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Chapter 21: Problem 5

Older stars are found farther from the midplane of a galactic disk because a. the disk used to be thicker. b. the stars have lived long enough to move there. c. the younger stars in the thick disk were more massive and have already died. d. none of the above

Short Answer

Expert verified
b. the stars have lived long enough to move there.

Step by step solution

01

Identify the Reason

The exercise asks for the reason why older stars are found farther from the midplane of a galactic disk. Carefully analyze each option to understand which one best explains this phenomenon.
02

Consider Option A

Option a states that 'the disk used to be thicker.' If this were true, older stars would have initially formed in a thicker disk and, as the disk thinned, these stars would appear in their current locations. However, there is no strong astronomical evidence supporting that the entire galactic disk was uniformly thicker in its past.
03

Evaluate Option B

Option b proposes that 'the stars have lived long enough to move there.' This means that stars migrate from their original positions over time due to gravitational interactions and other forces within the galaxy. This option is plausible as older stars would naturally disperse over long periods.
04

Analyze Option C

Option c suggests that 'the younger stars in the thick disk were more massive and have already died.' This explanation infers that only less massive, older stars remain, while more massive stars (which have shorter lifespans) have already undergone supernova explosions.
05

Assess Option D

Option d indicates 'none of the above.' Review if there is any other significant reason not covered by options a, b, or c that could explain the observed distribution of older stars. If no such reason is apparent, option d can be ruled out.
06

Conclusion

After evaluating the options, option b seems to be the most supported by current astrophysical understanding. Older stars have indeed had more time to move from their original positions due to galactic dynamics and gravitational interactions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Galactic Dynamics
Galactic dynamics refers to the complex behaviors and motions within galaxies. In a galactic disk, stars, gas, and dark matter all move according to gravitational forces. Over time, these interactions can change the positions of stars.
Key points about galactic dynamics include:
  • The Milky Way rotates, causing stars to orbit its center.
  • Stars can scatter due to gravitational encounters with other stars, gas clouds, or dark matter.
  • Density waves and spiral arms can influence star orbits.
All these phenomena together explain why older stars may be found farther from the midplane as they have experienced more interactions.
Gravitational Interactions
Gravitational interactions play a crucial role in stellar migration. The gravitational pull between celestial bodies can alter their paths within the galaxy.
Essential points about gravitational interactions are:
  • Encounters: Close approaches between stars can scatter them in different directions.
  • Clusters: Stars born in clusters may leave the group over time due to gravitational forces.
  • Galactic Tides: The collective gravitational pull from the Milky Way can stretch and alter star orbits.
The combination of these forces contributes to older stars moving to different locations within the galaxy.
Stellar Lifespans
Stellar lifespans vary greatly depending on a star's mass. Understanding these lifespans helps explain the distribution of stars within a galaxy.
Important aspects of stellar lifespans:
  • Massive Stars: Burn fuel quickly and have short lifespans, often ending in supernovae.
  • Less Massive Stars: Burn fuel slowly, living much longer, sometimes for billions of years.
  • Cool, Old Stars: These are often found farther from the galactic midplane, having survived for a long time.
Massive stars die young, so the stars currently found farther from the midplane are typically older and less massive.
Star Formation History
The history of star formation in a galaxy affects the current distribution of stars. It’s a vital piece of understanding why older stars are where they are.
Key points about star formation history:
  • Thick Disk Formation: Early periods of intense star formation created a thick disk of stars.
  • Migration Over Time: Stars formed in early galactic history have had more time to move from their birthplaces.
  • Evolving Structures: Galactic structures such as bars and spiral arms can evolve, influencing star movement.
Considering these factors gives us insights into why older stars are found in particular regions of the galaxy, farther from its midplane.

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Most popular questions from this chapter

Parallax measurements of the variable star RR Lyrae indicate that it is located 230 pc from the Sun. A similar star observed in a globular cluster located far above the galactic plane appears 160,000 times fainter than RR Lyrae. a. How far from the Sun is this globular cluster? b. What does your answer to part (a) tell you about the size of the galaxy's halo compared to the size of its disk?

Evidence of a supermassive black hole at the center of the Milky Way comes from a. direct observations of stars that orbit it b. X-rays from material that is falling in. c. strong radio emission from the region of the accretion disk d. all of the above

Detailed observations of the structure of the Milky Way are difficult because a. the Solar System is embedded in the dust and gas of the disk. b. the Milky Way is mostly dark matter. c. there are too many stars in the way. d. the galaxy is rotating too fast (about \(200 \mathrm{km} / \mathrm{s}\) ).

Use your imagination to describe how Earth's skies might appear if the Sun and Solar System were located a. near the center of the galaxy. b. near the center of a large globular cluster. c. near the center of a large, dense molecular cloud.

A most likely fate of the Large and Small Magellanic Clouds is that they will a. become part of the Milky Way. b. remain orbiting forever. c. become attached to another passing galaxy. d. escape from the gravity of the Milky Way.
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