StarDate

The brightest star of the Southern Cross is like a whole episode of “Dancing With the Stars.” It consists of perhaps six or more stars. They’re all twirling through their own ballroom, linked by the strong hands of gravity.

Alpha Crucis is 320 light-years away. To the eye alone, it looks like a single point of light – the 13th-brightest star in the night sky. But binoculars or a telescope show two stars. Both of them are at least a dozen times as massive as the Sun, and thousands of times brighter. They’re so far apart that it takes about 1300 years for them to complete a single orbit around each other – a slow turn across the dance floor.

But one of those stars is actually two stars on its own. They’re so close together that not even the biggest telescopes can see them individually – the second star reveals its presence only to special instruments. But it’s also bigger, heavier, and brighter than the Sun. The two stars are dancing to a faster tempo – one turn around each other every 76 days.

Those three stars might have three more companions. They’re a long way from the first trio, and they’re not as impressive. But they appear to share a common motion through the galaxy with the brighter trio. That means the two groups could be gravitationally bound to one another – dancing a waltz that would require a hundred thousand years to complete one turn across the floor.

Script by Damond Benningfield

The farther north you live, the less of the universe you can see. Earth itself blocks the view of a large swath of the southern celestial hemisphere. That’s the half of the sky that’s south of the celestial equator – the projection of Earth’s equator. So those of us in the United States miss out on at least part of the southern sky.

One of the treasures we miss is Crux, the southern cross. It’s the smallest of the 88 constellations. But it’s also one of the prettiest and most prominent.

Four of its stars are fairly bright, and they do form a shape that looks like a cross. If you include one more star in the pattern – the faintest of the five – the pattern looks more like a kite. It points the way to the south celestial pole.

Not surprisingly, that pattern has played a big role in the skylore of many southern-hemisphere cultures. Several saw the cross as the footprint of a big bird. Others saw it as a stingray, the anchor of a giant canoe, or some other prominent object or animal.

Today, Crux is featured on the flags of Australia, New Zealand, and Brazil. It’s also on the flag of the European Southern Observatory – which has a great view of the southern cross.

From the United States, Crux is barely visible from the Florida Keys, far-southern Texas, and Hawaii. At this time of year, it’s quite low above the southern horizon in early evening – pointing the way to the celestial pole.

More about Crux tomorrow.

Script by Damond Benningfield

Today, Saturn and its system of moons and rings look calm and peaceful. But things might have been much more chaotic in the fairly recent past. A collision between two moons might have destroyed one of them, changed the orbit of the other, and led to the birth of yet another moon and the planet’s rings.

Researchers have been trying to explain some oddities in the Saturn system. The planet itself is tilted far more than it should be, for example. The biggest moon, Titan, follows a more lopsided orbit than expected. And the moon is moving away from Saturn by about four inches per year.

A few years ago, a team proposed that Saturn once had another big moon, which the team called Chrysalis. Interactions between the moons might have kicked Chrysalis so close to Saturn that it was ripped apart, forming the rings.

But this year, another team came up with a slightly different scenario. It, too, involves a second moon. It collided with Titan a few hundred million years ago, changing Titan’s orbit. Debris from the impact formed the present-day moon Hyperion.

The activity caused two other moons to ram together as well. Both moons quickly re-formed, with the leftovers spreading out to form the rings as recently as 50 million years ago. This model explains many of the system’s oddities – bringing order to a chaotic arrangement.

Look for Saturn near our moon at dawn tomorrow. The planet looks like a bright star, low above the horizon.

Script by Damond Benningfield

Venus might be feeling a bit neglected. The last dedicated mission to the planet wrapped up its work two years ago. A couple of spacecraft have scanned the planet since then, but Venus wasn’t their main target. They were using the planet’s gravity to fling them toward their intended targets.

But Venus exploration could tick up over the next few years. Several missions are being developed. Most of them are big and complicated, so they won’t be ready until the next decade. But a craft the size of a beachball could head for Venus as early as this summer. It’ll probe the planet’s clouds for signs of organic compounds – the chemistry of life.

Venus Life Finder is a project of Rocket Lab and MIT – the first commercially developed mission to the planet. It’s a small, blunt cone. When it arrives at Venus, it will plunge through the planet’s clouds, shining a laser on the way down. The reflected light will reveal details about the cloud particles. Bits of organic matter might be set aglow.

Some recent observations have hinted that the clouds could contain microscopic life. Life Finder won’t actually search for life, but it could tell us if the building blocks of life lurk inside the planet’s clouds.

Venus is the “evening star.” It’s sneaking up on the star Elnath, at the tip of one of the horns of the bull. Tonight, the star is a little to the upper right of Venus. The planet will slip past it during the week.

Script by Damond Benningfield

The eyes of the dragon shine a third of the way up the northeastern sky at nightfall. Eltanin is the brightest star of Draco, with third-ranked Rastaban just above it. They circle high across the north during the night, and stand in the northwest at first light.

Although Eltanin looks brighter than Rastaban, that’s only because of their different distances. Eltanin is about 150 light-years away, while Rastaban is at 380 light-years. So if you lined them up side by side, Rastaban would shine about twice as bright as the dragon’s other eye.

Rastaban is only about 65 million years old, compared to four and a half billion years for the Sun. Yet it’s already passed the end of its “prime” lifetime.

That’s because it’s about six times as heavy as the Sun. Such massive stars “burn” through their nuclear fuel at a frantic rate. So Rastaban has already converted the hydrogen in its core to helium. Now, it’s probably getting ready to ignite the helium to make carbon and oxygen.

That’s caused the star to puff up – it’s about 40 times the diameter of the Sun. That’s the main reason it looks so bright – there’s a lot of surface area to beam light out into space.

Eventually, the nuclear reactions in the core will stop. The star’s outer layers will blow out into space. That will leave only the dead core – a cosmic ember as heavy as the Sun but only as big as Earth – closing one of the dragon’s bright eyes.

Script by Damond Benningfield