StarDate

The Milky Way is like a high-ranking politician or a popular music star: it travels with a big entourage. Astronomers have discovered about four dozen possible companion galaxies. A couple of them are visible to the unaided eye, but most are quite small and faint.The Milky Way and its entourage are all members of the Local Group — dozens of galaxies that are bound by their mutual gravitational pull. The Milky Way and another big spiral, the Andromeda galaxy, are its largest members.The Milky Way’s most prominent satellites are the Large and Small Magellanic Clouds. Each contains several billion stars, plus lots of gas for making more stars. From southern skies, they’re visible to the eye as small, glowing clouds.Most of the Milky Way’s other satellites are much smaller. In fact, the smallest contain only a few thousand stars.Although a few of these satellites have been known for a while, most were discovered only in the last few years. These late discoveries are quite faint, so you have to look carefully to see them. And a few others have been hidden behind clouds of stars or dust in the Milky Way.Some of these little guys are being pulled apart by the Milky Way, which is taking their stars and gas for itself — a fate that’s probably befallen many other small satellites during the galaxy’s long lifetime.A recently discovered satellite suggests that there could be many more of them out there even now. More about that tomorrow. Script by Damond Benningfield   

If you look at a map of just about any state, you’ll see a few major cities, a few more mid-sized ones, and a whole bunch of small towns. And the same thing happens if you look at a map of our region of intergalactic space. There are a couple of giant galaxies, a few mid-sized ones, and a whole bunch of little ones. And they are all bound to each other by their mutual gravitational pull.This collection of galaxies is known as the Local Group. It spans about 10 million light-years, and contains dozens of galaxies, with new ones added to the list every year.The “big cities” of the Local Group are our home galaxy, the Milky Way, and the Andromeda galaxy, M31. Each is a disk that’s at least a hundred thousand light-years wide, and contains hundreds of billions of stars.After these two behemoths, the list includes a few mid-size galaxies. The largest is the Triangulum galaxy. It’s about half the diameter of M31 and the Milky Way, but only about one-tenth as massive.A couple of other mid-size galaxies are satellites of M31. One is a spiral, while the other is a small elliptical galaxy, which looks like a fuzzy rugby ball. And two more members of the mid-size brigade orbit the Milky Way — the Large and Small Magellanic Clouds.Most of the remaining members of the Local Group are tiny. Some contain only millions of stars, not the billions found in the larger galaxies. And many of them are satellites of the Milky Way. More about that tomorrow. Script by Damond Benningfield

One of the most famous depictions of the night sky is “Starry Night,” a painting by Vincent Van Gogh. It depicts the Moon and stars as whorls of light and color.Unfortunately, the real night sky is a lot less dramatic. In fact, there’s little color to be seen at all — almost everything is black and white.If you look carefully, though, you can find some exceptions. A few stars look blue, a few look yellow, and a few look orange or red. That includes Aldebaran, the eye of Taurus the bull, which stands to the lower left of the Moon at nightfall.A star’s color tells you its surface temperature. Blue stars are the hottest — tens of thousands of degrees hotter than the Sun. White and yellow stars are in the middle, while orange and red stars are the coolest — up to a few thousand degrees cooler than the Sun.All of the orange and red stars that are visible to the unaided eye shine that color because of their size — they’re much bigger than the Sun. They’ve used up the hydrogen fuel in their cores, causing their outer layers to expand. That makes them cooler — and redder.Aldebaran itself is more than 40 times the diameter of the Sun. In the distant future, it’ll get even bigger, which will make it much brighter, and even redder than it is now. Eventually, though, Aldebaran’s outer layers will puff away into space, leaving only its hot but dead core — closing the bull’s ruddy eye.Tomorrow: our galactic neighborhood. Script by Damond Benningfield 

The star known as Castor is like a miniature star cluster. It consists of at least six stars, which are divided into three pairs. Only a few systems are known to have more individual stars.Castor represents the head of one of the twins of Gemini. Right now, the constellation is low in the eastern sky as night falls, with Castor above Pollux, Gemini’s other twin. They climb high overhead during the night.A telescope turns this point of light into two points, known as Castor A and Castor B. And a spectrograph — a device that splits a star’s light into its individual wavelengths — reveals that each point actually consists of two stars.One of the stars of each of these two pairs is bigger, brighter, and heavier than the Sun. In fact, the combined glow of these two stars is what we see as Castor with our eyes alone.But each of these stars has a companion that’s much smaller and fainter than the Sun. They’re in tight orbits with their more impressive companions, so it’s impossible to see them directly.The third set of Castor’s twins is known as Castor C. It’s billions of miles away from the other two. Each of its stars is about two-thirds as big and heavy as the Sun, but only about a tenth as bright. Their brightness changes, though, because they produce bright flares of energy and dark magnetic storms that are like sunspots — causing this pair of twins to flicker.The Castor sextet orbits the galaxy as a family — a small “cluster” of stellar twins. Script by Damond Benningfield

As the Sun drops below the horizon this evening, the sky will grow darker. Eventually, it’ll turn almost black — especially from locations away from the pesky glow of city lights.For several centuries, though, scientists wondered why the night sky turns dark.Most scientists thought the universe was infinite in both time and space. In other words, there was no point in time when the universe began, and it extended without end in every direction. If so, then we'd eventually see a star at every single point in the sky. That would make the night sky as bright as the Sun.In the last century, though, astronomers realized that the universe isn't infinite. It began 13.8 billion years ago, in the Big Bang, so it’s not infinite in time.And our view of the universe isn’t infinite, either. Because the speed of light is limited, there hasn't been enough time for us to receive light from stars that are more than 13.8 billion light-years away. So we see only a bubble of space, known as the observable universe.So while we see lots of stars in the night sky, we also see lots more empty space — making the night sky dark.If our eyes were attuned to microwaves, though, the sky would be bright. The afterglow of the Big Bang fills the entire universe. At the time it was produced, that energy was at visible wavelengths. But because the universe is expanding, the light waves have been stretched to longer wavelengths — invisible to the human eye. Script by Damond Benningfield 

“Outer space” is one of those things you know when you see it. The sky is dark and quiet, with no air to sustain life. But just where space begins is hard to say — there’s no single definition that’s accepted by one and all.In the last century, the National Advisory Committee for Aeronautics — the forerunner to NASA — said that space begins 50 miles up. At that altitude, the air pressure is only about a hundred-thousandth of the pressure at sea level. NASA and the Air Force have used that definition to award astronaut wings, beginning with the pilots of the X-15 rocketship in the 1960s.Later, Engineer Theodore von Karman devised a slightly higher altitude for where space begins: 100 kilometers — about 62 miles. At that altitude, the air pressure is just one-millionth of the pressure at sea level. In such thin air, a craft must move at orbital velocity to stay aloft.That altitude became known as the Karman Line, and today it’s the most commonly used definition for the edge of space. Anything that stays below the line is an aircraft, while anything that goes above it is a spacecraft. More than 500 people have flown above the line, qualifying them as space travelers.A few wisps of atmosphere extend far above the Karman Line — up to several thousand miles. The air is so thin, though, that it’s basically a vacuum — a region that, for most of us, would certainly qualify as “outer space.”We’ll talk about why space is dark tomorrow.

Script by Damond Benningfield

 

With the winter solstice just a couple of weeks behind us, there’s still not much sunshine to warm things up across most of the United States. For much of the country, early January is the coldest time of year — a fact that probably won’t come as much of a surprise.What may surprise you, though, is that we’re actually closest to the Sun for the entire year today — about one and a half million miles closer than the average distance of 93 million miles.Because Earth’s distance from the Sun changes, so does the amount of solar energy our planet receives. We get about seven percent more sunlight at this time of year than when we’re farthest from the Sun, in early July.Although that’s not a large number, you might still expect it to produce a pretty good swing in the planet’s climate — but it doesn’t.That’s largely because the oceans act as a climate storage battery — they store heat and circulate it around the globe. And it takes a lot of extra energy to make much of a change in the overall ocean temperatures. So by the time the oceans start to warm up from the extra sunshine, the planet is already moving farther away from the Sun. That means that any extra energy that’s been stored up begins to radiate back into space. That keeps the climate on a fairly even keel all year long.So if you find your own spot in the sunshine, soak up that extra bit of solar energy — and think ahead to the longer, warmer days of spring. Script by Damond Benningfield

The Moon probably owes its existence to a “big whack” — a collision between the young Earth and another planet about the size of Mars. The impact blasted a ring of debris into orbit around Earth. This material then coalesced to form the Moon.A recent study says that the impact initially produced a system that hardly resembles the current Earth-Moon configuration.Researchers created computer models of the collision that took into account the current configuration. Their simulations showed that Earth was knocked almost over on its side, and spun on its axis once every couple of hours. And the Moon orbited in the plane of Earth’s equator.Over time, a complex interplay of the gravity of Earth, Moon, and Sun caused some changes. The Moon moved away from Earth a bit, the tilt of its orbit changed, and Earth spun slower on its axis. Once the Moon reached a certain distance, its gravity pulled Earth into a more upright position. And tides in the Moon\'s crust, which are like ocean tides on Earth, changed the angle of the Moon’s orbit.The result of all of this is a system were Earth is tilted at about 23 degrees, and the Moon’s orbit is slightly askew relative to Earth’s orbit around the Sun — the result of a big whack and a lot of maneuvering.Look for the Moon in the southwest as night falls this evening. The orange planet Mars stands close to its lower right, with Venus, the brilliant “evening star,” farther along the same line.

Script by Damond Benningfield   

The crescent Moon starts the night off right, with a beautiful encounter with two planets. It then keeps the good skywatching going by setting around 9 or 10 o’clock — making way for the year’s first meteor shower.As the color of twilight begins to drain from the sky, look for the planet Venus to the lower right of the Moon, and Mars to the upper left. Venus is the brilliant “evening star,” so it really stands out. Mars is only about one percent as bright, but it’s also closer to the Moon, so you shouldn’t have any trouble finding it. It looks like a fairly bright orange star.Moonset clears the way for the Quadrantid meteor shower. It’s named for the extinct contellation Quadrans Muralis, which represented an early astronomical instrument. Today, that region of the sky lies within the borders of Bootes, the herdsman. It climbs into good view in the wee hours of the morning, marked by its brightest star, Arcturus. You don’t have to look at Bootes to see the shower, though — its meteors can shoot across any part of the sky.The shower is one of the shortest of all — its peak lasts just a few hours. But the Quadrantids can produce dozens of meteors every hour during that peak, so the shower is a good one to watch.To do so, find a dark, safe viewing site far from the glow of city lights. Then bundle up against the winter chill, look up, and enjoy the show.We’ll have more about the Moon and its planetary companions tomorrow. Script by Damond Benningfield