Thursday, May 9, 2013

What Kind of Star are you?

Despite constituting the vast majority of the matter in the universe, stars are surprisingly simplistic.  A star spends nearly all of its life in hydrostatic equilibrium, whereby gravity pulls it inward as thermal pressure pushes it outward.  This state is known as the main sequence, during which countless Hydrogen atoms fuse to form Helium ones, and release energy in the process.

OK, that might not seem simple.  But, a star is simplistic in the sense that its entire future is quite predictable given its mass when this thermonuclear fusion process begins.  We can predict how long this steady radiation will endure based on this mass.  One might think that more massive stars have more fuel to burn, and will burn for longer as a result.  Though more mass does mean more fuel, it also means higher temperature and pressure, which means a greater fusion rate.  Massive stars actually live a far shorter life than less massive ones - lifetime is roughly proportional to M^(-2.5), where M is mass.

In the graph shown below, the approximate time a star spends in its main sequence is shown as a function of its mass ratio to the Sun.

As can be seen from the graph, a star with the mass of the Sun (mass ratio of one) will spend 10 billion years in its main sequence.  A star with half this mass will burn for more than 50 billion years, while one with double this mass will burn for less than 2 billion years.  Incidentally, the Sun (having a mass of 2*10^30 kg) is currently about 40% through its 10 billion year life.
A person's life span is indeed much more unpredictable than that of a star, but there exists a useful analogy for remembering the above lifetime to mass relationship - famous people, like rock stars and movie stars often die younger than us 'dull normals'.  Studies have shown that fame leads to a shortened lifespan - about a five year reduction on average.
And just like the counter-intuitive nature of high-mass stars, which die young despite their excess of fuel, one might also suppose, incorrectly, that famous people would live longer lives on account of their additional fuel, er, money.  But, just like the thermonuclear variety of star, the extra luminosity, the fame, brings with it additional pressure, and a higher 'fusion' rate.  Famous people tend to live large, and too often see their flame extinguish early on account of excessive drug or alcohol intake.  Sometimes the pressure can seem overwhelming; some stars commit suicide.
Michael Jackson and Whitney Houston are recent examples of early death directly related to fame.  Consider the following famous musicians who all died at the age of 27: Kurt Cobain, Jim Morrison, Janis Joplin, Jimi Hendrix.
The star analogy also rings true when we consider the percentage of both by population.  Just as famous folks make up a tiny fraction of the total human population, only a tiny proportion of stars in the universe are of the high-mass variety.  That being said, more stuff tends to orbit high-mass stars, and in the case of rock stars, we may relate this to their entourage, or 'groupies'.
Indeed, most of us are like Red Dwarf stars, steadily burning through our fuel.  This kind of star has a lifetime as long as trillions of years.  Our Sun is an average star, a Yellow Dwarf, and is less common in the Universe, enduring billions of years.  The least common variety of star are Supergiants, and these endure as little as millions of years.
While fame sounds nice at first, being "larger than life" comes with a price.  Neil Young sang, "It's better to burn out than to fade away," but being a dim, low-mass star has its advantages.  You get to experience equilibrium for much longer, and can avoid a dramatic implosion, and the ensuing death by way of Supernova.  

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