StarDate

Russian astronomer Friedrich Wilhelm von Struve discovered and catalogued thousands of binary stars — pairs of stars that are gravitationally bound to each other. But a system that he first saw in 1829 was so striking that he gave it a special name: Pulcherrima — “the most beautiful.” The name honors the contrasting colors of the two stars. One looks pale orange, while the other looks blue-white or even green.

The system is also known by an even older name: Izar, “the girdle,” because it represents the middle of Boötes, the herdsman. Regardless of what you call it, most skywatchers agree with Struve: Seen through a telescope, the pair is quite a beauty.

The orange star is a giant. It’s burned through its original hydrogen fuel and is nearing the end of its life. As a result, it’s puffed up to many times the diameter of the Sun. That “puffiness” caused the star’s outer layers to cool, which is why it looks orange.

Its companion is much hotter, so it shines almost pure white. It looks blue or green only when it’s compared to its orange companion. It’s less massive than the companion, so it has a lot longer to go before it reaches its own “giant” phase of life.

Boötes is in the east as night falls. Look for its brightest star, brilliant yellow-orange Arcturus. Izar is the first noticeable star to the left of Arcturus. To the unaided eye, it looks like a single point of light. But a telescope reveals the true beauty of this colorful duo.

Script by Damond Benningfield

As night falls this evening, the southern sky is a big blob of darkness. The brilliant planet Jupiter is off in the southeast, with the star Spica close by. And Sirius, the brightest star in the night sky, is in the southwest, with some of the other leading lights of winter around it. But the space between them looks pretty empty. In fact, there’s only one modestly bright star in the whole region: Alphard, a name that means “the solitary one.”

Alphard is the brightest star of Hydra, the water snake, the longest of the 88 constellations. At nightfall, it stretches halfway across the southern sky.

Alphard’s appearance isn’t misleading — it really is bright. In fact, when you add up all wavelengths of light, it’s almost a thousand times brighter than the Sun.

That’s because the star is nearing the end of its life. It’s converted the original hydrogen fuel in its core to helium, the next-heavier element. Now, it’s converting the helium to carbon and oxygen. That’s caused the core to get much hotter. Its radiation pushes outward on the star’s outer layers, causing Alphard to puff up to giant proportions — more than 50 times the Sun’s diameter.

In the not-too-distant future, those outer layers will puff away into space. For perhaps 50,000 years or so, that’ll create a colorful bubble known as a planetary nebula. After that, though, only the star’s dead core will remain — and Hydra’s solitary bright light will disappear.

Script by Damond Benningfield

Aldebaran is a close neighbor. The bright orange eye of Taurus, the bull, is about 65 light-years away. In other words, the light we see from Aldebaran tonight actually left the star about 65 years ago. Despite decades of measurements, though, there’s still some uncertainty in that number — about one light-year. So if a starship headed for Aldebaran today, it could miss its target by up to six trillion miles.

That shows how tough it is to measure the distances to the stars.

The main technique that astronomers use is known as parallax. In essence, they look at a star’s position compared to the background of other stars when Earth is on opposite sides of the Sun. It’s like holding a pencil in front of your face, and then looking at it with first one eye, then the other. The pencil shifts position against the background of more-distant objects — and so does a star. The size of the shift reveals the star’s distance.

The parallax of Aldebaran is about 50 milliarcseconds — roughly one seventy-thousandth of a degree. That’s an extremely difficult angle to measure even with space telescopes, which avoid the blurring effect of Earth’s atmosphere. So even the tiniest flaw in measuring that angle can make a big difference in the star’s distance.

But no matter how far it is, Aldebaran is in good view early this evening. It stands below the Moon as night falls, so it’s easy to spot. The fainter planet Mars stands to the right of Aldebaran.

Script by Damond Benningfield

Every big galaxy is thought to have a big black hole at its heart. But just how those black holes were born is the subject of a lively debate. Some say the black holes formed from the collapse of giant clouds of gas. Others say they formed from “seed” black holes a few thousand times as massive as the Sun. But such black holes have been hard to find; astronomers have evidence for only a handful of them.

The most recent is in a star cluster known as 47 Tucanae. It’s in the far-southern constellation Tucana, the toucan. The cluster contains hundreds of thousands of stars, all packed into a ball about a hundred light-years across.

Several previous searches had found no evidence of an intermediate-mass black hole in the cluster.

But a new search took a different approach from the others. It measured the overall motion of stars within the cluster, as well as the orbits of pulsars — stellar corpses that emit regular pulses of energy. Both types of stars should sink to the center of the cluster. But examples of both were found well outside the center. That suggests something is stirring up the center, kicking heavy stars into unusually wide orbits.

By comparing these observations to mathematical models, researchers concluded that a black hole more than 2,000 times the mass of the Sun sits at the cluster’s heart.

If the finding is confirmed, it could provide good evidence that giant black holes can grow from medium-sized ones.

Script by Damond Benningfield

It’s not unusual to see a black hole feast on a passing star or gas cloud. But one black hole has managed to make the feast last for more than a decade — and it’s not over yet.

The black hole is known by a catalog name — XJ-1500 for short. It’s in a galaxy that’s almost two billion light-years away. The galaxy is much smaller than our home galaxy, the Milky Way. But it’s giving birth to stars at a much faster rate — and that could be one reason for the long meal.

The black hole is at the heart of the galaxy. It’s probably a million times as massive as the Sun — about a quarter the heft of the black hole at the center of the Milky Way.

In 2005, an orbiting X-ray telescope saw the galaxy flare up. Over the next few months, the X-rays got even brighter — about a hundred times brighter than before the flare-up. The outburst has faded since then, but the galaxy’s X-rays are still unusually strong.

An international team of astronomers interpreted the flare-up as the disruption of a star that passed close to the black hole, and was pulled apart by the black hole’s gravity. That created a stream of super-hot gas that spiraled closer to the black hole.

The star was probably about twice the mass of the Sun — an unusually large meal for a black hole. The odds of the black hole grabbing such a massive star might have been enhanced by the fact that its galaxy is giving birth to so many stars — providing the entree for a black-hole feast.

Script by Damond Benningfield

Our closest neighboring star was discovered a century ago. Ever since then, astronomers have suspected that it’s a member of a triple star system. And some recent research provides confirmation of that idea.

Despite its nearness, Proxima Centauri is so feeble that it’s invisible to the unaided eye. That’s why astronomers didn’t spot it until 1915. Today, we know that it’s just four-and-a-quarter light-years from Earth.

At the time of its discovery, astronomers noticed that Proxima Centauri was at nearly the same distance as a much brighter star — actually a pair of stars whose light blends together to make it the third-brightest star in the night sky.

This brilliant double star is Alpha Centauri. It’s only about a tenth of a light-year farther than Proxima, and it moves through space at nearly the same velocity as Proxima. So from the beginning, astronomers have suspected that Proxima Centauri is part of the Alpha Centauri system.

But Proxima is a long way from the other two stars. If it is bound to them, it must have almost exactly the same speed through space. Otherwise, the little star would escape Alpha Centauri’s gravitational grip.

Recent measurements of Proxima Centauri’s motion have shown that the star is indeed bound to the binary. It orbits its bright partners once every 550,000 years.

So the nearest star system to the Sun is triple, harboring two bright stars plus dim little Proxima.

Script by Ken Croswell, Copyright 2016

In the spring of 1967, the Soviet Union was getting ready to celebrate May Day, one of its most important holidays. And the celebration would be preceded by a spectacular mission in space: Two Soyuz spacecraft would rendezvous and dock, and two cosmonauts would walk in space.

Soyuz was designed to carry cosmonauts to the Moon. But three unmanned tests had all resulted in failure. Even so, there was pressure to get on with the first manned flights. So 40-year-old cosmonaut Vladimir Komarov launched aboard Soyuz 1 on April 23rd, 1967.

But the craft was a dud, rushed into space before it was ready. After launch, one of its solar panels failed to open, and systems designed to aim and maneuver the Soyuz failed as well.

The launch of Soyuz 2 was canceled, and controllers set about trying to get Komarov back home. More problems delayed retro-fire by two orbits. Finally, after a day in space, Komarov fired his rockets and headed home.

After reentry, though, the main parachute got stuck. A backup deployed, but got tangled with a smaller chute and never opened. Soyuz 1 hit the ground at 90 miles per hour, killing Komarov. Moments later, braking rockets fired, engulfing the capsule in flames.

It took 18 months to redesign the Soyuz and get it back into space — a delay that scuttled any chances of winning the Moon race. Today, though, the Soyuz continues to fly — carrying Russians, Americans, and others to the International Space Station.

Script by Damond Benningfield

This time of year is pretty inviting for a little evening skywatching. The evening hours are warm but not usually too hot, and thunderstorm activity generally hasn’t reached its peak — perfect conditions for watching the stars.

Unfortunately, though, one of the most beautiful star patterns is dropping from view about now, so there aren’t many more days to enjoy it.

Orion, the hunter, is low in the west as night falls. Its three-star belt stands almost parallel to the horizon. And its two brightest stars bracket the belt: orange Betelgeuse above, and blue-white Rigel below.

Orion always climbs into prominence in the evening sky around Thanksgiving and Christmas. At that time of year, in fact, it’s in view for most of the night.

As the months roll by, though, so does Orion. The constellation rises earlier each night, so by late February, it’s already halfway across the southern sky at nightfall.

And now, it’s about to disappear from view. There’s only about an hour of really good viewing time — between the end of twilight and the time Orion’s stars begin to set. And that viewing window gets shorter by the night. In a couple of weeks, it’ll be hard to see the constellation at all.

Fortunately, though, there’s still a lot to look at after Orion passes from view. And the hunter won’t stay gone forever. He’ll begin to climb into the morning sky in August — and will return to the evening sky during the long, chilly nights at year’s end.

Script by Damond Benningfield

The two brightest objects in the night sky team up at dawn the next couple of days: the Moon and the planet Venus. Venus stands close to the left or upper left of the crescent Moon tomorrow, and a little to the upper right of the Moon on Thursday. Venus is the brilliant “morning star.”

The planet has no moons of its own, although for a couple of centuries it seemed that it might. From the late 1600s to the late 1700s, many astronomers reported seeing a moon close to the bright planet. Some of those astronomers were among the best observers of their time, so others took the sightings seriously.

But the sightings were sporadic. And it was impossible to combine them all to calculate an orbit for the possible moon, so the subject fizzled out for a while.

Then, in 1884, another astronomer thought he had a solution. The sightings weren’t a moon, he decided, but a small planet. Its orbit was synchronized with Venus’s in such a way that they appeared close together once every three years. He named this rarely seen world Neith, after an Egyptian war and sky goddess who was sometimes known as the veiled goddess.

A few years later, though, the veil was lifted for good. A thorough analysis of all the sightings found that there was neither moon nor planet. Instead, they were all sightings of stars that just happened to line up close to Venus — simple coincidences. The subject was closed — and our closest neighbor world remained moonless.

Script by Damond Benningfield

An amazing mission of discovery is scheduled to enter its final phase tomorrow. Cassini, which has been orbiting the giant planet Saturn since 2004, will change its flight path. The new course will take the craft between Saturn’s cloudtops and the inner edge of its beautiful rings.

Cassini has studied the rings in great detail during its journey. It’s found that the system consists of hundreds or thousands of individual rings. Most of the particles in the rings are made of ice, with a smattering of rock and dust.

Cassini has also discovered many small “moonlets” in the gaps between rings. These bodies are like shepherds tending their flocks; their gravity keeps the ring particles in their proper place.

The innermost ring is fairly thin and faint. Its inner edge reaches to within just a few thousand miles of Saturn itself. Bits of material from this ring probably drop into Saturn’s atmosphere, where they burn up as meteors.

To protect itself against possible impact, Cassini will use its large radio dish as a shield as it plunges through the plane of the rings. That should keep its instruments and electronics safe.

Tomorrow, Cassini will fly close to Titan, Saturn’s biggest moon. That’ll change Cassini’s path, setting up its first loop inside the rings next week. It’ll make 21 more passages over the next five months, leading up to its final act in September: a fatal dive into Saturn’s atmosphere, bringing its mission to an end.

Script by Damond Benningfield