Night Sky

November 19, 2008, 5:30 PM

November 29, 2008, 5:30 PM

December 1, 2008, 5:30 PM

It's a measure of how crummy the weather has been here of late that I only noticed Venus last night. The most brilliant object in the sky (aside from the sun and moon) has been hanging low in the early evening sky for a month or so now, but it was only last night that I saw it. And what a site it was, paired, albeit somewhat distantly, with Jupiter, a planet whose brightness is second only to Venus.

Observers living in midnorthern latitudes need only look to the southwest on a clear evening as the sky is darkening after sunset to see these two jewels. You absolutely cannot miss them! Moreover, over the next two weeks they will get closer and closer together, rendezvousing with only a 2 degree separation on November 30. Meanwhile, a thin crescent moon will begin to rise beneath them, and on December 1 will have just overtaken them. It will be a dazzling site not to be missed, weather permitting. The three graphics accompanying this post show what it will look like on November 19 (tonight), November 29, and December 1. (Click on them for full-sized images.)

The brightness of Venus is astounding. Because it is covered in an atmosphere full of highly reflective clouds and is the closest planet to us, it blazes at about magnitude -4. Depending on the relative positions of the Earth and Venus in their orbits, it is sometimes brighter and sometimes dimmer, but always it is by far the brightest object in our sky aside from the sun and moon. (Right now, its magnitude is -4.1.) Also, viewed through binoculars or a telescope, it can be seen to go through phases like the moon and vary greatly in size depending, again, on where it is in its orbit as compared with the Earth.

Now Jupiter is also very bright, partly owing to its highly reflective clouds but also because of its great size. It's the largest of the planets in our solar system, with a volume of about 1400 Earths, so it catches and reflects a great deal of light from the sun. Right now, it shines at magnitude of -2.0. A difference of one magnitude is a difference of 2.5 times in brightness, so Venus is about 2.5 * 2.5 = 6.25 times as bright as Jupiter. For comparison, the brightest star in the night sky is Sirius, which shines at magnitude -1.4.

If the numbers give something to your understanding and enjoyment of what you're seeing, great. If they just frazzle you, don't worry about them. Just go outside in the early evening and look to the southwest. Watch Jupiter and Venus (and the moon) as they dance together over the next couple of weeks. It's a great way to end the day!

Perseus, one of the heros of the sky

Riding high in the northeast by about 10:00 PM (at midnorthern latitudes) you'll find one of the great Greek heroes who got his picture among the stars: Perseus. With six stars of magnitude 3.0 or brighter he's pretty obvious as he hangs around in the vicinity of Andromeda the Chained Maiden and Cassiopeia the Queen. His story is, of course, linked with theirs.

But there is another story in this constellation, one perhaps more fitting for a  Halloween tale. Of those six fairly bright stars, one is very spooky, or at least so thought the Arab astronomers who gave it its name: Algol, the Ghoul.

If you look at the star chart at the upper left (click on it for a full-sized version), you'll see that the brightest star in Perseus is magnitude 1.8 Mirfak. Following the upper branch of the constellation, you'll come to Algol, which most of the time shines at magnitude 2.1, just  tad fainter than Polaris. But you'll notice that I said "most of the time". Every 2.87 days, Algol suddenly dims over the course of a couple of hours, dropping to magnitude 3.4, or only about 30 percent of its normal brightness. It stays that way for about two hours, then brightens again to its usual value over the next couple of hours.

What causes this strange behavior? The Arab astronomers who named Algol couldn't have known this, but the reason is that it is what astronomers call an eclipsing binary star. That is, Algol is not one star but two, one bright and one very dim, locked in a tight orbit and aligned so that every 2.87 days the dim star passes between us and the bright star. When that happens the bright star is eclipsed and so the light we see is dimmed considerably.

Observing these eclipses is easy. All you need is information on when to expect them, and a star or two to compare with Algol. You can get timing information from Sky and Telescope's web site. They also publish the information in the magazine, which is available by subscription, on newsstands, or in your local library. As for comparison stars, the best one is nearby Almach, at the end of Andromeda. It's brightness is nearly identical to Algol's normal brightness. Epsilon Persei is a good star to use when comparing Algol in eclipse, as it is magnitude 2.9, about half a magnitude brighter than Algol's minimum. You'll find it about due east of Algol (straight down in the chart at the upper left), in the other main branch of stars in Perseus.

The spectrum of the sun (from the NOAO web site)

When you look up at the sky at night with just your eyes, mostly what you see are a lot of points of light set against a more or less dark background. (More or less depending on light pollution and the presence or absence of the moon.) The points of light, with only a few exceptions, are stars, and about all you can tell from looking at them is that some are brighter, some dimmer, and some have a bit of color to them.

Not very much to go on, you might think. But there is more information packed into that little bit of light than you might realize. For starters, the brightness or dimness of a star depends on two things: how far away it is, and how bright it actually is. If we can figure out the one, we can get a good handle on the other, and that in turn will tell us how stars are distributed in space and something about how they actually work. Also, color is related to temperature, so just knowing the colors of stars can give us more clues about how they work.

But something far more profound is hidden in starlight. If you've ever played with a prism or even simply looked at a rainbow, you'll know that most of the time light is composed of an array of colors ranging from red to blue. Light is electromagnetic radiation, which can be described as a wave, and the color of light is dependent upon its wavelength. Red light has a longer wavelength than blue light. The light coming from the sun is a mixture of all wavelengths. A prism causes different wavelengths of light to bend at different angles, with the result that when sunlight passes through a prism, it is split into a rainbow, revealing all the colors of which it is composed. The technical term for this rainbow is spectrum.

The odd thing is, if you pass the light through a thin slit, then into the prism, and look closely at the results, you'll see that there are dark lines in the sun's spectrum, as you can see in the graphic at the upper left. (Click on it for a full-sized version.) It's as though for some reason the sun isn't radiating certain wavelengths of light. What causes this?

As you know atoms are composed of a bundle of protons and neutrons in the nucleus, surrounded by orbiting electrons. But electrons don't orbit the nucleus the way planets orbit the sun. Quantum mechanics states that orbiting electrons act like standing waves. Because of this, they can only occupy certain orbits, each of which has a specific energy level associated with it. Electrons want to be in the lowest energy state possible. Everything in the quantum world can be described as both a wave and a particle, and light is no exception. So let's suppose a photon (a particle of light) happens along and strikes an electron. What happens? Well, the electron absorbs the energy of the photon, which knocks it into a higher, more energetic orbit. It doesn't want to stay there, however, so after a very short time it will reemit the photon and drop back down to the lowest possible energy state. In the process, the photon most likely careens off in a different direction than it had originally been traveling.

That's what causes the dark lines in the spectrum of the sun, or of any other star. They tell us the frequencies of light that are being absorbed and reemitted by atoms in the outer part of the star's atmosphere. That may not seem like much, but it is, because it turns out that every atom has a particular "signature" of dark lines it creates. The light from a star, in other words, encodes the chemical composition of the star's atmosphere! So, by looking at spectra, we can discover what stars are made of, and the strength of the lines (how light or dark they are) can tell us the relative abundance of each element.

Nor is that all. Because light is a wave, it is compressed when the source of light is moving towards us and stretched when the source is moving away from us. This is called the Doppler shift, and you've no doubt experienced it with sound waves. In the case of sound, wavelength corresponds to pitch, so the pitch made by a noisemaker (say, a train's horn or a siren) coming towards you sounds higher than one going away from you. The shift from high to low is readily apparent when it passes you by.

Same thing with light, only since wavelength corresponds to color an object moving towards us looks bluer and an object moving away from us looks redder. With stars we wouldn't be able to tell just by their color, because we don't know what their color is when they are standing still with relation to us. But the dark lines in the spectra know. They always occur at the same wavelengths, so if they appear shifted to the blue or to the red, we can tell whether the object is moving towards us or away from us, and how fast it is going.

So now, light is telling us something about the distance to stars, the absolute brightness of stars, the composition of stars, the temperature of stars, and the motion of stars. I've simplified some of this a bit, but clearly light is a very powerful thing! Moreover, from this basic information, we can figure out a great deal more. Indeed, what we know as light is only the tiny part of the electromagnetic spectrum registered by our eyes. Infrared light and radio waves are the same thing only longer in wavelength, and ultraviolet light, x-rays, and gamma rays are shorter in wavelength. Today we use all of these to understand the composition, structure, and workings of the universe.

So there it is. If you've ever wondered how astronomers know so much about the universe when no man-made device has ever been anywhere near even the nearest star beyond our solar system, that basically is it. Light carries vast amounts of information. All we need to do is figure out what it's trying to tell us.