Stars

 

Twinkle, twinkle, little star,
How I wonder what you are . . .

Stars have always fascinated humans.  At the dawn of history, and probably even before, wise men watched the stars and learned to read them as markers of the turning of the seasons.  They attributed magical powers to the stars (a belief enshrined to this day in newspaper horoscopes).  They thought the stars were everything from “the souls of the dead” to “lamps hung in the vault of heaven.”

Actually, the latter isn’t all that far off, except we now know those lamps are incredibly large, incredibly bright–and incredibly distant.

What stars are are large balls of hot gas, thousands to millions of kilometers in diameter, which give off huge amounts of light and heat from nuclear reactions inside them.  If that sounds familiar, it should; the closest star to Earth is better known as the sun.

Stars are not exactly uncommon–take a look at the night sky.  Even then, you’re only seeing a few thousand of the more than 100 billion stars in our galaxy (a galaxy is a large grouping of stars), and there are tens of millions of galaxies.  That means the total number of stars exceeds a billion billion, or, if you prefer to see your zeroes, 1,000,000,000,000,000,000.

The ancients noticed that stars remain fixed in their positions relative to each other night after night, unlike other heavenly bodies such as the Moon and planets.  However, the large telescopes developed in the 19th century showed that some of the stars aren’t quite as fixed as had always been thought.  In fact, if you could come back to Earth a few hundred thousand years from now, you would find the night sky radically altered.  You won’t notice this motion in your lifetime, however, because most of the changes are measured in fractions of a second of arc per year, and even a full second of arc is only the angular size of a pinhead 183 metres away.

Only the relatively nearby stars display this motion, because the truly distant ones are so far away that their movement (and our solar system’s movement) through space simply don’t make an appreciable difference.

Just how far away are the stars?  One way to find out is to measure “parallax.”  Parallax is the apparent motion of nearby stars, caused by the Earth’s orbit around the sun:  the star seems to shift, first one way, then the other, as the Earth moves from 150 million kilometres on one side of the sun to 150 million kilometres on the other.  Geometry tells you that if the shift in the apparent position of the star is one second of arc each way, the star is about 32 million million kilometres away.  This distance is one “parsec,” a term you may have heard on Star Trek, and it’s equal to 3.26 light years.  (A light year is the distance light travels in a year, at 298,000 kilometres per second.)

The parallaxes of several thousand stars have been measured.  The very nearest star, Proxima Centauri (visible only in the southern hemisphere) is a little over one parsec away.  Most of the measured distances are greater than 20 parsecs (65 light years).

The enormous distances involved is why stars are so much fainter than the sun, not because they put out less light.  In fact, our sun is a very ordinary and rather small star.  Some giant stars are 100,000 times more luminous than the sun and 400 times as large (large enough that if the sun were to suddenly swell to that size, it would engulf the Earth.)  Other stars, called white dwarfs, are a thousand times less luminous than the sun and only about one-100th as large.

Our sun is also run-of-the-mill heat-wise, with a surface temperature of about 6,000 degrees Celsius.  The hottest stars have a surface temperature up around 30,000; cooler ones (white dwarfs, again) run at about 3,000 degrees.

Our sun is a bit unusual in one respect, though; it’s solitary.  More than half of all stars are part of binary systems–two or more stars orbiting one another.

Stars are powered by nuclear fusion.  Deep in the interior of a star, the temperature and density are high enough (temperature of around 10 million degrees and density of 30 grams per cubic centimetre) for four hydrogen atoms to fuse into one helium atom, losing 0.7 percent of their mass in the process.  That mass is converted into energy–lots of energy, as Einstein’s famous equation about energy being equal to mass times the speed of light squared tells us.  In fact, the energy released is 60 million times greater than the energy that would be released by simply burning the hydrogen.  It’s a good thing, too, because otherwise the sun and other stars would have burned out a long time ago:  in the sun, for example, roughly 600 million tons of hydrogen are converted to helium every second.

But don’t worry; it’s still good for several billion years, and so are most other stars, except for the few that will flare into novas or supernovas or collapse into neutron stars or black holes.

Most of them will keep twinkling peacefully in our sky, and most of the time most of us won’t even think about the enormous size and power of these incredibly distant nuclear furnaces.

Except, of course, those of us who have read this column.

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