Astronomy Chapter 15, 16, 17, 18 and 19 Homework Questions

We found that mass must be inferred for the star described in Part A. However, we can measure a star's mass directly if __________. *we know its spectral type *it is unusually high in mass *it is a member of an eclipsing binary system *it is near enough for us to measure its distance with parallax

*Its a member of an eclipsing binary system.

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 Directly: Color, parallax angle, spectral type, apparent brightness. *Infer Indirectly: Surface temperature, luminosity, mass, radius.

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 Stars: Found in the upper right of the H-R diagram, Very cool but very luminous. *Main-Sequence Stars: the sun, for example, the majority of stars in our galaxy, the hottest and most luminous stars. *White Dwarfs: Not much larger in radius that Earth, very hto but very dim.

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. *The star must be a member of a binary system. *We have measured the star's spectral type. *No interstellar gas or dust absorbs or scatters light between us and the star. *We have measured the star's distance.

*We have measured the star's apparent brightness. *No interstellar gas or dust absorbs or scatters light between us and the star. *We have measured the stars's distance.

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? *The star must be located within the Milky Way Galaxy and not in another galaxy. *We have measured the star's spectral type. *We have determined that the star is a main-sequence star. *We have measured the star's velocity.

*We have measured the star's spectral type. *We have determined that the star is a main sequence star.

Approximately what is the parallax angle of a star that is 20 light-years away?

0.16 arcsecond

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

20 parsecs away

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

3 million years

Suppose that a star had a parallax angle of excactly 1 arcsecond. Approximately how far away would it be in light-years?

3.3 light years

Ten parsecs is about ____

32.6 light-years

Study this H-R diagram. What is the spectral type of the star Sirius?

A

Which of these stars has the coolest surface temperature?

A "K" star

Which of these star clusters is the oldest?

A cluster whose brightest main-sequence stars are yellow.

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

Which of these stars is the most massive?

A main-sequence "A"star

Which of these stars has the longest lifetime?

A main-sequence M star

Which of these stars has the largest radius?

A super giant "M" star.

Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s)

All of them are stacked on top of each other.

Five stars are shown on the following H-R diagrams. Rank the stars based on their luminosity from highest to lowest; notice that these are the same five stars shown in Part D. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).

All stacked on top of each other.

Study this H-R diagram. Which of the following stars is the largest in size (radius)?

Antares

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

Apparent brightness and distance.

Study this H-R diagram. Which of the following stars has (or had) the longest hydrogen burning lifetime?

Barnard's star

Study this H-R diagram. Which of the following stars is the most massive?

Beta Centauri

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, 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.

Listed following are several fictitious stars with their luminosities given in terms of the Sun's luminosity (LSun) and their distances from Earth given in light-years (ly). Rank the stars based on how bright each would appear in the sky as seen from Earth, from brightest to dimmest. If two (or more) stars have the same brightness in the sky, show this equality by dragging one star on top of the other(s).

Brightest to Dimmest: Nismo, Ferdinand and Shelby are the same, Enzo, Lotus

How did astronomers discover the relationship between spectral type and mass for a 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.

Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are __________.

Cooler and larger in radius.

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

Earth would be inside the supergiant.

What is the cause of stellar parallax?

Earth's orbit around the sun.

The following figure shows how four identical stars appear in the night sky seen from Earth. The shading is used to indicate how bright (white) or dim (dark gray) the star would appear in the sky from Earth. Rank the stars based on their distance from Earth, from farthest to closest.

Farthest to Closest: Darkest to lightest.

The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on their rotation rate, from fastest to slowest. (Assume that the angular momentum of the forming star is conserved throughout the formation process, though in fact it may shed some angular momentum by ejecting material into interstellar space.)

Fastest Rotation to Slowest Rotation: Main-sequence, protostar with jets, contracting cloud trapping infrared light, molecular-cloud fragment.

The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the order in which they occur, from first to last.

First to Occur to Last to Occur: Molecular-Cloud, fragment, Contracting cloud trapping infrared light, Protostar with jets, main-sequence star.

Part B: Use the labels to identify what kinds of stars inhabit each region of the H-R diagram.

From bottom to top: White Dwarfs, main sequence, Red giants, main sequence, super giants.

Consider the four stars shown following. Rank the stars based on their surface temperature from highest to lowest.

From highest to lowest: A blue white dwarf star, sun, an orange main-sequence star, a red super giant star.

Our sun is of spectral type ____

G

If the star Alpha Centauri were moved to a distance 10 times farther than it is now, its parallax angle would

Get smaller.

When a newly forming star is at its greatest luminosity, what is its energy source?

Gravitational contraction.

The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the central temperature, from highest to lowest.

Highest Temperature to Lowest Temperature: Main- sequence, protostar with jets, contracting cloud trapping infrared light, molecular-cloud fragment.

Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).

Highest to Lowest: Two high stars stacked on top of each other, Middle Dot by itself, Two low dots stacked on top of each other.

Five stars are shown on the following H-R diagrams; notice that these are the same five stars shown in Part F. Rank the stars based on their luminosity from highest to lowest. If two (or more) stars have the same luminosity, drag one star on top of the other(s)

Highest to Lowest: Two right dots stacked on top of each other, Middle dot in the middle, Two low dots stacked on top of each other.

Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).

Highest to Lowest: Dots that are furthest to the left to the dots that are furthest to the right.

Five stars are shown on the following H-R diagrams; notice that these are the same five stars shown in Part B. Ranks the stars based on their luminosity form the highest lowest. If two (or more) stars have the same luminosity, drag one star on top of the other(s).

Highest to Lowest: Highest dot to the lowest dot.

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.

Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are _____

Hotter and smaller in radius.

As a clump of interstellar gas contracts to become a main-sequence star, its changing position on the H-R diagram tells us __________.

How its outward appearance is changing.

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 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.

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

It mass and its stage of life.

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

What can we infer, at least roughly, from a stars luminosity class?

Its size (radius)

Watch the red dot representing the protostar in the animation. After it reaches its highest point on the diagram, how do the protostar's surface temperature and luminosity change as it approaches the main sequence?

Its surface temperature increases, but it luminosity decreases.

Compute the ratio between the luminosity from part B to the mass from part A. How does that ratio compare with the Sun's ratio of luminosity to mass?

L/M=1000: Lsun/Msun

What is the luminosity of the most luminous stars in the cluster?

L=1000Lsun

If star A is closer to us that 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 H-R diagram represent _____

Luminosity and surface temperature.

What is the approximate mass of the most massive stars left on the main sequence of this star cluster?

M=1Msun

How is the lifetime of a star related to its mass

More massive stars live much shorter lives than less massive stars.

Listed following is the same set of fictitious stars given in Part A. Rank the stars based on how bright each would appear in the sky as seen from Jupiter, from brightest to dimmest.

Nismo, Shelby and Ferdinand are the same, Enzo, Lotus.

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

OBAFGKM

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.

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

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.

The more distant the star, the______

Smaller its 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? *Apparent Brightness *Parallax Angle *Spectral Type *Mass

Spectral Type

Each choice below lists a spectral type and luminosity class for a star. Which one is a Red Supergiant?

Spectral type M2 luminosity class I

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

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

Star a

A star's color is related to it surface temperature because ____

Stars emit thermal radiation.

All the stars in this photo are at about the same distance from Earth (some 26,000 light-years away). Which stars in this picture are the largest in size (radius)?

The Bright Red stars

Before we can use parallax to measure the distance to a nearby star, we first need to know __________.

The Earth-Sun distance

The choice below each describe the appearance of an H-R diagram for a different star cluster. Which cluster ins the youngest?

The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.

The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?

The diagram shows main-sequence stars of spectral types G, K and M along with numerous giants and white dwarfs.

What is a white dwarf?

The remains of a star that ran out of fuel for nuclear fusion.

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 system.

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

The star is a member of an eclipsing binary star system.

What two pieces of information would you need in order to measure the masses of stars in an eclipsing binary system?

The time between eclipses and the average distance between the stars.

What the common trait of all main sequence stars?

They generate energy through hydrogen fusion in their core.

Based on the definition of apparent brightness, which units are appropriate for its measurement?

Watts per square meter

When does a newly forming star have the greatest luminosity?

When it is a shrinking protostar with no internal fusion.

Part A: The figure shows a standard Hertzsprung-Russell (H-R) diagram. label the horizontal and vertical axes using the two blanks nearest the center of each axis, and label the extremes on the two axes using the blanks on the ends of the axes

Y-Axis going up: fainter, luminosity X-Axis going from left to right: Hotter, Surface, Colder.

Which of these stars has the greatest surface temperature?

a 30Msun main-sequence star

The sketch below 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?

a: the rod looking thing

The sketch below 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?

b: the bottom left empty oval

The sketch below 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?

c: the left end of the rod looking thing.

On an H-R diagram stellar masses_____

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

The sketch below 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: The right end of the rod looking thing.

The sketch below 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 right end of the rod looking thing

The sketch below 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 right end of the rod looking thing.

The sketch below 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: the top right empty oval.

On an H-R diagram, stellar radii ____

increase diagonally from the lower left to the upper right

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.

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

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

orbital period and average orbital distance.

Estimate the maximum amount of time these very luminous stars can last as red giants from your answer to part C.

t=10^7 years

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

three quarters hydrogen, one quarter helium, no more than about 2% heavier elements