Question 1

Very hot stars have weak hydrogen spectral lines. Why is this?

Accepted Answer

The high temperatures in very hot stars ionize hydrogen atoms, stripping away their electrons and leaving them unable to emit the spectral lines characteristic of neutral hydrogen.

Question 2

For Balmer series lines to show up strongly in absorption in stellar spectra, significant numbers of hydrogen atoms must have electrons in the n = 2 energy level. What then does the appearance of these lines in a stellar spectrum describe about the temperature of the star's surface?

Accepted Answer

For Balmer series lines to be prominent in absorption, the star's surface temperature must be in a specific range. It needs to be high enough to excite hydrogen atoms' electrons to the n = 2 level but not so high that the atoms are ionized, which would prevent the absorption of light at the wavelengths of the Balmer series.

Question 3

A star has a surface temperature of 6000 K. Its spectrum registers a certain intensity in the green middle of the visible spectrum. Which of these must also be true of the star?

Accepted Answer

According to Wien's Law, a star with a surface temperature of 6000 K will have its peak emission in the visible spectrum around the green. Therefore, both violet and red, being further from the peak, will be less intense than green.

Question 4

The Sun's spectrum peaks in the green region (i.e., green is the most intense color in its spectrum). The spectrum of Rigel (the knee of Orion) peaks in the short-wavelength end of the visible spectrum. Compared with the Sun, Rigel is

Accepted Answer

Rigel's spectrum peaking in the short-wavelength end of the visible spectrum (which includes blue and violet colors) indicates that it is hotter than the Sun, whose spectrum peaks in the green region. Hotter stars emit more light at shorter wavelengths, making them appear bluer.

Question 5

Astronomers originally classified the spectra of stars according to the strengths of their hydrogen lines. What new physical insight enabled a reinterpretation of these patterns in terms of surface temperature during the 1920s?

Accepted Answer

The development of atomic physics provided insights into how atoms emit light, allowing astronomers to understand that the spectral lines were influenced by the temperature of the star's surface, leading to a reclassification based on temperature rather than hydrogen line strength.

Question 6

The star Rigel, in the constellation Orion, appears brighter through a blue filter than it does through a yellow filter. Suppose that a second star is found that is slightly brighter than Rigel through the yellow filter but is much brighter than Rigel through the blue filter. From this information, one can say conclusively that the second star has

Accepted Answer

The brightness of a star through different filters is related to its temperature. A star that appears brighter through a blue filter compared to a yellow filter is hotter, because blue light is emitted more by hotter objects. Therefore, if the second star is much brighter than Rigel through the blue filter, it conclusively indicates that the second star has a higher temperature.

Question 7

Spectral types of stars (e.g., O, B, A, F, G, K, and M) uniquely define their

Accepted Answer

Spectral types of stars are primarily determined by their surface temperatures, with the sequence O, B, A, F, G, K, M representing stars from the hottest to the coolest.

Question 8

The surface temperature of a nearby star can BEST be determined from spectral classification by examining the

Accepted Answer

The pattern of spectral absorption lines from various atoms can be used to determine the surface temperature of a nearby star. The hotter the star, the more ionized its atoms will be, and the fewer absorption lines will be present in its spectrum.

Question 9

Why are Balmer absorption lines very weak in the spectra of stars with low surface temperatures-significantly below 10,000 K, for example?

Accepted Answer

At low surface temperatures, most hydrogen atoms are in the ground state (n = 1) and not enough are excited to the n = 2 level, which is necessary for Balmer absorption to occur.

Question 10

A particular star appears approximately equally bright when viewed through a blue filter and through a yellow filter. What is the approximate surface temperature of this star?

Accepted Answer

Stars that appear equally bright in blue and yellow filters have a surface temperature that closely matches that of the Sun, which is approximately 6000 K. This is because the Sun emits a broad spectrum of light with a peak intensity in the green part of the spectrum, making it appear equally bright in adjacent parts of the spectrum such as blue and yellow.

Question 11

Which of these sequences of stellar spectral classifications is in the correct order of INCREASING temperature?

Accepted Answer

The correct sequence of stellar spectral classifications in order of increasing temperature is M, K, G, F, A, B, O. This sequence follows the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me," which helps to remember the order from coolest to hottest stars.

Question 12

If a certain star shows a strong absorption spectrum in hydrogen Balmer lines, the surface temperature of the star is hot enough that

Accepted Answer

The Balmer series of hydrogen is most prominent when many hydrogen atoms have electrons in the n = 2 energy level, as this series involves transitions from higher energy levels to n = 2. This indicates that the surface temperature of the star is such that it excites hydrogen atoms to this state without fully ionizing them.

Question 13

Why is there a limited range of stellar surface temperatures around 10,000 K at which neutral hydrogen gas absorbs visible light in the Balmer series?

Accepted Answer

Electrons in hydrogen atoms need to be in the n = 2 energy level for Balmer series absorption to occur. At temperatures much lower than 10,000 K, electrons predominantly occupy the ground state (n = 1), and at much higher temperatures, the hydrogen is ionized, leaving few or no electrons in the n = 2 level for absorption.

Question 14

The spectrum of an ordinary main-sequence star is a

Accepted Answer

The spectrum of an ordinary main-sequence star is characterized by a continuum of colors crossed by dark absorption lines. These absorption lines are created when light from the hotter, denser interior of the star passes through the cooler outer layers. Atoms and molecules in these cooler layers absorb specific wavelengths of light, leading to the appearance of dark lines in the spectrum. This phenomenon is known as absorption spectroscopy and is a key tool in astrophysics for determining the composition, temperature, density, and velocity of stars.

Question 15

Which of these MOST plausibly explains why the star Bellatrix in Orion looks bluish to the naked eye?

Accepted Answer

Bellatrix emits most of its light in the ultraviolet region, but within the visible spectrum, it emits more blue light than any other color, making it appear bluish to the naked eye. This is due to its high temperature, which results in a peak emission that falls in the ultraviolet, with significant visible light emission skewed towards the blue end of the spectrum.

Question 16

The surface temperature of a nearby star can be determined MOST precisely by measuring what parameters?

Accepted Answer

The relative strengths of absorption lines from different atoms and molecules in the star's spectrum provide information about the star's temperature, as different elements and molecules absorb light at specific wavelengths that are sensitive to temperature.

Question 17

Rigel, the bright star in the knee of Orion, has a surface temperature of about 12,000 K. Through which filter will it appear brightest?

Accepted Answer

Rigel, with a surface temperature of about 12,000 K, emits most of its light in the ultraviolet range due to its high temperature, according to Wien's displacement law.

Question 18

The star Regulus, in the constellation Leo, appears brighter through a blue filter than it does through a yellow filter. Suppose that a second star is found that has the same brightness as Regulus through the blue filter but is brighter than Regulus through the yellow filter. From this information, one can say conclusively that the second star has

Accepted Answer

The second star appears brighter through a yellow filter and has the same brightness through a blue filter compared to Regulus. This indicates that the second star emits more light in the yellow part of the spectrum, which is characteristic of a star with a lower temperature than a star that appears brighter in the blue part of the spectrum. Stars that are cooler emit more of their light in the red and yellow part of the spectrum, while hotter stars emit more in the blue and ultraviolet parts.

Question 19

Spectral classification of a star into the lettered categories, O, B, A, F, G, K, and M is carried out by

Accepted Answer

Spectral classification is primarily based on the absorption lines present in a star's spectrum, which are indicative of the star's temperature and the presence of various elements in its atmosphere.

Question 20

The chemical makeup of the Sun's surface can be determined by

Accepted Answer

Solar spectroscopy is the method used to determine the chemical makeup of the Sun's surface. It involves analyzing the spectrum of light emitted by the Sun to identify the elements present.

Quiz 12: Characterizing Stars