StarDate

Our solar system is passing through one of the spiral arms that makes the Milky Way Galaxy look like a pinwheel. But the key words there are “passing through.” The solar system moves through the galaxy a little faster than the spiral arms do. So over hundreds of millions of years, we cross all of the galaxy’s major arms. And those crossings could be dangerous. A spiral arm is a region where a passing wave squeezes giant clouds of gas and dust, triggering the birth of new stars. Many of the stars are especially hot and bright, so they light up the spiral arms. The star-forming clouds are dense and turbulent. And a recent study suggested that could be where the “danger” comes in. Researchers looked at tiny crystals in Earth’s crust. They found that the composition of the crystals varied over periods of hundreds of millions of years. During some of those periods, Earth’s crust appeared to be especially hot. And the timing of those periods may correspond to passages through the spiral arms. As we move through the dense clouds in the arms, big balls of ice and rock far from the Sun could be nudged inward. Some of them could ram into Earth, creating conditions that could account for some of the crystals. So while the Milky Way’s spiral arms may be beautiful, they may also be deadly. Look for the glowing band of the Milky Way curving across the east as night falls, and arching high overhead later on. Script by Damond Benningfield

The Milky Way shines at its best on summer nights. Right now, it arcs across the east as the sky gets fully dark, and passes high overhead later on. Under dark skies, it looks like a hazy band of light. That band outlines the disk of our home galaxy. So for the astronomers who study the Milky Way Galaxy, it’s the go-zone – there’s lots to look at. But for those who study other galaxies, it’s been the no-go zone. In fact, it’s called the Zone of Avoidance, because it’s hard to see anything through it. The main problem is giant clouds of dust scattered throughout the galaxy. The dust absorbs visible light. Depending on which part of the disk you look through, in fact, the clouds can block more than 99 percent of the light from objects behind them. The other problem is that the Milky Way is crowded – millions upon millions of stars everywhere you look. So when you look into the band of the Milky Way, it’s hard to know whether you’re seeing a star or gas cloud in the galaxy or something beyond it. Fortunately, some wavelengths that are invisible to the human eye do get through: infrared light and radio waves. The infrared is best seen from space, but the radio can be turned in by giant antennas on the ground. Galaxies typically emit more of both of those forms of energy than individual stars do – important ways to avoid problems from the Zone of Avoidance. More about the Milky Way tomorrow. Script by Damond Benningfield

It’s hard to map a forest when you’re standing in the middle of it. You see the trees that are close by, but most of the forest is blocked out. Astronomers have faced the same challenge when trying to map the Milky Way Galaxy. We’re right in the middle of it, surrounded by bright stars and dark dust clouds. So we can’t get an overall picture of the whole thing. But nature has provided a way to see the forest through the trees: galactic radio. Big clouds of hydrogen gas emit radio waves at a wavelength of 21 centimeters – eight and a quarter inches. The radio waves pass through the intervening material, giving us a good outline of the structure of the Milky Way. That wavelength is produced when hydrogen atoms get “bumped up” to a higher energy level. When the atoms drop back to their base level, they emit radio waves. This process plays out most commonly in clouds where new stars are being born. Mapping the clouds revealed that the Milky Way is a spiral galaxy – a beautiful cosmic pinwheel. And measuring the motions of the clouds reveals how that pinwheel spins. So a lot of what we know about the Milky Way has come to us through the broadcasts of “galactic radio.” The Milky Way arcs across the east as night falls. You need dark skies to see it. The center of the galaxy is in Sagittarius, which is low in the southeast. It’s easy to pick out because its stars form the outline of a teapot. Script by Damond Benningfield

The center of the Milky Way Galaxy is in good view as night falls this evening. And as the night ends, at dawn tomorrow, the galactic “anticenter” is in view – the point directly opposite the center. The Milky Way is our home galaxy. It’s a disk about a hundred-thousand light-years wide. Earth is about half-way between the center of the disk and its rim. In the night sky, the disk forms the faint path called the Milky Way. But you need nice dark skies to see it. The center is in the constellation Sagittarius. Its most prominent stars form the outline of a teapot. Puffs of “steam” appear to rise from the spout of the teapot. The center of the galaxy is immersed in the steam. We can’t see the center because intervening clouds of dust absorb its light. But if we could see it, it would be impressive. Billions of stars are jammed together – far more tightly packed than in our region of the galaxy. The anti-center is in Taurus, which is low in the east at dawn. That point is marked by the star Elnath. It’s the bull’s second-brightest star, at the tip of one of his horns. And it’s easy to spot tomorrow because it’s quite close to the crescent Moon. Except for Elnath, there’s not much to see in that direction. We’re looking toward the galaxy’s thinly settled outer precincts, with intergalactic space beyond. Enjoy the panorama of the Milky Way – our galactic home – all night long. Script by Damond Benningfield

Future spelunkers might want to explore caves on the Moon and Mars. Scientists have mapped many cave openings on both worlds. On the Moon, caves could provide shelter from radiation and meteorites for astronauts. On Mars, they could provide shelter for microscopic life that was born on the planet itself. Orbiting spacecraft have photographed some likely cave entrances on both worlds. They’re big holes in the ground. They may lead to larger chambers on the sides. On the Moon, the caves probably were excavated by lava flowing below the surface. After the lava disappeared, parts of the empty tubes they left behind caved in, providing the openings. The side chambers could be good places to set up lunar habitats. Most of the caves on Mars probably formed the same way. But a recent study found eight caves that might have been carved by water. The caves were seen in a region that’s marked by deep channels that carried water in the distant past. The surface water dried up long ago. But the caves could lead to buried pools of ice. If life ever evolved on Mars, it might have survived in those damp locations. So the caves could be a good place to check for life on the Red Planet. The Moon and Mars line up with the star Aldebaran in tomorrow’s dawn sky. Mars looks like a bright star to the lower right of the Moon. Aldebaran is an even brighter star, about the same distance to the lower right of Mars. Script by Damond Benningfield

When we gaze into the night sky, it’s like looking at a projection on a giant dome – we see two-dimensional pictures, with no perception of depth. Even astronomers can have a hard time plotting that third dimension. And that can skew their understanding of how stars work. Consider the Coathanger – a pattern of 10 stars that really does resemble a coat hanger. It’s in the constellation Vulpecula, the fox. For decades, astronomers thought those stars formed a cluster. A cluster’s stars are all the same age and same distance, and they formed from the same ingredients. But some of the stars in a cluster are small and light, while others are big and heavy. Seeing how the different weight classes have evolved helps astronomers understand how all stars age. But a study in 1970 found that only a few of the Coathanger’s stars were related. And a later study, which used a satellite to plot the distances to stars, found that none of them are related – they just happen to line up in the same direction. So plotting the third dimension robbed the Coathanger of some of its scientific value – but none of its beauty. The Coathanger is a great target for binoculars. Sweep them from the bright star Altair, which is low in the east at nightfall, toward even brighter Vega, far to its upper left. The Coathanger is about a third of the way along that line – a beautiful grouping that’s not really a group at all. Script by Damond Benningfield

61 Cygni has two distinctions. It moves across the sky faster than all but about a half-dozen other stars. And it was the first star to have its distance accurately measured. The system is in Cygnus, the swan. The constellation is high in the east at nightfall, marked by Deneb, the swan’s bright tail. 61 Cygni is to the lower right of Deneb. Under dark skies, it’s just visible to the eye alone. 61 Cygni consists of two stars. Both are smaller, lighter, and cooler than the Sun, and much fainter. They orbit each other once every 650 years or so. In 1804, Giuseppe Piazzi discovered that the system moves across the sky in a hurry. That suggested that the system is close by. So astronomers started trying to figure out just how close. They looked at the star at intervals of six months, when Earth was on opposite sides of the Sun. That slight change in perspective causes nearby stars to move a bit compared to stars that are farther away. Early attempts to measure that angle didn’t work – the equipment just wasn’t good enough. But in 1837 and ’38, Friedrich Bessel used a new instrument that provided a sharper view. It told him that 61 Cygni was 10.4 light-years away. That’s just one light-year off the true distance. So Bessel’s work provided the first good measurement of the distance to any star other than the Sun. Script by Damond Benningfield

The “evening star” nuzzles the lion the next few nights. Venus will pass quite close to Regulus, Leo’s brightest star. At their closest, they’ll be just one degree apart – the width of a pencil held at arm’s length. Despite their proximity in the sky, Venus and Regulus are nowhere close to each other in reality – they’re separated by many trillions of miles. Venus is a planet in our own solar system. Right now, it’s 92 million miles away. That’s about the average distance to the Sun – a distance known as the astronomical unit. It’s the basic “yardstick” for measuring the solar system. It’s a lot more convenient than miles or kilometers – a lot fewer zeroes to worry about. But it’s not a great yardstick for measuring the distances between stars. Regulus, for example, is more than five million astronomical units from us – five million times farther than Venus. Yet it’s one of our closer neighbors. So there are two other units for measuring those distances. The most common is the light-year – the distance light travels in one year – almost six trillion miles. And astronomers generally use parsecs; one parsec is three and a quarter light-years. So Regulus is about 24 parsecs from Earth. Regulus is close to the upper left of Venus at nightfall this evening. Venus will slide past the star over the next two nights. They’ll be closest together on Wednesday – but close only in appearance. Script by Damond Benningfield

Saturn is almost 10 times farther from the Sun than Earth is. At that distance, the Sun looks only about one percent as bright as it does from Earth, so Saturn is cold and dark. But it’s not quite as cold as you might expect. Saturn actually puts out more than twice as much energy as it receives – heat radiating from deep inside the planet. Saturn is made mostly of hydrogen and helium – the lightest and simplest chemical elements. But its core contains a lot of rock, metal, and other heavy materials – more than 15 times the total mass of Earth. Gravity squeezes the core tightly, causing it to shrink. That produces heat, which rises to the surface and shines out into space. But that doesn’t explain all of Saturn’s heat. The rest may come from an odd type of rainfall. Droplets of liquid helium may fall toward the core. As they fall, they produce friction, which produces heat. Saturn’s internal heat drives much of the weather in its atmosphere, including storms that can be as big as continents – swirling clouds on a cold, dark planet. Despite the lack of sunlight, Saturn looks bright in our sky because it’s huge – about nine times the diameter of Earth. It’s easy to see how bright the next couple of early mornings because it’s close to the Moon. Tomorrow, it’s to the lower left of the Moon at dawn. It’ll be a little farther to the right of the Moon on Wednesday. Script by Damond Benningfield

Earth will reach its farthest point from the Sun for the entire year around midday tomorrow. We’ll be about 3.1 million miles farther than we were at closet approach, in early January. That far point is known as aphelion or ap-helion. The term comes from a mash-up of Latin and Greek. “Ap” is from a Latin word that means “far away,” while “helion” is a Greek word for the Sun. The changing distance is a result of the shape of Earth’s orbit. Instead of a nice, round circle, it’s an ellipse – like a lopsided circle. Over tens of thousands of years, the shape changes – the result of the gravitational influence of the Sun, Moon, and planets. The orbit seesaws between being a little more circular and a little more lopsided. Right now, we’re toward the more-circular end of the seesaw. Today, aphelion always comes about two weeks after the summer solstice. But that won’t always be the case. Thanks to a slow wobble in Earth’s rotation, aphelion shifts an average of one day later every 58 years. So it occurred on the solstice about 900 years ago. And about 4600 years from now, it’ll occur on the fall equinox, in September. Incidentally, the lopsided orbit has a big impact on the length of the seasons. Our planet moves slowest when it’s farthest from the Sun. That makes summer in the northern hemisphere almost five days longer than winter. Script by Damond Benningfield