Thursday, February 19, 2009

Evening Star

Hanging high in the western sky at sunset is a brilliant star-like object that incites rubbernecking and double-takes. This marvel is the planet Venus.

The convention of referring to planet Venus as both the “Evening Star” and the “Morning Star” began long ago. The ancients, confused by what they observed at different times of the year, thought Venus was two separate bodies— two “wandering stars,” as planets were originally known. Whether we see Venus in the evening after sunset or in the morning before sunrise depends on where it is in its orbit around the Sun— to the “left” (east) or the "right" (west) of the Sun from our Earth-bound perspective.

In its current evening apparition, Venus will be visible in the western sky at sunset until nearly the end of March, after which it will become the “Morning Star,” visible in the eastern sky before sunrise.

Venus imaged in violet light (NASA)

Venus is the second planet from the Sun, orbiting between Mercury (closest to the Sun) and Earth (third rock). Venus is only slightly smaller than Earth. Like Earth, Venus is a terrestrial planet, that is, it's composed primarily of rock and metal. And that’s pretty much where the similarity ends.

Thick clouds of sulfuric acid blanket Venus, and its atmosphere is predominantly carbon dioxide. Its barren landscape is arid and covered with old lava flows. Air pressure at the surface is extremely high, comparable to the pressure we would experience ocean diving at 3,000 feet. This dense atmosphere coupled with the cloud blanket raise the planet’s surface temperature high enough to melt lead. An unsuspecting interplanetary tourist going ashore at Venus would be asphyxiated, crushed, and cooked in one fell swoop.

Lava flows on Venus (NASA)

But from a safe distance, Venus is an ornament for our sky, beautiful and stunningly bright. Why is Venus so darn bright? For one thing, it's the closest planet to Earth, so its apparent size (the amount of sky it covers from our perspective on Earth) is significant. For another thing, its thick cloud cover is extremely reflective, so most of the sunlight it receives bounces back.

Right now, Venus has a surprise for an observer with a telescope. Venus goes through phases, just like the Moon, and currently it’s exhibiting a lovely crescent phase. If you don’t have a telescope, beg a view from someone who does. You can also check with your local astronomy club, planetarium, or science center for scheduled observing opportunities.

Venus in crescent phase (NASA)

Venus in a crescent phase is much brighter than when the planet’s disk is more fully illuminated. This seems counter-intuitive, doesn’t it? We’re accustomed to, for example, a Full Moon being brighter than a crescent Moon. However, Venus in crescent phase is far closer to Earth than when it’s more illuminated, that is, when it’s heading behind the Sun from our perspective or re-emerging from behind the Sun. This proximity accounts for its increased brightness as a crescent.

At a recent public star party, a man took a good long look at Venus through my telescope while I explained that Venus goes through phases just like the Moon and that it was currently sporting a crescent. Finally, he looked up with a sheepish grin and said, “I thought it was the Moon."

Apparently, Venus can fool some of the people some of the time.

Astronomy Essential: Stars are nuclear reactors.

It’s true. What look like dainty little pinpricks of light sprinkled across the evening sky are in truth colossal glowing spheres. Blazing furnaces of unimaginably hot gas. Churning urns (to borrow from James Taylor) of burning funk.

A star begins as an unassuming clump of material— primarily the simple element hydrogen— inside a nebula, an immense cloud of gas and dust. Gravity causes the clump to contract until it’s a spinning sphere. Further contraction increases internal pressure and temperature until nuclear fusion ignites in the core in a complex series of energetic reactions that convert hydrogen into helium. As a by-product of these reactions, energy is emitted from the sphere in the form of light and heat, and a star is born.

Nuclear fusion is the engine that makes a star go.