It’s hard to see a pattern in most of the constellations. Their stars are too faint or too spread out, or the pattern is just too obscure. Perhaps the most prominent exception is Scorpius. It takes little imagination to see the curving body of a scorpion in its stars.

The scorpion skitters low across the south on summer nights. Its brightest star is Antares. The scorpion’s body and tail curl to the lower left. The head is to the upper right. It’s marked by a line of three stars. They’re about the same brightness, and they’re fairly evenly spaced.

From top to bottom, the stars are Beta, Delta, and Pi Scorpii. Delta is a bit brighter than the others.

All three stars are extraordinary. Each of them actually consists of more than one star. All of the member stars are quite young – no more than a few percent the age of the Sun. And most of them are big and heavy, with some of them fated to end their lives as supernovas – titanic explosions that will outshine billions of normal stars.

Delta Scorpii consists of two stars. At least one of them will become a supernova. Pi Scorpii is a triple system. It also features at least one future supernova.

Beta is the busiest of the systems – at least six stars, all orbiting each other in a complex gravitational ballet. Two of those stars are likely to become supernovas – briefly highlighting the head of the scorpion.

Script by Damond Benningfield

Our clocks tick off a steady 24 hours per day. But if a sundial could record the time with the same accuracy, it would show that the length of the day changes. The difference is called the equation of time.

Clocks measure the length of a day averaged over a full year – the Sun’s average motion across the sky. Sundials show the Sun’s true motion. Over the course of a year, the length of a solar day – the period from one local noon to the next – varies by almost a minute. And that adds up. In early February, a solar day lasts about 14 minutes less than 24 hours. In early November, it lasts about 16 and a half minutes more than 24 hours.

The change has a couple of causes. Earth’s orbit is lopsided, so our planet travels at different speeds. When we’re closest to the Sun, we move faster than average; when we’re farthest, we move slower. But the rate at which Earth spins on its axis remains the same. The difference in those two motions causes the Sun to move a little faster or slower across the sky, changing the length of a solar day.

And Earth’s axis is tilted, so the poles take turns dipping toward the Sun. Today is the June solstice, so the north pole is tilting sunward. The change in the Sun’s position as a result of that tilt adds to the complexity.

The solar day is exactly 24 hours long around June 13th. So now, the equation of time is almost zero – a close match between the sundial and the clock.

Script by Damond Benningfield

Summer arrives here in the United States in the wee hours of tomorrow morning – the moment of the June solstice. At the solstice, the Sun stands farthest north for the entire year. For people at about 23-and-a-half degrees north latitude, our star will pass directly overhead at local noon.

That line of latitude is known as the Tropic of Cancer. It was named a couple of thousand years ago. At the time, the Sun appeared against the constellation Cancer at the solstice. Today, though, the Sun’s almost directly astride the border between Taurus and Gemini. It’s on the Taurus side at the exact moment of the solstice, but it slides into Gemini a few hours later.

The change in address is the result of a slow “wobble” in Earth’s axis. As it wobbles, the Sun shifts position against the background of stars. It takes our planet about 26,000 years to complete a single wobble, so that’s how long it takes the Sun to move all the way across the zodiac. So the Sun will return to Cancer in about 24,000 years.

Earth’s axis also nods up and down a little over an even longer period – about 41 thousand years. That causes a shift in the latitude of the Tropic of Cancer. Right now, it’s moving southward at about 50 feet per year – changing the circle where the Sun stands overhead on the summer solstice.

We’ll have more about the summer solstice tomorrow.

Script by Damond Benningfield

A star seldom just flies apart – at least not when it’s in the prime of life. But some of them come close. One of the best examples is Regulus, the brightest star of Leo. It’s rotating so fast that it’s barely holding itself together.

Regulus consists of four stars, but only one of them is bright enough to see with the eye alone. It’s known as Regulus A. It’s more than four times wider and heavier than the Sun. And it spins much faster – about 200 miles per second at the equator – almost 200 times faster than the Sun.

According to studies, that’s 96 and a half percent of the speed required to make Regulus fly apart. The high speed pushes gas outward, so Regulus is about 30 percent wider through the equator than the poles.

The star was spun up by a now-dead companion star. That star was more massive than Regulus A, so it lived a shorter life. As it expired, it puffed up. Regulus A then pulled gas from its surface. As the gas piled up on Regulus A, it added momentum to the star’s rotation – like pushing harder and harder on a spinning globe.

The companion eventually lost all its outer layers. That left only its dead core, known as a white dwarf – a star that did fly apart, but not until the end of its life.

Regulus stands close to the right or lower right of the Moon at nightfall. They stay close together as they drop down the western sky. They set around midnight.

Script by Damond Benningfield

Allan Sandage once said that when he became a graduate student at Caltech, in the late 1940s, he was a “hick who fell off the turnip truck.” He fell at the feet of Edwin Hubble, the most famous astronomer of the time. Hubble was ill, so Sandage gathered data for him at the world’s largest telescope. When Hubble died, a few years later, Sandage took over much of his work. And like Hubble, he expanded the size and age of the universe, and shaped much of the debate over its fate.

Sandage was born 100 years ago today, in Iowa City. He got interested in astronomy while looking through the telescope of a boyhood friend.

Over the decades, he contributed to many areas of astronomy. As an example, he pioneered studies of globular clusters – large clumps of ancient stars.

That work led to a better understanding of the age of the universe. Many of the stars in globulars appeared to be older than the universe – an impossibility. Sandage used that and other lines of evidence to greatly increase the known age of the universe.

One line of evidence was the rate at which the universe is expanding – a number known as the Hubble constant. Hubble himself had come up with a number that was much too big, implying a much younger age. Sandage calculated a rate that was close to modern numbers.

Sandage wasn’t always right. But his work shaped the field of cosmology for decades – and still has an impact today.

Script by Damond Benningfield

As the Moon orbits Earth, its gravitational pull creates the ocean tides. As the “bulge” in the water laps against the continents, it creates drag that slows our planet’s rotation. That increases the length of a day by about 2.4 milliseconds per century. That doesn’t sound like much, but over the eons it adds up.

That rate can be affected by big changes in Earth itself, including powerful earthquakes, volcanic eruptions, tropical storms, and more. And over the past few decades, it’s become clear that one of those factors is climate change.

As Earth gets warmer, glaciers and polar ice sheets melt, raising sea level. The extra water increases the power of the tides, slowing Earth’s rotation.

According to a recent study, that’s extending the day by 1.33 milliseconds per century – the highest rate of change over the past 3.6 million years. And the rate could get even bigger by the end of the century. In fact, climate change could add more to the day than the effects of the Moon itself.

As Earth slows down, the Moon moves farther away. Right now, it’s receding at about an inch and a half per year. But climate change could speed things up – pushing the Moon away.

The crescent Moon is low in the west at sunset. And it has a bright companion: Venus, the brilliant “evening star.” They drop from sight a couple of hours later.

Tomorrow: measuring the age of the universe.

Script by Damond Benningfield

The crescent Moon charges through a rapidly disappearing group of bright stars and planets early this evening. Most of the group will be gone from view by the end of the month.

As twilight begins to fade, the planet Mercury is close below the Moon. Brighter Jupiter is the same distance to the left or upper left of the Moon. Pollux and Castor, the twins of Gemini, are to the upper right of the Moon. And the brightest member of the group is farther to the upper left of the Moon: Venus, the brilliant “evening star.”

Except for Venus, all the members of the group are dropping toward the Sun as seen from Earth. For Pollux and Castor, it’s because all true stars rise and set four minutes earlier each day. So every star disappears in the evening twilight at the same time every year.

For Jupiter and Mercury, the descent is due in part to the same thing – the daily shift of the starry background. But it’s also influenced by the relative motions of Earth and the planets themselves. Mercury is beginning a rapid dive toward the Sun, and will cross between Earth and Sun in a few weeks. Jupiter, on the other hand, is headed toward a passage behind the Sun as seen from Earth.

But Venus is actually moving farther from the Sun. It won’t reach its peak separation for two months, so it’ll remain in good view in the western evening sky into October.

We’ll have more about the Moon and Venus tomorrow.

Script by Damond Benningfield

Like a cosmic butterfly, a cluster of young stars is just emerging from its cocoon – a cloud of gas and dust. The cocoon `spans about 45 light-years. But some of the beautiful butterfly is already in view. Parts of the gas cloud are lit up by the brightest of the infant stars taking shape there. That creates a glowing patch of red and blue.

The whole complex is known as the Cocoon Nebula. It’s about 4,000 light-years away, in Cygnus. Hundreds of stars are being born inside it.

The most impressive of those stars is about 14 times as massive as the Sun, and tens of thousands of times brighter. It’s especially bright in the ultraviolet – wavelengths that are invisible to the human eye. The U-V zaps atoms of hydrogen in the nebula, splitting them apart. When the atoms re-combine, they emit red light – the main color of the nebula.

The hot star also illuminates dust grains in the nebula. It doesn’t set them aglow; instead, the light simply reflects off the grains. That colors the blue parts of the nebula.

Less-massive stars – stars like the Sun or even smaller – are still coming together. They won’t shine as fully formed stars for millions of years.

The Cocoon Nebula is low in the northeast at nightfall. It’s to the lower left of the bright star Deneb, which marks the tail of the swan. The nebula is too faint to see with the eye alone.

Script by Damond Benningfield