MCC PHS 142 M01 Astronomy Homework Ch.18-19      
Adj Prof Astronomy: Sam Wormley <>

Background Material

  Textbook - Read Chapters 18-19
  Textbook -
  Textbook -
      (take the Multiple Choice Quiz for for each chapter)

  Web - 
  Web - 
  Web - 
  Web - 

The furious expansion of a huge, billowing pair of gas and dust clouds
are captured in this NASA Hubble Space Telescope comparison image of
the supermassive star Eta Carinae. To create the picture, astronomers
aligned and subtracted two images of Eta Carinae taken 17 months apart
(April 1994, September 1995). The bright white region at the center of
the image results from the star and its immediate surroundings being
saturated in one of the images. This difference image shows that
material closer into the star (which is the bright blob at the image's
center) is blasting into space more quickly than material farther from
the star.

This picture is remarkable because most celestial objects barely change
noticeably over a span of many years. Eta Carinae is a dramatic
exception because it underwent a titanic explosion 150 years ago. The
twin lobes show white outer edges as the ejected material expands into
space at 1.5 million miles per hour. For the first time, astronomers
can track the motions of hundreds of small-scale structures in the
lobes which will allow them to characterize precisely how the lobes are
evolving. The new data may give clues as to how the lobes formed in the
first place, and shed light on the bipolar phenomenon in general. The
images were taken in violet light with Hubble s Wide Field Planetary
Camera 2 (WFPC2). The star is more than 8,000 light-years (2500 pc)
away in the southern constellation Carina. 

The cloud of gas and dust ejected by Eta Carinae contains at least two
solar masses of material and is moving away from Eta Carinae at 700
km/s. It is possible that this is not the first time people have
witnessed the ejection of mass from Eta Carinae. In 3000 B.C. the
Sumerians discovered a bright new star that barely rose above the
southern horizon. The "new" star was probably Eta Carinae during an
outburst. Eta Carinae is now losing mass at roughly 10-3 solar masses
per year. Obviously, it can't continue to do so for very long. Eta
Carinae is near the end of its evolution and is a good candidate for a
supernova explosion in the relatively near future. Maybe during your
lifetime... maybe next year!


Undersea corral? Enchanted castles? Space serpents? These eerie, dark
pillar-like structures are actually columns of cool interstellar
hydrogen gas and dust that are also incubators for new stars. The
pillars protrude from the interior wall of a dark molecular cloud like
stalagmites from the floor of a cavern. They are part of the "Eagle
Nebula" (also called M16 -- the 16th object in Charles Messier s 18th
century catalog of "fuzzy" objects that aren't comets), a nearby
star-forming region 7,000 light-years away in the constellation
Serpens. (see 10 p.m. on the 4th of July on your starwheel) 

The pillars are in some ways akin to buttes in the desert, where basalt
and other dense rock have protected a region from erosion, while the
surrounding landscape has been worn away over millennia. In this
celestial case, it is especially dense clouds of molecular hydrogen gas
(two atoms of hydrogen in each molecule) and dust that have survived
longer than their surroundings in the face of a flood of ultraviolet
light from hot, massive newborn stars (off the top edge of the
picture). This process is called "photoevaporation". This ultraviolet
light is also responsible for illuminating the convoluted surfaces of
the columns and the ghostly streamers of gas boiling away from their
surfaces, producing the dramatic visual effects that highlight the
three-dimensional nature of the clouds. The tallest pillar (left) is
about a light-year long from base to tip. Remember that our nearest
stellar neighbors are but four light-years away. 

As the pillars themselves are slowly eroded away by the ultraviolet
light, small globules of even denser gas buried within the pillars are
uncovered. These globules have been dubbed "EGGs." EGGs is an acronym
for "Evaporating Gaseous Globules," but it is also a word that
describes what these objects are. Forming inside at least some of the
EGGs are embryonic stars -- stars that abruptly stop growing when the
EGGs are uncovered and they are separated from the larger reservoir of
gas from which they were drawing mass. Eventually, the stars themselves
emerge from the EGGs as the EGGs themselves succumb to
photoevaporation. The picture was taken on April 1, 1995 with the
Hubble Space Telescope Wide Field and Planetary Camera 2. The color
image is constructed from three separate images taken in the light of
emission from different types of atoms. 

Star are born and stars die... just like us. The big massive stars have
but short lives, a few millions of years. Stars like our sun last for a
good 10 billions of years, and the little red stars like Barnard's Star
might last for 100 billion years. How long stars live, is determined by
their mass (which must be at least 80 Jupiter masses to sustain 
thermonuclear fusion of hydrogen). 

There are four (4) fates for the end of stars depending on their masses
and the masses of their cores: 

Red/Brown Dwarfs - less than 0.6 Ms  <==   Main Sequence 0.076-0.8 Ms
Stars less than about 0.6 solar masses, when nuclear fuel is used up,
gravitational collapse shrinks the star, but no more than the gas
temperature-pressure-volume laws of classical physics allow. We have
not found any white dwarf less massive than 0.6 solar masses. Part of
the answer is that the universe may not be old enough for lower mass
stars to have evolved off the main sequence. 

White Dwarfs - 0.08 and 1.44 Ms  <==   Main Sequence 0.8-8 Ms
Stars with core masses between 0.08 and 1.44 solar masses are
destined to become white dwarfs. White dwarfs are degenerate matter.
Further collapse is halted by electron degeneracy pressure. See pages
456-459 in your textbook. The vast majority of stars are in this mass
range and are destined to become white dwarfs

Neutron Stars - 1.44 and 2.9 Ms  <==   Main Sequence 8-30 Ms
Core masses between 1.44 and 2.9 solar masses overcome electron
degeneracy pressure and collapse to form neutron stars, a star that is
essentially one gigantic nucleus. Further collapse is halted by neutron
degeneracy pressure. 

Black Holes - 3 or more Ms  <==   Main Sequence > 30 Ms
But for cores with mass of 3 or more solar masses, neutron
degeneracy pressure does not stop the collapse and the star becomes a
black hole with zero physical size, but with all the mass. Gravity
really wins! 

In each case, gravity eventually wins, but, to what extent is
determined by the mass and the relative pressures of the quantum
mechanical forces, electron and neutron degeneracy pressure. Your book
has an excellent diagram on page 472 relating the original star mass to
that of the final core mass (for White Dwarfs). 

What Can We Measure

There are two basic things we can measure from stars, how bright they
are, and what color they are (their color spectra). It is these two
properties that tell us what kind of a star we are looking at. With
successive measurements we can tell if a star is variable, if it has
any measurable parallax and if it has a proper motion. 

From the color spectra, we can determine if a star has a component of
motion toward us or away from us. From spectral lines we can identify
elements and estimate relative abundances of those elements.

Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram, pioneered independently by Elnar
Hertzsprung and Henry Norris Russell, plots Luminosity as a function of
surface temperature for stars. The luminosity, or absolute magnitude,
increases upwards on the vertical axis; the temperature (or some
temperature-dependent characteristic such as spectral class or color)
decreases to the right on the horizontal axis. 

It is found that the majority of stars lie on a diagonal band that
extends from hot stars of high luminosity in the upper left corner to
cool stars of low luminosity in the lower right corner. This band is
called the main sequence. Stars called white dwarfs lie sparsely
scattered in the lower left corner. The giant stars-stars of great
luminosity and size (see red giant)-form a thick, approximately
horizontal band that joins the main sequence near the middle of the
diagonal band. Above the giant stars, there is another sparse
horizontal band consisting of the supergiant stars. The stars in the
lower right corner of the main sequence are frequently called red
dwarfs, and the stars between the main sequence and the giant branch
are called subgiants. 

The significance of the H-R diagram is that stars are concentrated in
certain distinct regions instead of being distributed at random. This
regularity is an indication that definite laws govern stellar structure
and stellar evolution. In population I regions (see stellar
populations) like the spiral arms of galaxies or open star clusters,
the stars fall almost exclusively on the main sequence. In population
II regions like the nuclei of galaxies and globular clusters, the stars
are older and have evolved significantly. The most luminous stars have
evolved furthest, and an H-R diagram of such a region will show the
upper end of the main sequence depopulated and will show a
well-developed giant branch. In such a diagram it appears that the main
sequence has "burned down from the top like a candle. Thus, the point
at which the main sequence terminates and the giant branch begins is an
indication of the age of a star cluster. A modified H-R diagram of the
stars in a cluster of unknown distance can be used to determine the
absolute magnitude, or luminosity, of the stars. Since the apparent
magnitude of a star of given absolute magnitude depends only on the
star's distance, the observed apparent magnitude of the stars can be
used to calculate the distance to the cluster.

Evolutionary path of our Sun on an H-R diagram. Note the Sun spends
most of its life on the main sequence--about 1010 years.

Homework Problems

Note the answers to the odd (Conceptual Questions, Problems and
Figure-Based Questions) are in the back of your textbook. It is
strongly suggested that you do some of those in every chapter so you
have immediate feedback as how well you are understanding the material.
There are online multiple choice quizzes for each chapter of your
textbook. Goto then click on

  Your book
  Student Edition
  Choose a chapter
  Multiple Choice Quiz
You are expected to do all of your own homework. Statistical patterns
showing copying or collaboration will result in no credit for the
homework assignment for all participants involved. The Code of Academic
Conduct for Iowa Valley Community College District is found in the
Student Handbook.

Physical Science classes require the use of mathematics. If you don't
know algebra, you sould NOT be taking this class. If you need to review,
look at Introduction to Algebra
WolframAlpha is way faster than a scientific calculator.

There is little excuse for turning homework in late. You have a whole
week between classes to read the chapters and do the homework. Homework
one week late - half credit. Two or more weeks late - no credit. Do the
homework during the week, not in class! You got homework questions,
email me 24/7.  Even if you don't have a homework 
question, email me anyway!

Problem 1: 
Why are radio and infrared radiation more useful than visible light
in studying molecular clouds?

Problem 2: 
What evidence do we have that stars form in the cores of giant
molecular clouds?

Problem 3: 
Why does a collapsing cloud remain cool as long as it is
transparent, but begins to heat up when it becomes opaque?

Problem 4: 
What evidence do we have that the planets formed as a result of
collisions of planetesimals?

Problem 5: 
Why can't hydrogen fusion and helium fusion go on at the same time
at the center of a star?

Problem 6: 
What is the relationship between the opacity of the gases in a star
and whether convection takes place within the star?

Problem 7: 
Suppose the temperature of a degenerate gas doubles while its density
remains the same. What happens to the pressure of the gas?

Problem 8: 
Why must there be a hot star at the center of a planetary

Problem 9: 
Star A is 3 times as massive and 60 times as luminous as star B. How
do the lifetimes of the two stars compare? Note that's a calculation.

Problem 10: 
Use Figure 19.9 to find the main sequence lifetime of a 5 solar-mass

Problem 11: 
Using your planisphere, note that the Sun is on both the Ecliptic
and the Celestial Equator two times a year. What is the Right Ascension
(RA) of the Sun at those two times?