StarDate

NGC 6397 seems like a pretty ordinary globular star cluster. It’s not especially big or bright, and nothing about its stars is going to strike anyone as odd. Yet it’s been an important astronomical laboratory. Among other things, it’s helped astronomers determine the age of the galaxy, and the minimum “weight” of a star.

NGC 6397 is in the constellation Ara, the altar. It’s below the stinger of Scorpius, the scorpion. That puts it so far south that it’s not visible from most of the United States. But it’s in great view for Hubble Space Telescope, which has monitored the cluster for decades.

NGC 6397 is a globular cluster — several hundred thousand stars packed into a ball only a few dozen light-years across. The Milky Way contains about 150 of these clusters, which are the galaxy’s oldest inhabitants.

Over the decades, Hubble has carefully monitored many of the cluster’s stars. One set of observations helped astronomers lock down the minimum mass required for an object to shine as a true star — about eight percent the mass of the Sun.

And a more recent set of observations helped measure the cluster’s distance with unprecedented accuracy — 7800 light-years, one of the closest of all globulars. That number is important for determining the cluster’s age — 13.4 billion years. That means that NGC 6397 — and its home galaxy, the Milky Way — took shape when the universe was just about 400 million years old.

Script by Damond Benningfield

The gods of ancient Greece weren’t especially good parents. One of the ancient gods known as the Titans, for example, swallowed his newborn children. Eventually, though, the kiddos escaped and overthrew him. Their victory is commemorated in the stars, as the altar where they swore allegiance to each other. It’s the constellation Ara, and it’s below the tail of the scorpion.

In the story, Cronus was the king of the Titans. He’d come to power by defeating his own father. A prophecy said that one of his children would, in turn, defeat him. To prevent that, he swallowed each of his first five children.

Eventually, though, his wife, Rhea, got tired of that plan. When the last child, Zeus, was born, she hustled him away to an island and gave Cronus a rock wrapped in a blanket.

When Zeus grew up, he returned home and forced his dad to cough up his brothers and sisters, who were fully grown gods and goddesses. They pledged to defeat the Titans, and won a 10-year war. Zeus became the king of the new gods and ruler of the sky. To celebrate their victory, he placed the altar among the stars.

The altar is so far south, though, that most of it is visible only from far-southern parts of the United States — places like Florida, south Texas, and especially Hawaii.

Ara is one of the smaller constellations, with only a couple of moderately bright stars. But it does have some good deep-sky objects, and we’ll talk about one of them tomorrow.

Script by Damond Benningfield

Solar eclipses roll around like clockwork — astronomers can predict them thousands of years in advance. But the clock is like something from the Middle Ages, with lots of complicated moving parts. So it took a long time to figure out how the parts all work together.

A solar eclipse occurs when the Moon passes directly between Earth and the Sun, blocking some of the Sun’s light. If the alignment is just right, the Moon completely covers the Sun, creating a total eclipse. If the alignment is off, though, then the Moon covers only part of the Sun.

Calculating an eclipse requires detailed knowledge of Earth’s orbit around the Sun, and the Moon’s orbit around Earth. Both orbits are slightly lopsided, so the distance between Sun and Earth, and between Earth and Moon, changes from day to day. And orbital speed changes depending on the distance.

The angle of the Moon’s orbit is important, too, because it tells us where on Earth that shadow will appear. So is Earth’s rotation on its axis, because it’s not steady. Thanks to the tides — caused by the Moon — the rate at which Earth spins can vary — one of the most complicated factors in calculating a solar eclipse.

And astronomers have calculated that there will be a partial eclipse early tomorrow. It’ll be visible from the Arctic Ocean down through Greenland, Scandinavia, and parts of Asia. The U.S. misses out on this one, though — skipped over by the clockwork motion of the solar system.

Script by Damond Benningfield

The south pole isn’t a very pleasant place to live, especially during the winter. It’s dark for six months, and temperatures average about 70 below zero. Yet those conditions make it a great place for astronomy. And scientists are taking advantage of those conditions now, during the last couple of months of southern winter.

One of their tools is the South Pole Telescope. It’s a 10-meter dish that looks at certain wavelengths of radio waves. Water vapor in the atmosphere interferes with those wavelengths. The south pole is so high and cold, though, that there’s almost no water vapor above it.

Among other things, the telescope is looking at the afterglow of the Big Bang, and at how the first clusters of galaxies interacted with it. It’s also contributing to the search for dark matter.

Scientists also operate an experiment called IceCube — an array of more than 5,000 light sensors buried a mile deep. The ice above it filters out radiation that can interfere with the detectors.

In particular, IceCube is studying neutrinos. They’re everywhere — a hundred trillion of them pass through your body every second. But they almost never interact with other matter. IceCube catches a few hundred of these interactions every year — from particles that have zipped all the way through Earth. The detections reveal details about the source of the neutrinos, and about the particles themselves — leading-edge science at the south pole.

Script by Damond Benningfield

Astronomers know that black holes exist. They’ve detected swirling disks of gas around them, measured their effects on nearby stars, and even “heard” them merging with other black holes. About the only thing they haven’t done is actually see one.

That could change soon, though, as astronomers turn more and more radio telescopes toward a couple of black holes. And one of those telescopes just went to work in Greenland.

The Greenland Telescope is a 12-meter radio dish. It was built as a prototype for a project in Chile. Astronomers refurbished it, then rebuilt it in Greenland. It took its first observations this year.

One of its goals is to help take the first picture of a supermassive black hole. Although these objects are heavy, they’re so far away that they’re tiny targets. Astronomers hope to actually see one by combining the light from radio telescopes around the world. Such an array can see smaller details than any single telescope.

Eventually, the Greenland Telescope may move to a higher site. Currently, it sits near sea level. The air above it holds a lot of water vapor, which blocks some radio wavelengths. Plans call for it to be moved to a spot near the center of Greenland, at an altitude of more than 10,000 feet. At that height, there’s much less water vapor to contend with, so the telescope will be able to see the universe much more clearly — maybe even a black hole.

We’ll talk about more astronomy on ice tomorrow.

Script by Damond Benningfield

The Milky Way is splotchy. This hazy band of light has jagged borders, along with dark zones and bright zones. That gives the Milky Way an interesting texture.

The Milky Way outlines the disk of our home galaxy. It’s the combined glow of millions of individual stars. It’s so faint, though, that you need to get away from city lights to see it. Over the next few evenings, it’s visible across the eastern sky, from the teapot of Sagittarius in the south, through the celestial swan high in the east.

The Milky Way is divided by a dark rift across its length. It’s formed by dense clouds of dust. Dust grains absorb the light from the stars behind them, making it look like there are almost no stars at all.

But some stretches of the Milky Way show little dust at all. These zones form several bright “star clouds.”

There are two in Sagittarius. They show us regions toward the center of the galaxy. These regions contain huge numbers of stars and star clusters. In particular, they are windows on the next spiral arm inward from our own.

The biggest star cloud is in Cygnus. It flows along the swan’s body, from its heart to its beak. It is a window into the Cygnus arm, another wide lane of stars. So we’re looking into another zone that contains lots of stars.

A pair of binoculars will reveal many individual stars in these bright star clouds. And a telescope will reveal many more — some of the billions of stars that make up our galactic home.

Script by Damond Benningfield

The band of light that outlines the disk of the Milky Way galaxy is a rare sight. To see it, you need especially dark skies — no moonlight, and no streetlamps or other artificial light sources. And as luck would have it, there’s no moonlight to spoil the show for the next few evenings.

Yet even then, only some of the Milky Way will shine through. Much of it is blocked by a dark feature that stretches across the middle of the Milky Way.

It’s known as the Great Rift. It looks like it’s empty — a lane where there are no stars at all. But that’s deceiving. There are plenty of stars, but they’re hidden behind dark clouds of gas and dust. The dust grains block the light of the stars behind them, just as clouds here on Earth can block the light from the Sun and stars.

The clouds are thousands of light-years from Earth. They’re between us and the Sagittarius Arm — one of the spiral arms that make the Milky Way galaxy look like a pinwheel. Some of the clouds are giving birth to new stars. And some of those stars will clear away some of the gas and dust around them, making the Milky Way shine a little bit brighter.

And if you can find a nice, dark skywatching site, look for the Milky Way in the east beginning as soon as twilight fades away. The region with the Great Rift stretches from teapot-shaped Sagittarius, low in the south, over to graceful Cygnus, the swan, high in the east.

We’ll talk about some bright clouds in the Milky Way tomorrow.

Script by Damond Benningfield

When a pot of vegetable soup simmers on the stove, about all you see at the top is the broth — the vegetables all stay at the bottom of the pot. Turn it up to a vigorous boil, though, and before long you see peas and corn and tomatoes all bubbling to the top — pushed upward by a process called convection.

The same thing is happening in the star that represents the eye of the bull. Aldebaran is nearing the end of its life, so the temperature in its core has cranked way up. That’s turned up the “boiling” action at the surface. And that is revealing elements created during Aldebaran’s long life.

Aldebaran has converted the original hydrogen in its core to helium through nuclear fusion. Right now, the helium core is getting smaller and hotter. At the same time, fusion is taking place in a thin layer of hydrogen around the core.

The heat from that process has caused Aldebaran’s outer layers to puff up. And it’s brought those layers to a vigorous boil. Hot gas bubbles up all the way up from the core to the surface. That’s dredged up smatterings of other elements created by fusion — carbon, oxygen, and nitrogen.

Eventually, fusion will ignite in the helium, converting the entire core to carbon and oxygen — creating more ingredients to bubble to the surface of Aldebaran.

And the bull’s eye follows the Moon as they climb the eastern sky in the wee hours of tomorrow morning. It’s the bright orange star to the lower left of the Moon.

Script by Damond Benningfield

The star system 61 Cygni has a couple of distinctions. It moves across the sky faster than all but about a half-dozen other stars. And it was the first star to have its distance accurately measured.

The system is in Cygnus, the swan. The constellation is high in the eastern sky at nightfall, marked by Deneb, the swan’s bright tail. 61 Cygni is to the lower right of Deneb. Under dark skies, it’s just visible to the eye alone.

61 Cygni consists of two stars. Both are smaller, lighter, and cooler than the Sun, and much fainter. They’re far apart, so it takes more than 650 years for them to complete a single orbit.

More than two centuries ago, Giovanni Piazzi discovered that the system moves across the sky in a hurry. This quick motion suggested that it’s close by. So astronomers started trying to figure out how close.

They looked at the star at intervals of six months, when Earth was on opposite sides of the Sun. That slight change in perspective caused nearby stars to move back and forth a bit compared to stars that were farther away.

Early attempts to measure that angle didn’t work — the equipment of the day just wasn’t good enough. But in 1837 and ’38, Friedrich Bessel used a new instrument that provided a sharper view. It told him that 61 Cygni was 10.3 light-years away. That’s just one light-year off the true distance. So Bessel’s work provided the first good measurement of the distance to any star other than the Sun.

Script by Damond Benningfield

The electric-vehicle business is facing a bit of bother. A key ingredient in the lithium ion batteries that power electric cars is in short supply. Prices doubled in 2017, and demand is predicted to jump by quite a bit this year.

Cobalt has found many applications over the centuries. It’s been used as a dye for ceramics, and in alloys for jet engines. It’s also a main ingredient in vitamin B12.

Like all but a few of the chemical elements, cobalt was forged in stars. It was made in the hearts of heavy stars, and in powerful stellar explosions.

In the hearts of stars, elements are bombarded with neutrons — the neutral particles in the nuclei of atoms. That builds heavier elements, including a radioactive form of iron. As the iron undergoes radioactive decay, it turns into the stable form of cobalt.

The element also is created when stars explode as supernovae. Such a blast releases enormous amounts of neutrons, which stick to atoms to make heavier elements.

Supernova blasts also make a radioactive form of cobalt. The initial blast produces enormous amounts of a radioactive form of nickel. In a few days, the nickel decays to cobalt. A supernova known as 1987A, for example, produced enough cobalt to make 20,000 planets as heavy as Earth.

But the radioactive cobalt doesn’t last long, either. It decays to a stable form of iron. That process heats the gas expelled by the dead star, keeping it shining brightly for several months.

Script by Damond Benningfield