StarDate

A planet known as Theia may be the most important world you’ve never heard of. No one has ever seen it — and no one ever will. Yet we see its possible handiwork almost every day: the Moon. Current thinking says Theia slammed into Earth when our planet was still an infant. The impact blasted out material that soon came together to form the Moon.

Most scientists agree on that basic outline. But they’ve been debating the details for decades: the size of Theia, the fate of Theia itself, the timing of the Moon’s formation, and much more.

The original scenario said Theia probably was about the size of Mars — roughly half the diameter of Earth. A recent study, though, has suggested it could have been almost as big as Earth itself.

Models of what happened to Theia have been all over the place. Some recent ones suggest that its core joined with Earth’s, making Earth’s final core unusually big. In addition, parts of the mantle could have sunk into Earth’s mantle, forming big blobs of dense rock.

The Moon probably took shape fairly quickly — in as little as a few years. And one recent model said it could have happened in a matter of hours.

No matter how long it took, you can see the result tonight: the Moon. It has a bright companion: Spica, the brightest star of Virgo. It’s quite close to the lower right of the Moon — the possible creation of the most important planet you’ve never heard of.
 

Script by Damond Benningfield

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One of the closest of the Milky Way’s globular star clusters is near the middle of Ophiuchus, the serpent bearer. The constellation is in the southeast at nightfall, and wheels across the south later on.

Messier 10 is a family of hundreds of thousands of stars. But it’s only about 80 light-years across. That means the stars in the cluster are jammed in far tighter than those in our region of the galaxy. So from any star in the cluster — especially those in its busy heart — hundreds or thousands of bright stars would decorate the night sky.

Globular clusters are among the oldest residents of the galaxy; Messier 10 is thought to be more than 11 billion years old. By comparison, the Sun is only four and a half billion years old.

That means the stars in the cluster formed not long after the Milky Way Galaxy itself. All of the big, bright stars in Messier 10 have long since expired, so most of its remaining stars are small and faint. There are a few bright stars, but they’re probably the results of mergers that “rejuvenated” smaller stars.

Messier 10 is only about 15,000 light-years away — closer than all but a few of the Milky Way’s 150 or so known globulars. Even so, it’s not bright enough to see with the eye alone. It is visible through binoculars as a hazy smudge of light. But you need a telescope to see any of its individual stars — members of a big but ancient stellar family.
 

Script by Damond Benningfield

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The Sun is pretty big as stars go — bigger than perhaps 80 or 90 percent of the stars in the entire Milky Way Galaxy. To get an idea of just how big, consider this: At the speed of a jet airliner, it would take about seven months to circle all the way around the Sun’s equator.

But if you really want to rack up the frequent-flier miles, try circling around the star at the tail of Cygnus, the swan.

Deneb is a white supergiant. The “white” designation means that its surface is thousands of degrees hotter than the Sun’s. And the “supergiant” designation means that it’s one of the biggest stars in the galaxy. In fact, it could be 200 times the diameter of the Sun. So even at the pace of a typical airliner, it would take more than a century to circle Deneb’s equator.

One problem you might have is deciding just where the star’s “surface” is. Supergiants are so puffed up that their outermost layers of gas are quite thin. And Deneb is blowing a powerful “wind” of material out into space, which makes it even harder to tell where the star ends and space begins.

But from our distance of 2600 light-years or so, that’s not a problem — Deneb looks like a sharp little point of light. It’s in the northeast at nightfall, at the left point of the bright Summer Triangle. And the rest of the swan stretches to its right, with the body roughly parallel to the horizon and the wings spread above and below.

Tomorrow: An ancient stellar family.
 

Script by Damond Benningfield

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One of the most graceful and beautiful of all constellations soars high overhead on summer nights. Cygnus, the swan, lines up in the east and northeast in early evening. The swan’s body is roughly parallel to the horizon, with its wings extended above and below. The bright star Deneb marks its tail, with the double star Albireo representing its beak.

Cygnus is also known as the Northern Cross. In fact, the pattern is so bright and obvious that quite a few cultures have shown it as a cross.

For the Maya of Central America, the cross may have played a role in the succession of kings.

Evidence of this role comes from a temple in the Mayan city of Palenque. Murals in the temple depict starry crosses, which some scientists have interpreted as Cygnus. The temple is aligned toward the northwest, to the point on the horizon where Deneb set.

Some of the artwork depicts the life of King Chan Bahlum. He was appointed heir to the throne in June of the year 641. From Palenque, Deneb was rising around sunset at that time of year. And Chan Bahlum became king in January of 684 — a time when Deneb was just becoming visible in the morning sky, and dropping from view at dusk — in perfect view from the temple.

Look for Deneb and the rest of Cygnus throughout the night. The swan is in the east and northeast as darkness falls, with its body roughly parallel to the horizon. It flies high overhead during the night.

More about Deneb tomorrow.
 

Script by Damond Benningfield

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We don’t know about thanking your lucky stars, but perhaps you should thank our lucky planet: Jupiter, the king of the planets. Without it, we might not be here to thank anything at all.

Jupiter is the largest and heaviest planet in the solar system — more massive than all the other planets and moons put together. And it’s that great mass that was lucky for Earth.

The infant solar system was filled with billions of leftover planetary building blocks — mountain-sized balls of ice and rock. But Jupiter’s gravity kicked out huge numbers of them. That’s important because the remaining blocks can ram into our planet even now, causing global death and destruction. The problem would be far worse if Jupiter hadn’t expelled most of them.

And a recent study says the giant planet did us one more favor: It blocked the formation of another planet between the orbits of Mars and Jupiter. It kicked out many possible building blocks, and kept the rest of them stirred up, so they couldn’t stick together.

If a planet had formed there, it might have influenced Jupiter’s orbit, making the inner solar system unstable — possibly sending Earth careening after all those balls of ice and rock into interstellar space.

Jupiter is staging a grand appearance in the early morning sky. It’s in the east at dawn, and looks like a brilliant star. It’s not, though — it’s a giant planet that’s had a giant influence on our own little world.
 

Script by Damond Benningfield

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Some people shine so brightly — they’re so smart, talented, effervescent, or attractive — that people overlook their siblings. The brothers and sisters might be standouts on their own, but no one seems to notice.

That happens with stars, too. Take Regulus, the brightest star of Leo, which is quite close to the Moon tonight. It’s actually a system of four stars. But one of them — Regulus Aa — gets almost all of the attention. It’s much bigger, brighter, and heavier than the Sun. And it’s the only member of the system we can see without a telescope.

One of the siblings is so faint, and so close to the brilliant star, that it was discovered just a couple of decades ago. Astronomers have devoted a lot of attention to it since then, even though they can’t actually see it — they have to use special instruments to scoop up its details.

The other two stars — Regulus B and C — have been known for much longer. But they’re smaller and less massive than the Sun, so they’re much fainter. So astronomers have paid them scant attention — few studies have ever been published on either star, and almost none in recent decades.

We do know that B and C orbit each other once every 600 years or so. And the pair is separated from Regulus Aa by at least 5,000 times the distance from Earth to the Sun. At that range, it takes several million years for the unheralded stars to orbit their attention-grabbing sibling.
 

Script by Damond Benningfield

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There’s a lot to talk about on this first day of summer. For one thing ... well, it’s the first day of summer! And for another, the worlds that pass closest to Earth form a tight triangle in the evening sky.

Summer arrives at 9:58 a.m. Central Time — the moment of the June solstice. For the northern hemisphere, the Sun stands highest in the sky for the entire year. And the solstice is the longest day of the year, so there’s more time to enjoy our high-climbing star.

After the solstice, the days start to get shorter, but not by much. It takes a while for the difference to add up to enough for us to notice.

At the same time, the Sun’s high point begins to creep southward — and so do its rising and setting points. Again, it’s a slow process at first, so it takes a few days or even weeks to notice the difference. In fact, the word “solstice” means “Sun stands still” — a reference to the barely noticeable change in the Sun’s position from day to day.

It’s a lot easier to see changes in the position of the Moon. Tonight, it’s quite close to Venus, the “evening star.” Fainter Mars is close to the upper left of the brilliant duo.

Venus and Mars pass closer to us than any other planets. Of all the major bodies in the solar system, only the Moon is closer.

By tomorrow night, the Moon will have moved away from Venus and Mars, and will be close to Regulus, the heart of the lion. More about that tomorrow.
 

Script by Damond Benningfield

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The Moon maneuvers through a crowded region of the sky the next few nights, past a couple of planets and some bright stars.

The Moon is in its waxing crescent phase. The “crescent” part of that description is obvious. The “waxing” part means the crescent is getting fatter by the night. Tonight, the Sun lights up only about eight percent of the Moon’s Earth-facing hemisphere. By tomorrow night, it’ll be about 13 percent, and by Thursday night, about 21 percent.

If we could see the Moon’s backside — the hemisphere that’s always hidden — the lighting would be just the opposite. It would be about 92 percent “full” tonight, but less full night by night.

Those changes are a result of the Moon’s orbit around Earth. The Moon passed between Earth and the Sun on Saturday night. Now, it’s pulling away from the Sun, so more and more of the Earth-facing hemisphere faces the Sun. That means more daylight — and a fatter crescent. The Moon will reach first quarter on Monday, moving into its gibbous phase — and headed toward full on July 3rd.

Watch the crescent Moon and its companions over the next few nights. Tonight, brilliant Venus, the “evening star,” stands to its upper left. Fainter orange Mars is farther along the same line, with Regulus, the heart of the lion, farther still. The twins of Gemini are close to the lower right of the Moon tonight — ready to disappear in the twilight.
 

Script by Damond Benningfield

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A spacecraft that’s scheduled to enter orbit around Mercury in late 2025 will tap the brakes tomorrow. It’ll fly past Mercury, using the planet’s gravity to slow it down a bit. It’ll take a peek at Mercury as it speeds by.

BepiColombo is a European mission. It consists of two separate spacecraft that are linked up for the ride. Once they arrive at Mercury to stay, they’ll go their own ways, following separate orbits around the planet. One of them will probe Mercury’s interior and its surface, while the other monitors the planet’s magnetic field.

Entering orbit around Mercury is tough. Mercury is the closest planet to the Sun, so a craft can use the Sun’s gravity to pull it inward. The problem is, that gravitational pull is too strong, so falling toward the Sun is like rolling down a steep hill. A craft really needs to ride the brakes to slow down enough to enter orbit.

There are a couple of ways to do that. One is to carry a big honkin’ rocket engine. But that’s expensive. The other is to use the gravity of the planets. If you target a craft just right, the planet exerts a slight “drag” on it, slowing it down.

BepiColombo flew past Earth once and Venus twice. And tomorrow, it’ll make its third pass by Mercury. Even with all of that, it’ll need three more encounters to get it just right — to slow down enough to enter orbit and begin exploring the Sun’s closest and smallest planet.
 

Script by Damond Benningfield

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All of the stars in the night sky are orbiting the center of the Milky Way Galaxy at a speedy clip. But most of the stars are so far away that even at their high speeds, we don’t see them change position — they don’t appear to move relative to the other stars.

One exception is Barnard’s Star. During the span of a human lifetime, it moves by about a quarter of a degree — half the size of the Moon in our sky. That makes it the speed demon of the night sky — it moves faster than any other star. That’s because Barnard’s Star is just six light-years away. Only the stars of Alpha Centauri are closer.

Barnard’s Star is named for Edward Emerson Barnard, who measured its blazing speed a century ago.

The star’s proximity and speed are its main talking points. It’s much smaller and lighter than the Sun, and much, much fainter — just a few hundredths of a percent of the Sun’s brilliance. That’s why it wasn’t discovered until 1888. And even then, it was recorded on a photograph that wasn’t analyzed for years.

Astronomers have been hunting for planets around the star for decades. And some have reported finding them. But none of the claims has stood up, so Barnard’s Star remains planetless.

Barnard’s Star is in Ophiuchus, the serpent bearer, which is in the east and southeast at nightfall. Barnard’s Star is well below the serpent bearer’s brightest star, Rasalhague. But it’s much too faint to see without a big telescope.
 

Script by Damond Benningfield

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