Stars

What are the stars?  The simple answer is they are huge bright spheres of hot gas, like our Sun, only much further away out in space.  Although the sun and the stars are similar, they are different in important ways.  We can explore these differences by investigating the distance, size, temperature and brightness of each.

LIFE AND DEATH OF STARS

The stars are apparently eternal, shining steadily night after night without change.  Based upon their energy consumption (that is, brightness) and their available fuel supply (their mass), we deduce that stars can live for billions, even trillions of years.  In fact, even with hundreds of billions of stars in our Milky Way galaxy, it has been 400 years since we observed a star burn out and die (Kepler’s supernova, 1604.) So how do we know that stars live and die?

The Milky Way

In our brief lifetimes, the view we have of our galaxy is like a snapshot capturing one moment in a story that has been unfolding for billions of years.  But, since there are billions of stars, of all different types and ages, we see a complete sample of the phases of their life cycles.

Imagine being an alien unfamiliar with humans, visiting Earth and observing for a short time.  Say you had enough time to take pictures of a representative fraction of the 6 billion+ people, captured in all of their various activities.  You would see old people, young people (children), babies, and a great deal of middle-aged people.  You would see some dying, some being born (and perhaps some being made!)  Even without observing long enough to see anyone get older, you could figure out the life cycle of humans.  This is how astronomers deduce the life cycle of stars.

H-R Diagram

One way to compare stars is to make a graph of their brightness vs. temperature.  Such a graph, like the one seen here, is called a Hertzsprung-Russell diagram, or simply an H-R diagram.  To determine a star’s true luminosity (absolute magnitude of brightness) we must know both its apparent magnitude and its distance from us.  The apparent magnitude is its measured brightness. Its distance can be determined from measurements of parallax (out to about 650 light years from Earth.)  Surface temperature can be determined from the star’s spectrum, and seven spectral classes have been defined based on spectral absorption features.  The classes are (from hottest to coolest) O, B, A, F, G, K, M, and each is subdivided into ten subclasses, numbered 0 to 9.

THE BRIGHTEST STARS

Below is a table of the 20 brightest stars as seen from Earth (not including our Sun.)  If you were to plot the brightest stars on a graph of luminosity vs. spectral class, using an asterisk (*), you would get an H-R diagram.  (Note: Some “stars” are actually pairs of stars, or triple, like Alpha Centauri.)

Would it surprise you to learn that most of the stars nearest Earth are not even visible?  Apparent magnitude is a scale of measure of the brightness of stars, as seen from Earth.  By tradition, the ten or so brightest stars are Magnitude 1 (or lower), and a magnitude of 6 is assigned to the faintest stars visible to the unaided human eye under ideal observing conditions (thus higher magnitudes mean fainter stars.)

If you study the H-R diagram here, you will see a pattern.  The line down the middle is called the Main Sequence.  Notice that faint stars are under-represented and bright stars are over-represented.  Actually detailed studies show that about 90% of stars are Main Sequence, 9% are white dwarfs, and 1% are giants.  The Main Sequence represents the major portion of a star’s lifetime (its adulthood), when it’s burning hydrogen (H) into helium (He.)

Each star represents a nearly endless battle between the forces of gravity and heat.  Gravity causes the material of the star to press inward toward its center, raising the density and temperature of the gas (and causing nuclear reactions which release energy), until gravity is balanced by the outward pressure of the superheated gas.  The balance, called hydrostatic equilibrium, is maintained as long as the star has fuel to burn.

A New Star

The defining characteristic of each star is its mass.  Mass determines what size, temperature and spectral class it will be on the Main Sequence.  It also determines how much fuel it has, how long it will live, and what kind of death it will have.

The more massive the star, the higher the temperature and pressure needed to balance gravity.  As a result, large stars are much hotter inside and consume their fuel much quicker (and as a result are much brighter).  The opposite is true for smaller stars.  The lifespan of the largest stars is millions of times shorter than that of the smallest stars.  In the 14 billion years since the Universe began, not a single M class star has run out of fuel.  Our Sun, at 4.5 billion years old, is only about halfway through its main sequence life. When it runs out of hydrogen in its core, the Sun will turn into a red giant, burning helium into carbon for a few million years more, then finally collapse into a white dwarf.

The largest stars, such as Rigel, live only a million years or so.  (The fact that we see any O or B stars is an indication that new stars are still forming.)  When such large stars exhaust all the fuel in their cores, they explode in a supernova, temporarily shining billions of times brighter than normal stars. Statistically speaking, we are overdue for a supernova in our galaxy.  Will we one day see a new star shining bright during the day?