StarDate

How long does it take to give birth to a giant?

If the giant happens to be Jupiter or Saturn, about four million years, according to a recent study. Not only that, but the giant planets passed through their period of youthful wanderlust in that same time, settling pretty much where they are today by the end of it.

Researchers at MIT studied meteorites that are more than 4.6 billion years old, which means they were around during the solar system’s formation. The older of these meteorites preserve traces of a magnetic field that permeated the early solar system. But those that are a bit younger show almost no magnetic field.

According to the researchers, that means the solar nebula — the cloud of gas and dust that gave birth to the planets — lasted no more than four million years. By the end of that time, the Sun’s magnetic field had weakened, allowing the leftover material to dissipate.

Jupiter and Saturn grew so large in part by sweeping up vast amounts of material in the nebula. When the nebula vanished, they stopped growing.

And the gravity of the nebula had caused the giants to move inward and then back out — a process that also stopped when the nebula went away. That left Jupiter and Saturn pretty much at their current distances from the Sun.

And Jupiter is in great view tonight. The planet is shining at its brightest for the year. And it’s quite close to the full Moon, so you can’t miss it.

More about Jupiter and the Moon tomorrow.

Script by Damond Benningfield

Astronomers and friends are gathering at McDonald Observatory in West Texas today to celebrate a major upgrade to the giant Hobby-Eberly Telescope. Over the last few years, much of the telescope’s optics and instruments have been replaced. That’s providing a wider look at the night sky, and allowing the telescope to use more of its main mirror. All of that enhances HET’s scientific capabilities, allowing astronomers to tackle new projects.

One of those projects is HETDEX — the Hobby-Eberly Telescope Dark Energy Experiment. It’s measuring the properties of a million galaxies that are roughly 11 billion light-years away. That means the telescope is seeing those galaxies as they looked when the universe was quite young.

The project’s goal is to see how the rate at which the universe is expanding has changed over the eons.

All but a few galaxies are moving away from us — the result of the expansion of the universe itself caused by the Big Bang. A couple of decades ago, astronomers discovered that the universe is expanding faster as it ages. That extra kick has been credited to “dark energy.” But just what dark energy is remains a mystery. It could come from the vacuum of space, or from one of several other possible explanations.

By looking at galaxies in the early universe, HETDEX will show how the rate of expansion has changed over time. That will help theorists narrow down the range of possibilities — and perhaps solve the mystery of dark energy.

Script by Damond Benningfield

It’s been almost a century since astronomers discovered that the universe is expanding — a result of the Big Bang in which it was born. But the details of that expansion are still being uncovered. In particular, astronomers are studying a phenomenon that appears to make the universe expand faster as it ages. It’s known as dark energy, and figuring it out is one of the great challenges of modern-day physics.

Astronomers discovered dark energy by measuring the speeds at which distant galaxies are moving away from us. That allowed them to plot how fast the universe was expanding at different times. They expected to find that the expansion rate was slowing down — the result of the gravitational pull of all the galaxies and dark matter.

Instead, they found that things are actually speeding up — the universe expands faster as time goes by.

So far, the cause of that faster expansion is still a mystery. Dark energy could be caused by an energy from the vacuum of space itself. It could also mean that our understanding of gravity is wrong — or at least incomplete. Or there could be some other explanation.

Several experiments are studying how the universe is expanding to try to solve the mystery. That includes a project at McDonald Observatory. HETDEX — the Hobby-Eberly Telescope Dark Energy Experiment — is measuring galaxies that are billions of light-years away, when the universe was young. We’ll have more about that tomorrow.

Script by Damond Benningfield

One of the brightest objects in the night sky is especially bright right now. That’s because Jupiter is at opposition — it’s lining up opposite the Sun. It’s closest to us now, and more of its light is reflected back in our direction, so it shines at its absolute best.

Jupiter is the largest planet in the solar system — about 11 times the diameter of Earth. And the clouds that top its atmosphere reflect a lot of sunlight. But even at its closest, it’s about 400 million miles away. That combination makes Jupiter the third-brightest object in the night sky — behind the Moon and the planet Venus.

It takes Jupiter more than 12 years to make one orbit around the Sun, which means it also takes it that long to complete one circuit against the background of stars. As a result, it spends an average of about a year in each constellation it crosses.

As Jupiter orbits the Sun, Earth is doing the same thing. Our orbit is much smaller, though, so it takes us much less time to make one turn around the Sun. It’s as if we’re on the inside lane of a race track, with Jupiter on the outside lane. It takes about 13 months for Earth to catch up to Jupiter and move past it, so that’s the gap between oppositions. In other words, the next opposition will be about 13 months from now — next May.

For now, look for Jupiter quite low in the east as night falls, and climbing across the south later on. It looks like a brilliant star.

Script by Damond Benningfield

Regulus, the bright star that marks the heart of the lion, is a lot older than it looks. Until about a decade ago, astronomers estimated its age at no more than a hundred million years. But the discovery of a dead companion star changed things. Today, the best estimate is a billion years.

In most cases, astronomers determine a star’s age by comparing its mass, its temperature, and its total brightness. Regulus is several times as massive as the Sun, thousands of degrees hotter, and hundreds of times brighter. Crunch the numbers, and that indicates a vigorous young star.

In this case, though, the numbers don’t tell the whole story. That’s because Regulus has a close companion — a tiny stellar corpse known as a white dwarf. It’s so small, faint, and close to Regulus that it’s impossible to see directly. Instead, it was discovered with the help of sensitive instruments.

When Regulus and the companion were born, the companion was the heavier of the two stars. That means it aged more quickly. As it aged, it got much bigger, so it began dumping the gas in its outer layers onto the surface of the star we see as Regulus. That made Regulus much bigger and heavier. In turn, that revved up the nuclear reactions in its core, making the star bluer and brighter. That change disguised the star’s true age: a billion years or older.

Look for Regulus tonight just a whisker away from the Moon — a bright star that’s older than it looks.

Script by Damond Benningfield

The Sun goes through an 11-year magnetic cycle. At the cycle’s peak, the Sun produces large numbers of dark sunspots and powerful explosions known as solar flares. At the cycle’s low point, the Sun is quiet, with little activity on its surface.

But no two cycles are alike. Some are especially active, while others are unusually quiet. And some of the quietest yet identified may have happened 7500 years ago.

An international team of scientists found this quiet period by studying tree rings. When the Sun is quiet, it allows more cosmic rays to enter the solar system. These particles come from exploding stars and other powerful sources. When they hit Earth’s atmosphere, they trigger showers of other particles. And when those particles reach the ground, they can alter atoms.

In particular, they can change carbon atoms to a radioactive form known as carbon-14. So the ratio of normal carbon to carbon-14 found in tree rings varies based on the amount of solar activity.

The new study found some of the highest levels of carbon-14 yet discovered in the rings of bristlecone pines from the White Mountains of California. The levels were especially high from about 5490 to 5460 BC.

That could mean that the Sun was especially quiet during that period. If that’s the case, then almost no sunspots would have mottled its surface. That would have allowed the cosmic rays to flood into the solar system and hit Earth — leaving traces in ancient trees.

Script by Damond Benningfield

Astronomers have cataloged thousands of black holes. Some of them are the remnants of collapsed stars. Others, found in the hearts of galaxies, are millions or billions of times the mass of the Sun. Yet so far, no one has actually seen even a single black hole.

In part, that’s because black holes produce no energy at all. And they’re also tiny targets; a black hole that’s a hundred times the mass of the Sun is only a few hundred miles in diameter. Supermassive black holes are much bigger, but they’re so far away that they’re too small for current telescopes to see.

So astronomers infer the presence of black holes from their effects on the visible matter around them. They discovered the black hole at the center of the Milky Way in part by plotting the orbits of the stars that are close to it. The speeds of those stars mean they’re being pulled by something that’s heavy, but smaller than our own solar system.

Astronomers are hoping to actually see that black hole soon. They’re combining a dozen radio telescopes across the globe to create the Event Horizon Telescope. It’ll look at the center of the galaxy to try to see the event horizon — the boundary between the black hole and the space around it. That technique could allow them to see the black hole, which would look like a silhouette against the background of stars and gas, with a thin ring of light around it.

The first observations are scheduled for this month — an attempt to see a black hole.

Script by Damond Benningfield

A black hole doesn’t have a “surface” — there’s nothing for you to crash into. Instead, it has a boundary — a point of no return for matter and energy. Anything that crosses the boundary is trapped forever inside the black hole.

That boundary is known as the event horizon. It’s the distance from the center of the black hole at which escape velocity — the speed you need to get away from it — equals the speed of light. Since nothing can travel faster than light, nothing within that distance of the black hole’s center can get away. So the event horizon looks like a black hole in space.

The size of the event horizon depends entirely on the mass of the black hole. For a black hole as massive as the Sun, the event horizon would span about three and a half miles. At a million times the mass of the Sun, the black hole would be three and a half million miles in diameter, and so on.

If you approached an event horizon, things would get a little weird. As seen by an outside observer, your clock would begin to tick slower and slower. At the event horizon, it would appear to stop. So anyone watching you approach the horizon would never see you cross it. Instead, you’d appear to stand still, with the light from your spaceship shifted to longer and longer wavelengths — just one of the strange effects around a black hole.

Even though there’s good agreement that black holes exist, no one has ever seen one. But that could change soon. More about that tomorrow.

Script by Damond Benningfield

Vela, the sail of the ship that carried Jason and the Argonauts, flutters quite low across the south on these early spring evenings, but only if you’re south of about Dallas or Tucson. And even then, it contains only a couple of moderately bright stars, so there’s not much to see.

If we could view that region in a few million years, though, it would be a different story. Several giant clouds of cold gas and dust appear to be giving birth to new stars. And some of those stars could be big, heavy, and extremely bright.

The biggest of these clouds is known as Vela C. It’s about 2300 light-years away, and more than a hundred light-years wide. In all, it’s more than 50,000 times as massive as the Sun.

Most of that mass is still in the form of gas and dust. But the cloud is splitting into smaller clumps of material. Gravity is causing the clumps to collapse. Eventually, some of them will condense enough to become new stars, making Vela C a stellar nursery.

In fact, there’s evidence that it’s already given birth to some stars. They’re extremely young — no more than a million years old, and probably a good bit younger. It’s hard to see them because they’re still embedded in cocoons of dust, which absorb most of their light.

Some of these stars are quite massive — perhaps 10 or more times the mass of the Sun. So when the dust around them clears away, they’ll shine thousands of times brighter than the Sun — lighting up the celestial sail.

Script by Damond Benningfield

Two colorful stars accompany the Moon down the western sky this evening. Aldebaran, the eye of the bull, is to the lower right of the Moon. And slightly brighter Betelgeuse, the shoulder of Orion, is a little farther to the left of the Moon.

If you look closely, you’ll notice that both stars are orange — two rare specks of color in the night sky. The color is a function of their surface temperatures. Both stars are thousands of degrees cooler than the surface of the Sun. Cool stars shine red or orange, while hot stars shine blue or white. Yellow stars like the Sun are in the middle of the temperature scale.

The surfaces of Aldebaran and Betelgeuse are cool for the same reason: the stars are huge.

Both stars are nearing the ends of their lives. They’ve undergone dramatic changes in their cores, causing them to puff up. As the stars expanded, the gas at their surfaces got cooler — and redder.

Aldebaran is more than 40 times the diameter of the Sun, so it’s classified as a giant. Betelgeuse, on the other hand, is more massive than Aldebaran, so it’s inflated even more — it’s almost a thousand times the Sun’s diameter, which makes it a supergiant.

A few billion years from now, the Sun will puff up as it nears the end of its life. It’s not massive enough to reach anywhere close to the proportions of Betelgeuse. But it will resemble present-day Aldebaran — a giant star shining bright and orange.

Tomorrow: Sailing to a nursery.

Script by Damond Benningfield