ASTR101: TEST 3

Suppose that you measure a galaxy's redshift, and from the redshift you determine that its recession velocity is 30,000 (3×104) kilometers per second. According to Hubble's law, approximately how far away is the galaxy?

1.4 billion light-years

If you tried to fly into a ____________ , you would be killed by tidal forces before you crossed the event horizon.

10-solar-mass black hole

By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____.

1150 AU

rank the galaxies based on their distance from earth, from farthest to closest.

130,000 km/s; 45,00 km/s; 18,730 km/s; 5,264 km/s; 1,577 km/s

Rank the galaxies based on the amount of redshift that would be observed in each galaxy's spectrum, from largest to smallest.

130,000 km/s; 45,000 km/s; 18,730 km/s; 5,264 km/s; 1,577 km/s

We could consider any of the stars shown in the figure, so let's consider the star with the highlighted orbit (chosen because its dots are relatively easy to distinguish). What is the approximate orbital period of this star?

20 years

Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?

200 million years

what is the Schwarzschild radius of a 10 solar mass black hole?

30 km

Based on the measurements discussed in part D, the mass of the central black hole is calculated to be about __________ times that of the Sun.

4 million

Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?

4 million solar masses

rank the galaxies based on the speed with which each should be moving away from the earth due to the expansion of the universe, from fastest to slowest.

5 billion light-years; 2 billion light-years; 800 million light-years; 230 million light-years; 70 million light-years

Imagine that when we looked out into the universe we found that the light from all galaxies was blueshifted (rather than redshifted) and that the light from the most distant galaxies was blueshifted by the greatest amount. Which statement best describes what we would conclude about the motions of galaxies in this case?

All are moving toward Earth, with distant galaxies moving faster than nearby galaxies.

Which of the following does not accurately describe what we observe toward the Galactic center?

At optical wavelengths, we see a cluster of old, red stars.

The Interactive Figure (in red) shows a graph of the Cepheid period-luminosity relation. This graph indicates that __________.

Cepheids with longer periods have higher luminosities

How does a black hole form from a massive star?

During a supernova, if the mass of the infalling core has enough gravity to overcome neutron degeneracy pressure, the core will collapse to a black hole.

Which statement concerning black hole masses and Schwarzschild radii is not true?

In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star.

Based on what you have learned, which of the following best describes the meaning of Hubble's constant (H0 )?

It describes the expansion rate of the universe, with higher values meaning more rapid expansion.

Which of the following three-step processes correctly describes how we use Cepheids as a tool to make cosmic distance measurements?

Step 1: Measure the period of the Cepheid's brightness variations. Step 2: Use the period-luminosity relation to determine the Cepheid's luminosity. Step 3: Calculate the Cepheid's distance from its luminosity and apparent brightness.

What actually causes a Cepheid to vary in apparent brightness?

The Cepheid varies in radius, and its luminosity is greater when its radius is larger.

What evidence supports the existence of a very massive black hole at the center of our galaxy?

The motions of the gas and stars at the center indicate that it contains 4 million solar masses within a region no larger than our solar system.

Based on observations, which of the following statements about stars in the Milky Way is generally true?

The older the star, the lower its abundance of heavy elements

By looking at the graph of Hubble's law, what can we say about the galaxies that have the lowest speeds?

They are moving away from Earth and are closer to Earth than galaxies with high speeds.

Imagine that radar had never been invented and that we instead had to rely on a less reliable method of measuring distances in our solar system. If that method led us to underestimate the Earth-Sun distance by 10%, how would it affect other measurements in the distance chain?

They would all be off by the same 10%.

What are the Magellanic Clouds?

Two small galaxies that orbit the Milky Way galaxy.

Why do we use Hubble's law to estimate the distances of most distant galaxies, rather than using white dwarf supernovae in all cases?

We have not observed white dwarf supernovae in most galaxies.

Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true?

We would not be able to measure the distances of distant galaxies

which of the following best describes a black hole?

a place from which the velocity exceeds the speed of light.

rank these objects based on their mass from largest to smallest...

a typical black hole; a typical neutron star; one-solar-mass white dwarf; main-sequence star of spectral type M; Jupiter; the moon

a _____ can form around a white dwarf, neutron star, or black hole in a binary system.

accretion disk

Consider the statement from Part A reading "a 3-solar-mass black hole may be hidden between Jupiter and Saturn." How do we know this statement is not true?

an object of that mass would disrupt the orbits of the planets in our solar system.

From an observational standpoint, what is a pulsar?

an object that emits flashes of light several times per second (or faster), with near perfect regularity.

What is the basic definition of a black hole?

an object with gravity so strong that not even light can escape

distinguishing characteristics of elliptical galaxies

are more reddish in color; contain primarily old, low-mass stars

If we could view our galaxy from a distance of several hundred thousand light-years, it would appear ________.

as a flattened disk with a central bulge and spiral arms.

what causes the radio pulses of a pulsar?

as the neutron star spends, beams of radio radiation sweet through space. if one of the beams crosses the earth, we observe a pulse

if you tried to visit a ________ , you would probably be killed by radiation well before you reached the black hole itself.

black hole in an X-ray binary system.

LIGO detects gravitational waves because the lengths of its arms change as gravitational waves pass by. About how much are these lengths expected to change when LIGO detects gravitational waves from the merger of two neutron stars or two black holes?

by an amount smaller than the diameter of a proton

How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?

by applying Newton's version of Kepler's third law (or the equivalent orbital velocity law) to the Sun's orbit around the center of the Galaxy

Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?

cepheids

which statements correctly summarize key differences between the disk and the halo?

clusters of young stars are found only in the disk; gas and dust are abundant in the disk but not in the halo; stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits; disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red.

which of the following statements about degeneracy pressure is not true?

degeneracy pressure can arise only from interactions among electrons.

Given such small length changes (as noted in Part D), what can give scientists confidence that they have really detected a gravitational wave signal?

detecting the same changes at more than one location

after a massive-star supernova, what is left behind?

either a neutron star or a black hole

a white dwarf can remain stable in size because of __________.

electron degeneracy pressure

which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true?

electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars.

the ________ marks the boundary between the inside and outside of a black hole.

event horizon

the boundary from within which light cannot escape from a black hole is called the black hole's_____.

event horizon

what makes up the interstellar medium?

gas and dust

Consider the statement from Part A reading "the singularity of a black hole has infinite density." Why is this statement in the "unknown" bin?

general relativity and quantum mechanics give different answers about the nature of singularity.

distinguishing characteristics of spiral galaxies

have a flattened disk of stars; are rare in central regions of galaxy clusters; contain many bright, hot stars; contain abundant clouds of cool gas and dust; have significant, ongoing star formation.

To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________.

if they observed for many more years, the dots would trace out ellipses

The Sun's location in the Milky Way Galaxy is ________.

in the galactic disk, slightly over halfway out from the center

All of the images except the first two show radio, infrared, or X-ray light. Why don't these images show visible light?

interstellar dust in the galactic disk prevents us form seeing the galactic center with visible light.

How does the interstellar medium affect our view of most of the galaxy?

it prevents us from seeing most of the visible and ultraviolet light from the galactic disk.

consider a binary system of two neutron stars. how should the emission waves affect this system?

it should cause the orbits of the two objects to decay with time.

what do we mean by the singularity of a black hole?

it's the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.

Which cosmic distance measurement techniques are considered standard candle techniques?

main-sequence fitting; white dwarf supernovae (distant standards); cepheids

rank these objects based on their diameter from largest to smallest...

main-sequence star spectral type A; Jupiter; one-solar-mass white dwarf; the moon; two-solar-mass neutron star; the event horizon of a two-solar-mass black hole

We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first calibrate it by __________.

measuring the distances to many distant galaxies with a standard candle technique

The graph of Hubble's law shows that galaxies with high speeds as measured from Earth are __________.

moving away from Earth and are farther from Earth than galaxies with lower speeds

a neutron star can remain stable in size because of __________.

neutron degeneracy pressure

a 10 solar mass main sequence star will produce which of the following remnants?

neutron star

Now consider its size. Based on what you can see in the diagram, you can conclude that the diameter of the central mass is __________.

no more than about 70 AU

a ________ occurs when fusion ignites on the surface of a white dwarf.

nova

properties and examples of halo stars

oldest stars; stars with the smallest abundance of heavy elements; stars whose orbits can be inclined at any angle; globular clusters

with current technology, we expect to be able to detect (directly) gravitational waves from a binary system of two neutron stars or two black holes __________.

only from the instant when the two object merge into one.

factors that leas to a spiral galaxy forming

protogalactic cloud has high angular momentum; most protogalactic gas settles into a disk.

a _______ is rapidly rotating neutron star.

pulsar

Astronomers are seeking to obtain an image of the region around the black hole's event horizon with a project called the Event Horizon Telescope. What type of light does this project seek to observe?

radio waves

rank the stars based on their current age, from oldest to youngest.

red main-sequence star in globular cluster M13 & red giant in globular cluster M13; the sun; hot, blue main-sequence star in disk.

If you wanted to observe the center of our galaxy, you would need to point a telescope in the direction of the constellation __________.

sagittarius

the ________ is the place to which all of a black hole's mass in principle located within the black hole.

singularity

rank these objects based on their density from highest to lowest

singularity of a black hole; typical neutron star; one-solar-mass white dwarf; main-sequence star.

Where is most of the dark matter in our galaxy thought to exist?

spread throughout the halo of the galaxy

factors that lead to an elliptical galaxy forming

stars form rapidly as the protogalactic cloud shrinks; protogalactic cloud has high density; a galaxy collision strips away gas; protogalactic cloud rotates very slowly.

Consider all of the observations shown in the video. Which of the following are reasonable conclusions?

stars near the galactic center are much closer together than stars around our sun; there are strong magnetic fields in the central region of the galaxy; gas orbits the radio source called Sgr A*

degeneracy pressure arises when ________.

subatomic particles are packed as tightly as the laws of quantum mechanics allow

ignoring any radiation, you could in principle survive the journey across the event horizon of a _______.

supermassive black hole

Which of the following observatories is most likely to discover a black hole in a binary system?

the Chandra x-ray observatory

Radar, the first link in the cosmic distance chain, is used to establish the baseline distance necessary for the second link, parallax. What baseline distance must we know before we can measure parallax?

the Earth-Sun distance

how does an accretion disk around a neutron star differ from an accretion disk around a white dwarf?

the accretion disk around a neutron star is much hotter and emits higher-energy radiation.

what caused the rapid spin of a neutron star that we see as a pulsar?

the conservation of angular momentum during the collapse of the original star's core.

Hubble's law expresses a relationship between __________.

the distance of a galaxy and the speed at which it is moving away from us

Keeping in mind that all the stellar positions were measured at approximately one-year intervals, which stars are moving the fastest in their orbits during the time period indicated by the dots?

the fastest stars are the ones with the dots farthest apart

which of the following accurately describe some aspects of gravitational waves?

the first direct detection of gravitational waves came in 2015; the existence of gravitational waves is predicted by Einstein's general theory of relativity; gravitational waves carry energy away from their sources of emission; gravitational waves are predicted to travel through space at the speed of light.

which of the following statements about gravitational waves are true?

the first direct detection of gravitational waves, announced in 2016, came from the LIGO observatory; two orbiting neutron stars or black holes will gradually spiral toward each other as a result of energy being carried away by gravitational waves; the emission of gravitational waves from merging black holes is predicted by Einstein's general theory of relativity.

In a photo like the Hubble eXtreme Deep Field (the figure in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?

the galaxies that are farthest away.

Gravitational waves were first detected directly in 2015. According to models, the source of these gravitational waves was __________.

the merger of two black holes

How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?

the stars that formed first could orbit the center of the galaxy in any direction at any inclination.

What characteristics of the orbiting stars do we need to measure in order to calculate the mass of the central object, Sgr A*?

their orbital periods and average orbital distances

Why do these facts lead astronomers to conclude that the central object is a black hole?

there is no known way to pack so much mass into such a small volume without it collapsing into a black hole.

suppose you drop a clock toward a black hole. as you look at the clock from a high orbit, what will you notice?

time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

Suppose that galaxy B is twice as far from Earth as galaxy A. Hubble's law predicts that galaxy B will be moving away from Earth with approximately _____.

twice the velocity of galaxy A

The first gravitational waves were detected in 2015 by the LIGO observatories in Washington and Louisiana. What event was thought to cause these gravitational waves?

two black holes merging

most stars in the milky way's halo are _______.

very old

Consider the statement from Part A reading "black holes make up 1% of the mass of the Milky Way Galaxy." Why is this statement in the "unknown" bin?

we cannot detect all black holes and therefore don't know the percentage of the galaxy's mass they make up.

Suppose you were unfortunate enough to fall into a black hole in a binary system where the black hole was accreting matter from its companion star. Which of the following is most likely to kill you first?

x-rays from the accretion disk

rank the stars based on the abundance of elements heavier than carbon that you would expect to find in each of the stars, from highest to lowest.

yellow main-sequence star in open cluster in disk & hot, blue main-sequence star in open cluster in disk; red giant in globular cluster M13 & red main-sequence star in globular cluster M13.

Suppose that future observations showed that the slope of Hubble's law on the graph is actually steeper than that shown. In that case, the age of the universe would be _________ than 14 billion years because the universe is expanding ______ than current data suggest.

younger // more rapidly

properties and examples of disk stars

youngest stars; the sun; stars that all orbit in nearly the same plane; high-mass stars