StarDate

The Sun belongs to a rare class of stars: Class G. Members of this class account for only seven or eight percent of all the stars in the Milky Way galaxy.

A star’s class is determined by its surface temperature, which we see as its color. G stars are in the middle of the temperature scale, so they shine yellow to yellow-white.

Most class G stars are in the prime of life — a span that puts them on the main sequence. They’re steadily “fusing” the hydrogen in their cores to helium. And most G stars will stay in that phase of life for 10 billion years or longer.

Most of them are within about 15 percent of the mass of the Sun. Any heavier and they’d get hot enough to move up to class F or above. Any lighter and they’d be cool enough to move down to classes K or M.

Main-sequence G stars are only a small fraction the size, mass, and brightness of the top stars. But the stars in higher classes are even more rare than G stars. So the Sun, which is near the top of its class, shines brighter than about 90 percent of the stars in the Milky Way.

Not all class G stars are on the main sequence, though. Some are bloated and heavy. Such stars are more massive than the Sun. They’ve burned through their hydrogen, and are fusing other elements. Most such stars are either red or blue. But some are making the transition from one to the other, so they briefly shine yellow-white — the monsters of class G.

Script by Damond Benningfield

Under a dark sky, far from city lights, the eye alone can see thousands of stars. They come in a variety of brightnesses and colors. Yet only a few of those pinpoints of light are like the Sun.

Stars are classified based on their surface temperature, which is an indication of their color. Hot stars are blue and white, while cool stars are orange and red. The Sun is in the middle, shining yellow-white, with a surface temperature of about 10,000 degrees Fahrenheit. Those traits mean it’s a member of class G.

From hottest to coolest, stars are classified with the letters O, B, A, F, G, K, M. Each class is broken up into 10 sub-categories. And each star is given a Roman numeral that indicates its size and its stage in life.

The Sun is class G2V. That means it’s toward the hot end of class G, and that it’s on the “main sequence” — it’s in the prime of life, converting the hydrogen in its core to helium.

A couple of moderately bright G main-sequence stars are in view by about 10 p.m. 51 Pegasi is in the south, almost directly above the Moon. At the same time, Tau Ceti is low in the southeast.

The brightest class G main-sequence star visible from Earth is Alpha Centauri A. It’s the leading light of a triple star system that’s about four light-years away — closer than any other stars. It’s so far south, though, that it’s visible from only a tiny sliver of the United States.

More about G stars tomorrow.

Script by Damond Benningfield

A meteor shower will be at its best this weekend. The Moon makes a pest of itself, though, especially on Sunday night, when the shower should reach its peak. It leaves a little better viewing window tonight and tomorrow night, though. And this is a shower that stretches things out a bit — the number of “shooting stars” stays near the peak for several nights in a row.

The Orionid shower is the offspring of Halley’s Comet. As the comet orbits the Sun, it sheds bits of dust and rock. Earth flies through clumps of this debris in May and October. Some of it slams into Earth’s atmosphere at high speeds, vaporizing as glowing streaks of light.

The Orionids get their name from the point where they appear to enter the atmosphere — Orion the hunter. The constellation climbs into view by about 1 a.m. You don’t have to look at Orion to see the meteors, though — they can streak across any part of the sky.

The shower is expected to reach its peak in the wee hours of Monday morning. By then, the Moon will be almost full, so it will overpower most of the meteors. And it won’t set until about 90 minutes before dawn.

But it sets a couple of hours earlier than that tonight, and about one hour earlier tomorrow night. And it won’t be as full. Since the shower stays fairly steady for several nights — perhaps a dozen or more meteors per hour — that makes the early part of the weekend a slightly better time to watch for the Orionid meteors.

Script by Damond Benningfield

Planet Earth turns on its axis like a spinning gyroscope — one turn every 23 hours and 56 minutes. And because of that spin, you’d weigh less at the equator than at the poles.

The spin is pretty fast — a point on the equator moves at more than a thousand miles per hour. That creates an outward “push” on material at the equator. That gives Earth a slight “bulge” in its waistline — about 27 miles wider through the equator than through the poles.

Since a person standing at one of the poles is a little bit closer to the center of Earth than a person at the equator, the person at the pole would feel a stronger gravitational pull.

The difference would be magnified by Earth’s rotation. A person at the equator would be pushed outward by the same force as Earth itself. That acceleration would decrease their weight.

When you add up the effects, a person who weighs 150 pounds at the poles would weigh almost a pound less at the equator.

The same thing would happen on the other planets of the solar system, although to varying degrees. Venus has the slowest rotation, and it’s the “roundest” planet, so the difference would be tiny.

On Jupiter and Saturn, though, it would be much more pronounced. Both planets are giant balls of gas, and they both spin much faster than Earth does. As a result, they’re much more “squashed” than Earth is. So if you floated through the clouds at the poles, you’d weigh many pounds more than at the equator.

Script by Damond Benningfield

The seasons are changing on Mars. Today is the first full day of winter in the planet’s northern hemisphere. The new season follows what turned out to be a nasty autumn.

Mars has seasons for the same reason Earth does: The planet is tilted on its axis. As it orbits the Sun, the north and south poles take turns dipping toward the Sun. As a result, the northern and southern hemispheres take turns getting more and less sunlight — creating seasons.

The seasons are more complicated than those on Earth, though. That’s because Mars follows a more lopsided orbit around the Sun. The planet is closest to the Sun during northern winter, and farthest during summer. So summers and winters are milder in the north than in the south.

The changes in sunlight can trigger big dust storms. As the surface warms, it creates wind currents that stir up the powdery dust that covers much of Mars.

That’s just what happened back in May. Just days after the spring equinox on Mars, a storm whirled to life in the northern hemisphere. By the end of June, it encircled the entire planet. Rovers on the surface saw day turn almost to night. The Sun vanished from view, and the sky turned dark red.

It’s taking months for the dust to settle out of the atmosphere — creating a hazy winter in the northern hemisphere.

Look for Mars to the left of the Moon as darkness falls this evening. It looks like a bright orange star. It’ll be even closer to the Moon as they set, after midnight.

Script by Damond Benningfield

A space rock the size of a building will buzz near Earth tomorrow. There’s no risk of it hitting us. But it’s a reminder that other members of the solar system often pass this way. And some of them could be hazardous.

Asteroid 2014 US7 will miss Earth by about 750,000 miles — three times the distance to the Moon. Most of the time, the asteroid stays well inside Earth’s orbit. But its orbital path is elongated, so it sometimes passes close to our planet.

2014 US7 probably is between about 40 and 100 feet wide. That’s about the same size as an asteroid that exploded over Russia in 2013. The explosion shattered windows and caused other damage.

Asteroids in this size range pass near Earth all the time. This year alone, in fact, several dozen of them have passed inside the Moon’s orbit. And one of them actually hit us. It was only about six feet wide, so it wasn’t discovered until a few hours before impact. It exploded harmlessly over southern Africa in June, raining small fragments on the surface.

Astronomers have been hunting and plotting the orbits of near-Earth asteroids over the past quarter century. The goal is to identify anything on a collision course long before it could hit us. And they’ve found most of the big asteroids, which could decimate life on a regional or global scale. Now they’re trying to find ones that are smaller, but still big enough to cause big problems — trying to keep Earth safe from space rocks.

Script by Damond Benningfield

Planetary systems can be dangerous. Like race cars zipping around a track, planets can ram into each other. That can destroy one or both of the planets, and create massive piles of debris. Some of that debris may then fall onto the parent star.

Astronomers have seen the likely aftermath of several such collisions. The most recent is in a star system in Auriga. The charioteer climbs into view in the northeast by about 10 p.m. It’s marked by the bright star Capella.

Astronomers have been intrigued by a star system known as RW Aurigae for decades. It consists of two stars in a tight orbit around each other. They’re only a few million years old.

The system periodically gets much fainter. In 2011, for example, it faded for about six months. It faded again in 2014, and stayed faint for more than two years.

A recent study says the dimmer switch probably was flipped by a collision between two small planetary bodies. The impact created a big cloud of dust and gas. The cloud passed in front of the star, blocking some of its light. And some of that material appears to be falling onto the star.

Such a discovery isn’t surprising. There’s evidence that young stars gobble up some of their close planets. Eventually, though, the systems calm down. Yet even then, collisions between planets and smaller bodies happen all the time — maintaining the danger level. We’ll have more about that tomorrow.

Script by Damond Benningfield

Saturn is the second-largest planet in the solar system — only mighty Jupiter outranks it. And it generates the second-largest magnetic “bubble” — a region of space dominated by Saturn’s magnetic field. It’s molded into a “teardrop” by the solar wind.

Saturn’s magnetic field is generated deep inside the planet. A core of rock and metal probably is surrounded by a layer of metallic hydrogen. The core and the hydrogen rotate at different rates. That creates a dynamo effect, like an electric motor, which generates the magnetic field.

Charged particles get trapped in the field. And many of those particles come from Enceladus, a small moon. It squirts geysers of water into space. Radiation zaps some of the water molecules, giving them an electric charge. The Cassini spacecraft found that some of the particles then flow toward Saturn in waves. So do particles from the planet’s rings.

Cassini recorded energy emitted by the waves. Mission scientists then converted some of the observations to sound. The sounds have been speeded up to make the patterns more audible.

Cassini’s observations confirm that an electric “circuit” connects Saturn and Enceladus — powered by the giant planet’s magnetic field.

And Saturn huddles quite close to our own Moon tonight. The giant planet looks like a bright golden star to the lower left of the Moon.

Script by Damond Benningfield

Many constellations have a star that represents its character’s tail. And the names of many of those stars begin with the Arabic word for tail, Deneb. The most famous are the tail of the swan, known simply as Deneb, and the tail of the lion, Denebola.

Two lesser-known tail stars are in the southern sky this evening: Deneb Algedi, in Capricornus, the sea-goat, and Deneb Kaitos, in Cetus, the whale.

Deneb Algedi is a good bit bigger and hotter than the Sun, and much farther along in life. In fact, it’s probably around the end of its “normal” lifetime — the phase when a star “fuses” the hydrogen in its core to make helium. If so, then it’s getting ready to puff up as it prepares to convert the helium to carbon.

Deneb Kaitos is already burning its helium, so it’s even bigger and brighter than Deneb Algedi. That extra light makes the star clearly visible across almost a hundred light-years of space.

The star will continue to consume the helium for another hundred million years or so. When it’s done, it’ll cast its outer layers into space. That will form a bright bubble known as a planetary nebula. When the nebula dissipates, only the star’s hot, dead core will remain.

Capricornus is in the south at nightfall. It’s especially easy to pick out right now because bright orange Mars is passing through. The sea-goat’s tail is at the left end of a wide triangle. The tail of Cetus is well to their lower left.

Script by Damond Benningfield

The hub of a galactic empire climbs the eastern sky on October evenings. M31, the Andromeda galaxy, is well up in the east-northeast at nightfall. Under dark skies, it looks like a smudge of light. But it’s actually a family of hundreds of billions of stars. It’s two and a half million light-years away — the farthest object that’s easily visible to the eye alone.

About three dozen other galaxies orbit M31. A couple of them are impressive on their own. But most of them are little more than puffballs. They contain no more than a few hundred million stars, so they’re small and faint.

In fact, some of them probably are ex-galaxies. The gravity of M31 has pulled them apart, so they’re just blobs of stars and gas that are spreading out.

About half of the satellite galaxies orbit M31 in a wide, thick disk, known as the Great Plane of Andromeda. And most of the members of the plane orbit M31 in the same direction, just as all the planets in the solar system orbit the Sun in the same direction.

That’s a bit of a surprise. According to models of how galaxies form, the smaller ones should be scattered around M31 at random — there shouldn’t be a pattern to them. But similar disks have been found around the Milky Way and another large galaxy.

There are several possible explanations for the orderly arrangement of the galaxies. But so far, none of them is convincing. So theorists have a lot of work to do to explain what’s going on.

Script by Damond Benningfield