Life of a Star

Stars are born in the enormous gas clouds which occupy the universe.  Once born they live their long lives in nuclear fusion and then die.  However, even in death the process continues with the remnants of the star…smaller stars, like our sun, leave behind a white dwarf….a white hot mass consisting of mainly carbon, hydrogen, and helium.  It will about the size of earth, have no nuclear reactions, and will slowly cool over a period of several thousand million years.  Larger stars, greater than 8 solar masses, explode in huge explosions called supernova and leave behind a neutron star, a mass so dense that a teaspoon portion would weigh thousands of tons.  It has no nuclear reactions taking place, but emits a narrow beam of electromagnetic radiation which is how the neutron star is detected.

A Star is born…….

Stars depend on fusion of hydrogen for the creation of energy.  The hydrogen comes from the abundant supply within the massive gas clouds in the Universe.  As these gas clouds cool and condense the atoms come under the influence of gravity and begin to collect and grow in size.   As this clump grows larger it attracts, by gravity, more and more hydrogen gas atoms and other, smaller, gas clumps. The increase in mass results in higher gravitational forces, which in turn results in the attraction of more mass, resulting in ever higher gravitational forces. and a continuing cycle.  As the mass increases the temperature increases.  When the mass is great enough to generate high enough temperatures – 10 million degrees – nuclear fusion results, converting the hydrogen to helium. The helium created is left in the core and the fusion reaction continues from the inside out.  Extra large stars (see below) will continue the fusion process to produce heavier and heavier elements.  This process takes billions of years.  The quantity of hydrogen is so large that the conversion to helium takes a very long time, in spite of the continuous fusion reaction.

Take a moment and appreciate the tremendous process that is taking place in our own Sun.  We have all seen the video of a hydrogen bomb detonation…..this is a relatively small amount of material that creates a huge amount of energy in an instant.  In our Sun this same process has been going on continuously for 5,000 million years, and will continue for another 5,000 million years!

A Star dies…..

Stars come in different sizes….pretty much divided into 4 groups….small, medium, large, and massive.  The massive stars are capable of producing the heavier elements.  Each of these groups have a unique characteristic of their death, they behave differently depending on their size. But for each group the first step in the death of a star is the same…they become a Red Giant.

The first step in the death…. a Red Giant.   As the hydrogen runs out and the fusion reaction slows, the star begins to collapse upon itself, due to gravity.  This collapse continues until the temperature rises (due to the collapse) and then the star expands outward to many times its original size.  For our sun this Red Giant phase will last for several thousand million years and will encompass the Inner planets. It stays this way for millions of years, slowly shedding layers of helium…

For low-mass stars….less than 0.4 times the mass of our sun… the Red Giant cools the core  this star will collapse to a white dwarf.  Its size shrinks to several thousand miles in diameter and remains that way for 1,000,000 million years, eventually losing all it’s mass to the space around.  Note that the life of a low-mass star can be longer than the age of the universe as we know it.  Therefore, it may be said that no low-mass stars have completed their life.

For medium-mass stars…. between 0.4 and 1.4 times the mass of the sun…..just as with a low-mass star, it will expand over a period of 100 million years to become a Red Giant, encompassing the Inner Planets.  Then, over a period of 0.1 million years, it will shed it’s outer layers until only a white hot core (30,000 degrees) remains, a white dwarf, about the size of earth.   It is very dim but has a very high surface temperature.  It remains that way for thousands of millions of years, eventually cooling to a cold, cold object in space.

For large-mass stars... from 1.5 to 8 times the mass of the sun….. the completion of burning the hydrogen fuel does not result directly in a Red Giant.  Rather, it will contract slightly, the core temperature will increase (100 million degrees) and additional fusion will occur, converting helium into carbon.  The star will continue to expand towards the red super giant  phase. At this point the physics is not certain but it is expected that thermonuclear processes continue, temperatures decrease as fuel runs out, collapse occurs resulting in an explosion during which huge amounts of energy are released within seconds as the star destroys itself (except for the super-dense core).  The super-dense remnants make-up a neutron star in which the density, and therefore gravity is so great that electrons and protons are combined to neutrons.  The size of the neutron star is very small….on the order of 10 km, a few miles!

For massive stars…greater than 8 times the mass of our sun (and up to 20 times the mass) these stars may become Black Holes.  The massive stars do not live as long as our Sun and the other smaller stars, maybe 10 million years before running out of nuclear fuel, but they burn brighter and hotter. Because they are hotter they can generate the internal temperatures (over a billion degrees) necessary to sustain successive nuclear reactions which produce heavier elements. This is how the heavier elements, such as gold, lead, and uranium, are formed.  Note that all heavy elements found on earth were formed in the successive nuclear reactions within these massive stars.

As these massive stars progress through successive stages of nuclear reactions they balloon to enormous size until they eventually run out of fuel at which point gravity takes over and they suddenly collapse, in a matter of seconds.  During the sudden collapse the core becomes more and more dense, it reaches a point where the material rebounds and explodes outward to become a Supernova, exploding in a huge flash and shining as brightly as 10 million of our Sun, if only for a short time….a matter of weeks.  The remaining core is usually a neutron star or small, Stellar mass Black Hole.  It is during this Supernova  process when the interstellar space is littered with the heavier elements.

While many Supernovas are observed each year, only five (5) Supernovas have been observed with the unaided eye…..4 in our galaxy and 1 in the nearby galaxy, Large Magellanic Cloud.  A Supernova is a rare sight, but one which can be seen with the unaided eye…so consider yourself fortunate indeed should you one night suddenly see a much brighter than normal object in the sky.