culmulative 3 stars and galaxy

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

20 parsecs (~ 65.2 light-years) away

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

3 million years

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

What is the overall result of the proton-proton chain?

4 H becomes 1 He + energy.

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

4.5 billion years

The Sun will exhaust its nuclear fuel in about __________.

5 billion years

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

5,800 K

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

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

Study this H-R diagram. What is the spectral type of the star Sirius? (The red arrow helps you locate Sirius on the diagram.)

A

Study this H-R diagram. Which of the following stars is the most massive? (The red arrows help you locate these stars on the diagram.)

Beta Centauri

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.

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

By measuring the masses and spectral types of main-sequence stars in binary systems.

Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?

Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.

In terms of the global warming that has been occurring on Earth over the past few decades, what is the key message from this figure?

Changes in the Sun's energy output (irradiance) cannot explain the observed warming.

How is the sunspot cycle directly relevant to us here on Earth?

Coronal mass ejections and other activity associated with the sunspot cycle can disrupt radio communications and knock out sensitive electronic equipment.

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.

Our Sun is a star of spectral type _____.

G

This photograph shows a small portion of the Sun's photosphere. What is going on in the bright regions (such as the bright region indicated by the arrow)?

Hot gas is rising up from the solar interior. Submit

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?

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?

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 gravityOutward pointing red arrows represent pressureLonger arrows represent a stronger push or pull

This star map shows stars as we see them in our sky from Earth, centered around the constellation Canis Major. Larger dots represent brighter stars, and a few of the brightest stars are identified. From this view alone, what can you conclude about Sirius?

It has the greatest apparent brightness of any star in this region of the sky.

What is a white dwarf?

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

What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?

It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.

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.

According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?

Its apparent brightness will decrease by a factor of 9.

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. Submit

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.

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. Submit

From hottest to coolest, the order of the spectral types of stars is __________.

OBAFGKM

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.

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.

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.

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.

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.

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.

Why do some of the bright stars (such as the one indicated by the arrow) in this photo have cross-shaped spikes over them?

The spikes are an artifact of photography through a telescope.

Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?

The star is a member of a binary star system.

Which of the following is not a characteristic of the 11-year sunspot cycle?

The sunspot cycle is steady, so that each 11-year cycle is nearly identical to every other 11-year cycle.

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.`

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.

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

They generate energy through hydrogen fusion in their core.

Why are neutrinos so difficult to detect?

They have a tendency to pass through just about any material without any interactions. Submit

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 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.

Which of the following correctly describes how the process of gravitational contraction can make a star hot?

When a star contracts in size, gravitational potential energy is converted to thermal energy. Submit

This x-ray image shows a loop of hot gas above the surface of the Sun. Suppose we took another photo looking at the same place 1 hour later. What would we see?

The loop would look about the same as it does in this photo.

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

What layer of the Sun are we seeing in this photograph, and in what wavelength band was it photographed?

corona, photographed in x-rays Submit

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

extremely hot plasma flowing along magnetic field lines Submit

What two physical processes balance each other to create the condition known as gravitational equilibrium in stars?

gravitational force and outward pressure

On an H-R diagram, stellar radii __________.

increase diagonally from the lower left to the upper right

What can we infer, at least roughly, from a star's luminosity class?

its size (radius)

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

larger than that of Star B

The total amount of power (in watts, for example) that a star radiates into space is called its __________.

luminosity

The axes on a Hertzsprung-Russell (H-R) diagram represent __________.

luminosity and surface temperature Submit

You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's _________.

mass

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

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

much shorter

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

nuclear fusion

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

nuclear fusion of hydrogen into helium

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

orbital period and average orbital distance

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

photosphere

What layer of the Sun are we seeing in this photo?

photosphere Submit

This photo shows an object located in the halo of our Milky Way galaxy. Notice the many bright red dots in the photograph. What are they?

red giant stars

Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

spectroscopic binary

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

stars emit thermal radiation

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

sunspots

This x-ray image shows a loop of hot gas above the surface of the Sun. If we took a visible light photo that looked in the Sun's photosphere just under the two points where the loop of gas comes down (arrows), what would we find?

sunspots

When we say that the Sun is a ball of plasma, we mean that __________.

the Sun consists of gas in which many or most of the atoms are ionized (missing electrons)

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 the stars in this photo are at about the same distance from Earth (some 25,000 light-years away). Which stars in this picture are the largest in size (radius)?

the bright red stars

Energy balance in the Sun refers to a balance between __________.

the rate at which fusion generates energy in the Sun's core and the rate at which the Sun's surface radiates energy into space

The proton-proton chain is __________.

the specific set of nuclear reactions through which the Sun fuses hydrogen into helium

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

On an H-R diagram, a red supergiant would be located in the __________.

upper right

To estimate the central temperature of the Sun, scientists __________.

use computer models to predict interior conditions Submit

What are the appropriate units for the Sun's luminosity?

watts

Which units are appropriate for measurement of apparent brightness?

watts per square meter

Study this H-R diagram. Which of the following stars is the largest in size (radius)? (The red arrows help you locate these stars on the diagram.)

Antares

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.

Study this H-R diagram. Which of the following stars has (or had) the longest hydrogen fusing lifetime? (The red arrows help you locate these stars on the diagram.)

Barnard's star

Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?

Earth would be inside the supergiant.

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

What happens to energy in the Sun's convection zone?

Energy is transported outward by the rising of hot plasma and sinking of cooler plasma.

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.

Why does the Sun emit neutrinos?

Fusion in the Sun's core creates neutrinos.

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.

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

Star A

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 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.

Which photo pair shows Earth correctly scaled in comparison to the Sun?

The Sun's diameter is more than 100 times Earth's diameter.

The light radiated from the Sun's surface reaches Earth in about 8 minutes, but the energy of that light was released by fusion in the solar core about __________.

a few hundred thousand years ago

This photo shows an object located in the halo of our Milky Way galaxy. What kind of object is it?

a globular cluster of stars

What is the solar wind?

a stream of charged particles flowing outward from the surface of the Sun

The sketch shows groups of stars on the H-R diagram, labeled (a) through (e); note that (a) represents the entire main sequence, while (c) and (d) represent only small parts of the main sequence.

bWhich group represents stars that have no ongoing nuclear fusion?

This graph shows how the apparent brightness of an eclipsing binary system changes with time. Which of the four labeled regions represents the system at a time when one star is eclipsing the other?

both II and IV

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

On an H-R diagram, stellar masses __________.

can be determined for main sequence stars but not for other types of stars

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