20. Cosmology quiz (PB)

What does the term 'stellar parallax' refer to? (Easy)

• The apparent shift in position of a nearby star against distant stars due to Earth's orbit

• The change in brightness of a star

• The rotation of a star on its axis

• The gravitational lensing effect

Answer explanation

Stellar parallax is the apparent shift in position of a nearby star against distant stars as observed from different points in Earth's orbit. This effect helps astronomers measure distances to stars.

What is a black hole? (Easy)

• A region of space with extremely high temperature

• A collapsed star with gravity so strong that not even light can escape

• A neutron star emitting X-rays

• A void in space with no matter

Answer explanation

A black hole is formed when a star collapses under its own gravity, creating a region where the gravitational pull is so intense that not even light can escape, making it invisible and detectable only through its effects on nearby matter.

What is a neutron star? (Medium)

• A star composed mainly of hydrogen

• A dense remnant of a massive star composed almost entirely of neutrons

• A star in the main sequence phase

• A white dwarf with high luminosity

Answer explanation

A neutron star is the remnant of a massive star that has undergone a supernova explosion, resulting in a highly dense object primarily made up of neutrons. This distinguishes it from other types of stars.

What does redshift indicate in cosmology? (Medium)

• Stars are rotating rapidly

• Galaxies are moving away from us, indicating expansion of the universe

• Stars are cooling down

• Galaxies are moving toward us

Answer explanation

Redshift occurs when light from galaxies shifts to longer wavelengths, indicating they are moving away from us. This observation supports the theory of the expanding universe, making the correct choice about galaxies moving away.

What is the definition of luminosity? (Medium)

• The apparent brightness of a star as seen from Earth

• The total energy emitted by a star per second

• The temperature of a star's surface

• The size of a star

Answer explanation

Luminosity refers to the total energy emitted by a star per second, distinguishing it from apparent brightness, which depends on distance and other factors. Thus, the correct definition is the total energy emitted by a star per second.

What is the Chandrasekhar limit? (Medium)

• The maximum mass of a neutron star

• The maximum mass of a white dwarf before it collapses

• The minimum mass for a star to become a black hole

• The maximum radius of a black hole

Answer explanation

The Chandrasekhar limit is the maximum mass (about 1.4 solar masses) a white dwarf can have before it undergoes collapse into a neutron star or black hole. Thus, the correct answer is the maximum mass of a white dwarf before it collapses.

What is dark energy? (Hard)

• Energy emitted by black holes

• A hypothetical form of energy causing the accelerated expansion of the universe

• Energy from dark matter

• Energy from supernova explosions

Answer explanation

Dark energy is a hypothetical form of energy that is believed to be responsible for the accelerated expansion of the universe, distinguishing it from other energy sources like black holes or supernovae.

What is Hubble's law? (Hard)

• The law stating that galaxies rotate at constant speed

• The law stating that the recessional velocity of galaxies is proportional to their distance

• The law stating that stars emit radiation proportional to their temperature

• The law stating that the universe is static

Answer explanation

Hubble's law states that the recessional velocity of galaxies increases with their distance from us, indicating that the universe is expanding. This is correctly described by the second answer choice.

What is the significance of isotropy in cosmology? (Hard)

• The universe looks the same in all directions on a large scale

• The universe has the same density everywhere

• The universe contains equal amounts of matter and antimatter

• The universe rotates uniformly

Answer explanation

Isotropy in cosmology means that the universe appears the same in all directions on a large scale. This uniformity supports the cosmological principle, which is fundamental in understanding the large-scale structure of the universe.

Stars rotate around the centre of their galaxy. Observations suggest that the stars at the edges of galaxies are moving at much higher velocities than expected. What is the name given to the current explanation for these observations?

• Chandrasekhar limit

• Dark matter

• The Cosmological principle

• Wien’s displacement law

Answer explanation

The higher-than-expected velocities of stars at the edges of galaxies suggest the presence of unseen mass, leading to the explanation of dark matter, which accounts for this discrepancy in galactic rotation.