Question 1

If a star is spinning rapidly, this may limit its final mass.

Accepted Answer

When a star is spinning rapidly it may lose mass due to centrifugal force, which can limit its final mass as it evolves and ages. When a star is spinning rapidly, the centrifugal force can counteract the gravitational force, which can limit its ability to accumulate additional mass. Therefore, a rapidly spinning star may have a lower final mass than a non-spinning star of the same initial mass.

Question 2

At the end of the Kelvin-Helmholtz contraction, a star is hundreds of times brighter than itwill be as a main sequence star.

Accepted Answer

During Kelvin-Helmholtz contraction, a star undergoes gravitational collapse, releasing energy in the form of radiation. This radiation makes the star hundreds of times brighter than it will be as a main sequence star. During the Kelvin-Helmholtz contraction, a star releases gravitational potential energy, causing it to become hundreds of times brighter than it will be as a main sequence star.

Question 3

The fundamental difference between a protostar and a star is that the latter has nuclear fusionas its energy source and the former does not.

Accepted Answer

This statement is true. A protostar is a collapsing cloud of gas and dust that has not yet reached the temperature and density required for nuclear fusion to occur. Once nuclear fusion begins, the object becomes a star. This statement is true. A protostar is a contracting clump of gas that has not yet achieved the temperature and density required for nuclear fusion to begin. Once a protostar has amassed enough mass and density at its core, nuclear fusion reactions will ignite and a true star is born.

Question 4

The evolution of a protostar slows significantly as it approaches the main sequence.

Accepted Answer

As a protostar contracts and heats up, its evolution becomes more rapid until it reaches the main sequence, at which point it settles into a stable state and evolves more slowly over a much longer timescale. As a protostar collapses and heats up, it follows a path on the Hertzsprung-Russell diagram called the Hayashi track. When the protostar reaches a certain temperature and luminosity, it begins to fuse hydrogen in its core and becomes a true star on the main sequence. This process is called the zero-age main sequence. As the protostar approaches the main sequence, its contraction and heating slow down significantly, leading to a much slower evolution.

Question 5

The birth of stars is a battle between gravity and radiation pressure.

Accepted Answer

Gravity pulls gas and dust together to form a protostar, but as it grows, the radiation pressure from the baby star begins to push away surrounding material. It is this battle between gravity and radiation pressure that determines the ultimate size and mass of the star. Gravity pulls gas and dust together, but as the particles become denser and hotter, they emit radiation that pushes against this gravitational pull. This battle between gravity and radiation pressure ultimately determines whether a protostar will continue to collapse and form a star, or disperse and never become a star.

Question 6

Rotation and magnetism both play key roles in protostar formation.

Accepted Answer

Rotation helps flatten the protostellar cloud and forms a disk around the star while magnetism helps to channel the gas and dust towards the center for star formation. Rotation helps to flatten the gas cloud during protostar formation, while magnetism helps to channel and focus the gas flow. Both of these factors contribute to the formation and stabilization of a protostar.

Question 7

A stage 4 protostar may temporarily be thousands of times more luminous than the Sun.

Accepted Answer

During the stage 4 of protostar formation, the protostar begins to collapse and heat up, leading to the ignition of nuclear reactions in its core. This results in a temporary increase in luminosity, which could make it thousands of times more luminous than the Sun. During the stage 4 of a protostar's formation, gravitational contraction heats up the core and the protostar emits a large amount of radiation, which can make it temporarily thousands of times more luminous than the Sun.

Question 8

Rotation causes the cocoon of dust to flatten into a dusty disk around protostars.

Accepted Answer

This is known as a protoplanetary disk, where the dust and gas eventually clump together to form planets. This is a known process in the formation of protostellar systems, called the protoplanetary disk. The rotation of the dust and gas cloud around the young protostar causes it to flatten into a disk, where planets can eventually form.

Question 9

Magnetic fields enhance the collapse of a gas cloud in star formation with bipolar flows.

Accepted Answer

Magnetic fields can actually support a gas cloud against collapse by providing an additional pressure component. They can channel outflows along magnetic field lines, leading to the formation of bipolar flows, but they do not enhance the collapse itself.

Question 10

It takes about 1032 hydrogen atoms to make a star.

Accepted Answer

Stars are formed from vast clouds of gas and dust in space, containing far more than 1032 hydrogen atoms. It typically takes a mass equivalent to many times that of our Sun (which itself contains approximately 10^57 hydrogen atoms) to form a star.

Question 11

As a protostar evolves from stage 4 to stage 6, it gets hotter but much less bright.

Accepted Answer

As a protostar evolves from stage 4 to stage 6, gravitational contraction causes an increase in temperature, but the protostar is still surrounded by its dusty envelope, which absorbs and scatters much of the radiation it emits. Therefore, the protostar becomes hotter but also less visible and bright. During stage 4, the protostar is still accreting mass and its energy is primarily released as visible light. As it evolves to stage 6, the central temperature increases and the energy is primarily released as infrared radiation, which is less bright in the visible spectrum. Therefore, the protostar gets hotter but much less bright as it evolves to stage 6.

Question 12

Stars evolve to the upper left along the main sequence, after forming in the middle.

Accepted Answer

Stars actually evolve from the main sequence by moving away from it, typically upwards and to the right on the Hertzsprung-Russell diagram, not along it or to the upper left. They start their lives on the main sequence and move off it as they exhaust their hydrogen fuel.

Question 13

A protostar of 20 solar masses will probably turn into a type O main sequence star.

Accepted Answer

A protostar of 20 solar masses is large enough to support the fusion of heavier elements, which results in the high temperatures and luminosities associated with O-type stars. A protostar of 20 solar masses has enough mass to become a type O main sequence star. Type O stars are the most massive and therefore have the shortest lifespan.

Question 14

Brown dwarfs most commonly form in the nebular disk around another object, much like aplanet.

Accepted Answer

Brown dwarfs are substellar objects that are too massive to be considered planets but not massive enough to sustain hydrogen fusion reactions in their cores like stars. They form directly from the collapse of gas and dust clouds, similar to stars, rather than in the nebular disk around another object.

Question 15

Rotation of the protostar helps generate the magnetic fields and bipolar flows.

Accepted Answer

The rotation of the protostar generates a dynamo effect, which amplifies the magnetic fields and produces bipolar outflows. The rotation of the protostar plays a crucial role in generating the magnetic fields and bipolar flows commonly observed in young stellar objects such as protostars. This is due to the dynamo effect, which arises from the interaction between the rotation and the complex convective motions within the protostar's interior. As the protostar continues to contract and spin faster, the magnetic fields and outflows become more pronounced, eventually giving rise to the final star and its surrounding planetary system.

Question 16

Many of the brightest stars we see are only a few million years old.

Accepted Answer

This is true because the brightest stars are often massive and have short lifetimes, typically a few million years or less. Therefore, they are relatively young compared to other stars that can live for billions of years. This is true because the brightest stars are typically massive and burn through their fuel at a faster rate than smaller stars, causing them to live shorter lives. Thus, they can be relatively young in astronomical terms, even if they appear bright in our night sky.

Question 17

Collisions between galaxies tend to destroy, not create, stars.

Accepted Answer

Collisions between galaxies often lead to intense regions of star formation. The gravitational interactions can compress gas and dust, leading to the creation of new stars.

Question 18

A protostar of 20 solar masses should form a star that will stay on the main sequence twentytimes longer than our Sun.

Accepted Answer

A protostar of 20 solar masses will form a star that burns through its nuclear fuel much faster than our Sun due to its higher mass and temperature, leading to a significantly shorter lifespan on the main sequence.

Question 19

Type M stars evolve to the main sequence faster than our G type Sun.

Accepted Answer

Type M stars, being smaller and cooler than our G type Sun, actually take longer to contract and heat up to the point where nuclear fusion begins in their cores, thus evolving to the main sequence more slowly.

Question 20

Brown dwarfs are failed stars, never igniting their hydrogen fusion.

Accepted Answer

Brown dwarfs are too small to ignite hydrogen fusion in their cores, which is necessary for them to become full-fledged stars. Therefore, they are considered failed stars. Brown dwarfs are often referred to as "failed stars" because they do not have enough mass to ignite and sustain hydrogen fusion in their cores, which is what powers stars.

Quiz 19: Star Formation: a Traumatic Birth