15-09-2007 Star Brightness’s (Article 103)

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Simon Kenny
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15-09-2007 Star Brightness’s (Article 103)

Post by Simon Kenny » Wed Feb 04, 2009 11:22 am

Star Brightness’s (Article 103)

This is a simplified overview of how the relative brightness’s of celestial objects are compared and given a value on a scale. It can be anything that shines in the sky, day or night, such as a planet, galaxy, nebula, our Sun and Moon, but for this discussion, the stars in particular.

Even a casual observer of the night sky will notice and wonder why some stars are brighter than others. From star maps you will find that each star is assigned a “mag.” with a numerical value e.g. during Sept. almost directly overhead are two quite bright stars close to each other, called Deneb and Vega. Deneb has a “mag.” of 1.3 and Vega is 0.03. The “mag.” referred to is apparent magnitude and this simply means “how bright the star appears as viewed from Earth” (instruments accurately measure the visual light output).

In 129BC Hipparchus, a Greek astronomer, invented an apparent magnitude scale by dividing all the stars visible to the naked eye into 6 classes of brightness, with the 1st magnitude being the brightest and the 6th being the faintest. (If Hipparchus had a 75mm refractor telescope he would have been able to see stars as faint as 11th magnitude). In the 19th century light measuring instruments showed that one magnitude was 2.5 times brighter or dimmer than the next magnitude. So, a magnitude 1 star was 100 times (2.5 x 2.5 x 2.5 x 2.5 x 2.5) brighter than a magnitude 6 star. Modern instruments can measure to thousandths of a magnitude. Objects brighter than magnitude 1 are given negative values e.g. planet Venus would be - 4.4, a full Moon -12.7, the Sun -26.8 and the brightest star in the sky, Sirius, is -1.5. With the unaided eye in a light polluted sky you can barely detect mag. 4.5 and from a very dark site mag. 6.0 is the limit. The Hubble Space Telescope can detect mag. 31.0, but then it is outside Earth’s blurring atmosphere.

Two things are worth remembering at this point, firstly, all the stars you can see are within our own galaxy (The Milky Way) and secondly, our galaxy is a very, very big place! So, a very bright luminous star located at a huge distance from Earth can appear fainter than a very dim star which is relatively close by. Distance matters!

Another magnitude measurement used by astronomers is called absolute magnitude. Imagine every star brought to a standard distance (33 light years) from Earth and their luminosities measured, you would have a fair comparison. Deneb, mentioned above, is 3,230 light years away with an apparent magnitude of 1.3. If placed 33 light years away it would have an absolute magnitude of - 8.73. Our Sun has an apparent magnitude of - 26.8, but an absolute magnitude of only 4.8. Deneb is a supergiant star and is 320,000 times more luminous than our Sun. Clear skies

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