StarDate

Some of the icons of winter nights are disappearing behind the curtain of evening twilight. They’ll vanish within days, so there’s not much time left to enjoy them.

Orion is quite low in the west as twilight begins to fade right now. In fact, the constellation’s brightest star, Rigel, is so low that you might not be able to see it at all. But Orion’s three-star belt is still in view, lining up parallel to the horizon. And bright orange Betelgeuse is well above the belt, and is still in view by the time the sky gets fully dark.

Off to the left of the belt, look for brilliant Sirius, the brightest star in the night sky. It’s the Dog Star — the leading light of Canis Major, the Big Dog. It’s still in view as the sky gets dark, too.

Like all stars, those of Canis Major and Orion rise and set about four minutes earlier each day. That’s why each star and constellation has its own season. So these bright stars will be a little lower in the sky each evening, dropping deeper into the twilight.

At the same time, though, the Sun is setting a little later each night as well. It’s not by as much as the stars, but the extra seconds help mitigate their disappearance. When you add it all up, it means that Sirius and Orion’s Belt should vanish in the twilight in the next week or two, with Betelgeuse following soon after.

Orion will return to view by the end of July — this time in the dawn twilight — with Sirius following a couple of weeks later.

Script by Damond Benningfield

If astronomers had discovered how meteor showers work a little sooner, then we might be in the midst of the Halley-id shower right now. Instead, it’s known as the Eta Aquarid shower — named after a star in Aquarius.

The shower should be at its peak in the wee hours of Monday morning, with perhaps a few dozen meteors per hour. But the shower is spread out, so you can see a fair number of meteors for a few nights before and after the peak. And the Moon sets not long after sunset, providing dark skies for the “shooting stars.”

The meteors can streak across any part of the sky. But they all appear to rain into the sky from near the star Eta Aquarii — hence the shower’s name.

All meteor showers are named for the constellation that contains their “radiant” — the point at which the meteors enter the atmosphere. Most showers use only the name of the constellation. But a few constellations host more than one shower. Those showers are named for the star that’s closest to the radiant.

At the time this scheme was devised, the source of meteor showers was unknown. Today, though, we know that showers come from streamers of debris shed by comets or asteroids. Had this been known earlier, then perhaps the showers would be named for their parent bodies — in this case, Comet Halley. But Halley spawns two showers. So this one would be the Spring Halley-ids, with the other, in October, known as the Fall Halley-ids.

Script by Damond Benningfield

Most of the matter in the universe is invisible — it produces no detectable energy. Yet we know it’s there because we see it pull on the visible matter around it. The most popular idea is that this “dark matter” consists of some new kind of subatomic particle. So far, though, efforts to find these particles have come up empty.

Some scientists suggest that instead of particles, dark matter consists of black holes. These black holes would have formed a fraction of a second after the Big Bang, when dense pockets of matter collapsed under their own weight. These “primordial” black holes would be up to a few dozen times the mass of the Sun. The recent discovery of mergers of pairs of black holes in that size range seems to bolster the idea.

One recent study said that we might be able to see the effects of these black holes in the “haloes” of small galaxies. The gravity of the black holes would change the way the normal stars in the galaxies are distributed.

Another study looked for primordial black holes to alter the view of exploding stars. The gravity of a black hole between Earth and such a star would act as a lens, changing the brightness of the explosion. But the study didn’t find a single instance of such an effect. Based on that, the researchers concluded that black holes could account for no more than 40 percent of dark matter.

So the search for dark matter — of whatever size — continues.

Script by Damond Benningfield

If you’ve ever wondered what it’s like to be a piece of spaghetti, just fall into a black hole. Its gravity will turn you into spaghetti in moments.

That “spaghettification” is the result of gravitational tides — just like the ones that create ocean tides here on Earth.

Ocean tides are caused mainly by the pull of the Moon’s gravity. That pull is slightly stronger on the side of Earth that faces the Moon than on the side that’s away from the Moon. The difference tugs on the ocean water, creating “bulges” that trail the Moon across the sky.

Anyone falling into a black hole would experience that same effect. If you fell feet first, your feet would feel a stronger gravitational pull than your head. For a black hole a few times the mass of the Sun, that effect would be uncomfortable at a distance of a few thousand miles. By a few hundred miles it would be deadly. The black hole’s gravity would stretch you from head to toe, while squeezing you from side to side. In essence, you’d be turned into a long strand of spaghetti.

But not for long. The strand would be pulled apart, forming more strands that would be stretched even more. In a fraction of a second, the black hole’s gravity would then pull the strands apart atom by atom. The atoms would plunge to the center of the black hole, where they’d be crushed out of existence — one strand of spaghetti at a time.

We’ll have more about black holes tomorrow.

Script by Damond Benningfield

Astronomers haven’t “seen” the black hole at the center of NGC 4696, a galaxy in the constellation Centaurus. Yet the black hole’s impact on the surrounding environment is quite clear. It produces shock waves, blows bubbles, and pushes hot gas into the fringes of the galaxy and beyond.

NGC 4696 is the brightest member of the Centaurus galaxy cluster. It’s a giant elliptical galaxy, which is bigger and heavier than our home galaxy, the Milky Way.

Like most big galaxies, NGC 4696 appears to have a supermassive black hole at its center. Astronomers haven’t measured its mass, although it’s much heavier than the black hole at the heart of the Milky Way.

Looking at the galaxy in X-rays and radio waves revealed that the black hole appears to “erupt” every five million to 10 million years. The eruptions probably come from a disk of superhot gas around the black hole. Something causes the disk to produce powerful “jets.” These high-speed streamers pump energy and particles into the space around the black hole.

The jets appear to race beyond the borders of NGC 4696. They clear out big bubbles in the hot gas between galaxies. And they push clouds of gas and dust out of the galaxy, preventing the clouds from forming new stars.

As the jets shut down, though, gas and dust near the black hole may get cooler. Some of the material then may fall into the disk around the black hole — perhaps triggering the next outburst.

Script by Damond Benningfield

The crescent Moon and Venus, the “morning star,” will peek through the dawn twilight early tomorrow. They’re quite low in the east about a half-hour before sunrise.

In close-up photographs, Venus looks featureless. It’s completely blanketed by yellowish clouds, which hide the surface.

But ultraviolet images of the planet show some details in the clouds. In particular, they reveal dark streaks, which may extend deep into the atmosphere. The stripes absorb a huge amount of ultraviolet energy from the Sun.

So far, scientists aren’t sure just what makes up those stripes. The surrounding atmosphere consists of layers of clouds made of sulfuric acid and other compounds. The dark regions appear to contain sulfur compounds as well. But something else must be contributing to them.

Some have suggested the extra “something” could be microscopic organisms living in the clouds. The chemistry of some microbes on Earth matches some of the observations of Venus’s atmosphere.

Today, Venus’s surface is hellish — the atmosphere is extremely hot, dense, and toxic. Early in the planet’s history, though, it could have been much more comfortable, with oceans of liquid water. If life evolved on Venus, it could have been carried to the clouds by winds or other processes. Over time, it could have adapted to changing conditions.

So it’s possible that life could help fashion the dark stripes in Venus’s atmosphere.

Script by Damond Benningfield

If you drew a bubble around the Sun 10 light-years wide, it would contain only three other stars — the triple system known as Alpha Centauri.

If you drew the same-sized bubble around a star near the center of Messier 3, though, it would contain many thousands of stars. The night sky of an Earth-like planet there would be ablaze with stars — enough to keep the night from getting truly dark. And some stars would be visible even in the daytime.

Messier 3 is a globular cluster — a ball-shaped collection of perhaps half a million stars that’s only a few hundred light-years across.

The cluster probably formed when the Milky Way was quite young. So its stars are all old, and most of them are feeble. They’re so tightly packed, though, that the view from inside the cluster would be spectacular.

And if you could see through all those stars, the view beyond the cluster would be even better. That’s because M3 is about 30,000 light-years above the plane of the galaxy’s disk. So any residents of the cluster would get a view of the Milky Way that looked like something out of a textbook — the grand expanse of our beautiful spiral galaxy.

M3 is a pretty nice sight on its own. It’s high in the east at nightfall. It’s to the upper left of the bright star Arcturus, by a little more than the width of your fish held at arm’s length. Binoculars reveal the cluster’s glow, while a small telescope shows some of its individual stars.

Script by Damond Benningfield

Hercules climbs into good view in the east and northeast a couple of hours after sunset. His torso is outlined by a lopsided square of stars known as the Keystone.

The constellation produced a big bang that was seen last June. It was cataloged as AT2018cow, and nicknamed “the Cow.”

The Cow was a fast-evolving luminous transient — a class of objects first seen only in the last few years. They flare up in a few days — far quicker than a typical supernova, which is a huge stellar explosion. And astronomers are having a tough time deciding what powers them.

A transient discovered in 2015, by the Kepler space telescope, flared up in just two days. Some astronomers suggested it was a supernova, but an odd one: The blast ran into gas around the star, producing a brilliant shock wave.

A couple of years later, another team produced another explanation for these transients: the merger of two stellar corpses known as white dwarfs.

The Cow produced even more possible explanations. One says that a massive star collapsed to form a black hole. “Jets” of material from the star rammed into the gas around it, creating the fireworks. Another explanation is that a black hole was eating a “normal” star. And yet another says the outburst was produced by a supernova with little gas around it. That allowed space telescopes to see gas falling onto a newborn neutron star or black hole — a possible explanation for some “flashy” stars.

Script by Damond Benningfield

The starry dome of the night sky seldom changes. The stars shine steadily night after night, century after century. Occasionally, though, a bright new star appears — an explosion known as a nova or supernova.

If our eyes could see deeper into the sky, and at wavelengths other than visible light, we’d see outbursts all the time — thousands of them every night. Astronomers have discovered many classes of these transient events — flashes that last anywhere from a fraction of a second to months or years.

The list includes gamma-ray bursts — the death of a massive star that briefly outshines everything else in the universe. It also includes the mergers of stellar corpses known as neutron stars, the destruction of stars by black holes, intense flares from normal stars, and many others.

The most recent class of transient is fast radio bursts. These objects emit flashes of radio waves that can last for less than a thousandth of a second. In that time, though, they can outshine hundreds of galaxies as big as the Milky Way.

Astronomers have seen a few dozen of these events. Most of them pop off just once. But a couple have repeated themselves. That means they can’t be produced by stellar explosions, because there’d be nothing left to flash again. They may be highly magnetic neutron stars or other exotic objects, perhaps near supermassive black holes — objects that flicker from across the universe.

More tomorrow.

Script by Damond Benningfield

A pair of ancient but faint constellations flows across the southern evening sky at this time of year. The two of them share a common story, which also involves a third constellation.

Corvus and Crater have been around for more than two millennia. Their story, in fact, comes from ancient Greece.

According to the myth, the god Apollo sent Corvus, the crow, to fill a cup — known as a crater — with water from a nearby spring. On the way, the crow saw a tree filled with unripe figs. Instead of fetching the water and coming back later, he waited for the figs to ripen. When they did, he gorged on the delicious fruit.

Corvus knew that Apollo wouldn’t be happy with him. So he filled the cup with water, then grabbed a water snake in his talons. He brought both back to Apollo, and blamed the snake for blocking his way. But Apollo wasn’t fooled. Instead, he was angry and vengeful. He cast crow, cup, and snake into the heavens, forming three constellations. As extra punishment, he decreed that the crow would suffer from thirst — with the water-filled cup forever just out of reach.

The constellations are in the southeast at nightfall right now. Corvus contains four moderately bright stars that form a shape that resembles a sail. Crater, to the upper right, looks like a goblet — but you need really dark skies to see it.

Both of them sit on the back of poor Hydra, the water snake — an innocent victim of the wrath of the gods.

Script by Damond Benningfield