StarDate

The two brightest objects in the night other than the Moon are getting ready to slide past each other in the early evening sky. Unfortunately, they’re so low in the sky that they’re tough to see, especially from the northern tier of states. But if you have a clear horizon, and you look at just the right time, you should be able to pick them out.

The brighter of the two is Venus, our nearest planetary neighbor. It passed behind the Sun as seen from Earth in early June, and remained hidden in the Sun’s glare for weeks. It’s beginning to emerge from that glare. But thanks to the relative motions of Venus and Earth, it’s a slow process. The brilliant planet is making little progress from night to night.

The other bright light is Jupiter, the solar system’s largest planet. It’s been in great view for months. But it’s getting ready to pass behind the Sun, so it’s dropping toward the Sun in a hurry.

The two worlds are due west about a half-hour after sunset, when there’s still plenty of color in the sky. They’re only a few degrees above the horizon, so any trees or buildings will blot them out. They’re easier to see from the southern tier of states because they climb into the sky at a little better angle.

Tonight, Jupiter is close to the upper left of Venus. On Saturday, they’ll stand almost atop each other. And after that, Jupiter will drop lower each day while Venus climbs higher, ready to put on a better show this fall and winter.

Script by Damond Benningfield

The Moon slides toward the eye of the bull early tomorrow. In fact, it’ll pass directly in front of the star for a while — but not until the middle of the day for those of us in the U.S. We’ll have to be content with a beautiful close encounter in the wee hours before daylight.

A similar encounter more than 1500 years ago helped prove that the stars move across the sky — sort of.

In the early 18th century, British astronomer Edmund Halley compared the positions of three stars to their positions almost 2,000 years earlier. The stars were Sirius, Arcturus, and Aldebaran — the star the Moon cozies up to early tomorrow. He also used Aldebaran’s position during a similar encounter with the Moon in the year 509.

Halley’s calculations showed that all three stars had moved a significant amount in that time — the width of the Moon or more. It was a crucial discovery, because it showed that the stars aren’t “fixed” in space — they move. Halley also suggested that because the three stars moved so far across the sky, they must be fairly close.

Modern calculations show that Halley’s numbers for Sirius and Arcturus were spot on. But his numbers for Aldebaran were off — it hadn’t moved nearly as much as he wrote. Even so, the bull’s eye does move across the sky — just like every other star.

Look for Aldebaran to the lower left of the Moon as they climb into good view by 1 or 2 in the morning. The Moon will move even closer to it by dawn.

Script by Damond Benningfield

Mars passes its legendary “rival” the next few evenings — the star Antares, at the heart of Scorpius, the scorpion. The two are in the southwestern quadrant of the sky, with the planet Saturn looking down on them.

Antares gets its name because it resembles Mars. Both bodies look reddish orange, and both are bright. Antares also lies along the path the Sun and planets follow across the sky, so every couple of years, Mars passes quite close to it.

When they put that all together, skywatchers thought of Mars and the star as rivals, so the name Antares means “rival of Ares,” the Greek version of Mars.

Of course, any rivalry between a planet and star is one-sided. Mars is a ball of rock and metal a bit more than half the diameter of Earth. It looks red or orange because its surface is coated with orange dirt.

Antares, on the other hand, is a supergiant, which makes it one of the biggest stars in our region of the galaxy. If Antares replaced the Sun in our own solar system, it would extend beyond the orbit of Mars — putting a quick end to its famous rival.

Look for these two rivals in the southwest as night falls. Mars is the brighter of the two, shining like a brilliant orange star. Antares is close to its lower left this evening, but almost directly below Mars by tomorrow evening. After that, Mars will slide slowly eastward, leaving Antares behind. The golden planet Saturn, which is fainter than Mars, stands just above them.

Script by Damond Benningfield

Tons of space dust rain down on Earth every day. Most of it comes from asteroids and comets in our own solar system — the debris from countless collisions. But some of it comes from outside the solar system. Millions of years ago, in fact, our planet was bombarded by debris from exploding stars — material that might have caused changes in Earth’s climate.

Scientists have found this debris in ocean sediments, and in samples of Moon dust gathered by Apollo astronauts.

In particular, they’ve discovered tiny bits of a radioactive form of iron. It’s produced by exploding stars, but it decays fairly quickly, so any that’s found on Earth must have arrived within the last few million years.

The first samples were discovered in ocean sediments in 1999. More were found later in samples from the Moon. And earlier this year, a team reported finding more iron in sediments from the Atlantic, Pacific, and Indian oceans.

There were high concentrations in samples from about 1.5 million and 2.3 million years ago, with a fainter concentration about 8 million years ago. That suggests that Earth was bombarded with debris from three different supernovae.

The more recent concentrations correspond to a time when Earth’s climate grew cooler, while the older one corresponds to a time of big changes in life on our planet. Cosmic rays from the explosions may have spurred cloud formation, altering the climate — dramatic changes caused by exploding stars.

Script by Damond Benningfield

If you’re looking for life in other star systems, it’s probably best to start with stars that resemble our own Sun. After all, it’s the only system where we know there’s life. And a couple of stars that fit the profile are in view in the western sky this evening. A few years back, in fact, they were identified as two of the best targets to search for signs of life, and for signs of intelligent life.

More than a decade ago, Margaret Turnbull and Jill Tarter published a catalog of more than 17,000 nearby star systems that could be habitable. The systems feature stars that are similar to the Sun.

Turnbull later picked five of those systems as good candidates for searches for radio signals from other civilizations. Those systems are especially close matches to the Sun.

At the top of the list was Chara, in Canes Venatici, the hunting dogs. It’s in the west-northwest at nightfall, between the bright star Arcturus and the Big Dipper. Chara is almost exactly the same mass as the Sun, and a bit bigger and brighter. It’s also older than the Sun, so there’s been plenty of time for life to develop there.

Another candidate was 18 Scorpii, in Scorpius, which is low in the south-southwest. It, too, is a near-twin to the Sun. It’s perhaps a billion years younger than the Sun, but it appears to have plenty of ingredients for planets.

So far, neither planets nor messages from ET have been detected in either system. Yet the search for planets — and life — continues.

Script by Damond Benningfield

Plenty of bright stars decorate the sky on these warm summer evenings. Among others, yellow-orange Arcturus is in the west, red-orange Antares is low in the south, and bright white Vega stands high overhead.

What’s remarkable, though, is that none of the bright stars visible on a summer’s eve is anything like our own star, the Sun. In fact, of all the stars visible to the unaided eye, only a couple of dozen can safely be described as “Sun-like.” And many of those are so faint that you need dark skies to see them.

In part, that’s because stars like the Sun aren’t that common. Stars that are relatively close to the Sun’s mass and stage of life account for only a few percent of all the stars in the galaxy. Most stars are much smaller and fainter than the Sun. Even so, Sun-like stars are so faint that they’re not visible beyond a few dozen light-years.

Most of the really bright stars in the night sky are giants or supergiants. These stars are nearing the ends of their lives, so they’ve puffed up to dozens or even hundreds of times the Sun’s diameter. Such big stars are rare, but they’re dazzlingly bright, so they can be visible from hundreds of light-years away.

Vega and a few other bright stars haven’t yet reached that stage of life — they’re still in middle age, as the Sun is. But they’re a good bit heavier than the Sun, which makes them shine much brighter.

Two Sun-like stars are visible in the western sky at nightfall; more about them tomorrow.

Script by Damond Benningfield

Over the centuries, astronomers have come from many backgrounds. Among many other professions, they’ve been astrologers, beer makers, and industrialists. But few can claim the background of Milton Humason. Before he became a staff astronomer at Mount Wilson Observatory in California, he was a “muleskinner” — he drove teams of mules that carried the materials for building the observatory.

Humason was born 125 years ago today in Dodge Center, Minnesota. His family moved to California a few years later. And when Humason was 14, he went to a summer camp on Mount Wilson. He talked his parents into letting him stay there for a year. So he dropped out of school — and never returned.

After his stint as a muleskinner, Humason became a janitor at the new observatory. And a couple of years later, he became a night assistant — someone who helped the astronomers use the telescopes.

Despite his lack of education, Humason was so smart and diligent that the observatory’s director, George Ellery Hale, hired him as an assistant astronomer, and later as a full-fledged astronomer.

It was a good hire. Humason was a skilled observer. He took spectra of hundreds of galaxies — work that, among other things, revealed how the galaxies were moving through space. Edwin Hubble used those observations to measure the expansion of the universe, and to work out how the expansion was changing — landmark discoveries made with the help of a one-time muleskinner.

Script by Damond Benningfield

Until a half-century ago, maps of the lunar surface were pretty coarse. Even the best telescopes couldn’t show features smaller than a couple of miles in diameter. And the lunar farside — the hemisphere that always faces away from Earth — had been seen in only a handful of crude pictures.

But all of that began to change 50 years ago today. That’s when Lunar Orbiter 1 started snapping pictures of the lunar surface — pictures that revealed details as small as a few feet across.

Lunar Orbiter was designed to support the Apollo missions to land astronauts on the Moon. It took pictures of possible landing sites to make sure they were flat and free of dangerous boulders.

Scientists wanted Lunar Orbiter to take its pictures on film, which provided sharper images than TV cameras. But there was no way to get the film back to Earth. So engineers devised an ingenious system. The camera took a couple of hundred pictures on 70-millimeter film. When the film reached the end of its roll, it was developed inside an on-board laboratory. It was then scanned, and the images were transmitted to Earth.

The five Lunar Orbiters also snapped highly detailed images of almost the entire lunar surface. The maps made from these pictures weren’t surpassed until fairly recently.

Lunar Orbiter 1 took one of its most famous pictures on August 23rd, 1966: a crescent Earth rising above the lunar surface — the first view of Earth from the vicinity of the Moon.

Script by Damond Benningfield

You don’t want to mess with the Milky Way. Our home galaxy is especially big and heavy, so its gravity can tear apart star clusters and even entire galaxies. And astronomers recently discovered one such victim: a globular star cluster that is no more.

A globular cluster contains hundreds of thousands of stars packed into a ball that’s only a few dozen light-years across. The Milky Way has more than 150 such clusters.

The constellation Draco, the dragon, features a globular that the Milky Way has torn to shreds. Astronomers discovered it after noticing a streamer of stars that were all racing through space at the same speed. The stars also share the same chemistry, and they’re all more than twice as old as the Sun — a trait of the Milky Way’s globular clusters.

Known as LAMOST-1, the shredded cluster is only 8600 light-years from Earth. That’s not much farther than the nearest surviving globular.

Although the cluster no longer exists, its stars keep shining. Today, they orbit the center of the Milky Way individually instead of as a group. At their closest point, they pass just 2500 light-years from the galactic center; at their farthest, they’re 26,000 light-years out — nearly the same distance as we are.

Once held close together by their mutual gravitational pull, the stars of this dead cluster now circle through the galaxy on their own — stripped from their stellar home by the gravity of the Milky Way.

Script by Ken Croswell, Copyright 2016

When you drive toward the downtown of any city on a clear, dark night, you can see a dome of light above it from miles away. Astronomers can see a similar glow from the downtown region of the Milky Way galaxy — a glow of gamma rays.

That region of the galaxy is in the constellation Sagittarius, which scoots low across the south on summer nights. Its brightest stars form the outline of a teapot.

The galaxy’s downtown is just above the spout of the teapot, although it’s hidden from human eyes behind clouds of dust. But gamma rays go right through the dust. In fact, they’re the most powerful form of energy.

The gamma-ray glow around the center of the Milky Way may be produced by dark matter — a form of matter that emits no detectable energy, but that exerts a gravitational pull on the matter around it. There appears to be about fives times more dark matter than the “normal” matter that makes up stars, planets, and people.

Dark matter may consist of heavy subatomic particles, known as WIMPs. They may occasionally ram together, producing gamma rays. And several teams of scientists say the gamma-ray glow from the Milky Way is a good match for what they’d expect collisions between WIMPs to look like.

Others, though, say the glow comes from thousands of rapidly spinning corpses of once-mighty stars.

Astronomers continue to watch the glow from the center of the Milky Way to better understand conditions in the galaxy’s busy downtown.

Script by Damond Benningfield