The Pleiades

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The Pleiades

• Lab 6

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The Pleiades
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(No Transcript)
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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.

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Open Star Clusters
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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

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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

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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

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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

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Why?

• 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.

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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

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And so it remained for 19 centuries..

• 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

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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

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Modern Version

• The modern system is no longer limited to 6
  magnitudes
• 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

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UBV system

• Magnitude is complicated by the fact that light
  is not monochromatic
• 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.

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Under-representation

• 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.

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Note!

• 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

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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

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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
  magnitude.
• The lower an object's absolute magnitude, the
  higher its luminosity.

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Mathematical relationship

• A mathematical equation relates apparent
  magnitude with absolute magnitude, via parallax
• M m5(1log10 p/p0
• where p stars parallax and p01 arcsec

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Diagram of parsec
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Absolute Magnitude

• Or by using the absolute magnitude of a star if
  given its apparent magnitude and distance
• M m5 log10 do/d
• where dois 10 parsecs ( 32.616 light-years) and
  d is the star's distance

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Apparent Magnitude

• Given the absolute magnitude, the apparent
  magnitude can be calculated from any distance
• m M 5 log10 d0/d

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Applet for U-B and B-V

• http//csep10.phys.utk.edu/astr162/lect/light/wien
  .html

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Standards

• In stellar and galactic astronomy, the standard
  distance is 10 parsecs ( 32.616 light years, or
  31014 km).
• A star at ten parsecs has a parallax of 0.1" (100
  milli arc seconds).
• 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)

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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

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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 parsecs.
• 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