Stars

Some objects in space just don't have what it takes to be a star. Which of the following is a "failed star", an object with too little mass to qualify as a star?

A brown dwarf

Astronomers arrange the stars into groups called spectral classes (or types) according to the kinds of lines they find in their spectra. These spectral classes are arranged in order of:

Decreasing surface temperature

Who was the astronomer who is the "H" in H-R diagram?

Hertzsprung

The first astronomer who did photometry in a systematic way (even though he did not have a telescope) was

Hipparchus

Studies of the spectra of stars have revealed that the element that makes up the majority of the stars (75% by mass) is

Hydrogen

Which law do astronomers use to determine the masses of the stars in a spectroscopic binary system?

Keplers 3rd law

After a lot of work, a group of graduate students has finally measured the wavelengths of many dozens of lines in the spectrum of a distant star. If a number of the lines come from molecules such as titanium oxide, the star is likely to be which spectral type:

M

Which of the following types of star is the coolest (has the lowest surface temperature)?

M

Two stars have the same luminosity, but star B is three times farther away from us than star A. Compared to star A, star B will look

Nine times fainter

Ninety percent of all stars (if plotted on an H-R diagram) would fall into a region astronomers call:

The main sequence

Which of the following has the smallest mass?

a planet

In recent decades, astronomers discovered stars even cooler than the traditional spectral type M stars recently. Astronomers gave these cool stars a new spectral type, L. If you wanted to go out and find more such type L stars, what kind of instrument would it be smart to use?

a sensitive infra-red telescope

A star moving toward the Sun will show:

a shift in the spectral lines toward the blue end (as compared to the laboratory positions of these lines)

Which of the following looks the brightest in the sky?

a star with magnitude -1

A star that is quite hot and has a very small radius compared to most stars is called

a white dwarf

In an H-R diagram, where can you see the spectral type of a star (whether it is an O type star or a G type star, for example)?

along the bottom (the horizontal axis)

One of your good friends asks you to point out the stars with the smallest mass on an H-R diagram that you are studying. Where are you sure to find the stars with the lowest mass on any H-R diagram?

among the stars at the bottom right of the main sequence

Which of the following statements about spectroscopic binary stars is FALSE?

an analysis of the ways the lines in the spectrum change allows us to calculate the star's distance directly

A team of astronomers takes spectra of thousands of different stars in different parts of the sky. The spectra show significant differences. The main reason the spectra of the stars do not all look alike is that the stars

are made of significantly different elements

Imagine that a brilliant but quirky scientist in the biology department manages to put you in a deep freeze and you wake up in a million years. Which of the following statements about the sky you would see in that future time is correct?

because of proper motion, a number of the familiar constellations will look somewhat different in a million years

Some "superstars" give off more than 50,000 times the energy of the Sun. Why are there no such stars among the stars that are close to the Sun?

because such very luminous stars are extremely rare, and thus any small neighborhood in the Galaxy is unlikely to contain one of them

Why are astronomers much more interested in the luminosity of a star than its apparent brightness?

because the luminosity tells us how bright a star really is, while apparent brightness only tells us how bright it happens to look from Earth

Two stars that are physically associated (move together through space) are called

binary stars

Which color star is likely to be the hottest?

blue-violet

Stars that do not have what it takes to succeed as a star (i.e. do not have enough mass to fuse hydrogen into helium at their centers) are called:

brown dwarfs

One key difference that astronomers use to distinguish between brown dwarfs and high-mass planets is that:

brown dwarfs are able to do deuterium fusion in their cores, while planets can't

At an astronomical conference, an astronomer gives a report on a star that interests astronomers because of hints that it may have a planet around it. In his report the astronomer gives the average speed with which this star is moving away from the Sun. How did the astronomer measure this speed?

by looking at the Doppler shift in the lines of the star's spectrum

For what type of star can astronomers measure the diameter with relative ease? (Pick best answer)

eclipsing binary stars

Astronomers identify the main sequence on the H-R diagram with what activity in the course of a star's life?

fusing hydrogen into helium in their cores

When an astronomer rambles on and on about the luminosity of a star she is studying, she is talking about:

how much energy the star gives off each second

Imagine that powerful telescopes in the future give us a truly representative sampling of all the stars in the Sun's cosmic neighborhood. Where on the H-R diagram would most of the stars in our immediate vicinity lie?

in the lower right, among the least luminous main sequence stars

Using a good pair of binoculars, you observe a section of the sky where there are stars of many different apparent brightnesses. You find one star that appears especially dim. This star looks dim because it is:

it could be more than one of the above; there is no way to tell which answer is right by just looking at the star

Measurements show a certain star has a very high luminosity (100,000 x the Sun's) while its temperature is quite cool (3500o K). How can this be?

it must be quite large in size

Which of the following characteristics of a single star (one that moves through space alone) is it difficult to measure directly?

its mass

The most common kinds of stars in the Galaxy have

low luminosity compared to the Sun

Stars on the main sequence obey a mass-luminosity relation. According to this relation,

luminosity is proportional to mass to the fourth power (luminosity increases strongly with mass)

Stars that lie in different places on the main sequence of the H-R diagram differ from each other mainly by having different:

masses

An exhausted-looking astronomer comes off the mountain where her observatory is located and tells you she has been doing photometry all night. What has she been up to?

measuring the brightness of different stars

Which of the following is a method for measuring the diameter of a star?

more than one of the above

Where on the H-R Diagram would we find stars that look red when seen through a telescope?

only on the right side of the diagram and never on the left

Astronomers call the motion of a star across the sky (perpendicular to our line of sight) its

proper motion

A graduate student has done a careful analysis of the spectrum of a star. While she has found lines from many elements, there was not a trace of the element helium in the spectra she has been analyzing. From this she can now conclude:

since helium shows lines only in hot stars, this star must be relatively cool

A white dwarf, compared to a main sequence star with the same mass, would always be:

smaller in diameter

Two stars have the exact same luminosity, but star Y is four times dimmer looking that star X. This means that

star Y is twice as far away as star X

Why can astronomers not measure the diameters of stars directly?

stars are so far away, we cannot resolve (distinguish) their diameters

An astronomer whose secret hobby is riding merry-go-rounds has dedicated his career to finding the stars that rotate the most rapidly. But the stars are all very far away, so none of them can be seen to spin even when he looks through the largest telescopes. How then can he identify the stars that rotate rapidly?

stars that rotate have much wider lines in their spectra than stars that do not

An H-R Diagram plots the luminosity of stars against their:

surface temperature

I am measuring the spectrum of the stars in a spectroscopic binary system. When one of the stars is moving toward the Earth in its orbit, we observe

that the lines in its spectrum show a blue-shift

When an astronomer measures a color index for a star, what is she measuring? (be careful here, this question is tricky)

the difference between how bright a star looks at two different wavelength regions

Most of the really bright stars in our sky are NOT among the stars that are very close to us. Why then do they look so bright to us?

these stars are intrinsically so luminous, that they can easily be seen even across great distances

Which of the following can astronomers NOT learn from studying the spectrum of a star?

you can't fool me, all of the above can be learned from studying the spectrum