The Pleiades

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					The Pleiades

    Lab 6
The Pleiades
                The Pleiades
• An open cluster is a group of up to a few
  thousand stars that were formed from the same
  giant molecular cloud, and are still gravitationally
  bound to each other
• Open clusters are found only in spiral and
  irregular galaxies, in which active star formation
  is occurring.
• The Pleiades is an open cluster, contains over
  3000 stars, is ~400 light years away, and only
  13 light years across
• Low mass, faint, brown dwarfs have recently
  been found in the Pleiades.
Open Star Clusters
         The Pleiades stars
• The stars in the Pleiades are thought to
  have formed together around 100 million
  years ago, making them 1/50th the age of
  our sun, and they lie some 130 parsecs
  (425 light years) away
• The Pleiad(e)s were the seven daughters
  of Atlas and Pleione
                 The Myth
• One day the great hunter Orion saw the Pleiads
  as they walked through the countryside, and
  fancied them
• He pursued them for seven years, until Zeus
  answered their prayers for delivery and
  transformed them into doves placing them
  among the stars
• Later on, when Orion was killed, he was placed
  in the heavens behind the Pleiades,
  immortalizing the chase
            Pleiad members
• The 9 most prominent stars have individual
  Greek names and represent each of the 7
  sisters and their parents
• The brightest member in the Pleiades is
  Alcyone, which has an apparent magnitude of
  about 2.9
• The other stars in the cluster that have Greek
  names include Merope, Celaeno, Sterope
  (which is actually a double star), Taygeta, Maia,
  Electra, Atlas, and Pleione
• Their apparent visual magnitudes range from 3.8
  to 5.5
        What is Magnitude?
• In astronomy, magnitude refers to the
  logarithmic measure of the brightness of
  an object, measured in a specific
  wavelength, usually in optical or near-
  infrared wavelengths
• The brightness of a star depends not only on
  how bright it actually is, but also on how far
  away it is
• a street light appears very bright directly
  underneath it, but not as bright 1/2 a mile away
• Therefore, astronomers developed the
  "absolute" brightness scale.
• Absolute magnitude is defined as how bright a
  star would appear if it were exactly 10 parsecs
  (~33 light years) away from Earth.
          The Magnitude Scale
• Magnitude is measured by sorting stars visible to the
  naked eye into six magnitudes
• The brightest stars were considered first magnitude, or
  (m = +1), while the faintest were of sixth magnitude or
  (m = +6)
• Why? Because the limit of human visual perception is
  sixth magnitude
• Each grade of magnitude was considered to be twice the
  brightness of the following grade
• Since the response of the eye to light is logarithmic, the
  resulting scale is also logarithmic
        And so it remained for 19
• In 1856, Pogson formalized the system by defining a first
  magnitude star as a star which is 100 times brighter than
  a sixth magnitude star
• So a first magnitude star is about 2.512 times brighter
  than a second magnitude star.
• The fifth root of 100, an irrational number ~ 2.512 is
  known as Pogson's Ratio.
• Pogson's scale was originally fixed by assigning Polaris
  a magnitude of 2.
• Astronomers later discovered that Polaris is slightly
  variable, so they switched to Vega as the standard
  reference star
           How interesting!
• When astronomers began to accurately
  measure the brightness of stars using
  instruments, it was found that each
  magnitude is about 2.5 times brighter than
  the next greater magnitude.
• This means a difference in magnitudes of
  5 units (as in from magnitude 1 to
  magnitude 6) corresponds to a change in
  brightness of 100 times
             Modern Version
• The modern system is no longer limited to 6
• Really bright objects have negative magnitudes
• Sirius, the brightest star of the celestial sphere,
  has an apparent magnitude of −1.44 to −1.46
• The Moon has an apparent magnitude of −12.6
• The Sun has an apparent magnitude of −26.8
  The Hubble and Keck telescopes have located
  stars with magnitudes of +30
                    UBV system
• Magnitude is complicated by the fact that light is not
• For this purpose the UBV system is widely used, in
  which the magnitude is measured in three different
  wavelength bands
   – U (centred at ~350 nm, in the near ultraviolet)
   – B (~435 nm, in the blue region)
   – V (~555 nm, in the middle of the human visual range)
• The V band gives magnitudes closely corresponding to
  those seen by the human eye
• When an apparent magnitude is given without any
  further qualification, it is usually the V magnitude that is
  meant, more or less the same as visual magnitude.
• Since cooler stars, such as red giants and
  red dwarfs, emit little energy in the blue
  and UV regions of the spectrum their
  power is often under-represented by the
  UBV scale
• In fact, some L and T class stars would
  have a magnitude of well over 100 if we
  could see in the infrared.
• On traditional photographic film, the relative
  brightnesses of the BLUE supergiant Rigel and
  the RED supergiant Betelgeuse are reversed
  compared to what our eyes see since film is
  more sensitive to blue light than it is to red light
• For an object with a given absolute magnitude, 5
  is added to the apparent magnitude for every
  tenfold increase in the distance to the object
    apparent magnitude (m)
• a measure of its apparent brightness
  which is the amount of light received from
  the object
• The dimmer an object appears, the higher
  its apparent magnitude
• Hundred times less bright - the same
  object ten times as far - corresponds to an
  apparent magnitude that is five more
         Absolute Magnitude
• In defining absolute magnitude it is necessary to
  specify the type of electromagnetic
  radiationbeing measured.
• When referring to total energy output, the proper
  term is bolometric magnitude.
• The dimmer an object (at a distance of 10
  parsecs) would appear, the higher its absolute
• The lower an object's absolute magnitude, the
  higher its luminosity.
    Mathematical relationship
• A mathematical equation relates apparent
  magnitude with absolute magnitude, via
     M = m+5(1+log10 π/π0
where π = star’s parallax and π0=1 arcsec
Diagram of parsec
        Absolute Magnitude
• Or by using the absolute magnitude of a
  star if given its apparent magnitude and
            M= m+5 log10 do/d
• where dois 10 parsecs (≈ 32.616 light-
  years) and d is the star's distance
       Apparent Magnitude
• Given the absolute magnitude, the
  apparent magnitude can be calculated
  from any distance
      m = M – 5 log10 d0/d
      Applet for U-B and B-V
• In stellar and galactic astronomy, the standard distance
  is 10 parsecs (~ 32.616 light years, or 3×1014 km).
• A star at ten parsecs has a parallax of 0.1" (100 milli arc
• Many stars visible to the naked eye have an absolute
  magnitude which is capable of casting shadows from a
  distance of 10 parsecs; Rigel (-7.0), Deneb (-7.2), Naos
  (-7.3), and Betelgeuse (-5.6).
• For comparison, Sirius has an absolute magnitude of 1.4
  and the Sun has an absolute visual magnitude of 4.83 (it
  actually serves as a reference point)
               Parsec Review
• The parsec is a unit of length used in astronomy
• It stands for "parallax of one arc second".
• It is based on the method of trigonometric parallax, an
  old standard method of determining stellar distances
• The angle subtended at a star by the mean radius of the
  Earth's orbit around the Sun is called the parallax.
• The parsec is defined to be the distance from the Earth
  of a star that has a parallax of 1 arcsecond
• Alternatively, the parsec is the distance at which two
  objects, separated by 1 astronomical unit appear to be
  separated by an angle of 1 arcsecond
        Parsecs and Parallax
• There is no star whose parallax is 1 arcsecond.
• The greater the parallax of the star the closer it
  is to the Earth, and the smaller its distance in
• Therefore the closest star to the Earth will have
  the largest measured parallax.
• This belongs to the star Proxima Centauri, with a
  parallax of 0.772 arcseconds, and thus lying
  ~1.29 parsecs, or 4.22 light-years, away

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