By David Ward
On a clear winter night, go outside and look up into that star-filled dome above. There seems to be millions of them, but if your eyesight is excellent and the sky is really dark, you can see about 4,500 stars. Now here is an interesting fact: Almost all of those stars are bigger and brighter than our sun. Our sun would actually be invisible to the naked eye from most of them.
Before you start feeling inferior about having a 98-pound weakling star for our sun, there are some advantages to being a small star. You get to live longer, a whole lot longer.
Take Betelgeuse, for example. You can see it this month coming up in the east, a bright orange star in the upper left corner of Orion. Betelgeuse has a diameter over 1,000 times that of our sun. If it were a giant orange beach ball as tall as a 10-story building, Earth would be the size of the period at the end of this sentence.
Here is the catch: Bigger stars weigh more, and more weight and pressure on the core of a star where nuclear fusion takes place means they burn through their fuel a lot faster. Betelgeuse is a candle burning at both ends. Astronomers estimate it is only 8 million years old and that perhaps in a mere 20,000 years from now it is going to blow its guts out in a supernova explosion. That is not nearly enough time to incubate that delicate commodity we call life. Here on this little planet, life has been brewing for several billion years. If Betelgeuse had any planets, life would not have had a chance to even get started on one of them.
Here is another interesting fact. Our sun is brighter and bigger than most of the stars out there. I know that does not make any sense, but for every star you can see, there are perhaps 10 or more too dim to be visible. The night sky would be a very crowded place if our eyes were sensitive enough to see them all.
These little stars are called red dwarfs, and the closest star to us is one. Known as Proxima Centauri, this small star is 100 times fainter than the dimmest stars our eyes alone can see. At a little over four light years away, it is literally our next-door neighbor. But how far away is that?
The Voyager 1 spacecraft is the most distant man-made object from Earth. Launched in 1977 and traveling at over 38,000 miles per hour, it has passed the orbit of Pluto. Yet to get to Proxima Centauri, if it were headed in that direction, would take it another 73,000 years!
Red dwarf stars are the Methuselahs of the universe. Our sun is roughly halfway through a 10-billion-year life cycle, but red dwarf stars will shine 10 trillion years or more, far longer than the current age of the universe.
Life on other planets?
Whether life could exist on a planet orbiting one of these stars is a subject hotly debated in the field of astrobiology, the study of life on other planets, but the prospects do not look great. First of all, a planet in the habitable zone, where water can remain liquid, would have to be very close to that red dwarf star. That close orbit would result in the planet being tidally locked, having one side always facing the star.
Our moon is tidally locked to Earth. That is why we only see one side of the moon. So that planet would have one very hot side and the other in a perpetual deep freeze. No one knows if such a planet could even have an atmosphere. Also, red dwarfs are inherently unstable, shooting out giant flares and damaging radiation as well as fluctuating wildly in temperature. Add to that the idea that photosynthesis does not work very well in red light and our good old steady sun is looking better and better all the time.
Astronomers often refer to Earth as the “Goldilocks Planet,” if you remember the story of “Goldilocks and the Three Bears.” We orbit just the right kind of star at just the right distance. That and hundreds of other seeming coincidences make our little world a rare oasis in a very hostile and violent universe.
December is not known for a lot of clear nights, but if you find yourself out in one be sure to look up. Over in the east, the beautiful Pleiades star cluster glimmers like little jewels sprinkled on the blackness of the cosmos. These stars are 100 times farther away than our closest neighbor, Proxima Centauri, and shine far brighter than our sun.
Also check out the bright white star Rigel in the lower right corner of Orion. It is 200 times farther away than our near neighbor and gleams 120,000 times brighter than our sun.
Finally, at the head of the Northern Cross, which is standing upright in the northwest these December evenings, look for the bright star Deneb. At 400 times farther away than Proxima Centauri, this star may shine 200,000 times brighter than our sun. No one knows for sure, because it is too far away.
Dec. 21 is the date of the winter solstice when the sun reaches its lowest point in the southern sky. After that it is all uphill for the next six months until the summer solstice arrives in June.