Our sun is a star, and the stars are suns.
While some are hotter or cooler than others, and some are larger or smaller, all work the same way.
Each is a thermonuclear cauldron that converts hydrogen into helium and releases a tremendous amount of energy, some of which we see as light.
For centuries folks wondered how the sun worked — they postulated that it might be coal- or wood-powered — but it wasnct until Albert Einstein’s famous equation E=mc2 hit the presses in 1905 that we finally understood the process, that they were all like our sun.
This understanding also made it possible for us to measure stellar distances. By sending a star’s light through a spectrograph, we can determine a star’s temperature and intrinsic brightness.
Once we know how luminous the star is, we can ask (mathematically, of course) how far that star would have to be to appear as faint as it does. And that’s its distance.
Surprisingly, it was the ancient Greeks who first did this. They didn’t have telescopes and spectrographs, but some thinkers presumed that the stars were like our sun.
And when they wondered how far they would need to drag our sun away from us before it appeared as small and faint as the stars we see at night, they were shocked by the answer.
It was the equivalent of trillions of miles. Well, this was just too mind-boggling for them, so they dropped the whole thing!
As it turns out, they were right. The stars are trillions of miles from us, but modern astronomers don’t use miles when computing stellar distances.
These units are much too small for cosmic distances, and the numbers we calculate come out insanely large (just as the ancient Greeks discovered).
Instead, we talk about how long it takes for a beam of light from the stars to reach us.
Take Sirius, for example, the brightest star in the night sky, now appearing low in the south after dark.
Sirius has been measured to be about 50.6 trillion miles away, and that means that its light takes 8.6 years to travel to Earth, so we say that Sirius is 8.6 light-years from us.
In other words, the light we see of Sirius tonight left that star in the summer of 2016, and we see the star as it was then.
Other stars are considerably farther, with some of their light requiring hundreds or thousands of years to reach us. And that means that we view the stars as they were in the distant past.
Now you may ask if the stars we see are even there anymore, and this is a great question.
The answer is yes, they are. Stars operate for billions of years, so a few measly centuries or millennia to a star would be equivalent to only a few seconds or minutes to a human.
Stars just don’t change much in such a short time.
In a sense, the night sky is a cosmic time machine in which we can view light from the distant past.
Check out the winter sky this week and enjoy traveling back through history!
