Chapter 17: The Stars Part 1

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Chapter 17 The Stars Part 1

• Alyssa Jean-Mary

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The Stars

• Toward the end of the eighteenth century, the
  study of the stars intensified with William
  Herschels work. Herschel looked for some sort of
  order in the stars. He began by taking
  observations, and thus spent many years
  cataloging stars and measuring their apparent
  motions. This study allowed him to verify a
  structure for the universe that is very similar
  to what is believed to be correct by todays
  astronomers.
• There are billions of stars in the universe. All
  of these stars, besides the sun, appear as no
  more than a point of light, even when viewed by
  the most powerful telescopes. Due to
  spectroscopic analysis, there is a great deal of
  detailed information known about thousands of the
  stars in the universe. In addition to this
  information, spectroscopic analysis has allowed
  scientists to trace the evolution of a star all
  the way from its birth, through maturity, to its
  last agonies and eventual fate.

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The Sun

• The sun is the dominating body of the solar
  system.
• Its life cycle is closely connected to the origin
  and destiny of the earth.
• The sun is in many ways a typical star, a rather
  ordinary member of the 1020 stars that make up
  the universe. Thus, the properties of the sun are
  interesting not only because they apply to the
  sun, but because they can also be applied,
  generally, to the rest of the stars in the
  universe.

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The Properties of the Sun 1

• The mass of the sun can be obtained using the
  characteristics of the orbital motion of the
  earth around the sun. Thus, the mass of the sun
  is 1.99 x 1030kg, which is more than 300,000
  times the mass of the earth.
• The radius of the sun can be obtained using
  simple geometry based on the fact that the
  angular diameter of the sun as seen from the
  earth is 0.53. Thus, the radius of the sun is
  6.96 x 108m.
• The volume of the sun is so large that 1,300,000
  earths could fit into it.

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The Properties of the Sun 2

• The gases that are in the interior of the sun are
  very hot. Thus, they emit a lot of light. Since
  the gases are also dense, the light that they
  emit is reabsorbed almost immediately. Both the
  temperature and the density of these gases
  decreases with distance from the center of the
  sun. Because of this, there is a region in the
  sun where the gases are hot enough to radiate a
  large amount of light, but not dense enough to
  reabsorb this light and thus prevent this light
  from escaping. This region of the sun is called
  the photosphere. The photosphere is what we see
  as the surface of the sun. The temperature of
  the photosphere is 5800K. This temperature was
  found both by using the shape of the spectrum of
  the sun and by using the rate at which the sun
  gives off energy.
• The spectrum of the sun has thousands of lines.
  Half of these lines have been traced to specific
  elements. It is thought that the other lines come
  from highly excited energy levels in atoms or
  energy levels of ions instead of atoms. Only a
  few lines are from compounds since the
  photosphere is so hot that it breaks up almost
  all molecules.
• Even though the conditions on the sun are very
  different from the conditions on the earth, it
  appears that they both have the same basic
  matter. The relative amounts of different
  elements are similar, but there is much more of
  the lighter elements hydrogen and helium on the
  sun. On the sun, hydrogen is actually 72 by
  mass, and helium is 27 by mass. Since the
  temperatures on the earth are relatively low,
  most of the elements have combined to form
  compounds. On the sun, however, since the
  temperature is so high, most of the elements are
  individual atoms or ions.

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The Properties of the Sun Above the Photosphere

• After the photosphere is a rapidly thinning
  atmosphere that consists mainly of hydrogen and
  helium. In this atmosphere, great flamelike
  prominences sometimes project out into space.
  Prominences occur in a variety of forms. They are
  normally about 200,000km long, 10,000km wide, and
  50,000km high. Prominences are quite often
  associated with sunspots. They also seem to have
  magnetic fields associated with them.
• When the moon obscures the disk of the sun during
  a total eclipse of the sun, a wide halo of pearly
  light is seen around the dark moon. This halo is
  called the corona. The corona can extend out as
  much as a solar diameter, and it seems to have a
  great number of fine lines that extend outward
  from the sun. The corona consists of ionized
  atoms and electrons that are in extremely rapid
  motion. The temperature of the corona reach as
  high as 1 million K. The corona that is seen
  during an eclipse is relatively near the sun. The
  corona is also found in a very diffuse form much
  farther out from the sun, actually beyond the
  orbit of the earth. This outward flow of ions and
  electrons which is an extension of the atmosphere
  of the sun is called the solar wind. The solar
  wind was detected by spacecraft. It is what helps
  deflect comet tails away from the sun, and it is
  also what causes auroras in the upper atmosphere
  of the earth.

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The Aurora 1

• One of natures most awesome spectacle is the
  aurora, also called the northern lights.
• During an aurora, colored streamers seem to race
  across the sky, and glowing curtains of light
  pulsate as they change their shapes into weird
  forms and images. The climax of an aurora makes
  the heavens appear as if they are on fire, with
  silent green and red flames that are dancing
  everywhere. After awhile, the aurora fades away,
  with only a faint reddish arc remaining.
• Auroras are most common in the far north and the
  far south. When an aurora is in the northern
  hemisphere, it is called an aurora borealis. When
  an aurora is in the southern hemisphere, it is
  called an aurora australis.

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The Aurora 2

• Auroras are caused by the solar wind. The ionized
  atoms and electrons from the sun take about a day
  to reach the earth in the solar wind. The ionized
  atoms and electrons interact with the nitrogen
  and oxygen as they enter the upper atmosphere.
  This interaction causes light to be given off.
  The gas molecules (i.e. the nitrogen and oxygen
  molecules) are excited by the charged particles
  (i.e. the ionized atoms and electrons) that are
  moving close to them. The energy from this
  excitation is radiated as light in the
  characteristic wavelengths of the particular
  element. The green hues of an aurora are from
  oxygen, the blue hues are from nitrogen, and the
  red hues are from both oxygen and nitrogen.
• The way in which the ionized atoms and electrons
  of the solar wind are affected by the magnetic
  field of the earth is complicated. Thus, must
  auroras occur within doughnut-like zones that are
  about 2000km in diameter that are centered about
  the geomagnetic north and south poles. At times,
  however, when the cloud of particles that comes
  from the sun is so immense, auroras can also be
  seen in other places.
• Even when the auroras are not obvious, there is
  still a faint glow in the night sky because there
  is a less concentrated stream of particles from
  the sun that are interacting with the upper
  atmosphere. This is referred to as the airglow.
  The airglow is always present to some extent, but
  its brightness varies with the amount of solar
  activity. In addition to occurring during the
  night, both auroras and the airglow occur in the
  daytime, but are too dim to be seen.

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Sunspots 1

• Sunspots are dark patches that occur on the
  surface of the sun, and thus tarnish the intense
  luminosity of the sun. Sunspots are cooler
  portions of the sun, and they only appear dark
  because they are seen against a bright
  background. When a spot has a temperature of
  5000K, it is hot enough to glow brilliantly, but
  it doesnt appear so because it is considerable
  cooler than the rest of the surface of the sun.
• The lifetime of a sunspot ranges from 2 to 3 days
  to more than a month. During its lifetime, a
  sunspot continually changes form, growing rapidly
  and then shrinking. A sunspot can be as large as
  many thousands of km across, which is actually as
  large as several earths.

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Sunspots 2

• Galileo was one of the first to study sunspots.
  He noticed that sunspots appear to move across
  the disk of the sun. He interpreted this as a
  sign that the sun rotates on its axis. Doppler
  shifts in the spectral lines of radiation that
  comes from the edges of the disk of the sun
  further confirm the rotation of the sun on its
  axis. The sun actually rotates faster at its
  equator that near its poles. It takes 27 days for
  a complete rotation at its equator and 31 days
  for a complete rotation near its poles.
• Usually, sunspots appear in groups. Each group
  has a single large spot, with several smaller
  spots. Some groups have as many as 80 separate
  spots.
• Sunspots usually occur in two zones. These zones
  are located on either side of the equator.
  Sunspots are rarely seen either near the equator
  or at latitudes on the sun higher than 35.
• Strong magnetic fields are associated with
  sunspots. It is thought that these fields are
  actually involved in the formation of sunspots.
• The number of spots on the sun increases and
  decreases with time in a regular cycle. This
  cycle covers about 11 years. There is evidence
  that other stars also have cool spots on their
  surfaces, and on some of these stars, the spots
  appear to come and go periodically, just like the
  spots on the sun. Many of these cool spots on
  other stars, starspots, are much larger than
  sunspots.

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Sunspots and the Earth

• Some occurrences on the earth seem to follow the
  same cycle as the sunspots. These occurrences
  include disturbances in the terrestrial magnetic
  field, shortwave radio fadeouts, changes in
  cosmic-ray intensity, and unusual auroral
  activity.
• It is thought that the radio transmissions are
  affected by the ionosphere changes that occur due
  to the intense bursts of ultraviolet and
  x-radiation that happen more often when sunspots
  are at a maximum.
• It is thought that the magnetic, cosmic-ray, and
  auroral effects are caused by solar storms that
  occur due to vast streams of energetic protons
  and electrons that shoot out from the sun near
  the groups of sunspots at the same time.
• Also, some aspects of weather and climate seem to
  be in sync with sunspot activity.
• For example, between 1645 and 1715, there was
  only a few sunspots that appeared. During this
  time on earth, the temperatures were lower than
  usual. This coincides with a time that is
  referred to as the Little Ice Age.
• Thus, it appears that the events that cause
  sunspots on the surface of the sun are linked to
  slightly higher energy output from the sun. Even
  a small change in the amount of energy output
  from the sun would cause the climate effects that
  are observed on the earth.

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Solar Energy 1

• An area of 1m2 on the earth that is exposed to
  the vertical rays of the sun received energy at a
  rate of about 1.4 kW. Thus, when all the energy
  that is received over the entire surface of the
  earth is added up, it is a staggering total. But
  this total is actually only a tiny fraction of
  the total radiation that the sun puts out. For
  billions of years, the sun has been emitting a
  extremely large amount of energy at this same
  rate.
• The energy that is emitted by the sun comes from
  processes that take place inside of the sun. Near
  the center of the sun, the temperature is
  estimated to be 14 million K, and the pressure is
  estimated to be 1 billion atm. Also, the density
  of matter that is near the center of the sun is
  thought to be nearly 10 times the density of lead
  on the surface of the earth. Under these
  conditions, the atoms of lighter elements have
  lost all of their electrons, and the atoms of
  heavier elements only have their innermost
  electrons. Thus, the interior of the sun consists
  of atomic debris (i.e. free electrons and
  positive nuclei that are either surrounded by a
  few electrons or none at all).
• At ordinary temperatures, the atomic fragments
  that are in the interior of the sun move far more
  rapidly than gas molecules. Since they are moving
  so fast, two atomic nuclei may get close enough
  to each other, even though there are repulsive
  electric forces between them because they are
  both positively charged, to react with each other
  and form one large nucleus. When this happens
  between two light elements, the new nucleus that
  is formed will have a little less mass than the
  combined masses of the nuclei that combined to
  make this new nucleus. This missing mass is
  converted to energy (i.e. E0 mc2). These
  reactions that give off such a large amount of
  energy are nuclear fusion reactions. They are the
  reactions that are responsible for the energy
  that is emitted from the sun.

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Fusion Reactions in the Sun

• Most of the energy of the sun comes from the
  conversion of hydrogen into helium. This
  conversion occurs both directly and indirectly.
  It occurs directly when two hydrogen nuclei (i.e.
  protons) collide with each other. It occurs
  indirectly when a carbon nuclei absorbs a
  succession of hydrogen nuclei in a series of
  steps.
• Whether directly or indirectly, the entire fusion
  reaction makes about 0.007kg of matter disappear
  for every kg of helium it produces. This
  disappearance of matter corresponds to an energy
  release of 6.3 x 1014J. In perspective, to obtain
  this amount of energy, about 20 million kg of
  coal would have to be burned.
• The relative likelihoods of the carbon and
  proton-proton cycles depend on the temperature in
  the sun. Since the interior of the sun and other
  stars like it have temperatures that are as high
  as 14 million K, the proton-proton cycle
  predominates over the carbon cycle. The stars
  that are hotter than this obtain most of their
  energy from the carbon cycle.
• The sun converts more than 4 billion kg of matter
  into energy every second. The sun actually has
  enough hydrogen to be able to release this amount
  of energy for billions of years into the future.
  The amount of matter that has been lost in all of
  geologic history is not enough to have changed by
  much the amount of radiation that comes from the
  sun. This confirms that the surface temperature
  of the earth hasnt changed by much.

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The Origin of the Heavy Elements

• The reactions that convert hydrogen into helium
  are not the only reactions that take place in the
  sun and the other stars. Since there is hydrogen
  and helium to be used as raw materials, and high
  temperatures and pressures, most of the other
  elements are formed in reactions also. The
  heaviest elements, however, need even more
  extreme conditions to form. These conditions
  occur during the supernova explosions of heavy
  stars. In addition to forming the heavier
  elements, these explosions also scatter into
  space the elements that were already formed in
  the parent star. Once these elements are
  scattered, both the heavy and the light elements
  mix with the hydrogen and helium in interstellar
  space and then become incorporated into new stars
  and their planets. Thus, we are all made of star
  dust.

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Stellar Distances 1

• Aristotle concluded that the earth is stationary
  based on the observation that the stars dont
  seem to shift in position. He thought that if the
  earth did revolve around the sun, then the stars
  should seem to shift in position, just like trees
  and buildings seem to shift in position as we
  ride past them.
• Copernicus proposed another explanation as to why
  the stars dont seem to shift in position. He
  said that the stars are just too far way to be
  able to easily detect such shifts in position.
• In 1838, a shift for a star was discovered by
  Friedrich Bessel, a German astronomer. After
  this, other shifts were found as well. These
  shifts are so very small that it is not
  surprising that it took so long to detect them.

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Stellar Distances 2

• The discovery that Bessel made allowed the direct
  measurement of distances to the nearer stars. The
  position of the star is determined twice, each
  time being six months apart. From these
  observations, along with the measured change in
  the angle of the telescope and the fact that the
  telescope was moved by the 300 million km
  diameter of the orbit of the earth during the six
  months, the distance to the star is calculated.
• The apparent shift in position of a star is
  called the parallax. The parallax of only a few
  thousand of the nearer stars is large enough to
  be measured. The parallax of the closest star,
  Proxima Centauri, is equal to the diameter of a
  dime that is seen from a distance of 6km. The
  distance from the earth to Proxima Centauri is
  about 4 x 1016m.
• A light-year is a unit that is sometimes used to
  express stellar distances. A light-year is the
  distance that light travels in a year. It is
  equal to 9.46 x 1012km. Thus, the closest star to
  the earth, Proxima Centauri, is a little over 4
  light-years away. This means that when we see the
  star, we see it as it was 4 years ago, not as it
  is now. There are only about 40 stars that are
  within 16 light-years of the solar system.

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Apparent and Intrinsic Brightnesses 1

• The measurement of the parallax of a star is only
  possible for distances up to about 300
  light-years away. There are some indirect
  methods, however, that are used to measure the
  distances of stars that are father away than 300
  light-years. One of these is based on a
  comparison of the apparent and intrinsic
  brightnesses of stars.
• The apparent brightness of a star is its
  brightness as seen from the earth. It expresses
  the amount of light from the star that reaches
  the earth.
• The intrinsic brightness of a star is the true
  brightness of the star. It depends on the total
  amount of light that it radiates into space.
• The apparent brightness of a star actually
  depends on both its intrinsic brightness and its
  distance from the earth. Thus, a star that is
  actually very bright might appear as if it is
  faint because it is far away from the earth, and
  also, a star that is actually faint might appear
  as if it is very bright because it is close to
  the earth.

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Apparent and Intrinsic Brightnesses 2

• As long as both the apparent and intrinsic
  brightness of a star is known, the distance to
  the star can be calculated by finding out how far
  away an object with the same intrinsic brightness
  must be located in order to send us the amount of
  light that we observe (i.e. the apparent
  brightness).
• Walter Adams, an American astronomer, discovered
  one way to find out the intrinsic brightness of a
  star. While he was studying the spectra of the
  near stars, stars that have their intrinsic
  brightness known, he noticed that the spectra of
  stars that have high intrinsic brightnesses have
  certain relationships among the strengths of
  their lines. The spectra of stars that have low
  intrinsic brightnesses also have certain
  relationships, but different relationships than
  the spectra of the stars that have high intrinsic
  brightnesses. Thus, Adams was able to establish
  the intrinsic brightness of a star just by
  looking at its spectrum.
• This method of comparing the apparent and
  intrinsic brightnesses of stars allowed stellar
  distances to be determined up to several thousand
  light-years away.