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Celestial Sphere Aug 23, 2025

If you watch the stars on a dark night, it’s easy to think of the sky as a great dome. But as the night goes on, the dome rotates. New stars rise in the east, while others disappear in the west. So ancient skywatchers thought of the sky not as a dome, but a sphere that completely encircles us – the celestial sphere.

To the Greeks, the sphere was real – a perfect crystalline surface, with the stars hanging from it like lanterns. Earth stood still at the middle of the sphere, which turned around it.

Today, of course, we know that Earth is turning, and the stars are so far away that they appear to be fixed in place. Yet astronomers still use the celestial sphere. Their coordinate system is based on it. The system has lines of latitude and longitude, an equator, and north and south poles – all of which are projections of Earth’s coordinates.

The celestial poles, for example, are based on the projection of Earth’s poles – the directions in which our planet’s axis is pointing.

There’s also a celestial equator – an extension of Earth’s equator. As darkness falls tonight, it arcs from Aquarius, in the east; through Aquila, in the south; and down to Virgo, in the west.

Only those who live near the equator can see the entire celestial sphere. For everyone else, it’s clipped. And at the poles, only half of the sphere is ever visible – a great dome showing the same stars all year long.

Script by Damond Benningfield

Ophion Aug 22, 2025

Many “open” star clusters arch high overhead on summer nights. They’re lined up along the glowing band of the Milky Way – the outline of our home galaxy. Each cluster is a family of stars – from a few dozen to a thousand or more. But open clusters don’t stay together for long. Their stars eventually spread out, so the cluster disappears.

Some families begin to spread out early – before many of their stars are even fully formed. One recently discovered example is called Ophion. It consists of more than a thousand stars.

Astronomers found the group by analyzing data from Gaia, a space telescope. They looked through observations of more than 200 million stars. Then they narrowed their search to stars that are cooler than the Sun, and no more than 20 million years old. And Ophion just popped out.

The stars form a giant clump that’s centered about 650 light-years away. But all of its members are going their own way. So they don’t form an obvious “cluster” – a tight grouping that’s easy to pick out.

Ophion is on the edge of a region that’s given birth to many thousands of stars. Exploding stars in that region – or within Ophion itself – might have scattered the stars like bowling pins, keeping the family from sticking together.

Ophion is near the middle of Ophiuchus, which is well up in the south-southwest at nightfall. You can see many clusters there – but not a hint of the ill-fated Ophion.

Script by Damond Benningfield

Eos Aug 21, 2025

Human eyes are perfectly tuned to see sunlight. But that’s a thin slice of the total range of light. As a result, we miss a lot of what’s out there – even objects that are big and close.

A recently discovered example is a cloud of gas and dust that’s been named Eos. It spans about 40 times the width of the Moon. But it’s thinly spread, and it produces most of its light in the far-ultraviolet – wavelengths we can’t see. And even if we could see them, Earth’s atmosphere blocks them. So Eos wasn’t discovered until astronomers combed through observations made two decades ago by a Korean space telescope.

The cloud’s inner edge is about 300 light-years away. It’s along the rim of the Local Bubble – a giant void around the solar system that’s been cleared out by exploding stars.

Eos is about 170 light-years across. It contains enough gas to make more than 5,000 stars as heavy as the Sun. But there’s no evidence that it’s ever given birth to any stars at all. And while it could spawn stars in the future, that’s not likely. The cloud is evaporating, and should vanish in about six million years.

Eos is centered along the border between the northern crown and the head of the serpent. That point is high in the west-southwest at nightfall, to the upper left of the bright star Arcturus. But unless you have your own space telescope, there’s no way to see this giant neighbor.

Script by Damond Benningfield

Moon and Mercury Aug 20, 2025

Many centuries ago, people knew of only seven metals. That also was the number of known “planets” – the five true planets that are visible to the naked eye, plus the Sun and Moon. So each metal was associated with a planet – gold with the Sun, silver with the Moon, for example.

Another metal with a good match was quicksilver. It’s the only metal that’s liquid at everyday temperatures, so it was associated with the quickest planet: Mercury. And it was even given the planet’s name.

The planet moves back and forth between the morning and evening sky every few months. That quick motion is where the planet got its name. Mercury was the Roman messenger god, who flitted across the heavens on winged heels.

The only spacecraft to study the planet from orbit didn’t find any trace of the metal mercury on its surface. And if there’s any of it near the planet’s equator, it would go through all three everyday phases of matter. At night, the planet is so cold that the metal would be frozen solid. At noon, it’s so hot that it would vaporize, forming a gas. And for much of the rest of the daytime, it would be a liquid – quicksilver puddles on a quicksilver planet.

Mercury will stand close to the Moon during the dawn twilight tomorrow. It looks like a fairly bright star, to the lower right of the Moon. The brighter planets Venus and Jupiter align to their upper right – the planets of copper and tin.

Script by Damond Benningfield

Moon, Venus, and More Aug 19, 2025

Early risers are in for a treat tomorrow. Venus, Jupiter, and the twins of Gemini congregate around the Moon. The group climbs into good view a couple of hours before dawn. Venus is close to the lower right of the Moon, Jupiter is farther to the upper right, and Gemini’s twins are to the upper left of the Moon. The brighter twin, Pollux, is especially close to our satellite world.

Venus is the “morning star” – the brightest member of the group after the Moon. It shines so brightly because it’s close to Earth and the Sun, and because it’s topped by clouds of sulfuric acid. They reflect about three-quarters of the sunlight that strikes them.

Jupiter is the next brightest – mainly because it’s the largest planet in the solar system. It’s about 11 times the diameter of Earth, and it’s more than twice as massive as all the other planets and moons put together. And Earth is moving closer to Jupiter now, so the planet will grow even brighter over the next few months.

Pollux and Castor, the twins, are true stars. But they’re hundreds of thousands of times farther than the planets, which dulls their countenance. Even so, they’re easy to see through the moonlight – part of a beautiful panorama in the early morning sky.

Another bright light rises well below the group: Mercury, the Sun’s closest planet. The Moon will stand close to it on Thursday, and we’ll talk about that tomorrow.

Script by Damond Benningfield

Moon and Jupiter Aug 18, 2025

The crescent Moon will slide past three bright planets over the next three mornings, growing thinner as it does so. First up is Jupiter, the largest planet in the solar system. It looks like a bright star below the Moon at dawn tomorrow.

The Moon is in the part of its orbit that carries it between Earth and the Sun. It’ll reach that point on Friday night. As it drops toward the Sun, the Earth-Moon-Sun angle changes. So the Sun lights up less and less of the lunar hemisphere that faces our way. As a result, the crescent gets thinner day by day.

Tomorrow, for example, about 15 percent of the lunar disk will be in the sunlight. By Wednesday, as it poses near Venus, it’ll be down to eight percent. And by Thursday, when it’s close to Mercury, it’ll be the barest of fingernails – it’ll be daylight across only about three percent of the visible disk.

On the other hand, as the crescent gets smaller, the dark portion of the Moon will get brighter. That’s because that part of the Moon is bathed in earthshine – sunlight reflected from our own planet.

As the Moon gets thinner and thinner in our sky, Earth will get fatter and fatter in the lunar sky, so earthshine will get brighter. It’ll reach its peak when Earth is full – at the same moment that the Moon is new. The Moon will be lost in the Sun’s glare then, but it will return to view a couple of days later – as a thin crescent in the evening sky.

Script by Damond Benningfield

Moon and Elnath Aug 17, 2025

The Moon butts up against the tip of one of the horns of Taurus early tomorrow. They’ll appear to almost touch as they climb into good view, around 2:30 or 3 a.m. They’ll be closest as viewed from the East Coast, especially the northeast.

The tip of the horn is represented by Elnath. It’s the second-brightest star in the constellation. It’s outranked only by Aldebaran, the bull’s eye.

Based on the calendar alone, Elnath is a youngster – roughly 100 million years old. That’s only about two percent the age of the Sun. But the star is well into middle age. That’s because it’s about five times the mass of the Sun. Heavier stars “burn” through their nuclear fuel much faster than lighter stars. So Elnath probably is about halfway through its prime phase of life.

Right now, it’s fusing hydrogen to make helium. So is the Sun. The process is more complicated for heavier stars. But the result is the same: the nuclei of four hydrogen atoms fuse together to make one helium atom. Almost one percent of the mass of the hydrogen is converted to energy, making the star shine.

The Sun converts more than four million tons of mass to energy every second. Elnath fuses its hydrogen at a much faster rate, and it has a lot more hydrogen to start with. So it converts hundreds of millions of tons of matter to energy per second – making “the butting one” almost 600 times brighter than the Sun.

Script by Damond Benningfield

Changing Planet Aug 16, 2025

Planets can really get around. In the early days of our own solar system, for example, the giant outer planets may have moved toward or away from the Sun by hundreds of millions of miles. And many of the planets seen in other star systems probably have spiraled inward from their birthplaces.

One example is a planet orbiting the star Gliese 1214. The star is smaller and less massive than the Sun, and just one-third of one percent as bright.

The planet is a “mini-Neptune” – bigger and heavier than Earth. It’s so close to the star that it’s extremely hot – about 535 degrees Fahrenheit on the dayside, and 325 degrees on the nightside.

Astronomers studied the planet a couple of years ago with Webb Space Telescope. They found that it’s blanketed by shiny clouds or haze. They reflect half of the starlight that strikes them back into space.

The composition of that layer suggests the planet has gone through some changes during its long lifetime. It might have formed much farther from the star – out beyond the “snow line,” where there was a lot of frozen water and other ices. Over time, it spiraled inward and heated up. The heat changed its atmosphere, producing the hot, shiny brew seen today.

Gliese 1214 is in Ophiuchus, which is high in the south at nightfall. But the star is much too faint to see without a telescope.

Script by Damond Benningfield

RS Ophiuchi Aug 15, 2025

A star system in the constellation Ophiuchus keeps blowing up. Every 15 years or so, it flares about 1500 times brighter than average. And it could be building up to an even bigger outburst – a final act that would make it shine billions of times brighter.

RS Ophiuchi consists of two stars. One of them is a white dwarf – a small, hot stellar corpse. The other is a red giant – a dying star that’s much bigger than the Sun.

Gas from the giant flows toward the white dwarf. It forms a swirling disk that’s millions of miles across. Gas in the disk spirals inward, and settles on the white dwarf. When enough gas builds up, it gets hot enough to trigger a nuclear explosion – a nova. Gas blasts outward at millions of miles an hour. That destroys the disk around the white dwarf – but only for a while. It regenerates in about nine months, starting the process all over again.

Astronomers have recorded as many as nine outbursts from the system – the first in 1898, the most recent just four years ago. The gap between them has ranged from nine to 27 years, with an average of about 15.

It’s possible that not all of the gas that piles up on the white dwarf gets blasted away, so the star keeps getting heavier. Eventually, it may pass the weight limit for such a star. If that happens, the white dwarf will blast itself to bits as a supernova – the final demise of a dead star.

Script by Damond Benningfield

Ophiuchus Clusters Aug 14, 2025

For a few weeks in the spring of 1764, Charles Messier was a star-cluster-discovering machine. He found five globular clusters in Ophiuchus, the serpent bearer. He cataloged them as Messier 9, 10, 12, 14, and 19.

Messier wasn’t interested in the clusters – or even in the stars. Instead, he was looking for comets. At the time, finding a comet was a way to fame and fortune. Kings offered prizes to those who found comets. And comets were named for their discoverers – a bit of immortality.

But Messier and others kept coming across fuzzy objects that resembled comets. Figuring out if they really were comets wasted time. So the French astronomer decided to compile a catalog of these distractions. He logged more than a hundred objects. They included star clusters, galaxies, stellar nurseries, and the final gasps of dying stars. Today, Messier’s list is the most famous of all astronomical catalogs.

The globular clusters all look about the same. They’re tight balls of stars. Today, we know that the typical globular contains a hundred thousand stars or more. And they’re among the oldest residents of the Milky Way – more than 10 billion years old.

Ophiuchus is a large constellation that stands well up in the southern sky at nightfall. Messier’s globulars are scattered across it. They’re all visible through binoculars – just don’t mistake them for comets.

Script by Damond Benningfield

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