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Chapter 17: Problem 15

A fellow student expresses the opinion that since the Sun's spectrum has only weak absorption lines of hydrogen, this element cannot be a major constituent of the Sun. How would you enlighten this person?

Short Answer

Expert verified
The Sun mainly consists of hydrogen. Its absorption lines in the sun's spectrum are weak, not because hydrogen is rare in the Sun, but because conditions in the photosphere (where the spectrum is formed) are such that most of the hydrogen is not in a state to absorb many photons. Therefore, the absorption spectrum does not directly reflect the Sun's overall element composition.

Step by step solution

01

The Nature of the Sun's Structure

Firstly, it is important to realize that the Sun consists mainly of hydrogen (about 73% of its mass, or about 92% of the number of atoms is hydrogen). The Sun has different layers; the core, the radiative zone, the convective zone, and the photosphere, which is the visible surface of the Sun that emits the light we see.
02

Spectrum Analysis

The spectrum of the Sun is an absorption spectrum, characterized by a continuum of colors crosscut by dark absorption lines, called Fraunhofer lines. Each of these lines corresponds to a particular element's light absorption. This happens when light from the photosphere passes through the cooler gases above it. These gases absorb certain wavelengths of light and make dark lines at those wavelengths on the spectrum. This is why we see absorption lines corresponding to different elements.
03

Hydrogen in the Sun's Spectrum

Hydrogen is the most abundant element in the Sun, but its absorption lines are weak in the Sun's spectrum because the surface of the Sun (photosphere), where the light emitting from the Sun is formed and escapes into space, contains less hydrogen compared to deeper layers. Also, the temperature condition in the photosphere is such that most of the hydrogen is in the ground state (not ionized or excited), which means it's not capable of absorbing much of the high energy photons from the continuous spectrum.
04

Summary of explanation

Therefore, it is not valid to assert that the weak absorption lines for hydrogen in the Sun's spectrum indicate that hydrogen is not a major constituent of the Sun. Absorption lines are more about the conditions in the Sun's photosphere and less about its overall composition.

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Most popular questions from this chapter

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 \(8 \mathrm{~km} / \mathrm{s}\) ?

. Stars \(C\) and \(D\) both have the same luminosity, but \(C\) is \(32 \mathrm{pc}\) from Earth while \(D\) is 128 pc from Earth. Which star appears brighter as seen from Earth? How many times brighter is it?

Find the average distance from the Sun to Neptune in parsecs. Compared to Neptune, how many times farther away from the Sun is Proxima Centauri?

. What is the mass-luminosity relation? Does it apply to stars of all kinds?

Kapteyn's star (named after the Dutch astronomer who found it) has a parallax of \(0.255\) arcsec, a proper motion of \(8.67\) arcsec per year, and a radial velocity of \(+246 \mathrm{~km} / \mathrm{s}\). (a) What is the star's tangential velocity? (b) What is the star's actual speed relative to the Sun? (c) Is Kapteyn's star moving toward the Sun or away from the Sun? Explain.
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