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A Universe of Stars
One of the most universal human experiences is the awe we all felt the first time, when as children we saw the universe of stars on a clear, moonless night. A breathtaking array of bright and faint, red and blue glittering points of light on a dark, but not black, sky background. If it was dark enough, we saw a branch of the Milky Way sweeping across the sky with so many faint stars that it seemed like a continuous irregular band of light, intermixed with dark, mysterious lanes.
What are these sparkling diamonds that so regally adorn the night sky? Are they forever or do they come and go with the seasons? They seem to be constant night after night—do they have no birth, no death, no middle age? Are the faint stars young stars that grow into brighter, older stars or is each star its own special brightness?
What is the intrinsic nature of these wondrous nightly beacons? Are they made of the same material as the Earth or the Sun? Why do they shine so brightly and how long can they keep up whatever fire fuels that shining?
In an amazing scientific adventure story, much of the nature of stars has been discovered in the 20th century.The Basic Properties of Stars
Unlike the investigations of most sciences, stellar astronomy is restricted entirely to observations. It is hard to capture a star and kick it around the parking lot until it breaks open and we can investigate its insides and figure out what makes it tick. Investigating the stars is even more difficult because they seem almost perfectly changeless over the memories of humans. Those stars that do change seem to do so on a regular basis, like clocks, and it seems, in them, we are only watching smoothly functioning machines ticking through their paces.
MIRA image of the open cluster NGC 6871. Vertical streaks from bright stars are artifacts of the electronic camera.
So before we can know the inner secrets of stars -- what they were and what they hope to become -- we need to measure their most obvious characteristics. How about an easy question for starters: how far away are the stars?
We have immediately come to one of the most obvious questions in astronomy and one of the most difficult to answer. What is the distance to the stars? You'd think that if you pointed to any star in the sky and asked an astronomer to tell you exactly how far away it is you'd get an exact answer. Actually, we know the approximate distances to many stars but the exact distance to only one: our Sun.
Once we know how far away they are, we can combine that measure with their apparent brightness to determine their intrinsic brightness (see Brightness of the Stars). Of course, since we don't know their distances very well, we don't know how bright they actually are very well either.
How about another obvious question: how big are stars? Since they are too far away to measure in an ordinary way we must combine a measurement of their distance with their apparent size, or measure their sizes by indirect techniques. Once we know how big they are, perhaps we can determine how massive they are.
Once we know their distances and their brightness, we can answer the question: are all stars the same? As expected, they are not all the same, but just how big or how small, or how bright can an object be, and still be a star?The Evolution of Stars
Stars change on time scales that are comparable to geological time scales: millions and billions of years. These changes, driven by the masses of the stars, is called the evolution of the stars. To decipher this puzzle is like going to a zoo frozen in time. All the creatures, big and small, are there to tell the story, but how can you tell, without the luxury of watching the life cycles of the animals that mice are not baby elephants? The first step, of course, is to be able to assign an age to stars. Then we can assign a time scale to their evolution.
Out of swirling dark clouds of interstellar matter, stars condense. After time, they become fainter but stable for a long period of time, fueled by a slow but steady nuclear fire. Eventually even that fuel, the best in the universe, is exhausted. Some stars react to this exhaustion by ending their lives in a most violent manner while others slip away, slowly cooling to invisibility. Some become black holes.