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Chapter 23: Problem 4

What are RR Lyrae stars? Why are they useful for determining the distance from our solar system to the center of the Galaxy?

Short Answer

Expert verified
RR Lyrae stars are variable stars with a known period-luminosity relationship, that makes them act as standard candles for distance measurements in astronomy. They are useful in determining the distance from our solar system to the center of the Galaxy because their presence in globular clusters, which orbit the galactic center, allows for distance measures that help to map the Galaxy's structure and size.

Step by step solution

01

Definition of RR Lyrae Stars

RR Lyrae stars are a type of variable star, meaning that their brightness varies over time. They are most commonly found in globular clusters and they have a very specific period-luminosity relationship. This period-luminosity relationship, where the period is the time it takes for the star's brightness to go through a full cycle of variation, is what makes them especially interesting for astronomers.
02

Distance Determination Using RR Lyrae Stars

Since RR Lyrae stars have a known period-luminosity relationship, the period of the star's brightness variation can be used to calculate its absolute magnitude, or intrinsic brightness. By comparing this absolute magnitude to the apparent magnitude, or brightness as seen from earth, astronomers can apply the distance modulus equation to determine the distance to the star.
03

Application to Galactic Distance Measurement

By observing RR Lyrae stars in globular clusters, which orbit the center of our Galaxy, astronomers can determine the distance to these clusters. This provides a measure of the distance from our solar system to the center of the Galaxy. By measuring the distances to a number of globular clusters surrounding the Galaxy, astronomers can build up a three-dimensional picture of the Galaxy's structure and size.

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Most popular questions from this chapter

What proposals have been made to explain the nature of dark matter? What experiments or observations have been made to investigate these proposals? What are the results of this research?

In our Galaxy, why are stars of spectral classes \(\mathrm{O}\) and \(\mathrm{B}\) only found in or near the spiral arms? Is the same true for stars of other spectral classes? Explain why or why not.

Use the Starry Night Enthusiast \(\mathrm{TM}^{\mathrm{M}}\) program to observe the Milky Way. (a) Display the entire celestial sphere by selecting Favourites \(>\) Guides \(>\) Atlas. Select View \(>\) Stars \(>\) Milky Way to display this galaxy. Select Options \(>\) Stars \(>\) Milky Way, move the Brightness slide-bar to the far right to brighten the Milky Way and click OK. In the View menu, ensure that the Scrollbars are activated and use them to look at different parts of the Milky Way. Can you identify the direction toward the galactic nucleus? In this direction the Milky Way appears broadest. Open the Find pane, enter Sagittarius in the Query box and press Enter to center on this constellation to check your identification. (b) Use this full-sky view to determine the orientation of the plane of the Galaxy with respect to the celestial sphere. Move the vertical scrollbar to its central position to display the Celestial Equator as a horizontal line across the lower part of the view. Move the horizontal scrollbar until the Milky Way is centered upon the view. Estimate the angle between the Milky Way and the celestial equator on the screen. How well aligned is the plane of the Milky Way with the plane of the Earth's equator? (c) A third plane of interest is that of the ecliptic, which is shown as a green line. Use the scrollbars to adjust the view so that the ecliptic appears as a straight line rather than as a curve, thereby ensuring that you are viewing in a direction that lies in the ecliptic plane. Use the horizontal scrollbar to move the view to where you can see where the ecliptic crosses the Milky Way. Estimate the angle between the Milky Way and the ecliptic on the screen. How well aligned is the plane of the Milky Way to the ecliptic, the plane of the Earth's orbit around the Sun? (d) Click on Home in the toolbar to return to your home view, stop Time Flow and set the local time to midnight (12:00:00 A.M.). Select Options > Stars > Milky Way, move the Brightness slide-bar to the far right to brighten the Milky Way and click OK. Adjust the date to January 1, then February 1, and so on. In which month is the galactic nucleus highest in the sky at midnight, so that it is most easily seen from your location?

The mass of our Galaxy interior to the Sun's orbit is calculated from the radius of the Sun's orbit and its orbital speed. By how much would this estimate be in error if the calculated distance to the galactic center were off by \(10 \%\) ? By how much would this estimate be in error if the calculated orbital yelocity were off by \(10 \%\) ? Explain your reasoning.

In what way are the orbits of stars in the galactic disk different from the orbits of planets in our solar system? What does this difference imply about the way that matter is distributed in the Galaxy?
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