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.

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.

What’s Up in the Sky April, 2009
By Peter Burkey

Sputnik, Voyager, Spirit and Opportunity are all probably names that are familiar to many with an interest in space science. These, along with the Hubble Space Telescope and the International Space Station have gained notoriety through media exposure and famous discoveries. But what about Stardust, J-MAPS, or Dawn? Although not as well known, these are spacecraft that are no less significant.

Orbiting the Sun since 1999, NASA’s Stardust spacecraft collected a tiny sample of dust as it flew by Comet Wild-2 in 2004. Two years later it sent its sample back to Earth in a capsule and receded into space. In 2005 the Deep Impact probe fired a projectile into Comet Tempel-1 but was unable to image the resulting crater due to obscuring dust and debris from the impact. Now NASA is sending Stardust back to Tempel-1 for a better look. It should arrive in February, 2011.

J-MAPS is a future mission that will map the distances, positions, and motions of millions of stars to extreme accuracy. While this does not have the glamour of Hubble photos, such a vast database is extremely important to astronomers’ understanding of our Milky Way galaxy.

The next Mars rover will dwarf Spirit and Opportunity – missions that were, incidentally, designed to last three months and are still going strong after more than five years! The Mars Science Laboratory, now scheduled for launch in the fall of 2011, will be the size of a small car, five times larger than the current rovers. Its main mission will be to assess the “habitability” of the planet to determine whether Mars’ environment is or ever was able to support life. It will be a mobile science lab with state-of-the-art instruments to evaluate the atmosphere, geology, and surface conditions of the planet.

The Dawn probe has been in space a little over a year. This spacecraft is unique in that it uses an ion propulsion system. This type of engine accelerates Xenon gas to a very high velocity for a very long time so it gets a small thrust for a long duration rather than a short, large push like that of a chemical rocket. Dawn will orbit and study the two largest protoplanets (asteroids), Ceres and Vesta beginning in 2011.

These spacecraft are just a few of the current and future missions that will further our understanding of what’s up in the sky.

This month in history:
April 1: Comet Hale-Bopp nearest Sun – 1997
April 8: Gemini 1 launched – 1964
April 12: Yuri Gagarin becomes first human in space – 1961
April 12: Columbia is first space shuttle to be launched – 1981
April 14: KSC and Vandenberg AFB chosen as shuttle launch sites – 1972
April 19: Surveyor 3 lands on Moon – 1967
April 21: First space funeral: cremated remains of 24 people launched into orbit aboard Pegasus rocket – 1997
Apr. 25: Deployment of Hubble Space Telescope – 1990

Here are this month’s viewing highlights:
Planets this month: After sunset Saturn is in SE below the constellation Leo. Mercury becomes visible near western horizon mid – month. Jupiter and Venus are easily visible in the predawn sky.
April 2: First quarter Moon.
April 9: Full Moon.
April 16-24: Look for Mercury above western horizon at dusk.
April 17: Last quarter Moon.
April 22: See crescent Moon near Venus before dawn near eastern horizon.
April 24: New Moon.