Stars

1
Stars

• Chapter 30

2
Properties of the Sun

• The Sun is the largest object in the solar
  system, in both size and mass.

• The Sun contains more than 99 percent of all the
  mass in the solar system, which allows it to
  control the motions of the planets and other
  objects.

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Properties of the Sun

• The solar interior is gaseous throughout because
  of its high temperatureabout 1 107 K in the
  center.

• Many of the gases are in a plasma state
• The outer layers of the Sun are not quite hot
  enough to be plasma.

4
The Suns Atmosphere

• The photosphere, approximately 400 km in
  thickness, is the lowest layer of the Suns
  atmosphere.

• This is the visible surface of the Sun because
  most of the light emitted by the Sun comes from
  this layer.

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The Suns Atmosphere

• The chromosphere is above the photosphere.

• The corona, which is the top layer of the Suns
  atmosphere, extends several million kilometers
  outward from the top of the chromosphere.

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The Suns Atmosphere

• Solar Wind

• Gas flows outward from the corona at high speeds
  and forms the solar wind which consists of
  charged particles, or ions, that flow outward
  through the entire solar system.

• The charged particles are trapped in two huge
  rings in Earths magnetic field, called the Van
  Allen belts, where they collide with gases in
  Earths atmosphere, causing an aurora.

7
Solar Activity

• The Suns magnetic field disturbs the solar
  atmosphere periodically and causes new features
  to appear in a process called solar activity.

• Sunspots are cooler areas that form on the
  surface of the photosphere due to magnetic
  disturbances, which appear as dark spots.

8
Solar Activity

• Solar Activity Cycle

• The number of sunspots changes regularly, and on
  average reaches a maximum number every 11.2
  years.
• The length of the solar activity cycle is 22.4
  years.
• There were severe weather changes on Earth during
  the latter half of the 1600s when the solar
  activity cycle stopped and there were no sunspots
  for nearly 60 years.

9
Solar Activity

• Other Solar Features

• Solar flares are violent eruptions of particles
  and radiation from the surface of the Sun that
  are associated with sunspots.

• prominence, sometimes associated with flares, is
  an arc of gas that is ejected from the
  chromosphere, or gas that condenses in the inner
  corona and rains back to the surface.

10
The Solar Interior

• Fusion occurs within the core of the Sun where
  the pressure and temperature are extremely high.

• Fusion is the combining of lightweight nuclei,
  such as hydrogen, into heavier nuclei.

• In the core of the Sun, helium is a product of
  the process in which hydrogen nuclei fuse.
• At the Suns rate of hydrogen fusing, it is about
  halfway through its lifetime, with about another
  5 billion years left.

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The Sun
12
Spectra

• A spectrum is visible light arranged according to
  wavelengths.

• A continuous spectrum is produced by a hot solid,
  liquid, or dense gas. When a cloud of gas is in
  front of this hot source, an absorption spectrum
  is produced. A cloud of gas without a hot source
  behind it will produce an emission spectrum.

13
Solar Composition

• The Sun consists of hydrogen, about 73.4 percent
  by mass, and helium, 25 percent, as well as a
  small amount of other elements.

• The Suns composition represents that of the
  galaxy as a whole.

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Groups of Stars

• Constellations are the 88 groups of stars named
  after animals, mythological characters, or
  everyday objects.

• Circumpolar constellations can be seen all year
  long.
• Summer, fall, winter, and spring constellations
  can be seen only at certain times of the year.

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Aries, Cancer, Canis Major, Draco, Hercules,
Hydra, Leo, Libra, Orion, Pegasus, Pices, Taurus
Ursa Minor, Virgo
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Groups of Stars

• Star Clusters

• A group of stars that are gravitationally bound
  to each other is called a cluster.

• In an open cluster, the stars are not densely
  packed.
• In a globular cluster, stars are densely packed
  into a spherical shape.

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Groups of Stars

• Binaries

• A binary star is two stars that are
  gravitationally bound together and that orbit a
  common center of mass.

• More than half of the stars in the sky are either
  binary stars or members of multiple-star systems.

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Constellations
19
Stellar Position and Distances

• Astronomers use two units of measure for long
  distances.

• A light-year (ly) is the distance that light
  travels in one year, equal to 9.461 1012 km.
• A parsec (pc) is equal to 3.26 ly, or 3.086
  1013 km.

20
Stellar Position and Distances

• To estimate the distance of stars from Earth,
  astronomers make use of the fact that nearby
  stars shift in position as observed from Earth.

• Parallax is the apparent shift in position of an
  object caused by the motion of the observer.

• As Earth moves from one side of its orbit to the
  opposite side, a nearby star appears to be
  shifting back and forth.

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Stellar Position and Distances

• The distance to a star, up to 500 pc using the
  latest technology, can be estimated from its
  parallax shift.

22
Basic Properties of Stars

• The diameters of stars range from as little as
  0.1 times the Suns diameter to hundreds of
  times larger.
• The masses of stars vary from a little less than
  0.01 to 20 or more times the Suns mass.

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Basic Properties of Stars

• Apparent Magnitude

• The ancient Greeks established a classification
  system based on the brightnesses of stars.
• The brightest stars were given a ranking of 1,
  the next brightest 2, and so on.

• In this system, a difference of 5 magnitudes
  corresponds to a factor of 100 in brightness.
• Negative numbers are assigned for objects
  brighter than magnitude 1.

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Basic Properties of Stars

• Absolute Magnitude

• Apparent magnitude does not actually indicate how
  bright a star is, because it does not take
  distance into account.
• Absolute magnitude is the brightness an object
  would have if it were placed at a distance of 10
  pc.

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Basic Properties of Stars
Luminosity

• Luminosity is the energy output from the surface
  of a star per second.

• Specta

• Stars also have dark absorption lines in their
  spectra and are classified according to their
  patterns of absorption lines.

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Spectra of Stars

• Classification by Spectra

• All stars, including the Sun, have nearly
  identical compositionsabout 73 percent of a
  stars mass is hydrogen, about 25 percent is
  helium, and the remaining 2 percent is composed
  of all the other elements.

• The differences in the appearance of their
  spectra are almost entirely a result of
  temperature effects.

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Spectra of Stars

• Wavelength Shift

• Spectral lines are shifted in wavelength by
  motion between the source of light and the
  observer due to the Doppler effect.

• If a star is moving toward the observer, the
  spectral lines are shifted toward shorter
  wavelengths, or blueshifted.
• If the star is moving away, the wavelengths
  become longer, or redshifted.

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Spectra of Stars

• H-R Diagrams

• A Hertzsprung-Russell diagram, or H-R diagram,
  demonstrates the relationship between mass,
  luminosity, temperature, and the diameter of
  stars.

• An H-R diagram plots the absolute magnitude on
  the vertical axis and temperature or spectral
  type on the horizontal axis.

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Spectra of Stars

• H-R Diagrams

• The main sequence, which runs diagonally from the
  upper-left corner to the lower-right corner of
  an H-R diagram, represents about 90 percent of
  stars.

• Red giants are large, cool, luminous stars
  plotted at the upper-right corner.
• White dwarfs are small, dim, hot stars plotted in
  the lower-left corner.

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Basic Structure of Stars

• The mass and the composition of a star determine
  nearly all its other properties.

• Hydrostatic equilibrium is the balance between
  gravity squeezing inward and pressure from
  nuclear fusion and radiation pushing outward.

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Basic Structure of Stars

• Fusion

• Inside a star, the density and temperature
  increase toward the center, where energy is
  generated by nuclear fusion.
• Stars on the main sequence all produce energy by
  fusing hydrogen into helium, as the Sun does.
  Stars that are not on the main sequence either
  fuse different elements in their cores or do not
  undergo fusion at all.

32
Stellar Evolution and Life Cycles

• Star Formation

• A nebula (pl. nebulae) is a cloud of interstellar
  gas and dust.

• Star formation begins when the nebula collapses
  on itself as a result of its own gravity.
• A protostar is a hot condensed object that forms
  at the center of the disk that will become a new
  star.

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Stellar Evolution and Life Cycles

• Fusion Begins

• Eventually, the temperature inside a protostar
  becomes hot enough for nuclear fusion reactions
  to begin converting hydrogen to helium.

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The Suns Life Cycle

• What happens during a stars life cycle depends
  on its mass.

• When the hydrogen in its core is gone, a star
  has a helium center and outer layers made of
  hydrogen-dominated gas.
• Some hydrogen continues to react in a thin layer
  at the outer edge of the helium core causing the
  outer layers to expand forming a red giant.

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The Suns Life Cycle

• While the star is a red giant, it loses gas from
  its outer layers while its core becomes hot
  enough for helium to react and form carbon.
• When the helium in the core is all used up, the
  star is left with a core made of carbon.

• The outer layers expand once again and are driven
  off entirely by pulsations that develop, becoming
  a shell of gas called a planetary nebula.
• In the center of a planetary nebula, the core of
  the star remains as a white dwarf made of
  carbon.

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The Suns Life Cycle

• A Nebula Once Again

37
The Suns Life Cycle

• Pressure in White Dwarfs

• A white dwarf is stable because it is supported
  by the resistance of electrons being squeezed
  close together and does not require a source of
  heat to be maintained.
• A star that has less mass than that of the Sun
  has a similar life cycle, except that helium may
  never form carbon in the core, and the star ends
  as a white dwarf made of helium.

38
Life Cycles of Massive Stars

• A massive star begins its life high on the main
  sequence with hydrogen being converted to
  helium.

• A massive star undergoes many reaction phases and
  produces many elements in its interior.

• The star becomes a red giant several times as it
  expands following the end of each reaction stage.

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Life Cycles of Massive Stars

• As more shells are formed by the fusion of
  different elements, the star expands to a larger
  size and becomes a supergiant.

• A massive star loses much of its mass during
  its lifetime.
• White dwarf composition is determined by how
  many reaction phases the star went through before
  reactions stopped.

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Life Cycles of Massive Stars

• Neutron stars and Pulsars

• A star that begins with a mass between about 8
  and 20 times the Suns mass will end up with a
  core that is too massive to be supported by
  electron pressure.
• Once no further energy-producing reactions can
  occur, the core of the star violently collapses
  in on itself and protons and electrons in the
  core merge to form a neutron star.

41
Life Cycles of Massive Stars

• Supernovae

• A neutron star has a mass of 1.5 to 3 times the
  Suns mass but a radius of only about 10 km.

• Infalling gas rebounds when it strikes the hard
  surface of the neutron star and explodes outward.
• A supernova is a massive explosion in which the
  entire outer portion of the star is blown off and
  elements that are heavier than iron are created.

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Life Cycles of Massive Stars

• Black Holes

• A star that begins with more than about 20 times
  the Suns mass will not be able to form a neutron
  star.
• The resistance of neutrons to being squeezed is
  not great enough to stop the collapse, so the
  core of the star simply continues to collapse
  forever, compacting matter into a smaller and
  smaller volume.
• A black hole is a small, extremely dense remnant
  of a star whose gravity is so immense that not
  even light can escape its gravity field.