Two stars that sound a bit scary peek above the southern horizon on summer nights. Together, they form the “stinger” – the end of the curving body of the scorpion.

The stars are Lambda and Upsilon Scorpii. Lambda is the brighter of the two. It’s also the more complicated – it consists of three stars.

The system’s details are a bit uncertain. That’s largely because its distance is uncertain. Estimates range from about 365 light-years to almost twice that range. Without a good measurement of the distance, it’s tough to figure out how big and heavy the stars really are.

We do know that the system consists of a tight binary – two stars that orbit each other once every six days – plus a third star that orbits the others every three years.

One of the stars in the binary is much bigger and brighter than the Sun, and 10 to 14 times the Sun’s mass. That means the star will end its life with a colossal explosion.

Its close companion is maybe twice the mass of the Sun. But it’s not yet fully formed. The distant companion is another big guy – roughly 8 to 10 times the mass of the Sun. If it’s at the high end of that range, then it, too, will explode as a supernova. If not, its fate is less certain. It could become a supernova, but it also might expire in a more gentle process – a fate similar to the Sun’s.

More about the scorpion tomorrow.

Script by Damond Benningfield

A star system on the far side of the galaxy keeps blowing up. Since 1863, astronomers have recorded 12 outbursts from the system – the most recent just four years ago. The flare-ups are powered by a complicated interplay between a dead star and a companion that may be dying.

U Scorpii probably is more than 60,000 light-years away, far on the other side of the heart of the Milky Way Galaxy. Most of the time, the system is quite faint. But during the outbursts, it can flare 10,000 times brighter in just a few hours. It starts to fade quickly, but it takes about two months to return to its “quiet” state.

The system consists of two stars in a tight orbit. One of them is a white dwarf – the dead core of a Sun-like star. The other star is headed toward the same fate. It’s at the end of the prime phase of life, so it’s starting to puff up. The white dwarf “steals” some of the gas from its surface, forming a swirling disk.

Some of the gas piles up on the white dwarf. Eventually, the gas gets so hot, it sets off a nuclear blast, making the system flare up. The outburst blows away all or part of the disk. Before long, though, the process starts all over again – leading to another explosion a few years later.

U Scorpii is in Scorpius, which is low in the south-southeast at nightfall. The system is above the curving line of stars that outlines the scorpion’s body and tail.

Script by Damond Benningfield

About 10,000 light-years from Earth, a dead star is devouring its living companion. The process creates a disk of gas that’s heated to millions of degrees, so it shines brightly. Some of the gas is fired back into space at almost the speed of light, adding to the fireworks. The system is so powerful that it’s classified as a microquasar – a smaller version of some of the brightest objects in the universe.

GRO J1655-40 consists of a black hole about six or seven times the mass of the Sun, plus a close companion star more than twice the Sun’s mass.

The black hole probably began as a star about 25 times the Sun’s mass. It evolved quickly, with its core collapsing to form the black hole. Its outer layers were blasted into space. Some of that material fell on the companion. Today, the black hole is pulling some of that gas away from the companion.

The same thing happens in the cores of many remote galaxies. Supermassive black holes create monster disks as they pull in gas, dust, and stars. Such a disk can shine billions of times brighter than the Sun – forming a quasar. GRO J1655-40 is a smaller version of that.

The system is in Scorpius, which crawls across the south on summer evenings. The microquasar is near where the scorpion’s body curves to form its tail.

Script by Damond Benningfield

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