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Chapter 17: Problem 1

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

Short Answer

Expert verified
Apparent brightness is how bright the star appears to be from Earth and depends on both its distance and its luminosity. Luminosity, on the other hand, is a measure of the total amount of energy emitted by a star per unit of time and is independent of the star's distance from an observer. Therefore, a star can be very luminous yet appear dim if it is far away, or it can be less luminous but appear very bright if it is closer.

Step by step solution

01

Defining Apparent Brightness

Apparent brightness, also known as apparent magnitude, is a measure of how bright a star appears from Earth. It depends not only on a star's intrinsic brightness but also on its distance from the Earth. The closer a star is to Earth, the brighter it appears.
02

Defining Luminosity

Luminosity of a star is the total amount of energy emitted by it per unit of time. It is also known as the intrinsic brightness of a star. Unlike apparent brightness, luminosity does not depend on the distance of the star from the observer.
03

Explaining the Difference

The main difference between a star's apparent brightness and its luminosity is that while apparent brightness is how bright the star appears to an observer from Earth, luminosity is the actual amount of light the star is giving off. A star can have high luminosity but may appear dim if it is far away, while a less luminous star may appear bright if it is closer to Earth. Thus, the apparent brightness of a star is influenced by both its distance and its luminosity.

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Most popular questions from this chapter

Some giant and supergiant stars are of the same spectral type (G2) as the Sun. What aspects of the spectrum of a G2 star would you concentrate on to determine the star's luminosity class? Explain what you would look for.

As seen from the starship Enterprise in the Star Trek television series and movies, stars appear to move across the sky due to the starship's motion. How fast would the Enterprise have to move in order for a star \(1 \mathrm{pc}\) away to appear to move \(1^{\circ}\) per second? (Hint: The speed of the star as seen from the Enterprise is the same as the speed of the Enterprise relative to the star.) How does this compare with the speed of light? Do you think the stars appear to move as seen from an orbiting space shuttle, which moves at about \(8 \mathrm{~km} / \mathrm{s}\) ?

Search the World Wide Web for information about Gaia, a European Space Agency (ESA) spacecraft planned to extend the work carried out by Hipparcos. When is the spacecraft planned to be launched? How will Gaia compare to Hippar\(\cos\) ? For how many more stars will it be able to measure parallaxes? What other types of research will it carry out?

The Sun experiences solar flares (see Section 16-10). The amount of energy radiated by even the strongest solar flare is not enough to have an appreciable effect on the Sun's luminosity. But when a flare of the same size occurs on a mainsequence star of spectral class \(M\), the star's brightness can increase by as much as a factor of 2 . Why should there be an appreciable increase in brightness for a main-sequence M star but not for the Sun?

Suppose a star experiences an outburst in which its surface temperature doubles but its average density (its mass divided by its volume) decreases by a factor of 8 . The mass of the star stays the same. By what factors do the star's radius and luminosity change?
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