A white dwarf is _________.
what most stars become when they die
A typical white dwarf is _________.
as massive as the Sun but only about as large in size as Earth
If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh _________.
as much as a truck
The maximum mass of a white dwarf is _________.
about 1.4 times the mass of our Sun
What is an accretion disk?
A disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole
According to our modern understanding, what is a nova?
An explosion occurring on the surface of a white dwarf in a close binary system
Suppose that a white dwarf is gaining mass because of accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit?
The white dwarf will explode completely as a white dwarf supernova.
A neutron star is _________.
the remains of a star that died in a massive star supernova (if no black hole was created)
A typical neutron star is more massive than our Sun and about the size (radius) of _________.
a small asteroid (10 km in diameter)
If you had something the size of a sugar cube that was made of neutron star matter, it would weigh _________.
about as much as a large mountain
Pulsars are thought to be _________.
rapidly rotating neutron stars
How is an X-ray burst (in an X-ray binary system) similar to a nova?
both involve explosions on the surface of stellar corpse
What is the basic definition of a black hole?
A black hole is an object with gravity so strong that not even light can escape.
Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly _________.
3 solar masses
What do we mean by the event horizon of a black hole?
It is the point beyond which neither light nor anything else can escape.
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.
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.
What makes us think that the star system Cygnus X-1 contains a black hole?
It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.
What does the Schwarzschild radius of a black hole depend on?
Only the mass of the black hole
Scientists have now detected thousands of gamma ray bursts. The evidence suggests that most or all of these bursts _________.
have occurred in distant galaxies
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 radius of a white dwarf is determined by a balance between the inward force of gravity and the outward push
electron degeneracy pressure
a _____ occurs when hydrogen fusion ignites on the surface of a white dwarf in a binary system
a _______ occurs when fusion creates iron in the core of a star
massive star nova
a white dwarf in a close binary system will explode as a supernova if it gains enough mass to exceed the _________________
white dwarf limit (1.4 solar masses)
an ___________ consists of hot, swirling gas captured by a white dwarf (or neutron star or black hole) from a binary companion star
a __________ can occur only in a binary system, and all such events are thought to have the same luminosity
white dwarf supernova
characteristics of white dwarf
typically the size (diameter) of earth
has a mass no greater than 1.4 MSun
in a binary system, it can explode as a supernova
supported by electron degeneracy
characteristics of neutron star
usually has a very strong magnetic field
sometimes appears as a pulsar
How does the diameter of the disk of Milky Way Galaxy compare to its thickness?
The diameter is about 100 times greater than the thickness.
What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?
The galaxy's bulge
The Sun's location in the Milky Way Galaxy is _________.
in the galactic disk, roughly halfway between the center and the outer edge of the disk
How do disk stars orbit the Galaxy?
Disk stars orbit the Galaxy, all staying in the same thick plane-shaped region in space.
What do we mean by the interstellar medium?
Interstellar medium is gas and dust that lies between stars in the Milky Way Galaxy.
What are the Magellanic Clouds?
Two small galaxies that probably orbit the Milky Way Galaxy
How do we know how much mass there is in the Milky Way galaxy, inside the Sun's galactic path?
By measuring stellar orbits around the center of the Galaxy.
Fill in the blank: Elements heavier than hydrogen and helium constitute about _________ of the mass of the interstellar medium.
What do we mean by the star-gas-star cycle?
It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.
What are cosmic rays?
They are subatomic particles that travel close the speed of light
The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with _________.
radio telescopes observing at a wavelength of 21 centimeters
Which of the following molecules is the most abundant in molecular clouds?
Molecular hydrogen (H2)
Interstellar dust consists mostly of _________.
microscopic grains of carbon and silicon minerals
Which of the following analogies best describes a spiral density wave?
Traffic slowing to look at an accident in the on-coming lane.
What do we mean by a protogalactic cloud?
A cloud of hydrogen and helium that contracts to become a galaxy.
Most stars in the Milky Way's halo are _________.
What is an ionization nebula?
It is a colorful cloud of gas that glows because it is heated by light from nearby hot stars.
What do halo stars do differently from disk stars?
They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
Where does most star formation occur in the Milky Way Galaxy?
It occurs in the galaxy's spiral arms.
Which of the following statements about stars in the Milky Way appears to be generally true, based upon observations?
The older the star, the lower its abundance of heavy elements.
What strange object do we think lies in the center of the Milky Way Galaxy?
A more than 2 million solar mass black hole.
the circular but relatively flat portion of the galaxy is the ________
the first portion of the galaxy to form was the ____________
tightly packed group of a few hundred thousand very old stars is a
a _______ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust
our milky way galaxy is a _______
stars orbiting in the _____ near the galaxys center can have orbits highly inclined to the galactic plane
disk stars (yellow)
high mass stars
stars that all orbit in nearly the same plane
halo stars (green)
stars hose orbit can be inclined at any angle
stars with the smallest abundance of heavy metals
Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in your text), the number of galaxies in our observable universe is estimated to be somewhere around _________.
Which of the following is NOT one of the three major categories of galaxies?
A standard candle is _________.
light source of known luminosity
What is main-sequence fitting?
A method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram
What is a Cepheid variable?
A type of very luminous star that makes an excellent standard candle
What two properties of a Cepheid variable are directly related to one another?
The period between its peaks of brightness and its luminosity
What does Hubble's law tell us?
The more distant a galaxy, the faster it is moving away from us.
When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?
The inside of the balloon does not represent any part of our universe.
If we say that a galaxy has a lookback time of 1 billion years, we mean that _________.
its light traveled through space for 1 billion years to reach us
Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called _________.
the cosmological horizon
Current estimates place the age of the universe at about _________.
14 billion years
Telescopes being planned for the study of the earliest stages in galactic lives will be optimized for observations in ______.
What is an important starting assumption in most models for galaxy formation?
Some regions in the universe start out denser than others.
Galaxies that we see as they were 2 billion years after the Big Bang tend to look like __________.
What do we mean by a protogalactic cloud?
It is a cloud of matter that contracts to become a galaxy.
Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______.
may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies.
Collisions between galaxies typically unfold over a period of _________.
hundreds of millions of years
Why are collisions between galaxies more likely than collisions between stars within a galaxy?
Relative to their sizes, galaxies are closer together than stars.
Which of the following features is NOT a feature of a central dominant galaxy?
They are often spiral galaxies.
Why should galaxy collisions have been more common in the past than they are today?
Galaxies were closer together in the past because the universe was smaller.
The distinguishing feature of a starburst galaxy is _________.
a rate of star formation that may be 100 or more times greater than that in the Milky Way
The unusually bright centers found in some galaxies are called _________.
active galactic nuclei
According to current understanding, what is a quasar?
An active galactic nucleus that is particularly bright
Which of the following phenomena is probably NOT related to the presence of a supermassive black hole?
The large sizes of central dominant galaxies
The mass of the supermassive black hole thought to power a typical active galactic nucleus would be around _________.
1 billion solar masses
According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______.
light emitted by hot gas in an accretion disk that swirls around the black hole.