H-R DIAGRAM

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H-R DIAGRAM

• HERTSPRUNG-RUSSELL
• DIAGRAM

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• The surface temperatures of main sequence stars
  range from about 3000 K (spectral class M) to
  over 30,000 K (spectral class O).
• This relatively small temperature rangeonly a
  factor of 10is determined mainly by the rates at
  which nuclear reactions occur in stellar cores.
• In contrast, the observed range in luminosities
  is very large, covering some eight orders of
  magnitude (that is, a factor of 100 million) and
  ranging from 10-4 to 104 times the luminosity of
  the Sun.

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• Astronomers can use the radius to luminosity to
  temperature relationship (L IS PROPORTIONAL TO
  Re2 X T e 4 ) to estimate the radii (R) of
  main-sequence stars from their temperatures (T)
  and luminosities (L).
• They find that in order to account for the
  observed range in luminosities, stellar radii
  must also vary along the main sequence.
• The faint, red M-type stars in the bottom right
  of the HR diagram are only about 1/10 the size
  of the Sun, whereas the bright, blue O-type stars
  in the upper left are about 10 times larger.

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• We see a very clear trend as we traverse the main
  sequence from top to bottom.
• At one end, the stars are large, hot, and bright.
  Because of their size and color, they are
  referred to as blue giants.
• At the other end, stars are small, cool, and
  faint. They are known as red dwarfs. Our Sun lies
  right in the middle.

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WHITE DWARFS AND RED GIANTS

• About 90 percent of all stars in our solar
  neighborhood, and probably a similar percentage
  elsewhere in the universe, are main-sequence
  stars.
• About 9 percent of stars are white dwarfs
• About 1 percent are red giants.

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LUMINOSITY CLASS

• The width of a spectral line can provide
  information on the density of the gas where the
  line formed.
• The atmosphere of a red giant is much less dense
  than that of a main-sequence star, and this in
  turn is much less dense than the atmosphere of a
  white dwarf.
• By studying the width of a star's spectral lines,
  astronomers can usually tell, with a high degree
  of confidence, whether or not it is on the main
  sequence.

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• Over the years, astronomers have developed a
  system for classifying stars according to the
  width of their spectral lines.
• Because the line width is particularly sensitive
  to density in the stellar photosphere, and the
  atmospheric density in turn is well correlated
  with luminosity, the class in which a star is
  categorized has come to be known as its spectral
  class.

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• This classification provides a means for
  astronomers to distinguish supergiants from
  giants, giants from main-sequence stars, and
  main-sequence stars from white dwarfs by studying
  a single spectral propertythe line broadeningof
  the radiation received.

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• CLASS DESCRIPTION
• Ia Bright supergiants
• Ib Supergiants
• II Bright giants
• III Giants
• IV Subgiants
• V Main-sequence stars/dwarfs

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BINARY STARS

• Most stars are members of multiplestar
  systemsgroups of two or more stars in orbit
  around one another.
• The majority of stars are found in binary
  clusters, which consist of two stars in orbit
  about their common center of mass, held together
  by their mutual gravitational attraction.
• Most complex of all are the star clusters

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• VISUAL BINARIES have widely separated members
  that are bright enough to be observed and
  monitored separately
• Spectroscopic binary A binary-star system which
  from Earth appears as a single star, but whose
  spectral lines show back-and-forth Doppler shifts
  as two stars orbit one another.
• Recall that motion toward an observer blueshifts
  the lines, and motion away from the observer
  redshifts them.

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• In a doubleline spectroscopic binary, two
  distinct sets of spectral linesone for each
  component starshift back and forth as the stars
  move. Because we see particular lines alternately
  approaching and receding, we know that the
  objects emitting the lines are in orbit.
• In the more common single-line systems, one star
  is too faint for its spectrum to be
  distinguished, so only one set of lines is
  observed to shift back and forth.

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• In the much rarer , the orbital plane of the pair
  of stars is almost edgeon to our line of sight.
  In this situation, we observe a periodic decrease
  of starlight as one component passes in front of
  the other. By studying the variation of the light
  from the binary systemthe binary's astronomers
  can derive detailed information not only about
  the stars' orbits and masses but also about their
  radii.

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• optical doubles are just chance superpositions
  and carry no useful information about stellar
  properties.

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• Mass, more than any other stellar property,
  determines a star's position on the main
  sequence. Stars that form with low mass will be
  cool and faint they lie at the bottom of the
  main sequence. Very massive stars are hot and
  bright they lie at the top of the main sequence.

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• With few exceptions, main-sequence stars range in
  mass from about 0.1 to 20 the mass of the Sun.
• The hot, O and B type, stars are generally about
  10 to 20 times more massive than our Sun.
• The coolest, K and M type stars, contain only a
  few tenths of a solar mass.
• the mass of a star, at the time of formation,
  determines its location on the main sequence.

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messier 8
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globulars m2
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m3
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STELLAR LIFETIMES

• We can estimate a star's lifetime simply by
  dividing the amount of fuel available (the mass
  of the star) by the rate at which the fuel is
  being consumed (the star's luminosity)
• STELLAR LIFETIME IS PROPORTIONAL TO
• STELLAR MASS
• STELLAR LUMINOSITY cubed

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• Because the massluminosity relation tells us
  that a star's luminosity is roughly proportional
  to the cube of its mass, we can rewrite this
  expression to obtain, approximately,
• STELLAR LIFETIME IS PROPRTIONAL TO
• _______ 1________
• STELLAR MASS squared

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• massive stars can survive only for short times.
  Their nuclear reactions proceed so rapidly that
  their fuel is quickly depleted despite their
  large masses.
• We can be sure that all the O and B stars we now
  observe are quite youngless than a few tens of
  millions of years old. Massive stars older than
  that have already exhausted their fuel and no
  longer emit large amounts of energy. They have,
  in effect, died.

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• At the opposite end of the main sequence, the
  cooler K and Mtype stars have less mass than
  our Sun. With their low core densities and
  temperatures, their protonproton reactions churn
  away rather sluggishly, much more slowly than
  those in the Sun's core.
• Many of the K and Mtype stars now seen in the
  sky will shine on for at least another trillion
  years.

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STAR CLUSTERS

• If we observe a group of stars that all lie at
  the same distance from us, then comparing
  apparent brightnesses is equivalent to comparing
  absolute brightnesses.
• Star clusters can include anywhere from a few
  dozen to a million stars in a region a few
  parsecs across
• OPEN CLUSTERS - PLIEADES CLUSTER

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GLOBULAR CLUSTERS

• Globular clusters are much more tightly knit than
  the loose groups of stars that make up open
  clusters. All globular clusters are roughly
  spherical (which accounts for their name) and
  contain hundreds of thousands, and sometimes
  millions, of stars spread out over about 50 pc.

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• The distance to this cluster has been determined
  by a variation on the method of spectroscopic
  parallax but applied to the entire cluster rather
  than to individual stars. From calculations of
  the distance at which the apparent brightnesses
  of the cluster's stars taken as a whole best
  match theoretical models, the cluster is found to
  lie about 5000 pc from Earth.

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• We now know that, although low-mass red stars and
  intermediate-mass yellow stars abound, globular
  clusters contain no main-sequence stars with
  masses greater than about 0.8 the mass of the
  Sun.
• Apparently, globular clusters formed long ago
  the more massive O through F stars have long
  since exhausted their nuclear fuel and
  disappeared from the main sequence (becoming the
  red giants and other luminous stars above the
  main sequence

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• Other factors confirm that globular clusters are
  old. For example, their spectra show few heavy
  elements, implying that these stars formed in the
  distant past when heavy elements were much less
  abundant than they are today.

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• On the basis of these and other observations,
  astronomers estimate that all globular clusters
  are at least 10 billion years old. They contain
  the oldest known stars in the Milky Way Galaxy.
  As such, globular clusters are considered to be
  remnants of the earliest stages of our Galaxy's
  existence.