StarDate

A stellar vanishing act led to a revelation about an iceball far from the Sun: It has a thin atmosphere. It’s the smallest object with a known atmosphere in the entire solar system.

The object is 2002 XV93. It’s in the Kuiper Belt – a wide “doughnut” beyond the orbit of Neptune, the Sun’s outermost major planet. The belt contains millions of icy bodies, including Pluto.

XV93 is about 300 miles in diameter – just one-fifth as big as Pluto.

Astronomers in Japan watched the little iceball in January of 2024. They’d calculated that XV93 would pass in front of a star, hiding the star from view. As they watched the star disappear, it didn’t blink out instantly. Instead, it dimmed a bit before it passed behind XV93. And it took a moment to reach full brightness when it returned to view. That meant that something was partially obscuring the star: an atmosphere. It’s only about one percent as thick as Pluto’s atmosphere, which is a bare wisp.

XV93’s gravity is extremely weak, so it can’t hang on to any atmosphere for long. So the gases might vent into space from its interior, which would keep the atmosphere going. Or they might have surrounded the object after it was hit by a small comet. In that case, the atmosphere would start to vanish quickly – eventually leaving XV93 airless.

Script by Damond Benningfield

After teaming up with the Sun to produce a solar eclipse today, the Moon will make another skywatching spectacle possible tonight – the Perseid meteor shower. It’s expected to be at its peak tonight. And since the Moon is new, it won’t be around to interfere with the sparklers.

The Perseids occur every August, as Earth flies through the orbital path of Comet Swift-Tuttle. The comet is a ball of frozen gases mixed with bits of rock and metal. As it approaches the Sun, some of its gas vaporizes. That releases some of the heavier material, known as comet dust.

As Earth zips through this material, some of the dust grains slam into our atmosphere at more than a hundred thousand miles per hour. They vaporize instantly, creating meteors – streaks of light across the sky.

Swift-Tuttle returned to the inner solar system a third of a century ago. That was its first appearance since the Civil War, so the Perseids were especially good for a while. With the comet retreating into the outer solar system, though, there’s less comet dust to feed the shower.

The meteors enter the atmosphere from the direction of Perseus – hence the name. But they can streak across any part of the sky, so you don’t have to look to a specific region to see them.

To view the Perseids, find a safe, dark observing site away from the pesky glare of city lights. Then watch the sky for cosmic sparklers.

Script by Damond Benningfield

The top of the world will see a total solar eclipse tomorrow. A partial eclipse will encompass a wider slice of the globe, including Alaska and parts of the northeastern United States.

A solar eclipse occurs when the Moon passes directly between Earth and the Sun, blocking the Sun from view. The Sun’s hot but faint outer atmosphere, the corona, looks like silvery tendrils radiating away from the Moon.

This eclipse begins when the lunar shadow first touches Earth, over northeastern Russia. The eclipse path then passes near the north pole, slides across parts of Greenland and Iceland, and finally over northern Spain before vanishing over the Mediterranean Sea. At its peak, off the coast of Iceland, totality will last for 2 minutes, 18 seconds.

A partial eclipse, where the Moon covers only a portion of the solar disk, will flank that path. In addition to parts of North America, it’ll encompass much of western Europe and western Africa.

From far-northern Alaska, the Moon will cover more than half of the Sun’s disk, at about 8:30 a.m. From Anchorage, about a quarter of the Sun will be covered. And from the northeast, the greatest eclipse takes place in northern Maine, shortly before 2 p.m.

If you’re in one of those areas, remember to protect your eyes. Watch the eclipse only through special glasses or dark welder’s glass, and never look at the unfiltered Sun.

Script by Damond Benningfield

A total solar eclipse is coming up on Wednesday. The Moon will cover the Sun, turning day to night across a narrow slice of the globe. Parts of Alaska and the northeastern United States will see a partial eclipse.

The timing of the eclipse is known down to the second, and has been for decades; astronomers can predict eclipses far into the future. But making such predictions isn’t easy. It requires a detailed knowledge of the Moon’s orbit around Earth, Earth’s orbit around the Sun, and even the shapes of Earth and the Moon.

Just when the first prediction was made is unclear.

There’s no doubt that people have been predicting lunar eclipses for thousands of years. But they’re easier to forecast. Earth’s shadow is roughly 35 times wider than the Moon’s, so you don’t need to be quite as precise to get it right.

There are stories that the Chinese were predicting solar eclipses more than 4,000 years ago, but no confirmation. An eclipse in 585 BC that stopped a war supposedly was predicted by a Greek scientist. But many present-day scientists doubt that.

The first confirmed prediction was made by Edmond Halley, using the laws of gravity recently devised by Isaac Newton. Halley forecast that an eclipse would cross England on May 3rd, 1715. And he was right. So the event is known as Halley’s Eclipse – honoring the prediction of an astronomical spectacle.

Script by Damond Benningfield

The Moon forms a nice triangle with the twins of Gemini at dawn tomorrow – the stars Castor and Pollux. Castor is to the left of the Moon, with brighter Pollux about the same distance to the lower left.

Castor consists of six known stars. We haven’t found planets orbiting any of them. Pollux is a single star. And it has one likely planet – a giant that’s much heavier than any of the planets in our own solar system.

The planet was discovered 20 years ago, by astronomers at McDonald Observatory. They’d seen evidence of the star earlier, but they couldn’t confirm it.

The planet is known as Pollux b. It also has a formal name – Thestias – a variation of the name of Pollux’s mother in Greek mythology. The planet is almost three times the mass of Jupiter, the Sun’s largest planet. It’s farther from Pollux than Earth is from the Sun. But that’s much too close for the planet to be habitable.

That’s because Pollux has passed beyond the prime phase of life. It’s puffed up to about nine times the Sun’s diameter, so it’s much brighter than the Sun. That extra energy makes Thestias much too hot to have liquid water at its surface – a key ingredient for life.

The temperature could have been more comfortable in the past. The planet itself probably is a big ball of gas – not a good place to live. But if it has any moons, it’s possible that they could have offered better environments for life.

Script by Damond Benningfield

Mars and the Moon hang out in tomorrow’s early-morning sky. Mars looks like a bright star to the right of the Moon at first light.

The Moon is a thin crescent, and it’s getting thinner by the day. That means the Moon is about to cross between Earth and the Sun, so the Sun is lighting up less of the lunar hemisphere that faces our way. But you can make out the nighttime part of the disk because it’s lit up by earthshine – sunlight reflected from Earth.

If anyone were standing on that part of the Moon, the earthshine would be bright enough to read by. That’s because Earth is in its gibbous phase – it’s almost fully in daylight. Earth is much bigger than the Moon, and its surface is much more reflective. So a gibbous Earth is dozens of times brighter than a gibbous Moon.

From Mars, both Earth and the Moon appear gibbous. That’s because Earth and Mars line up more or less on opposite sides of the Sun. But over time, Earth and the Moon would appear to go through a full cycle of phases, just as the Moon does as seen from Earth. And their phase would always be the same.

As the Moon orbits Earth, the angle between them would vary. So sometimes, Earth and the Moon would appear so close together that they’d form a single bright point. But at their widest separation, it would be easy to see them as individual points, with Earth especially bright, and the Moon a good bit fainter.

Script by Damond Benningfield

The terms that describe the phases of the Moon can be a little confusing. Take tomorrow morning, for example. From most of the United States, the Sun will light up one-quarter of the lunar hemisphere that faces Earth. You might think that would make it a “quarter” Moon – but no.

A “quarter Moon” doesn’t describe the Moon’s appearance – it describes the position in its month-long cycle of phases.

The cycle begins when the Moon is new. The Moon crosses the line between Earth and the Sun then, so it’s hidden in the Sun’s glare. A bit more than a week after that, the Moon is a quarter of the way through the cycle – it’s at first quarter. It lines up at a right angle to the Earth-Sun line, so the Sun lights up half of the visible disk.

A week later comes full Moon. And a week after that comes last quarter – three-quarters of the way through the cycle. Once again, half of the lunar disk is illuminated – but it’s the opposite half from the first-quarter Moon. Then the Moon rolls back around to new – starting a new cycle.

The next cycle starts on Wednesday. And this time it brings a rare treat – a total solar eclipse.

In the meantime, look for the Moon passing through Taurus tomorrow. Aldebaran, the bull’s bright eye, is to the right of the Moon. Elnath, the tip of one of its horns, is closer to the lower left of the Moon. As a bonus, Mars is below them, and the Moon will pass close to it on Sunday.

Script by Damond Benningfield

The Dog Star is just beginning to emerge from the Sun’s glare in the dawn sky. By tradition, that means we’re just getting into the Dog Days of summer.

Sirius is the brightest star in the night sky. It’s climbing into view in the morning glow of the brightest star in all the sky – the Sun. Sirius is quite low in the east-southeast shortly before sunrise. It’s best seen from the southern tier of states, although it’ll pull into better view from the northern states over the coming days and weeks.

Sirius is also the leading light of the constellation Canis Major, the big dog, so it’s known as the Dog Star. And that’s where we get the name for this part of summer – the Dog Days.

Sirius makes its closest approach to the Sun during summer, disappearing for a few weeks in the Sun’s glare. In ancient times, it returned to view in the morning by early July – just as the weather was reaching its hottest. At the time, it seemed logical to associate the bright star with the sweltering weather. So the people of ancient Greece and Rome named that stifling time of year in the star’s honor.

Thanks to an effect known as precession, the stars shift position with respect to the Sun over the centuries. So Sirius now returns to view a few weeks later than it did thousands of years ago. So the heat is already in full force at northern latitudes – the Dog Days of summer.

Script by Damond Benningfield

NGC 6397 seems like a pretty ordinary globular star cluster. It’s not especially big or bright, and nothing about its stars is going to strike anyone as odd. Yet it’s been an important astronomical laboratory. Among other things, it’s helped astronomers determine the age of the galaxy, and the minimum “weight” of a star.

NGC 6397 is in the constellation Ara, the altar. It’s below the tail of Scorpius, the scorpion. That puts it so far south that it’s visible only from the southernmost United States.

Hubble Space Telescope has monitored the cluster for decades. It contains several hundred thousand stars, packed into a ball only a few dozen light-years across. The Milky Way contains about 150 of these clusters, which are the galaxy’s oldest inhabitants.

Over the decades, Hubble has monitored many of the cluster’s stars. The observations helped astronomers lock down the minimum mass for an object to shine as a true star – about eight percent the mass of the Sun.

Another set of observations helped measure the cluster’s distance – just 7800 light-years – one of the closest of all globulars. That number is important for determining the cluster’s age – 13.4 billion years. That means that NGC 6397 – and its home galaxy, the Milky Way – took shape when the universe was just 400 million years old.

Script by Damond Benningfield

The gods of ancient Greece weren’t great parents. One of the ancient gods known as the Titans, for example, swallowed his newborn children. Eventually, though, the kiddos escaped and overthrew him. Their victory is commemorated in the stars, as the altar where they swore allegiance to each other. It’s the constellation Ara, and it’s below the tail of the scorpion.

In the story, Cronus was the king of the Titans. He’d come to power by defeating his own father. A prophecy said that one of his children would defeat him. To prevent that, he swallowed each of his first five children.

But his wife, Rhea, got tired of that plan. When the last child was born, she hustled him away to an island and gave Cronus a rock wrapped in a blanket.

That child was Zeus. When he grew up, he returned home and forced his dad to cough up his brothers and sisters. They pledged to defeat the Titans, and won a 10-year war. Zeus became the king of the new gods and ruler of the sky. To celebrate the victory, he placed the altar among the stars.

The altar is so far south that most of it’s visible only from far-southern parts of the United States – places like Florida, South Texas, and especially Hawaii.

Ara is one of the smaller constellations, with only a couple of moderately bright stars. But it does have some good deep-sky objects, and we’ll talk about one of them tomorrow.

Script by Damond Benningfield