StarDate

Five years ago, astronomers at McDonald Observatory searched for dark matter in a small companion galaxy of the Milky Way. They found darkness all right, but not much dark matter. Instead, they found a black hole that’s almost as big as the one at the heart of the Milky Way.

Leo I is more than 800,000 light-years away. That makes it one of the Milky Way’s most-distant satellite galaxies. And it’s tiny: just one or two percent the width of the Milky Way, and about one fifty-thousandth of the Milky Way’s mass.

Such galaxies are common, though, and many of them appear to contain a lot of dark matter — matter that we can’t see, but that exerts a gravitational pull on the visible matter around it. So astronomers from Texas and Germany examined Leo I to plot its dark matter.

They used a telescope at McDonald Observatory to measure the motions of stars. The way the stars move reveals how much matter is pulling the stars, and how the matter is distributed.

The observations revealed a compact but dark source at the galaxy’s heart — a black hole. It’s 3.3 million times the mass of the Sun — three-quarters the mass of the black hole in the Milky Way. It hints that even bigger black holes grow from the mergers of galaxies like Leo I — more darkness from small galaxies.

Leo I is just a whisker away from Regulus, the brightest star of Leo, the lion. Regulus is due east as night falls, and springs high across the south during the night.
 

Script by Damond Benningfield

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The brightest and closest exploding star in centuries was first seen 35 years ago tonight. Supernova 1987a was bright enough to see with the unaided eye. To understand how bright that really was, consider that the supernova was in another galaxy — more than 160,000 light-years away. It’s faded from view since then, but astronomers still keep a close eye on it through telescopes on the ground and in space.

Several stars in our own galaxy, the Milky Way, are likely to explode as supernovas fairly soon. On the astronomical time scale, that means any time within the next hundred thousand years or so. And when they go, they’ll be bright enough to see in the daytime for weeks or months.

One of the best candidates is Antares, the leading light of Scorpius. The brilliant star is quite close to the Moon at first light tomorrow.

Antares is actually a binary — two giant stars locked in a mutual orbit. The brighter star is roughly a dozen times the mass of the Sun. At that great heft, it’s destined to explode as a supernova. Its core will collapse to form a neutron star. Its outer layers will fall inward, then rebound, blasting out into space.

The star is roughly 15 million years old. That’s quite young compared to most stars. But for a star of its mass, Antares is quite late in life. So the nuclear reactions that power the star are quickly building to a climax — setting up Antares for a brilliant and powerful demise.
 

Script by Damond Benningfield

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Supernova 1987a is a gift that keeps on giving. The exploding star was discovered 35 years ago this week. In the decades since, it’s revealed a lot about many topics: the stars that become supernovas, the processes that drive the explosion, the mechanism that keeps them bright for a long time, and more.

Supernova 1987a was in a galaxy that’s more than 160,000 light-years away. Even so, the explosion was bright enough to see with the unaided eye.

Pictures taken before the blast revealed that the exploding star was a blue supergiant. Until then, astronomers thought such stars would have to expand and cool to become red supergiants before they could explode.

Shortly before the supernova became visible, detectors recorded a small outburst of neutrinos. These ghostly particles rarely interact with other matter. But theory says a supernova should produce huge numbers of them, providing much of the power for the explosion. The neutrino discovery confirmed that idea.

Supernova 1987a remained quite bright for many years, mainly through the decay of radioactive elements forged in the blast — another confirmation that models were on the right track.

The models also said that a supernova should leave behind a neutron star — the star’s crushed core. It took time to find it. But just last year, astronomers confirmed its discovery — one more “gift” from an exploding star.
 

Script by Damond Benningfield

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A half a century ago, the Soviet Union got a “do-over” in its lunar exploration program. A spacecraft designed to gather samples from the rugged lunar highlands had crashed. But 50 years ago today, its replacement touched down on target.

The do-over mission was Luna 20. It launched on February 13th, 1972. It spent a couple of days in lunar orbit, then landed on February 21st.

Its target was the Apollonius highlands, a mountainous region near the edge of the Sea of Fertility. Luna 18 had been sent to the same site just a few months earlier, but it slammed into the surface and was destroyed.

Luna 20, on the other hand, got it just right. After landing, a robotic arm drilled into the surface. It scooped up 55 grams of dirt — about two ounces — and placed the sample in a small capsule. After just one day on the Moon, the capsule headed back to Earth. It parachuted to a safe landing in Kazakhstan, allowing scientists to study its precious cargo.

The Luna program ended in 1976, with Luna 24. But it’s about to resume. Russia plans to launch Luna 25 later this year — picking up where it left off almost half a century earlier.

Look for the Moon climbing into good view after midnight. Zubenelgenubi, one of the brighter stars of Libra, rises close below it. The Sea of Fertility is hidden on the night portion of the lunar disk — the site of a “do-over” in space.
 

Script by Damond Benningfield

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Here on Earth, a birthday comes once every 365 days — once a year. On planets in some other star systems, though, the birthdays would be fast and furious, because a year on these planets lasts less than one Earth day.

Astronomers have discovered several of these “ultra-short-period” planets. None of the planets is more than a few million miles from its star. Most of them probably are “locked” so the same hemisphere always faces their star, just as the same hemisphere of the Moon always faces Earth.

One example is Gliese 367b. It was discovered by a space telescope in 2019, and astronomers reported its details late last year.

The planet orbits a small, cool star once every eight hours. It’s about two and a half times as far from the star as the Moon is from Earth. The system is 30 light-years away, in Vela, the sail, which just peeks above the southern horizon in late evening.

Gliese 367b is smaller than Earth, and about half as massive. That makes it almost as dense as iron. In fact, most of the planet probably is iron, wrapped in a thin layer of rock. Radiation and winds from the nearby star may have stripped away most of its outer layers. Or perhaps they were blasted away by collisions with other planets.

Gliese 367b is so close to its star that its surface may be molten. If so, then gases from the hot rock may form a thin atmosphere — a blanket of iron-rich air for a world where the birthdays never stop.
 

Script by Damond Benningfield

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Measuring the distances to the stars isn’t easy — even for the brightest stars of all. Consider Spica, the leading light of Virgo and the 16th-brightest star system in the night sky. It climbs into good view by 11 or 11:30. It rises below the Moon tonight, and closer to the upper right of the Moon tomorrow night.

Spica consists of two big, hot, bright stars. They’re so close together, though, that their light blurs into a single pinpoint.

The best estimate for the distance to Spica is 250 light-years, with a margin of 10 light-years either way. Astronomers came up with those numbers by measuring the system’s parallax — an apparent shift in its position.

Astronomers measure the direction of Spica six months apart, when Earth is on opposite sides of the Sun. That causes a tiny shift in Spica’s location against the background of more-distant stars. It’s like holding a pencil in front of you and looking at it with first one eye, then the other. As you blink back and forth, the pencil appears to move against the background. The size of that shift tells you the distance to the pencil.

For the stars, the angle is tiny. So it takes many observations with sharp instruments to get an accurate measurement. And the farther the star, the more difficult the task. So even though Spica is one of the brighter stars in the night sky, there’s still some wiggle room in its distance: roughly 250 light-years.
 

Script by Damond Benningfield

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Our closest planetary neighbors will huddle close to each other over the next few weeks. They’re low in the southeast at dawn. The brighter world is Venus, the “morning star.” It passes closer to Earth than any other planet. The other is Mars, which stays a little farther away. It’s to the lower right of Venus.

Both worlds could be volcanically active today, or they could have been active in the recent past.

A region on Mars known as Tharsis contains several giant volcanoes. The largest is about 15 miles tall and as big as Missouri. These volcanoes may have been active within the past few million years.

A recent study found that Tharsis also contains more than a thousand small volcanoes — no more than a few hundred feet tall. The youngest could be just a few million years old. Together, they may have belched as much lava as the big volcanoes. They may sit on a patch of crust that’s thicker than the patch below the big volcanoes. That makes it harder to build big volcanoes, so the region produces a bunch of little ones instead.

Venus is covered with many old volcanoes. The thinking has been that it’s a dead world, with a single solid crust instead of separate plates like those on Earth. But a study found evidence that parts of the crust are moving in different ways. That suggests that Venus does have separate plates, but they move more slowly than those on Earth. So Venus may not be dead at all — just a little bit slow.
 

Script by Damond Benningfield

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A couple of cosmic boulders will slide past Earth early tomorrow. Neither one is big enough to see without a large telescope. And neither is a threat to hit us. But they show that Earth shares this region of space with some potentially nasty neighbors.

The boulders are asteroids — chunks of rock and metal that orbit the Sun. Their paths cross Earth’s, so they occasionally fly quite near our planet.

Asteroid 2020cx1 will miss us by 1.7 million miles — 600,000 miles farther than its previous encounter, exactly one year ago. The asteroid probably is no more than a couple of hundred feet in diameter.

Another asteroid will pass even closer just a couple of hours earlier. 2018cw2 will miss us by about half a million miles. It’s probably not as big as 2020cx1, but it could be big enough to cause localized damage if it were to hit Earth. It’s in the same size range as an asteroid that exploded in the skies above Russia in 2013, shattering windows and injuring more than a thousand people.

2018cw2 isn’t expected to pass this close again until 2193. But the timing of that encounter is uncertain. So it’s possible that the boulder could miss Earth by as little as a couple of hundred miles — or it might not miss us at all.

To nail down its orbit, NASA is tracking its current passage. That will give us a better idea of whether future generations will need to worry about this cosmic boulder.
 

Script by Damond Benningfield

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As we gaze into the night sky, we see only one close neighbor: the Moon. It’s a quarter of a million miles away — millions of miles closer than any other large body. But a lot of little bodies inhabit our region of the solar system. They’re too small to see with our eyes. But they follow orbits that can bring them quite close to Earth — perhaps dangerously close.

They’re known as Near-Earth Asteroids. Astronomers have discovered about 28,000 of them. They’re split into groups based on their orbits. Some stay inside Earth’s orbit, while others say just outside.

But two groups follow orbits that cross Earth’s. They’re known as Apollo and Aten asteroids, for the first known members of each group. Several hundred of them are at least one kilometer in diameter — big enough to cause a global catastrophe if they hit us. The odds of a collision are small. But hundreds of the asteroids are listed as “potentially hazardous,” so we can’t rule out impacts.

Perhaps the best known of these asteroids is Apophis, one of the Atens. It’s about a thousand feet across, and it’ll pass less than 20,000 miles from Earth in 2029. Scientists originally feared it might hit us then or later in the century. They’ve since ruled out that chance.

Two other asteroids will pass close to Earth on Friday. One will miss us by almost two million miles. But the other will pass just half a million miles away. More about that tomorrow.
 

Script by Damond Benningfield

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A giant comet may have broken up about 20,000 years ago. And we see the effects of that break-up today.

The main remnant of the object appears to be Comet Encke. It’s about three miles wide, and orbits the Sun once every 3.3 years. It’s the source of several meteor showers. It sheds grains of rock and dirt as it orbits the Sun. They spread out to form wide clouds. When Earth flies through such a cloud, it spawns a meteor shower. The best known is the Taurids, in November.

Astronomers have supposed that Encke was the remnant of a larger body for decades. But a study last year found that it has a lot of company. Researchers found 88 small comets and asteroids that appear to be related. When they traced the orbits of these bodies back in time, they found that all of them appear to meet up about 20,000 years ago.

The original body could have been up to 75 miles wide. It probably was a cosmic “rubble pile” — big chunks of rock held together by ice. That made it easier to pull apart. As it split up, it sent Encke and many other bodies off on their own.

One of those bodies might have been the asteroid that exploded above Tunguska, Russia, in 1908. It was only a couple of hundred feet wide, but it leveled several square miles of forest. And it’s possible that more bodies like that could lurk in the debris — cosmic missiles that could someday hit Earth.

We’ll talk about the remnants of another large body tomorrow.
 

Script by Damond Benningfield

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