is the only female constellation of the zodiac . She lived on the earth during the golden age, and is thought to provide justice for mortals. The bright star spica has a magnitude of almost exactly 1. Its 220 light years away and has over 2000 times the luminosity of the sun.

Taurus the bull

When Zeus fell in love with Europa, princess of Phoenicia, he took on the form of a bull. Europa came to like the gentle beast, and eventually Zeus revealed himself as a god. Two large star clusters can be seen in this constellation. They are known as Hyades and the Pleiades. The brightest star in the constellation Taurus is named Aldeberan.

Spiral structure

Within the disk itself are stars, but even more than that there is interstellar dust. This dust obscures our view of a lot of our galaxy. A breakthrough in this field came in 1951, when astronomers were able to detect radio waves from hydrogen atoms. These radio waves have a wavelength of 21 cm and are often referred to as 21 cm hydrogen radiation. This radiation can penetrate the dust clouds of our galaxy. By studying this, astronomers have been able to map out our galaxy and see the spiral structure. The sun is revolving around the center of the galaxy at a speed of about half a million miles per hour. At this rate it takes the sun 200 million years to complete one revolution! Knowing the period, we can use Kepler's laws to determine the mass of the galaxy. It is about 94,000,000,000 times the mass of our sun.

Conduction

is very familiar to anyone who has tried to grasp a metal spoon that has been left in a hot pan. Heat is transferred through the atoms or molecules of the metal directly. The virbration of molecules forces the next one to vibrate more, and the next ones and so on. The ability of substances to conduct heat varies. As a means of heat transfer fro the sun as a whole, conduction is the least significant.