NES Profile: Earth and Space Science (307)

Which of the following physical characteristics of a main-sequence star is the most important factor in determining its luminosity?

1. total mass
2. size of the corona
3. chemical composition
4. strength of the magnetic field

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A. This question requires the examinee to demonstrate knowledge of the physical characteristics of stars. The luminosity of a star is a measure of the amount of energy it radiates each second. Based on research conducted during the 1920s, the astrophysicist A.S. Eddington discovered that the luminosity of a main-sequence star is determined by its mass. Eddington's discovery is the basis of the mass-luminosity relationship L = M ³L equals M cubed, where the luminosity of a main-sequence star is equal to the star's mass cubed. Analysis of the energy emitted by a star continues to be an essential tool that astronomers use to deduce stellar properties, including composition, temperature, size, and mass.

The orbit is elliptical with the sun at one focus of the ellipse. On either side of the sun along the long axis of the ellipse, there is a symmetrical area shaded gray around the long axis. At the end where the sun is close to the ellipse, the shaded area is defined by 2 short radii and a long distance of arc on the perimeter of the ellipse. At the other end, the shaded area is defined by 2 long radii and a short distance of arc on the perimeter of the ellipse.

The diagram above shows the distance traveled by a planet at two different intervals in its solar orbit. The astronomer Johannes Kepler compared the two shaded gray areas of the ellipse in order to determine the:

1. mass of planets with different orbital periods.
2. diameter of the orbits of other planets.
3. gravitational pull that the planet exerts on other objects.
4. changing speeds at which a planet orbits the sun.

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D. This question requires the examinee to recognize the law developed by Johannes Kepler to describe the changing orbital speed of a planet as it travels around the sun. This law states that the orbital speed of a planet varies so that a line joining the sun and the planet will sweep over equal areas in equal time intervals. The graphic in the question represents the two equal areas that a hypothetical planet covers during two different but equal time periods in its elliptical orbit. Since the time periods are the same, but the distance covered during the two intervals is different, the orbital speed must be different.

The large dark areas of the moon known as mares are easily visible from Earth. Which of the following explains the presence of these dark areas?

1. Massive volcanic eruptions emptied underlying magma caverns, causing their collapse and the formation of calderas.
2. Shortly after the moon's formation, Earth's strong gravitational pull on the moon caused the release of magma from fissures in the crust.
3. Large meteorites excavated huge craters that filled with low-viscosity basalt flowing from the fractures created in the underlying crust.
4. When the moon was still semi-molten, thin areas of crust collapsed due to tectonic activity, creating low areas that trapped sediments.

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C. This question requires the examinee to demonstrate knowledge of the physical characteristics of the moon. The mares on the moon are large dark areas that formed when meteorites slammed into the moon, producing huge craters that then filled with basalt that flowed up into the craters through fractures in the moon's crust.