StarDate

Every star in the night sky looks like nothing more than a dot against the darkness of space. Some are brighter than others, and some show a bit of color, but they’re all still just pinpoints of light. As modern telescopes allow us to zoom in more closely, though, we see that there’s a lot more to a star system than meets the eye.

An example is Fomalhaut, the leading light of Piscis Austrinus, the southern fish. It’s low in the southeast at nightfall, and at its highest, due south, a few hours later. It’s pretty bright, and there are no other bright stars around it, so it stands out.

Fomalhaut itself is bigger, heavier, and brighter than the Sun, and billions of years younger.

It’s surrounded by a disk of debris that spans tens of billions of miles. Much of it consists of grains of dust coated with ice that are warmed by Fomalhaut, causing the disk to glow in the infrared. But the disk also contains lots of icy comets and rocky asteroids. They stage frequent collisions, renewing the supply of material in the disk.

The disk also contains a giant planet. It’s been photographed by Hubble Space Telescope, although all we see is a small orange dot. The planet follows an elongated orbit around Fomalhaut. That may mean that another planet that’s closer to the star is pushing the visible planet around. That would give the system at least two planets, plus a vast cloud of debris — meaning there’s a lot more to Fomalhaut than meets the eye.

Script by Damond Benningfield

Planets are like bacteria — they fill every ecological niche we can think of. They orbit big stars and little stars, young stars and old stars. Some even orbit dead stars. And some don’t orbit a star at all — they inhabit the cold and darkness of interstellar space.

Such nomads probably were born just like other planets — from disks of debris around young stars. But the environment around a young star is busy and crowded. Planets may pass so close to each other that their gravity whips both of them around. A planet may be hurled into the star — or into deep space.

Or a planet in a distant orbit may be kicked out by the gravity of a passing star. And yet another possibility is that planets may collide, knocking one or both of them out of the system like billiard balls.

Only a few of these “rogue” planets have been observed. So there’s no agreement on just how many there are.

A study released this year looked at “microlensing” events, in which the brightness of a star changes as it passes behind another object; the intervening object’s gravity acts like a lens, bending and amplifying the starlight. Based on the number of events likely caused by planets, the study concluded that billions of nomads could be scattered throughout the galaxy.

With no star to warm them, such worlds probably are lifeless. But it’s possible that internal heat sources could make some of them habitable — life-bearing worlds hiding in the darkness.

Script by Damond Benningfield

With the Major League Baseball playoffs cranking up in early October, perhaps it’s not surprising that Mother Nature tosses out her own slider. The Harvest Moon, which usually graces the skies of mid to late September, this year slides into early October.

The Harvest Moon is the full Moon closest to the autumnal equinox, which marks the beginning of fall. This year, the equinox took place on September 22nd. September’s full Moon occurred on the 6th — about 16 days before the equinox. This month’s full Moon, on the other hand, comes today, about 13 days after the equinox, giving October the Harvest Moon honors.

The “Harvest Moon” name seems be have been bestowed by farmers in northern Europe, who relied on its light to help them complete the harvest.

On average, the Moon rises and sets about 50 minutes later each day. At this time of year, though, the full Moon rises only a few minutes later on succeeding nights from high northern latitudes. So when the Sun sets, the rising Moon lights up the fields, allowing farmers to continue harvesting their crops.

The full Moon after the Harvest Moon is the Hunter’s Moon. It illuminates the now-bare fields, making it easier for hunters to track game. That’s not especially important today. In bygone centuries, though, it was a big help to hunters who were trying to store food for the long, dark winter. And since this year’s Harvest Moon moved into October, the Hunter’s Moon slides into November.

Script by Damond Benningfield

Our two closest planetary neighbors slip past each other in the early morning sky the next couple of days. The closest neighbor is Venus, which shines as the brilliant “morning star,” low in the east at first light. The next-closest neighbor, Mars, will be a smidgen to the lower right of Venus tomorrow, and a bit farther to the upper right of Venus on Friday. Mars is only about one percent as bright as Venus, but its proximity to the brighter world will help you pick it out.

As Earth, Venus, and Mars orbit the Sun, the distance to both worlds varies by quite a bit.

At its absolute closest, Venus passes just 27 million miles from Earth. Now, though, it’s more than a hundred million miles farther. That’s because Venus is on the far side of the Sun. It’ll pass behind the Sun early next year, when it’ll disappear from view for several weeks.

Mars passed behind the Sun a few months ago. So right now, the planet is about 235 million miles away. At that distance, it’s at its faintest. As the months roll by, though, Mars will move closer to Earth, so it’ll get brighter. It’ll also rise earlier each day, so it will be in view for a longer and longer period each night. Mars will be at its absolute best in late July, when it will pass just 36 million miles from Earth.

Again, look for these two close neighbors heading in opposite directions in the eastern sky at first light.

Tomorrow: sliding back the Harvest Moon.

Script by Damond Benningfield

The almost-full Moon is near the eastern edge of the constellation Aquarius tonight. That puts it just a few degrees from the most populous planetary system other than our own: Trappist-1. Unfortunately, the star at the center of the system is tiny and feeble, so it’s far too faint to see with the eye alone.

The system consists of seven known planets. All of them are rocky worlds, like Earth. The smallest is a small fraction the size and mass of Earth, while the largest is more than half-again Earth’s mass. They orbit the star at ranges of one million to six million miles. And they probably are locked so that the same hemisphere of each world always faces the star.

This configuration means that you’d see quite a show in the night sky of any of these planets. The other planets would all pass quite close, so they’d be big targets. In fact, you’d be able to make out surface features on at least some of the other worlds.

No one knows what the planets might look like. But some of them are at the right temperature for liquid water, so they could have lakes or oceans. The inner planets might be covered by volcanoes — perhaps shooting out plumes that are tall enough to see from the other worlds.

And all of the planets would have great views of the star. Although small and faint, the star might produce magnetic storms that would blast out great streamers of hot gas. That’s a danger to anything that might live on these planets — but an amazing sight.

Script by Damond Benningfield

By almost any standard, Trappist-1 is an underachiever. The star is only about eight percent as heavy as the Sun. Any less massive and it wouldn’t be a star at all. And because of its tiny mass and size, it’s quite feeble — less than a tenth of a percent of the Sun’s brilliance.

In one way, though, Trappist-1 is an over-achiever. It has seven known planets — more than any star system other than our own. As befits a tiny star, though, it’s a tiny planetary system. There are no giant worlds, and all of the planets are within a few million miles of the star — just a fraction of the distance between the Sun and its closest planet, Mercury.

At least three of the planets appear to lie within the star’s “habitable zone.” That’s the distance from the star where temperatures are just right for liquid water, which is a key ingredient for life.

That doesn’t mean other conditions are right for life, though. In fact, a couple of studies released earlier this year suggest the planets probably aren’t habitable.

One study says the planets are bombarded by intense levels of ultraviolet radiation, which could strip away their atmospheres. The other says they’re bombarded by a “wind” of charged particles that’s a hundred thousand times stronger than the wind from the Sun. That, too, could erode a planet’s atmosphere — and perhaps even the planet itself — making Trappist-1 an unlikely home for life.

We’ll have more about Trappist-1 tomorrow.

Script by Damond Benningfield

Earth is a “Goldilocks” planet. It’s neither too hot nor too cold for liquid water, which is a key ingredient for life. In other words, it’s just right.

Part of what makes Earth just right is its distance from the Sun. It’s in the middle of the habitable zone — the distance where temperatures allow liquid water to exist on the surface.

Astronomers have found several planets in other star systems that are in the habitable zone, and they expect to find many more in the years ahead.

But just because a planet is in the habitable zone doesn’t mean it’s actually habitable. Habitability involves many factors, including the planet’s gravity, the composition of its atmosphere, and the age of its star.

The star’s age is important because as a star gets older, it gets brighter. That changes the distance of its habitable zone. So a planet that started as an iceball could become more comfortable as the star gets older.

A recent study, though, found that a planet can quickly jump from too cold to too hot. As the planet warms, it takes a long time for its ice to start to melt. When it does, much of the water can quickly evaporate. Water vapor traps energy from the star, making the planet warm up in a hurry. It can get so warm that the oceans evaporate, and water vapor at the top of the atmosphere escapes into space.

So it takes a fine balance of temperature, along with the right mixes of gases in the atmosphere, to make a planet just right.

Script by Damond Benningfield

Most of the stars in the Milky Way galaxy are members of binaries — two stars that are bound to one another by their mutual gravitational pull. But a system in Lyra, the harp, is a rarity: two binaries that are held together by their gravity, creating a family of four stars.

Epsilon Lyrae stands quite close above Vega, one of the brightest stars in summer and autumn skies, which is high in the west at nightfall.

Through binoculars, it’s easy to see the system as two pinpoints of light, known as Epsilon 1 and Epsilon 2. And a small telescope reveals the binary nature of each of those points. Since all four stars can be seen on their own, Epsilon Lyrae is also known as the Double Double.

One of the most remarkable things about the system is that all four of its stars are quite similar. All of them are bigger, hotter, and heavier than the Sun. It’s rare to find a double binary in which all of the stars are so much alike.

Current estimates say the system is about 800 million years old — roughly one-sixth the age of the Sun. That’s a long time for two binaries to remain stuck together. But they may not cling to each other forever. As Epsilon Lyrae orbits the center of the galaxy, it’s tugged by the gravity of passing stars and gas clouds, as well as “tides” created by the galaxy as a whole. Someday, that may pull the binaries apart, sending them on their separate ways — turning the Double Double into two single doubles.

Script by Damond Benningfield

Ceres doesn’t look especially hospitable. The solar system’s biggest asteroid has no atmosphere, and its surface is covered by impact craters — a landscape that looks as desolate as the Moon.

But things aren’t always what they seem. The top layer of dirt and rock on Ceres appears to be mixed with a large amount of frozen water. Liquid water may lie below the surface. And organic compounds have been detected on Ceres as well — some of the basic building blocks of life.

The Dawn spacecraft has been orbiting Ceres for more than two years. Its earliest pictures revealed some brilliant white spots on the surface. The spots are deposits of salts. Impacts by big space rocks may have cracked the crust, allowing salty water to bubble to the surface. The water quickly evaporated, leaving behind the salt.

The craft also has photographed several types of landslides on Ceres. The contours of some of the landslides resemble the aftermath of avalanches here on Earth — a mixture of ice and rock. Others look like they were made by a mixture of rock and liquid water; impacts may have melted some of the ice near the surface, forming a muddy sludge that flowed downhill.

Dawn also has found organic compounds on Ceres. Those chemical building blocks may also have come from inside Ceres, just as the water did.

So Ceres has some of the key ingredients for life — perhaps making the little world more hospitable than it looks.

Script by Damond Benningfield

 

Many regions of the night sky look fairly dark and empty. They look that way because they don’t contain any of the bright planets of our solar system, or any stars that are close and bright. If you look deeper into those regions, though, you’re almost certain to come across a cosmic wonder — something that’s both beautiful and intriguing.

An example is a small group of galaxies known as Seyfert’s Sextet. It’s in the west as night falls, in Serpens, the serpent — one of those regions that looks dark and uninteresting.

Seyfert’s Sextet is about 200 million light-years away — far outside our home galaxy, the Milky Way. It’s named for astronomer Carl Seyfert, who discovered the sextet almost seven decades ago.

The sextet is a bit deceiving. One of its galaxies is actually a background object — it’s hundreds of millions of light-years farther than the others. And what looks like two galaxies is really two parts of a single galaxy.

Even so, it’s a fascinating group. The four galaxies are so close together that they’re interacting — each galaxy’s gravity is having an effect on the others. The interaction has pulled a huge streamer of stars out of one of the galaxies, for example, making it look like two galaxies. It’s also triggering the birth of many new stars.

Over the next few billion years, the four galaxies are likely to merge. They’ll form a single super galaxy — an impressive sight far from our own cosmic neighborhood.

Script by Damond Benningfield