Perhaps the most important characteristic of a star is its luminosity —the total amount of energy at all wavelengths that it emits per second. Earlier, we saw that the Sun puts out a tremendous amount of energy every second. And there are stars far more luminous than the Sun out there. For example, the luminosity of Sirius is about 25 times that of the Sun. In a later chapter, we will see that if we can measure how much energy a star emits and we also know its mass, then we can calculate how long it can continue to shine before it exhausts its nuclear energy and begins to die.
Astronomers are careful to distinguish between the luminosity of the star the total energy output and the amount of energy that happens to reach our eyes or a telescope on Earth. Stars are democratic in how they produce radiation; they emit the same amount of energy in every direction in space. Consequently, only a minuscule fraction of the energy given off by a star actually reaches an observer on Earth.
If you look at the night sky, you see a wide range of apparent brightnesses among the stars. Most stars, in fact, are so dim that you need a telescope to detect them. If all stars were the same luminosity—if they were like standard bulbs with the same light output—we could use the difference in their apparent brightnesses to tell us something we very much want to know: how far away they are.
Imagine you are in a big concert hall or ballroom that is dark except for a few dozen watt bulbs placed in fixtures around the walls. Since they are all watt bulbs, their luminosity energy output is the same.
But from where you are standing in one corner, they do not have the same apparent brightness. Those close to you appear brighter more of their light reaches your eye , whereas those far away appear dimmer their light has spread out more before reaching you. In this way, you can tell which bulbs are closest to you. In the same way, if all the stars had the same luminosity, we could immediately infer that the brightest-appearing stars were close by and the dimmest-appearing ones were far away.
To pin down this idea more precisely, recall from the Radiation and Spectra chapter that we know exactly how light fades with increasing distance. The energy we receive is inversely proportional to the square of the distance. If, for example, we have two stars of the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one. If it is three times farther away, it will look nine three squared times dimmer, and so forth.
Alas, the stars do not all have the same luminosity. Actually, we are pretty glad about that because having many different types of stars makes the universe a much more interesting place. But this means that if a star looks dim in the sky, we cannot tell whether it appears dim because it has a low luminosity but is relatively nearby, or because it has a high luminosity but is very far away. To measure the luminosities of stars, we must first compensate for the dimming effects of distance on light, and to do that, we must know how far away they are.
Distance is among the most difficult of all astronomical measurements. Some stars are simply more luminous than others are, and the brightness level varies greatly. The other factor is proximity. The sun is technically the brightest star when viewed from Earth, but it is also the closest star to Earth. In reality, the sun is in the middle of the brightness range when compared to other stars. Stargazers might wonder then, why is Venus, which is not a star at all, much brighter than many stars when viewed from Earth?
The answer is simple, and it is a factor when considering brightness. Venus does not give off its own light, but it does reflect sunlight toward Earth. In fact, this is the same reason the moon is visible at night. All Star Registration Documents are ready for you to download and delivered to the gift recipient immediately after the star is named.
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All Rights Reserved — Terms and Conditions. For a limited time, NameAStarLive. When you look at the night sky, you can see that some stars are brighter than others as shown in this image of Orion. A searchlight puts out more light than a penlight. That is, the searchlight is more luminous. If that searchlight is 5 miles 8 kilometers away from you, however, it will not be as bright because light intensity decreases with distance squared.
A searchlight 5 miles from you may look as bright as a penlight 6 inches 15 centimeters away from you. The same is true for stars. Astronomers professional or amateur can measure a star's brightness the amount of light it puts out by using a photometer or charge-coupled device CCD on the end of a telescope. If they know the star's brightness and the distance to the star, they can calculate the star's luminosity:.
Luminosity is also related to a star's size. The larger a star is, the more energy it puts out and the more luminous it is.
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