Louie Countryman, age 11, of Remsen, New York, for her question:
Why are the biggest stars not the brightest?
The heavenly bodies tend to fool our eyes and what we see up there is not necessarily so. The brightest star we see from the earth is sparkling white Sirius, 300 times more brilliant than the dimmest star our eyes can detect. However, some of the dimmer stars actually are far bigger and brighter than Sirius. They look pale because they are much farther away. When astronomers compute the true brilliance of a star, they must allow for distance, which is something our eyes cannot do.
The brilliance of a star is termed "magnitude" and there are two ways to estimate it. We can judge the magnitude of this or that star from what we see across the vast reaches of space. This visual evidence is termed "apparent magnitude." Astronomers compute apparent magnitude and distance with a lot of other factors to estimate the "real," or "absolute," magnitude of a star. No two stars are exactly alike, but it is possible to sort similar types into classes. They are graded according to color and temperature and these qualities reveal other interesting facts.
Color and temperature help to reveal how fast a star is burning up its supply of gaseous fuel. The faster it burns, the hotter it tends to be and the more light and other radiation it tends to shed. These factors help to estimate its age and predict its future.
As a rule, the stars in each class tend to be approximately the same age and at the same stage of their history. For example, one class includes a group of super hot, planet sized dwarf stars. These are very ancient stars made of densely packed gases. For their sizes, they shed intensely brilliant light. But they are too small for our eyes to see them across the vast reaches of space. Another class includes a group of red giants and super giants. These are young, rather cool stars that shed less light for their size. The whopping bluish stars in another class are seething hot and dazzling brilliant.
Actually, some of the biggest stars are rather dim and other whoppers rate among the most brilliant. Naturally, the light reflected from a neighboring planet cannot compare with the brilliance of the dimmest star. Yet, because it is so close, sometimes the planet Venus outshines all the stars in the sky. Similarly, our sun is a medium sized star, yet from the earth no other star can compare with its razzle dazzle brilliance. Out there are smaller stars that are brighter and others that are dimmer. There also are whoppers with enormous magnitudes and others that shed less light for their sizes.
Sparkling Sirius actually is much larger than our sun, a lot hotter and far more brilliant. It appears to be so bright because it happens to be merely 8.6 light years away. The red giant Antares is 250 light years away. It is a cool young star, only half as hot as the sun and 390 times wider. Antares shines big and bright in our summer skies because of its enormous size. Giant blue white Rigel is more than three times hotter than the sun and intensely brilliant. It shines brightly in our winter skies, even from a distance of more than 500 light years.