The Life Cycles of Stars

November, 2009

By Tom Koonce

Antelope Valley Astronomy Club, Inc.

Lancaster, California

“The bigger they are, the harder they fall…â€?  This is certainly true of stars.  When single stars condense from a star forming nebula, their life history is pre-written based upon their initial mass and the cloud’s composition.  High mass stars burn very hot, have very short stellar lifetimes then explode in spectacular Supernovae, forming either Neutron Stars or Black Holes.  On the other end of the mass scale, low mass single stars have relatively cool temperatures, but live extremely long lifetimes and may radiate dimly for many, many billions of years

Image1Over time, higher density regions within giant nebulae like the Orion Nebula or the Eagle Nebula begin to contract gravitationally, and as they do, the cloud rotates.  As the gas contracts and rotates faster, the gas begins to heat up to become a Protostar.  Once its temperature reaches approximately 15,000,000 Celsius, nuclear fusion initiates in the cloud’s center causing the Protostar to begin to radiate brightly.   The smallest stellar objects that form in the star forming regions are called Sub-Stellar Objects.  These form with masses between 0.013 and 0.08 times the mass of our own Sun (our Sun = one solar mass).  These stars radiate briefly as a dim star, but gradually collapse, cool as they evolve further into Brown Dwarf stars.  Eventually the Brown Dwarf will cool further and it will cease radiating at all.

The stars known as “Red Dwarfâ€? stars have between 0.08 and 0.4 solar masses when they form.  These are the most common type of stars in the observable universe and have lifetimes longer than 13 billion years.  As these small, long living stars eventually cool, they die and become Black Dwarf stars.

Stars approximately the size of our Sun with 0.4 to 8 solar masses are called “Intermediateâ€? stars and will swell into Red Giant stars as their fuel is expended.  Eventually, these stars will end their lives as White Dwarf stars.

Image2Nebulae and stars are typically composed of 74% hydrogen, 25% helium and 1% everything else in the periodic table by mass.  A star’s initial mass is determined by the amount of material available within the nebula from which the star forms.  Very dense nebulae can produce the most massive stars – true giants with 8 times (or greater than) our Sun’s mass.  Those stars with between 8 and 25 solar masses will expand into Super Giant stars then explode as supernovae and end their lives as Neutron Stars; those stars with greater than 25 solar masses will expand into Super Giant stars, explode as supernovae and become Black Holes.  It isn’t known what the upper limit is to a star’s initial mass is, but in the early 1990’s, a star nicknamed the “Pistol Starâ€? was discovered by the Hubble Space Telescope near the center of the Milky Way galaxy with a mass of 100 solar masses and a radius of 100 million miles, comparable to the Earth-Sun distance of 93 million miles.  The Pistol Star is called a Blue Hyper Giant and is so hot that its gravity can’t stabilize it and it is expected to go supernova within only 1 to 3 million years.  A great deal of gas and matter is expelled during these supernovae explosions which then give rise to future generations of stars, repeating the cycle of stellar birth.

Smaller stars burn dimly, but may burn for billions and billions of years.  Giant stars burn with incredible intensity, but go through their hydrogen and helium fuel in as little as millions of years, and then end their lives in dramatic supernovae explosions.  I can think of a few analogous Hollywood situations…but that’s for another type of “Starâ€? article altogether.

References and image credit: NASA StarChild initiative, NASA Hubble Space Telescope, Wikipedia.

Other Newsletter inserts:

The zodiac names we use today are actually the names our ancestors gave to special star groups known as constellations. How many of the ancient constellation names can you correctly identify? Place the constellation’s letter on the line next to its description.

A. Gemini _____ The Water Carrier
B. Cancer _____ The Crab
C. Aries _____ The Goat
D. Libra _____ The Twins
E. Ursa Major _____ The Dragon
F. Capricornus _____ The Winged Horse
G. Leo _____ The Scorpion
H. Draco _____ The Bull
I. Pegasus _____ The Archer
J. Taurus _____ The Fish
K. Pisces _____ The Hunter
L. Aquarius _____ The Lion
M. Sagittarius _____ The Scales
N. Scorpius _____ The Ram
O. Orion _____ The Great Bear

STAR SIGNS ANSWER KEY

A. Gemini – The Twins

B. Cancer – The Crab

C. Aries – The Ram

D. Libra – The Scales

E. Ursa Major – The Great Bear

F. Capricornus – The Goat

G. Leo – The Lion

H. Draco – The Dragon

I. Pegasus – The Winged Horse

J. Taurus – The Bull

K. Pisces – The Fish

L. Aquarius – The Water Carrier

M. Sagittarius – The Archer

N. Scorpius – The Scorpion

O. Orion – The Hunter

How many star terms can you find hidden in the puzzle below? Words may be written

horizontally, vertically, diagonally, left to right or right to left. Circle each word as you

find it.

Star Terms:

hot, atoms, nebula, supernova, neutron, red giant, cycle, sphere, energy, fusion,

core, galaxy, hydrogen, evolve, gas, cloud, glow, x-ray.

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