 Students will be able to determine how distances between stars are measured.  Students will be able to distinguish between brightness and luminosity

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  • Students will be able to determine how distances between stars are measured.Students will be able to distinguish between brightness and luminosity.Students will be able to identify properties used to classify stars.

  • Star: A self-luminous celestial body consisting of a mass of gas held together by its own gravity.

  • Page 837Main Idea: Stellar classification is based on measurement of light spectra, temperature, and composition.

  • Page 837Constellations: Groups of stars.There are 88 of them.Circumpolar Constellations: Visible in Northern Hemisphere all year. Summer, Fall, Winter, Spring: Only visible during certain seasons.

    Constellations are used by people to know when to prepare for planting, harvest, and ritual celebrations.

    Circumpolar StarTrails

  • Hercules: Visible in NH only during summerOrion: Visible in NH only during winterSeasonal Constellations

  • Page 838Star Clusters: Groups of stars that are gravitationally bound to each other.2 kinds:Open Cluster: Not densely packed.Globular Cluster: Densely packed into spherical shape.

  • Open Star ClusterGlobular Star Cluster

  • Page 838Binary Stars: Two stars that are gravitationally bound together and that orbit a common center mass.Hard to tell two stars apart.One star often is much brighter than the other.

  • Constellation in a cup

  • Page 840Stellar Position and Distances:2 Units used to measure long distances:Light Year (ly): The distance that light travels in 1 year = 9.461 x 10 km.Parsec (pc): 3.26 ly.12

  • Distances in Light Years

  • Page 840Parallax: Apparent shift in position of an object caused by the motion of the observer. Used to calculate distance.Earths orbit around the Sun means that an observers position in space is always changing.Distance to a star can be estimated using the stars parallax (angle of change.)Closer the star, the larger the shift.Estimates are accurate up to 500 pc away.

  • Page 8414 Basic Properties of Stars: MassDiameterLuminosityTemperature Estimated by finding spectral type of star. Controls nuclear reaction rate and governs luminosity and magnitude.

  • H-R Diagram

  • Page 842Magnitude: Apparent Magnitude: How bright a star appears.Brightest stars = +1Next brightest = +2A difference of 5 magnitudes = 100 x more bright.Ex) +1 is 100 x more bright than +6.

  • Page 842Magnitude: Absolute Magnitude: Accounts for distance (apparent does not.)How bright a star would appear if it were 10 pc away.

  • Page 842Luminosity: Energy output from surface of star per second a stars power.Requires that one know a stars apparent magnitude and distance away.Measure in watts energy emitted per second.Suns luminosity = 3.85 x 10 w.


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  • Page 483Classification of Stars: Word of the Day:Spectral Line: An isolated bright or dark line in a spectrum produced by emission or absorption of light of a single wave length.

  • Page 483Spectral Lines: Provide information about a stars temperature and composition.

  • Page 483Temperature: Corresponds to a stars spectral lines.Types: OHot50,000 K B A F G K MCool 2,000 KEach type is subdivided into more specific divisions by numbers 0 9.

  • Spectral Lines of StarsHotter StarsCooler Stars

  • Page 485Composition: All stars have similar compositions.Hot Stars: Less lines in spectra (fewer kinds of gasses.)Cool Stars: More lines in spectra (more kinds of gasses.)

    Typical Composition of Stars:73% hydrogen (H)25% helium (He)2% other elements.

  • Composition of StarsHeavier Elements = Anything heavier than Helium

  • Page 485Hertzprung Russell Diagram (H-R Diagram): Shows the lifetimes of stars.A graph on which absolute magnitude is plotted on vertical axis and temperature or spectral type is plotted on horizontal axis.

    Each class of star has a specific mass, luminosity, magnitude, temperature and diameter.

  • Page 485H-R Diagram: Plots evolution of stars from one class to another.Main Sequence: Area of the H-R Diagram in which most stars are located.Runs diagonally from upper left corner (Hot Bright Stars) to lower right corner (Cool Dim Stars.)

  • Page 486Main Sequence: Contains 90% of stars including Sun (which is at the center because it has an average temperature and luminosity.)Stars here fuse hydrogen.As hydrogen runs out stars fuse helium and begin to evolve off of the main sequence.

  • Page 486Mass determines a stars lifetime.More Mass = More Pressure = Shorter LifetimeHigher mass stars burn hydrogen faster.Lower Mass = Less Pressure = Longer LifetimeLower mass stars burn hydrogen slowly.

  • Page 486Red Giants and White Dwarfs:Red Giants: Top right of H-R diagram = Cool, Bright Stars.Because these stars are cool, but still bright, they must have large surface areas.Mass is more than 100 x greater than the Sun.

  • Page 486Red Giants and White Dwarfs:White Dwarfs: Lower left of H-R diagram = Hot, Dim Stars.Appear dim, though they are very hot, must have small surface areas.Size of Earth, but has a mass same as the Sun.


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