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Exam 3: 15
Stellar Properties
25
Astronomy
11/13/2011

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Term
What do we need to measure in order to determine a star's luminosity?
Definition
apparent brightness and distance
Term
If the distance between us and a star is doubled, with everything else remaining the same, the luminosity...
Definition
remains the same, but the apparent brightness is decreased by a factor of four.
Term
How  would  the  apparent  brightness  of  Alpha   Centauri  change  if  it  were  three  times   farther  away?
Definition
 It  would  be  only  1/9  as  bright.
Term
What two pieces of information would you need in order to measure the masses of stars in an eclipsing binary system?
Definition
the time between eclipses and the average distance between the stars
Term
Why is a star's birth mass its most fundamental property?
Definition
A star's birth mass determines its quantity of hydrogen fuel, its central pressure and temperature, and therefore its luminosity. These in turn set the star's main-sequence lifetime and surface temperature. These relationships mean that the birth mass of the star sets most of its other properties.
Term
Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is __________.
Definition
less luminous, cooler, smaller, and less massive

Term
Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are __________.
Definition
cooler and larger in radius
Term
Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are __________.
Definition
hotter and smaller in radius

Term
Approximately, what basic composition are all stars born with?
Definition
three-quarters hydrogen, one-quarter helium, no more than 2 percent heavier elements

Term
The spectral sequence in order of decreasing temperature is
Definition
OBAFGKM
Term
Why is the spectral sequence of stars not alphabetical?
Definition
The original alphabetical labeling did not correspond to surface temperature and thus had to be reordered
Term
Suppose you see two main-sequence stars of the same spectral type. Star 1 is dimmer in apparent brightness than Star 2 by a factor of 100. What can you conclude? (Neglect any effects that might be caused by interstellar dust and gas.)
Definition
Star 1 is 10 times more distant than Star 2.
Term
On a Hertzsprung-Russell diagram, where would we find stars that are cool and luminous?
Definition
upper right
Term
In order to understand star clusters, we need to be able to estimate their ages. What technique do scientists use for this?
Definition
finding the main-sequence turnoff point of the stars
Term
What prevents a brown dwarf from undergoing nuclear fusion?
Definition
Degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.
Term
Which of the following best describes the axes of a Hertzsprung-Russell (H-R) diagram?
Definition
surface temperature on the horizontal axis and luminosity on the vertical axis
Term
On a Hertzsprung-Russell diagram, where would we find stars that have the largest radii?
Definition
upper right
Term
You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to determine all of the following, except the
Definition
rotation rate of the star.
Term
how  do  we  determine  the  luminosity  of  the  Sun   from  its  flux?    
Definition
luminosity  =  flux  *  area  of  sphere  at  your  distance
Term
what allows  us  to   measure  the  distance  for   nearby stars?
Definition
parallax, sin  p  =  1  AU  /  d  
where  p  will  be  either  in   radians  or  degrees
Term
hot stars should appear...
Definition
brighter and bluer
Term
how does one determine the masses of stars in binary systems?
Definition
Newton's form of Kepler's 3rd Law
Term
where a star lies on the main sequence is a function of....
Definition
mass
Term
when a star's mass increases...
Definition
its gravity increases and its thermal energy increases to keep the star from collapsing
Term
when thermal pressure increases...
Definition
temperature and density in the core increase, leading to an increase in the rate of fusion