StarDate

The heaviest known star is a bit of a mind-bender — it’s more massive than most models of star formation allow.

The idea of a stellar weight limit is pretty simple. It says that as a growing star adds more material, its surface gets hotter. That produces more energy and a stronger “wind” of charged particles blowing into space. At a certain point, the wind and radiation should push away any more in-falling gas, preventing the star from getting any heavier. And in the modern-day universe, that limit should be about 150 times the mass of the Sun.

But a star known as R136a1 appears to tip the scales at more than 250 times the Sun’s mass. The star is a member of a cluster in a nearby galaxy. The cluster contains several stars that are among the heaviest ever seen.

R136a1 is not only the most-massive of those stars, but it’s also the brightest. If you add up all wavelengths of light, it’s more than eight million times brighter than the Sun. It emits as much energy in just four minutes as the Sun does in an entire year.

The star is so hot that it’s blowing itself apart. In its two-million-year lifetime, a powerful wind has whisked about 50 Suns-worth of material off of its surface.

R136a1 is likely to expire in a few million years — but no one is sure just how. It could produce an especially powerful explosion, or collapse to form a black hole, or do some combination — more mind-bending uncertainty from the heavyweight champion of the stars.

Script by Damond Benningfield

A giant tarantula creeps through a nearby galaxy. It’s not trying to be stealthy, though — it’s the galaxy’s brightest feature. In fact, it’s the brightest stellar nursery in the entire Local Group — the cluster of dozens of galaxies that includes the Milky Way.

The Tarantula Nebula is in the Large Magellanic Cloud, a companion to the Milky Way that’s just 160,000 light-years away.

Over the last few million years, the nebula has given birth to millions of stars. That’s probably the result of a close encounter with a smaller galaxy. The gravity of the other galaxy caused large clouds of gas and dust to collapse, forming new stars.

The Tarantula incorporates several star clusters — groups of stars that all formed at about the same time. The most impressive is R136. It contains at least a half-million stars, most of which are no more than three million years old.

Most of those stars are the mass of the Sun or smaller. But a few are monsters that are many times heavier than the Sun. At least nine of them are more than a hundred times the Sun’s mass. And the biggest of all is more than 250 times the Sun’s mass — the heaviest star yet seen in any galaxy, including our own. More about that tomorrow.

Within the next few million years, many of these stars are likely to blast themselves to bits as supernovae. In fact, a star on the outskirts of the nebula did just that back in 1987 — a brilliant outburst from the brilliant tarantula.

Script by Damond Benningfield

[SFX: fireworks]

Fireworks will light up the skies of many American cities and towns tonight — celebrations of Independence Day.

For a real fireworks display, though, you might want to visit one of the Milky Way’s companion galaxies. It’s giving birth to many thousands of new stars, including some of the biggest and brightest yet seen anywhere — a result not of independence, but of a close relationship with another galaxy.

The Large Magellanic Cloud is too far south to see from the continental United States. In southern-hemisphere skies, though, it’s quite a sight — a bright cloud that’s several times bigger than the full Moon.

The galaxy is much smaller and fainter than the Milky Way. But it’s right next door — just 160,000 light-years away. That’s one of the reasons it looks so bright.

Another is that the galaxy contains millions of vigorous young stars — stars that are thousands of times brighter than the Sun. And it’s giving birth to more. In fact, it contains the largest and brightest stellar nursery in the entire Local Group — the family of galaxies to which the Milky Way belongs. More about that tomorrow.

The fireworks probably are the result of a gravitational interaction with another galaxy, the Small Magellanic Cloud. The smaller galaxy passed close to the bigger one, squeezing vast clouds of gas and dust. The clouds then gave birth to new stars — triggering fireworks in a busy galaxy.

Script by Damond Benningfield

AUDIO: 3, 2, 1, ignition and liftoff! of the Atlas V with Juno on a trek to Jupiter.

It’s been a long trek for the Juno spacecraft — almost five years and more than 1.7 billion miles. But the journey is almost over — Juno is scheduled to enter orbit around Jupiter tomorrow.

Its instruments will probe Jupiter’s interior, its atmosphere, and its magnetic field. Among other things, those observations should help scientists decide how Jupiter and the other planets took shape.

Scott Bolton is Juno’s lead scientist:

BOLTON: If we want to go back in time and understand where we came from, and how the planets were made, Jupiter holds this secret, because it’s got most of the leftovers after the Sun formed, and so we want to know that ingredient list. What we’re really after is discovering the recipe for making planets, and we’re back at the first step of making sure we have all the ingredients in that recipe.

When our solar system was born, the young Sun was surrounded by a disk of gas and dust. One theory says that some of this material coalesced to form small chunks, which merged to form bigger and bigger bodies — the planets. But another says the planets formed from the collapse of unstable zones of material within the disk.

Juno will help determine the composition of Jupiter’s core and measure the contents of its atmosphere. A dense, rocky core and a lot of water and ammonia in the atmosphere would support formation from smaller chunks of material — a major discovery from a trek across the solar system.

Script by Damond Benningfield

Jupiter blazes in the west at nightfall. It’s the brightest pinpoint of light in the night sky, so you can’t miss it.

The giant planet is about 540 million miles away. At that distance, it takes 48 minutes for the sunlight that reflects off Jupiter’s clouds to reach Earth. For skywatching purposes, that doesn’t really matter. But it does matter to scientists and engineers at NASA, who are guiding a spacecraft toward the planet.

Juno is scheduled to arrive at Jupiter on Monday. It’ll fire its main engine for 30 minutes, slowing it into orbit around Jupiter’s poles. But because it’s so far away, the craft will be on its own — no one will be able to help if something goes wrong.

If things go right, Juno will slip into an even lower orbit over the following days and weeks. Eventually, it’ll drop to within about 3,000 miles of the planet’s poles.

From that orbit, Juno will probe Jupiter’s interior, its atmosphere, and its magnetic field. Those observations will yield new details on how Jupiter is put together, what its atmosphere is like far below the cloudtops, and how the planet interacts with the solar wind. Scientists hope the findings will help reveal how Jupiter and the solar system’s other planets took shape. More about that tomorrow.

Juno also carries a camera that will photograph Jupiter when the craft is closest to the planet. That’ll provide the sharpest views of Jupiter’s cloudtops to date — pictures from half-a-billion miles away.

Script by Damond Benningfield

Most of the skywatching action has been concentrated in the evening sky for the last few months. But there’s a nice bit of business in the morning sky tomorrow: the crescent Moon snuggles close to the eye of the bull. There’s not much time to look, though — the dawn twilight will overpower them in a hurry.

The eye of Taurus, the bull, is Aldebaran, a star that shows us what the Sun will look like in a few billion years: big, fluffy, bright, and orange.

All of those traits are part of the natural aging process for stars like Aldebaran and the Sun. Such a star spends most of its life steadily “fusing” the nuclei of hydrogen atoms in its core to make helium — just what the Sun is doing right now. When that process is done, the core shrinks and gets hotter. That causes the star’s outer layers to puff up.

That makes the star get much bigger. But since it doesn’t add any material, it gets less dense, so its outer layers get fluffy. The star eventually swells to many times its original size. With that much surface area, it gets brighter. And the puffed-up gas gets cooler, which makes the star glow orange or red.

And that’s where Aldebaran is right now: big, fluffy, bright, and orange — a picture of what the Sun will look like in the distant future.

Look for Aldebaran close to the upper right of the Moon, quite low in the eastern sky at first light tomorrow. They’re still low in the sky as twilight begins to overpower them.

Script by Damond Benningfield

It’s been only a few months since the first direct detection of gravitational waves — “ripples” in space and time created by the merger of two black holes. But astronomers have already come up with several scenarios to explain the history of the black holes themselves.

The basic facts are that one black hole was 36 times the mass of the Sun, the other 29 times the Sun’s mass. In the instant before they merged, they were orbiting each other at about half the speed of light. And the merger took place more than a billion light-years from Earth.

But how and where the black holes formed is less certain.

One idea says they were born 10 billion years earlier, from the collapse of mighty stars in the core of a giant cluster. Close encounters with other black holes kicked the tightly bound pair into intergalactic space.

Another idea says the black holes were “primordial” — they were born before the formation of any stars or galaxies, from the collapse of dense clumps of matter shortly after the Big Bang.

And yet another idea says the black holes were born from a single giant star. The star was spinning so fast that its core split into two clumps, each of which collapsed to form a black hole. Within a minute, material in the original star’s outer layers fell onto the black holes, increasing their heft. And in a few minutes more, the two black holes merged — creating waves in the universe that rippled past Earth last year.

Script by Damond Benningfield

A star like the Sun can live for more than 10 billion years. It can survive the shockwave from a nearby supernova, or the impact of an infalling planet. One thing it can’t survive is a close encounter with a supermassive black hole. The black hole’s gravity can rip the star apart, leaving a streamer of super-heated gas that will fall into the black hole.

That was the case with an unlucky star in a galaxy about 300 million light-years away. The star passed within a few hundred million miles of the black hole at the galaxy’s heart. The black hole is only about a million times the mass of the Sun — pretty small for a supermassive black hole. Yet its gravity is still so powerful that it ripped the star apart, producing a brilliant flare.

A space telescope saw the flare in late 2014. Teams of astronomers quickly began watching the flare with other telescopes in space and on the ground. That allowed them to determine the nature of the flare: the destruction of a star roughly the size of the Sun.

As they watched the flare over the following weeks and months, they detected an outburst of radio waves — an act that came after the star’s demise. The star’s hot gas joined a disk of material around the black hole. As the gas spiraled around the black hole, some of it was funneled back into space — a “fountain” of charged particles racing outward at almost the speed of light: the last remains of a dead star.

More about black holes tomorrow.

Script by Damond Benningfield

The black holes that inhabit the cores of most big galaxies come in a range of sizes — from big to wow! The one at the heart of the Milky Way is in the “big” category — about four million times as massive as the Sun. But those in the “wow!” category can be more than a thousand times heavier. In fact, one recent study said that, under the right circumstances, such black holes could max out at about 50 billion times the Sun’s mass — heavier than some galaxies.

One recently discovered super-monster is in the galaxy NGC 1600, a couple of hundred million light-years away. It’s perhaps 17 billion times as massive as the Sun. But observations of stars that orbit the black hole suggest that it may actually be two black holes instead of one. If so, they’d likely be remnants from the merger of two galaxies. Eventually, the black holes themselves may merge as well.

A black-hole binary has been confirmed in a galaxy known as OJ 287. But it’s not an even pairing. The more-massive of the two is 18 billion times the Sun’s mass, while the other is less than one percent as heavy. The smaller black hole orbits the bigger one once every dozen years or so, on a stretched-out path. When it gets close, it plunges through a disk of hot gas that surrounds the bigger black hole twice — once on the way in, and again on the way out. That stirs up the disk and causes bright flares of energy — powered by a dark monster.

More about black holes tomorrow.

Script by Damond Benningfield

The constellation that marks the center of the Milky Way galaxy scoots low across the south on summer nights. Its brightest stars form the outline of a teapot, with the glowing band of the Milky Way rising from the spout like steam. The galaxy’s heart is inside the steam.

A supermassive black hole sits at the center of this region. It’s known as Sagittarius A-star, after the constellation. It weighs as much as four million Suns.

Astronomers determined that mass by measuring the orbits of nearby stars. As the stars get closer to the black hole they speed up. The speed indicates the presence of a small, heavy concentration of matter that tugs at the orbiting stars.

Yet no one has seen the black hole itself. Its event horizon — the point of no return for matter that falls into the black hole — is about 14 million miles in diameter. That’s a big target. But from a distance of about 27,000 light-years, that’s too small to measure with any existing telescope.

Yet there’s an effort underway to see the black hole nonetheless. Astronomers are linking radio dishes around the world in a project known as the Event Horizon Telescope. Combining several widely spaced telescopes produces images as sharp as those made by a single telescope as big as Earth.

The goal is to see the outline of the black hole. The black hole’s gravity should distort the view of the stars and gas behind it, producing a glowing ring — outlining a black hole.

Script by Damond Benningfield