As darkness falls on these balmy July evenings, the famous "Summer Triangle" is high in the eastern sky. The triangle is composed of three of the brightest stars in the sky, each the brightest star in its own constellation.
The brightest is the bluish-white star Vega, in Lyra the Lyre. Next in brightness is yellow-white Altair in Aquila, the Eagle. Finally there is white Deneb, in Cygnus, the Swan.
Sometimes when I find myself outdoors and giving an impromptu talk on the night sky, Ill direct my audiences attention to the Summer Triangle. Ill then ask two questions.
First, Ill ask in what order of brightness are the three stars, from the brightest to the faintest? This question is rather easy to answer since it is obvious that Vega is by far the brightest of the trio, appearing to shine twice as bright as Altair and more than three times brighter than Deneb.
able --> Then, Ill follow up with my second question:
In terms of luminosity, what would be the ranking of the three stars? In other words, which star is the most luminous of the trio?
Shedding light
If someone fails to understand the difference between brightness and luminosity, Ill bring up the following analogy: Pretend youre outside at night with two friends. One of them has a powerful 5-cell flashlight and the other a tiny penlight flashlight. Your friend with the 5-cell flashlight walks a distance of 600-feet before he turns the light on and ultimately shines it toward you.
Now your other friend walks a distance of only ten feet, and from that distance shines the tiny penlight bulb in your direction.
Question: from your vantagepoint, which appears brighter . . . the light from the 5-cell flashlight or the much-smaller penlight flashlight? Obviously, it is the penlight.
But now . . . which light is the more luminous? Obviously, it is 5-cell flashlight. But since its 60 times farther away, it doesnt appear as bright as the much closer penlight.
Back to space
The same explanation can be used when speaking of the luminosity of the three stars in our Summer Triangle.
Astronomers know that Vega clearly is more luminous compared to Altair, because its situated at a greater distance from us. Altair is just 17 light-years away, while Vega is 25. So, the light you see from Vega now started on its journey to Earth back in 1978; from Altair were looking at light from 1986. As compared to our Sun, Altair is about 1 times larger and nine times brighter. Vega, however, is more than three times larger and 58 times more luminous.
But both Vega and Altair pale in comparison with Deneb, one of the greatest supergiant stars known. Deneb is 1,467 light years from Earth; its is more than 85,000 times that of our Sun.
Astronomers utilize absolute magnitudes for the stars. This is the brightness that a star would have if all the stars were placed at the same distance from us. That distance is equal to 10 parsecs, or 32.6 light years.
Trading places
Were we able to move our Sun out to this distance, it would appear to shine at a paltry magnitude of +4.9, meaning that it would appear as a moderately faint star; most people would need a sky chart in order to identify it.
Altair would appear to shine at magnitude +2.1, just a trifle fainter than Polaris, the North Star. Vega would glow at magnitude +0.6, just a little brighter than Altair appears to us.
But Deneb would appear absolutely dazzling. Shining at magnitude 7.5, it would be readily visible both night and day, and appearing nearly 13 times brighter than Venus! But because its light takes nearly 15 centuries to reach us, Deneb merely appears in our summer sky as a fairly conspicuous but by no means particularly notable star.
Which only goes to prove that even in astronomy, perhaps especially in astronomy, looks can be deceiving.
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