Sections Review

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Vocabulary

blueshift doppler effect proper motion
tangential velocity

Formulae

• redshift: > 0. Object moving away from you.
• blueshift: < 0. Object moving toward you.
• Radial velocity = (/ rest) × c, where is the doppler shift, rest is the wavelength you would see if the star was at rest. The rest wavelength is also the wavelength of light emitted by the star. Larger doppler shift means greater radial velocity.
• Tangential velocity = k × (the star's distance) × (d/dt), where k is a conversion factor and (d/dt) is the angular velocity or proper motion of the star.
• Total velocity = Sqrt[(radial velocity)2 + (tangential velocity)2]

Review Questions

1. How is spectroscopy used to find the composition of stars?
2. How do scientists know that the laws of physics are the same everywhere in the Galaxy and have been the same for billions of years?
3. How does the amount of doppler shift in the spectral lines depend on the relative speed? How does the direction of shift of the spectral lines depend on the direction of motion?
4. Given three stars moving with the same total speed of 40 kilometers/second but in three different directions: (A) moving away at an angle=45° from our line of sight, (B) moving directly away from us, (C) moving perpendicular to our line of sight, put the stars in order of increasing amount of shift you see in their spectral lines (the smallest shift first).
5. The H-alpha spectral line is at 656.3 nanometers. Star (A) has that line at 656.8 nanometers, star (B) has that line at 656.0 nanometers, star (C) has that line at 656.3 nanometers. Which star is moving the fastest (along the line of sight) and what is the three stars' directions of motion?
6. What must you first know about a star before you can convert its angular velocity to a tangential velocity in kilometers/second?
7. Two stars have proper motions of 0.5 arc seconds/year. Star (A) is 20 parsecs away and star (B) is 30 parsecs away. Which one is moving faster in space?
8. Two stars orbit about a common point in a plane that is oriented parallel to our line of sight. What will you see happening to the spectral lines of each star? If there is a change, will both stars' spectra be changing in the same direction at the same time?

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last updated: 23 May 2001