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. Rigel is around 800 light years from Earth and is the brightest star in the constellation of Orion. We couldn‘t live as close to Rigel as we live to our sun, because Rigel is nearly twice as hot, a blue-white star, and about 40,000 times brighter than our sun.
20) Polaris another name for the North Star; the front two stars of the Big Dipper point at this; located at the end of the Little Dipper in the constellation Ursa Minor; positioned almost directly over Earth‘s North pole. Polaris is the largest star in the constellation Ursa Minor (which contains the little dipper), which is 430 light years from Earth. Polaris cannot be seen at all from the southern hemisphere.
21) parallax this is the apparent shift in position of an object when viewed from two different places, used to determine distances. The distances to nearby stars can be measured by using parallax. Astronomers can measure a star's position once, and then again 6 months later and calculate the apparent change in position. The star's apparent motion is called stellar parallax.
22) H-R diagram The Hertzsprung-Russell diagram is a graph that shows the relationship between a star‘s absolute magnitude and temperature. The hottest stars in space are blue in color. The majority of stars, including our Sun, are found along a region called the ‗Main Sequence‘. Main sequence stars vary widely in Our Sun temperature but the hotter they are, the more luminous they are, hence the main sequence tends to follow a band going from the bottom right of the diagram to the top left. These stars are fusing hydrogen to helium in their cores. Stars spend the bulk of their existence as main sequence stars so 80 – 90% of stars are main sequence stars. Other major groups of stars found on the H-R diagram are the giants, supergiants and the white dwarfs.
23) giant stars ( red giants) A red giant star is a star that started with a mass like our Sun but is in the last phase of its life, or late stage of the star cycle. This is a star that has exhausted most its supply of hydrogen so fusion reactions are slowing. The core contracts and the outer layers of the star expand and cool. After a few more millions of years, the star will collapse into a white dwarf-planetary nebula system
24) supergiant as heavier elements are formed by fusion, a massive star expands into one of these; These start out as very massive stars and are seen just before their supernova stages. Betelgeuse, Antares and Arcturus are such stars. Like red giants, Supergiant stars still have nuclear fusion going on inside them, but they change helium atoms into carbon, or carbon into oxygen.
25) spiral galaxies two types of these galaxies are barred and normal; they have spiral arms that wind out from inner section. Spiral galaxies get their name from the shape of their disks. In a spiral galaxy, the stars, gas and dust are gathered in spiral arms that spread outward from the galaxy's center. Spiral galaxies have a lot of gas, dust and newly forming stars. Since they have a lot of hot, young stars, they are often among the brightest galaxies in the universe. About 20% of all galaxies are spirals. We live in a spiral galaxy called the Milky Way.
26) irregular galaxies these galaxies have many different shapes and are usually smaller and less common than other types of galaxies; two of these, called the Clouds of Magellan, orbit the Milky Way. Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright.
27) Milky Way the solar system in which we live is in the this galaxy; is part of a galaxy cluster named the Local Group; usually classified as a spiral galaxy; may contain one trillion stars. The Milky Way is Our a barred spiral galaxy some 100,000–120,000 light-years in Sun diameter, which contains 100–400 billion stars. The Solar System is located within the disk, about 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm.
28) Doppler Shift also called the Doppler effect. This effect shows changes in the light coming from distant stars and galaxies and is used as a major fact for the support of the big bang Theory. Starlight moving toward the Earth has its electromagnetic wavelengths of light compressed and shifted to the shorter wavelength blue end of the spectrum. Starlight moving away from the Earth has its electromagnetic wavelengths of light stretched and shifted to the longer wavelength red end of the spectrum. These are also called red-shift and blue-shift. The faster the object is moving, the bigger the red or blue shift.
29) white dwarf when a star has no fuel left ( has burned up all of its hydrogen forming helium) and its outer layers escape into space; a small, hot star consisting of a hot, dense core contracting under the force of gravity; forms from the giant star; about the size of Earth but with matter that is 500,000 times as dense. A teaspoon full of material would weigh as much as an elephant.
30) main sequence star classification of about 90 percent of the stars, including our earth; stars that fit into a band that runs from the upper left to the lower right in the H-R diagram; contains hot, blue, bright stars in the upper left and cool, red, dim stars in the lower right. A main sequence star becomes a giant after it uses up the hydrogen in its core. Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores. Most of the stars in the universe — about 90 percent of them — are main sequence stars. The sun is a main sequence star. 31) Local Group A group of more than 30 galaxies that includes 2 large spiral galaxies, the Milky Way and Andromeda, as well as numerous smaller galaxies, many of which are dwarfs. The Local Group has a diameter of approximately 10 million light-years, with its gravitational center located between our galaxy and the Andromeda galaxy. It is part of a larger grouping known as the Virgo supercluster.
32) photosphere lowest layer of the suns atmosphere; gives off light and is about 6,000 K; often called the surface of the sun. The photosphere (meaning ―sphere of light‖) is the lowest layer of the Sun visible from Earth. It is about 300 miles (500 kilometers) thick. Energy finally escapes the Sun from the photosphere, so it is significantly cooler than the solar interior. The temperature at the visible surface is about 5,800 K (10,000° F). This layer is where the sun's energy is released as light. Because of the distance from the sun to Earth, light reaches our planet in about eight minutes. Sunspots are cooler regions of the photosphere that appear dark because of their lower temperatures of 3,800 degrees K versus 5,780 degrees K. Sunspots can vary in size up to 50,000 km in diameter. The photosphere is also the source of solar flares: tongues of fire that extend hundreds of thousands of miles above the sun's surface. Solar flares produce bursts of X-rays, ultraviolet radiation, electromagnetic radiation and radio waves.
33) chromosphere is the reddish, gaseous layer of the Sun‘s atmosphere between the corona and the photosphere that is 2,000 to 5,000 miles thick. The chromosphere emits a reddish glow as super-heated hydrogen burns off, but the red rim can only be seen during a total solar eclipse. At other times, light from the chromosphere is too weak to be seen against the brighter photosphere. The name of this layer comes from the Greek chroma meaning color, a reference to its reddish appearance.
34) solar prominences huge, arching gas columns that may be caused by the intense magnetic fields of the sunspots. A solar prominence is an arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space. Prominences are held above the Sun's surface by strong magnetic fields and can last for many months. Though these structures appear to be very bright and hot, they are actually hundreds of times cooler and denser than the surrounding gases in the Sun's corona or outermost atmosphere.
35) coronal mass ejection A coronal mass ejection is a solar event which can emit a relatively large burst of electrons and protons in the form of plasma. Unlike a solar flare, a CME doesn't produce intense light. But it does produce a magnetic shockwave that extends billions of miles out into space. It takes about two days for the cloud of material produced by a coronal mass ejection to reach and interact with Earth's magnetosphere. On Earth, such ejections can deplete the ozone layer and cause particularly strong polar aurora. These events can disrupt radio transmissions and damage satellites and electrical transmission lines. They can appear as a halo around the Sun when emitted in the Earth‘s direction. CME‘s can happen 2-3 times a day when the number of sunspots are at the maximum. CME‘s are huge, balloon-shaped plasma bursts that come from the Sun. As these bursts of solar wind rise above the Sun's corona, they move along the Sun's magnetic field lines and increase in temperature up to tens of millions of degrees. These bursts release up to 220 billion pounds (100 billion kg) of plasma. CME's can disrupt Earth's satellites. CME's usually happen independently, but are sometimes associated with solar flares.
36) elliptical galaxies An elliptical galaxy is a galaxy having a football shape. They are the most common shape. They are one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Stars found inside of elliptical galaxies are very much older than stars found in spiral galaxies. One of these elliptical galaxies ( Messier 32 ) is about 2.9 million light years away in the Andromeda constellation.
37) Betelgeuse Betelgeuse is the brightest star in Orion and marks the western shoulder of the constellation. It is the largest of the known stars. Betelgeuse emits almost 7,500 times as much energy as the Sun. The combination of size and temperature tells astronomers that the star is a kind of star called a red super giant. Red super giants are stars that are close to the end of their life. Probably within the next ten to hundred thousand years Betelgeuse will end its life in a supernova explosion.
38) Canis Major constellation showing the faithful dog companion of Orion the hunter; contains the star Sirius, the brightest star that is visible from the Northern hemisphere.
39) Proxima Centauri the nearest star to the Earth other than the sun. It is 4.3 light years, or about 400 trillion km away. It is the third star in the Alpha Centauri system and is a red dwarf.
40) Alpha Centauri system Alpha Centauri is not a star, but really a star system. Of the three stars in the system, the dimmest is a red dwarf called Proxima Centauri. Proxima Centauri is actually our sun‘s closest neighbor. This triple star system is the closest star system to the sun.
41) fusion Fusion is the process that powers our sun and other stars. All of the heat and light coming from the Sun comes from the fusion process happening deep inside the core. The Sun produces energy by fusing hydrogen atoms into helium atoms in its core. This fusion reaction releases a lot of energy causing heat and light. All main-sequence stars generate their energy by nuclear fusion of hydrogen nuclei into helium.
42) binary star system Our Sun is NOT part of a binary star system. More than four- fifths of the single points of light we observe in the night sky are actually two or more stars orbiting together. The most common of the multiple star systems are binary star systems. A binary star system is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star. It is believed that at least 1/4 of all stars are at least binary systems, with as many as 10% of these systems containing more than two stars (trinary, ternary, etc.) Stars travel around the galaxy, and sometimes a massive star captures a passing one, creating a new binary pair.