PHYS202 HW Questions Chapter 17-23

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 color star is likely to be the hottest?

blue-violet

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 any of these; there is no way to tell which answer is right by just looking at the star

The most common kinds of stars in the Galaxy have

low luminosity compared to the Sun

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)

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

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

it must be quite large in size

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

because the luminosity tells us how much energy the star emits, while apparent brightness only tells us how bright it happens to look from Earth

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

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)

How do fragile structures like acetaldehyde molecules survive in the harsh environment of interstellar space and avoid being destroyed by high-energy radiation from stars?

such molecules are found only in dense clouds that have a lot of dust; the dust keeps the radiation from hot stars from reaching the molecules

The dust in the dust clouds in interstellar space consists of

tiny solid grains

An astronomer is interested in a galaxy called M31, the nearest galaxy that resembles our Milky Way, about 2 million light-years away. Which technique would be able to give us a distance to this galaxy?

period-luminosity relation for Cepheid variables

How far away would a star with a parallax of 0.2 arcsec be from us?

5 parsecs

Some of the interstellar gas in our Galaxy has been heated to millions of degrees. How do we now think that gas between stars gets that hot?

very powerful shock waves from exploding stars heat the gas they come into contact with

When observing a binary star system, a few spectral absorption lines do not shift at all while others show a Doppler shift. How can we explain the non-shifting lines?

the lines come from interstellar matter between us and the star, not from the stars themselves

Among interstellar clouds, the hotter the cloud, the

lower the density of particles in it

Which property of a star typically requires a knowledge of distance before it can be determined?

its luminosity

Why did it take astronomers until 1838 to measure the parallax of the stars?

because the stars are so far away that their annual shift of position in the sky is too small to see without a good telescope

What is the baseline that earth-bound astronomers use to measure the parallax (the distance) of the nearest stars?

½ the diameter of the Earth’s orbit around the Sun

Planets in the habitable zone of their stars:

are at a temperature where water can exist as a liquid

A star whose temperature is increasing but whose luminosity is roughly constant moves in what direction on the H-R diagram?

to the left

Why was the Kepler mission not able to find planets smaller than Mars?

Such planets make dips in the light of the star that are too small for Kepler to detect

The Orion Nebula is:

a large cloud of gas and dust illuminated by the light of newly formed stars within it

The big surprise about the first planet discovered around another regular star was that it

orbited so close to its star it took only 4 days to go around

What observations about disks of dusty material around young stars suggest that planets may be forming in such disks?

the disks show lanes that are empty of dust within them

The telescope that allowed astronomers to discover most of the planets found with the transit method was called

the Kepler mission

When a star settles down to a stable existence as a main-sequence star, what characteristic determines where on the main sequence in an H-R diagram the star will fall?

its mass

Proxima Centauri is an M-type main sequence star with a planet in its habitable zone. How would its habitable zone differ from our Sun's?

it would be significantly closer to Proxima Centauri than ours is to the Sun

What technique did astronomers use to make the first confirmed discovery of a planet around another star like the Sun?

measure the Doppler shift of the lines in the star's spectrum and look for periodic changes in this shift due to the pull of the planet as it orbits the star

On an H-R diagram of a cluster of stars, which characteristic do astronomers use as a good indicator of the cluster's age?

the point on the main sequence where stars begin to "turn off" -- to move toward the red giant region

How are globular clusters distributed in our Milky Way Galaxy?

mostly in a large spherical halo (or cloud) surrounding the flat disk of the Galaxy

Really massive stars differ from stars with masses like the Sun in that they

can fuse elements beyond carbon and oxygen in their hot central regions

If we look back to the first generation of stars made when the Galaxy was first forming, how do they differ from stars being formed today?

the first generation stars contain little or no elements heavier than helium

Which of the following types of stars will spend the longest time on the main sequence? (Options: G, K, O, A)

K

A type of star cluster that contains mostly very old stars is

a globular star cluster

Why can a star with a mass like our Sun not fuse (produce) further elements beyond carbon and oxygen?

because they just cannot get hot enough for the fusion of heavier nuclei

The event in the life of a star that begins its expansion into a giant is

almost all the hydrogen in its core that was hot enough for fusion has been turned into helium

When a star begins the path toward becoming a red giant, a layer of hydrogen around the core begins to undergo fusion because:

the core is collapsing under its own weight and heating up from the compression; this heats the next layer up

Where are heavier elements like silicon produced in the universe if stars like the Sun cannot make elements heavier than oxygen?

heavier elements are made in the cores of significantly more massive stars than the Sun, which can get hotter in the middle

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

smaller in diameter

A neutron star is as dense as

the nucleus of an atom

When a single star with a mass equal to the Sun dies, it will become a

white dwarf

How do astronomers know that the visible filaments in the Crab Nebula are moving toward us at great speed?

from the Doppler shift in the line radiation from the nebula

Elements heavier than iron can be created during:

a supernova explosion

What allows a nova explosion to happen to a star more than once?

the star that goes nova has a companion star near it, which dumps material onto the first star and continues to do so even after the first nova explosion

How can the presence of white dwarfs in a young star cluster be explained so soon in the cluster's life?

stars lose a lot of mass on their way to becoming white dwarfs; and so the white dwarfs could have started out as quite massive stars

If astronomers want to observe radiation from a dying giant star surrounded by its own expelled dust and gas, which spectral band is best?

infrared

Why have astronomers on Earth not seen a supernova explosion in our Galaxy since 1604 despite them happening every 25-100 years?

the disk of our Galaxy contains a great deal of dust, which tends to block the light of supernova explosions from more distant parts of our Galaxy

The most stable (tightly bound) atomic nucleus in the universe is:

iron