StarDate

Other than the Sun, we’ve seen the surfaces of only a few stars in any detail. The stars are all so far away that they’re just pinpoints of light — it’s hard to see them as disks with real features. One of the few we can see is Antares, the bright orange heart of Scorpius. It snuggles especially close to the Moon as they climb into good view by about 10:30 or 11 tonight.

Antares is easier to study because it’s a supergiant — one of the biggest stars in our region of the galaxy. Even so, the details we can see are huge — hundreds of times the diameter of the Sun. Those details are hotspots where pockets of gas are bubbling to the surface.

It’s hard to know the exact size of those features because we don’t know the exact size of Antares, either. Astronomers have measured the star’s apparent size — how big it looks on the sky — with a high degree of accuracy. But to translate that to the star’s true size, we have to know how far away it is. Measuring the distance to a supergiant is hard, though, so estimates vary by quite a bit. But the star probably is somewhere around 600 light-years away — a magnificent star that reveals a few of its features.

Antares is to the right or lower right of the Moon as they rise. They’re separated by only a couple of degrees — the width of your finger held at arm’s length. They’ll move slightly farther apart before they set, in the wee hours of the morning.

Tomorrow: the head of the serpent.
 

Script by Damond Benningfield

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One of nature’s great spectacles will play out tonight: a total lunar eclipse. The Moon will pass through Earth’s dark inner shadow, turning the Moon orange or red. All or most of the eclipse will be visible from all of the U.S. except Alaska.

Lunar eclipses occur at full Moon, when the Moon aligns opposite the Sun in our sky. But the Moon’s orbit around Earth is tilted a bit, so we don’t see an eclipse every month. The full Moon has to be crossing the plane of the Sun’s path across the sky to create an eclipse.

And even then, it’s not always a total eclipse. Sometimes, the Moon only dips part of the way into the shadow, creating a partial eclipse. There are at least two lunar eclipses every year, though, and sometimes more. This year we have two total eclipses.

Tonight’s gets under way when the Moon first touches the inner shadow, beginning the partial eclipse, at 9:28 p.m. Central Daylight Time. The Moon will be fully eclipsed an hour later. Totality will last an hour and 25 minutes. And the partial phase will end at 12:55 a.m., when the Moon leaves the shadow behind.

All of that action will be visible from most of the U.S. — from Arizona to Montana and points east. Skywatchers to the west of that line — including Hawaii — will see all of the total eclipse but will miss out on the end of the partial eclipse. Alaska will miss out on the whole thing. But it’s in prime position for the next eclipse, in November.
 

Script by Damond Benningfield

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The edge of the realm of the planets is marked by Neptune, the Sun’s fourth-largest major planet. It’s almost four times Earth’s diameter. But you need strong binoculars or a telescope to pick it out. For the next few mornings, though, it’s a little easier to spot than average. That’s because it’s passing close to bright Mars.

Neptune looks so faint because it’s remote — an average of 2.8 billion miles from both Earth and the Sun. At that range, the Sun is just one-tenth of one percent as bright as it appears from Earth. That means there’s not much sunlight to illuminate the planet. And despite its size, Neptune is a tiny target in our skies.

Neptune is only slightly less reflective than the other giant planets — Jupiter, Saturn, and Uranus. Methane in its upper atmosphere absorbs red light, giving the planet a rich blue color. The methane also hides much of the detail in Neptune’s clouds.

Neptune is one of four planets bunched fairly close together in the dawn sky right now. They line up in the east and southeast. Venus, the “morning star,” is lowest in the sky. Bright Jupiter is to its upper right, with Mars a little farther along the same line. It looks like a bright orange star. Neptune will stand a couple of degrees to the left of Mars tomorrow — the width of your finger held at arm’s length. It’ll pass just half a degree above Mars on Wednesday. The gap will widen after that, leaving Neptune all alone.
 

Script by Damond Benningfield

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The Moon will slip through Earth’s long shadow on Sunday night, creating a total lunar eclipse. It comes exactly one year after another total eclipse — not one calendar year, but one lunar year, which is about 11 days shorter.

A lunar eclipse occurs when the full Moon passes through Earth’s shadow, darkening at least part of the lunar disk. It’s a total eclipse when the shadow covers the entire disk.

Eclipses don’t happen every month, however, because the Moon’s orbit around Earth is tilted. So most months, the Moon passes above or below the shadow, so there’s no eclipse. An eclipse takes place only when the full Moon is crossing the plane of Earth’s orbit.

Eclipses can take place at intervals of one, five, or six “lunations.” A lunation is the period from one full Moon to the next — about 29 and a half days. Six lunations make a semester. And two semesters make a lunar year, which is 354.4 days long. Lunar eclipses can repeat a semester apart, in a series of eight eclipses. They aren’t all visible from the same part of Earth, but there is some overlap from one eclipse to the next.

Sunday’s eclipse comes one semester after the last one, on November 19th, and a full lunar year after another. Another total eclipse will take place one semester from now, on November 8th, and one more eclipse in a lunar year — a barely there eclipse in which the Moon just dips into Earth’s faint outer shadow.
 

Script by Damond Benningfield

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For a space telescope, Earth orbit isn’t a great viewing spot. As a craft passes in and out of Earth’s shadow, it changes temperature, which can mess with its instruments. Earth itself can shine into the telescope, spoiling the view. And activity on the Sun can cause Earth’s atmosphere to expand, increasing drag and shortening the ’scope’s lifetime.

So in recent decades, several space telescopes have been sent to a spot almost a million miles away. It provides an open view of the heavens, steady temperatures, and easy contact with Earth.

The spot is known as L2. It’s one of five “Lagrange points” — places where the gravity of Earth and Sun are balanced. They’re named for Joseph-Louis Lagrange, a mathematician and astronomer who discovered them 250 years ago.

L2 is on the opposite side of Earth from the Sun, at one percent of the Sun’s distance. The point itself isn’t stable. But a craft can orbit L2, where it needs little fuel to stay in place.

Some pioneering telescopes have studied the universe from L2. W-MAP studied the afterglow of the Big Bang — work that earned its creators a Nobel Prize. And several craft are there now, including Gaia, which is mapping more than a billion stars, and James Webb Space Telescope, which arrived early this year. Engineers are still tweaking Webb to get it ready to look for early stars and galaxies, study exoplanets and stellar nurseries, and more — from a million miles away.
 

Script by Damond Benningfield

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Rattlesnakes, vampire bats, and James Webb Space Telescope have something in common: They can all detect the infrared — wavelengths of light that are too long for the human eye. That helps the snakes and bats find and catch prey. And it will help the telescope find some of the coolest and most remote objects in the universe.

The light that’s visible to human eyes forms only a tiny sliver of the electromagnetic spectrum. Other forms range from radio waves — the longest and least energetic — to gamma rays — the shortest and most energetic.

Infrared is right in the middle. Although we can’t see it, we do feel it — as heat. A fireplace and a pot of soup simmering on the stove both produce a lot of infrared. So do people and other mammals.

Yet infrared is the main way to study some of the cooler objects in the universe. It’s produced in large amounts by cool stars, failed stars, and stellar nurseries. Many planets emit infrared as well. And the earliest stars and galaxies also glow mainly in the infrared — their light stretched to longer wavelengths by the expansion of the universe.

Webb will study all of these objects. Its infrared sensors also will peer through giant clouds of dust and let us see what’s beyond. And they’ll try to measure the composition of atmospheres of planets in other star systems — perhaps telling us whether any of them show hints of life.

We’ll talk about the telescope’s viewing spot tomorrow.
 

Script by Damond Benningfield

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If you gaze into the early evening sky now, you might notice a few spots of color. The bright star Betelgeuse is orange. Arcturus is yellow orange. And Spica has a hint of blue.

The colors tell us that the stars shine brightest at different wavelengths. Orange stars are cool, so they shine at longer wavelengths. Blue stars are hot, so they shine at shorter wavelengths.

Yet that range represents only a tiny sliver of the electromagnetic spectrum. And astronomers use the entire spectrum to study the universe.

Electromagnetic waves are produced by everything from clouds of dust to exploding stars. The longest are radio waves. Each wave — the point from one peak to the next — spans yards to miles. So astronomers use giant radio dishes — some of them hundreds of feet across — to catch the waves. Among other things, the waves tell us about some of the coldest objects in the universe, such as big clouds of gas and dust.

The shortest waves are gamma rays. A wave can be smaller than the nucleus of an atom. That makes them hard to catch — they pass right through conventional detectors. So astronomers use special detectors in space, or telescopes on the ground that catch the effects of gamma rays.

Visible light is in the middle. A typical wave is about the size of a large virus. Among the many detectors we use to study them are the human eye — perfectly tuned to observe a sliver of the electromagnetic spectrum.

More tomorrow.
 

Script by Damond Benningfield

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As many as a dozen robotic landers are scheduled to head for the Moon in the next three years or so. They’ll study the rocks and soil, look for signs of water, and drive across the surface. And several of them will continue an experiment that’s been going on for more than half a century: lunar laser ranging.

The technique bounces laser beams off special reflectors carried to the Moon by Apollo astronauts and Soviet rovers. Scientists use the technique to probe the Moon’s interior, measure the Moon’s motion, and even study Albert Einstein’s theory of gravity.

The first lunar-laser experiment didn’t have those reflectors, though — it came before anything had landed on the Moon. Instead, scientists bounced the laser beams off the Moon itself. And the first “shot” took place 60 years ago tonight.

The experiment was called Luna See — S-E-E. Scientists and engineers from MIT aimed a red laser at a region of jumbled terrain known as the southern highlands. Over three nights, they fired the laser 80 times and used a telescope to measure its reflection.

Luna See didn’t provide much scientific return. But it did show that you could bounce a laser off the Moon. And that set up the current laser experiment, which should just get better in the years ahead.

The gibbous Moon is high in the sky at sunset. The bright star Regulus — the heart of the lion — stands close below it. They’ll stay close together for the rest of the night.
 

Script by Damond Benningfield

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The star that anchors the Big Dipper’s handle is a bit of an outcast. The five stars that make up the dipper’s middle are members of the same family — the Ursa Major Moving Group. They were born from the same cloud of gas and dust, and they move through space together.

The stars at the ends of the dipper are different. Tens of thousands of years from now, they will have moved away from the others as seen from Earth, pulling the “dipper” apart.

Alkaid is the end of the dipper’s handle. It’s farther away than the members of the moving group — about a hundred light-years. And it’s only about 10 million years old, which is far younger than the members of the moving group.

Ten million years isn’t long on the astronomical time scale — the Sun is four-and-a-half billion years old. Even so, Alkaid is getting well along in life because it’s more than six times as massive as the Sun. The cores of heavy stars are much hotter than those of lighter stars, which revs up their nuclear reactions. They consume their nuclear fuel at a much faster rate, so they burn out much more quickly. Alkaid, for example, will live a “normal” lifetime of less than a hundred million years, versus 10 billion years for the Sun — a short life for a stellar outcast.

Look for Alkaid in the north as night falls, with the rest of the dipper extending to its left. The dipper rotates down toward the northern horizon during the night, ready to scoop up a dipperful of stars.
 

Script by Damond Benningfield

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If not for its location, the star Megrez would attract little attention. It doesn’t look especially bright, and while the star itself is bigger and brighter than the Sun, it’s still not all that remarkable.

Yet we do pay attention to the star, because it’s a celestial thumbtack — it connects the bowl and handle of the Big Dipper.

In astronomical parlance, Megrez is a class A3 dwarf. The “A3” part of that designation means its surface is thousands of degrees hotter than the Sun’s, so it shines pure white. And the “dwarf” part means that it’s in the prime of life, steadily converting the hydrogen in its core to helium. A star in that phase of life isn’t exactly tiny — Megrez is a couple of million miles in diameter. But later in life, it will puff up to many times that size — it’ll be not a dwarf, but a stellar giant.

Megrez and four other members of the Big Dipper are siblings — they were born from the same cloud of gas and dust, and they move through space together. The other Dipper stars in the group are the two at the bottom of the bowl, and the two in the handle that are closest to Megrez. They’re all about 80 light-years from Earth.

The Big Dipper is high overhead as night falls, with the handle to the right of the bowl, linked by Megrez. It’s the faintest star in the entire dipper, but perhaps the most important: Without it, there would be no Big Dipper at all.

We’ll talk about another star in the Big Dipper tomorrow.
 

Script by Damond Benningfield

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