HW6_Ch9-Stars&H-R

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Astronomy

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Dec 6, 2023

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ASTR 1304 (Solar System) Homework Assignment #6 (Ch. 9 – Stars and the H-R Diagram) Name: ___Joshua Gutierrez____________________ One of the most useful and powerful plots in astronomy is the Hertzsprung-Russell diagram (or H-R diagram). It originated in 1911 when the Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars against their color (consequently effective temperature). Independently in 1913 the American astronomer Henry Norris Russell used spectral class versus absolute magnitude. Their resultant plots showed that the relationship between temperature and luminosity of a star was not random but instead appeared to fall into distinct groupings of stars. Please consult the lecture slides and eText for additional information on stellar origins and evolution as well as the H-R diagram. 1

Figure (right). In a Hertzsprung-Russell (H–R) diagram, a star is represented by a dot at a position that shows the star’s luminosity and temperature. The background color in this diagram indicates the temperature of the stars. The Sun is a yellow-white G2 star. Most stars (including the Sun) have properties along the main-sequence strip running from hot, high- luminosity stars at upper left to cool, low-luminosity stars at lower right. 2

Figure (above). Note that the H-R diagram has four discreet star fields: main sequence, giant, supergiant, and white dwarf . To begin, stars in the upper left on the main sequence are luminous and have very high temperature. And from the mass-luminosity relationship, we also know that these are the most massive stars on the main sequence. Main sequence stars in the lower right have low luminosity, are cool, and have masses smaller than the Sun. Next, according to our knowledge of blackbodies, a star which is cool can only be luminous if it has a large surface area (and, therefore, large radius) from which energy is emitted; thus, their position on this diagram requires giants and red supergiants to be large. This means that these giant stars are luminous but have cool outer envelopes. Finally, examine the lower left portion of the H-R diagram. These stars are hot but have relatively low luminosity. Using our understanding of blackbody radiation, such a hot star will have limited total energy emitted if it is small in radius. Multiple choice (2 points each). Circle, mark, or highlight correct letter. 1. The Hertzsprung-Russell diagram can be labeled in many different ways. Which coordinates CANNOT be used on an H-R diagram? a. luminosity versus spectral type b. luminosity versus temperature c. radius versus temperature d. absolute magnitude versus temperature e. luminosity versus color 2. Select the answer that does NOT apply correctly. The horizontal axis (X-axis) of the H-R diagram is labeled with ___________. a. temperature b. spectral type c. color d. radius 3. Select the answer that does NOT apply correctly. The vertical axis (Y-axis) of the H-R diagram is labeled with ___________. a. spectral type b. luminosity c. absolute magnitude d. brightness 4. The ___________ run(s) from the top left to the bottom right in the H-R diagram. These are all “ordinary stars”, which like the Sun, are burning nuclear fuel at their centers. a. red dwarfs b. main sequence c. green goblins d. neutron stars 3

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