StarDate

There’s just one place to catch a wave outside Earth: Titan, the largest moon of Saturn. It’s the only other world in the solar system with bodies of liquid on its surface. And the waves there could reach heights of 10 feet. But future surfers will need to dress carefully; the surface temperature is almost 300 degrees below zero.

Titan is half-again the size of Earth’s moon, and it has a dense atmosphere. Rain fills its lakes and seas with liquid methane, ethane, and nitrogen. The largest sea covers about 200,000 square miles – twice the area of all the Great Lakes combined. It could be several hundred feet deep.

The Cassini spacecraft used radar to measure Titan’s ponds. It saw waves of no more than an inch or so high. But a recent study says the waves could get a lot bigger.

Based on 20 years of observations of waves on Lake Superior, scientists built a computer model of how waves might work on other worlds. The model accounted for gravity, the pressure of the atmosphere, and the properties of the liquid. When the model was applied to Titan, it showed that even gentle breezes could whip up some big waves. But the waves would move more slowly than those on Earth – a slow-motion trip across the sea.

Saturn looks like a bright star close to our moon the next couple of mornings. It’ll stand to the left of the Moon at dawn tomorrow, and the lower right of the Moon on Tuesday.

Script by Damond Benningfield

The dawn twilight has a bright visitor the next few mornings – the planet Mercury. It’s farthest from the Sun for its current morning appearance. It looks like a bright star, but it’s so low in the sky that it’s tough to find.

Mercury holds an important spot in the history of astronomy and physics. It provided some of the first confirmation of General Relativity – Albert Einstein’s theory of gravity.

Mercury’s orbit around the Sun is lopsided, so the planet’s distance from the Sun varies. For a long time, astronomers had seen that the orbit’s closest point shifted a tiny bit over time. Isaac Newton’s laws of gravity explained most of the difference. But there was still a tiny amount that couldn’t be accounted for.

Einstein’s theory of gravity held that massive bodies warp the space around them. Since Mercury is the Sun’s closest planet, its orbit is influenced by that “warpage” more strongly than any other planet’s. In fact, general relativity accounted precisely for the shift in the orbit. So Mercury’s orbit provided some of the first evidence to support general relativity – a new way of thinking about gravity.

Look for Mercury quite low in the eastern sky during the waxing twilight. It’ll shine a little brighter each day over the next few mornings. But it’ll also drop a little closer to the Sun, so you’ll need a clear horizon to spot it.

Tomorrow: catching waves.

Script by Damond Benningfield

The Sun is getting bigger, hotter, and brighter. The change isn’t enough to notice during a human lifetime – or even a thousand lifetimes. It plays out over hundreds of millions of years. And it’s all the result of changes deep inside our star.

Like all stars in the prime phase of life, the Sun is “fusing” atoms of hydrogen in its core to make helium. That generates the energy that makes the Sun shine. As the amount of helium builds up, the core gets denser, so gravity squeezes it more tightly. That speeds up the fusion reactions, making the core even hotter.

Radiation from the hotter core pushes on the Sun’s outer layers, making the Sun bigger. It also makes its surface hotter. The combination of bigger and hotter makes the Sun brighter. So over its four-and-a-half-billion-year lifetime, our star has grown about 15 percent wider, and perhaps 40 percent brighter.

That should mean that the young Earth would have been an iceball. But studies suggest the atmosphere was much thicker when Earth was young. The atmosphere also contained much more carbon dioxide and other greenhouse gases. They trapped more heat, keeping Earth from freezing over.

The Sun’s bigger-hotter-brighter trend will continue. In perhaps a billion to two billion years, it’ll be so hot and bright that Earth’s air and oceans will boil away. That will reduce our planet to a bare cinder.

Script by Damond Benningfield

Eta Aquilae is big, bright, and unsteady. Over a bit more than seven days, the star pulses in and out like a beating heart. That causes its brightness to change. How it changes makes the star a good “standard candle” – a type of object that astronomers use to measure the scale of the universe.

Eta Aquilae is a Cepheid variable – the first one ever discovered. Such stars brighten and fade in a predictable way. By timing the cycle, astronomers can determine the star’s true brightness. Comparing that to how bright the star looks allows them to calculate the star’s distance. Cepheids are bright enough to see hundreds of millions of light-years away – in galaxies beyond the Milky Way.

To go even farther, astronomers rely on another type of standard candle: the exploding stars known as Type Ia supernovas. Like the Cepheids, the way they brighten and fade reveals their true brightness. Some of them appear in galaxies with Cepheids, where we already know their distance. That provides a way to calibrate all of the supernovas, which can be seen from billions of light-years away.

Of course, it’s all a little more complicated than that. There are different classes of Cepheids, for example. So astronomers have to understand all the details – making sure that a standard candle really is a good distance marker.

Eta Aquilae is high above the Moon in early evening, near Altair, its constellation’s brightest star.

Script by Damond Benningfield

Saturn feels like it has a big region of the early morning sky practically to itself right now. The giant planet climbs into good view after midnight, and stands high in the south at first light. It looks like a bright golden star. You have to scan a long way in every direction to find another planet or star that rivals it.

Saturn is traveling through Pisces, skimming along the border with Cetus. That region of the sky is well below the Milky Way – the hazy band of light that outlines the disk of the Milky Way Galaxy. When we look at the Milky Way, we’re looking into the most heavily populated part of the galaxy. So not only are there a lot more stars in and around that band, there are a lot more bright stars.

At the same time, Saturn’s location is a quarter of the way around the sky from the center of the galaxy, which is in Sagittarius. Again, that means we’re looking into more thinly settled parts of the galaxy. It’s like looking toward the suburbs of a major city instead of its busy downtown – there’s just a lot less to see.

Saturn is so far from the Sun that it takes the planet about 30 years to make one full circle against the starry background. So the planet will stay in this dimly settled region of the sky for a couple of years – making it especially easy to find as you look into the darkness.

Tomorrow: bright “mile markers” for measuring the scale of the universe.

Script by Damond Benningfield