StarDate

Three bright objects stair-step up the sky at first light tomorrow: the Moon, the planet Jupiter, and the star Spica. Jupiter looks like a brilliant star below the Moon, with fainter Spica below Jupiter.

Jupiter has more than 60 moons of its own. But from Jupiter itself, you’d be able to see only a few of them. Most are so small, dark, and far away that they’d be invisible from the top of Jupiter’s atmosphere. Four of the moons are big and bright, though, making them easy targets.

The easiest of all is Io, the closest of the four. It’s the same size as our moon, but it’s a little closer to Jupiter than the Moon is to Earth, so it forms a slightly larger target. It’s a more interesting target as well, because its surface is a maelstrom of volcanic activity. About 400 active volcanoes belch out gas and super-hot lava. Some of the volcanic debris reaches hundreds of miles high, so from Jupiter, they’d be visible to the eye alone.

The next moon out is Europa. It’s the smallest of the four, but still big enough to make out details on its surface. The main features are giant tiles of ice floating atop a hidden ocean — features unlike anything on our moon.

The other two big satellites, Ganymede and Callisto, are bigger than the Moon, but they’re far away from Jupiter. Even so, they’re close enough to see as disks, and perhaps to make out a few details — features on big moons in an alien sky.

More about the Moon and Spica tomorrow.

Script by Damond Benningfield

If you don’t like winter and you live in the northern hemisphere, then give a little thanks to the laws of orbital mechanics. Because of Earth’s lopsided path around the Sun, winter is the shortest season north of the equator — several days shorter than summer.

Earth’s orbit around the Sun isn’t a perfect circle. Instead, it’s an ellipse, which looks like a flattened circle, with the Sun slightly away from the center. That means our distance to the Sun changes. And that’s where the laws of orbital motion come into play.

Johannes Kepler devised those laws more than four centuries ago. One of them says that if you draw a line from the center of the Sun to the center of a planet, as the planet orbits the Sun that line will sweep out equal areas over equal periods of time.

To do that, a planet must move fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. Earth is closest to the Sun in early January — the start of winter — and farthest at the start of summer, in early July. So Earth moves around the Sun in a hurry during winter, making the season shorter.

This winter, for example, starts tomorrow, at 4:44 a.m. Central Standard Time. That’s the time of the December solstice, when the Sun stands farthest south in the sky for the year. And winter ends 89 days later. By comparison, this past summer lasted almost 93 days — a longer season thanks to the science of orbits.

Script by Damond Benningfield

Space observatories that don’t take pretty pictures or discover planets in other star systems don’t get a lot of attention. But a couple of European missions that have been operating this year are making big scientific contributions.

Gaia completed the first part of its mission this summer. It plotted the locations of more than a billion stars. It also plotted the distances to more than two million stars. That information will help astronomers produce the most accurate map of our region of the galaxy to date.

Gaia is continuing to watch the stars, and scientists hope that by next year, it’ll have distance measurements to all of its target stars.

LISA Pathfinder tested the concept for a space-based observatory for catching gravitational waves. These waves are produced by merging black holes and other events. They’re so weak, though, that they’re almost impossible to detect. In fact, they weren’t seen directly until just this year. A space observatory could see different kinds of events from those detectable by ground-based observatories.

LISA Pathfinder contained two small cubes of gold and platinum. The goal was to see if the cubes could drift without touching or being pushed around by the surrounding spacecraft — and they did. A full-scale LISA mission would use more than one spacecraft, linked by lasers. Any motion of the cubes inside them should be caused by gravitational waves — ripples in the universe detected from space.

Script by Damond Benningfield

Few sights in the universe rival that of a dramatic barred spiral galaxy. These grand galaxies feature two massive spiral arms that sweep away from the ends of a huge bar, which forms the galaxy’s heart. As a result, such a galaxy looks like a sweeping letter S.

By coincidence, the southern sky tonight features two of the most dramatic barred spirals in the heavens. Through a telescope, you can see them standing due south in mid-evening. One of the galaxies, NGC 1300, is in the large constellation Eridanus, the river. The second galaxy, NGC 1365, is south of the first, in the neighboring constellation Fornax, the furnace.

Each galaxy is about 60 million light-years from Earth. The galaxies have no connection to each other, though, because they’re 20 million light-years apart.

In both galaxies, the enormous spiral arms contain giant clouds of gas and dust. New stars are born from these clouds. Some of these new stars are especially hot and bright, so their light illuminates the spiral arms. The long bar that connects the arms contains mainly older stars, which are fainter and redder than many of their young stellar cousins.

Our own galaxy, the Milky Way, is also a barred spiral. But the bar isn’t all that prominent, so astronomers recognized it only in the latter half of the twentieth century. If we instead lived in a dramatic barred spiral like NGC 1300 or 1365, we probably would have discovered the bar long ago.

Script by Ken Croswell, Copyright 2016

Many of the stars that sparkle across a dark sky are deceptive — there’s much more to them than the eye can see. That’s because most of those points of light are the combined glow of two or more stars.

A famous example is Regulus, the bright heart of Leo, the lion. It stands close to the lower left of the Moon as they climb into good view by around 11 o’clock tonight.

What we see as Regulus is a star that’s bigger and hotter than the Sun, and hundreds of times brighter. But the star has at least three companions. They’re all so faint, though, they they add little or nothing to the light we see from the system’s main star.

One of the companions is a white dwarf — the dead core of a star that was once more impressive than the visible star. It’s so close to the main star that it’s impossible to see through the glare.

The two other known stars also form a pair. They’re hundreds of billions of miles away from the bright pair, so they are visible through a telescope.

The brighter member of this pair is a bit smaller, cooler, and fainter than the Sun. And its companion is cooler and fainter still — a bare cosmic ember that’s only a small fraction of the Sun’s brightness. The two stars are separated by about a hundred times the distance between Earth and the Sun, so it takes them centuries to complete a single orbit around each other — illuminated by the brilliant glow of the star we know as Regulus.

Tomorrow: twisted spirals.

Script by Damond Benningfield

Under dark skies, two galaxies are in good view tonight: Andromeda and our own galaxy, the Milky Way. As night falls, the Milky Way forms a faintly glowing band across the northern sky, arcing from Orion in the east to the Summer Triangle in the West. And Andromeda stands just above the apex of the Milky Way’s arch. It looks like a small, hazy patch of light.

These and many other galaxies formed the basis of one of the most important discoveries of the 20th century: dark matter. It emits no detectable energy, but its gravity pulls at the visible matter around it.

Astronomers discovered dark matter in part by measuring the motions of stars. If galaxies contained only stars, gas, and dust, then the stars at their edges should move slower than the stars near the center. But observations showed that the stars were all moving at about the same speed, suggesting that something else was accelerating the ones at a galaxy’s rim: dark matter.

A recent study, though, suggests otherwise. The study found that stars at the rims of 153 galaxies move just as they would if they were being pulled along only by the galaxies’ visible matter — the stuff that shows up in telescopes. No dark matter is needed to explain their motions.

The observations are unlikely to mean that dark matter doesn’t exist — there’s just too much other evidence that it does. But they could mean that it takes a different form than expected — adding to the mystery of dark matter.

Script by Damond Benningfield

Astronomers have discovered many black holes that are loud and showy. They’re gobbling gas from companion stars, or even entire stars, which produces a lot of energy, making them easy to find. But a recent discovery is calm and quiet. The black hole is barely feeding at all, so it’s tough to see.

The black hole is known as VLA J2130+12 — a catalog designation that means it was discovered by a radio telescope known as the VLA — the Very Large Array.

At first, it was thought to be a galaxy, far beyond our own Milky Way. But follow-up observations with the VLA and other telescopes found that it’s only 7200 light-years away — well within the Milky Way.

The energy that’s coming from the system means that it probably consists of a black hole that’s a few times the mass of the Sun, and a companion star that’s much smaller than the Sun. The black hole is slowly pulling gas from the companion. The gas forms a thin, faint disk around the black hole. Some of this material trickles into the black hole, slowly increasing its heft.

The system is so far and faint that it’s impossible to see. But you can find its position early tonight with the help of two guiding lights. Orange Mars is in the south-southwest as night falls. And the bright star Deneb is far to its upper right, at the top of the prominent Northern Cross. VLA 2130+12 is about half way between them — a faint black hole slowly snacking on a stellar companion.

Script by Damond Benningfield

At first glance, there’s nothing remarkable about the galaxy NGC 1600. It’s a little bit bigger than our home galaxy, the Milky Way, but it looks plain and uninspiring. It’s an elliptical galaxy, which means it looks a bit like a glowing, fuzzy rugby ball.

When astronomers peered deep into the heart of NGC 1600, though, they found something that is remarkable: one of the biggest black holes yet seen. It’s about 4,000 times the mass of the black hole at the center of the Milky Way.

The astronomers studied the galaxy with several telescopes on the ground, including one at McDonald Observatory, as well as with Hubble Space Telescope. In particular, they measured the motions of stars near the galaxy’s heart. How fast the stars move reveals the mass of the object they’re orbiting — in this case, a supermassive black hole.

The astronomers say the black hole may have grown so large by merging with the black holes in several smaller galaxies. That process also cleared out most of the stars in the center of NGC 1600. Without stars to feed on, the black hole is dark and quiet.

NGC 1600 is about 200 million light-years away, in the constellation Eridanus, the river, which meanders across the southern sky tonight. Not long after night falls, the galaxy stands above Rigel, the brightest star of the adjoining constellation Orion. It’s so faint, though, that you need a telescope to see it.

We’ll have more about black holes tomorrow.

Script by Damond Benningfield

December marks the start of winter in the modern western calendar. The season begins on the solstice, which is the shortest day of the year. In many older cultures, though, the solstice marked the middle of winter. And with the promise of longer days ahead, it was a time to celebrate.

In Scandinavia, for example, December 13th was Saint Lucy’s Day, a commemoration of the year’s longest nights. In Sweden, girls dressed in white and served pastries, and people sang songs to celebrate the return of light in the weeks ahead.

And in Rome, the solstice was celebrated with Saturnalia, a festival that honored Saturn, a god of the harvest. It began as a single-day affair, on December 17th. It was so popular, though, that it quickly expanded to six days.

Saturnalia was a time of feasting and high spirits. People decorated trees with sweets and ornaments. They performed acts of charity, and exchanged small gifts. Houses were decorated with candles and lamps, and villages built big bonfires, to cheer up the long winter nights.

Many of the customs of Saturnalia are observed today as part of the celebration of Christmas. In fact, there may be a connection between the two.

The early Church was looking to co-opt the many pagan solstice celebrations throughout Europe. So it set December 25th as the date to celebrate the birth of Christ. And it blended some of the trappings of the older festivals into the new celebration of Christmas.

Script by Damond Benningfield

The bright Moon has a bright companion tonight — at least for most of the night. The Moon will actually pass in front of the companion for a while, blocking it from view.

The companion is Aldebaran, the star that marks the eye of Taurus the bull. It’s close to the lower left of the Moon at sunset. Not long afterward, though, the Moon will cross in front of the star — an event known as an occultation. After an hour or so, Aldebaran will return to view on the other side of the Moon. The timing depends on your location. It happens a little later from the East Coast than from the western states.

The Moon itself is just a day away from full. The full Moon of December is known as the Moon Before Yule. It’s also known as the Long-Night Moon. That’s because it’s in view longer than any other full Moon of the year.

You can think of the full Moon as the opposite of the Sun. At this time of year, the Sun moves low across the south for those of us in the northern hemisphere. It’s also in view for the shortest period of time — as little as eight or nine hours from northern latitudes.

The full Moon does just the opposite of that. It soars high across the sky. And since it’s in view from sunset to sunrise, there’s a lot of time to enjoy the view — anywhere from 14 to 16 hours for most of the United States.

So enjoy all of those hours of moonlight over the next few nights — and tonight’s occultation of the eye of the bull.

Script by Damond Benningfield