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Chapter 10: Problem 1

Explain the difference between a star's apparent brightness and its luminosity.

Short Answer

Expert verified
Answer: Apparent brightness is the measure of how bright a star appears when observed from Earth, depending on the star's intrinsic brightness (luminosity) and its distance from the observer. Luminosity is the total amount of energy emitted by a star per unit of time, being an intrinsic property of the star that does not depend on its distance from the observer. Distance affects the apparent brightness but not the luminosity of a star.

Step by step solution

01

Define Apparent Brightness

Apparent brightness is the measure of how bright a star appears to be when observed from Earth. It depends on the star's intrinsic brightness (luminosity) and its distance from the observer on Earth. The apparent brightness of a star is measured in units called magnitudes; lower magnitudes represent higher brightness.
02

Define Luminosity

Luminosity is the total amount of energy emitted by a star per unit of time. It is an intrinsic property of the star and does not depend on the star's distance from an observer. Luminosity is typically measured in watts, which express the amount of energy emitted per second. To compare the luminosity of different stars, it is common to use the unit solar luminosity (\(L_\odot\)), where one solar luminosity is the energy emitted by the Sun per second.
03

Explain the Role of Distance in Apparent Brightness and Luminosity

Distance plays a crucial role in determining a star's apparent brightness, but it does not have any impact on its luminosity. A more luminous star might appear less bright if it is further away from Earth, and a less luminous star might appear brighter if it is closer to Earth.
04

Give an Example to Illustrate the Difference

Let's consider two stars, Star A and Star B. Star A has a luminosity of 5 solar luminosities (\(5L_\odot\)), while Star B has a luminosity of 1 solar luminosity (\(1L_\odot\)). If both stars are at the same distance from Earth, Star A will appear brighter than Star B, as it has a higher luminosity. However, if Star B is closer to Earth than Star A, it might have a higher apparent brightness and appear brighter than Star A, even though its intrinsic luminosity is lower. This situation illustrates the difference between the apparent brightness and the luminosity of a star.
05

Conclusion

To sum up, luminosity and apparent brightness are two distinct concepts used to describe a star's brightness. Luminosity is the innate energy output of a star and does not depend on the star's distance from the observer. In contrast, apparent brightness depends on both the star's luminosity and its distance from the observer.

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Most popular questions from this chapter

Why is the magnitude scale called a "backward" scale? What is the difference between apparent magnitude and absolute magnitude?

If a red star and a blue star both have the same radius and both are the same distance from Earth, which one looks brighter in the night sky? Explain why.

Sketch a Hertzsprung-Russell diagram. Indicate the regions on your diagram occupied by (a) main-sequence stars, (b) red giants, (c) supergiants, (d) white dwarfs, and (e) the Sun.

The star Zubenelgenubi (from the Arabic for "scorpion's southern claw" ) has apparent magnitude \(2.75,\) while the star Sulafat (Arabic for "tortoise") has apparent magnitude \(3.25 .\) Which star appears brighter? From this information alone, what can you conclude about the luminosities of these stars? Explain your answer.

What information about stars do astronomers learn from binary systems that cannot be learned in any other way? What measurements do they make of binary systems to garner this information?
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