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Stars and Their Characteristics


Stars and Their Characteristics 28.2 * * * * * * Constellations Constellation- groups of stars that appear to form patterns 88 constellations can be seen from n. and ... – PowerPoint PPT presentation

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Title: Stars and Their Characteristics

Stars and Their Characteristics
  • 28.2

  • Constellation- groups of stars that appear to
    form patterns
  • 88 constellations can be seen from n. and s.
  • So far away that only after thousands of years
    might the motions be observed
  • Big Dipper- asterism (small-star grouping)
  • Part of Ursa Major- Great Bear

  • Polaris- north star
  • Doesnt appear to move
  • Circumpolar stars- seem to move around Polaris
    because Earth rotates
  • Position of Earth

Distance to Stars
  • Light year- distance light travels in a year
  • Used instead of kilometers/astronomical units
  • Parallax- basic way to measure star distance
  • The nearest stars have the largest parallax
    angles, while those of distant stars are too
    small to measure

Elements in Stars
  • sphere of mostly hydrogen and helium gases
  • small percentage may be heavier elements (oxygen,
    carbon, nitrogen, etc..)
  • no two stars contain exactly the same elements in
    the same proportions
  • wavelengths depend on both composition and

Mass, Size, and Temperature of Stars
  • mass can be determined by the inertial properties
    of the body or by its gravitational effect on the
    bodies around it
  • more mass more gravitational effect
  • stars vary more in size than they do in mass and
    even more in density
  • range of colors a star emits depends on its
    surface temperature
  • Blue hottest red coolest

Luminosity and Absolute Magnitude
  • Luminosity (apparent magnitude)- brightness of a
    star at its current distance from Earth
  • depends on its size and temperature
  • Bigger stars tend to be brighter
  • Bluer stars tend to be brighter
  • absolute magnitude- measure of how bright the
    star would be if all stars were at the same
    distance from Earth
  • The more negative the number, the brighter the

Object mv Mv
Sun -26.8 4.83
Sirius -1.47 1.41
Vega 0.04 0.5
Betelgeuse 0.41 -5.6
Polaris 1.99 -3
Variable Stars
  • variable stars are those stars that show regular
    variation of brightness
  • pulsating stars change brightness as they expand
    and contract
  • Cepheid variable- yellow supergiants whose cycles
    of brightness range from about 1 day to 50 days
  • the slower the cycle, the greater the luminosity
    of the star
  • can calculate the distances to galaxies in which
    they can identify Cepheid stars
  • non-pulsating star can change brightness because
    is part of a binary star system
  • results in eclipses

H-R Diagram
  • Hertzsprung-Russell Diagram
  • diagram plots the luminosity of stars against
    their surface temperatures
  • most stars (90) are in a band that runs from the
    upper left (high, high) to the lower right (low,
  • main sequence stars
  • main sequence stars vary in surface temperature
    and absolute magnitudes
  • commonality actively fusing hydrogen into helium

H-R Diagram cont
  • giant stars- great luminosity and diameter
    10-100x greater than sun
  • supergiants- higher luminosity diameter gt 100x
    than sun
  • white dwarfs- stars near end of life
  • once red giants that lost atmosphere

HR Diagram
Stellar Evolution
  • Birth of a Star
  • begins as a nebula- cloud of dust and gas (99
  • nebula may condense when an outside force acts
    upon it
  • particles move closer together under gravity
  • increase density increase temperature
  • if nebula glows, called protostar
  • center will become hotter until fusion takes
    place and a star is born

Nebula and Protostar
Stellar Evolution
  • Death of a Star (size of the sun)
  • remain same size (main sequence) for billions of
    years because energy produced through fusion
    equals gravitational pull
  • hydrogen is used up so gravity takes over, which
    then produces heat by contraction
  • entire star expands (Red Giant)
  • core temperature rises enough for helium to fuse
    into heavier elements, producing a carbon-oxygen
  • surface gases are blown away, leaving core (white
  • planetary nebula- glowing halo of gases
  • fades as gases dissipate into space leaving white
    dwarf behind
  • once all fuel is used up, the core will no longer
    glow, becomes a black dwarf

Main Sequence and Red Giant
White Dwarf, Planetary Nebula, and Black Dwarf
Stellar Evolution
  • Death of a Massive Star
  • fusion process will continue until iron nuclei
    are formed
  • absorbs energy, so iron core quickly collapses
  • supernova- explosion that produces a brilliant
    burst of light
  • produces many elements copper, uranium, silver,
    and lead
  • Remnants of Massive Stars
  • neutron star-dense mass of neutrons formed
    through gravity
  • pulsar- a distant neutron star that emits rapid
    pulses of light and radio waves instead of steady
  • black hole- remnant of a star at least 15 times
    as massive than the sun
  • gravitational force is so strong that light
    cannot escape

Supernova and Neutron Star
Pulsars and Black Holes