StarDate

You don’t always need a telescope to enhance the beauty of the night sky. For many objects, a pair of binoculars will do just fine. In fact, some objects are better through binoculars.

One example is the star cluster Messier 39. It’s high in the northeast at nightfall, to the lower left of Deneb, the bright star that marks the tail of Cygnus, the swan.

M39 is a bit sparse. It contains about 30 stars, which are spread across about seven light-years of space. Seen from the cluster’s distance of 800 light-years, that span is roughly the size of the full Moon. And that’s why M39 is best seen through binoculars. A good pair will allow you to see the entire cluster in a single view. Three bright stars form a wide triangle, with the other members of the cluster roughly filling in the space between them. A telescope typically sees a smaller field of view, so it can’t take in the entire cluster all at once.

And if you have especially dark, clear skies, you might just make it out with your eyes alone. It looks like a faint, hazy cloud of light below the swan.

In the future, M39 will undergo periods where it’s a lot more prominent. A few of its brighter stars are nearing the ends of their “normal” lifetimes. When they reach that end, they’ll puff up to many times their current size, making them shine much brighter. That’ll make M39 stand out — no matter how you look at it.

Tomorrow: ending a voyage of discovery with a fatal touchdown.

Script by Damond Benningfield

There are no oceans or seas on Venus, so there’s no sea level for measuring the height or depth of mountains, canyons, or other features. Instead, scientists calculate an average elevation, then measure any deviations from that. And what they’ve found is that most of the planet’s surface is remarkably smooth and flat.

The entire surface was shaped by volcanoes and volcanic events. In fact, there’s evidence that the whole planet was repaved by molten rock a few hundred million years ago. Since then, many thousands of volcanoes have built up across the surface. They’re not as dramatic as the volcanoes on Earth, though. They’re wide but generally not very tall, so their slopes are gentle.

Other prominent features also have a volcanic origin. Collapsed lava tubes form canyons, and molten rock pushing up from below created domes as wide as states but no taller than a good-sized hill.

Even with these features, about 80 percent of the surface of Venus is no more than one kilometer above or below the average elevation. The tallest mountain is taller than Mount Everest, but the lowest spot is only a bit more than a mile deep. That’s far shallower than the ocean trenches here on Earth — helping to give our planetary neighbor a smooth complexion.

Look for Venus shortly after sunset this evening, to the lower right of the crescent Moon. It’s brilliant but low in the sky, and sets by about the time the last blush of twilight fades away.

Script by Damond Benningfield

Our complexions change as we age. Spots, wrinkles, and other features come and go. And the same thing is true on the giant planets of the solar system. Big spots can appear and disappear over timescales of years or even decades.

Over the last year, for example, Hubble Space Telescope has watched a big new dark spot appear on Neptune, the outermost of the solar system’s major planets. Hubble first saw the spot a year ago, then saw it again in May. It’s a high-pressure system — a big whorl at the top of the planet’s thick atmosphere. It’s similar to the Great Red Spot on Jupiter. Streaks of white clouds stream around it.

The Voyager 2 spacecraft saw an even bigger spot on Neptune when it flew past the planet in 1989. The Great Dark Spot was as big as Earth. Even so, Neptune is so far away that ground-based telescopes of the day couldn’t see enough detail on Neptune to follow it. By the time Hubble could take a look at it a few years later, the Great Dark Spot had disappeared. It’s unclear why it vanished — or why it formed. Yet it is clear that such spots are common features on this solar system giant.

And Neptune is putting in is biggest appearance of the year. It’s lining up opposite the Sun, so it rises at sunset, is in the sky all night, and shines brightest for the year. It’s so far away, though, that you need help to spot it. Through a telescope it looks like a tiny blue star near the center of the constellation Aquarius.

Planet Earth is sometimes described as a Blue Marble — a beautiful sphere of blue and white spinning around the Sun. But another world in the solar system is even bluer: Neptune, the most distant of the Sun’s eight major planets. It’s almost entirely blue, with no continents and almost no white clouds to alter the color palette.

Neptune is shining at its brightest right now. The planet is at opposition — it lines up opposite the Sun in our sky. It rises around sunset and is in view all night, in the constellation Aquarius. It’s almost three billion miles away, though, so at a minimum you need strong binoculars to see it. And you need a telescope to detect its blue color.

Neptune is the fourth-largest planet in the solar system — almost four times the diameter of Earth. It probably consists of a heavy core of rock surrounded by layers of ice and gas. It’s topped by a thick atmosphere that’s rich in water vapor, ammonia, and other compounds. Methane in the top of the atmosphere absorbs red wavelengths of light, allowing the blue to shine through.

Most of the blue is a beautiful shade somewhat darker than turquoise. Bands of clouds sometimes form streaks that are brighter or darker. And large high-pressure systems are sometimes seen to form ovals of dark blue; we’ll have more about that tomorrow. These colors make Neptune one of the most beautiful worlds in the solar system — a blue marble spinning far from the Sun.

Script by Damond Benningfield

2016 is a poor year for eclipses, especially from here in the United States. There are no total eclipses of either the Sun or Moon, and no partial eclipses, either.

Residents of Africa, though, will get a pretty good eclipse tomorrow: an annular eclipse of the Sun. The Moon will pass directly between Earth and Sun, blocking most of the Sun’s disk from view. But the Moon is a little farther from Earth than average right now, so it won’t be big enough to cover the entire disk. That’ll leave a thin but brilliant ring of sunlight around the Moon.

On average, the Moon is less than 240,000 miles from Earth. But its orbit is a bit lopsided, so the distance varies by almost 30,000 miles. When the Moon is closest to Earth, it’s more than big enough to cover the entire Sun. When it’s farther than average, though, it’s not quite big enough to get the job done.

For this eclipse, which will last a bit more than three hours, the Moon will cover about 95 percent of the Sun. The ring of sunlight is still bright enough to cause eye damage, though, so eclipse watchers will need to use proper eye protection to look at it directly.

The annular eclipse will cover a narrow path across southern Africa, with the rest of the continent seeing a partial eclipse. But those of us in North America are out of luck — we get no eclipse at all. Things will change next year, though, when a total solar eclipse will slice across the American heartland in August.

Script by Damond Benningfield

The main system that astronomers use to mark the positions of stars and other objects is basically a projection of Earth’s coordinates on the sky. The celestial equator, for example, is a projection of Earth’s equator. And the celestial north pole is the projection of Earth’s pole, marked by the star Polaris.

But astronomers also use a second system. It’s based not on Earth, but on the Milky Way. The galactic equator is the plane of the Milky Way galaxy, and the poles are the points 90 degrees north and south of the galactic center.

This system was adopted in 1958. The starting point is roughly the center of the galaxy, where the galactic plane crosses the Sun’s path across the sky.

The north galactic pole is in the constellation Coma Berenices, which is in the northwest at sunset, to the lower left of the Big Dipper. There’s no bright star to mark the pole, though.

The pole is a popular region for the astronomers who study other galaxies. It’s away from the busy star fields and dark dust clouds in the Milky Way’s disk, so it’s easier to see other galaxies.

In fact, a search for dark energy that’s getting under way at McDonald Observatory is looking not far from the pole, around the Big Dipper. The project will measure a million galaxies that are billions of light-years away. That will tell astronomers how the universe was expanding long ago, and help them understand how the expansion has changed — a key to understanding dark energy.

Script by Damond Benningfield

So far, the leading candidate in the search for dark matter has wimped out. No one is giving up on it, but scientists are pondering some other possibilities just in case.

Dark matter emits no detectable energy. Yet astronomers “see” it by measuring its gravitational pull on the visible matter around it. Those observations tell us that there’s a lot more dark matter than normal matter.

The leading explanation for dark matter is a heavy subatomic particle known as a WIMP. But years of searches haven’t produced a single confirmed WIMP, and none have been seen in the experiments with the Large Hadron Collider. A gamma-ray glow from the center of the Milky Way galaxy could be produced by colliding WIMPs, but there are other possible explanations as well.

So researchers are looking for particles that would be much less massive than WIMPs, which makes them even more difficult to detect. And others are going in the opposite direction — looking for enormous numbers of black holes.

When black holes merge, they emit gravitational waves, which create “ripples” in space. Detectors have seen waves from two such mergers, between black holes that are up to about 30 times heavier than the Sun. A recent study said that the number of such mergers could tell us the total number of these black holes, which were formed in the early universe. If there are enough of them, they could account for all or part of the dark matter — leaving WIMPs in the dark.

Script by Damond Benningfield

This is one of the best times of year to watch the Big Dipper. It’s in the northwest as night falls, with its bowl a little below the handle. It looks as though it’s about to scoop up a big dipperful of stars.

Using the bowl’s handle, you can use one of skywatching’s most famous directions to find one of the sky’s brightest stars: Arc to Arcturus. In other words, follow the curve of the handle away from the dipper until you come to the brilliant yellow-orange star Arcturus, which is well up in the west as night falls. It’s the brightest true star in the sky on summer evenings, and there are no other bright stars around it, so it’s hard to miss.

In skylore, there’s a connection between Arcturus and the dipper. The dipper’s stars are the brightest members of Ursa Major, the great bear. And the name “Arcturus” means “the bear watcher.” As the stars rotate across the night sky, Arcturus always follows Ursa Major, so skywatchers imagined that the star was keeping an eye on the bear.

There’s no real association between Arcturus and the bear, though. Arcturus is only 37 light-years away, while the seven bright stars that outline the Dipper all are at least twice that far.

And for skywatchers, there’s one other important difference. Arcturus will set not long after midnight tonight. But for much of the northern hemisphere, the Big Dipper never sets. So for awhile, the bear can roam freely — without the prying eye of the bear watcher.

Script by Damond Benningfield

The world’s biggest single-dish radio telescope always points straight up. The Arecibo telescope in Puerto Rico is built in a bowl-shaped depression, so it can’t turn from side to side. Instead, it looks straight up — at the point in the sky called the zenith — and it uses instruments atop the dish to track stars.

Arecibo isn’t the first observatory built to look straight up. Many centuries ago, the Maya and other cultures built observatories for tracking the Sun as it crossed the zenith.

To these cultures, the cardinal directions were sacred. And “straight up” was the fifth cardinal direction. That was especially true at latitudes where the Sun passed directly overhead at certain times of the year. From Mesoamerica, for example, it crossed the zenith a few weeks before and after the summer solstice.

In the city of Monte Alban, built by the Zapotec, one building contains a small room with a 15-foot tube to the surface. When the Sun passed through the zenith, it shone straight down the tube, filling the room with light. And a smaller tube illuminated a room at a Mayan city. These devices helped residents celebrate the Sun — and the sacred zenith.

And from the northern half of the United States, a couple of bright stars are passing through the zenith right now. From the latitude of about Denver or Kansas City, brilliant Vega crosses the zenith around 10 or 10:30 p.m. And from farther north, the slightly fainter star Deneb is at the zenith around midnight.

Script by Damond Benningfield

We live in a galaxy-eat-galaxy universe. Big galaxies frequently grab smaller ones, then make the stars and gas of the little ones their own. That may cause a dramatic change in the bigger galaxies.

A good example is M63, the Sunflower galaxy. It’s a stunningly beautiful spiral galaxy that resembles a sunflower. It’s about 27 million light-years away, and it’s about the same size as our own galaxy, the Milky Way.

One odd feature of the Sunflower is a streamer of stars that loops at least half-way around the galaxy. It’s about 10,000 light-years wide, and it contains hundreds of millions of stars — enough to make a small galaxy.

And in fact, that may be where the streamer came from. A couple of billion years ago, a dwarf galaxy may have passed close to the Sunflower. The gravity of the bigger galaxy grabbed the smaller one and pulled it apart, forming the streamer.

The encounter may have had some dramatic effects on the Sunflower as well. One edge of the galaxy’s disk is warped — a possible effect of the small galaxy’s gravity. Even more impressive, the encounter may have produced ripples in the Sunflower’s disk that created or enhanced its spiral structure. Those ripples may have triggered the birth of many new stars throughout the Sunflower — enhancing the beauty of this amazing galaxy.

The Sunflower is in the west-northwest at nightfall. It’s visible through a telescope, to the left of the handle of the Big Dipper.

Script by Damond Benningfield