Astroworld Final Takeoff
Which of the following statements correctly summarize key differences between the disk and the halo?
- Disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red. - Stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits. - Gas and dust are abundant in the disk but not in the halo. - Clusters of young stars are found only in the disk.
Consider all of the observations shown in the video. Which of the following are reasonable conclusions?
- Gas orbits the radio source called Sgr A*. - There are strong magnetic fields in the central region of the galaxy. - Stars near the galactic center are much closer together than stars around our Sun.
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. - The emission of gravitational waves from merging black holes is predicted by Einstein's general theory of relativity. - Two orbiting neutron stars or black holes will gradually spiral toward each other as a result of energy being carried away by gravitational waves.
Halo Stars
- globular clusters - oldest stars - stars whose orbits can be inclined at any angle - stars with the smallest abundance of heavy elements
Spiral Galaxy
- most protogalactic gas settles into a disk - protogalactic cloud has high angular momentum
Elliptical Galaxy
- stars form rapidly as the protogalactic cloud shrinks - protogalactic cloud has high density - a galaxy collision strips away gas - protogalactic cloud rotates very slowly
Which of the following accurately describe some aspect of gravitational waves?
- the first direct detection of gravitational waves came in 2015 - gravitational waves carry energy away form their sources of emission - gravitational waves are predicted to travel through space at the speed of light - the existence of gravitational waves is predicted by Einstein's general theory of relativity
Which cosmic distance measurement techniques are considered standard candle techniques?
- white dwarf supernovae (distant standards) - Cepheids - main-sequence fitting
disk stars
- youngest stars - the sun - stars that all orbit in nearly the same plane - high-mass stars
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
A Cepheid with a period of 30 days has an average luminosity that is about __________ times the luminosity of the Sun. (You will have to click on the blue Cepheids link on the main screen of the Interactive Figure to get back to the relevant plot.)
10,000
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
Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately
100 billion
Look again at the orbit of the star with the highlighted orbit. 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
Click on the blue Cepheid label, then click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Then click the Next button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. Notice that the window with the light curve also has a box that tells you the apparent brightness of the Cepheid, which shows a value of 1.3×104. (The units are shown in the box, but you only need to focus on the numerical value.) Based on this apparent brightness and the luminosity you found in Part D, what is the approximate distance to this Cepheid?
2.5 million light-years
To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. 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 yr
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
Part complete 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
The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (M) of a central object, in solar masses, from an orbiting object's period (p) in years and semimajor axis (a) in astronomical units: M=a3p2 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
Click on the blue Cepheid label, then click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Then click the Next button in the Interactive Figure to bring up the screen that shows a Cepheid light curve in the upper left. What is the approximate luminosity of the Cepheid whose light curve is shown in the graph?
9000 LSun
Which of the following best describes a black hole?
A place from which the escape velocity exceeds the speed of light.
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.
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.
What causes the radio pulses of a pulsar?
As the neutron star spins, beams of radio radiation sweep through space. If one of the beams crosses the Earth, we observe a pulse
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.
Which technique is the most useful for measuring the distance to a galaxy located 10 million light-years away?
Cepheids
To get started, click on the blue Cepheids link on the main screen of the Interactive Figure. 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
Which of the following statements about degeneracy pressure is not true?
Degeneracy pressure can arise only from interactions among electrons.
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 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.
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.
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.
Consider the portion of the video that starts with the all-sky view of the Milky Way and then zooms in to the galactic 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 from seeing the galactic center with visible light.
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.
What do we mean by the singularity of a black hole?
It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.
Part complete 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 of gravitational waves affect this system?
It should cause the orbits of the two objects to decay with time.
If you wanted to observe the center of our galaxy, you would need to point a telescope in the direction of the constellation __________.
Sagittarius
Which of the following three-step processes correctly describes how we use Cepheids as a tool to make cosmic distance measurements? In all cases, assume that the Cepheid's apparent brightness has been carefully measured through observations.
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.
Click on the blue Cepheid label, then click the label on the bottom of the Interactive Figure that reads Cepheids as Standard Candles to bring up the next screen. Read the instructions that appear in the upper left hand corner and study the animation. 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.
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 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.
Part complete 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.
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.
Why do spiral galaxies appear blue in color?
Their light output is dominated by hot, massive blue stars.
You've now found that the central object has a mass of about 4 million solar masses but is no more than about 70 AU in diameter—which means it cannot be much larger than the size of our planetary system. 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.
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%.
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 we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
We can learn the distance to the galaxy.
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
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.
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
This figure shows how the luminosity of supernovae change over time. How long does it take a white dwarf supernova to decrease in luminosity by a factor of 100 from its peak?
about 200 days
A (n) ______ can form around white form around a white dwarf, neutron star, or black hole in a binary system
accretion disk
From an observational standpoint, what is a pulsar?
an object that emits flashes of light several times per second (or even 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
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
if you tried to a visit ________, 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
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
The most massive galaxies in the universe are
elliptical
The most basic difference between elliptical galaxies and spiral galaxies is that ________.
elliptical galaxies lack anything resembling the disk of a spiral galaxy
Which of the following types of galaxies appear reddest in color?
ellipticals
The boundary from within which light cannot escape from a black hole is called the black hole's __________.
event horizon
the ____ marks the boundary between the inside and outside of a black hole
event horizon
What makes up the interstellar medium?
gas and dust
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
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
From Part E you know the mass of the central object. 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 (n) _____ occurs when fusion ignites on the surface of a white dwarf
nova
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 objects merge into one
A (n) ____ is rapidly rotating neutron star
pulsar
Part complete 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
the ______ is the place to which all of a black hole's mass is in principle located within the black hole.
singularity
Where is most of the dark matter in our galaxy thought to exist?
spread throughout the halo of the galaxy
Degenearcy 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
Part complete 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
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
Notice that some of the stars on the diagram are represented by a series of dots that are very close together, while others have their dots farther apart. 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 dots farthest apart
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
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
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
What are the Magellanic Clouds?
two small galaxies that orbit the Milky Way Galaxy
Most stars in the Milky Way's halo are _______
very old
The age of the universe is related to the slope of the graph of Hubble's law, and current data put the age of the universe at about 14 billion years. 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. (Each choice gives words to fill in the two blanks, separated by a slash.)
younger / more rapidly
