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Download Astronomy Activity: The Life-Line of the Stars
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Astronomy Activity: The Life-Line of the Stars Activity: Using the concepts of stellar brightness and color, you will graphically analyze the relationships between bright stars, near stars, and temperature (color). Materials: graph paper (provided) 2 different colored pencils, pens, or crayons Exploration: When you look up at the stars of the night-time sky, you will immediately notice that all of the stars do not have the same apparent brightness. Here are some possible hypothesis for you to consider: Hypothesis 1: All stars are the same size, brightness, and temperature, it's just that dim stars are far away and bright stars are close. Hypothesis 2: All stars are the same distance away, but some are naturally bright while others are just dim. Hypothesis 3: Bright stars are far away and dim stars are close. Which one do you think is correct and why? Write your answer in the form of a sentence, then compare your answer with another student. Concept Introduction: A. Background: Brightness of Stars The brightness that a star has as seen from the Earth is called the apparent brightness . Stars which are very bright are called magnitude 1 stars . The next brightest are magnitude 2 stars. Then comes magnitude 3, 4, 5, and down to the very faintest stars visible with the naked eye, magnitude 6 stars*. All stars are not the same distance away. Obviously, a star which is far away will appear dimmer than a similar star which is closer. We adjust for the distance of stars by giving stars an absolute brightness . The absolute magnitude of a star is the apparent magnitude that a star would have at a distance of 32.6 light years (10 parsecs). Stars that are really bright are also very large! B. Background: Colors of Stars Just like a fire flame (like a burning match) has different colors, so do the stars. This is because stars have different temperatures. Really hot stars are blue or white-hot. Cool stars are red or redish-orange in color. Astronomers classify stars based on what they are made of (in addition to hydrogen). An "A" class star has lots of Iron, a "B" class star has lots of calcium, and so on. This classification system is called a spectral class system . IF you arrange the stars from hot to cold, the order of the stars are: OBAFGKM C. Directions for the Graph: 1. Using the list of Bright Stars in Table 1 and the list of Near Stars in Table 2, plot their absolute magnitude vs. spectral class on the attached graph. Use different colors for the near stars and for the bright stars. 2. What general trends or concentrations do you see in the data? Are there generalizations you can make about bright stars? How are they different from stars that are close to the Earth? Write your answers using sentences. Concept Application: 3. Our star, the Sun, is a G2 spectral class star with an absolute magnitude of 4.8 . How does it compare to the locations of the Near Stars on the diagram? How does it compare to the locations of the Bright Stars on the diagram? Write your answers using sentences. 4. Which spectral class is most common? Which spectral class is the least common? 5. Most of the stars seem to be along a line from the upper left corner to the lower right corner of the HR Diagram. Stars which fall into this category of stars are called main sequence stars . Does our Sun fit into this category? 6. Consider the extra stars in the upper right hand corner ( bright-cool stars) and the lower left hand corner (hot- dim stars). What would you name the bright-cool stars? What would you name the hot-dim stars? (Compare your answer with another student). Name the bright-cool stars: Name the hot-dim stars: 7. Why is it that black holes do not appear on the HR Diagram? Table 1 Bright Stars (Stars which appear very bright as seen from the Earth) Star Name Spectral Absolute Class Magnitude Sirius B B8 +11.5 Canopus F2 -3.1 Arcturus K2 -0.3 Capella G6 -0.7 Rigel A B8 -6.8 Betelgeuse M2 -5.5 Antares M8 -4.5 Spica B4 -3.6 Deneb A2 -6.9 Procyon B A10 +13.0 Table 2 Near Stars (Stars which are very close to the Earth) Star Name Spectral Class Absolute Magnitude Sun G2 +4.8 Centari Alpha G2 +4.4 Sirius A A6 +1.4 Ross 154 M4 +13.3 Ross 248 M8 +14.7 Luyten 789-6 M4 +14.7 Ross 128 M2 +13.8 61 Cygnus A K4 +7.5 61 Cygnus B K8 +8.3 Procyon A F6 +2.7 Note : Several of the star names appear twice followed by an A or a B. These are mostly double stars - two stars which are revolving around each other together in space. They seem to frequently have similar spectral types. Can you guess why they have similar spectral types? Background Information for Activity * Stellar Magnitude For historical reasons, astronomers still call the brightest stars magnitude 1 stars, the next brightest 2, then 3, 4, 5, and on down to the dimmest stars, magnitude 6 stars. This backwards scale is now defined as mathematically as: brightness of star 1 = difference in magnitude brightness of star 2 (2.512) Using this sort of scale, we can actually describe the brightness of stars in real numbers. Really bright objects, like the full moon have a negative magnitude. Object Apparent Magnitude Sun -26.5 Full Moon -12.5 Venus -4.1 Sirius -1.4 North Star 2.0 limit of naked eye 6 Pluto 15 limit of HST 28 ** Spectral Classes Stars are easily classified by the amount of hydrogen which they contain as revealed to us by the intensity of their spectrum. Stars with the most hydrogen are called A stars, the remaining letters designate varying amounts of different chemical stellar compositions. If we consider the temperature of stars, as revealed to us by their color, we find that the O stars, are hottest, followed by B stars, A stars, F, G, K, and M stars. A star which has an intermediate temperature between an O star and a B star is called an O5 star. This layout of temperature is given from hottest to coolest as: O0, O1, O2, O3, O4, O5, O6, O7, O8, O9, B0, B2, B3, B4, B5, B6...... Astronomers sometimes humorously teach their students to remember this sequence from hottest to coolest as: O h, b e a f ine g uy (or gal), k iss m e! Our Sun is G2 star, which means that it is a rather intermediate temperature star.