Astronomy Hw 7
Provided following are the spectral types of four different main-sequence stars. Rank the stars based on the strength of the radiation pressure that pushes outward as they are forming, from highest pressure to lowest pressure.
big to small-O9-A5-G2-M6
Star A is identical to Star B, except that 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
Assume that all four H-R diagrams below represent a star in different stages of its life, after it starts to fuse hydrogen in its core. Rank the HR diagrams based on when each stage occurs, from first to last.
first stage -last stage -star in sequence -slightly above sequence -above sequence -below sequence
Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are __________.
hotter and smaller in radius
What does the apparent magnitude of a star tell us about that star?
how bright it appears from Earth
The particles ejected from the Sun during a coronal mass ejection, if directed at Earth, will reach us
in several days
Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
infrared
We found that mass must be inferred for the star described in Part A. However, we can measure a star's mass directly if __________.
it is a member of an eclipsing binary system
All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the characteristics of a star?
its mass and its stage of life
Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is __________.
less luminous, cooler, smaller, and less massive
The Hertzsprung-Russell (H-R) diagram shows the relationship between a star's
luminosity and surface temperature.
On the H-R diagram, most stars fall into the region labeled ____________.
main sequence
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
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
If the distance between us and a star is doubled, with everything else remaining the same, its luminosity
remains the same, but its apparent brightness is decreased by a factor of four.
The following figures show the spectral types of four main-sequence stars. Rank them based on the time each takes, from longest to shortest, to go from a protostar to a main-sequence star during the formation process.
small to big-M6-G2-A5-O9
The more distant a star, the __________.
smaller its parallax angle
Compared to a nearer star, a more distant star will have a __________.
smaller parallax angle
From Part A, you know that surface temperature is a stellar property that we infer indirectly. What must we measure directly so that we can infer a star's surface temperature?
spectral type
To place a star on an H-R diagram, we must know its __________.
surface temperature and luminosity
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
Before we can use parallax to measure the distance to a nearby star, we first need to know __________.
the Earth-Sun distance
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
Among main sequence stars, those with the highest surface temperatures have ___________.
the highest masses and shortest lifetimes
A main-sequence star's luminosity can directly inform us of
the rate at which it converts hydrogen to helium.
What is a white dwarf?
the remains of a star that ran out of fuel for nuclear fusion
What is the only force that can overcome the repulsion between two positively charged nuclei and bind them together?
the strong force
Since all stars begin their lives with the same basic composition, what characteristic is most important in determining how they will differ?
their initial mass
Most interstellar clouds remain stable in size because the force of gravity is opposed by ________ within the cloud.
thermal pressure
Stars with masses greater than 150 times our Sun probably cannot form because
they would generate so much energy that they would blow off their outer layers.
The GAIA spacecraft is capable of measuring parallax angles as small as about 0.00002 arcsecond (20 microarcseconds). Based on this fact, GAIA should in principle be able to measure the distances of stars located __________.
throughout the Milky Way Galaxy, but not in the Andromeda galaxy or other more distant galaxies
Recent claims that global warming is due to changes in solar irradiance related to sunspot activity are
unlikely since global temperatures have continued to rise even though solar irradiance has not.
Based on its surface temperature of 6,000 K, most photons that leave the Sun's surface lie in which region of the electromagnetic spectrum?
visible
Which of the following facts about stars in a cluster make it possible to determine the approximate age of the cluster?
- All the stars in a cluster are at about the same distance from us. - Stars in a cluster all formed at approximately the same time. - The mass of a star determines how long it will be on the main sequence.
Provided following are four different ranges of stellar masses. Rank the stellar mass ranges based on how many stars in each range you would expect to be born in a star cluster, from highest number to lowest number.
-less than 1 solar mass -between 1 and 10 solar masses -between 10 and 30 solar masses -between 30 and 60 solar masses
How is the lifetime of a star related to its mass?
More massive stars live much shorter lives than less massive stars.
Which of the following statements about globular clusters is true?
Most stars in the cluster are yellow or reddish in color.
Based on its location on the HR diagram, what can we say about Rigel's mass and lifetime?
Nothing, because it is not on the main sequence.
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.
Consider a relatively nearby, single star, that is, a star that is not a member of a binary system and has no known orbiting planets. Listed below are a few properties of this star. Classify each property as either something that we can observe or measure directly (with the aid of a telescope and instruments such as cameras or spectrographs) or something that we must infer indirectly (and hence is correct only if all of our assumptions are correct).
Observe - color, parallax angle, spectral type, apparent brightness Infer - luminosity, surface temperature, mass, radius
Which of the following statements comparing open and globular star clusters is not true?
Open and globular clusters each typically contain a few hundred stars.
The faintest star visible to the naked eye has an apparent magnitude of about
6
Rigel's radius is about _____ times the Sun's radius.
80
Approximately what is the parallax angle of a star that is 20 light-years away?
0.16 arcsecond You can confirm this answer by using the animation "Parallax and Distance" in the Interactive Figure. Move the distance slider until distance shown is 20 light-years, then read the corresponding parallax angle in arcseconds. Alternatively, you can use the parallax formula d(in light-years)=3.26×1p(in arcseconds)d(in light-years)=3.26×1p(in arcseconds) or p(in arcseconds)=3.26×1d(in light-years)p(in arcseconds)=3.26×1d(in light-years) Using a distance of d = 20 light-years, this formula gives the parallax angle to be p=3.26/20=0.163p=3.26/20=0.163 arcsecond.
Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.
1. The MAIN-SEQUENCE LIFETIME is greater for low-mass stars than it is for high-mass stars. 2. The stars known as SUPERGIANTS are the very largest and brightest of all the stars. 3. Most of the stars near the Sun are MAIN-SEQUENCE STARS. 4. Stars are classified on the basis of their spectral type and LUMINOSITY CLASS. 5. Cepheid's are examples of PULSATING VARIABLE STARS. 6. WHITE DWARFS are no longer generating energy through nuclear fusion. 7. Stars that are cooler than the Sun yet 100 to 1,000 times as luminous as the Sun are classified as GIANTS.
Approximately what core temperature is required before hydrogen fusion can begin in a star?
10 million K
About what is Rigel's surface temperature?
10,000 K
Rigel's luminosity is about _____ times the Sun's luminosity.
100,000
You measure the parallax angle for a star to be 0.5 arcseconds. The distance to this star is
2 parsecs
Parts D through F use the parallax formula: d (in light-years)=3.26 × 1p (in arcseconds)d (in light-years)=3.26 × 1p (in arcseconds) Suppose there was a star with a parallax angle of 1 arcsecond. How far away would it be?
3.26 light-years
Suppose that a star had a parallax angle of exactly 1 arcsecond. Approximately how far away would it be, in light-years?
3.3 light-years A more precise animation would show that the distance of an object with a parallax angle of 1 arcsecond is 3.26 light-years, which astronomers call a parsec (short for "parallax second"). One parsec is defined as the distance of an object with a parallax angle of exactly 1 arcsecond, and its value in light-years is 1 parsec = 3.26 light-years. This is the reason why there is the 3.26 in the parallax formula given at the end of Part E.
The Sun's average surface (photosphere) temperature is about ________
5,800 K
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.
Parts D through F use the parallax formula: d (in light-years)=3.26 × 1p (in arcseconds)d (in light-years)=3.26 × 1p (in arcseconds) Use the parallax formula to complete the following statements correctly.
A star with a parallax angle of 0.1 arcsecond is 32.6 light-years away from us. A star with a parallax angle of 0.01 arcsecond is 326 light-years away from us. A star with a parallax angle of 0.001 arcsecond is 3260 light-years away from us. A star with a parallax angle of 0.0001 arcsecond is 32,600 light-years away from us.
The brightest main-sequence star in a cluster has a temperature of 30,000 K and a luminosity about 10^4 times greater than the sun. What is the approximate age of the cluster?
About 10^7 years
Which of the following statements about open clusters is true?
All stars in the cluster are approximately the same age.
Which of the following statements about brown dwarfs is not true?
Brown dwarfs eventually collapse to become white dwarfs.
Suppose you observe two clusters of stars: Cluster A contains main-sequence stars of nearly all masses. Cluster B contains only low-mass main-sequence stars, along with many giants. What can you conclude about their ages?
Cluster A is younger than Cluster B.
Why do disks form around young stars?
Collisions between rotating gas particles flatten the gas cloud along its axis of rotation.
Which of the following makes it possible for us to observe stellar parallax (from Earth)?
Earth's orbit around the Sun
What is the cause of stellar parallax?
Earth's orbit around the Sun.
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 more drag on satellites in low orbits
Listed following are the different layers of the Sun. Rank these layers based on their distance from the Sun's center, from greatest to least.
Greatest distance corona, chromosphere, photosphere, convection zone, radiation zone, core. least distance.
Which of the following is a valid way of demonstrating parallax for yourself?
Hold up your hand in front of your face, and alternately close your left and right eyes.
You learned about the general concept of parallax in Chapter 2, and the video offers a review of the basic ideas. To check your understanding of the concept, which of the following is a valid way of demonstrating parallax for yourself?
Hold up your hand in front of your face, and alternately close your left and right eyes.
What is the fate of an isolated brown dwarf?
It will remain a brown dwarf forever.
The three bins represent three important properties of stars. Drag the items that we must measure in order to determine each property into the appropriate bin.
LUMINOSITY-distance-apparent brightness SURFACE TEMPERATURE-color or spectral type MASS-orbital period in binary system-orbital distance in binary system
Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category.
Red Giant or Supergiant Starts: Very cool but very luminous Found in the upper right of the H-R diagram Main-sequence stars: The majority of stars in our galaxy The Sun, for example A very hot and very luminous star White dwarfs: not much larger in radius than Earth Very hot but very dim
Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?
The cloud must radiate much of its thermal energy.
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 choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants
Which of the following is not a characteristic of the 11-year sunspot cycle?
The sunspot cycle is very steady, so that each 11-year cycle is nearly identical to every other 11-year cycle.
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 is the common trait of all main-sequence stars?
They generate energy through hydrogen fusion in their core.
At the time the hypothesis was suggested, what major problem was identified with the idea that the Sun was powered by gravitational contraction?
This process would power the Sun for only about 25 million years, but geologists already had evidence the Earth was much older than that.
What is the approximate chemical composition (by mass) with which all stars are born?
Three quarters hydrogen, one quarter helium, no more than 2% heavier elements
You should now see that the reason the mass of the star in Part A must be inferred is that the star has no known orbiting objects, which means we cannot apply Newton's version of Kepler's third law. Which of the following must be true if the star's inferred mass is to be accurate?
We have determined that the star is a main-sequence star; We have measured the star's spectral type
Which of the following must be true if we are to infer (calculate) a star's luminosity directly from the inverse square law for light?
We have measured the star's apparent brightness; We have measured the star's distance; No interstellar gas or dust absorbs or scatters light between us and the star.
It takes ________ for the sun to progress through one sunspot cycle from a maximum area covered with sunspots, to a minimum, and back to a maximum. Refer to the graph above.
about 11 years
Which of the following is the most numerous type of main-sequence star?
an M star
The sketch above 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. Which group represents stars that are extremely bright and emit most of their radiation as ultraviolet light?
c
The sketch above 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. Which group represents the most common type of stars?
d
(STOP STUDYING HERE THIS IS CH.13) The following figures show various stages during the life of a star with the same mass as the Sun. Rank the stages based on when they occur, from first to last.
contracting cloud of gas and dust,protostar,main-sequence G star,red giant,planetary nebula,white dwarf
Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are __________.
cooler and larger in radius
In which of the following layer(s) of the Sun does nuclear fusion occur?
core
Rank the layers of the Sun based on their density, from highest to lowest.
core, radiation zone, convection zone,photosphere, chromosphere, corona.
Rank the following layers of the Sun based on the pressure within them, from highest to lowest.
core, radiation zone, convective zone, photosphere.
Rank the following layers of the Sun based on their temperature, from highest to lowest.
core, radiation zone, convective zone, photosphere.
Which of the following layers of the Sun can be seen with some type of telescope? Consider all forms of light, but do not consider neutrinos or other particles
corona, chromosphere, and photosphere.
The sketch above 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. Which group represents stars that are cool and dim?
d
The sketch above 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. Which group represents stars with the longest main-sequence lifetimes?
d
The sketch above 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. Which group represents stars of the largest radii?
e
