astrology test 4

In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot because of gravitational contraction. What was the major drawback to this idea?

It predicted that the Sun could shine for about 25 million years, but geologists had already found that Earth is much older than this.

What do we mean when we say that the Sun is in gravitational equilibrium?

There is a balance within the Sun between the outward push of pressure and the inward pull of gravity.

How does the Sun's mass compare to Earth's mass?

The Sun's mass is about 300,000 times the mass of the Earth.

The Sun's surface seethes and churns with a bubbling pattern. Why?

We are seeing hot gas rising and cool gas falling as a result of the convection that occurs beneath the surface.

According to modern science, approximately how old is the Sun?

4.5 billion years

The source of energy that keeps the Sun shining today is _________.

nuclear fusion

What is the Sun's approximate composition (by mass)?

70% hydrogen, 28% helium, 2% other elements

From the center outward, which of the following lists the "layers" of the Sun in the correct order?

core, radiation zone, convection zone, photosphere, chromosphere, corona

The Sun's visible surface (that is, the surface we can see with our eyes) is called the __________.

photosphere

The Sun's average surface (photosphere) temperature is about _____.

5,800 K

The arrows in this diagram are meant to show how gravitational equilibrium works in the Sun. What do the different colors and different arrow lengths represent?

Inward pointing green arrows represent gravity

Outward pointing red arrows represent pressure

Longer arrows represent a stronger push or pull

The dark spots in this photo (such as the one indicated by the arrow) represent what we call __________.

sunspots

If the Sun's core suddenly shrank a little bit, what would happen in the Sun?

The core would heat up, fusion rates would increase, the core would re-expand.

The fundamental nuclear reaction occurring in the core of the Sun is __________.

nuclear fusion of hydrogen into helium

What causes the cycle of solar activity?

changes in the organization of the Sun's magnetic field

Why do sunspots appear dark in pictures of the Sun?

They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere.

What do sunspots, solar prominences, and solar flares all have in common?

They are all strongly influenced by magnetic fields on the Sun.

Study this figure and its axis labels. What is this graph showing us?

The number of sunspots on the Sun tends to increase and decrease with an approximately 11-year cycle.

All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two factors are most important in determining the current appearance of a star?

mass and stage of life

Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?

Rigel has a higher surface temperature than Sirius.

What is the approximate chemical composition (by mass) with which all stars are born?

3/4 hydrogen, 1/4 helium, no more than about 2% heavier elements

If star A is closer to us than star B, then Star A's parallax angle is __________.

larger than that of Star B

Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 5. Which star is brighter in our sky?

Star A

What do we mean by a star's spectral type, and how is spectral type related to surface temperature and color.

Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the hottest and M are the coolest. Hotter stars look bluer to us, and cooler stars look redder.

What do we need to measure in order to determine a star's luminosity?

apparent brightness and distance

Which of these stars has the coolest surface temperature?

a K star

Which of these stars is the most massive?

a main-sequence A star

The approximate main-sequence lifetime of a star of spectral type O is _________.

3 million years

High-mass stars have __________ lifetimes than low-mass stars.

much shorter

What is the common trait of all main-sequence stars?

They generate energy through hydrogen fusion in their core.

What is a white dwarf?

It is the remains of a star that ran out of fuel for nuclear fusion.

Which of the following statements comparing open and globular star clusters is true?

Stars in open clusters are relatively young, while stars in globular clusters are very old.

Which of these star clusters is oldest?

a cluster whose brightest main-sequence stars are yellow

How do we learn the chemical composition of the interstellar medium?

By studying spectra of interstellar gas clouds

What do we mean by the interstellar medium?

the gas and dust that lies in between the stars in the Milky Way galaxy

Which of the following statements about the first stars that were born in the universe is expected to be true?

They were made only from hydrogen and helium.

This figure shows frames from a computer simulation of star formation, starting at the left with a large molecular cloud measuring more than a light-year across. What is happening as time passes (from left to right)?

The cloud is fragmenting into smaller pieces that will form stars.

During the time before fusion begins, a protostar's core gradually ________.

shrinks and heats

What is a protostar?

a star that is still in the process of forming

Which kinds of stars are most common in a newly formed star cluster?

M stars

Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?

A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.

The vast majority of stars in a newly formed star cluster are __________.

less massive than the Sun

This diagram shows the life tracks from protostar to the main sequence for several stars of different masses. Which track represents the star that takes the longest amount of time to reach the main sequence?

Track 1

What is the main idea captured by this graph?  

Low-mass stars are much more common than higher-mass stars.

Which of the following stars will live longest?    

1 solar mass star

What is a planetary nebula?

gas ejected from a low-mass star in the final stage of its life

The ultimate fate of our Sun is to __________.

become a white dwarf that will slowly cool with time

Assume the following stars all have the same mass as our Sun. Which one does not have fusion occurring in its central core?

a red giant

What would you be most likely to find if you returned to the solar system in 10.0 billion years?

a white dwarf

Carbon fusion occurs in high-mass stars but not in low-mass stars because _________.

the cores of low-mass stars never get hot enough for carbon fusion

Which of the following stars will end its life in a supernova?

a 10 solar-mass star

To predict whether a star will eventually fuse oxygen into a heavier element, what do you need to know about the star?

its mass

After a supernova explosion, the remains of the stellar core can be __________.

either a neutron star or a black hole

Which of these stars has the hottest core?

a white main-sequence star

What happens to the core of a high-mass star after it runs out of hydrogen?

It shrinks and heats up.

The more massive a white dwarf, the _________.

smaller its radius

The white dwarf that remains when our Sun dies will be mostly made of ______.

carbon

According to present understanding, a nova is caused by _________.

hydrogen fusion on the surface of a white dwarf

Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?

No, because it is not orbited by another star.

A white dwarf is __________.

what most stars become when they die

If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh __________

as much as a truck

A neutron star is __________.

the remains of a star that died in a massive star supernova (if no black hole was created)

If you had something the size of a sugar cube that was made of neutron star matter, it would weigh __________.

about as much as a large mountain

Pulsars are thought to be __________.

rapidly rotating neutron stars

What is the basic definition of a black hole?

an object with gravity so strong that not even light can escape

What do we mean by the event horizon of a black hole?

It is the point beyond which neither light nor anything else can escape.

In the late 1800s, Kelvin and Helmholtz suggested that the Sun stayed hot because of gravitational contraction. What was the major drawback to this idea?

It predicted that the Sun could shine for about 25 million years, but geologists had already found that Earth is much older than this.

When is/was gravitational contraction an important energy generation mechanism for the Sun?

It was important when the Sun was forming from a shrinking interstellar cloud of gas.

What do we mean when we say that the Sun is in gravitational equilibrium?

There is a balance within the Sun between the outward push of pressure and the inward pull of gravity.

What do we mean when we say that the Sun is in energy balance?

The amount of energy released by fusion in the Sun's core equals the amount of energy radiated from the Sun's surface into space.

Which of the following is the best answer to the question, "Why does the Sun shine?"

As the Sun was forming, gravitational contraction increased the Sun's temperature until the core become hot enough for nuclear fusion, which ever since has generated the heat that makes the Sun shine.

How does the Sun's mass compare to Earth's mass?

The Sun's mass is about 300,000 times the mass of the Earth.

Which of the following best describes why the Sun emits most of its energy in the form of visible light?

Like all objects, the Sun emits thermal radiation with a spectrum that depends on its temperature, and the Sun's surface temperature is just right for emitting mostly visible light.

The Sun's surface seethes and churns with a bubbling pattern. Why?

We are seeing hot gas rising and cool gas falling as a result of the convection that occurs beneath the surface.

Which of the following correctly compares the Sun's energy generation process to the energy generation process in human-built nuclear power plants?

The Sun generates energy by fusing small nuclei into larger ones, while our power plants generate energy by the fission (splitting) of large nuclei.

Every second, the Sun converts about 600 million tons of hydrogen into 596 million tons of helium. The remaining 4 million tons of mass is __________.

converted to an amount of energy equal to 4 million tons times the speed of light squared

Which of the following best explains why nuclear fusion requires bringing nuclei extremely close together

Nuclei normally repel because they are all positively charged and can be made to stick only when brought close enough for the strong force to take hold.

If the Sun's core suddenly shrank a little bit, what would happen in the Sun?

The core would heat up, fusion rates would increase, the core would re-expand.

Which of the following best explains why the Sun's luminosity gradually rises over billions of years?

Fusion gradually decreases the number of independent particles in the core, allowing gravity to compress and heat the core, which in turn increases the fusion rate and the Sun's luminosity.

Which of the following choices is not a way by which we can study the inside of the Sun?

We can send a space probe into the Sun's photosphere.

Why does the Sun emit neutrinos?

Fusion in the Sun's core creates neutrinos.

If the Sun suddenly stopped emitting neutrinos, what might we infer (after checking that our neutrino detectors were still operational)?

Fusion reactions in the Sun have ceased within the past few minutes.

A salesman attempts to convince you to purchase a "solar neutrino shield" to protect you and your family. (It's even "on sale!") Why do you turn down this excellent offer?

Neutrinos rarely, if ever, interact with other matter.

Why do sunspots appear dark in pictures of the Sun?

They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere.

How can we best observe the Sun's chromosphere and corona?

The chromosphere is best observed with ultraviolet telescopes and the corona is best observed with X-ray telescopes.

The intricate patterns visible in an X-ray image of the Sun generally show _________.

extremely hot plasma flowing along magnetic field lines

How can we measure the strength of magnetic fields on the Sun?

By looking for the splitting of spectral lines in the Sun's spectrum.

Satellites in low-Earth orbits are more likely to crash to Earth when the sunspot cycle is near solar maximum because _________.

Earth's upper atmosphere tends to expand during solar maximum, exerting drag on satellites in low orbits

We can essentially rule out the Sun as a cause of global warming over the past few decades because __________.

the amount of incoming sunlight has decreased while Earth's average temperature has increased

All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two factors are most important in determining the current appearance of a star?

mass and stage of life

Which units are appropriate for measurement of apparent brightness?

watts per square meter

Star A is identical to Star B, but Star A is twice as far from us as Star B. Therefore, ______________.

both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A

A star with a parallax angle of 1/20 arcsecond is __________.

20 parsecs (≈ 65.2 light-years) away

A star's color is related to its surface temperature because _________.

stars emit thermal radiation

Which of the following statements about spectral types of stars is not generally true?

The spectral type of a star can be used to determine its distance.

Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?

Rigel has a higher surface temperature than Sirius.

To calculate the masses of stars in a binary system, we must measure their _________.

orbital period and average orbital distance

Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes significantly dimmer for about 2 hours. The most likely explanation is that _________.

the star is a member of an eclipsing binary star system

Which group represents stars that are cool and dim?

d

Which group represents stars of the largest radii?

e