Stars come in all colors and sizes. But our eyes limit us to seeing very few of those colors. And the range of apparent magnitudes of stars we can see with our naked eye is also rather limited. Depending on how dark it is and how good your eyes are, the dimmest visible stars are a magnitude of about 6.5. The brightest star, Sirius, at magnitude -1.47, is more than 1,000 times brighter than the dimmest stars, but that is still a much smaller factor than should be possible for stars.
But how bright a star looks depends greatly on how far away that star is. In order to study the wide variety of stars, astronomers have come up with a classification system that overcomes the differences in distance and helps to understand not only the physical characteristics of a star but also how old a star might be and how it might evolve during its lifetime.
One of the most important features of a star is its luminosity, which is how much energy that star is producing. We could give that energy in watts, so we could compare it to light bulbs, but the numbers would be rather unwieldy. For instance, the sun produces about 3.8x(10 to the 26th) watts.
Instead of watts, we can use the more familiar magnitude scale. When we look at a star, we assign the apparent magnitude simply by how bright it looks. To compare the luminosity between one star and the next, we need to use what is called its absolute magnitude. Absolute magnitude is the magnitude the star would appear to be if it was located a distance of 10 parsecs, or about 33 light years, away from the solar system. That is a very arbitrary distance, but at least it is consistent and would tell you how bright stars would be if they were all the same distance away.
If the sun was 10 parsecs away, its apparent magnitude would be 4.83. Such a fifth magnitude star would be easily visible but rather dim. That means the absolute magnitude of the sun is 4.83, which is a good reference point for comparing it to other stars.
Conveniently, the sun is near the middle of the range of observed absolute magnitudes. And for all observed stars, this range is tremendous. The dimmest stars can be more than 10,000 times less luminous than the sun. However, there is a lower limit, because if a star is too small, then fusion would never start in its core. We call such objects brown dwarfs and their characteristics are more like Jupiter than the sun.
The most luminous stars easily produce more than 10,000 times more energy than the sun. But burning fuel that fast gives the most luminous stars a very short lifetime. If one of these most luminous stars was only 10 parsecs from the solar system, it would appear in our sky brighter than a full moon.
Besides luminosity, the other parameter of interest in classifying a star is its spectral type. This basically describes the color of the star, and the color is directly related to its temperature. But I will have to save the discussion of spectral type for next month.
This monthThe Lyrid meteor shower is one of average intensity and peaks on the evening of April 22. But Lyrid meteors, from the dust of comet C/1861 G1 Thatcher, might be seen from Tuesday to April 25.
Venus is now rising a little before 6 a.m. as a morning star. For the next week or so, Mercury can be seen within 5 degrees of Venus – below and to the left of it.
Mars is in Taurus, which is in the western sky in the early evening. It is between the two open clusters Pleiades and Hyades. At magnitude 1.5, it is much dimmer than it was last summer. Nearby Aldebaran, the bright red star in the Hyades, at magnitude 0.84 is much brighter than Mars now.
Jupiter and Saturn are both near the Milky Way and rising well after midnight.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at firstname.lastname@example.org.