StarDate

The Moon and three bright planets huddle in the southwestern sky after sunset this week. One planet drops from sight early on, but the others are in good view until well after night falls.

This evening, start looking about 30 or 40 minutes after sunset for Venus, the brilliant “evening star.” It’s to the lower right of the Moon, quite low in the sky, so you need a clear horizon to spot it.

Once you do find it, you might need to keep an eye on it for a while to convince yourself that it really is a planet. Venus is so bright that it often looks like an approaching airplane. Unlike an airplane, though, it maintains its position. Its only motion is a slow descent toward the horizon as Earth turns on its axis. Venus drops from view not long after the sky gets good and dark.

The second planet is farther to the upper left of the Moon. Saturn lacks the luster of Venus. The planet itself is much bigger, but it’s also hundreds of millions of miles farther. A bright star poses below Saturn: Antares, the heart of Scorpius.

After darkness falls, you might notice the star’s orange color. That hue is matched by the third planet in the lineup, Mars, which is far to the upper left of Saturn. Mars is still shining brighter than Antares, though, so it’s easy to spot.

The Moon will move past Saturn over the next couple of nights, and Mars over the weekend, leaving behind its companions — until next month.

More about the Moon and Saturn tomorrow.

Script by Damond Benningfield

Dozens of cosmic invaders blazed into Earth’s atmosphere 20 years ago tonight. Some formed bright fireballs that were visible across the western United States. Others weren’t as bright, but they were detected by satellites or special detectors on the ground.

The first fireball streaked across southeastern New Mexico and western Texas, leaving a trail of green sparks behind it. Its passage was detected by instruments that were designed to listen for evidence of covert nuclear explosions.

A second fireball appeared an hour and 44 minutes later over California. It exploded with a force equal to perhaps a few hundred tons of TNT. The blast was recorded by seismometers across the state.

At first, scientists thought the two fireballs were caused by a single space rock. They thought it skipped off the upper atmosphere over New Mexico and Texas, but was slowed down enough to circle around Earth and plunge toward the ground over California.

But a later analysis of videos, photos, eyewitness accounts, and the shockwaves painted a different picture. Scientists determined that Earth was hit by a barrage of debris — perhaps the remains of a fractured asteroid. The biggest chunks formed the bright fireballs. But about 60 other bits of debris also hit Earth. A few were seen across the United States over the following 24 hours or so. Others were recorded by satellites or the network of nuclear-bomb detectors — evidence of a cosmic invasion 20 years ago.

Script by Damond Benningfield

Thanks to satellites, you don’t have to see the ocean floor to know what it looks like. The surface of the ocean bulges upward over underwater mountains, and forms depressions above ocean basins. So measuring contours in the ocean surface can provide a rough look at what’s below.

And a satellite around Venus has showed that you don’t have to see that planet’s surface to know what it looks like.

A blanket of clouds hides the surface of Venus from view. A few spacecraft have used radar to peek through the clouds, allowing them to map the surface. But the last of these missions ended more than two decades ago.

A more recent mission found clues to the surface contours in the clouds. Venus Express orbited the planet for eight years. It found ripples in the clouds downwind of a mountain range known as Aphrodite Terra. It also found that the clouds in that region contained high concentrations of water vapor.

Scientists say those features are caused by winds at the surface flowing over the mountains. The winds create waves that ripple all the way to the top of the atmosphere. The rising air also carries water vapor. So looking at the clouds helps reveal what’s happening far below.

Venus stands in the west-southwest as darkness falls this evening. It’s quite low in the sky, but it’s the brilliant “evening star,” and it’s close to the left of the crescent Moon, so you shouldn’t have much trouble spotting it.

More about Venus and the Moon tomorrow.

Script by Damond Benningfield

Our galaxy, the Milky Way, is bigger and brighter than most others. Its hundreds of billions of stars emit roughly 20 billion times the light of the Sun. But a few rare galaxies make our galactic home seem modest. The brightest galaxy yet seen, for example, is 10,000 times brighter than the entire Milky Way.

This brilliant galaxy, which is identified only by a catalog name, is in the faint constellation Aquarius, which is low in the southeast at nightfall on these early autumn nights.

The galaxy is about 12-and-a-half billion light-years away, so we see it as it was when the universe was just 1.3 billion years old.

The galaxy owes most of its brilliance not to its stars, but to a black hole. A supermassive black hole at the galaxy's center attracts gas. As the gas plunges in, it gets so hot that it radiates profusely. Dust blocks our view of the galaxy, but the dust itself gets heated and emits infrared light. In fact, astronomers discovered the galaxy with an infrared telescope in space.

We can’t see what the galaxy looks like today. But it led such an accelerated lifestyle in its youth that it probably blew away much of its gas and converted the rest into stars. As a result, today it may be a dull type of galaxy known as a giant elliptical. If so, then it has no gas, no young stars, and no evidence that it once reigned as one of the brightest galaxies in the cosmos.

Script by Ken Croswell, Copyright 2016

If you have dark skies and a pair of binoculars, you can scan for some of the oldest stars in the entire galaxy this evening. The stars may have formed as early as 13 billion years ago — when the universe was less than a billion years old.

The stars belong to globular clusters, which reside in the Milky Way’s halo, outside the galaxy’s bright disk.

Each cluster is a tightly packed ball of a hundred thousand stars or more. And all of those stars are among the most ancient in the galaxy.

Astronomers infer a star’s age by measuring its composition. The first stars in the universe consisted almost entirely of hydrogen and helium — elements forged in the Big Bang. Nuclear reactions in those stars made heavier elements. When the stars died, they expelled some of the heavy elements into space.

Over time, those elements were incorporated into new stars, which in turn made and distributed even more heavy elements. So stars like the Sun, which formed billions of years after the Big Bang, contain fairly high concentrations of these elements. But the stars in globular clusters contain almost none, suggesting that they’re extremely old.

Three clusters are in the southeast at nightfall. M15 is highest in the sky, to the upper right of the Great Square of Pegasus. M2 is not far to its lower right. And M30 is down in Capricornus. Through binoculars, each cluster looks like a fuzzy star — the glow of some of the oldest stars in the universe.

Script by Damond Benningfield

There’s a lot more to the Milky Way than meets the eye.

Most of what we think of as the Milky Way galaxy is contained in a thin disk of stars and gas that’s about a hundred thousand light-years wide. But there’s a lot more material outside the disk than inside.

The region outside the disk is known as the halo. It forms a ball that completely surrounds the disk, and it’s several times wider than the disk.

There aren’t all that many individual stars in the halo. But the halo is home to the galaxy’s globular clusters — tightly packed balls of hundreds of thousands of stars. Most of the clusters lie outside the disk, but even those that are inside the disk are just passing through — they belong to the halo.

The stars in globular clusters are the oldest in the entire Milky Way; we’ll have more about that tomorrow.

A second component of the halo is hot gas. It produces an X-ray glow that was discovered a few years ago. The gas may equal the mass of all the stars in the galaxy — doubling the amount of the Milky Way’s “normal” matter.

Most of the stuff that makes up the Milky Way is dark matter, which is the halo’s main component. Dark matter produces no energy, but its gravity tugs at the visible matter around it. It may consist of subatomic particles that are still awaiting discovery.

The dark matter is several times more massive than all the galaxy’s stars and gas combined — so there’s a lot more to the Milky Way than meets the eye.

Script by Damond Benningfield

The next few evenings offer a great chance to see the Milky Way — the hazy band of light that outlines the disk of our home galaxy. It arcs high across the sky at nightfall, and there’s no Moon around to spoil the show. As always, though, you need dark skies to see it — streetlamps and other artificial light sources quickly overpower its milky glow.

The center of the galaxy consists of a large “bulge” of stars. Most of the stars are billions of years older than the Sun. But there are also clusters of hot young stars.

The bulge is hidden from view behind thick clouds of dust. But its location is easy to spot. Look in the south-southwest after the sky gets good and dark for teapot-shaped Sagittarius. The center of the galaxy is just above the spout of the teapot. And right now, the planet Mars stands at right about the same spot, shining like a bright orange star.

A disk of stars and gas surrounds the central bulge. It spans about a hundred-thousand light-years. Our solar system is a bit more than half way from the center of the galaxy to its edge.

The disk contains hundreds of billions of stars. The hottest and brightest congregate in spiral arms that wrap around the bulge. There are four major arms, plus at least a couple of partial ones. We’re near the edge of one the partial arms, which incorporates many of the bright stars of winter nights.

The galaxy doesn’t end at the edge of the disk, though. More about that tomorrow.

Script by Damond Benningfield

If you like sunrises and sunsets, the planet Mercury would be an interesting world to visit. The planet has no atmosphere, so there’s none of the vivid color that comes with sunrise and sunset here on Earth. But Mercury has something that Earth doesn’t. It can have two sunrises and sunsets, or two sunsets and sunrises, all at the same point along the horizon.

Mercury is visible through the glow of our own sunrise right now. It’s quite low in the east about 45 minutes before sunrise. It looks like a fairly bright star, but you need a clear horizon to see it. It’ll stand to the lower left of the Moon tomorrow, and quite close above the Moon on Thursday.

The odd mixture of sunrises and sunsets is a result of Mercury’s long day and its orbit around the Sun. Mercury spins slowly, so, on average, there’s a gap of about six Earth months between one sunrise and the next. But Mercury’s orbit is stretched out, so its orbital speed varies.

As a result, the Sun moves across the sky at an uneven pace. It can stop and even reverse direction — creating the odd sequences of sunrises and sunsets.

Although there’s no air to create twilight, the sky would be a bit brighter before sunrise or after sunset — the glare of the Sun’s hot outer atmosphere, called the corona. The silvery corona would form a hazy dome of light above the horizon — one more reason to enjoy a sunrise on the little planet Mercury.

Tomorrow: Mapping out the Milky Way.

Script by Damond Benningfield

The Moon cozies up to a “dwarf” in the wee hours of tomorrow morning: Regulus, the brightest star of Leo, the lion. They’re well up in the east at first light, with Regulus close to the lower left of the Moon.

In astronomical jargon, terms like “dwarf” are relative. There are the tiny stars known as red dwarfs, the failed stars known as brown dwarfs, and the dead stars known as white dwarfs.

Most of the other stars are also known as dwarfs, although in a different way.

The main system for classifying stars uses the letters O, B, A, F, G, K, and M. The letters are based on the star’s color or surface temperature. The letter is followed by a number from zero to nine, breaking each class down into subcategories. And after that comes a Roman numeral from 1 to 5, with supergiants at the low end, then giants, subgiants, and dwarfs.

In this scheme, stars like Regulus and the Sun are classified as dwarfs, even though they’re bigger than most of the other stars in the galaxy. In fact, Regulus is about three times as massive as the Sun, and about four times wider. Only a few percent of the stars in the entire galaxy outrank it.

Dwarf stars are in the prime of life, converting hydrogen to helium in their cores. At the ends of their lives, when the hydrogen is gone, they’ll puff up like balloons — moving from the dwarf section of the scale to the giant section.

We’ll talk about the Moon and another bright morning light tomorrow.

Script by Damond Benningfield

One of the oddest experiments of the 20th century began 25 years ago tomorrow. Eight volunteers sealed themselves inside Biosphere 2, a miniature replica of Earth built in the desert north of Tucson. It was designed to study environmental problems confronting Earth itself. But a secondary goal was to look at how we might build self-sustaining colonies on other worlds.

The structure itself looked like a cross between a Space Age greenhouse and an ancient Egyptian monument. A multi-level steel framework was covered with thousands of panes of glass. Inside, it included an artificial mountain, an ocean, and livestock pens. About 3800 species of plants and animals were scattered through a desert, a rainforest, and other habitats.

In the first major experiment, the eight “biospherians” were to live in the structure for two years, completely sealed off from the rest of the world.

Yet things didn’t work as planned. Carbon dioxide built up in the air, so they had to use chemicals to remove it. They couldn’t grow enough food, so everyone lost weight. Some plants and animals died, but cockroaches thrived.

The grand experiment wasn’t a grand success. Yet it did show that maintaining a sealed habitat on another world will be even harder than expected.

Today, Biosphere 2 is still operating, although without the two-year runs. Scientists conduct ecological experiments, and it’s a popular tourist attraction — a small-scale Earth in the Sonoran Desert.

Script by Damond Benningfield