StarDate

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