Chapter 25 Vocabulary & Study Guide 1) constellation A constellation is a group of stars visible within a particular region of the night sky that appears to form a pattern. The positions of the constellations appear to change throughout the year because Earth revolves around the Sun. As the earth turns you can see different constellations. Some constellations were named after animals and some mythological characters. Some constellations were named after some scientific instruments. Constellations can be viewed after sunset and before sunrise. 2) circumpolar constellations Constellations that circle Polaris in the Northern sky and are visible all year round. If you live in the Northern Hemisphere, the constellations that circle around the North Star are visible all year. They are called circumpolar constellation because they travel in circles around the North Star. The main circumpolar constellations are Ursa Major, the Great Bear; Ursa Minor, the Little Bear; Draco, the Dragon; Cepheus, the King; and Cassiopeia, the Queen. 3) spectroscope An instrument used to break visible light from a star into its component colors or spectrum. The spectrum indicates the elements that are in the stars atmosphere. 4) absolute magnitude a measure of the amount of light given off by a star. Although it has a greater absolute magnitude than Sirius, Rigel does NOT look as bright in the night sky. 5) apparent magnitude a measure of the amount of light received on Earth. Its apparent magnitude makes Sirius the brightest star in the night sky. The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, it is what it LOOKS like. 6) red shift stretching of the light waves that occurs when stars are moving away from earth causes the dark absorption lines in the spectrum to shift toward the red end; this is seen when a spectroscope is used to study the light from galaxies beyond the local group; tells us that the galaxies are moving away from earth and that the universe is expanding 7) Big Bang theory An explanation for the beginning of the universe; how the universe began with an enormous explosion; holds that the universe began 13.7 billion years ago with huge explosion that caused expansion everywhere at the same time The Big Bang theory of the formation and expansion of the universe is supported by the observed red shift. 8) galaxy a large group of stars, gas, and dust held together by gravity. There are billions of Galaxies in the Universe. Some are very small with only a few million stars, others could have as many as 400 billion stars, or even more. There are three main kinds of Galaxies, Spiral, Elliptical, and Irregular. The only difference between the three is what shape they are. Our Milky Way Galaxy is a barred spiral galaxy about 120,000 light-years in diameter containing up to 400 billion stars and possibly just as many planets. Our Milky Way Galaxy is part of a 'Local Group' of galaxies in which the galaxies move relative to each other. 9) corona the largest layer of the Sun‘s atmosphere; extends millions of km into space; has temperatures as high as 2 million K; charged particles continually escape from this layer and move through space as solar wind. Outside the Sun‘s surface, or photosphere, are two further layers of light gases, the chromosphere and the corona (Corona means ―crown‖ in Latin). The corona is the luminous "atmosphere" of the Sun extending millions of kilometers into space, most easily seen during a total solar eclipse. 10) black hole an object so dense that nothing can escape its gravity field, not even light; caused by the mass of a tremendously big supernova core collapsing to a point. A black hole is a region so dense that nothing, including light, can escape its gravity field. Although black holes cannot be seen, we know they exist from the way they affect nearby dust, stars and galaxies. Most galaxies, including the Milky Way, have supermassive black holes at their centers. Black holes are formed when giant stars explode at the end of their lifecycle. This explosion is called a supernova. The center of a black hole, where all its mass resides, is a point called a singularity. Our Sun won‘t and cannot become a black hole, only stars with much more mass than the Sun end up black holes. 11) nebula a large cloud of gas and dust that is the beginning of a star, earliest stage of a star‘s formation is in a nebula. Some nebulae (more than one nebula) are regions where new stars are being formed, while others are the remains of dead or dying stars. These are considered a stellar nursery! 12) sunspot - areas of the Sun‘s surface that appear dark because they are cooler than surrounding areas; they are temporary features which come and go over days, weeks, or months; they increase and decrease in a 10 to 11 year patterns called solar activity cycles; they are related to intense magnetic fields. Sunspots have a dark center called the Umbra and are surrounded by a lighter colored region called the Penumbra. ( sound familiar?) 13) neutron star - the collapsed core of a supernova that contains only neutrons. These are so dense that a teaspoonful would weigh more than 600 million tons. Neutron stars are city-size ( average 20 KM ) stellar objects with a mass about 1.4 times that of the sun. When stars that are four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons, yielding the name "neutron star." 14) Ursa Major the Big Dipper is part of this constellation; ‗big bear‘, Ursa Major is a circumpolar constellation visible throughout the year in the northern hemisphere. It is the 3rd largest constellation in the night sky. Its name means "Great Bear" in Latin. Its seven brightest stars form the famous Big Dipper, because the major stars can be seen to follow the rough outline of a large ladle, or dipper. 15) Sirius the brightest star in the sky. It is in the constellation Canis Major the Greater Dog. Sirius is sometimes called the Dog Star. It is relatively close to Earth (8.6 light-years). If the star were placed next to Earth's sun, Sirius would outshine it more than 20 times over. To find Sirius, use the belt of Orion as a pointer. The three stars point downward toward Sirius to the left. 16) light year the distance of about 9.5 trillion kilometers that light travels in one year. Light is the fastest-moving stuff in the universe. It travels at an incredible 300,000 kilometers (186,000 miles) per second. A light-year is how astronomers measure distance in space. It‘s defined by how far a beam of light travels in one year. Traveling at light speed travel to the moon takes about a second-and-a-half, and travel to the sun takes about eight minutes. If you could travel at the speed of light, you could circle the globe of Earth almost eight times in one second. 17) spectra astronomers study these to learn about the properties of stars. When light passes through a prism, it separates into the colors that make it up. White light separates into a rainbow called a spectrum. Scientists build special instruments to separate light into a spectrum, these instruments are called spectrographs. When astronomers pass the light of a star through a spectrograph, they get a spectrum of the star. The spectrum looks like a regular rainbow of colors—except that there are dark lines in it. Here is a spectrum of our sun: Like a ‗Chemical Fingerprint‖ the colors in a star‘s spectrum help to identify which elements that are present in the star. Spectroscopy also lets you determine if an object is moving towards or away from you by the change in frequency of the wavelength . ( Doppler effect – Red Shift – Blue Shift) . Spectroscopy also tells us the age of a star by looking at the amount of its matter made up of chemical elements other than hydrogen and helium. The earliest stars were composed of just hydrogen and helium because they were the first elements to form after the Big Bang. 18) Orion this constellation, named after a mythical hunter, includes the star Betelgeuse. Orion is one of the largest constellations in the sky and is also one of the easiest to find. The easiest way to find Orion is to look for the three stars that make up his "belt". The third "star" down in Orion's sword is not a star at all, but the Orion Nebula. If you get a pair of binoculars and look closely at this "star", you will see not one, but many stars. This nebula is also one of the very few places in the sky where the Hubble Space Telescope has been able to spot disks of dust around some the young stars. Stars with these disks may be forming their own solar systems. Betelgeuse is also the largest star in the constellation, it has 1,000 times the radius of the sun. The brightest star in the constellation is Rigel, which is 40,000 times brighter than the sun and emits 100,000 times the energy 19) Rigel even though this star has an absolute magnitude greater than that of Sirius, it looks dimmer from Earth since it is 100 times farther away.