Have you ever wondered if the stars you see tonight are the same ones you saw last night? Or last summer? Or ten summers before that?
Stars are suns, like our sun. They last for millions, sometimes billions of years. Even though they are moving through the arms of our galaxy, the change in position of the stars is so ponderous it’s completely unnoticeable over several human life spans. That’s why the ancient constellations like Pegasus, Scorpius and Andromeda are still there just the way the Greeks saw them.
Over a time span of thousands of years, there are a few stars that would be in slightly different positions, enough to be detected by a seasoned stargazer. Two of the brightest stars, Sirius and Arcturus, are in this group. But these are the exceptions. It would take tens of thousands of years for the starry sky to begin to look different enough that some of the constellations would be unrecognizable.
For instance, the best known of all the star groups, the Big Dipper, would have a bent-down handle and bent-open bowl after 20,000 years, but it would still be recognizable in that distant future.
Next time you look at the Big Dipper, think about this: As the tiny, fragile spears of light from each of the Dipper’s stars enter your eyes, a journey that began decades ago comes to an end. Five of the seven stars in the Big Dipper are relatively nearby stars about 75 light-years away. The light from them takes a human lifetime to reach Earth. Light from the other two, at the extreme ends of the configuration (the ones that will appear to distort the Dipper’s shape in the remote future), are about two lifetimes away.
Use the two stars on the outer edge of the Dipper’s bowl to point to Polaris, the North Star, one Dipper length away out the “open” side of the bowl. Polaris appears as bright as the Dipper stars to us, but it is 430 light-years from Earth, three to six times farther than the Dipper stars. Polaris is an enormously brilliant star, 4,000 times more luminous than our sun, significantly brighter than any of the Dipper stars.
Although astronomers have learned the brightness, temperatures and distances of the stars, even their approximate ages, the actual sizes of the very largest stars have remained elusive. Estimates from different experts sometimes vary by a factor of three because the outer extent of these stars are difficult to measure.
In 2005, a team of astronomers who had spent years studying giant stars announced more accurate estimates of the sizes of the biggest stars. They say that three stars, KW Sagittarii, V354 Cephei, and KY Cygni, are the largest known stars, each about 14 times wider than the diameter of the Earth’s orbit. That’s enormous–almost as large as the orbit of Saturn–and bigger than most researchers had expected.
None of the big three are visible without a telescope. Of the well-known stars, the largest is Antares, the orange star at the heart of the constellation Scorpius, the scorpion. It varies in size over time and can bloat out to four times the diameter of the Earth’s orbit, or 850 times the size of the Sun. If Antares replaced the Sun, the Earth would orbit deep inside it.