StarDate

The Moon and the bright star Aldebaran are scheduled for a last rendezvous tomorrow night. The Moon will pass in front of the bright star, briefly blocking it from view. It’s the last time they’ll meet that way until 2033.

Such an encounter is known as an occultation. And the Moon and Aldebaran began a series of them in January of 2015.

Occultations between the two bodies are possible because of their positions relative to the ecliptic, which is the Sun’s path across the sky. Aldebaran — the bright orange eye of Taurus, the bull — is about five and a half degrees south of the ecliptic. The Moon, on the other hand, moves back and forth across that path. At its maximum extent south of the ecliptic, it just overlaps Aldebaran.

That doesn’t happen every time the Moon passes Aldebaran. Instead, the occultations occur over a period of about three and a half years, with the middle of any two cycles separated by 18.6 years. So the next cycle begins in 2033 and ends in 2037. And because of the viewing angle, occultations of Aldebaran are visible only from the northern hemisphere.

This occultation will be visible from some of the islands of far-northern Canada and a bit of Greenland. Here in the United States, we’ll have to settle for a close pairing of the two bodies. Aldebaran will stand to the lower left of the Moon at first light tomorrow, and even closer to the upper right of the Moon on Monday.

Script by Damond Benningfield

The center of the Milky Way galaxy could be a bit hazardous. It’s home to a supermassive black hole — about four million times the mass of the Sun. Anything that gets too close is pulled into the black hole, where it’s lost forever.

A recent study says there could be hundreds of smaller black holes there as well.

These black holes are collapsed cores of once mighty stars, so they’re up to a few dozen times the mass of the Sun. They produce no energy, but they sometimes have companion stars. A black hole can steal gas from the surface of the companion. The gas forms a disk around the black hole. As the gas spirals toward the black hole, it gets extremely hot, so it emits X-rays.

Astronomers looked for the glow of these disks with Chandra X-Ray Observatory, a space telescope. They found more than a half-dozen likely suspects. All of them are within about three light-years of the supermassive black hole. Since only the brightest sources are likely to show up at that distance, that suggests there could be many more black holes in that same volume of space. That would make the center of the galaxy an even more dangerous region for visitors.

We can safely view that region, though. It’s in Sagittarius the archer, which scoots low across the southwest at this time of year. The constellation’s brightest stars form the outline of a teapot. The center of the galaxy is above the spout of the teapot — 27,000 light-years away.

Script by Damond Benningfield

In the modern world, lithium is a vital element. It’s a key ingredient in batteries that power smartphones and cars, for example. And it’s also an important ingredient in treatments for some psychiatric problems.

Some recent research says the element may be created by exploding stars — enough of it to account for all the lithium in the galaxy.

Small amounts of lithium were created in the Big Bang. And some of those atoms are still floating around. But lithium atoms are a bit delicate. When they’re incorporated into stars, they’re easily destroyed. And so are atoms created by the stars. But a couple of exploding stars known as novae created a lot of the stuff.

A nova occurs when a small dead star steals gas from a companion star. When enough gas builds up, it triggers a nuclear explosion. The nuclear reactions can forge new elements.

That’s what astronomers have detected in a few novae. The most recent was seen a couple of years ago in Sagittarius.

To be precise, the astronomers actually saw lots of another element, beryllium. But it’s a radioactive form of the element, which quickly decays to form lithium. Given the amount of lithium formed in the explosions, and the number of novae over the lifetime of the galaxy, that could mean that stellar explosions are the main source of this important element.

And Sagittarius is low in the south at nightfall. The constellation’s brightest stars form the outline of a teapot.

Script by Damond Benningfield

The “teapot” of Sagittarius steams low across the southwest on late summer evenings. The spout of the teapot lines up in front of the center of the galaxy, which is about 27,000 light-years away.

Astronomers have discovered about 50 exoplanets in that region — planets orbiting stars other than the Sun. And most of them were discovered using an unusual technique, known as microlensing.

It takes advantage of the fact that the gravity of a massive object, such as a star or planet, can act as a lens. It both bends and magnifies the light of objects behind it. So if a star and planet pass in front of another star, the background star gets two “bumps” in its light — a big one from the foreground star, and a smaller one from its planet.

From the size and length of the bump, astronomers can learn about the planet’s mass and its orbit. But they don’t get a second look. Most of the planets discovered through microlensing are thousands of light-years away, which is too far to study with other techniques.

Two microlensing projects have discovered most of the planets in this region. One project, called OGLE, uses a telescope in Chile. The other, MOA, is in New Zealand. They look toward the galactic center because there are huge numbers of stars in that direction. That gives them a better chance of seeing the random alignments that produce microlensing events — alignments that can reveal distant planets.

More about Sagittarius tomorrow.

Script by Damond Benningfield

A big asteroid, named Florence, flew about four-and-a-half million miles from Earth back in September. Astronomers took a look at it with radio telescopes. And they discovered that it wasn’t alone. Two small moons orbit the asteroid. One is about the length of two football fields, while the other spans three football fields.

No one was really surprised to find the little moons. That’s because more than 300 asteroids have known moons, including quite a few with two of them, and some that have three.

The first asteroid moon was discovered in pictures that were snapped 25 years ago today. The Galileo spacecraft was passing through the asteroid belt on its way to Jupiter. It flew close to Ida, an asteroid that’s about 35 miles long. When mission scientists looked at the pictures of Ida a few days later, they discovered a mile-wide companion. They named it Dactyl, after creatures from Greek mythology that lived on Mount Ida.

The most likely way for an asteroid to get a moon is through a collision with another asteroid. If the impact is at the right speed and angle, it can chip off a chunk of rock and send it into orbit. If the impact is too strong, though, the debris sails away into space, leaving the parent asteroid behind.

In some cases, a collision may split an asteroid in two. That creates a pair of twins — two asteroids of roughly equal size orbiting the Sun together.

Tomorrow: finding planets toward the heart of the galaxy.

Script by Damond Benningfield

The “evening star” is sneaking up on a true star. Spica, the leading light of Virgo, is close to the upper right of brilliant Venus as darkness falls this evening. They’ll get even closer between now and Friday night. After that, Venus will move away from the star, adding separation each night.

Such an encounter is possible because both Venus and Spica lie near the ecliptic, which is the Sun’s path across the sky.

Spica’s position relative to the ecliptic is fixed — it never changes. Spica is moving around the center of the galaxy at hundreds of thousands of miles per hour. But it’s also 250 light-years away. That distance is so incredibly vast that it takes many human lifetimes for any change in position to become apparent.

Venus, on the other hand, is our closest planetary neighbor. As Venus and Earth orbit the Sun, our viewing angle changes quickly. So Venus changes position from night to night. That motion is quite easy to follow, especially when Venus is passing another celestial body, such as Spica.

In a few weeks, Venus will reverse course and begin to drop back toward the Sun. It’ll pass between Earth and the Sun in late October, moving from evening sky to morning sky. By the time it climbs into good view, in early November, it’ll once again be close to Spica — a frequent companion for our close planetary neighbor.

Tomorrow: discovering moons around some chunks of cosmic rubble.

Script by Damond Benningfield

In centuries past, an expedition to view a rare astronomical event could take years, and there was no guarantee that everyone would make it back alive.

Consider the expedition of the Endeavour. The ship left Plymouth, England, 250 years ago today. Her destination was Tahiti, to observe a transit of Venus across the face of the Sun.

Endeavour was commanded by James Cook, an experienced sailor and mapmaker. The ship carried 94 men, including Charles Green, the assistant to England’s astronomer royal.

Endeavour arrived in Tahiti in April of 1769. The crew set up an observing site, known as Fort Venus. Green, Cook, and other scientists observed the transit with telescopes and other instruments.

Astronomers hoped the transit would help them measure the distance from Earth to the Sun, which would allow them to map the solar system. In addition, the Royal Society hoped to get measurements that would help sailors determine their longitude.

When the transit was done, Endeavour continued exploring. It ventured to New Zealand, where Cook made the first map of its coastline. And it explored the east coast of Australia.

Endeavour then journeyed to the East Indies for repairs and supplies. Most members of the crew contracted malaria or other diseases there. About 30 of them died, including Green. The survivors sailed around Africa, then returned to England in 1771 — completing a journey of exploration that took them around the globe.

Script by Damond Benningfield

The full Moon is passing through Aquarius tonight. The constellation is home to one of the most interesting planetary systems yet discovered. Known as Trappist-1, it contains at least seven planets. All of them are roughly as big as Earth, and three of them are in the “habitable zone” — the distance from the star where temperatures are just right for liquid water.

Most of the system’s planets were discovered or confirmed by Spitzer Space Telescope, which was launched 15 years ago today. It observes the sky at infrared wavelengths, which are invisible to the human eye.

Infrared is produced by objects that are relatively cool. That includes clouds of gas and dust, and the nurseries that give birth to stars.

It also includes planetary systems. When Spitzer was built, the study of exoplanets was in its infancy, so they weren’t even on its list of targets. It turns out, though, that the telescope is good at finding and studying planets in other star systems. In addition to discovering some of the planets of Trappist-1, for example, it found that several of them contain a lot of water.

Spitzer is still operating, but the job is getting harder. It long ago used up the liquid helium that cools its instruments, so some of them have shut down. And it’s moving farther from Earth, which complicates its operations. Still, it’s been approved to continue watching the skies into next year — giving it time to study many more planets.

Script by Damond Benningfield

The Sun’s closest planet is putting on a pretty good show in the early morning sky right now. It’s a bit north of due east as twilight paints the dawn, and quite low. That means you need a clear horizon to spot it. It’s pretty bright, though, and it’ll get brighter over the next couple of weeks.

Mercury is at a point in its journey across the sky known as elongation. That means it stands farthest from the Sun as seen from Earth. Because Mercury is closer to the Sun than any other planet, though, it doesn’t climb all that high. But it does get pretty bright. Tomorrow, it’ll shine at magnitude zero — brighter than all but a half-dozen other planets and stars.

We’re helped out a bit by the geometry of the solar system. At this time of year, the path followed by Mercury and the other planets is at a high angle, so Mercury pops almost straight up as it rises. And the Sun is rising later each morning, so that extends the viewing window.

Mercury is getting brighter by the day, too. Paradoxically, that’s because Mercury is moving farther from Earth. As it moves away, though, the side of the planet that faces Earth receives more and more sunlight. Tomorrow, for example, only about a third of that hemisphere is illuminated. By Tuesday morning, though, about half of it will be lit up. So Mercury will become an easier target over the next 10 days or so, before it once again drops too close to the Sun to pick it out.

Script by Damond Benningfield

The recent volcanic eruption in Hawaii reminds us that Earth is still an active planet — there’s a lot going on. And that’s the case for several other worlds as well. There’s evidence of active volcanoes on Venus, for example. And Io, one of the big moons of Jupiter, is covered with hundreds of them.

No one is quite sure, though, about whether there’s any volcanic activity on Mars. No one’s ever seen any eruptions on the planet. But there is some evidence of activity fairly recently — on the geological time scale. And that suggests that Mars isn’t dead yet.

Mars is home to several giant volcanic mountains. There’s no doubt that they were quite busy in the distant past. But Mars has lost most of the heat left over from its formation, so there’s not a lot to keep them going. And the Martian crust isn’t made of overlapping plates, which create volcanic activity on Earth.

The biggest volcano is Olympus Mons. It covers an area half the size of New Mexico, and towers about 15 miles above the surrounding plains. It probably formed billions of years ago, and kept on going. Some studies say it last erupted about 20 million years ago. Others say it staged some small eruptions just two million years ago. Either way, that means it could erupt again in the future.

Look for Mars close to the lower right of the Moon this evening, and sticking close to the Moon throughout the night. The planet looks like a bright orange star.

Script by Damond Benningfield