What’s Up in the Sky May, 2009
By Peter Burkey
I was recently asked why the sun is not “burning out” and getting dimmer as it ages. This seems reasonable, sort of like what happens to a campfire as the night progresses. But stars shine by a process quite different from that of a campfire. Whereas a fire’s energy output is the result of a chemical reaction (wood burning), a star’s energy is released through the thermonuclear reaction of hydrogen fusing into helium and the conversion of mass into energy according to Einstein’s famous equation, E = mc2. Stars shine with a near constant energy outflow for billions of years.
Stars are born, they live, and they die through a process known to astronomers as “stellar evolution”. The critical factor that determines a star’s life cycle is its mass. Massive stars live fast and die young. Low mass stars have longer, more stable lifetimes.
One of the defining stages of a star’s life is when it is fusing hydrogen into helium in a manner such that the outward pressure of the reaction balances the inward force of gravity in the star. This is known as “hydrostatic equilibrium” and we say the star is on the “Main Sequence”. The Sun is an example of such a star. It has been on the main sequence for about 4.5 billion years.
We classify stars according to their surface temperature which depends on how fast the fusion process occurs. This, in turn, is dependent on the star’s mass. The different spectral classes are designated by a letter – O, B, A, F, G, K, M – with the type O stars being blue, hot and massive and the type M stars being red, cool, and least massive.
The spring sky offers examples of several different types of stars. In the south, Spica is a type B star with a surface temperature about four times that of the Sun and about ten thousand times as luminous and with ten times the mass. Right above Spica is Arcturus, a type K red giant 3.5 times more massive and over 200 times more luminous than the sun.
A distinguishing characteristic of the different types of stars is their color. Look closely to see if you can notice any differences in these objects that you see up in the sky.
This month in history:
May 1: Comet Hyakutake makes closest approach to sun – 1996
May 9: Hyabusa, first spacecraft to bring back sample from an asteroid, is launched – 2003
May 20: Pioneer -Venus 1 launched – 1978
May 25: President Kennedy gives speech challenging nation to land astronaut on Moon before the end of the decade – 1961
May 29: First experimental test of Einstein’s General Theory of Relativity performed during total solar eclipse – 1919
Here are this month’s viewing highlights:
Planets this month: Mercury is still visible in WNW after sunset early in May. Saturn remains below Leo in SW. Jupiter rises around 3 a.m. All month, Venus and Mars remain low above the eastern horizon before dawn.
May 1: First quarter Moon.
May 9: Full Moon.
May 17: Last-quarter Moon. Telescope reveals shadows of Io and Callisto on Jupiter between 4 and 5 a.m.
May 21: Venus, Mars, and crescent Moon form lovely triangle in East one hour before sunrise.
May 24: New Moon.
May 25-29: Jupiter passes less than a full Moon diameter south of Neptune. Use a telescope at 40 to 80 power to view both planets.
