Scientists don’t know what dark matter is. But they have some ideas of what it isn’t. And they took a big step in ruling out some possibilities with the release of a study last year.
Dark matter produces no energy – the reason it’s described as “dark.” But we know it’s there because its gravity pulls on the visible matter around it. In fact, it appears to make up about 85 percent of all the matter in the universe.
The leading idea says dark matter consists of some kind of subatomic particle. A top candidate is called a WIMP – a weakly interacting massive particle.
Although dark matter almost never interacts with normal matter, it might occasionally do so – ramming into the nucleus of a normal atom. That would produce a tiny spark of light, which detectors might see.
One experiment is LUX-ZEPLIN. It’s in a former gold mine, almost a mile below the town of Lead, South Dakota. The rock above it blocks other types of particles from reaching the experiment.
Its detectors are inside a vat filled with about 8,000 tons of liquid xenon. The hope is that a WIMP will hit a xenon molecule and trigger that spark of light.
Project scientists conducted 280 days of observations. And they didn’t find any indication of WIMPs. But their test was the most sensitive yet for certain types of WIMPs. So the experiment rules out some candidate particles – narrowing the possibilities for dark matter.
Script by Damond Benningfield
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Ceres
At first glance, the dwarf planet Ceres doesn’t seem like a friendly home for life. It’s small, dark, and scarred by impact craters. Yet a deeper look presents a more optimistic picture. It has more water than any body in the inner solar system besides Earth. It has an abundance of organic compounds – the chemical building blocks of life. And it should be warm enough below the surface to sustain microscopic life.
Ceres is the largest member of the asteroid belt – a wide band of debris between the orbits of Mars and Jupiter. It’s about a quarter the diameter of the Moon. It probably consists of a dense core and mantle surrounded by an icy crust.
The Dawn spacecraft studied Ceres from orbit a decade ago. It saw big patches of bright, salty minerals. It also saw mountains, including one that’s three miles high; if you scaled Ceres to the size of Earth, the mountain would be 40 miles high. And the craft discovered that much of the surface consists of minerals that formed in a wet environment. So Ceres has water, heat, and organic compounds – the basic ingredients for life in what looks like an unfriendly world.
Ceres is at a point called opposition – it lines up opposite the Sun in our sky. That means it rises around sunset and is in view all night. It’s also closest to us at opposition, so it shines at its brightest. Even so, you need binoculars or a telescope to pick it out, in the constellation Cetus.
Script by Damond Benningfield
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Armed Constellations
The constellations are well armed. Several of the star patterns that depict people or gods are carrying weapons. And some of them are in good view at this time of year.
As darkness falls, look low in the west for the brilliant star Arcturus. It stands at the base of Botes the herdsman. Like many of the ancient star figures,
Botes has different stories, and is drawn in different ways. In most depictions, he’s holding something long and straight against his right side. In some cases, it’s a staff. But in others, it’s a spear.
Well above Botes is Hercules, marked by a lopsided box of four stars. He’s wrestling the multi-headed hydra. And in some depictions, he’s holding up a club.
In the south, look for Sagittarius. To modern eyes, it forms the outline of a teapot. But to the ancients, those stars formed an archer. The star at the outer edge of the spout is the point where he’s gripping both bow and arrow.
And low in the northeast there’s a figure with a unique weapon. Perseus the hero is holding the head of Medusa. In mythology, anyone looking at Medusa was turned to stone. Perseus managed to sever the head, then used it to save the princess Andromeda from a monster.
And if you’re stargazing before dawn, there’s another armed figure, well up in the south: Orion the hunter. He has two weapons. He’s holding a club in an upraised arm, with a sword strapped to his belt – a heavily armed figure in the stars.
Script by Damond Benningfield
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More Fomalhaut
The star Fomalhaut is a bit of a disappointment. Almost two decades ago, astronomers announced the discovery of a giant planet orbiting the star – the first exoplanet actually seen at visible wavelengths of light. Almost from the beginning, though, other astronomers questioned the discovery. And they were right. It wasn’t a planet at all, but a big clump of dust – the aftermath of a giant collision.
Fomalhaut is about twice as big and heavy as the Sun, and quite a bit brighter. It’s encircled by wide bands of dust. Most of the dust is at least a hundred times the distance from Earth to the Sun.
Fomalhaut is only about one-tenth the age of the Sun. Even so, it’s old enough that it should have blown away most of the dust. The fact that the belts are so prominent – especially the outer belt – means that they’re being renewed. The most likely source is collisions between large comets or asteroids. As those bodies are destroyed, they spew dust out into space.
One estimate says it would take the destruction of 2,000 comets that are one kilometer in diameter every day to keep the belts going. The would-be planet was the result of a collision between two even larger objects – briefly creating the illusion of a giant planet around this bright star.
Fomalhaut is low in the southeast at nightfall, and climbs across the south later on.
Script by Damond Benningfield
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Fomalhaut
The southern evening sky is pretty bare at this time of year – lots of dark, empty spaces, but few bright stars. The one notable exception is Fomalhaut. It’s the brightest star of Piscis Austrinus, the southern fish. It’s low in the southeast at nightfall, and arcs across the south later on.
The star we see as Fomalhaut is 25 light-years away. It’s about twice as big and heavy as the Sun, and more than 15 times brighter. It’s young – about 10 percent the age of the Sun. And it’s encircled by wide bands of dust, which may contain planets; more about that tomorrow.
Fomalhaut has two companion stars – bound to it by their mutual gravitational pull. Both stars are smaller, cooler, and fainter than the Sun. One of them is barely visible to the eye alone, but you need a telescope to see the other.
Both stars are a long way from Fomalhaut itself. One is almost a light-year away, while the other is two and a half light-years. Astronomers know they’re bound to Fomalhaut because they’re moving in the same direction and at the same speed. Their composition is similar to Fomalhaut’s as well, and so is their age.
Fomalhaut itself will shine for another few hundred million years. But the companions will last much longer – billions of years for the larger one, and hundreds of billions of years for the other. So they’ll still be shining across the galaxy long after the demise of their showy companion.
Script by Damond Benningfield
Australia