ASTR Homework 9
According to present scientific understanding of the Milky Way's formation, which of the following statements are true? Select al that apply.
-Halo stars formed before disk stars. -The protogalatic cloud(s) contained essentially no elements besides hydrogen and helium.
Click the icon "Spiral arm and star motion" in the interactive figure and watch the animation of the galaxy's rotation over several hundred million years. Which of the following statements accurately describe the motion? Select all that apply.
-Individual stars orbit around the center of the galaxy. -Individual stars move in and out of spiral arms over time.
Consider all of the observations shown in the video. Which of the following are reasonable conclusions? Check all that apply.
-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*.
Which of the following accurately describe some aspect of gravitational waves? Select all the statements that are true.
-The existence of gravitational waves is predicted by Einstein's general theory of relativity. -The first direct detection of gravitational waves came in 2015. -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 are you more likely to find within (or on the edges of) spiral arms of a spiral galaxy than in between these arms? Select all that apply.
-ionization nebulae -young stars -dense, dusty gas clouds -massive stars (spectral types O and B)
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
The disk of the Milky Way Galaxy is about ___________ in diameter and ___________thick
100,000 light-years, 1,000 light-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
Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest.
A globular cluster in the outskirts of the halo, a cloud of gas and dust in the outskirts of the disk, our solar system, the edge of the central bulge
Which of the following best describes a black hole?
A place from which the escape velocity exceeds the speed of light.
Which of the statements below about black holes is not true?
A spaceship passing a few million kilometers from a 10 solar mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar mass main-sequence star.
Imagine a photon of light traveling the different paths in the Milky Way described in the following list. Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest.
Across the diameter of the galactic halo, across the diameter of the galactic disk, from the Sun to the center of the galaxy, across the diameter of the central bulge, through the disk from top to bottom
The expelled gas forms a(n) _____ that is very hot and expands in size.
Bubble
What causes the glow of the reddish areas visible primarily in the spiral arms?
Clouds of gas are being heated by ultraviolet light from nearby, recently formed stars.
Star formation occurs primarily in the galaxy's ________.
Disk
Our galaxy consists of a large, nearly flat ______ with a central _______, all surrounded by a vast _________.
Disk, Bulge, Halo
Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit?
Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit.
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
We find_________of stars primarily in the galaxy's halo
Globular clusters
Based on the association of star formation with spiral arms, we can conclude that the gas in spiral arms __________ than it does in regions between the arms.
Has greater density
Listed following are several stars found in the disk and halo of the Milky Way Galaxy. Assume that both the blue and yellow disk stars are members of the same open cluster. 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. If you expect two (or more) stars to have approximately the same abundance, rank them as equal by dragging one on top of the other(s).
Hot blue main-sequence star in open cluster in disk = Yellow main-sequence star in open cluster in disk, Red giant in globular cluster M13 = Red main-sequence star in globular cluster M13
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.
We refer to the gas and dust that resides in our galaxy as the ___________
Interstellar medium
What do we mean by the event horizon of a black hole?
It is the boundary beyond which light cannot escape.
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.
From Part B, you know that from afar you'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?
Its light will become so redshifted that it will be undetectable.
Further cooling allows the gas to become a(n) ____ , in which star formation occurs.
Molecular cloud
Sort each item into the appropriate bin according to whether it is more common in spiral arms or about equally common within and between the spiral arms in a spiral galaxy's disk.
More Common in Spiral Arms -Ionization nebulae -Dense, dusty gas clouds -Star formation -Young stars Equally Common Within and Between Spiral Arms -Old stars
Will our Sun ever undergo a white dwarf supernova explosion? Why or why not?
No, because it is not orbited by another star.
Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit?
Nothing; Earth's orbit would remain the same.
A ______ occurs when fusion ignites on the surface of a white dwarf
Nova
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 following figures show several stars found in the disk and halo of the Milky Way Galaxy. Rank the stars based on their current age, from oldest to youngest. If two (or more) stars have approximately the same age (that is, ages within a few million years), rank them as equal by dragging one on top of the other(s).
Red giant in globular cluster M13 = Red main-sequence star 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
Watch the video that comes up when you click the icon "Star formation in spiral arms" in the interactive figure. Which of the following best describes what spiral arms are?
Spiral arms are waves of higher density that move outward through a galaxy, triggering star formation as they pass.
Ignoring any radiation, you could in principle survive the journey across the event horizon of a _____________
Supermassive black hole
If we begin from star birth, the next key stage in the star-gas-star cycle occurs when a(n) _______ returns gas and new elements into space.
Supernova
Which of the following correctly describes how light will be affected as it tries to escape from a massive object like a neutron star?
The light will be redshifted.
Imagine what would happen if Jupiter was suddenly replaced by a black hole with the same mass as Jupiter.
The orbits of the other planets in the solar system would be unaffected.
Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova?
The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.
Consider the four features of spiral arms that you identified in Part A. What do they all have in common that explains why they are found together in spiral arms?
They are all associated with star formation.
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.
From the viewpoint of an observer in the orbiting rocket, what happens to time on the other rocket as it falls toward the event horizon of the black hole?
Time runs increasingly slower as the rocket approaches the black hole.
From our location, we cannot see far into the disk with ____ because our view is blocked by ____.
Visible light, dark, dusty gas clouds
Listed following are distinguishing characteristics of different end states of stars. Match these to the appropriate consequence of stellar death.
White Dwarf -Supported by electron degeneracy pressure -Typically about the size (diameter) of Earth -Has a mass no greater than 1.4 MSun -In a binary system, it can explode as a supernova Neutron Star -Sometimes appears as a pulsar -Usually has a very strong magnetic field Black Hole -Size defined by its Schwarzschild radius -Viewed from afar, time stops at its event horizon
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
A multi-dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era) from among the choices given.You are on the surface of an object, and you have a fairly clear view out into space. Unfortunately, you are also very squashed. The light you observe from distant objects is slightly blueshifted. The surface of the object is composed primarily of carbon and oxygen, and the distance to the horizon seems about the same as that on Earth. By observing the sky for a few weeks, you realize that there are several planets orbiting your object. Where are you?
You are on the surface of a white dwarf.
Degeneracy pressure stops the crush of gravity in all the following except
a very massive main-sequence star.
What kind of star is most likely to become a white-dwarf supernova?
a white dwarf star with a red giant binary companion
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
If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would __________.
accelerate as you fall and cross the event horizon completely unhindered
An ____________can 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
If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running __________.
at a constant, normal rate as you approach the event horizon
As the gas cools and merges with other interstellar gas, it forms a(n) ______.
atomic hydrogen cloud
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
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
A neutron star can remain stable in size because of _________
neutron degeneracy pressure
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 __________ is rapidly rotating neutron star
pulsar
Spiral arms are __________.
regions of active star formation
The _______ is the place to which all of black hole's mass is in principle located within the black hole
singularity
As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket __________.
slows down as it approaches the event horizon and never actually crosses the event horizon
Degeneracy pressure arises when ________.
subatomic particles are packed as tightly as the laws of quantum mechanics allow
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
A neutron star is ________.
the core remnant of a star that died in a massive star supernova
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
A white dwarf is ________.
what most stars become when they die
