StarDate

Venus doesn’t have any moons. But it does share its orbit around the Sun. Astronomers have discovered 20 asteroids known as “co-orbitals,” but there could be many more. These big space rocks follow roughly the same path as Venus. But they won’t stay in that lane forever. And when they leave it, they could threaten Earth.

These objects are nudged along by the gravity of Venus and the Sun. They generally stay well ahead of or behind Venus. Only one follows exactly the same orbit as the planet. The others move in and out a bit, getting closer to the Sun, then moving farther away.

Over the long term, though, their orbits aren’t stable, so they can break free and head elsewhere. A recent study found that of the 20 known objects, six could threaten Earth within the next 12,000 years. And three of them are especially dangerous. All three are at least a thousand feet in diameter, so they could cause major damage if they hit our planet.

A study also found that there could be many more of these Venus groupies. They stay so close to the Sun in our sky that they’re hard to see through the solar glare. And they move quickly, making them even harder to find. But a new telescope in Chile might pick out some of them – helping us find potential threats far in advance.

Look for Venus near the Moon in the dawn sky tomorrow. It’s the brilliant “morning star,” so you can’t miss it.

Tomorrow: an autumn meteor shower.

Script by Damond Benningfield

California is the land of the stars. It’s also in the stars as the California Nebula – a cloud of gas and dust that looks like the outline of the state. It’s more than a thousand light-years away, in Perseus.

The nebula belongs to a giant star-forming complex – the Perseus O-B-2 association. The region has given birth to many class O and B stars – the biggest and brightest of all stars.

The California Nebula probably is energized by one of those stars, known as Xi Persei. The star is more than 30 times the mass of the Sun, and tens of thousands of degrees hotter. At that temperature, it produces huge amounts of ultraviolet energy.

When that radiation zaps hydrogen atoms, it splits them apart. When they link back up, the atoms produce red light – the main color of the nebula. Oxygen and other elements produce their own colors, but they’re not nearly as common as hydrogen.

The California Nebula probably is about a hundred light-years long. It’s likely to split into smaller clumps that will collapse to form even more stars. But radiation and winds from Xi Persei and other big stars will blow away much of the nebula’s material – limiting the number of new stars for this cosmic California.

Perseus climbs into good view, in the northeast, in early evening. Xi Persei is visible to the naked eye, near the bottom of the constellation. But you need a telescope to see the faint outline of the California Nebula.

Script by Damond Benningfield

Xi Persei doesn’t look all that imposing. The star shines at fourth magnitude, so it’s visible under dark skies, but not from cities and towns. But that’s only because it’s a long way off – about 1200 light-years. In reality, it’s one of the most impressive stars visible to the human eye.

Perseus climbs the eastern sky on autumn evenings. It consists of a couple of ribbons of stars that join at Mirfak, the constellation’s leading light. And it contains the most famous variable star in the sky: Algol, the Demon Star, which fades and brightens every three days.

Yet neither can compare with Xi Persei, which is near the bottom of the longer ribbon. At visible wavelengths, it’s about 13,000 times brighter than the Sun.

But it’s tens of thousands of degrees hotter than the Sun, so it emits most of its light in the ultraviolet. When you add that in, Xi Persei is a quarter of a million times the Sun’s brightness.

The key to that showiness is the star’s mass – roughly 30 times the Sun’s mass. At that great heft, gravity squeezes its core tightly, revving up its nuclear engine. Energy works its way to the surface, making Xi Persei hot and bright.

Before long, it’ll get even hotter and brighter. It’ll explode as a supernova, briefly shining brighter than billions of normal stars – a brilliant demise for an impressive star.

Xi Persei energizes a nearby cloud of gas, and we’ll have more about that tomorrow.

Script by Damond Benningfield

For decades, Regulus had astronomers fooled. The star is bright, hot, and blue – an indication that it was quite young. Most estimates put its age at no more than a hundred million years – about two percent the age of the Sun. Instead, it’s at least a billion years old. But like a vampire, it’s been rejuvenated by taking the life’s blood of a companion, making it look much younger.

The star we see as Regulus is about four times the size and mass of the Sun, and more than 300 times brighter. A few decades ago, astronomers discovered its companion – a “dead” star known as a white dwarf. The two stars are so close together that the corpse was hidden in the glare of the bright star.

The presence of the companion means the system has to be at least a billion years old – old enough for the companion to evolve to its present state. As it evolved, it puffed up. Gas flowed from its surface over to the other star. That made the star we see today much bigger and heavier. It also made the star hotter, which made it bluer. Hot blue stars usually are quite young. So astronomers were fooled into thinking that bright Regulus was still a youngster – not an older star that’s been rejuvenated.

Look for Regulus close to the Moon at dawn tomorrow. The distance between them will narrow as you move westward. They’ll be especially close as seen from Alaska or Hawaii.

Script by Damond Benningfield

The Milky Way is a giant among galaxies – a hundred thousand light-years in diameter. But a few galaxies make the Milky Way look like a mere bauble by comparison. They span millions of light-years – puffed up by the action of supermassive black holes.

These monsters are known as giant radio galaxies. Not only are they large, but they produce enormous amounts of radio waves.

The black hole in such a galaxy’s heart is encircled by a massive disk. As material in the disk spirals into the black hole, magnetic fields fire “jets” of some of its particles like water from a firehose. These jets can streak far into space. They end as they plow into the material between galaxies, forming “lobes” that are bright sources of radio waves.

A recent study found 15 of these giants. They’re in the constellation Sculptor, which creeps low across the south on October evenings.

It actually has two sets of jets – one nested inside the other. The longer set is older – powered up by the black hole millions of years ago. But the black hole might have slowed down its eating for a while, shutting off that flow of particles. Later, it started chowing down again, powering the second set of jets, which continue to expand – sustaining this galactic monster.

Script by Damond Benningfield

Deneb, the brightest star of Cygnus, stands high overhead as night falls at this time of year. And it really is a brilliant star – tens of thousands of times brighter than the Sun. But if we could tune our eyes to see radio waves, Deneb wouldn’t even register. Instead, the swan’s leading light would be Cygnus A – one of the brightest radio galaxies in the universe.

A radio galaxy produces huge amounts of radio waves. It’s usually a large elliptical galaxy, which looks like a fat, fuzzy football. It has a supermassive black hole at its center.

Gas, dust, and stars spiral into the black hole. But powerful magnetic fields eject some of that material back into space. It forms “jets” that fire out at almost the speed of light. The jets can span hundreds of thousands of light-years. Electrons spiral through a jet’s magnetic field, producing radio waves. Eventually, the jets plow into gas and dust between galaxies, forming wide bubbles that emit even more radio waves.

Cygnus A was the first radio galaxy ever discovered, in 1939. It’s about 760 million light-years away. Its black hole is two and a half billion times the mass of the Sun. The entire complex – galaxy, jets, and bubbles – spans more than 600,000 light-years. That’s six times the diameter of our home galaxy, the Milky Way – one of the biggest, brightest radio galaxies in our part of the universe.

More about radio galaxies tomorrow.

Script by Damond Benningfield

Floating through the clouds at Jupiter’s equator sounds like a celestial carnival ride. The equator spins at about 28,000 miles per hour – 28 times faster than Earth’s equator. So the Sun, moons, and stars would zip across the sky in a hurry.

Jupiter is the largest planet in the solar system – 11 times Earth’s diameter. It also spins faster than any other planet – so fast that it bulges outward at the equator. At that speed, a day on Jupiter is less than 10 hours long. So the equator always sees about five hours of daylight followed by five hours of darkness.

It might not sound right, but Jupiter spins so fast because it’s so big. As it swept up more material while it was taking shape, gravity compressed it, making it smaller. The planet had to spin faster to balance the books – like a skater spinning faster as it pulls in its arms.

Some studies have suggested that Jupiter might actually have been slowed down early on by its magnetic field. The young planet was encircled by a disk of gas and dust that gave birth to its moons. As the gas swirled through the magnetic field, some of it developed an electric charge. The charged-up gas grabbed on to the field, acting like a brake – slowing down the solar system’s biggest and still fastest planet.

Jupiter looks like a brilliant star below the Moon at dawn tomorrow. The twin stars of Gemini are closer to the left and lower left of the Moon.

Script by Damond Benningfield

If you’d like to thank your lucky stars for a bit of good fortune, we have two stars for you to look at. They’re the brightest stars of Aquarius. Both of them have names that mean “lucky.”

The brighter of the two is Sadalsuud. The name comes from an Arabic phrase that means something along the lines of “luckiest of the lucky.” When the name was bestowed, the star first appeared in the dawn sky around the spring equinox. The days were getting longer and warmer, and spring rains were settling in – bringing life-giving water to the fields. So the star was considered a sign of good fortune.

The other lucky star is Sadalmelik – “luck of the king.” The exact reason for its name is unclear, although it, too, may relate to the seasons.

Both stars are class-G supergiants. They’re about the same temperature and color as the Sun, but much bigger, heavier, and brighter. Both stars have passed through the prime phase of life, so their luck is running out – they’re nearing the end. Each will shed its outer layers and leave behind a massive white dwarf – a corpse about as heavy as the Sun, but only as big as Earth.

Aquarius is in the southeast at nightfall. The “lucky” stars line up parallel to the horizon, with Sadalmelik on the left. The stars are separated by about the width of your fist held at arm’s length. But they’re so far from us that they don’t look all that bright – a bit of bad luck for skywatchers.

Script by Damond Benningfield

Stars like the Sun go through several distinct phases of life, from embryo to corpse.

Consider Aldebaran, the bright eye of Taurus, which accompanies the Moon tonight. It’s more than six billion years old – older than the Sun. And it’s well into “old age.”

Aldebaran was born when a cloud of gas and dust collapsed. For millions of years, it shined as a result of the heat generated by that collapse – its “embryonic” phase. Eventually, its core got hot enough to ignite the fires of nuclear fusion, and Aldebaran entered the prime phase of life – fusing hydrogen to make helium.

A few hundred million years ago, it used up the hydrogen in the core. The core got smaller and hotter, and Aldebaran began fusing the hydrogen in a shell around the core. At the same time, its outer layers puffed up, so Aldebaran is more than 40 times wider than the Sun. This is the giant phase of life.

Eventually, the core will get hot enough to fuse the helium to make carbon and oxygen. But when the helium is gone, fusion will stop. The core will get smaller and hotter, and its radiation will push the star’s outer layers into space. Only the hot, dead core will remain – a white dwarf.

Even that isn’t the end, though. The white dwarf will cool and fade. Hundreds of billions of years from now – and perhaps much longer – it’ll stop producing any visible light at all. That will make it a black dwarf – the final stage for the eye of the bull.

Script by Damond Benningfield

The Moon barrels through the Pleiades star cluster this evening. It’ll pass directly in front of the cluster, briefly blocking most of its stars from view.

The Pleiades is the most famous of all star clusters. It’s also known as the Seven Sisters, but under dark skies – with no Moon in the way – you might actually see nine stars or more.

But that’s only the beginning. The cluster contains more than a thousand stars. In fact, it was the first cluster to be recognized as a cluster – a group that’s moving through the galaxy together.

That recognition came in 1767. John Mitchell, a clergyman and scientist, was looking at several tightly packed groups of stars. He studied the Pleiades in the greatest detail. And he calculated that there was only a one-in-500,000 chance that the grouping could be random. Instead, something had to be holding the stars together.

His idea was confirmed when astronomers measured the motions of the cluster’s stars. They’re all moving in the same direction, and at the same speed.

Today, we know that’s because they were born together, from a single giant complex of dust and gas. They’re bound to each other by their mutual gravitational pull. But they won’t stay together. The cluster is being pulled apart by the gravity of the rest of the galaxy. So the Pleiades probably will dissipate in about 250 million years – with its member stars going their own way.

Script by Damond Benningfield