StarDate

If you ever want to chat with someone on another planet, you better have a lot of patience. It takes a long time for a message from Earth to reach another world, and just as long for the reply to reach Earth.

That’s because radio waves travel at the speed of light. And although light is pretty swift, its speed is limited – 670 million miles per hour. Given the scale of the solar system, that means there’s a long pause between halves of a conversation.

Consider the planets that flank the Moon at dawn tomorrow: brilliant Venus to the lower left of the Moon, and fainter Saturn to the upper right.

Right now, Venus is more than 58 million miles away. At that distance, it would take a radio signal from Earth about five and a quarter minutes to get there – a round-trip time of 10 and a half minutes.

Saturn is about 930 million miles away. So the round-trip travel time is about two hours and 45 minutes.

That’s a big concern for the folks who send probes to these and other planets. Despite what you might see in sci-fi movies and TV shows, there’s no way to have a real-time conversation. So spacecraft are programmed to do much of their work without direct help from Earth. And if they encounter a problem, they shut down most of their systems and place a call for help – then settle in for the long wait to hear from home.

More about the Moon and Venus tomorrow.

Script by Damond Benningfield

A 900-mile-wide, two-toned walnut orbits the planet Saturn. It’s Saturn’s third-largest moon, and definitely the most eye-catching. One hemisphere is as dark as coal, while the other is as bright as sea ice. And a mountain ridge wraps around the equator, making it look like a walnut.

Iapetus was discovered in 1671. And right away, astronomers realized there was something odd about it. It was easy to see when it was on one side of Saturn, but invisible on the other. Today, we know why that’s the case: the planet’s leading hemisphere is 10 to 20 times brighter than the trailing hemisphere.

The leading idea says that long ago, the darker side was pelted by dust and rocks blasted off some smaller moons. The darker material trapped the Sun’s heat, vaporizing ices. The vapor drifted to the other side, where it froze, making that side bright. And that process continues today – making Iapetus the “yin and yang” of moons.

The ridge around the equator is about six miles high. It might have formed long ago when Iapetus rotated much faster than it does today. Or it might be the remains of a ring that collapsed onto the surface – making Iapetus look like a walnut.

Saturn appears quite close to our own Moon at dawn tomorrow. It looks like a bright star to the lower left of the Moon. The much-brighter planet Venus is farther to the lower left. More about this morning lineup tomorrow.

Script by Damond Benningfield

For the most part, black holes come in two varieties: small and jumbo. The small ones are the remnants of dead stars. They range from a few to about a hundred times the mass of the Sun. The jumbos inhabit the hearts of galaxies. They range from about a hundred thousand to several billion times the Sun’s mass. But there just isn’t much in the middle.

In fact, astronomers have logged only a few dozen medium-sized black holes. And many of those are controversial – they’re hard to confirm. Most of them are in other galaxies, so it’s hard to see their influence on the stars and gas around them.

One recent study may have added to the tally. Using observations designed to study dark energy, scientists said they discovered about 300 medium-sized black holes. About 70 of them inhabit the centers of small galaxies. Those black holes are gobbling gas and dust around them, making them brighter and easier to find.

Theory says there should be many more black holes in size “medium.” They may be the remnants of the ultra-massive stars that populated the early universe. Or they may have formed when dense clumps of gas collapsed under their own gravity. Either way, it’s possible that such black holes were the “seeds” from which the jumbo black holes grew.

For now, the search continues for these hard-to-find medium-sized black holes.

Script by Damond Benningfield

The closest galaxy we can see other than our own Milky Way may be inside the Milky Way. Its outer precincts have been stripped away, leaving only its core – a tight ball of 10 million stars. And a rare type of black hole appears to lurk in its middle.

Omega Centauri rolls low across the south during the night. The view is better from the southern half of the country. To the eye alone, it looks like a fuzzy star.

Omega Centauri is classified as a globular cluster – a family of very old stars. It’s the biggest one in the galaxy. But it probably wasn’t born in the Milky Way. Instead, it began as a separate galaxy. But it was reeled in by the Milky Way’s gravity, which also pulled away most of its stars. Only the stars in the galaxy’s core stuck together.

An intermediate-mass black hole appears to inhabit the center of the cluster. Such beasties are rare. Most black holes are either no more than about a hundred times the mass of the Sun, or a few hundred thousand times the Sun’s mass or more.

A study a couple of decades ago reported a possible black hole in Omega Centauri weighing 40,000 times the Sun’s mass. Later work suggested that number was too high. The most recent estimate was compiled from 20 years of observations by Hubble Space Telescope. It puts the black hole at about 8,000 times the Sun’s mass – a rare black hole in the remnant of a dead galaxy.

More about mid-sized black holes tomorrow.

Script by Damond Benningfield

Mighty Hercules stands well up in the east and northeast as night falls. His most prominent feature is the Keystone, a lopsided square of stars that represents his body. But the constellation’s brightest star isn’t part of the Keystone. Instead, it represents the entire strongman: Its name, Kornephoros, comes from a Greek word that means “the club bearer” – Hercules himself.

Like many of the stars in the galaxy, there’s more to Kornephoros than meets the eye: It consists of two stars, not one. One star is smaller and fainter than the Sun, so it’s not visible to the eye alone.

The visible star, on the other hand, is about three times the mass of the Sun, almost 20 times the Sun’s diameter, and 150 times its brightness. So the star is an easy target even though it’s about 140 light-years away.

The star is nearing the end of its life. It’s probably consumed the hydrogen fuel in its core, converting it to helium. That’s caused the core to get smaller and hotter. The extra radiation pushes on its outer layers, causing them to puff up to giant proportions.

Today, Kornephoros is fusing the helium to make carbon and oxygen. Eventually, that process will end. The star will lose its outer layers, leaving only its dead core – and the “club bearer” will vanish from sight.

For now, though, look for it due east at nightfall, halfway up the sky – the first modestly bright star to the right of the Keystone.

Script by Damond Benningfield

A pretty semicircle of stars crowns the sky on spring and summer nights: Corona Borealis, the northern crown. It’s in the east as night falls now, and stands high overhead a few hours later. In a couple of months, it’ll be overhead at nightfall.

Most of the semicircle isn’t very bright – you need pretty dark skies to see it. It stands out because of the tight pattern, with a fairly bright star at the center: Alphecca, “the bright one.”

Alphecca is really a binary – two stars locked in a gravitational embrace. The heavier of them is about three times as massive as the Sun, thousands of degrees hotter, and dozens of times brighter. Its companion is a little smaller and fainter than the Sun.

The stars are quite close together – an average of about half the distance between the Sun and its closest planet, Mercury. The stars orbit each other once every 17 and a half days.

And they’re lined up in such a way that we see the fainter star pass in front of the brighter one – an eclipse. When that happens, Alphecca dims by a few percent. That’s not enough for most of us to notice with the eye alone, but it’s an easy catch for astronomical instruments.

Instruments also see a disk of debris around the stars. It extends billions of miles into space. It consists mainly of small grains of dust – material left over from the formation of Alphecca itself.

We’ll talk about a pair of stars in Hercules tomorrow.

Script by Damond Benningfield

Hubble Space Telescope had many “parents” – people who conceived it, lobbied for it, designed it, and mapped out its science mission. But none was more important than Nancy Grace Roman. She served as NASA’s first chief astronomer, and later as director of one of its field centers. She pushed, prodded, and cajoled for the telescope for decades. And once it was approved, she helped get it running.

Roman was born 100 years ago today, in Nashville. In sixth grade, she founded her school’s first astronomy club. A year later, she decided to become an astronomer. Despite discouragement from teachers, she stuck with it. She earned her Ph.D., from the University of Chicago, in 1949. Over the next few years she studied the stars, using telescopes at McDonald Observatory and elsewhere.

Academia didn’t offer much opportunity for women at the time, so Roman went into government work. And soon after NASA was established, she was hired as chief astronomer. Among other things, she led the development of the first space telescopes – one series to watch the Sun, another to study the stars.

Roman died on Christmas Day in 2018. But her legacy is far from over. NASA’s next big space telescope will hunt for planets in other star systems, probe the nature of dark energy, plot the evolution of the universe, and more. The telescope is scheduled for launch in two years: the Nancy Grace Roman Space Telescope.

Script by Damond Benningfield

For the most part, the star cluster NGC 2281 has escaped the attention of astronomers. It hasn’t been studied in a lot of depth over the years. So many of its details haven’t really been locked down.

So far, astronomers have cataloged more than 200 stars in the cluster. And they’ve ruled out many more stars that happen to line up in the same direction. That makes NGC 2281 a fairly puny cluster.

The cluster’s distance is a bit uncertain as well. Measurements have been getting better in recent years, thanks in part to the Gaia space telescope. It’s obtained precise details on several of the stars in the cluster, including their distance. Those observations put NGC 2281 at more than 1700 light-years.

And its age is still debated, too. Estimates in recent years have ranged from about 275 million to 630 million years. Various studies have used different techniques to plot the age. That includes the types of stars found in the cluster, the number of dead stars, and even how fast the Sun-like stars in the cluster spin; stars slow down as they age.

NGC 2281 is in Auriga the charioteer, in the west-northwest as night falls. The “twins” of Gemini stand to its upper left, with the brilliant star Capella farther to its lower right. Under clear, dark skies, it’s visible to the unaided eye as a hazy patch of light – a star cluster that we’re still getting to know.

Script by Damond Benningfield

The “halo” that surrounds the Milky Way Galaxy is dark but heavy. It’s much more massive than the galaxy’s bright disk, but we don’t see much there. So the halo must be filled with dark matter. It produces no detectable energy, but it reveals its presence through its gravitational pull on the matter we can see.

The leading idea says dark matter consists of some type of exotic particle. But efforts to find such particles have come up empty.

Astronomers have also looked to see if the dark matter might consist of MACHOs – massive compact halo objects. The list of candidates includes faint stars, dead stars, black holes, free-floating planets, and brown dwarfs.

Such objects are extremely faint. But they can sometimes brighten – not directly, but by magnifying the light of stars behind them.

The technique is known as gravitational lensing. When one massive body passes in front of another, it causes the background object to get much brighter. The flare-up can last from hours to months. How long it lasts, and how much the background star brightens, reveals details about the lensing object. And that reveals the type of object.

Searches for gravitational lenses have found many planets, faint stars, and even the first “rogue” black hole – one that couldn’t be seen any other way. But there just aren’t enough MACHOs to account for more than a small fraction of the dark matter – leaving us in the dark about its nature.

Script by Damond Benningfield

The human eye is amazing. It can focus on objects near and far, provide a three-dimensional look at the world, and see under both brilliant sunlight and the faint glow of the stars. It also sees all the colors of the rainbow – from red and orange to blue and violet.

Yet there’s a lot more that the eye can’t see – wavelengths that are beyond its range. That means we’re missing much of what the universe is showing us.

Consider Antares, the heart of the scorpion. The star is just a whisker away from the Moon as they climb into view this evening.

Antares is one of the brightest pinpoints in the night sky. And it shines with a distinctly orange hue. But there’s a lot more to it than that.

For one thing, Antares consists of two stars, not one. The one we see is a supergiant – many times the Sun’s mass, and hundreds of times its diameter. At visible wavelengths, it shines about 10,000 times brighter than the Sun, with a distinctly orange color.

But the star is much cooler than the Sun. Such stars produce most of their energy in the infrared – wavelengths too long for the human eye. So when you add that in, Antares is about a hundred thousand times the Sun’s brightness.

The other star of Antares is much hotter than the Sun. So most of its light is in the ultraviolet – wavelengths that are too short for the eye. So it is about three thousand times the Sun’s brightness – much more than the eye can see.

Script by Damond Benningfield