GIZMO Big Bang (1).

Question: How do Cepheids allow astronomers to measure intergalactic distances? 1. Collect data: Locate and select the yellow Cepheid variable star (A-091) in the lower left section of the Andromeda Galaxy. Click the Collect data button. You will see a graph of the apparent brightness of the star over time. A. How does the star’s apparent brightness change over time? B. Turn on Show time probes. Set the left probe at the first brightness peak, and the right probe at the second brightness peak. List the time represented by each probe: Left probe time: Right probe time: C. What is the time difference between the two brightness peaks? This is the period of the Cepheid. D. In the DATA tab, record the name of this star and its period. Do the same on your paper Data worksheet, located on the last page of this document. 2. Collect data: The apparent brightness of the star is shown on the y-axis of the graph. The brightness is given as the ratio of the star’s brightness to the Sun’s brightness if viewed from a standard distance of one megaparsec (1 Mpc), which is about 3.26 million light years. For example, a brightness of “4,000” means that the star appears 4,000 times as bright as the Sun would appear if observed from a distance of 1 Mpc. A. What is your estimate of the mean apparent brightness of star A091? B. Turn on Show mean brightness. What is the mean brightness of A-091? Record this value in the Gizmo and on your Data worksheet. 3. Observe: Click Record spectrum. Stars emit light at a variety of wavelengths. Just as white light is separated into a rainbow when it passes through a prism, starlight can be separated into its constituent Reproduction for educational use only. Public sharing or posting prohibited. © 2020 ExploreLearning™ All rights reserved colors when it passes through an instrument called a spectrograph. The result is a pattern of colors and black bars called an absorption spectrum. The bars represent wavelengths of light that are absorbed in the star’s atmosphere. Each star’s unique spectrum depends on the temperature and composition of its atmosphere. Look at the spectrum of A-091. ️Sketch the lines of its stellar spectrum in the diagram below. Click to select Edit to use the drawing tool. 4. Record: Click Done. Select the other Cepheid variable star in the Andromeda Galaxy (A-171). Determine and record the period, mean brightness, and spectrum of this star. Then, click Return to map and select Region B. There are a total of ten Cepheids hidden in the nine regions of the Gizmo. Record the period, mean brightness, and spectrum of all ten stars both in the Gizmo data table and on your own data table. 5. Compare: By comparing Cepheid variables that were all about the same distance away, Henrietta Leavitt discovered that larger, more luminous Cepheids had longer periods. Look at the periods of the ten stars in your data table. A. Based on the periods of the stars, which three stars are the most luminous? B. Find two stars with similar periods, and therefore similar size. List these stars, their periods, and their mean brightness below. Star: Period: Mean brightness: Star: Period: Mean brightness: C. Compare the apparent brightness of these two stars. Which star do you think is farther away? Explain. Activity B: Luminosity and distance Get the Gizmo ready: ● Select the DISTANCE tab. ● Check that Luminosity vs. Period is selected. Reproduction for educational use only. Public sharing or posting prohibited. © 2020 ExploreLearning™ All rights reserved Introduction: Henrietta Leavitt observed Cepheids inside the Small Magellanic Cloud, a star cluster just outside the Milky Way. All of these stars are about the same distance away. Leavitt found that the most luminous Cepheids had the longest periods. By relating each Cepheid’s luminosity to its period, Leavitt discovered a way to find the luminosity of a Cepheid in any galaxy. By comparing the apparent brightness of a Cepheid variable to its known luminosity, the distance to any Cepheid (and its host galaxy) could then be determined. Goal: Find the mean luminosity and distance of Cepheid stars. 1. Collect data: On the DISTANCE tab, check that the Luminosity vs. Period graph is selected. Notice that as the period of a Cepheid variable increases, so does its luminosity. A luminosity of 2,000 Suns means that the star is 2,000 times brighter than the Sun. A. Based on the graph, what is the approximate mean luminosity of a Cepheid variable star with a period of 5 days? 10 days? B. Turn on Show draggable point. Move the point so that its x–coordinate is equal to the period of star A-091. What is the mean luminosity of this star? Record this value in the data table of the Gizmo and on your own data table. C. Use the draggable point to find the mean luminosity of the other stars. If necessary, use the +/- zoom controls to zoom in or out on the graph. Record your results in the Gizmo and in your own data table. 2. Calculate: The distance to a star can be found by comparing its luminosity to its apparent brightness. A star that is far away will be very dim compared to its luminosity, and the ratio of its luminosity to apparent brightness will be greater than for a nearby star. A. Stars B-618 and C-197 have about the same mean brightness. Calculate the “luminosity ratio” of each by dividing its mean luminosity by its mean brightness. B-618 luminosity ratio: C-197 luminosity ratio: B. Based on these ratios, which star is farther away? Explain. 3. Infer: Below the data table, click the right arrow once. You should now see a column labeled Luminosity/brightness. The ratio of each star’s luminosity to its brightness has been calculated for you. Based on these ratios, which stars are closest, and which stars are farthest away? Closest: Farthest: Explain: Reproduction for educational use only. Public sharing or posting prohibited. © 2020 ExploreLearning™ All rights reserved 4. Record: On the DISTANCE tab, select Distance vs. Luminosity Ratio. This graph allows you to estimate the distance of each star, measured in megaparsecs (Mpc), based on its luminosity ratio. A. Based on the graph, how does the ratio of a star’s luminosity to its apparent brightness relate t