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Groups of Stars. Constellations are the 88 groups of stars named after animals, mythological ... Aries, Cancer, Canis Major, Draco, Hercules, Hydra, Leo, Libra, ... – PowerPoint PPT presentation

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Title: Stars

  • Chapter 30

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

Properties of the Sun
  • The solar interior is gaseous throughout because
    of its high temperatureabout 1 107 K in the
  • Many of the gases are in a plasma state
  • The outer layers of the Sun are not quite hot
    enough to be plasma.

The Suns Atmosphere
  • The photosphere, approximately 400 km in
    thickness, is the lowest layer of the Suns
  • This is the visible surface of the Sun because
    most of the light emitted by the Sun comes from
    this layer.

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.

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.

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.

Solar Activity
  • Solar Activity Cycle
  • The number of sunspots changes regularly, and on
    average reaches a maximum number every 11.2
  • The length of the solar activity cycle is 22.4
  • 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.

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.

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.

The Sun
  • A spectrum is visible light arranged according to
  • 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.

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.

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

Aries, Cancer, Canis Major, Draco, Hercules,
Hydra, Leo, Libra, Orion, Pegasus, Pices, Taurus
Ursa Minor, Virgo
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
  • In a globular cluster, stars are densely packed
    into a spherical shape.

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.

Stellar Position and Distances
  • Astronomers use two units of measure for long
  • 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.

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.

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

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.

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.

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

Basic Properties of Stars
  • 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.

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.

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.

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

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
  • 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.

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.

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.

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

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.

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.

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

The Suns Life Cycle
  • A Nebula Once Again

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.

Life Cycles of Massive Stars
  • A massive star begins its life high on the main
    sequence with hydrogen being converted to
  • 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.

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.

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.

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.

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
  • 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.
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