Leeds Astronomical Society LAS Meetings Observing Membership

 

 

Sadr region

(James Clark - RGB)
(James Clark - RGB Starless)
(James Clark - HSO palette)
(James Clark - greyscale)

Information...

Here is a widefield image of the region around the star Sadr (γ Cyg) in the constellation of Cyngus, with Sadr being the bright star on the middle-left hand side. Also in the image, to the middle-right, is NGC6888 - the Crescent nebula.

This image was taken with a Canon zoom lens and a narrow band filter that captures light from Hydrogen (Hα at 656.3nm & Hβ at 486.1nm) and doubly ionised Oxygen (OIII at 500.7 & 495.9nm). It has then been processed in a number of different ways, with the HSO palette being where Hα is mapped to the Red channel, Hβ & OIII are mapped to Blue, and a blend of Red/Blue is used to create the Green channel. The RGB images likewise make use of this artificial Green channel and for the greyscale image, the luminance has been extracted.

 

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Map

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Measuring Angles

Hold your arm at full length, then close one eye & use the hand shapes shown above to measure the angular distance between the stars.

(Ain't anatomy wonderful!)

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Apparent Magnitude

The apparent magnitude of a star is a measure of how bright it appears from Earth. The scale was introduced over 2,000 years ago by the Greek astronomer Hipparchus, who grouped stars into six categories. The brightest 20 or so were deemed to be 'first magnitude', slightly dimmer stars 'second magnitude', and so on until the barely visible stars were classed as 'sixth magnitude'.

Later it was recognised that our eyesight, once it has been given time to get used to darkness, has a logarithmic response. i.e. a Mag. 1 star is actually 2.512 times brighter than a Mag. 2 star, or 6.310 times brighter than a Mag. 3 star (2.512 x 2.512 = 6.310).

The six Magnitudes thus corresponds to a 2.5126 difference in brightness or 100x.

Apparent magnitude

Today the scale has now been extended, so that brighter objects can have an apparent magnitude of 0 or even negative. The brightest star Sirius, for example, has an apparent magnitude of -1.44 and the Sun is a whopping -26.74, due to it's close proximity to Earth.