StarDate

The two planets that bracket Earth as it orbits the Sun perch above the Moon this evening. Venus is the brilliant “evening star.” Fainter Mars is close to the upper left of Venus, and looks like a modestly bright orange star.

Earth is the third planet from the Sun. Venus is the second planet out, while Mars is the fourth.

On average, there’s about a 75-million-mile span from the orbit of Venus to the orbit of Mars. On the astronomical scale, that’s not much. Yet it’s enough to make a huge difference in the habitability of the three worlds.

When Venus was young, it might have been relatively Earth-like, with oceans of liquid water on its surface. Venus was so close to the Sun, though, that the water soon boiled away. That triggered a runaway greenhouse effect that turned Venus into an inferno. Today, it has a dense carbon-dioxide atmosphere, and surface temperatures of 860 degrees Fahrenheit — one of the most inhospitable places in the solar system.

When Mars was young, it probably was fairly warm and wet as well. But its weak surface gravity couldn’t hold on to its water, so much of it leaked out into space. Today, the Martian atmosphere is thin and bitterly cold. So Mars is more comfortable than Venus, but still not especially inviting.

And in the middle, there’s Earth — at just the right distance from the Sun for liquid water at the surface, and enough heft to hold on to it — the coziest planet in the solar system.

Script by Damond Benningfield

Our Sun is a sedate middle-aged star. It’s been shining for about four and a half billion years, and it’s expected to keep shining for billions of years more.

But some stars that are well into the final stages of life are only a fraction of the Sun’s age. These stars are much heavier than the Sun, and it’s a star’s mass that determines its timeline.

An example is Arneb, the leading light of Lepus, the rabbit. The constellation stands below the feet of Orion, the hunter, which is in the southeast at nightfall.

Arneb is only about 13 million years old, yet it’s well into old age. In fact, it’s likely to expire within the next million years or so.

That’s because Arneb is about 14 times as massive as the Sun. Its gravity squeezes its core tightly. That makes the core extremely hot, which revs up the nuclear reactions that power the star.

The extreme radiation from the core pushes outward on the surrounding layers of gas, making the star puff up like a balloon. So Arneb is more than a hundred times the diameter of the Sun. If it took the Sun’s place in our own solar system, it would gobble up the planet Mercury and come close to Venus, the second planet from the Sun. And the star may get even bigger in the future.

Eventually, though, Arneb will no longer be able to generate nuclear reactions in its core. The core will then collapse, and the star’s outer layers will blast into space — bringing the short, brilliant life of Arneb to a fiery end.

Script by Damond Benningfield

The celestial rabbit occupies a perilous position in the sky: It’s below the feet of Orion, the hunter, and to the west of Canis Major, one of Orion’s hunting dogs.

Lepus is low in the southeast as night falls right now. It’s to the lower right of Orion and his three-star belt, and to the upper right of Sirius, the leading light of Canis Major and the brightest star in the night sky.

Lepus consists of a few moderately bright stars, although you might have a hard time connecting them to see a rabbit.

One of its most intriguing denizens is invisible to the eye alone. But telescopes have revealed that Gliese 229 may consist of at least three objects: a faint star, a brown dwarf, and a giant planet.

The star is smaller and cooler than the Sun, and much fainter. Because of its low mass, though, it’ll live billions of years longer than the Sun.

The star has an even smaller, fainter companion: a brown dwarf — one of the first ever discovered. It’s more massive than a planet, but not massive enough to shine as a true star.

And there’s evidence for a third member of the system — a planet that’s bigger and heavier than Neptune, one of the giants of our own solar system. The planet appears to be about the same distance from Gliese 229 as Earth is from the Sun. Because the star is so faint, though, that puts the planet well outside the habitable zone — the region where conditions are just right for life.

We’ll have more about Lepus tomorrow.

Script by Damond Benningfield

Some star names are ancient — they’ve been around since the dawn of civilization. Others are more recent. Three names, for example, have been around for only half a century. They were created as practical jokes by the crew of the first planned Apollo mission. Later, they were retained as a memorial to the astronauts after they died in their Apollo 1 spacecraft 50 years ago today.

Gus Grissom, Ed White, and Roger Chaffee were scheduled to head for space in February of 1967. On January 27th, they were going through a dress rehearsal at Cape Kennedy. Several hours in, a fire started inside the cockpit, which was quickly engulfed in flames. Seconds later it ruptured, spreading fire and smoke through the gantry and slowing rescue efforts. By the time the hatch was opened, the astronauts were dead.

An investigation found many problems. There were exposed wires in the cockpit, lots of flammable materials, a pure-oxygen environment, and a hatch that was hard to open. Investigators concluded that NASA got slopply as it raced to the Moon.

The Apollo spacecraft was redesigned, and carried the first astronauts to space in October of 1968 — and to the Moon just two months later.

Those crews navigated through space with the help of several dozen stars, including three named Navi, Dnoces, and Regor — parts of the names of Grissom, White, and Chaffee spelled backwards. And the names are sometimes used even today — a memorial to the crew of Apollo 1.

Script by Damond Benningfield

For the casual skywatcher, Camelopardalis, the giraffe, is a dud. Only a few of its stars are visible to the unaided eye, and you need dark skies to see even those.

With binoculars, though, you can pick out several pretty sights within the giraffe’s borders. One example is Kemble’s Cascade — a string of about 20 stars along the giraffe’s neck. The stars aren’t related to each other — they just happen to line up in the same direction in the sky.

Near one end of the cascade, though, is a group of several dozen stars that are related: the star cluster NGC 1502. It stands high above the North Star in mid evening.

The cluster is a few thousand light-years away, although the exact distance is uncertain. The cluster’s age is uncertain, too, although it’s probably only a few million years.

One clue to the age is the types of stars the cluster contains. Many of the stars of NGC 1502 are much bigger, brighter, and hotter than the Sun. Such stars live no more than a few tens of millions of years, compared to billions of years for stars like the Sun. Since quite a few of these stars are still around, it means that the cluster can’t be very old.

The brightest member of the cluster is called SZ Cam. It’s a family of six stars, split into three tight pairs. Two of the pairs huddle close together, while the third is much farther out. SZ Cam helps make the cluster worth looking for, in a constellation that’s a little short of skywatching wonders.

Script by Damond Benningfield

The relationship between stars and the material around them — the interstellar medium — is complicated. Stars are born from this material — wispy clouds of hydrogen, helium, and other elements, plus tiny grains of dust. And when stars die, they expel much of their material back into the interstellar medium, enriching it with elements forged inside the stars themselves.

And between birth and death, a star can have a powerful influence on the interstellar medium around it.

An example is the star known as Alpha Camelopardalis. It’s one of the brightest stars of the faint constellation Camelopardalis, the giraffe, which is high in the north on January evenings.

Alpha Cam is probably about 30 times as massive as the Sun. That great heft makes its surface extremely hot, driving a strong “wind” of gas off the surface and into space. This material forms a big bubble around Alpha Cam.

The star is racing through the galaxy at millions of miles per hour. As it plows through the interstellar medium, its wind squeezes the cooler gas and dust ahead of the star, forming a bow shock — like water piling up in front of a ship. The shock wave spans several light-years.

Over time, it’s possible that the shock wave could help compress the interstellar medium enough to give birth to new stars — continuing the complex relationship between the stars and the material around them.

We’ll have more about Camelopardalis tomorrow.

Script by Damond Benningfield

As twilight begins to paint the dawn sky tomorrow, a couple of planets appear near the crescent Moon. One of them is easily visible from the entire country. The other, though, is best seen from southern latitudes — south of about Oklahoma City or Raleigh, North Carolina.

The easy target is Saturn, the second-largest planet in the solar system. It looks like a golden star to the upper right of the Moon at first light. And the difficult target is Mercury, the smallest planet. As the dawn brightens, it stands below the Moon. It looks like a fairly bright star, but it’s quite low in the sky, so you need a clear horizon to spot it.

Although Mercury is little, it’s also quite dense — the planetary equivalent of a bowling ball. More than half of its volume consists of a core that’s made of iron and nickel.

There are several possible reasons for that. One is that the region where Mercury was born was so hot that most of the lightweight materials near the Sun vaporized and were blown farther into space by the Sun’s radiation. That left mainly the heavier materials to make up the little planet.

Another is that Mercury had more of the lightweight materials when it was born, but they there were blasted into space by a collision with another planet-sized body. That left mainly the dense, heavy core.

Whatever the reason, Mercury is a heavy little bowling ball of a planet rolling around the Sun.

Tomorrow: the influence of a “shocking” star.

Script by Damond Benningfield

Every year or two, it seems, there’s a new explanation for the rings of Saturn — how and when they formed. Some researchers have said they formed at about the same time as the planet itself, perhaps from the shattered fragments of a large moon. Others have said they formed much more recently, from the fragments of two moons that rammed together.

And late last year, another group came up with yet another idea: the rings formed about four billion years ago, from a Pluto-sized object that wandered too close to Saturn and was partially pulled apart by Saturn’s gravity.

Saturn’s rings are made mainly of ice, with smaller amounts of rock and dust. And that’s the key to the new idea.

Researchers in Japan simulated encounters between Saturn and objects like Pluto, which should have been plentiful in the early solar system. Their models showed that if one of these objects passed close to Saturn, the planet’s gravity could have sheared off the object’s outer layers, which consisted mainly of ice. Some of this material could then have been captured into orbit around Saturn — enough to account for its rings.

And Saturn is part of a beautiful pairing in the pre-dawn sky tomorrow. The giant planet looks like a fairly bright golden star, quite close to the lower right of the crescent Moon. A telescope will reveal Saturn’s rings — which are beautiful no matter how they formed.

We’ll talk about the Moon and another planet tomorrow.

Script by Damond Benningfield

Early risers, take note: The crescent Moon will slide down past several bright companions over the next few mornings, creating some beautiful configurations in the pre-dawn sky.

First up are the star Antares and the planet Saturn. Bright orange Antares stands to the lower right of the Moon at first light, with golden Saturn about the same distance to the lower left of the Moon.

Antares and Saturn are among the few objects in the night sky to show any color. Antares looks orange because of its surface temperature — it’s thousands of degrees cooler than the surface of the Sun. Saturn, on the other hand, looks yellow because of the composition of its upper atmosphere.

The giant planet is a big ball of hydrogen and helium gas, with a tiny mixture of other chemical elements. Those elements combine to form layers of clouds at the top of the atmosphere.

The clouds in the top layer are made of ammonia. Those in the layers below probably are made of water, ammonia, and a compound that includes ammonia and sulfur.

These compounds are all white. But above Saturn’s visible cloudtops, several compounds react with sunlight to form hydrocarbons, which have an orange or red color. Tiny amounts of them fall into the ammonia clouds, adding a bit of color. When we see Saturn in the night sky, the colors blend together to give the giant planet a subtle golden hue — a rare bit of color in the starry night sky.

More about Saturn and the Moon tomorrow.

Script by Damond Benningfield

Two “dog stars” hunker low in the east and southeast as night falls about now, and scamper higher across the sky later on. One of them rises a bit earlier than the other — a fact reflected in its name: Procyon means “before the dog.”

Procyon is the brightest star of Canis Minor, the little dog. It rises a few minutes earlier than Sirius, the brightest star in all the night sky. Sirius is also the leading light of Canis Major, the big dog, so it’s known as the Dog Star.

Because of their relative locations in the sky, Procyon always rises before Sirius — but only from north of roughly 30 degrees north latitude. Below that line, Sirius rises first.

Although Procyon isn’t as bright as Sirius, it’s still one of the most prominent stars in the night sky. In fact, the two stars look so bright for the same reasons — they are fairly bright, but more important, they’re both close neighbors.

Sirius is about 26 times brighter than the Sun, but it’s also less than nine light-years away — closer than only a few other star systems. Procyon is only about a third as bright as Sirius, and it’s about three light-years farther, so it looks a good bit fainter as it leads the Dog Star across the night sky.

The two stars are quite low as the sky gets good and dark tonight. Procyon stands due east, with Sirius well to its right, posing a little lower in the sky as seen from most of the United States.

Tomorrow: subtle color for a big planet.

Script by Damond Benningfield