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Moon and Regulus
Regulus, the bright heart of Leo, the lion, huddles close to the upper right of the Moon as darkness falls this evening. They're low in the sky, so we're seeing them through a thicker layer of atmosphere than if they were high overhead.
Dust and other particles in the air absorb much of their blue light, allowing the reds to shine through. That gives the Moon and Regulus a yellow or orange tint as they drop toward the horizon. It also makes them look fainter. By the time they're just a couple of degrees up, the atmosphere will have absorbed so much of their light that Regulus will disappear.
Because of that, astronomers don't usually look at objects that are that low in the sky. Even higher up, though, there's frequently a problem with dust -- not in the air, but out in space.
Giant clouds of gas and dust permeate our home galaxy, the Milky Way. Some of them are giving birth to new stars, while others are too spread out to do so, spanning many light-years. These giant clouds absorb some of the light of the stars and galaxies behind them. And like the dust in the air, they also make them look redder.
Astronomers map these clouds so they know just how much of the light from distant objects is absorbed, and how much redder they look -- key bits of information for determining distances and making other measurements.
That's not a problem for Regulus, though. The star is just 79 light-years away, so the view is unblocked.
Script by Damond Benningfield, Copyright 2011