How Bright are the Stars?

Some stars are brighter than others. If you look up at the sky in Lynbrook you only see the brightest stars and there is a good chance the brightest “star” you see is actually a planet but more about that in a moment. How bright a star appears to us depends on several things - the brightness of the star itself and how far the star is from us are two major factors.

Although all the stars (except the Sun) are very far away from us, some are much further away than others. A very bright star that is very far away from us can appear dim while a relatively dim star that is closer to us can appear very bright. A star’s inherent brightness is called absolute magnitude while how bright the star appears to us on Earth is called the apparent magnitude. The absolute magnitude of a star depends on its size and where it is in its life cycle.

Hipparchus (190 to 120 BC), a Greek astronomer and mathematician, was the first to set up the stellar magnitudes. He is considered to be the greatest ancient astronomical observer and did a lot of observing of the Sun and the Moon. He also created a star catalog that was used, and improved upon, by many famous astronomers, such as Ptolemy and Copernicus.

Magnitude is recorded as 1st, 2nd, 3rd, etc. with the 1st magnitude being brighter than the second and so on. The lower the magnitude number, the brighter the star. Hipparchus’ magnitude scale was refined and formalized by Norman Pogson, an English astronomer, in 1856 by making each magnitude about 2.5 times as bright as the prior. So a magnitude 4 star is about 2.5 times as bright as a magnitude 5 star and a magnitude 3 star is about 2.5 times as bright as a magnitude 4 star. Once the magnitudes were formalized in this way some stars were found to be much brighter than others and we wound up with negative magnitudes.

For example, Sirius, in the constellation Canis Major, is the brightest star in our night sky with an apparent magnitude of -1.46. Pollux, in the constellation Gemini, has an apparent magnitude of +1.14. So Sirius appears to be much brighter than Pollux; however, that is just how it appears to us from Earth. Pollux has an absolute magnitude of +0.7 while Sirius has an absolute magnitude of +1.4. So if Pollux is actually a brighter star than Sirius, why doesn’t it appear to be brighter when we look at them in the night sky? The reason is that Sirius is much closer to Earth (8.6 light years) than Pollux which is much further away (40 light years). 8.6 light years is pretty far away but it is a lot closer than 40 light years.

So if you go outside on a clear night you will see the brightest stars, such as Sirius (-1.46) in the constellation Canis Major, Arcturus (-0.04) in the constellation Bootes, Vega (+0.03) in the constellation Lyra, Cappela (+0.08) in the constellation Auriga and Rigel (+0.12) in the constellation Orion. These are the 5 brightest stars we can see from Lynbrook at varying times during the year but you may see some “stars” that are even brighter. These other “stars” are actually planets. At varying times during the year we can easily see Venus, Mars, Jupiter and Saturn. With our naked eyes these planets appear to be bright stars but with a small telescope or binoculars it is easy to see that the planets are much different than the stars.

Stars, other than the Sun, are very far away from us and they all generate light by nuclear fusion. The planets do not generate light, they just reflect light from the Sun to us. When we look at a star in a telescope or binoculars we see a small dot of light because they are so far away but when we look at a planet we see a disk and sometimes we can see other things, as well, like the rings of Saturn, the bands of Jupiter, the polar ice cap of Mars and the phases of Venus. The planets are much closer to us than the stars and generally much more fun to look at.

Since planets change their distance from us quite a bit in their orbit around the Sun (and Earth’s orbit around the Sun) they appear brighter at certain times. The brightest planets are Venus (-4.9 to -3.8), Jupiter (-2.6 to +1.6), Mars (-2.9 and +1.8) and Saturn (-0.24 to +1.2). These planets can easily be seen from Lynbrook at various times of the year. Right now Jupiter is very visible in the early evening. The apparent magnitudes of the Sun (-27) and the Moon - full Moon (-13) and crescent Moon (-6), have also been measured.

Some stars also change their magnitude over time and are called variable stars. Stars can vary for several reasons such as binary stars can block each other at various times in their orbits around each other and a star may be going through changes in its life cycle causing it to expand or contract and become brighter or dimmer.

See if you can find a bright star or planet on the next clear night, but don’t stay out long, it’s pretty cold out there.

Hipparchus

http://en.wikipedia.org/wiki/Hipparchus

Norman Pogson

http://en.wikipedia.org/wiki/N._R._Pogson

Stellar Magnitudes

http://www.skyandtelescope.com/howto/basics/Stellar_Magnitude_System.html

Variable Stars

http://www.aavso.org/variables-what-are-they-and-why-observe-them

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