StarDate

The Sun is larger and brighter than most other stars in the galaxy. In fact, only a few percent of the Milky Way’s stars are about the same size, temperature, and brightness as the Sun. And at this time of year, if you have a dark sky and a good star chart, you can see two of them for yourself.

The stars form a binary system known as 16 Cygni. The system is about 70 light-years away, in the constellation Cygnus, which is in the northeast in early evening. But the stars are so close to each other that their light blends together to form a single point -- one that’s just barely visible to the unaided eye.

If you aim a small telescope at 16 Cygni, though, you’ll see both stars. They’re almost identical to each other -- and to our Sun: about the same color, mass, and brightness. The fainter of the two stars even has a giant planet in orbit around it.

One big difference between the Sun and the stars of 16 Cygni is age. While the Sun is about four and a half billion years old, the stars of 16 Cygni are about seven billion years old. Scientists know this because the structure of a star changes as the star ages. Oscillations inside the star cause tiny changes in its light that reveal the structure of the star’s interior.

The stars of 16 Cygni show us what the Sun would look like if we could view it across many light-years of space -- and if it had a matching partner to keep it company on its long journey through the galaxy.

Script by Ken Croswell

People don’t always shine at their brightest as they near the ends of their lives. But stars do. They can shine hundreds of times brighter than when they were in the prime of life -- but only for a little while. After that, they quickly fade away.

An example is Tarazed, in Aquila, the eagle. The constellation’s brightest star, Altair, is in the east at nightfall, at the lower right corner of the bright Summer Triangle. Tarazed is just a couple of degrees above Altair.

The star is only about a hundred million years old, compared to four and a half billion years for the Sun. But it’s passed the end of its “normal” lifetime because it’s almost six times as massive as the Sun.

Heavier stars “burn” the hydrogen in their cores at a furious rate, quickly converting it to helium. When that happens, the core gets smaller and hotter, allowing it to burn the helium to make carbon. That causes the star’s outer layers to puff up to gigantic proportions.

And that’s what’s happened to Tarazed. It’s about a hundred times the diameter of the Sun. That makes it shine about 2500 times brighter than the Sun. And that makes it easy to see even though it’s almost 400 light-years away.

Before long, though, Tarazed will cast its outer layers into space. That will leave only its dead core -- a ball of carbon and oxygen that’s as massive as the Sun but only about as big as Earth: a white dwarf -- a hot but small ember slowly cooling through the long cosmic night.

Script by Damond Benningfield

The signature star pattern of summer climbs high across the sky on July nights. It’s so big that it incorporates stars from three constellations. And it’s so bright that it’s easily visible even from light-polluted cities.

The Summer Triangle is well up in the east and northeast as night falls. It passes directly overhead after midnight.

The stars that outline the triangle are Vega, Deneb, and Altair. Vega is the brightest, and it stands at the top of the triangle in early evening. Deneb marks the left point of the triangle, with Altair to the lower right of the other two.

Altair is the closest member of the triangle -- less than 17 light-years away. It’s bigger and brighter than the Sun, but not amazingly so. And it spins much faster than the Sun does, so it’s much fatter through the equator than through the poles.

Vega is about nine light-years farther than Altair. Since it looks brighter, that means it’s much more impressive. In fact, when you add up all wavelengths of light, it’s almost four times Altair’s brightness, and 40 times the Sun’s. And Vega also has a bulging waistline.

The real stunner of the trio, though, is Deneb. It’s about 20 times heavier than the Sun, 200 times wider, and 200,000 times brighter. So while Deneb looks meeker than the others, that’s an illusion -- caused by its distance of about 2600 light-years -- dozens of times farther than the other members of the beautiful Summer Triangle.

Script by Damond Benningfield

The gibbous Moon has a famous companion tonight — the planet Mars. It’s to the lower right of the Moon as they climb into good view, around midnight, and almost directly below the Moon at first light tomorrow. Mars looks like a bright orange star.

Mars has two moons of its own. They’re nowhere near as impressive as our moon — Phobos is only about 14 miles in diameter, with Deimos just about half that. Yet they may have something in common with the Moon — the way they were born.

Our moon probably was born after the young Earth was hit by another planet. The impact blasted debris into orbit around Earth. Some of that material coalesced to form the Moon.

Phobos and Deimos might have formed as the result of a big impact as well. A recent study at the Southwest Research Institute says the impactor probably was a few hundred miles in diameter. That’s a good bit smaller than suggested by earlier research.

The collision would have blasted a ring of material into orbit. Some of the material that was close to Mars formed fairly large moons. They were so close, though, that Martian gravity quickly pulled them back down. Some of the debris farther from Mars formed Phobos and Deimos, which are still around today.

The new study says the moons should be made of almost the same material as Mars, with one big exception: The heat of the impact should have vaporized any water. So as Phobos and Deimos coalesced, they were left high and dry.

 
Script by Damond Benningfield

There’s no privacy for Mars. As of late May, six spacecraft were in orbit around the Red Planet, with two rovers on the surface. There’s never been a time when more craft were operating at Mars or anywhere else beyond Earth.

The most recent craft to get to work is Trace Gas Orbiter. It arrived at Mars in October of 2016. Engineers spent a few months adjusting the angle of its orbit. Then, starting in March of 2017, they adjusted its altitude. The European craft used a technique called aerobraking. On each orbit, it dipped into the outer fringes of the atmosphere. Friction with the atmosphere dropped the high point of its orbit. It completed those maneuvers and entered its final science orbit in April.

Now, Trace Gas Orbiter is trying to sniff out certain compounds in Mars’s upper atmosphere. In particular, it’s looking for methane. On Earth, most methane comes from living organisms, or from deposits of hydrocarbons. A small amount comes from volcanoes.

Other probes have detected small amounts of methane on Mars. But they couldn’t tell where it came from. Trace Gas Orbiter will make far more precise measurements. That will help scientists determine where on Mars the methane comes from, and what’s producing it — life or geology.

And Mars is growing brighter by the day. It’s in good view in the southeast by midnight, and looks like a bright orange star. Tonight, it’s to the lower left of the Moon.

More about Mars and the Moon tomorrow.

Script by Damond Benningfield

 

Not many of us build something that’s classified as the world’s largest. But George Ellery Hale did it four times. He led the design and construction of four different telescopes that were the biggest in the world.

Hale was born 150 years ago tomorrow in Chicago, the son of a wealthy industrialist. Hale built his first telescope at age 14. And while he was in college, he built a new instrument for studying the Sun. Later, he used that type of instrument to discover that sunspots are magnetic storms.

After college, Hale joined the staff of the University of Chicago. While there, he convinced another industrialist to fund a new observatory. Its centerpiece was a telescope with a 40-inch lens. It’s still the world’s largest refracting telescope.

The observatory was built in Wisconsin, where conditions weren’t that great for skywatching. So Hale established a new observatory in California. He built special telescopes for watching the Sun, and the world’s biggest reflector, which uses a glass mirror to gather starlight. And a few years later, he built an even bigger one — 100 inches in diameter. Edwin Hubble used that telescope to discover that the universe is expanding.

Although Hale was in poor health by then, he spearheaded a 200-inch reflector atop Palomar Mountain. It wasn’t completed until after Hale died, in 1938. But it reigned as the world’s largest telescope for decades — a final testament to George Ellery Hale.

Script by Damond Benningfield

Saturn is at opposition today. That means the giant planet lines up opposite the Sun. Such an alignment provides the best viewing conditions for a planet. It rises at sunset and is in view all night. It’s closest to Earth at opposition as well, so it shines brightest for the entire year.

Tonight, Saturn will stand just a degree or two to the lower right of the Moon at nightfall. Coincidentally, the Moon is also at opposition, so it’s full.

Not every planet can go through opposition. Mercury and Venus orbit the Sun inside Earth’s orbit, so they can never appear opposite the Sun. As a result, you’ll almost never see either planet at true local midnight. Instead, they’re in view for only a few minutes or hours before sunrise or after sunset.

Saturn, Mars, and Jupiter, on the other hand, orbit outside Earth’s orbit, so they can circle all the way across the nighttime sky.

Saturn’s brightness can vary by quite a bit from one opposition to the next. That’s because we view its rings at different angles. Right now, the rings are at a fairly open angle, making Saturn especially bright. It’ll reach magnitude zero, making it brighter than all but the Moon and a handful of other planets and stars.

If you can’t see Saturn tonight, though, don’t worry. The planet will stay just about as bright for weeks after opposition — there’s not a sudden spike in its brilliance. So Saturn will remain a beautiful sight throughout the summer.

Script by Damond Benningfield

In the busy hearts of globular star clusters, black holes may pair up and merge over and over again. That process could produce especially heavy black holes. And upgraded detectors should be able to “hear” those mergers.

Globular clusters are balls of hundreds of thousands of stars. Most of the clusters are more than 10 billion years old. Their most massive stars burned out a long time ago, leaving behind black holes. Many of them settle into the cluster’s core — perhaps thousands of them.

A team of researchers recently used a supercomputer to simulate how these black holes interact. In particular, they looked at the effects of general relativity — Albert Einstein’s theory of gravity. The researchers found that the black holes can become quite chummy.

As two black holes pass near each other, they emit bursts of gravitational waves — ripples in space and time. That causes them to lose a bit of their speed, so they settle into orbit around each other. As they orbit, they continue to emit gravitational waves, causing them to spiral together and merge.

The merged pair could be shot out of the cluster. If not, then a merged black hole can pair up with another black hole, leading to another merger — a process that can repeat multiple times.

Scientists are upgrading the leading gravitational-wave detectors. The new capabilities should allow them to “hear” the mergers of these larger black holes, in the hearts of ancient star clusters.

Script by Damond Benningfield

 

The scorpion has a bit of a heart murmur. Over a period of several years, the star that represents its heart fades and brightens. That’s probably caused by changes in the size of the star — its diameter may vary by millions of miles.

Antares is one of the brightest stars in the night sky. It’s near the middle of the curved body of Scorpius, which is in the south-southeast at nightfall. Tonight, Antares is not far to the lower right of the almost-full Moon.

The star is one of the biggest and brightest in our region of the galaxy. If you combine all wavelengths of light, it shines tens of thousands of times brighter than the Sun.

But its brightness varies. At visible wavelengths, in fact, it’s been shown to vary by a factor of more than two, over a period of more than four years. But the exact period has been hard to pin down, with estimates varying by about a year and a half.

The change in brightness is caused by a change in the size of Antares — it pulses in and out like an irregularly beating heart. Gas around the core of Antares may get so hot that it expands, puffing up the star’s outer layers, making Antares bigger. Those layers then expand and cool, so they shrink, making Antares smaller.

The pulsations also may be driven by bubbles of hot gas that rise to the surface — some of them many times bigger than the Sun. Some bubbles may surge farther than others, helping drive the “murmur” in the heart of the scorpion.

Script by Damond Benningfield

Earth is zipping past the outer planets like a race car on the inside lane of the track. We raced past Jupiter last month, and we’ll take on Mars next month. And on Wednesday, we’ll pass Saturn, the faintest of the three worlds.

Saturn is low in the southeast as night falls, and looks like a bright star. Its luster is dimmed by the gibbous Moon, far to Saturn’s upper right. Even so, Saturn outshines all but a few other objects in the night sky, so it’s hard to miss.

Earth passes by these planets because it follows a smaller, faster orbit around the Sun. We’re the third planet out from the Sun, while Mars is fourth, Jupiter is fifth, and Saturn is sixth. As Earth catches up to these worlds, they line up opposite the Sun — an alignment known as opposition. The planets are closest to Earth about then, and they shine brightest for the year.

At the same time, the planets appear to move in reverse against the background of distant stars. It’s like passing a slower-moving car on the highway. When the car is way ahead of you, it appears to move forward against the background of buildings and trees. As you pass it, though, it appears to move backward against that same background for a while.

So Saturn and Jupiter are in the “retrograde” phase of their motion across the sky now — appearing to move in reverse as Earth speeds past them. And Mars will enter retrograde in a few days.

We’ll have more about Saturn’s opposition on Wednesday.

Script by Damond Benningfield