StarDate

A star can be old and new at the same time. In fact, it can even be dead and new. The dead star can gain a bit of new life from a companion. That turns the star into a nova — a Latin word that means “new.”

The term was first used more than four centuries ago. At first, it applied to stars that suddenly became visible where no star had been seen before. Such stars were “new” to the night sky.

As astronomers developed the tools to study the stars in detail, though, they discovered that the stars weren’t new at all. Instead, they were dead stars that gained a brief bit of glory from a close companion.

The dead star in such a system is a white dwarf — the hot, dense core of a once-normal star. The white dwarf’s powerful gravity pulls hydrogen off the surface of its companion — usually a bloated star that soon will become a white dwarf as well.

Some of the gas piles up on the surface of the white dwarf. This new layer gets hotter and hotter. Eventually, it gets hot enough to explode like a giant H-bomb. That blasts the new material out into space. In a few hours, the system can flare to thousands of times its normal brightness — creating a nova.

The blast interrupts the flow of hydrogen from the companion. Before long, though, it starts again. And that can produce another explosion — usually thousands of years later. But some novas can pop off again in just a few decades. And we’ll talk about the best-known example tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

Studying one of the farthest known galaxies is like taking a trip through time. Its light has taken 13.4 billion years to reach us. So we see it as it looked when the universe was just 400 million years old.

GN-z11 was discovered five years ago. Astronomers determined its distance by measuring a shift in the wavelength of its light. The shift is caused by the expansion of the universe itself, which causes galaxies to move farther apart.

We’ve learned that the galaxy was small but busy. It was giving birth to new stars at a furious rate, mainly because it still had huge amounts of hydrogen and helium — the raw materials for making stars.

Despite its youth, some of its stars had already expired. As they died, they peppered the galaxy with heavier elements created by the stars themselves. That means the light we see from GN-z11 comes from second-generation stars — those that incorporated elements produced by the first generation. So astronomers continue to look for galaxies that are even farther — populated by the very first stars.

Since we see GN-z11 as it looked early on, we have no idea what it looks like today — and we never will. Because of the continued expansion of the universe, it’s moving away from us so fast that the light it shines into space today can’t travel fast enough to reach Earth. So sometime in the future, GN-z11 will simply fade from sight — lost forever in the distant universe.

Script by Damond Benningfield

Support McDonald Observatory

You can organize a constellation’s stars in many ways — by brightness, color, or position, for example. But a system that astronomers have been using for centuries can’t quite make up its mind. In most constellations, it goes by brightness. In others, it’s based on position. And in some, there doesn’t seem to be a guiding principle at all.

An example of the latter is Corvus, the crow. It’s low in the southeast at nightfall, to the right of the bright star Spica, the leading light of Virgo.

Four stars form an angled box that looks like a sail. A fainter star is near the right point of the sail. Yet it receives top billing.

The naming system was devised more than four centuries ago by German astronomer Johann Bayer. He labeled the bright stars in each constellation with the letters of the Greek alphabet. “Alpha” usually designated the brightest star. In some instances, though, the title went to a star that was the starting point for an easily defined pattern.

But in the case of Corvus, Bayer doesn’t seem to have followed any pattern at all. The stars aren’t named from top to bottom, left to right, or in a circle. They don’t form any logical order within the figure of the crow, either.

And the Alpha star is only the fifth-brightest in the constellation — a faint pinpoint that’s visible only under dark skies.

So no one is quite sure what Bayer was thinking when he named the stars of this pretty little constellation.

Script by Damond Benningfield

Support McDonald Observatory

It’s often the case that an artist’s work isn’t fully appreciated until after the artist is gone. And the same thing is true in the stars. We appreciate a star’s beauty as it twinkles through the night. But a star’s true glory — a masterpiece of color and form — is also its final creation.

An example is the Cat’s Eye Nebula, in Draco, the dragon. The faint constellation slithers around the north and northeast at nightfall, and moves high across the sky later on.

Photographs of the nebula reveal swirls and loops of green and red — more like a flower than a cat’s eye. But the nebula’s real beauty is not as a photograph, but as a sculpture. It consists of a long bubble of hot gas attached to two bigger bubbles. Jets of gas shoot into space from the tops of the bubbles.

The process that sculpted the Cat’s Eye is the death of its star, which was several times as massive as the Sun. Thousands of years ago, it began throwing off big shells of gas and dust. And about one thousand years ago, it began ejecting its outer layers — a process that eventually will leave nothing but its dead core.

Ultraviolet energy from the core zaps the expanding bubbles, making them glow. And the shape of the nebula may be sculpted by the gravity of a small companion star.

The Cat’s Eye is expanding at hundreds of thousands of miles per hour. Eventually, it’ll spread out so much that it will fade away — and a work of cosmic art will die with its artist.

Script by Damond Benningfield

Support McDonald Observatory

Being too close to an exploding star can spoil your day. But being just close enough could make your day. A study a couple of years ago, for example, suggested that nearby exploding stars could have sped up the evolution of human ancestors.

Several massive stars within a couple of hundred light-years of Earth have exploded as supernovas over the last few million years. They’ve created a huge bubble of hot but thin gas around us, known as the Local Bubble.

The remnants of these stars produce huge amounts of cosmic rays — particles accelerated to almost the speed of light. They zip through the Local Bubble, pelting the stars and planets inside it. When they hit Earth, they give the atmosphere a stronger electric charge.

According to the study, that could result in more lightning bolts hitting the surface, starting more wildfires. And there’s evidence of more soot in the atmosphere starting about seven million years ago, and peaking a few million years later. Combined with other evidence, that supports the idea both of nearby supernovas and of increased wildfires.

The wildfires would have consumed forests in parts of the world, converting them to grasslands. And that could have been one factor that led human ancestors to begin walking on two legs.

Scientists continue to study supernovas in our neighborhood to learn more about their impact on Earth and its life — the possible influence of exploding stars.

Script by Damond Benningfield

Support McDonald Observatory

When you turn up the heat in your oven, the pie cooks faster. And if you turn it up too much, the pie can burn up in a hurry.

The same thing happens in the “ovens” at the hearts of massive stars. As such a star ages, the temperature in its core goes up. That causes things to cook faster — and, eventually, to burn up in a titanic explosion: a supernova.

All stars generate energy by “fusing” lighter elements to make heavier ones, releasing energy in the process. They start with hydrogen, the lightest element, which fuses to make helium.

Stars like the Sun end with carbon and oxygen. But stars that are at least eight times the Sun’s mass generate even heavier elements — a process that speeds up over time.

As the core finishes with one element, it gets smaller and hotter, triggering fusion of the next element. It takes millions of years to fuse the hydrogen to make helium. It then takes about one million years to fuse the helium to make carbon; a thousand years to fuse the carbon to make oxygen; and just a few months to fuse the oxygen.

Finally, the process produces silicon, which takes about a day to convert to iron. The iron absorbs energy instead of producing it. So the star’s core collapses to form a neutron star or a black hole. The outer layers plunge inward, then rebound. Those layers blast into space — ripping the star to bits as a supernova.

More about supernovas tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

No star lasts forever. But some stars will expire more impressively than others: They’ll blast themselves to bits in titanic explosions. And several stars that are visible in April’s evening skies face that ending.

A bunch of them are in Orion, which is in the southwest at nightfall. Its belt of three stars is parallel to the horizon. Four brighter stars form a rectangle around the belt.

All seven of those stars will become supernovas. That’s because all of them are supergiants — they’re many times the mass of the Sun.

Such stars “burn” through their nuclear fuel in a hurry. They quickly reach a point where the reactions stop. When that happens, the star’s core collapses to form a neutron star or a black hole. Its outer layers fall inward, then rebound. They blast into space at up to a few percent of the speed of light. For a while, the explosion can outshine an entire galaxy of normal stars.

In Orion, the stars closest to that end probably are Betelgeuse, the bright orange light above the belt, and Rigel, a blue-white star below the belt. They could explode at just about any time. But they’ll certainly blow in the next few million years.

As Orion drops toward the horizon in early evening, another future supernova climbs skyward in the east-southeast: Spica, the leading light of Virgo. It probably has more time left than Rigel or Betelgeuse. Yet it can’t escape its fate — as an exploding star.

More about supernovas tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

Jupiter may produce its own snowballs. And getting hit by one would sting — not just from the cold or the impact, but because the snowballs contain ammonia.

Jupiter is a giant ball of gas that’s topped by clouds. The clouds are made of ammonia, water, and other compounds. And they produce lightning.

Until recently, spacecraft had seen only “super-bolts” of lightning. But Juno, which is orbiting the planet, has photographed bolts comparable to the ones here on Earth. Those may be created much higher in the clouds, where it’s too cold for liquid water. But the high clouds contain ammonia. Crystals of water ice can be blown into those clouds, with some interesting results, says Heidi Becker, a Juno scientist:

BECKER: And when those crystals hit the ammonia vapor, the ammonia acts like an antifreeze and lowers the melting point of the water, letting it become a new ammonia-water liquid kind of like a Windex. So you can almost think of it like Windex raindrops colliding with water-ice crystals and charging up to create the conditions necessary for lightning.

The ammonia and water mix to form hailstones. As they cycle through the clouds they get bigger, creating “mushballs.” Eventually, they get heavy enough to fall out of the clouds at hundreds of miles per hour — high-speed mushballs on a giant planet.

Look for Jupiter above the Moon at first light tomorrow. It looks like a bright star. The fainter planet Saturn stands to their upper right.

Script by Damond Benningfield

Support McDonald Observatory

Some planets in other star systems appear to be even fluffier than the meringue atop a lemon pie. Such planets are called super-puffs. But some of them might not be as puffy as they look. Instead, they may be encircled by large sets of rings.

Astronomers have measured the size and mass of many exoplanets — planets that circle stars other than the Sun. They use those measurements to calculate a planet’s density. And they’ve found that quite a few of them are as light as balsa wood — and, in some cases, much lighter.

Some of the planets are especially close to their stars, so they’re quite hot. And as they heat up, their outer layers expand. That makes them a lot bigger.

But other super-puffs are harder to explain. And that’s where the rings come in.

Astronomers usually measure the size of a planet by watching it pass in front of its star, blocking some of its light. The amount the star fades reveals the size of the planet. In some cases, though, they may be seeing rings pass in front of the star as well. That would make the planet look much bigger than it really is. So some “super-puffs” may not be puffy at all.

Saturn — the planet with the biggest ring system in our own solar system — perches just above the Moon at first light tomorrow. It looks like a fairly bright star. The much brighter planet Jupiter stands off to their left. It will stand close to the Moon on Wednesday, and we’ll have more about that tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

Many stars — and perhaps most of them — travel in pairs. The stars are siblings — they were born together, from the same cloud of gas and dust. In most cases, the twin stars aren’t much alike. In a few cases, though, they’re almost identical. One example is Gamma Virginis, one of the brighter stars of Virgo.

Both stars are about half-again the mass of the Sun. They’re also wider and hotter than the Sun. And they’re probably a little more than a billion years old — just a quarter the age of the Sun.

On average, the two stars are separated by about the distance between the Sun and Pluto. Their orbit is quite lopsided, though, so the distance varies by billions of miles.

Tracking that orbit confirmed that the two stars are related, and don’t just happen to line up in the same direction. The fact that the system consists of two stars was discovered in 1689. And by 1803, astronomer William Herschel wrote that the stars were “intimately held together by the bonds of mutual attraction” — the attraction of their gravitational pull.

The stars were closest to each other about 15 years ago — so close that it was nearly impossible to see them as individual stars through most telescopes. But now they’re moving away from each other, making it much easier to see these identical twin stars.

As the sky gets good and dark, Gamma Virginis stands directly above bright Spica, Virgo’s leading light, in the east-southeast. It’s to the right of Spica at first light.

Script by Damond Benningfield

Support McDonald Observatory