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Introduction to Astronomy
Exam 2
50
Astronomy
03/14/2010

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Term
What is interstellar reddening?
Definition
Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.
Term
By mass, the interstellar medium in our region of the Milky Way consists of
Definition
70%hydrogen 28% helium and 2%heavier elements
Term
The typical size of an interstellar dust grain is
Definition
Microscopic, usually less than 1 micrometer across which is smaller than a single cell of bacteria.
Term
If you wanted to observe a molecular cloud, in what wavelength of light would you most likely observe?
Definition
Infared
Term
The thermal pressure of a gas depends on
Definition
Density and temperature
Term
What prevents the pressure from increasing as a cloud
contracts due to its gravity?
Definition
Thermal energy is converted to radiative energy via molecular collisions and released as photons.
Term
What property of a molecular cloud does not counteract gravitational contraction?
Definition
Thermal Pressure
Term
What is the likely reason that we cannot find any examples of the first generation stars?
Definition
The first generation stars were mostly very massive and exploded as supernova.
Term
Why do we think the first generation of stars would be different from stars born today?
Definition
Without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse.
Term
What happens to the rotation of a molecular cloud as it collapses to form a star?
Definition
The rotation rate increases and results in a disk of material around a protostar.
Term
When does a protostar become a true star?
Definition
when nuclear fusion begins in the core
Term
What is the difference between brown dwarfs and Jupiter?
Definition
to be cont.
Term
No stars have been found with masses greater than 100 times our Sun because
Definition
they would generate so much power that they would blow themselves apart.
Term
Which of the following may be caused by a protostellar disk?
Definition
Protostellar jets, Protostellar winds, Accretion of material onto the star, All of the above
Term
What do astronomers mean when they say that we are all “star stuff”?
Definition
that the carbon, oxygen, and many elements essential to life were created by nucleo synthesis in stellar cores
Term
Which two energy sources can help a star maintain its internal thermal pressure
Definition
nuclear fusion and gravitational contraction
Term
What type of star is our Sun?
Definition
yellow, dwarf star
Term
What is the range of star masses for high‐mass stars?
Definition
between 8 and 100 solar masses
Term
Which of the following statements about degeneracy pressure is not true?
Definition
Degeneracy pressure can only be created by interactions among the electrons in an object.
Term
What happens when a star exhausts its core hydrogen supply?
Definition
Its core contracts, but its outer layers expand and the star becomes bigger and brighter.
Term
At approximately what temperature can helium fusion occur?
Definition
100 billion K
Term
How many helium nuclei fuse together when making carbon?
Definition
3
Term
The helium fusion process results in the production
Definition
of carbon
Term
What is a planetary nebula?
Definition
It is a shell of gas ejected from a star late in its life.
Term
What happens to the core of a star after a planetary nebula occurs
Definition
It becomes a white dwarf.
Term
Which of the following sequences correctly describes the stages of life for a low‐mass star?
Definition
protostar, main-sequence, red giant, white dwarf
Term
Compared to the star it evolved from, a white dwarf is
Definition
hotter and dimmer.
Term
What is the CNO cycle?
Definition
a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts
Term
Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?
Definition
iron
Term
Which event marks the beginning of a supernova?
Definition
the sudden collapse of an iron core into a compact ball of neutrons
Term
Degeneracy pressure is the source of the pressure that stops the crush of gravity in all the following except
Definition
a very massive main-sequence star
Term
White dwarfs are so called because
Definition
they are both very hot and very small.
Term
A teaspoonful of white dwarf material on Earth would weigh
Definition
a few tons.
Term
Why is there an upper limit to the mass of a white dwarf?
Definition
The more massive the white dwarf, the greater the degeneracy pressure and the faster the speeds of its electrons. Near 1.4 solar masses, the speeds of the electrons approach the speed of light, so more mass cannot be added without breaking the degeneracy pressure.
Term
What is the ultimate fate of an isolated white dwarf?
Definition
It will cool down and become a cold black dwarf.
Term
How does a 1.2‐solar‐mass white dwarf compare to a 1.0‐ solar‐mass white dwarf?
Definition
It has a smaller radius.
Term
Which of the following is closest in size (radius) to a white dwarf?
Definition
the earth
Term
Observationally, how can we tell the difference between a white‐dwarf supernova and a massive‐star supernova?
Definition
The spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white-dwarf supernova does not.
Term
After a massive‐star supernova, what is left behind?
Definition
either a neutron star or a black hole
Term
Which of the following is closest in size (radius) to a neutron star?
Definition
a city
Term
From an observational standpoint, what is a pulsar?
Definition
an object that emits flashes of light several times per second or more, with near perfect regularity
Term
What is the basic definition of a black hole?
Definition
any object from which the escape velocity equals the speed of light
Term
How does the gravity of an object affect light?
Definition
Light coming from a compact massive object, such as a neutron star, will be redshifted.
Term
How does a black hole form from a massive star
Definition
During a supernova, if a star is massive enough for its gravity to overcome neutron degeneracy of the core, the core will be compressed until it becomes a black hole.
Term
A 10‐solar‐mass main‐sequence star will produce which of the following remnants?
Definition
neutron star
Term
If you were to come back to our Solar System in 8 billion years, what might you expect to find?
Definition
to be cont.
Term
Black holes, by definition, cannot be observed directly. What observational evidence do scientists have of their existence?
Definition
Their effects on nearby light and matter (Gravitational interactions)
Term
How do we know that pulsars are neutron stars?
Definition
No massive object, other than a neutron star, could spin as fast as we observe pulsars spin.
Term
What causes the radio pulses of a pulsar?
Definition
As the star spins, beams of radio radiation sweep through space. If one of the beams crosses the earth, we observe a pulse.
Term
From a theoretical standpoint, what is a pulsar?
Definition
a rapidly rotating neutron star