Astronomy Unit 3

Which star spends the longest time as a main-sequence star?

0.5 M

A Type Ia supernova can be as luminous as

1 billion suns.

In what two ways does temperature affect the rate of nuclear reactions?

1. increased likelihood of atomic nuclei collisions 2. higher probabilities that the atomic nuclei will collide with enough energy to overcome their mutual electric repulsion and fuse Temperature Effects As the average kinetic energy increases, the particles move faster and collide more frequently per unit time and possess greater energy when they collide. Both of these factors increase the reaction rate. Hence the reaction rate of virtually all reactions increases with increasing temperature.

A Type Ia supernova occurs when a white dwarf exceeds a mass of

1.4 M

Neutron stars have masses that range from

1.4 M to 3 M

The evolutionary cutoff between low- and high-mass stars occurs at approximately

10 M

A 10 M star will evolve through the same phases as a

20?

If the Sun suddenly turned into a black hole, what would be the radius of its event horizon?

3 km

Place the evolutionary stages shown in the figure in order from youngest to oldest

3,1,2

List the H-R diagrams in the figure shown from oldest to youngest.

4, 3, 1, 2

The Sun will become a red giant star in about

5 billion years

The Sun will likely stop being a main-sequence star in

5 billion years.

What is the difference between the singularity and the event horizon of a black hole?

A singularity is the center of a black hole where space time ceases to exist. The event horizon surrounds the singularity, and is the point at which the escape velocity equals the speed of light

What does singularity mean in the context of black holes?

A singularity is the point of a black hole that has infinite density and zero volume, and when something gets too close to it, it is essentially lost due to curved space time. At this point, space time ceases to exist

A 1-M star in a binary system could create which chemical element and eject it into the interstellar medium?

ALL OF THE ABOVE carbon helium iron

Explain why a blue star cluster is likely younger than a red star cluster

As the star clusters age, they cool down, making them dimmer and cooler, causing them to turn redder. Thus, blue clusters are most likely younger, because they are hotter.

Why are star clusters helpful for testing our ideas about star formation and stellar evolution?

Assuming that the stars in a cluster all formed at the same time from the same cloud of gas and have the same composition, the only difference between these stars would be their mass, which determines their life span. We can see many stages of stellar evolution among the members of a single cluster, and we can see whether our models can explain why the H-R diagrams of clusters of different ages look the way they do.

Considering how long it took for life to arise on Earth, which of the stellar spectral types shown below would be the least likely to have planets with life?

B0

Why does the core of a main-sequence star have to be hotter to burn helium into carbon than hydrogen into helium?

Carbon is a heavier element than helium, so it requires a higher temperature to fuse helium into carbon.

https://www.chegg.com/homework-help/questions-and-answers/question-1-main-sequence-star-gaining-mass-interacting-binary-system-would-happen-star-s-l-q8842699

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You observe three different star clusters and find that the main-sequence turnoff stars in Cluster 1 have spectral type F, the main-sequence turnoff stars in Cluster 2 have spectral type A, and the main-sequence turnoff stars in Cluster 3 have spectral type G. Which star cluster is the youngest and which one is the oldest?

Cluster 2 is the youngest and Cluster 3 is the oldest

What is one way in which high-mass stars differ from low-mass stars?

Convection is important in their cores, which mixes up helium throughout the core.

___________ keeps the core of a red giant star from collapsing.

Degenerate pressure from electrons

About 2 percent of the mass of the Sun is in elements heavier than helium. Where did these elements come from?

Elements heavier than helium in the sun come from lighter elements fusing into heavier elements.

__________ is the result of mass distorting the fabric of spacetime.

Gravity

Briefly explain why planets like ours could not have formed at about the same time as the very first generations of stars in the universe.

Heavy elements found on planets such as our had to have been formed as a result of supernovae explosions, therefore very early planets would not contain such heavy elements if they existed before such explosions and nucleosynthesis.

As a main-sequence star burns its core supply of hydrogen, what happens?

Helium builds up in the core

Give two major arguments for the scarcity of stars in the high-mass, high-luminosity, main-sequence blue tail

High mass stars live shorter lives on the main sequence than low mass stars, so more stars will tend to be found on the low mass end of the main sequence, since more stars stay on that end for longer. evolve off of the main sequence more quickly

As a white dwarf star gradually cools, its radius stays approximately constant. What is happening to the white dwarf's luminosity?

It decreases.

How do we understand the very fast rotation of neutron stars?

It is a consequence of the conservation of angular momentum applied to collapsing objects

What is the meaning of the word spacetime?

It is the combined treatment of space and time in the theory of relativity

What is the significance of the Schwarzschild radius around a black hole?

It is the radius at which the escape velocity equals the speed of light

Why can an object with a nonzero mass never travel as fast as the speed of light?

It would take an infinite amount of energy to accelerate it to a speed of c.

Describe the structure of a red giant star just before the helium flash takes place. How does this compare with the structure of a horizontal-branch star?

Just before the helium flash, a red giant star has an inert helium core, before temperatures are high enough to convert helium into carbon, as well as a hydrogen burning shell. In the horizontal branch, the core is now helium burning and creating a carbon inert core, as well as a hydrogen burning shell.

A low-mass star that burns helium in its core and hydrogen in a shell surrounding the core is ________ than a similar star that burns hydrogen only __________.

Less luminous ; in a shell around a degenerate core

Where did the iron in your blood come from?

Nuclear reactions in the cores of massive stars

How can the core of a star be degenerate with respect to the electrons but nondegenerate with respect to the nuclei?

Quantum mechanics allows electrons and nuclei to occupy the same physical space so nuclei essentially don't notice the jam-packed electrons.

How does relativity compare with Newtonian physics?

Relativity gives the same result as Newtonian physics when objects are moving slowly.

What is a planetary nebula, and why are many planetary nebulae not symmetrical?

The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different.[48] Nevertheless, the reason for the huge variety of physical shapes is not fully understood.[46] Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae

If a main-sequence star were gaining mass by being in an interacting binary system, what would happen to that star's luminosity and why?

The luminosity would increase because the star's central pressure would rise and the rate of nuclear reactions would increase

As a subgiant star becomes a red giant, its luminosity increases while its temperature remains approximately constant. What does this mean?

The radius is increasing

Suppose that an object is moving and it is emitting light toward you, in vacuum. What would you notice about the light you observe?

The speed stays constant.

Explain what the triple-alpha process is and when it takes place in evolving stars.

The triple-alpha process is a process in which Helium is fused to form carbon once temperatures in a stars core reach 100 million K. This takes place in red giant stars that have left the main sequence and used up all of their core hydrogen and have reached core temps of 100 million K.

Which of the following is not true about neutron stars?

Their densities are comparable to that of atomic nuclei.

When a star burns hydrogen in a shell, how does the energy released compare with when the star burned hydrogen in the core?

There will be more energy produced (?)

What might be true about the oldest stars in the Milky Way?

They would have few heavy elements, since there was not much chance for earlier generations of stars to explode as supernovae before these stars were formed

Explain why the pulsars we see might be only a fraction of the neutron stars in the Milky Way.

We only see the beams of light emitted from neutron stars when they are directly pointed towards our field of view. If they are not lined up just right, we will not see them.

Describe the helium flash that occurs when helium fusion ignites in the core of a red giant star.

When helium fusion ignites in the core of a red giant star at 100 million K, the core heats rapidly without expanding, and emits a flash that we refer to as the helium flash.

How does the formation of elements by nuclear fusion depend on the mass of the star?

With increasing mass, progressively heavier nuclei are forged deeper and deeper inside the star

A neutron star contains a mass of up to 3 M in a sphere with a diameter approximately the size of

a small city.

Hawking radiation is emitted by a black hole when

a virtual pair of particles is created near the event horizon.

Gravitational lensing occurs when _____________ distorts the fabric of spacetime.

any massive object

Compared with a clock on the surface of the Earth, a clock on the International Space Station runs

at approximately the same rate, but slightly slower.

A red giant star is found to be orbiting an unseen object with a short orbital period. By measuring the speed at which it orbits, astronomers deduce that the unseen object has a mass of 10 M. This object is probably a ______________ because __________________________.

black hole; its mass is too large to be a neutron star or a white dwarf

Essentially all the elements heavier than iron in our Milky Way were formed

by supernovae.

An iron core cannot support a massive main-sequence star because iron

cannot fuse to make heavier nuclei and produce energy

In the CNO cycle, carbon is used as a catalyst for the fusion of hydrogen into helium. This means that

carbon facilitates the reaction but is not consumed in it

Once silicon burning initiates in the core of a high-mass star, the star has only a few __________ left to live.

days

What mechanism provides the internal pressure inside a neutron star?

degeneracy pressure from neutrons

If you measure the average brightness and pulsation period of a classical Cepheid variable star, you can also determine its

distance.

A star like the Sun will eventually become a(n) __________ star.

electron degenerate white dwarf

In a white dwarf, what is the source of pressure that halts its contraction as it cools?

electrons packed so closely that they become incompressible

A person would experience __________ as he or she approached the event horizon of a black hole.

extremely strong tidal forces

According to the theory of relativity, a clock on top of Mount Everest would run _______ compared with a clock at sea level because _________________________________ .

faster; of the high altitude, which means a slightly weaker gravity

We can identify only a fraction of all the radio pulsars that exist in our Galaxy because

few swing their beam of synchrotron emission in our direction.

Massive stars explode soon after fusion to iron initiates because

fusion of elements heavier than iron requires energy, so the star runs out of fuel and cannot hold itself up against gravity

The bending of light paths near a massive object is the essence of

gravitational lensing.

General relativity predicts that coalescing (merging) massive objects would trigger

gravitational waves.

When a star depletes its core supply of hydrogen, _________ causes the core to collapse while increased gas _________ is exerted on the atmosphere

gravity; pressure

A main-sequence star of 25 solar masses has about 25 times the luminosity of a 10-solar-mass star (recall the mass-luminosity relation presented in the previous chapter). This is because the more massive star

has a hotter core, and therefore nuclear burning proceeds more rapidly

Asymptotic giant branch stars have high-mass loss rates because they

have low surface gravity.

As a red giant star evolves, hydrogen shell burning proceeds increasingly faster due to

heat released from the core's contraction.

A 1-M solar mass red giant star's energy comes from

helium burning to carbon

Degenerate refers to a state of matter at

high density.

During which phase of the evolution of a low-mass star does it have two separate regions of nuclear burning occurring in its interior?

horizontal branch

Explain why stars are able to lose mass as they expand to red giant size.

https://www.astronomynotes.com/evolutn/s5.htm

The gas in a planetary nebula is composed of

hydrogen and heavier elements like helium and carbon processed in the core of the post-asymptotic giant branch star.

A main-sequence star is unique because

hydrogen burning occurs in its core

Once the core of a low-mass main-sequence star runs out of hydrogen, ____________ stops until the core temperature is high enough for helium fusion to begin.

hydrogen fusion in the core

Which of these fusion reactions begins first in the core of a massive star?

hydrogen fusion to helium

According to Special Relativity, spacecraft that would travel faster than the speed of light are

impossible, because nothing can travel that fast.

If a main-sequence star's core temperature increased, fusion reaction rates would ________ because the protons would be moving _______ .

increase; faster

The luminosity of a star depends on

its mass and age

The collapse of the core in high-mass stars naturally explains all but which one of the following neutron stars properties?

large distance from earth

Which of the following is not a common characteristic of a neutron star?

large radius

Asymptotic giant-branch stars have _________ luminosities, _________ radii, and _________ escape velocities.

large; large; small

When a spectral-type G2 star like the Sun leaves the main sequence, its

luminosity increases and its surface temperature decreases.

In young clusters the light is dominated by

luminous hot, blue, and some red supergiants.

The fundamental stellar property that determines the major evolutionary differences in the life history of stars is

mass

What factor is most important in determining a star's position on the main sequence and subsequent evolution?

mass

A star's surface temperature during the horizontal branch phase is determined primarily by its

mass and chemical composition

What are two ways that Type Ia supernovae can be produced?

mass transfer and stellar mergers

A nova is the result of which explosive situation?

mass transfer onto a white dwarf

Nucleosynthesis refers to the formation of

massive atoms from less massive ones

The __________ a main-sequence star is, the more hydrogen it has to burn, and the ______ its main-sequence lifetime lasts.

more massive; shorter

When the first pulsar was discovered, scientists thought it might be a signal from a distant extraterrestrial civilization. However, this idea was quickly discarded because

more pulsars were discovered, which meant that these were natural phenomena.

Each stage of nuclear burning in a 25 M star is __________ in duration than in a star of 15 M

much shorter

As a high-mass main-sequence star evolves off the main sequence, it follows a __________ on the H-R diagram

nearly horizontal path

A few hours before a high-mass star is blasting its outer layers in a colossal explosion, specialized detectors on Earth would be able to reveal a spike in the number of

neutrinos.

Iron has 26 protons in its nucleus, and gold has 79 protons. Where did all the gold on the Earth come from?

nucleosynthesis that took place in supernova explosions

The collapse of the core of a high-mass star at the end of its life lasts approximately

one second.

Massive stars synthesize chemical elements going from helium up to iron

only in the core of the star.

The Crab Nebula is an important test of our ideas about supernova explosions because

people saw the supernova and later astronomers found a pulsar inside the nebula.

For main-sequence stars in hydrostatic equilibrium, at any interior radius there exists a balance between the downward gravitational force at that radius and the

pressure from a degenerate electron core

When the core of a massive star collapses, a neutron star forms because

protons and electrons combine to make neutrons.

One reason why we think neutron stars were formed in supernova explosions is that

pulsars sometimes have material around them that looks like the ejecta from supernovae.

What causes massive stars to lose mass at a high rate through stellar winds?

radiation pressure

Massive stars explode when they

run out of nuclear fuel in their core, and the cores collapse

During the main-sequence evolution of a massive star, progressively more massive elements are fused in the core, giving the core support for

shorter and shorter times

One star in a binary will almost always become a red giant before the other because

small differences in main-sequence masses yield large differences in main-sequence ages

Novae and Type Ia supernovae can occur in binary star systems because __________ can mean large differences in their __________ .

small differences in the stars' masses; main-sequence lifetimes

When helium fusion begins in the core of a red giant star, the situation quickly gets out of control because electron-degeneracy pressure does not respond to changes in

temperature.

The dominant mechanism by which high-mass stars generate energy on the main sequence is called

the CNO cycle.

The gravitational redshift of light should be smallest for light emitted from the surface of

the Sun??? A planet like the earth???

A low-mass main-sequence star's climb up the red giant branch is halted by

the beginning of helium fusion in the core

What characteristic of a star cluster is used to determine its age?

the color of the main-sequence turnoff in the cluster

What is a planetary nebula?

the ejected envelope of a giant star surrounding the remnant of a star

What would you need to measure in a planetary nebula to determine how long ago its parent star died?

the nebula's radius and expansion velocity

The luminosity of a Cepheid star varies in time because

the outer envelope of the star contracts and expands radially

Hawking radiation from black holes refers to

the radiation of particles created near the event horizon

The event horizon of a black hole is defined as

the radius at which the escape speed from the black hole equals the speed of light

If the Sun were to be instantly replaced by a 1 M black hole, the gravitational pull of the black hole on Earth would be

the same as it is now

Helium burns in the core of a horizontal branch star via _________ and produces _________.

the triple-alpha reaction; carbon

In the post-main sequence stages, high-mass stars follow quasi-horizontal paths in the H-R diagram. This means that

their color gets redder because they expand

Photons have no mass, and Einstein's theory of general relativity says

their paths through spacetime are curved in the presence of a massive body

Stars evolve primarily because

they use up the fuel in their cores.

Even if a black hole emitted no light, we can still detect it

through its gravitational effect on surrounding gas or stars

Light is increasingly redshifted near a black hole because

time is moving increasingly slower as viewed from the observer's frame of reference.

What ionizes the gas in a planetary nebula and makes it visible?

ultraviolet photons emitted by a white dwarf

Black holes that are stellar remnants can be found by searching for

variable X-ray sources.

When do the predictions of Special Relativity match those of Newtonian physics?

when objects are moving slowly

The Sun eventually will become a(n)

white dwarf.

Type Ia and Type II supernovae are respectively caused by what types of stars?

white dwarfs; massive stars