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Chapter 17: Problem 18

Pulsations in a Cepheid variable star are controlled by a. the spin. b. the magnetic field. c. the ionization state of helium. d. the gravitational field.

Short Answer

Expert verified
c. the ionization state of helium.

Step by step solution

01

Understand the Nature of Cepheid Variable Stars

Cepheid variable stars are a type of star that varies in brightness in a regular cycle. Understanding why these variations occur is essential to solving the exercise.
02

Review Pulsation Mechanism

The key lies in the ionization of helium within the star. As the star expands and contracts, helium ionizes and recombines, driving these pulsations.
03

Analyze the Options

Option a (spin) and option b (magnetic field) are not associated with the pulsation mechanism in Cepheid variables. Although the gravitational field (option d) plays a general role in star behavior, it's not the specific cause of the pulsations.
04

Identify the Correct Answer

From the review, it becomes clear that the variations are directly linked to the ionization state of helium.

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Key Concepts

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

stellar pulsations
Cepheid variable stars exhibit a fascinating phenomenon known as stellar pulsations. These stars continually expand and contract in a rhythmic pattern, causing their luminosity to vary.
The pulsations occur due to the intricate balance of internal forces within the star. Gravitational forces cause compression, while thermal pressure causes expansion. The cycle of these forces leads to a periodic change in the star's size and brightness.
Key points to grasp are:
  • The star pulsates, meaning it expands and contracts.
  • This pulsation happens in a regular pattern or cycle.
  • The brightness varies as the star's size changes.
Understanding stellar pulsations is fundamental when studying variable stars like Cepheids, as it helps explain the periodic brightness changes.
helium ionization
One of the core reasons behind the pulsations in Cepheid variable stars is the process of helium ionization.
Ionization refers to the process where atoms gain or lose electrons, creating ions. In Cepheids, helium atoms undergo cycles of ionization and recombination.
  • When the star compresses, the interior heats up, causing helium atoms to lose electrons (ionization).
  • As the star expands, the interior cools, allowing helium ions to recombine with electrons (recombination).
This ionization and recombination cycle acts like a 'heat engine,' providing the energy needed for the pulsations.
Knowing this cycle is pivotal. It elucidates how internal processes of stars impact their overall brightness.
variable star brightness
The brightness of Cepheid variable stars changes in a predictable manner, a characteristic that makes them extremely valuable in astronomy.
Their pulsations lead to periodic changes in luminosity. When the star expands, it becomes brighter because its surface area increases.
Conversely, as the star contracts, it becomes dimmer. This regular variation in brightness is very useful for measuring distances in space. Astronomers use Cepheid variables as 'standard candles' because their intrinsic brightness (absolute magnitude) is well-understood.
  • Brightness varies based on the pulsation cycle.
  • These variations are used to calculate distances in the universe.
  • Cepheid variables serve as critical tools for understanding cosmic scales.
Understanding how and why a Cepheid's brightness changes is essential for grasping broader concepts in stellar and galactic astronomy.

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Most popular questions from this chapter

Elements heavier than iron originated a. in the Big Bang. b. in the cores of low-mass stars. c. in the cores of high-mass stars. d. in the explosions of high-mass stars.

Recordings show that SN 1987A was detected by neutrinos on February \(23,1987 .\) About 3 hours later it was detected in optical light. What was the reason for the time delay?

Cepheids are highly luminous, variable stars in which the period of variability is directly related to luminosity. Why are Cepheids good indicators for determining stellar distances that lie beyond the limits of accurate parallax measurements?

T/F: Uranium forms in the core of a star.

The approximate relationship between the luminosity and the period of Cepheid variables is \(L_{\text {star }}\) \(\left(\operatorname{in} L_{\odot}\right)\) \(=335 P\) (in days). Delta Cephei has a cycle period of 5.4 days and a parallax of 0.0033 arcsecond (arcsec). A more distant Cepheid variable appears \(1 / 1,000\) as bright as Delta Cephei and has a period of 54 days. a. How far away (in parsecs) is the more distant Cepheid variable? b. Could the distance of the more distant Cepheid variable be measured by parallax? Explain.
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