phys202 exam 4

Which of the following is evidence that the formation process of our Galaxy may have included collisions with smaller neighbor galaxies?

(is this right?)the observation of long moving streams of stars that continue to orbit through our Galaxy's halo

Roughly what is the diameter of the milky way galaxy?

100,000 light years

What is the thickness of the disk of the milky way galaxy?

1000 lightyears

Factoring in everything we currently know about the history of the universe, our best estimate for the age of the universe is

13.8 billion

Approximately how far is the sun from the center of the galaxy?

25,000 lightyears (28k more precisely)

At first, right after the Big Bang, the universe was too hot for nuclei and electrons to combine into the kinds of neutral atoms that are familiar to us today. How soon after the beginning did it become cool enough for neutral atoms to form?

3 minutes

True or false; the supermassive blackhole at the galactic center makes up roughly 90% of the mass of the milky way galaxy

False

Why do we believe that 90% of the mass of the milky way galaxy is in the form of dark matter?

From the Galaxy's rotation curve, a graph of the orbital speed of stars from the galactic center, we calculate these stars are feeling gravitational effects from unseen matter

Compared with our sun, how would you describe stars in the galactic halo?

Old, red, dim, and metal-poor

What two quantities did Hubble plot against each other to discover the expansion of the universe

Recessional speed and distance

What is a quasar?

The extremely luminous center of a distant galaxy,powered by a supermassive black hole

The disk component of a spiral galaxy includes which of the following parts?

The spiral arms

True or false; A collapsing object becomes a black hole when the escape velocity from its surface exceeds the speed of light

True

True or false; The event horizon is the boundary around the black hole at which the escape velocity equals the speed of light

True

True or false; Accretion of material onto a white dwarf can cause it to explode

True (Chandrasekhar limit)

How do we learn what is going on in the center of our own galaxy, the milky way?

We cannot see the galactic center with visible light or UV light, but infrared, radio waves and x-rays from the galactic center can be detected

Objects orbiting around the center of the Milky Way obey Kepler's 3rd Law. Therefore:

a cloud of gas or star that is further from the center will generally take more time to orbit

How do astronomers know that there aren't significant amounts of dark matter within our solar system?

a lot of dark matter would affect the motions (orbits) of our spacecraft as the move through the solar system, and see no such effect

A Congressman from Texas visits our National Observatory in Tucson, Arizona, and wants to be shown an object in the universe with the "biggest darn redshift you ever saw". What type of object should the astronomers show him?

a quasar

What type of main sequence star is most likely to become a black hole?

an O-type star

Astronomers now know that surrounding the main body of our Galaxy (which our various kinds of telescopes have shown to us) and our fainter halo of stars there is

an invisible halo made of what astronomers are calling "dark matter"

Astronomers have concluded that growing supermassive black holes (which have millions of times the Sun's mass or more) is pretty unlikely at our location in the Milky Way Galaxy. Where do they think is the most likely place in the Milky Way for such a supermassive black hole?

at the center of the Milky Way Galaxy, where matter is more crowded

When Einstein proposed his General Theory of Relativity, he suggested some pretty strange ideas about space, time, and gravity. How did scientists in 1919 show that Einstein's theory described the behavior of the real world and wasn't just a crazy hypothesis?

by observing starlight coming close to the Sun during an eclipse

According to the general theory of relativity, the presence of mass:

causes a curvature (or warping) of spacetime

Why do galaxies collide, while stars almost never do?

compared to the size of a star, the stars are very far apart; but compared to the size of a galaxy, galaxies are close together

Which type of galaxy is observed to contain mostly older stars?

elliptical

If you are in a freely falling elevator near the top of a tall building, as the elevator falls, your weight would be:

equal to zero - you would be weightless

True or false; Earth is located in the globular cluster M-13

false

True or false; Quasars are powered by the intense production of large number of stars that can only be sustained for a relatively short time

false

True or false; astronomers have a pretty good idea what the dark matter is made of

false

true or false; cosmic photon background radiation is the result of the radiation from many independent sources such as stars and galaxies

false

An astronomer needs to measure the distance to a globular cluster of stars that is part of a neighboring galaxy nearby the Milky Way Galaxy. What method should she try to use to find the distance?

find a variable star (cepheid or RR Lyrae) in the cluster

One of the main projects being carried out by the Hubble Space Telescope is to measure the distances of galaxies located in groups dozens of millions of lightyears away. What method do astronomers use with the Hubble to find such distances?

finding Cepheid variables and measuring their periods

What method would astronomers use to find the distance to a galaxy so far away that individual stars are impossible to make out (resolve)?

finding the redshift and using Hubble's Law

Compared with elliptical galaxies, how would you describe spiral galaxies?

flatter shapes with younger and bluer stellar populations

When one member of a binary star system is a black hole, and astronomers detect flickering x-rays coming from the system, where are these x-rays usually coming from?

from a disk of material (accretion disk) around the black hole (material that has been pulled from the companion star and is falling toward the black hole)

Astronomers believe that the large elliptical galaxies formed

from the collision and merger of many smaller fragments

What objects did Harlow Shapley use as "signposts" to figure out the extent of the Milky Way Galaxy and the location of its center?

globular clusters

Where would you look for the youngest stars in the Milky Way Galaxy?

in the disk

Where is our solar system located within the milky way?

in the galactic disk roughly halfway from the galactic center to the outskirts of the galactic disk

What was especially noteworthy about the Sagittarius Dwarf Galaxy when it was discovered among the small galaxies near the Milky Way?

it was on a collision course with the Milky Way and would be swallowed by it eventually

To predict whether a star will ultimately become a black hole, what is the key property of the star we should look at?

mass

A graduate student in astronomy needs to measure the mass of a spiral galaxy she is studying for her PhD thesis. Which of the following observations would be important for her to make?

obtain the speed at which stars or gas near the outer regions of the galaxy are moving around

How are galaxies and quasars related?

quasars are active supermassive black holes at the centers of galaxies

Our Milky Way Galaxy is what type of galaxy?

spiral

The new instrument that made it possible for Edwin Hubble to demonstrate the existence of other galaxies in the early 1920's was:

the 100-inch reflector on Mount Wilson

What is good evidence that the universe began with a Big Bang?

the 3-degree cosmic microwave background radiation

What is NOT part of the growing chain of evidence that makes many astronomers suspect there is a black hole at the very center of the Milky Way Galaxy?

the Hubble Space Telescope has shown us a visible-light image of an accretion disk at the center of the Galaxy

According to the most recent data from satellites making precise measurements of the properties of the cosmic background radiation (CBR),

the dark energy makes up just a little less than 70% (about 68%) of the density of the universe, making it the most significant constituent of the mass-energy

William Herschel thought that the Sun and Earth were roughly at the center of the great grouping of stars we call the Milky Way. Today we know this is not the case. What was a key reason that Herschel did not realize our true position in the Milky Way?

the dust that extends throughout the disk of the Galaxy only allowed Herschel to see the small part of the Milky Way that surrounds us

A friend of yours who is a science fiction fan hears you talk about the fact that astronomers now believe that the mechanism for the large energy output of quasars involves a supermassive black hole. He challenges you, saying something like "Oh come on, every science fiction fan knows that nothing, not even light, can escape from a black hole! How can a black hole be an energy source?" How would you respond to his objection?

the energy we see from quasars comes from regions where matter is falling in; these regions are still outside the event horizon

Edwin Hubble was able to show that (with the exception of our nearest neighbors) the farther a galaxy is from us, the

the faster it is moving away from us

With enormous effort, a team of astronomers manages to collect enough light from a galaxy far, far away to produce a spectrum. That spectrum has lines from the elements carbon, silicon, and sulfur. This tells the team that

the galaxy must have had an entire generation of stars that was born, lived, and died

In the first direct detection of gravitational waves by LIGO in 2015, the waves came from

the merger of two black holes

In the far future, a starship becomes trapped inside the event horizon of a black hole. Although the crew discovers that their ship cannot out, they at least want to send a message to other ships in the area to stay away from the danger zone. If they send out a message in the form of a radio wave, what will be its fate?

the message will never emerge from the event horizon

The first time that astronomers observed both gravitational waves and electro-magnetic waves from the same event, what they were observing was:

the spiraling toward each other of two neutron stars

If quasars often resemble little blue stars, what was it about them that so surprised astronomers when they were discovered?

their spectral lines were at first hard to recognize and then turned out to have large redshifts

Among irregular galaxies, what makes the Large and Small Magellanic Cloud especially useful for astronomers?

they are (for galaxies) very close to us, so they are easy to study

The reason type Ia (white dwarf) supernovae are useful to astronomers for determining distances to other galaxies is that

they are very bright, and generally reach the same peak luminosity

One important way astronomers can learn in some detail about what happens when galaxies collide is

to simulate galaxy collisions on a large computer and watch what the simulation predicts

true or false: collisions and mergers between galaxies can sometimes change a galaxy's type (shape)

true

true or false; Some quasars are more than a thousand times more luminous than the milky way galaxy

true

true or false; cosmic photon background radiation appears essentially the same in all directions (isotropic) with only tiny variations

true

true or false; cosmic photon background radiation had a much higher temperature in the past

true

true or false; cosmic photon background radiation has a present day temperature of about 3 kelvins above absolute zero

true

true or false; cosmic photon background radiation was first detected by Penzias and Wilson in NJ in the 1960's

true

true or false; gravitational lensing sometimes produces more than one image of a single quasar

true

true or false; observations of white dwarf supernovae allow us to measure the distances of very distant galaxies

true

true or false; quasars are powered by the energy radiated by matter falling into a central, supermassive black hole

true

true or false; the observed composition of "ordinary" matter in the universe was roughly 75% H and 25% he, closely matches the theoretical calculations based on the big bang model

true

true or false; young stars and open star clusters are generally found only in the galactic disk and not in the galactic halo

true

After several decades of observation, astronomers have concluded that quasars are

very powerful and compact sources of energy at the centers of distant galaxies

Before you can use Hubble's Law to get the distance to a galaxy, what observation must you make of that galaxy?

you must take a spectrum of the galaxy and measure the red shift