quiz 6

30. Galaxy B1 is four times more distant than Galaxy B2. According to the Hubble law, Galaxy B1 is ___ as Galaxy B2.

receding four times as fast

Elliptical galaxies are classified as E0, E1, E2, and so on. The main difference between an E1 and an E7 is the ___ of the galaxies.

shape

For a black hole, the location where all the mass is thought to be concentrated is called the:

singularity

Which stage has carbon fusion in the core? a. Supergiant c. Main Sequence Star e. Red Giant b. Horizontal Branch Star d. Protostar

supergiant

Of the following objects, on which one would time on its surface run most slowly?

the Schwarzschild radius of a black hole

One of the important predictions of general relativity is:

the bending of light by gravity

Which part of the galaxy contains mostly older Population I stars?a. the disk b. the nucleus c. the halo d. the central bulge e. the spiral arms

the disk

Globular clusters lie primarily within which part of the Galaxy?a. the disk b. the nucleus c. the halo d. the central bulge e. the spiral arms

the halo

Which part of the galaxy contains a possible supermassive black hole?a. the disk b. the nucleus c. the halo d. the central bulge e. the spiral arms

the nucleus

We measure the mass of the black hole at the galactic center from:

the orbits of stars in the galactic center.

Within which region of the Galaxy does the solar system reside?a. the disk b. the nucleus c. the halo d. the central bulge e. the spiral arms

the spiral arms

The tremendous explosion at the end of a high mass star's life is the result of:

the sudden collapse of the core as fusion ends

29. Recent observation of the clustering of galaxies show that:

there are gigantic regions in space with little or no galaxies

he first indication of the true location of the center of the Milky Way galaxy came when H. Shapley observed:

e the distribution of globular clusters

Which of these objects has the largest radius? a 1.2MSunwhite dwarfa 1.5MSun neutron star a 3.0MSun black hole

1.2MSunwhite dwarf

The universe is approximately ___ years old and today has an overall temperature of ___.

10-20 billion, 3K

What is a black hole? What do we mean by the event horizon of a black hole?

A black hole is a bottomless pit in spacetime. Nothing can escape the black hole once it is within the event horizon, the point of no return around the black hole.

Why is a black hole called a "black hole"? Why is the event horizon called an "event horizon"? In other words, what is the meaning of their names?

A black hole is called "black" because the mass singularity at its center curves space time so intensely that not even light can escape its gravity when it gets closer to the singularity than a certain distance known as the Schwarzschild radius. Because light cannot escape the black hole and bring with it information about what is looks like, we call the hole in space and time "black". We envision it as a hole because of the strong curvature of space time the closer we come to the mass singularity. The Schwarzschild radius is also the size of the "event horizon". Since light cannot escape the black hole once it has entered the event horizon it is not able to bring back information to us from the inside. In that sense, the events occurring within the black hole are causally disconnected from the events outside.

In what sense is a black hole like a hole in the observable universe? Define the event horizon andSchwarzschild radius, and describe the three basic properties of a black hole.

A black hole is like a hole in the observable universe because it represents a part of the universe we can never observe and from which we could never return if we went in. The event horizon is the boundary between the inside of the black hole and the outside universe. The radius of the event horizon is called the "Schwarzschild radius". Black holes have only three measurable properties: mass, spin, and charge.

Describe the light house model for neutron stars.

A light house projects a powerful beam of light in one direction. The light source is rotating at a regular rate so that the beam sweeps across the land and sea once for every rotation. An observer will see a regular pulse of light whenever the light house beam sweeps past him/her. This behavior is analogous to that of a pulsar. A pulsar is a neutron star. A neutron star has a strong magnetic field and rotates very fast. Near its magnetic poles are hot spots due to high energy electrically charged particle that are trapped by the powerful magnetic field. These particles emit radiation. If the magnetic axis is not aligned with the rotational axis of the neutron star, the beam of radiation will sweep through space just like the beam of a light house. If Earth happens to be in the path of this sweep, we will see the neutron star as a pulsar.

What do we mean by the singularity of a black hole? How do we know that our current theories are inadequate to explain what happens at the singularity?

A singularity is a mathematical concept where the math becomes undefined (like division by zero, or infinity). In this chapter, a singularity is the point where all of the mass of a black hole is crushed into a point. Unfortunately, general relativity (which describes gravity) disagrees with quantum mechanics (which describes the physics of the very small) at this extreme limit of tiny sizes but the F_g = "infinity" at the center of a black hole, so our current theories are inadequate to describe what happens precisely at the singularity.

Suppose you discover a star made purely of hydrogen and helium. (a) How old do you think it would be? Explain your reasoning.

A star made of only helium and hydrogen would have to be among the first generation of stars ever born, arising out of the primordial mix of elements that came from the Big Bang. The oldest stars we know of are about 13 billion years old-a star made of only helium and hydrogen would have to be at least this old. (No such star has ever been discovered.)

How would the curvature of spacetime change in the region of Earth's orbit?

Because the mass of the black hole is the same as the mass of the Sun, there would be no change to the curvature of spacetime at distances far from the object, such as at the orbit of Earth.

c) What effect would the change have on Earth's orbit? Explain.

Because there is no change to the curvature of spacetime at Earth's orbit, Earth's orbit would not be affected.

What would happen if the Sun suddenly became a black hole without changing its mass?

Earth would remain in the same orbit.

What are gamma-ray bursts, and how do we think they are produced?

Gamma ray bursts are brief but incredibly energetic outbursts of radiation (primarily in the gamma rays) coming from locations well outside our own galaxy. We hypothesize that gamma-ray bursts may be caused by a supernova that forms a black hole, releasing many times more gravitational potential energy than the kind of supernova that forms a neutron star. It is also possible that some gamma-ray bursts are caused by neutron stars merging with each other.

What is gravitational time dilation? What determines how much time is slowed in a gravitational field?

Gravitational time dilation refers to the slowing of time due to gravity. The amount that time is slowed depends on the strength of the gravitational field: a stronger field results in more slowing.

2. How can we use orbital properties to learn about the mass of the galaxy? What have we learned?

If we know a star's orbital period and orbital radius, we can use Newton's version of Kepler's third law to determine the mass of the galaxy, with a minor warning: We get only the mass of the galaxy within the orbit of the star we examine. What we have learned from our studies to find the distribution of mass in the galaxy is that the stars in the galactic disk orbit at about the same speeds. This observation tells us that most of the galaxy's mass resides far from the center and is distributed throughout the halo.

Viewed from a distance, how would a flashing red light appear as it fell into a black hole?

Its flashes would shift to the infrared part of the spectrum.

Which stage has no fusion in the core?a. Supergiant c. Main Sequence Star e. Red Giant b. Horizontal Branch Star d. Protostar

Protostar

What is Sgr A*? What evidence suggests that it contains a massive black hole?

Sgr A* is the name of the radio source we find at the center of our galaxy. The motions of the stars in this region indicate that it contains a few million solar masses of matter within a small space. Observations show that there are not nearly enough stars to account for all of the mass, so we suspect that it contains a supermassive black hole.

Suppose the Sun were magically replaced by a black hole of precisely the same mass. A) How would the curvature of spacetime change in the region where the Sun used to be located?

Spacetime would be much more highly curved in the region that used to be within the Sun, with the curvature becoming more extreme near the center.

What triggers star formation within a spiral arm? How do we think spiral arms are maintained?

Stars are created more readily in spiral arms because collisions between gas clouds compress the gas in the clouds, increasing the strength of gravity and triggering star formation.

1. Describe the basic characteristics of stellar orbits in the bulge, disk, and halo of our galaxy.

Stars in the disk of the galaxy have nearly circular orbits that are mostly in the plane of the galactic disk. The disk stars have vertical motions out of the plane, making them appear to bob up and down, but they never get "too far" from the disk. Orbits of stars in the bulge and the halo of the galaxy are much less orderly, traveling around the galactic center on elliptical orbits with more or less random orientations.

Explain what the term "Onion-ring structure" means with respect to high mass super giants.

The core of a high mass super giant will undergo fusion of successively heavier elements at increasingly higher temperatures. Whenever one type of fusion in the core ends, the core will contract and heat up to the point where the next higher fusion will start. Because of the higher temperature in the core, the temperature in a thin shell around the core will increase to the point where the fusion of that element begins that the core has just run out of. This causes the next outer thin shell to heat up to the point where fusion of the lighter element will occur. The result is a structure where the core fuses the heaviest element, and successively further shells fuse successively lighter elements. This structure of nested shells looks very similar to that of an onion.

What characteristics distinguish disk population stars from spheroidal population stars?

The disk population of stars contains both young and old stars, all of which are made up of about 2% heavy elements. The spherical population stars are always old and have low masses. The stars in the halo are also lower in heavy elements than the disk population of stars.

What is special about the element iron? Where did all the elements smaller than iron come from? Where did all the elements heavier than iron come from?

The iron nucleus has the highest binding energy. That means it its protons and neutrons are most tightly bound to each other and are hardest to break apart. That means that elements lighter than iron can be fused into heavier elements up to iron and release energy in the process. This fusion occurs in stellar cores. Elements heavier than iron will undergo radioactive decay into smaller nuclei and release energy in the process. As a result, the fusion of iron into even heavier nuclei requires an input of an additional energy source. Stellar cores don't have an additional energy source but when the core of a high mass super giant collapses and comes to a dead stop when it has reached neutron degeneracy, the kinetic energy of the infalling matter is then released into the envelope which is ripped apart in an explosion that facilitates the fusion into elements heavier than iron.

How do we know that spiral arms do not rotate like giant pinwheels? What makes spiral arms bright?

We know that the spiral arms cannot rotate like pinwheels around the center of the galaxy because the inner stars would finish several orbits while the outer stars complete one. This would wind the spiral pattern up, which we do not see. The arms appear bright because of the enhanced star formation in them. The bright, bluer stars in particular make the arms bright since these stars do not live long and so die before spreading far from the arm in which they formed.

Why do we think that supernovae should sometimes form black holes? What observational evidence supports the existence of black holes?

We think that black holes should sometimes be formed by supernovae because models indicate that in some supernovae the outer layers of the star are not completely blown away. The extra mass can push the neutron star core over the mass limit for neutron stars and make it a black hole. We have strong evidence for black holes in X-ray binary systems where the mass of the unseen companions is larger than the mass limit for neutron stars. The only kind of object we currently know of that these companions could be are black holes. We also have evidence for supermassive black holes in the centers of many galaxies.

Compared to an Sc galaxy, an Sa galaxy has a ___ central bulge and ___ spiral arms.

a larger, tighter

25. Which method of distance determination is the most accurate for the closest galaxies, like Andromeda?

a. Cepheid Variables

Where do elements heavier than iron come from?

a. They are created during a supernova explosion

Which object is the result of the death of a star? a. black hole c. white dwarfb. neutron star d. all of the above

all of the above

Which kind of star is most likely to be part of the spheroidal population?

an M star

By observing the gravitational influence of a galaxy on its neighbors, astronomers can determine the ___ of the galaxy.

b mass

Which object cannot have a mass more than 3 solar masses (3 M)? a. black hole c. white dwarfb. neutron star d. all of the above e. none of the above

b. neutron star

Which object is supported against gravity by neutron degeneracy?a. black hole c. white dwarfb. neutron star d. all of the above

b. neutron star

Why do disk stars bob up and down as they orbit the galaxy?

because the gravitational pull of other disk stars always pulls them toward the disk

Which object would be formed from a 5 solar mass (5 M) star? a. black hole c. white dwarfb. neutron star d. all of the above e. none of the above

black hole

According to the standard Big Bang model, the matter now present in the universe was created

by high energy photons

The best candidates for black holes are found:

by observing material from a nearby star falling into it

21. Which galaxy type shows a random, disorganized shape?a. all types b. S and/or SB c. Irr d. S0 e. E

c

31. The concept of a steady-state universe has apparently been disproved by the discovery of:

cosmic microwave background

The primary difference between Population I and Population II stars is that Population I stars have:

d more heavy elements (metals)

Which galaxy type can be the most massive and have the most stars?a. all types b. S and/or SB c. Irr d. S0 e. E

e

Which galaxy type has the largest percentage of old, Population II stars?a. all types b. S and/or SB c. Irr d. S0 e. E

e

Which of the following will not be a part of the evolution of a high mass star (25 Mo)?a. main sequence c. supergiant e. shell He fusion b. helium fusion in core d. red giant

d. red giant

We determine the distance of a Cepheid by

determining its luminosity from the period-luminosity relation and then applying the inverse square law for light.

How do we determine the Milky Way's mass outside the Sun's orbit?

from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun

The peculiar galaxy shapes resulting from collisions are due to:

gravitational tidal forces

Where would you expect to find stars with the highest proportions of elements heavier than hydrogen and helium?

in the disk

Where are most of the Milky Way's globular clusters found?

in the halo

Immediately prior to its death, the core of a high mass star is mostly: a. Carbon b. Helium c. Hydrogen d. Silicon e. Iron

iron

Our best estimates show that the overall density is less than critical density. If this is accurate, the universe:

is an open universe

Which stage has hydrogen fusion in the core? a. Supergiant c. Main Sequence Star e. Red Giant b. Horizontal Branch Star d. Protostar

main sequence

The only measurable properties of a black hole are:

mass, charge, rotation

Which object is typically the size of Atlanta, GA? a. black hole c. white dwarfb. neutron star d. all of the above e. none of the above

neutron star

The material inside a black hole typically consists of:a. electron degenerate material d. superheated iron nuclei b. neutron degenerate material e. none of the other choices c. superconducting material

none

The center of the universe is located:

nowhere, there is no center

A star that is roughly 25 times the mass of our sun (25 M) will end its life _____ and the dead core will become a _____.

with a supernova explosion, black hole