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

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 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

• http://csep10.phys.utk.edu/astr162/lect/light/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