Astronomy Exam #2

Star-forming clouds appear dark in visible-light photos because the light of stars behind them is absorbed by __________.

interstellar dust

The basic requirement for a cloud to collapse to form a star is that __________.

gravity must be strong enough to overpower the cloud's internal pressure.

If a protostar has a mass too small for it to sustain nuclear fusion, it becomes the type of object known as a

Brown Dwarf

The collapse of a protostar with less than 0.08 timAs a protostar shrinks in size, its central temperature rises along with its

es the mass of the Sun is halted by .

degeneracy pressure

A star is born when it comes into ______, in which the energy released by fusion matches the energy it radiates into space.

Energy Balance

As a protostar shrinks in size, its central temperature rises along with its?

Thermal Pressure

A forming star spins more rapidly as it collapses because of conservation of?

Angular Momentum

A star that has not yet finished forming is called a?

Protostar

A gas cloud in which gravity is perfectly balanced against pressure is said to be in?

Gravitational Equilibrium

Which of the following represents key observational evidence that stars are born in clouds of gas and dust?

Young star clusters are always associated with clouds of gas and dust.

Why does it make sense for star-forming clouds to be cold and dense?

these conditions help gravity overcome gas pressure.

Which of the following best explains the current state of agreement between theoretical models of star formation and observations of star formation?

Current models can reproduce general features of star birth, but not all the details.

The main illustration in the video shows the life track of a one-solar mass star. Each point along this track represents __________.

the star's surface temperature and luminosity at one time in its life.

Is a protostar in energy balance? Why or why not?

No, because its core generates less energy than its surface radiates into space.

What event brings the forming star into energy balance?

the onset of nuclear fusion in the core

The fundamental lesson that we learn from the life track is that the star's outward appearance at any moment in its life reflects __________.

the way it is generating in its core

The star expels its outer layers into space almost immediately upon the end of Stage _____.

5

What element(s) should you expect to be most abundant in the star when it reaches Stage 6?

carbon

At which of the first five labeled stages is the star in energy balance?

main sequence and the next closest to main sequence

How do the properties of long-lived stars compare to those of short-lived stars?

Long-lived stars begin their lives with less mass and a smaller amount of hydrogen fuel.

Long-lived stars are less luminous during their main-sequence lives.

A main-sequence star twice as massive as the Sun would last __________.

much less than half as long as the Sun

If stars A and B are both main-sequence stars and star A has a greater fusion rate than star B, which of the following statements hold(s)?

Star A must be more luminous than star B.

Star A must be more massive than star B.

According to our modern understanding of the origin of elements, why are hydrogen and helium so much more abundant than any other elements?

They were produced in the Big Bang.

Notice that nitrogen is less abundant than either carbon or oxygen. This is an example of the more general observation that __________.

elements with even atomic numbers are more abundant than the elements with odd atomic numbers that come between them.

Overall, careful study of the patterns revealed in the graph of measured element abundances has allowed scientists to __________.

test and validate models of how elements are produced by stars.

The diagram indicates that the third most abundant element in the Milky Way Galaxy is _____.

oxygen

According to the diagram, the approximate abundance of oxygen atoms in the galaxy is __________.

1/1000 that of hydrogen

According to the diagram, what is the most abundant element with an atomic number greater than or equal to 20?

iron

According to current understanding, the two most abundant elements in the universe were made __________.

In the big bang.

What did Carl Sagan mean when he said that we are all "star stuff"?

that the carbon, oxygen, and other elements essential to life were created by nucleosynthesis in stellar cores

Which two energy sources can help a star maintain its internal thermal pressure?

nuclear fusion and gravitational contraction

What eventually halts the gravitational collapse of an interstellar gas cloud if it is massive enough to become a star?

The central object becoming hot enough to sustain nuclear fusion in its core

What is the source of luminosity for protostars that have not yet become hot enough for fusion in their cores?

gravitational energy released by infalling matter

What can we learn about a star from its life track on an H-R diagram?

what surface temperature and luminosity it will have at each stage of its life.

When does a star become a main-sequence star?

when the rate of hydrogen fusion within the star's core is high enough to sustain gravitational equilibrium.

What is the fate of an isolated brown dwarf?

It will remain a brown dwarf forever.

You observe a red giant star. What is happening to that star?

It is increasing in size, and is fusing hydrogen in a shell around its core.

Why are the very lowest mass stars unable to fuse helium in their cores?

Their cores will never get hot enough to fuse helium.

If the Sun somehow acquired more hydrogen and helium and doubled in mass, what would happen to it?

It would evolve more quickly than it is now.

What is a planetary nebula?

gas ejected from a 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

What percentage of a star's total lifetime is spent on the main sequence?

90%

What happens after the helium flash in the core of a star?

The core quickly heats up and expands as helium fusion begins all at once throughout the core.

Which of the following sequences correctly describes the stages of life for a one solar mass star?

protostar, main-sequence, red giant, white dwarf

Why is Supernova 1987A particularly important to astronomers?

It was the nearest supernova detected in nearly 400 years.

Which event marks the beginning of a supernova?

the sudden collapse of an iron core into a compact ball of neutrons.

The figure above shows the abundance of elements in the galaxy relative to hydrogen. What is the general trend in elemental abundance?

Elements with more protons are less abundant.

The figure above shows the abundance of elements in the galaxy relative to hydrogen. What is the least abundant of the elements with an atomic number less than iron's?

beryllium

Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?

iron

The following question refers to the H-R diagram below that shows the life track of a 1-solar-mass star, with various stages labeled with Roman numerals.

At the end of its life, the remaining core of this star will be left behind as?

a white dwarf made primarily of carbon and oxygen.

The following question refer to the H-R diagram below that shows the life track of a 1-solar-mass star, with various stages labeled with Roman numerals.

A star at the point labelled iii is

on the main sequence.

The following question refers to the sketch below of an H-R diagram for a star cluster.

Based on its main-sequence turnoff point, the age of this cluster is

about 10 billion years.

The following question refer to the sketch below of an H-R diagram for a star cluster.

Which statement about this cluster is not true?

It is the type of cluster known as an open cluster.

The following question refer to the sketch below of an H-R diagram for a star cluster.

Consider the star to which the arrow points. How is it currently generating energy?

by hydrogen shell fusion around an inert helium core.

Consider the star to which the arrow points. Which of the following statements about this star is not true?

It is significantly less massive than the Sun.

In science, it is now well accepted that the speed of light is always the same and therefore nothing can travel faster than light. These ideas gained acceptance primarily because __________.

they have been verified through actual observations and experiments

Based on these thought experiments, what can you conclude?

Jackie can never reach or exceed the speed of light.

A white dwarf is?

the exposed core of a dead star, supported by electron degeneracy pressure.

Why is there an upper limit to the mass of a white dwarf?

The more massive the white dwarf, the greater the degeneracy pressure and the faster the speeds of its electrons. Near 1.4 solar masses, the speeds of the electrons approach the speed of light, and no more mass can be supported.

Which of the following hypothetical observations would contradict our theories about the formation and evolution of white dwarfs?

discovery of a white dwarf with a mass 1.5 times that of the Sun (1.5 Msun).

Imagine comparing a 1.2 solar mass white dwarf to a 1.0 solar mass white dwarf. Which of the following must be true

The 1.2 solar mass white dwarf has a smaller radius.

What kind of pressure supports a white dwarf?

electron degeneracy pressure

Which of the following is not true about differences between novae and supernovae?

Supernovae eject gas into space but novae do not.