StarDate

A perfect spiral galaxy would include a bright, round “bulge” of stars in the middle; glittering spiral arms wrapping around it; dark lanes of dust lacing through the arms; and bright star clusters sprinkled about like lights on a Christmas wreath. In other words, it would look just like Messier 81, one of the best examples of a “grand design” spiral galaxy. It’s about 12 million light-years away, and appears close to the bowl of the Big Dipper. It’s a bit smaller and less massive than our own home galaxy, the Milky Way. M81’s “bulge,” though, is much larger and brighter than the one in the center of the Milky Way. And the black hole in the galaxy’s heart is almost 20 times as massive as the Milky Way’s. The spiral arms are outlined by the galaxy’s youngest, brightest stars. Over the past 600 million years or so, a major bout of starbirth has brightened the arms. That outburst is the result of gravitational interactions between M81 and two companion galaxies. The encounters compress big clouds of gas and dust. The clouds break into clumps, which then collapse to form stars – stars that make Messier 81 one of the most beautiful galaxies of all. Under clear, dark skies, you can spot M81 with binoculars. Find the Big Dipper, which is high in the north at nightfall. M81 hangs below the bowl at that hour. It looks like an oval smudge of light that’s almost as wide as the Moon. Script by Damond Benningfield

The stars look like they’re stuck in position – like fairy lights thumbtacked to a giant black canvas overhead. And over the course of a human lifetime – or many lifetimes – that’s true – there’s no way to see any motion without the help of sensitive instruments. But that’s only because the stars are so far away. Every one of those little lights is moving – fast. They’re all orbiting the center of the Milky Way Galaxy, for example. And they’re moving either toward or away from Earth. So over millions of years, the configuration of the stars changes – constellations come and go. And the pattern of brightness changes as well – some stars fade, others grow brighter. An example is Eltanin, the brightest star of Draco, the dragon. In fact, its name means “the great serpent.” It represents one of the dragon’s glowing eyes. Today, Eltanin is a bit more than 150 light-years away. But it’s moving more or less toward us at more than 60,000 miles per hour. On the scale of the galaxy, that’s tiny – but it adds up. In about one and a half million years, it’ll be just 28 light-years away. If the star doesn’t change much over that period, it could be the brightest star in Earth’s night sky. And it could maintain that rating for hundreds of thousands of years. Look for Eltanin high in the northeast at nightfall. It’s to the upper left of Vega, one of the brighter stars in the night sky – for now. Script by Damond Benningfield

Two fairly bright lights are headed for an especially close meet-up: the planet Mars and the star Regulus, the heart of the lion. They’re a few degrees apart tonight, but they’ll draw even closer over the coming evenings. Right now, Mars and Regulus are almost the same brightness. One way to tell them apart is their color – Mars looks pale orange, while Regulus is white with a hint of blue. Binoculars accentuate the colors. Another way to tell them apart is to look for them to twinkle. Regulus does, but Mars doesn’t. That’s because Mars is a bigger target in our sky. Regulus is thousands of times the size of Mars. But it’s so far away that we see it as nothing more than a pinpoint. That tiny beam of light is bent and twisted as it passes through the atmosphere. That causes the star to “twinkle.” It twinkles more when the air is more unsettled. Mars, on the other hand, is close enough that it appears as a tiny disk, made up of many pinpoints. Each one twinkles, but they even out. So Mars appears to hold steady as it shines through even the most un-steady skies. Look for Mars and Regulus about a third of the way up the western sky at nightfall. Regulus perches to the left or upper left of Mars. They’ll pass closest to one another on Monday and Tuesday. After that, they’ll move apart. At the same time, Mars will fade. A couple of weeks from now, Regulus will clearly outshine the Red Planet. Script by Damond Benningfield

Anchorage, Alaska, isn’t quite the “land of the midnight Sun.” Tonight, there are about five hours between sunset and sunrise. But it is a land of midnight sunlight, because twilight never completely fades. Twilight is the transition between day and night. Earth’s atmosphere scatters sunlight from the dayside to the fringes of the nightside. But when, exactly, does twilight end? When is the sky really dark? As you might expect, astronomers have their own definition. Astronomical twilight begins or ends when the Sun is 18 degrees below the horizon – about twice the width of your fist held at arm’s length. That’s when the sky’s as dark as it’s going to get. Because of the Sun’s motion, astronomical twilight lasts a minimum of about an hour and 10 minutes. But because the Sun usually rises and sets at an angle, twilight can last a good deal longer. During much of June and July, when the days are longest, twilight for much of the northern hemisphere lasts all night. The Sun never drops far below the horizon, so even though it’s out of sight, its light never disappears. So the people of Anchorage need some good blackout curtains to get a dark night’s sleep. If you want a few hours of darkness, head south – someplace like Miami Beach. It gets a full seven hours between evening and morning twilight – hours that might be illuminated by the neon lights of South Beach, but not by the Sun. Script by Damond Benningfield

Cold hands, warm heart. Still waters run deep. Feed a cold, starve a fever. The list of pithy old sayings goes on and on. But here’s a new one for you: long Sun, short Moon. It means that the full Moon does the opposite of what the Sun does. So when the Sun is in the sky for a long time, the full Moon makes itself scarce. And that’s the case tonight. The full Moon of June has many names, including Flower, Strawberry, and Honey Moon. But it’s also known as the Short-Night Moon. That’s because it’s in view for less time than any other full Moon of the year. From the northern hemisphere, the Sun passes highest across the sky at this time of year, and remains in view longest. But because of the trail they follow, the Sun and the full Moon are like opposite ends of a seesaw. When one is up, the other is down. So right now, the full Moon passes low across the sky. And it rises around sunset and sets around sunrise. The difference is greater as you go farther north. From Miami, the Sun will be above the horizon for almost 14 hours today, with the Moon popping into view for less than 10 and a half hours. From Duluth, Minnesota, it’s almost 16 hours versus less than eight hours. And from Anchorage, the Sun graces the sky for 19 hours, with the Moon showing up for a stingy hour and a half – an especially short appearance for the Short-Night Moon. More about night and day tomorrow. Script by Damond Benningfield

On average, Antares is the fifteenth-brightest star in the night sky. It looks like an orange-red gem at the heart of the scorpion. Tonight, though, it looks a little feeble. It hasn’t gotten any fainter. Instead, it stands especially close to the almost-full Moon. It looks a little washed out in the powerful moonlight. Antares really is one of the more impressive stars in the galaxy. It’s probably 12 to 15 times the Sun’s mass, hundreds of times the Sun’s diameter, and tens of thousands of times its brightness. Ere long – at least on the astronomical timescale – Antares will get even more impressive – but only for a while. Sometime in the next million years or so, it’s expected to blast itself apart in a titanic explosion – a supernova. For a few months, it’ll shine brighter than the combined glow of billions of normal stars. As it fades, its demolished outer layers will form a nebula – a colorful cloud of gas and dust, energized by the blast and by the decay of radioactive elements. Over thousands of years, the nebula will expand and fade. That will leave only the star’s dead core – a tiny, super-dense corpse known as a neutron star – the almost-invisible remnant of the mighty heart of the scorpion. Antares stands close to the Moon at nightfall, and the Moon will move closer to it during the night – washing out this brilliant star. We’ll have more about the Moon tomorrow. Script by Damond Benningfield

The rings of Saturn are among the most beautiful features in the solar system. They’re wide enough to span the distance between Earth and the Moon. And they’re made of bits of ice and dust – like tiny “moonlets” orbiting the giant planet. The first person to propose that idea was Giovanni Cassini, who was born 400 years ago today. He worked in several fields, from astrology to engineering. But Cassini’s greatest love was astronomy. He became director of the Paris Observatory, and studied the Moon and planets – especially Saturn. He discovered four of its moons, plus a dark “gap” between its two most prominent rings. That gap was named the Cassini Division in his honor. Although it looks empty, the gap contains a smattering of dark particles. It probably was mostly cleared out by a small moon that orbits inside the gap. Its gravity pushes ring particles away. A study a few years ago said the gap could have started much smaller than it is today. Over the past few million years, the moon moved closer to Saturn, clearing a wider region. But now, it’s moving away from Saturn. So the Cassini Division could close up – in about 40 million years. Saturn is in the southeast at dawn, and looks like a bright star. We’re viewing the rings almost edge-on, so there’s not much to see even through large telescopes. But the view will improve over the coming months – revealing both the rings and the dark divide between them. Script by Damond Benningfield

The world has known many great astronomers, but only a few great astronomy dynasties. One of those celebrates an anniversary tomorrow – the birth of its patriarch 400 years ago. Giovanni Cassini was born in a small Italian village just across the border from Nice, France. Cassini the first, as he’s often called, was the first of four generations of Cassini astronomers. And he definitely was the most productive. He’s best known for his discovery of a gap in the rings of Saturn. It’s named the Cassini Division in his honor. Later, Cassini and his son Jacques theorized that the rings were made of “swarms of tiny satellites” moving at different speeds. They understood the truth many years before it was actually confirmed. More about Cassini and Saturn’s rings tomorrow. Cassini made many other contributions to astronomy. He was among the first to realize that light travels at a limited speed, although he didn’t believe the speed could be calculated. And he developed a law to explain why we always see the same side of the Moon: The Moon takes the same amount of time to orbit Earth as it does to complete one turn on its axis. For most of his career, Cassini was director of Paris Observatory. His descendants kept the job in the family for more than 120 years. Cassini the fourth ended the streak in 1793 – when he left the observatory to write the “stellar” history of this astronomical dynasty. Script by Laura Tuma

There was a lot of talk earlier this year about an asteroid with the highest odds of hitting Earth ever calculated. The chances of an impact by asteroid 2024 YR4 in December of 2032 peaked at about three percent. The asteroid is big enough to cause major damage if it hit. As astronomers tracked it a little longer, though, they realized that’s not going to happen – the asteroid will miss by at least 60,000 miles. Such close calls aren’t rare. Hundreds of asteroids pass within a few million miles of Earth every year. This weekend, in fact, asteroid 2014 LL26 will miss us by just two million miles. Its orbit overlaps Earth’s orbit, so it passes close fairly often. And it could hit our planet at some point in the future – though not anytime soon. Astronomers have discovered more than 27,000 potentially hazardous asteroids. And they discover more all the time. The one that caused the kerfuffle earlier this year, in fact, was just discovered in December. Several automated searches scan the sky every night. Those efforts yield thousands of asteroids every month. But it takes observations over a period of days or weeks to give us a good measurement of an asteroid’s orbit. Most of the new discoveries are in the asteroid belt, between the orbits of Mars and Jupiter. But some are close enough to keep an eye on – potential hazards to life on Earth. Script by Damond Benningfield

The Pilbara region of Western Australia is big, dry, and wide open. And it may contain the oldest cosmic “scar” on Earth: an impact crater gouged three and a half billion years ago. Scientists discovered evidence of the crater during a brief expedition in 2021. They found some rock formations called shatter cones. Some of the cones are as tall as a house. The only known way to make them is in giant collisions with space rocks. Follow-up work last year revealed many more of these formations. The cones were found in a rock layer that’s miles wide, but only a few dozen feet thick. The layer also contains tiny “beads” that formed when molten rock was blasted high into the sky. The flight through the air sculpted droplets of the molten rock into balls. Geologists found that the layer formed three and a half billion years ago, so that’s when the impact must have taken place – more than a billion years earlier than the previous record holder. The asteroid could have been miles wide, and blasted a crater more than 60 miles across. The effects of the collision would have been felt around the world. In fact, researchers say the impact could have helped shape the world. Major asteroid impacts could have traveled deep, churning things up far below the surface. That could have created the “seeds” that gave birth to the continents when Earth was young. We’ll talk about potential future impacts tomorrow. Script by Damond Benningfield