Astronomy (C894)

Correct Answer D. cluster.

Explanation

A galaxy cluster is a group of galaxies that are gravitationally bound together. These clusters can contain anywhere from a few to thousands of galaxies. The galaxies in a cluster are held together by the mutual gravitational attraction between them, and they can move as a group through space. Clusters are one of the largest known gravitationally bound structures in the universe.

Why other options are wrong

A. supernova.

A supernova is the explosive death of a star, not a group of galaxies. It is an event that occurs when a star exhausts its fuel and undergoes a catastrophic explosion, which can briefly outshine an entire galaxy.

B. supercluster.

A supercluster is a much larger structure than a galaxy cluster, containing multiple clusters of galaxies. While both are gravitationally bound, a supercluster refers to a large collection of galaxy clusters, not just a single cluster.

C. solar system.

A solar system is a collection of celestial bodies, including a star and the planets and other objects that orbit it. It is much smaller in scale compared to a galaxy cluster, which involves multiple galaxies.

D. cluster.

This is the correct term for a group of galaxies bound together by gravity.

Correct Answer B. Galaxies are moving apart from each other due to the expansion of the universe.

Explanation

The primary motion of galaxies in the Universe is that they are moving apart due to the expansion of the universe. This phenomenon was first observed by Edwin Hubble, who noted that distant galaxies appear to be moving away from us, with their speed increasing as their distance from us increases. This is consistent with the concept of the expanding universe, where space itself is stretching, causing galaxies to recede from each other over time.

Why other options are wrong

A. Galaxies are stationary and do not move relative to each other.

This is incorrect because galaxies are not stationary. Instead, they are in motion relative to each other, with the overall trend being the expansion of space itself, causing galaxies to move away from each other.

C. Galaxies are all moving towards each other at a constant speed.

This is incorrect because while some galaxies, such as those in the Local Group, are gravitationally bound and may move towards each other (like the Andromeda Galaxy approaching the Milky Way), the overall motion in the universe is that galaxies are moving away from each other due to the expansion of space.

D. Galaxies orbit around a central point in the universe.

This is incorrect because there is no central point in the universe. The universe is expanding uniformly, and galaxies are not orbiting a central point. The motion of galaxies is more accurately described by the expansion of space itself rather than orbital movement around a single center.

Correct Answer A. have an orbital period that does not depend on their mass

Explanation

The orbital period of a satellite around Earth is determined by its distance from the Earth, not its mass. This is a direct consequence of Kepler's Third Law of planetary motion, which states that the orbital period depends on the radius of the orbit, not the mass of the orbiting object.

Why other options are wrong

B. must have small thrusters aimed toward Earth to act against Earth's gravity

This is incorrect because satellites in orbit around Earth are in free-fall. They are constantly falling toward the Earth but also moving sideways fast enough to avoid hitting the ground, which keeps them in orbit. They do not need thrusters to counteract gravity once they are in orbit.

C. must have small thrusters turned sideways to constantly push them sideways

This is incorrect because once a satellite is in orbit, it doesn't need continuous sideways thrust. The initial velocity given to the satellite at launch is usually sufficient to keep it moving in a stable orbit.

D. must have small thrusters aimed away from Earth so they don't fly off into space

This is incorrect because a satellite in orbit is already moving at a sufficient velocity to stay in orbit. Small thrusters are typically used for adjustments or to correct the satellite's orbit, not to prevent it from flying off into space.

Correct Answer B. An icy body orbiting the sun

Explanation

A comet is primarily made of ice, dust, and rocky material. It orbits the Sun in highly elliptical orbits. As a comet approaches the Sun, the heat causes the ice to vaporize, creating a glowing coma and sometimes a tail. This distinguishes comets from other small bodies like asteroids, which are mostly rocky or metallic.

Why other options are wrong

A. A rocky body orbiting the sun

This is incorrect because a comet is not primarily made of rock. While it may contain some rocky material, its main composition is ice and dust. This differentiates it from an asteroid, which is primarily composed of rock and metal.

C. A gaseous body orbiting the sun

This is incorrect because a comet is not composed mostly of gas. The visible part of a comet (its coma and tail) is created by the sublimation of ice and dust, not by gaseous material.

D. A very small planet

This is incorrect because a comet is not considered a planet. Planets are large bodies that orbit stars, while comets are much smaller, icy bodies that have highly elliptical orbits around the Sun.

E. An asteroid

This is incorrect because asteroids are typically rocky and metallic, whereas comets are icy bodies. Although both are small Solar System bodies, their composition and behavior differ.

Correct Answer A. Light has a finite speed

Explanation

When we look through a telescope, the light from distant stars and galaxies takes time to reach us. Because light travels at a finite speed, the farther away the object, the longer it takes for its light to arrive. This means that when we observe distant objects in the universe, we are actually seeing them as they were in the past, not as they are right now.

Why other options are wrong

B. The distances in the universe are enormous

While the vast distances in the universe contribute to the delay in receiving light, it is the finite speed of light itself that is the main reason we see into the past. This option does not directly explain the phenomenon.

C. Light bounces off distant objects

This is not the reason we see into the past. Light reflects off objects, but the time it takes for light to travel from the object to the observer is what causes the delay. The reflection is just part of the process, not the cause.

D. Light contains many different wavelengths

While light does contain different wavelengths (which give us information about the object's characteristics), it is not the reason we see into the past. The delay is due to the speed at which light travels, not the wavelengths it contains.

Correct Answer A. No. From this position, all stars cross the horizon at some time, rising and setting at intervals of 12 hours.

Explanation

At the equator, all stars rise and set every 12 hours, meaning no star is circumpolar. Circumpolar stars are those that never set below the horizon and can be seen all year round, but this phenomenon only occurs at higher latitudes. At the equator, the celestial poles are on the horizon, so stars will always rise and set.

Why other options are wrong

B. Of course. All stars would be visible for 24 hours a day from this location if it were not for daylight.

This is incorrect because not all stars are visible for 24 hours at the equator. Stars do rise and set, and they are not always visible at any given time of the day or year.

C. It depends on the time of the year, or the season. Earth's axis tilts at different angles to the equator at different times of the year, making some stars circumpolar at certain times of the year.

This is incorrect because at the equator, no stars are circumpolar regardless of the time of year. The tilt of the Earth's axis does not affect the observation of circumpolar stars at the equator.

D. Yes. Stars within 23.5° of the celestial equator are circumpolar as seen by someone on Earth's equator.

This is incorrect because no stars are circumpolar at the equator. Stars near the celestial equator rise and set throughout the year. Only at higher latitudes can stars be circumpolar.

Correct Answer B. Asteroids are small, rocky bodies that do not have enough mass for gravity to shape them into a sphere.

Explanation

Asteroids are small, rocky bodies that orbit the Sun, primarily found in the asteroid belt between Mars and Jupiter. They are too small and lack sufficient mass to have gravity that could shape them into spherical forms, unlike planets. Instead, they often have irregular shapes. Asteroids differ from comets in that they are composed mostly of rock and metal, rather than ice and gas.

Why other options are wrong

A. Asteroids are large, spherical objects that orbit the Sun.

Asteroids are not typically large or spherical. They are usually much smaller than planets, and their irregular shapes prevent them from being spherical.

C. Asteroids are primarily composed of gas and ice, similar to comets.

This statement is inaccurate. Asteroids are primarily made of rock and metal, not gas and ice. Comets, on the other hand, contain ice, dust, and volatile gases.

D. Asteroids are celestial bodies that only exist in the outer regions of the Solar System.

While many asteroids are found in the outer regions of the Solar System, particularly in the asteroid belt, they can exist in other regions as well. There are also Near-Earth asteroids and those found beyond the Kuiper Belt.

Correct Answer C. Because the galaxies of the Local Group are gravitationally bound together.

Explanation

The observation that the Andromeda Galaxy is moving toward us does not contradict the expansion of the universe because the galaxies in the Local Group, including the Milky Way and Andromeda, are gravitationally bound. While the general expansion of the universe causes galaxies to move away from each other, the gravitational attraction within the Local Group is strong enough to overcome the expansion, causing the galaxies to move toward each other instead.

Why other options are wrong

A. Because the Local Group is located at the center of the expansion.

This is incorrect because the universe does not have a center; expansion occurs uniformly throughout space. There is no central point from which galaxies are expanding away. The Local Group's motion toward Andromeda is not due to a central location in the expansion but due to gravitational forces.

B. Because the Andromeda Galaxy is a spiral galaxy.

The type of galaxy, whether spiral or elliptical, has no impact on the expansion of the universe. The motion of galaxies toward one another is primarily due to gravitational forces rather than their shape or structure.

D. Because expansion is expected to cause some galaxies to move toward us.

This is incorrect. The expansion of the universe generally causes galaxies to move away from each other. Galaxies only move toward each other if they are gravitationally bound, as in the case of the Local Group.

Correct Answer D. the universe is expanding.

Explanation

The observation that distant galaxies are moving away from us at faster rates than those closer to us is the basis for the concept of the expanding universe. This is a direct consequence of Hubble's Law, which shows that the farther a galaxy is, the faster it appears to recede. This phenomenon suggests that the entire universe itself is expanding, not just the Milky Way or any specific galaxy.

Why other options are wrong

A. the Milky Way Galaxy is expanding.

This is incorrect. The Milky Way Galaxy itself is not expanding. While the universe is expanding, galaxies are gravitationally bound systems, and the expansion does not cause them to grow in size.

B. we are located at the center of the universe.

This is incorrect. The expansion of the universe is uniform from all points. There is no central point in the universe, and we are not at the center of it. Every observer, no matter their location, would observe similar expansion.

C. the farthest galaxies will eventually be moving faster than the speed of light.

This is incorrect. While it may seem like distant galaxies are moving faster than the speed of light due to the expansion of the universe, this is a result of the stretching of space itself, not the motion of objects through space. Therefore, it does not violate the laws of physics.

E. the universe is shrinking.

This is incorrect. The observation of galaxies moving away from each other contradicts the idea that the universe is shrinking. Instead, it is expanding.