StarDate

The key to a star’s appearance and lifespan is its mass. Heavier stars shine brighter but live shorter lives than less-massive ones.

An example can be found in the two bright stars of Alpha Centauri, the leading light of the constellation Centaurus. It’s the closest star system to our own, at a distance of about four light-years.

One of the stars is about 10 percent more massive than the Sun. The other is about 10 percent less massive than the Sun. With so little difference, you might expect them to be almost identical — but they’re not.

The heavier star, Alpha Centauri A, burns through the nuclear fuel in its core faster than its companion, Alpha Centauri B. As a result, it shines three times brighter. The surface of Alpha Centauri A is also hundreds of degrees hotter, so it looks yellow-white versus the yellow-orange of Alpha Centauri B.

While “A” is far brighter, “B” has a much longer future to look forward to. Its “normal” lifetime should last about half again as long as that of its companion.

There’s actually a third star in the Alpha Centauri system — Proxima Centauri — that’s a long way from the other two. It’s a more extreme example of what a difference mass can make. It’s only 15 percent as massive as the Sun, so it’s only about one thousandth as bright as the Sun. Yet it will be shining a trillion years after the other two stars have faded into the cosmic night.

More about Alpha Centauri tomorrow.

Script by Damond Benningfield
 

A beautiful trio highlights the after-midnight sky tonight: the Moon and the planets Jupiter and Saturn. Jupiter is the brighter of the two worlds, but Saturn stands closer to the Moon.

A telescope will reveal Saturn’s largest moon, Titan. In fact, if you look at Titan with even a modest backyard telescope, you’ll know almost as much about the little world as astronomers did until well into the 20th century.

Dutch astronomer Christaan Huygens discovered Titan in 1655. In the 1880s, George Hill determined its mass by measuring its gravitational effects on the orbit of another of Saturn’s moons.

The first big breakthrough in understanding Titan came in 1944. Using the brand-new 82-inch telescope at McDonald Observatory, Gerard Kuiper detected the chemical “fingerprint” of methane — revealing that Titan has an atmosphere. It was the first moon in the solar system known to have an atmosphere, and it’s the only moon with a thick atmosphere.

Bigger telescopes allowed astronomers to see Titan more clearly, yet they revealed few details. That’s because a thick “haze” at the top of the atmosphere conceals the surface. It took visits from spacecraft to reveal the surface. The Cassini mission, for example, used radar to peer through the clouds. It showed us lakes and seas filled with liquid hydrocarbons, plus giant sand dunes and ice volcanoes — wonders on a world we’ve known about for a long time, but are just starting to know.

Script by Damond Benningfield
 

Earth isn’t the only world in the solar system that’s wet. Jupiter and some of its big moons are awash in water. In fact, several of the moons have hidden oceans. And each of those oceans has more water than all the oceans on Earth combined.

A recent study found that there’s quite a lot of water at Jupiter as well. The planet doesn’t have a solid surface, so it doesn’t have oceans. But there’s a fair amount of water vapor in the atmosphere. Some of it condenses to form clouds. The clouds produce rainfall, as well as giant blasts of lightning.

The moons Europa, Ganymede, and Callisto appear to have oceans below their icy crusts. Europa is the most intriguing of the moons because its crust is fairly thin – in some places, as little as a mile or two. There’s evidence that water occasionally squirts through cracks in the crust to reach the surface. There’s no atmosphere to speak of, though, so the water either freezes onto the surface or vaporizes into space.

Jupiter and its moons follow our moon across the sky tonight. Jupiter looks like a brilliant star. It’s to the left of the Moon as they climb into good view around midnight. It’s to the upper left of the Moon at first light tomorrow. The planet’s four largest moons, including the watery ones, are visible through binoculars. They look like tiny stars close to the giant planet.

As a bonus, the planet Saturn is close to the upper left of Jupiter. We’ll have more about that tomorrow.

Script by Damond Benningfield

Ursa Major, the great bear, may be the most famous constellation that isn’t well known. That’s because most of us know it for a pattern formed by only some of its stars: the Big Dipper. But few know that the dipper’s bowl forms the body of a bear, while its handle represents the bear’s tail.

The Dipper is high in the sky at nightfall right now, with the bowl hanging upside down. But there’s much more to the bear than just the dipper. His long legs and the rest of his body extend well below and to the left of the Big Dipper.

The star at the bear’s nose is far below the bowl — by about twice the width of your fist held at arm’s length. It’s known as Muscida — a Latin name that means “the muzzle.” And it’s about to undergo a big change.

The star is classified as a giant. As you might expect, that means it’s much bigger and brighter than the Sun. It’s so bright that it’s easily visible even though it’s about 180 light-years away.

Right now, the star’s heart is quiet. It’s converted its original hydrogen fuel to helium. It hasn’t yet moved on to the next step — but it’s about to. The core is getting smaller and hotter. That will trigger the next round of nuclear reactions — converting the helium to carbon and oxygen. As that happens, the star’s outer layers will puff up even more. That’ll make the star brighter and redder. For a while, the bear’s nose will resemble that of a certain famous reindeer — bright, shiny, and red.

Script by Damond Benningfield

Venus is hidden from view — in more ways than one. It’s passing between Earth and the Sun right now, so it’s hidden in the Sun’s glare. And even if we could see it, we couldn’t really see it. The planet is blanketed by clouds, so its surface is always hidden. The only good looks we have are from a few Soviet landers — which survived for only minutes — and a few orbiters that used radar to peer through the clouds.

The view could get better in a few years, though. NASA’s considering two new missions to the planet — plus two to the outer solar system. They would explore Io, the volcanic moon of Jupiter, or Triton, the largest moon of Neptune.

The missions are part of the Discovery program. It sends out fairly inexpensive craft to study the solar system and beyond. Its missions have orbited an asteroid, grabbed samples of a comet, found evidence of water on the Moon, and made detailed maps of Mercury.

This year, the program selected four proposals for detailed study, including the Venus missions. Veritas would use radar to make 3D maps from orbit. And Davinci-Plus would measure the clouds as it drops to the surface, snapping pictures all the way down. The Io craft would fly by the active moon several times, learning more about how its volcanoes are powered. The Triton mission would fly by it a single time, mapping the moon and learning whether it has a hidden ocean — making more discoveries about the solar system.

Script by Damond Benningfield

The folklore of Australia’s native cultures tells of a young man who got himself in trouble — but escaped to the stars. In the story, he was going through an initiation. As part of the ceremony, his body was painted red. His brother’s wives were attracted to him, so they seduced him. Their angry husband then burned down their hut. But the young man threw a spear into the Milky Way, then pulled himself and the women to safety in the heavens. There they remain, with the young man sometimes growing brighter — a signal to his people to calm their own desires.

A study a couple of years ago said the story represents Antares, the brightest star of Scorpius, the scorpion. Antares shines bright orange, so it represents the young man. It’s flanked by two fainter white stars — the sisters-in-law. And Antares grows brighter and fainter, just as in the story.

The tale may indicate that aboriginal cultures were paying attention to variable stars long ago. While most stars change very little, a few can change a bit more. Antares’s brightness, for example, can change by 20 percent or more. But such a change plays out over months or years. So it would take decades of patient skywatching to notice it — and to pass it on to later generations.

Antares is pretty easy to spot tonight because it rises below the full Moon. The bright star will be even closer to the Moon at first light.

Tomorrow: Pondering new missions of exploration.

Script by Damond Benningfield

Venus has gone away for a while. After reigning as the brilliant “evening star” for months, the planet has disappeared in the twilight. It’s just too close to the Sun for us to see it. In fact, it passes between the Sun and Earth today, moving into the morning sky as it does so. It’ll become visible as the “morning star” in a few days.

Venus repeats its path across the sky every eight years. During that time, it makes five appearances in both morning and evening sky. And each appearance traces the same loop against the background of stars. That makes it easy to predict where the planet will appear long into the future.

Easy, that is, once you figure out the pattern. It took a while for most cultures to start getting the idea.

One of the earliest to get it was the Mayan culture of Central America. Mayan astronomers built special observatories for observing the sky. From those perches, they plotted Venus’s position over many years. They then compiled tables of Venus’s motion across the sky — details that were important to the Mayan calendar. With those tables, they could forecast Venus’s appearances hundreds of years into the future.

For the immediate future, look for Venus in the east-northeast shortly before sunrise in a week or so. Although it’ll be immersed in the twilight, it’s so bright that it will stand out. It will climb away from the Sun over the following weeks — beginning a long reign as the morning star.

Script by Damond Benningfield
 

Stars that are born as twins don’t always remain apart. Sometimes, they merge to form a single star. That can happen while the stars are still young and vigorous, or even after they’re “dead.”

Astronomers recently discovered the remnant of the merger of two dead stars. It’s 150 light-years away, in the constellation Phoenix.

The remnant is a white dwarf. It’s heavier than the Sun, but smaller than Earth. It’s one of the heaviest white dwarfs ever seen.

When astronomers took a good look at it, they found a few other odd things. Its outer layers, for example, contain a high level of carbon. And the star appears to be especially old.

The astronomers plugged those traits into models of how stars work. They found that the most likely explanation was a merger between two white dwarfs more than a billion years ago.

The merger probably began when one of the original stars reached the end of its “normal” lifetime. The star puffed up, enveloping its companion. That dragged the two stellar cores closer together. The star then shed its outer layers, leaving only its dead core — a white dwarf. Later, the second star went through the same process, pulling them even closer. Over billions of years, the stars shed energy through gravitational waves — causing them to spiral together.

The combined stars were almost heavy enough to explode as a supernova. Instead, though, the merged stars settled in — beginning a long “life” as a dead star.

Script by Damond Benningfield

The biggest ball of stars in the Milky Way Galaxy creeps low across the south on June evenings. But it might not have been born in the galaxy. Instead, it may be the core of a smaller galaxy that was grabbed by the Milky Way billions of years ago.

Omega Centauri is classified as a globular cluster. It’s the biggest and brightest in the galaxy. It contains about 10 million stars. They’re all packed into a ball that’s about 150 light-years in diameter. In the cluster’s core, stars may be just a tenth of a light-year apart. By comparison, the Sun’s closest neighbors are more than four light-years away.

Several lines of evidence support the idea that Omega Centauri was born in another galaxy. For one thing, its orbit around the center of the Milky Way is backwards from the general flow. For another, its chemistry doesn’t quite match that of the Milky Way’s other stars.

So it seems likely that the cluster was the core of a dwarf galaxy that passed close to the Milky Way. The Milky Way’s gravity grabbed the smaller galaxy and pulled it in. The stars in the galaxy’s outer regions were absorbed into the body of the Milky Way. But the core stuck together – forming a bright globular cluster.

Omega Centauri is quite low in the south as night falls. From the northern third of the United States, in fact, it’s below the horizon, so it’s not visible at all. From other locations, it looks like a faint, fuzzy star – the remnant of a galaxy.

Script by Damond Benningfield

Ophiuchus, the serpent bearer, climbs high across the southern night sky at this time of year. The constellation’s main pattern resembles a coffee urn. It’s on its side, low in the east, as night falls, but stands upright after midnight.

Ophiuchus is north of Scorpius and Sagittarius, which point the way to the center of the Milky Way Galaxy. As a result, many star clusters are found inside its borders. The list includes 10 fairly bright globular clusters — tight balls of hundreds of thousands of stars.

The one that’s most impressive to look at is Messier 10. It’s near the middle of the urn, and spans about half the size of the Moon. It’s a good target for binoculars, and a very good one for telescopes.

The cluster looks so impressive mainly because it’s close — about 14,000 light-years away. It’s about 225,000 times the mass of the Sun. That’s a lot of stars. But it’s puny as globulars go. Most of the other clusters in Ophiuchus are much heavier.

Messier 14, for example, is a million times the Sun’s mass. It doesn’t look as bright as M10, though, because it’s twice as far. And Messier 62 is even more impressive — about one-point-two million times the Sun’s mass. It’s closer than M14, but there’s more dust between M62 and Earth, which blocks some of its light. Even so, it’s visible through binoculars — one of the beautiful clusters of the serpent bearer.

Script by Damond Benningfield