What’s Up in the Sky June, 2009
By Peter Burkey
I usually try to write about a wide variety of topics in this column and not repeat myself month to month. However, I received the most positive response from last month’s column ever, so I decided to follow up on the same subject this month.
That subject is stars or, more specifically, the characteristics of different types of stars. Now I will focus on the life cycles of various stars.
To understand stars, one must understand a few basic concepts. The first is mass, or the amount of matter that makes up a star. The mass of the sun = one solar mass. The second important concept is hydrostatic equilibrium or the balance between the inward force of gravity and the outward pressure of the gases within the star. This outward pressure is caused by the enormous energy released when hydrogen fuses into helium at the core of a star. Such a star is said to be on the “main sequence” and remains stable for a long time.
How long depends on the mass. Stars range in mass from about one-tenth to one hundred solar masses. The sun has been on the main sequence for about 4.5 billion years and will remain so for about that long. Low mass stars will remain stable for up to 200 billion years (much longer than the age of the universe) and high mass stars for only one million years! Massive stars live fast and die young.
As the fusion process in a star’s core progresses, the chemical composition of the star changes. Helium builds up in the core and the star expands and cools, becoming a red giant. Eventually gravity forces the helium to fuse into carbon. In massive stars, the mass on the outer layers is enough to compress the carbon into neon, then oxygen, silicon and finally iron. All this happens very quickly compared to the main sequence life of the star.
At this point, the fusion stops because it takes more energy to fuse iron than is released in the process. So, the hydrostatic equilibrium is disrupted, gravity takes over and the star collapses. What happens next is again dependent on the star’s mass.
A star like the sun will shed its outer layers and its hot, dense core becomes what we call a white dwarf. Low mass stars simply collapse into a brown dwarf and slowly cool off. Very massive stars, though, end their lives in a spectacular explosion known as a supernova where they blow themselves to smithereens, spewing heavy elements into interstellar space. In fact, all atoms of elements heavier than iron were formed in a supernova explosion.
So, the next time you put on your gold jewelry, think about the fact that it was created when a star blew up long before the earth was formed. As Carl Sagan said, “we are made of star stuff”.
This month in history:
June 5: Regular observations of Neptune begun by Voyager 2 – 1989
June 8: Giovanni Cassini born – 1625
June 13: Pioneer 10 leaves solar system on journey through interstellar space – 1983
June 22: Evidence of liquid water on Mars announced by NASA – 2000
June 25: Progress spacecraft collides with MIR – 1997
Here are this month’s viewing highlights:
Planets this month: Venus, Mars, and Jupiter are visible in the predawn sky. Saturn is high in the SW after sunset.
June 7: Full Moon
June 15: Last-quarter Moon.
June 19: Venus and Mars are below crescent Moon low in E before dawn.
June 20-21: Midsummer Night – shortest night of the year. Summer solstice 1:46 a.m.
June 22: New Moon.
June 29: First quarter Moon.