You’ve probably never seen two donkeys perched on the back of a crab. Unless, that is, you’ve spent much time looking at the stars. Two faint stars in Cancer have names that mean the northern and southern donkeys — Asellus Borealis and Asellus Australis. They’re on opposite sides of the crab’s back. They flank the constellation’s most famous member — a star cluster known as both the manger and the beehive.

The star names come from Greek mythology. The donkeys carried gods who were involved in a war between two sets of gods — those of Olympus, and their predecessors, the Titans.

The northern donkey is about 180 light-years away. It’s a little more than twice the size and mass of the Sun. But it’s billions of years younger than the Sun.

The southern donkey is a little closer — about 130 light-years. It appears to consist of more than one star. The main star is bigger and heavier than the Sun, and about half the Sun’s age. It’s nearing the end of its life, so it’s puffed up to become a giant — a stage in life that awaits the Sun in several billion years.

Cancer is in the east at nightfall. Pretty much everything in the constellation is faint, so you need a dark sky to see much. A moonless night like tonight is a good time to look. The Beehive star cluster looks like a hazy patch of light. The donkeys flank it — the northern one to the left, and the southern one to the right.

Script by Damond Benningfield

The Donkeys

StarDate, the longest-running national radio science feature in the U.S., tells listeners what to look for in the night sky.

Natural Sciences

Science & Medicine

Education

Training

Language Courses

Higher Education

K-12

Educational Technology

Science

Astronomy

Millions of black holes inhabit our home galaxy, the Milky Way. Most of them probably roam the galaxy alone, so we never see them. Luckily for science, though, many of them have companion stars. That makes it possible for astronomers to “see” and learn about the black holes.



An example is a system called Swift J1357. It was discovered in 2011 by Swift, a space telescope that studies the X-ray sky. The system is thousands of light-years away, and appears to be outside the galaxy’s disk of stars.



J1357 consists of two objects: a black hole and a small, faint companion star. The black hole’s gravity pulls in hot gas from the companion. The gas spirals around the black hole, heating up and forming a faint disk.



The black hole appears to be at least nine times the mass of the Sun. It and the companion orbit each other once every two and a half hours — one of the tighter orbits of any known black-hole binary system.



J1357 produces outbursts of X-rays every few years. They may occur when too much gas piles up in the disk. It gets so hot that it causes an eruption of particles and energy — an outburst that reveals more about this intriguing system.



Swift J1357 is much too faint to see with the eye alone, but we can spot its location. It’s close to the left of Spica, the brightest star of Virgo. It’s about a third of the way up the southeastern sky at nightfall.



Script by Damond Benningfield

Black-Hole Binary

Jupiter is missing. The solar system’s biggest and heaviest planet is nowhere to be seen. And it won’t return to view for a few weeks.



Jupiter is passing behind the Sun as seen from Earth, so it’s hidden in the Sun’s brilliant glare. It’ll be closest to the Sun on Saturday afternoon. That point is known as conjunction, and it happens every 12 and a half months.



Jupiter is known as a “superior” planet. That means its orbit is outside Earth’s orbit around the Sun. So Jupiter sometimes lines up directly opposite the Sun in our sky, shining brightly all night long. About half a year later, though, it passes behind the Sun and out of sight.



The planet doesn’t necessarily pass directly behind the Sun. This time, for example, it slides about half a degree from the Sun — less than the width of your finger held at arm’s length. Even so, it’s so close to the Sun that astronomers can’t point their telescopes toward it. And radio “static” from the Sun means that flight controllers lose contact with spacecraft at Jupiter for a while. The craft operate on their own, and save their observations to beam to Earth when the Sun is out of the way.



For casual skywatchers, Jupiter should return to view in a month or so. It’ll be quite low in the eastern sky during the dawn twilight. It’ll be in better view as we move into summer — climbing farther from the Sun, and shining like a brilliant beacon in the early morning sky.



Script by Damond Benningfield

Missing Giant

The Moon and the heart of the lion stand close together this evening. The lion’s heart is the star Regulus, to the lower right of the Moon at nightfall.



We see Regulus and the Moon for different reasons. Regulus is a star — a brilliant ball of hot gas. It generates its own light — so much light that it’s visible across almost 80 light-years. That means we see Regulus as it looked almost 80 years ago. So if anything big were to happen to the star tonight, Earth wouldn’t know it until early in the 22nd century.



The Moon doesn’t produce any light of its own. Instead, it reflects sunlight. The Moon looks big and bright only because it’s far closer to us than any other celestial object — an average of about a quarter of a million miles. That’s a bit more than one light-second, so we see the Moon as it looked a bit more than one second earlier.



Tonight, the Sun lights up a little more than half of the lunar hemisphere that faces our way. Many people think of that phase as a “half” Moon. And based on its appearance alone, that’s perfectly correct.



But a “half” Moon is also known as a quarter Moon. And at the current angle, it’s the first quarter Moon. That may sound confusing, but there’s a simple explanation: At first quarter, the Moon is one-fourth of the way through its month-long cycle of phases. It’s half of the way through the cycle when it’s full. And the cycle starts over at new Moon — when it’s not visible at all.



Script by Damond Benningfield

Moon and Regulus

A binary star system about 30 million light-years away is an impostor. When astronomers first saw it, they thought it was a supernova — the titanic explosion of a massive star. They even called it one: Supernova 2000ch. But the system is still there.



The system is in the spiral galaxy NGC 3432, in a clump of young, bright, heavy stars.



By the end of last year, 2000ch had produced 23 outbursts. They’ve been about six and a half months apart. Based on that interval, the way the system brightens and fades, and other details, astronomers have developed a model of what’s going on.



The system’s main star is dozens of times the mass of the Sun, and a million times brighter. It’s also unstable — it puffs in and out.



A smaller companion star follows a lopsided orbit. During its close approaches, it stirs up the brighter star, triggering an outburst. And if the main star is in its “puffier” phase, then the outburst is especially bright.



The system may be building up to a much bigger outburst in the fairly near future. And after that, the massive star could explode as a supernova — no longer an “impostor,” but the real thing.



NGC 3432 is in Leo Minor, the little lion. It’s too faint to see without a telescope. But it’s high above the Moon as darkness falls tonight. The bright star Regulus — the heart of the big lion — is to the left of the Moon. More about that meeting tomorrow.



Script by Damond Benningfield

Impostor!

Beta Pictoris is one of the more famous of all star systems. It’s also one of the most complex. It includes a young star, at least two giant planets, lots of gas and dust, and a “cat’s tail” that might be the result of a giant impact.



The star is almost 65 light-years away. It’s about twice as big and heavy as the Sun, and almost 10 times as bright.



In 1983, an infrared space telescope discovered a disk of gas and dust around Beta Pic. It spans hundreds of billions of miles, and contains several wide belts. The belts may consist of debris left over from the collisions of comets and asteroids.



Gaps between the belts contain two known planets. Both of them are more than 10 times the mass of Jupiter, the giant of our own solar system. One of them orbits the star once every three and a third years; the other, once every 24 years.



A smaller second disk also encircles Beta Pic. Last year, Webb Space Telescope discovered a “cat’s tail” of debris curling away from it. It might have formed about a century ago, when a collision between giant chunks of rock and ice shattered the bodies, splashing debris up and away from the disk.



Beta Pictoris is still an infant — it’s only about 25 million years old. So the building blocks around it could still be coming together to make more planets, while others could be destroyed in more big collisions — adding to the complexity of this well-known star system. 



Script by Damond Benningfield

The Donkeys

Download our free app to listen on your phone