Question 1

An astronomer discovers an object at a distance of 28 Mpc from Earth. At this distance, what is the object MOST likely to be?

Accepted Answer

28 Mpc (megaparsecs) is a vast distance, much larger than the scale of our solar system, our galaxy, or the region around Earth where artificial satellites orbit. This distance is typical for distant galaxies, making option D the most likely choice.

Question 2

Suppose that, at the same time on the same night, an astronomer sees two supernovas (exploding stars) explode in the night sky. If one is in the Andromeda Galaxy, 2 million light-years away from us, and the other is in the galaxy M82, 6 million light-years away from Earth, which of these statements concerning the actual explosion times of these supernovas is correct?

Accepted Answer

The answer of Suppose that, at the same time on...

Question 3

One au, or 1 astronomical unit, is defined as the

Accepted Answer

The astronomical unit (au) is defined as the mean distance between the Earth and the Sun, providing a convenient measure for distances within the solar system.

Question 4

Suppose an astronomer were to observe two supernovas (exploding stars) in the night sky on a particular night. If one was in the nearby arm of the Milky Way Galaxy, approximately 2000 light years away from Earth, while the other was in the Andromeda Galaxy, M31, 2 million light-years away from Earth, which of these statements concerning the actual explosion times of these supernovas is correct?

Accepted Answer

The light from the supernova in the Andromeda Galaxy, being 2 million light-years away, took 2 million years to reach us, whereas the light from the supernova in the Milky Way, being 2000 light-years away, took 2000 years to reach us. Therefore, the supernova in the Andromeda Galaxy actually occurred before the one in our Galaxy.

Question 5

If a supernova was first seen in the year of Christ's birth (the Star of Bethlehem?) and its distance from Earth was 3 kpc, approximately when did the supernova actually explode?

Accepted Answer

The answer of If a supernova was first seen in...

Question 6

An astronomical unit (au) is a unit of

Accepted Answer

An astronomical unit (au) is defined as the average distance between the Earth and the Sun, approximately 149.6 million kilometers. This unit is used to measure distances within our solar system.

Question 7

One light-year is the

Accepted Answer

A light-year is a measure of distance, not time. It represents the distance that light travels in one year in a vacuum, which is about 9.461 trillion kilometers or about 5.879 trillion miles.

Question 8

In 2014 A.D., an inhabitant of a planet orbiting a distant star observes the flash of the first nuclear explosion on Earth, which occurred in July 1945. Approximately how far away is his solar system from Earth?

Accepted Answer

The time difference between 1945 and 2014 is 69 years. Light travels at a speed of 1 parsec per 3.26 light-years, so 69 light-years is approximately 21.2 parsecs.

Question 9

What is the distance between Earth and the nearest star?

Accepted Answer

The answer of What is the distance between Earth and...

Question 10

Why do astronomers need special units, in addition to the metric system of units such as meters and kilograms, to describe the universe?

Accepted Answer

Astronomical distances and masses are extremely large, making the use of metric units impractical for these measurements. Special units like light-years for distance and solar masses for mass simplify these representations and make calculations more manageable.

Question 11

Comet Halley returns to the Sun's vicinity approximately every 76 years in an elliptical orbit. According to Kepler's third law, what is the semimajor axis of this orbit?

Accepted Answer

Kepler's third law states that the square of the orbital period (in years) of a planet is directly proportional to the cube of the semi-major axis of its orbit (in astronomical units, au). For Comet Halley, with an orbital period of approximately 76 years, the semi-major axis can be calculated using the formula $P^2 = a^3$, where $P$ is the period in years and $a$ is the semi-major axis in astronomical units. Solving for $a$ gives a semi-major axis of approximately 17.9 au.

Question 12

The Crab Nebula is the result of a supernova explosion of a star that occurred at a distance of about 1.84 kpc from Earth. If people first saw the explosion in A.D. 1054, when did the explosion actually occur?

Accepted Answer

The light from the explosion took 1.84 kpc / 300,000 km/s = 6133 years to reach Earth. So the explosion actually occurred in 1054 A.D. - 6133 years = 4946 B.C.

Question 13

Light from fires that were lit at the time of the Battle of Hastings in England in A.D. 1066 has traveled out into space at the speed of light. How far out into space has this light now reached, compared to the distances to the 20 brightest stars?

Accepted Answer

The light from the fires of 1066, having traveled for nearly 957 years (as of 2023), would have covered a distance of approximately 957 light-years. Sirius is about 8.6 light-years away, Betelgeuse is about 642.5 light-years away, and Rigel is about 860 light-years away, all of which are within 957 light-years. Deneb, however, is about 2,600 light-years away, far beyond the reach of this light.

Question 14

The semimajor axis of Earth's orbit is

Accepted Answer

The semimajor axis of Earth's orbit around the Sun is defined as 1 astronomical unit (au), which is the standard unit of measurement in astronomy for distances within our solar system.

Question 15

If light takes about 8.3 minutes to travel from the Sun to Earth, approximately how much longer would light take to travel from Jupiter to Earth when Jupiter is at conjunction (appearing closest to the Sun in the sky) than when it is at opposition (on the opposite side of Earth from the Sun)? (Assume that light from Jupiter is not blocked by the Sun at conjunction. A diagram might help.)

Accepted Answer

When Jupiter is at conjunction, it is approximately one astronomical unit (AU) closer to Earth than when it is at opposition, because at opposition, Earth and Jupiter are on opposite sides of the Sun, essentially adding Earth's distance from the Sun (1 AU) to the distance light must travel. Since light takes about 8.3 minutes to travel 1 AU (the average distance from the Earth to the Sun), the difference in light travel time between these two positions is roughly double that time, or 16.6 minutes.

Question 16

The time taken for light to travel from a galaxy that is 10 Mpc away is

Accepted Answer

The answer of The time taken for light to travel...

Question 17

One light-year is a distance of approximately

Accepted Answer

One light-year is the distance that light travels in one year, which is approximately 9.5 × 10^12 km.

Question 18

A light-year is a measure of

Accepted Answer

A light-year is a measure of distance, specifically the distance that light travels in one year in a vacuum.

Question 19

An inhabitant of a planet orbiting the star Betelgeuse (a distance of 197 parsecs from Earth) observes the flash from the first nuclear weapon on Earth, exploded in July 1945. If this extraterrestrial being were to send a signal to Earth immediately to confirm this sighting by exploding a bomb of equivalent brightness, and astronomers on Earth were watching his planet, when would they expect to see the flash?

Accepted Answer

The answer of An inhabitant of a planet orbiting the...

Question 20

If a person sent a radio message toward the star nearest to the Sun, and intelligent beings from a planet near the star sent back a reply immediately on receipt of the message, how long after the transmission would this person have to wait for a reply?

Accepted Answer

The nearest star to the Sun is Proxima Centauri, which is approximately 4.2 light-years away. A radio message would take 4.2 years to reach it, and the reply would also take 4.2 years to return, totaling 8.4 years.

Quiz 2: Gravitation and the Motion of the Planets