The Nighttime Sky Holds Many Stars

The nighttime sky looks like it has millions of stars visible to observers. Thats because we live in a galaxy that has hundreds of millions of them. However, we cant really see all of them with the naked eye from our backyards. It turns out that the skies of Earth have, at most, around ten thousand stars that can be spotted with the naked eye. However, not everyone can see all the stars; they see only whats overhead in their own region. Light pollution and atmospheric hazes reduce the number of stars that can be seen even more. On average, however, the most anyone can really see (with very good eyesight and from a very dark viewing area) is around three thousand stars. People living in very big cities still see a few stars, while those in country areas away from lights can see more.   The best places to see stars are dark-sky sites, such as Canyonlands National Park or from onboard a ship in the middle of the ocean, or high in the mountains. Most people do not have access to such areas, but they can get away from most city lights by going out into the countryside. Or, if viewing from in the city  is someones only choice, they can pick an observing spot that is shaded from nearby lights. That increases the chances of seeing a few more stars.   If our planet was in a region of the galaxy with a lot more stars, chances are stargazers really WOULD see tens of thousands of stars at night. Our section of the Milky Way is, however, less well-populated than the core for example. If our planet could be in the center of the galaxy, or perhaps in a globular cluster, the sky would shimmer with starlight. In fact, in a globular cluster, we might never have dark skies! In the center of the galaxy, we might be stuck in a cloud of gas and dust, or perhaps be subjected to forces from the black hole at its heart. So, in a way, while our location in the outskirts of the Milky Way reveals fewer stars to stargazers, its a safer place to have a planet with dark skies.   Stargazing Among the Visible Stars So, what can be learned from the stars that observers CAN see? For one thing, people often notice that some stars appear white, while others are bluish, or orangey or reddish.   Most, however, appear to be a dull white.  Where does the color come from? The stars surface temperature gives a clue—the hotter they are, the more blue and white they are. The redder they are, the cooler they are. So, a blue-white star is hotter than a yellow or orange star, for example. Red stars are usually fairly cool (as stars go). Its important to remember, however, that a stars color isnt vivid, its more likely very pale or pearlescent. Also, the materials that make up a star (that is, its composition) can make it look red or blue or white or orange. Stars are primarily hydrogen, but they can have other elements in their atmospheres and interiors. For example, some stars that have a lot of the element carbon in their atmospheres look redder than other stars.   Figuring out Brightness of Stars Among those three thousand stars, observers can also notice differences in their brightnesses. A stars brightness is often referred to as its magnitude and thats simply a way to put numbers to the different brightnesses we see among all the stars. What affects that brightness? A couple of factors come into play. A star can look bright or dim depending on far away it is. But, it can also look bright because its very hot. Distance AND temperature play a role in magnitude.   A very hot, bright star that lies very far away from us appears dim to us. If it was closer,   it would be brighter. A cooler, intrinsically dim star might look very bright to us if it was very close by. Most stargazers are interested in something called visual (or apparent) magnitude, which is the brightness it will appear to the eye. Sirius, for example, is -1.46, which means that its quite bright. It is, in fact, the brightest star in our night sky. The Sun is magnitude -26.74 and is THE brightest star in our daytime sky. The dimmest magnitude anyone can detect with the naked eye is around magnitude 6.   The intrinsic magnitude of a star is how bright it is due to its own temperature, regardless of distance. Astronomy researchers are much more interested in this number since it gives some clue about conditions inside the star. But, for backyard stargazers, that figure is less important than visual magnitude.   While our viewing is limited to a few thousand stars (with the naked eye), of course, observers can seek out more distant stars using binoculars and telescopes. With magnification, new populations of stars widen the view for observers who want to explore more of the sky. Edited and expanded by Carolyn Collins Petersen.