StarDate

Bellatrix, Spica, and Omega Herculis have something in common. They’re all good “semiquincentennial” stars. That has nothing to do with the stars themselves – only their distance. All three of them are roughly 250 light-years away. In other words, we see them as they looked roughly 250 years ago – around the time of the American Declaration of Independence.

One light-year is the distance light travels in a year – almost six trillion miles. So each of the anniversary stars is about 1.5 quadrillion miles away – the number 15 followed by 14 zeroes.

The distances were measured with a technique known as parallax. Astronomers look at a star when Earth is on opposite sides of the Sun. That causes a tiny shift in the star’s position against the background of more-distant objects. The size of that shift reveals the star’s distance. But there’s a slight margin of error, so the distances might not be exact.

The best measurements say that Bellatrix is a little more than 250 light-years away. Omega Herc is a little less. Spica is in the middle – almost exactly 250 light-years from Earth. So we see it as it looked around the year 1776.

Spica is the brightest member of that trio. It’s low in the southwest as night falls. Earth won’t see the light it emits tonight until the American quincentennial – 250 years from now.

Script by Damond Benningfield

The next big meteor shower is weeks away – at least, here on Earth. But some predictions say that a big shower could take place this weekend on the planet Venus – a result of the long-ago break-up of an asteroid.

Regardless of where it takes place, a meteor shower happens when a planet flies through the orbital path of a comet or asteroid. Bits of rock and dust shed by the small body ram into the planet’s atmosphere at high speed. They heat up and vaporize, forming the glowing streaks known as meteors.

In this case, the source of the particles could be two asteroids. They have a similar composition, and they follow similar paths around the Sun.

A team of European astronomers recently plotted the orbits of the two bodies a hundred-thousand years into the past. The study found that, about 20,000 years ago, both asteroids passed especially close to the Sun many times. And the asteroids were so close to each other that they could have been one larger asteroid.

Warmed by the Sun, the surface of the asteroid could have cracked and splintered. The way the asteroid heated up could have caused it to spin faster. The weakened asteroid then split apart, spewing debris into space. Venus periodically slips through this trail of debris – perhaps triggering a meteor shower.

From Earth, only the brightest of those meteors might be visible through a telescope – streaking through Venusian skies.

Script by Damond Benningfield

The planets Mars and Uranus will stage an especially close encounter the next couple of mornings. Uranus is quite faint, but its proximity to Mars can help you pick it out.

Uranus is a giant – about four times the diameter of Earth. But it’s so far away that it’s faint. Under especially dark skies, people with good eyesight can just make it out. Moonlight or light pollution mask it from view, so you need binoculars to find it.

And even then, it looks like a meager star, perhaps with a hint of blue-green. Methane in its upper atmosphere absorbs red light, so only the blue and green can reach us.

Mars is much smaller – only a bit more than half of Earth’s diameter. But it’s also much closer, which makes it easier to see.

Even with the eye alone, it’s no problem to make out the planet’s orange color. That’s produced by iron oxide in the rocks and the tiny dust grains that coat much of the surface. That color will become easier to make out over the coming months, as Earth and Mars get closer and closer.

For now, look for Mars low in the east-northeast beginning a little before dawn. It’s quite easy to make out. Tomorrow, Uranus will stand a little to the lower left of Mars, so both of them will fit in a binocular field of view. And Uranus will be even closer above Mars on Saturday – a faint giant appearing to almost touch the Red Planet.

We’ll talk about a much brighter planet tomorrow.

Script by Damond Benningfield

A Japanese spacecraft is scheduled to pay a call on an asteroid this weekend. The encounter won’t last long – the craft will buzz by at about 12,000 miles per hour.

This is the second asteroid encounter for Hayabusa2, which launched in 2014. Its main mission was a detailed study of the asteroid Ryugu. It orbited the asteroid for a year and a half. It gathered a few grams of dust and pebbles and dropped them off at Earth in late 2020. Studies have shown that the samples contain all the key ingredients of DNA – the building blocks of life.

Hayabusa then continued its trek. And this weekend, it’ll pass by the asteroid Torifune – a chunk of rock about a quarter of a mile in diameter.

Torifune’s average distance from the Sun is just a fraction farther than Earth’s distance. But its path is lopsided, so it regularly crosses Earth’s orbit. It’s not currently a threat to hit our planet. But it could be sometime in the distant future.

Right now, the asteroid is at its closest point to the Sun, and about 60 million miles from Earth. Hayabusa will scan it with several instruments as it swings by. But the high speed makes that tough. The entire spacecraft has to turn to keep the asteroid in view. And it can’t turn fast enough to keep an eye on it through the entire encounter. So it’ll have to settle for a quick glimpse as it blazes by this potentially hazardous asteroid.

Script by Damond Benningfield

The bright star that marks the tail of the swan is big, bright, and heavy. If you want to know how big, bright, and heavy, well, the most precise answer we can give you is “very.”

We know for sure that Deneb is one of the more imposing stars around. But the details are a little hazy because the star’s distance is hazy. In part, that’s because of Deneb’s impressiveness.

The most accurate stellar distances come from Gaia, a space telescope. But Deneb is so bright that Gaia can’t look at it directly. Instead, it has to study the star indirectly – sort of like checking something out through the corner of its eye. It takes a lot of work to translate those glimpses into solid numbers.

In addition, the star’s outer layers are puffy and unstable, so it’s hard to know just where its surface is. And you need a good idea of the surface to measure the star’s location.

The best estimates say that Deneb is about 1500 light-years away – one of the most remote stars that’s visible to the eye alone. But other estimates say it’s almost twice that far. So based on that range, Deneb is about 15 to 20 times the mass of the Sun, 100 to 200 times the Sun’s diameter, and 50 thousand to 200 thousand times its brightness – impressive numbers for a very impressive star.

Deneb is a third of the way up the northeastern sky at nightfall, and soars high overhead during the night.

Script by Damond Benningfield

When the Moon covers the Sun during a total eclipse, a couple of rare sights greet viewers. One is the corona – the Sun’s hot but thin outer atmosphere, which looks like a silvery halo. The other is the short red or pink tendrils known as prominences – eruptions of gas into the corona. They’re actually there all the time, but they’re impossible to see against the brilliance of the Sun’s disk.

Prominences can span many thousands of miles; the largest are about half the size of the Sun itself. They’re actually thousands of degrees cooler than the surface of the Sun. They look bright only when they’re seen against the dark background of space. When they’re seen against the Sun, they form dark streaks.

Prominences are powered by the Sun’s magnetic field. Strands of the field can levitate above the surface. The strands can be filled with plasma – hot gas that has an electric charge.

Some prominences are common around magnetically active regions. They can erupt in minutes, loop into the corona, then collapse within hours.

Some prominences don’t stop at the corona. They can send huge clouds of plasma into the solar system. If they hit Earth, these outbursts can trigger brilliant auroras and disrupt technology – prominent impacts from the Sun.

Script by Damond Benningfield

To predict the lifespan of a star, you don’t need a crystal ball – a bathroom scale will do just fine. Heavier stars age faster, so if you know the star’s mass, you have a good idea of its future.

Consider Tarazed, the second-brightest star of the eagle. It’s only about six percent the age of the Sun. But because it’s about three and a half times the Sun’s mass, it’s already completed the “prime” phase of life. Now, it’s well into the next phase – as a red giant.

Mass is critical because, as the star’s mass increases, so does its gravity. Stronger gravity squeezes the star’s core more tightly, increasing its temperature. That revs up the rate of nuclear reactions in the core.

When a star is born, its core is mostly hydrogen. In the prime phase of life, the star “fuses” the hydrogen atoms to make helium. When the hydrogen is gone, the core shrinks, so it gets even hotter. That causes the star’s outer layers to puff up, which is what’s happened to Tarazed – it’s more than 90 times the Sun’s diameter.

Higher core temperatures trigger the next round of reactions. So today, Tarazed is fusing the helium to make heavier elements. Eventually, that will end as well. Tarazed will shed its outer layers, leaving only its tiny, dead core – ending the star’s fairly short but bright life.

Tarazed is low in the east at nightfall. It’s close above even brighter Altair, at the southern point of the Summer Triangle.

Script by Damond Benningfield

Altair is one of the highlights of summer. It’s at one point of the Summer Triangle, and it’s the twelfth-brightest star in the night sky. And it’s just 16.7 light-years away.

Because Altair is so close and bright, we know quite a bit about it. And we’re learning more all the time. A study a couple of years ago, for example, refined the likely age of the star – 88 million years, give or take 10 million. That’s just two percent the age of the Sun.

Altair is about twice the size and mass of the Sun. And because it’s so young, it spins in a hurry – one turn every eight hours or so, versus 25 days for the Sun. That whirling rotation makes the star look squished – it’s about 25 percent wider through the equator than the poles.

In 2022, astronomers “listened” to the star with a space telescope. It measured vibrations on the surface of the star. Combined with observations from the ground, that revealed a total of 34 vibration modes – like 34 different musical notes.

The vibrations travel deep into the star. Each “note” reveals details about Altair’s interior. Piecing together the whole symphony, the astronomers found that the core of Altair contains more than 97 percent of the hydrogen it was born with. As a star ages, it converts its hydrogen to helium. So the amount of hydrogen reveals that Altair is just starting out.

Script by Damond Benningfield

A wave of starbirth is sweeping through Scorpius and some nearby constellations. It started about 20 million years ago, and is continuing today. It’s triggered the birth of thousands of stars, including some of the most impressive in our part of the galaxy.

The complex is called the Scorpius-Centaurus association. It consists of three distinct regions that appear to be related. Giant clouds of gas and dust in one of those regions began collapsing tens of millions of years ago. That gave birth to the first stars in the association.

The most massive members of that group produced winds and radiation that squeezed the surrounding clouds. And some of those early stars exploded, creating shockwaves that compressed the clouds even more. That triggered the birth of many more stars.

Many stars in the association are extremely big, bright, and massive. They account for many of the bright lights that outline the scorpion, the centaur, and other constellations in the region. For every one of those stars, there are hundreds of less-massive stars.

From Earth, the most impressive member of the association is Antares, the heart of the scorpion, which is quite near the Moon tonight. Antares is more than a dozen times the mass of the Sun, and tens of thousands of times brighter. In the next million years it’s likely to explode as a supernova – perhaps triggering the birth of even more stars in this busy complex.

Script by Damond Benningfield

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