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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 nearinfrared wavelengths
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.
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
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
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
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
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.
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.
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
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
magnitude.
• The lower an object's absolute magnitude, the
higher its luminosity.
Mathematical relationship
• A mathematical equation relates apparent
magnitude with absolute magnitude, via
parallax
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
distance:
M= m+5 log10 do/d
• where dois 10 parsecs (≈ 32.616 lightyears) 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
• http://csep10.phys.utk.edu/astr162/lect/ligh
t/wien.html
Standards
• 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
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)
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
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