Alison C. Rohde
A Closer Look: Going Supernova12/18/09 A supernova is most often described as the explosion of a star in space. However, this definition is slightly inaccurate.
What actually happens during a supernova is better described not so much as an explosion, but as a massive ejection of the star's mass into outer space.
When a star can no longer withstand its own gravitational force due to either an accretion of too much mass, or the inability to continue nuclear fusion of its atoms, all the mass of the star collapses down, hits the core, and bounces back out in an enormous shockwave.
When it reaches maximum brightness, the supernova may even outshine the galaxy in which it resides.
There are two main types of supernovae, Type I and Type II. Type I supernovae take place in a binary system where two stars closely orbit each other. Over time, the gravitational force from the smaller star, called a white dwarf, will pull mass from its larger companion.
Eventually, the white dwarf will reach the Chandrasekhar limit, which is a mass of about 1.4 times that of the sun, at which point it collapses into itself and erupts into a supernova.
Type II supernovae occur when a single, very massive star dies. When the star begins to reache the end of its life, nuclear fusion slows down and then stops in its core, causing it to collapse. This collapse releases enormous amounts of energy in the form of neutrinos and electromagnetic radiation.
The force of this energy causes the outer layers of the star to shoot out into space in a bright "explosion".Sometimes, the remnants of supernovae will form beautiful planetary nebulae or even a black hole.
In other cases, a very dense object called a neutron star may be left behind.
Last Chance to See Venus This Week
12/3/09 On March 27th of last year, the planet Venus once again became visible in the dawn skies as the morning star. For months it has danced along the horizon, moving closer to the sun. If you haven't yet enjoyed this spectacle, this week and the next may be your last chance for a good while. Over the next several days, Venus will drift too near the sun to be observed. Venus will not return to our skies until March of 2010, when it will reappear on the western horizon as an evening star.
Venus, often referred to as our sister planet because of the similarity in size to earth, is also the brightest object in our sky (excluding the sun, of course).When it is the evening star, it is the first object to appear in the sky after sunset. Conversely, it is the last celestial object visible before dawn. Because the orbit of Venus lies between Earth and the sun, it always appears to us to be either leading or following the sun very closely and has phases similar to those of the moon.
Right now, the phase of Venus is very close to full. Currently, Venus is traveling towards its superior conjunction on January 12, at which time it will be directly behind the sun from our vantage point.
After this point it will shrink in phase, reappearing from behind the sun in March and reaching its half-full phase in August. Between August and October, it will continue to wane until it reaches its crescent phase, which is an excellent time for telescope viewing.
How Dark are Your Skies?
12/3/09 On a dark, moonless night with perfectly clear skies, approximately 2,500 stars are visible to the naked eye. However, when factors such as city smog or ambient light are added into the equation, the number of visible stars can be greatly diminished.
In New York City for example, an average of only about 15 stars are visible in the night sky.If you want to measure how dark your local skies are, look to the constellation Ursa Minor, also known as the Little Dipper. The two stars that form the side of the dipper are call Kochab and Pherkad. These are known as the "guardian stars" because they circle around Polaris, keeping watch over the North Star. These two are also brighter than the other stars which make up the dipper, with Kochab reaching a magnitude of 2 and Pherkad a magnitude of 3. The other stars have magnitudes of 4 and 5.
If your skies are dark, all the stars of the dipper should be visible.
If only the Guardians are visible, your sky can be considered fair to poor.Light pollution is often defined as too much, or misdirected, light in an area. The primary sources of light pollution are uncovered lampposts, street lights, or outdoor lighting fixtures, such as flood or security lights. In wintertime, the lamps which illuminate ski slopes can have a significant negative effect on dark skies, creating a hazy glow of light which blots out a large portion of the stars.
Because of these interferences, most Americans will never experience a perfectly dark sky.
Although this may seem like a gloomy prediction, it's important to remember that this is a problem which can be helped. Each of us can do our part by turning off outdoor and any unnecessary lights and urging town officials to make smart lighting choices, such as covered lamps and street lights.
When I Heard the Learn'd AstronomerWHEN I heard the learn'd astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts, the diagrams, to add, divide, and measure them,
When I sitting heard the learned astronomer where he lectured with much applause in the lecture room,
How soon unaccountable I became tired and sick,
Till rising and gliding out I wander'd off by myself,
In the mystical moist night-air, and from time to time,
Look'd up in perfect silence at the stars.Walt Whitman (1865)
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