As seen from most of the United States, the Big Dipper is plunging toward the northern horizon as night falls, as if it’s about to dip into a pail of water.

If you line up the stars at the outer edge of the dipper’s bowl, and follow that line to the upper right, the first moderately bright star you come to is Polaris, the Pole Star or North Star. Earth’s north pole aims toward it, so Polaris forms the hub of the northern sky – all the other stars appear to rotate around it. And it’s always at the same point above the horizon – night and day, all year long.

There’s a southern pole star, too. It’s not as prominent as Polaris, though. In fact, it’s barely visible.

The star is Polaris Australis. It’s also known as Sigma Octantis because it’s in the constellation Octans, which depicts a navigational instrument known as an octant.

Polaris Australis isn’t as impressive as Polaris mainly because Polaris is huge and brilliant.

Compared to most stars, though, the southern pole star is impressive, too. It’s more than half again the mass of the Sun. It’s expanding as it nears the end of its life, so it’s several times wider than the Sun. And its outer layers puff in and out, so it brightens and fades a tiny bit every couple of hours. On average,

it’s more than 40 times brighter than the Sun. But it’s almost 300 light-years away. So that keeps Polaris Australis from being a better pointer to the celestial south pole.

Script by Damond Benningfield

The Blue Danube has been performed for some pretty lofty audiences – kings and queens, emperors and empresses, presidents and prime ministers. But a performance earlier this year topped them all: it was aimed at the stars.

The waltz was composed by Johann Strauss II, who was born 200 years ago today. His birthday was one of the motivations for the performance. The other was the 50th anniversary of ESA – the European Space Agency. So the broadcast was mostly symbolic – not a real attempt to contact other civilizations.

The waltz was performed by the Vienna Symphony Orchestra in late May. It was transmitted to space by one of ESA’s tracking stations.

The waltz was beamed toward Voyager 1. It’s the most-distant working spacecraft in history – more than 15 billion miles from Earth – so far that it took 23 hours for the waltz to reach it. Voyager carries a golden phonograph record inscribed with several musical works – but not the Strauss waltz.

Voyager is passing through Ophiuchus, near the constellation’s brightest star, Rasalhague. It’s about half way up in the west-southwest at nightfall, and it’s easy to see. It’s a bit more than 48 light-years away. So if anyone there happens to point a radio telescope toward Earth in late 2073, perhaps they’ll hear the strains of The Blue Danube waltzing through the galaxy.

Script by Damond Benningfield

A giant companion to a giant star faces an uncertain fate. The star is dying. As it expires, it will blast the companion, drag it inward, zap it with radiation, then loosen its grip on whatever remains.

Mirach is the second-brightest star of Andromeda. It’s passed through the prime phase of life, and now is in the red-giant phase. It’s puffed up to about 85 times the diameter of the Sun, making it shine about 1700 times brighter than the Sun.

Two years ago, astronomers discovered that Mirach has a companion. It’s probably a “failed star” known as a brown dwarf. It’s twice as far from Mirach as Earth is from the Sun.

Before long – astronomically speaking – the star’s outer layers will flow into space at tens of thousands of miles per hour. That will “sandblast” the companion, stripping away some of its bulk. And friction from that material will drag the companion toward the star.

After that, only the star’s hot but dead core will remain – a white dwarf. It’ll pelt the companion with ultraviolet radiation, vaporizing more of it. But the white dwarf will be much less massive than the present star, so it will loosen its gravitational grip on the companion.

No one knows for sure how all of this will play out, so we can’t predict the fate of Mirach’s giant companion.

Mirach is a third of the way up in the east-northeast at nightfall. It’s easy to see, even from most light-polluted cities.

Script by Damond Benningfield

The black hole at the heart of a distant quasar has the biggest appetite astronomers have ever seen. It gobbles down the equivalent of one Sun per day – more than any other known black hole. It’s fed by the widest disk of gas and dust yet seen. And it outshines everything else in the known universe – 500 trillion times the Sun’s brightness.

The quasar is so far away that we see it as it looked when the universe was a little more than one-tenth of its current age. It was discovered in the early 1980s, but astronomers thought it was a star. They deciphered its true nature just a couple of years ago.

The heart of the quasar is a black hole 17 billion times the mass of the Sun. That’s not a record, but it’s near the top of the list. The black hole’s enormous gravity pulls in gas, dust, and stars. They form a spinning disk around the black hole. The disk is seven light-years across – half again the distance from the Sun to its closest neighboring star.

As material in the disk funnels toward the black hole, it’s heated to millions of degrees. So the disk shines brilliantly – allowing us to see it across most of the visible universe.

The quasar is in Pictor, the painter’s easel. For skywatchers in the far-southern United States, the constellation is barely in view, low in the south, before dawn. Despite the quasar’s great power, though, it’s much too faint to see without a telescope.

Script by Damond Benningfield

We got our first picture from the surface of another planet 50 years ago today, when the Soviet Union’s Venera 9 landed on Venus. It transmitted data from the surface for 53 minutes, including a wide panorama.

Venus is completely covered by thick clouds, so we can’t see its surface from Earth, or even from orbit around Venus – orbiters use radar to peer through the clouds. Venus also has a hot, dense atmosphere, so landing there is tough.

Venera 9 parachuted through the clouds, measuring their thickness and composition. At the surface, it measured the density of the atmosphere – about 90 times the density of Earth’s atmosphere. And it measured the surface temperature – about 900 degrees Fahrenheit.

The lander was supposed to take a full 360-degree view of the landscape. But the lens cap on one of its cameras didn’t pop off as planned, so Venera photographed only half of the scene around it. The image revealed a flat landscape covered with wide, flat rocks. And the lighting was comparable to a cloudy summer day on Earth.

Venera 9 relayed its findings to Earth through an orbiter. Communication ended when the orbiter moved out of range – ending our first direct view of the surface of Venus.

Venus is the beautiful “morning star” this month. It’s low in the east at dawn, and slowly fades from view in the waxing twilight.

Tomorrow: the most ravenous black hole.

Script by Damond Benningfield

A star in the constellation Cetus brightens and fades dramatically every 11 months. At its brightest, it’s fairly easy to see. At its faintest, it’s visible only through a telescope. Because of that change, a 17th-century astronomer called the star Mira – from the Latin word for “wonderful.”

The star changes because it pulses in and out like a beating heart. Mira’s in the final stages of its red-giant phase of life. Its core is no longer producing nuclear reactions. Instead, it’s fusing hydrogen and helium in thin shells around the core.

Mira’s outer layers are puffed up by radiation from the shells. At the maximum, that inflates the star to about 400 times the diameter of the Sun. That’s also when its surface is coolest and faintest. As the outer layers cool, they fall inward, making the surface hotter and brighter. At minimum, the star is about 330 times the Sun’s diameter.

Each time it puffs up, Mira loses a little of the gas at its surface. Within the next million years or so, it’s likely to expel all the gas in its outer layers. That will leave only its hot but dead core – a white dwarf.

Astronomers have discovered thousands of stars like Mira. And many others will undergo the same phase, including the Sun – in about six billion years.

Script by Damond Benningfield

Algol does something amazing. Every 2.9 days, the star fades to just one-third of its usual brightness. In centuries past, the stars were thought to be unchanging. A star that changed so blatantly was a bit scary. So it was given a name to match: “Algol” comes from an Arabic phrase that means “head of the demon.”

But the star’s odd behavior isn’t scary it all – Algol fades as the result of eclipses.

The system consists of three stars. Two of them form a tight binary. The members of the binary orbit each other once every 2.9 days. We see the system edge-on, so the two stars eclipse each other. One star is much brighter than the other. When the fainter star crosses in front of it, the system fades dramatically. When the bright star covers up the faint one, though, the difference is tiny – much too subtle to see with the eye alone.

Astronomers have cataloged hundreds of eclipsing binaries. And the eclipses are important. They reveal the relative sizes and masses of the two stars, details about their orbit, and more. So there’s nothing to fear from these up-and-down star systems.

Algol is low in the northeast at nightfall, in Perseus. It should be at its brightest tonight. The faint part of its cycle will happen during daylight for the next few cycles. It’ll be visible during nighttime later in the month.

Sometimes, a star can change brightness all on its own, and we’ll have more about that tomorrow.

Script by Damond Benningfield

The Orionid meteor shower should be at its most active the next few nights. And there’s no Moon to get in the way, so it should be a pretty good show.

The shower is named for Orion because its meteors appear to “rain” into the sky from Orion the hunter. The constellation climbs into good view after midnight, so that’s when the shower is at its best – between midnight and dawn. You don’t have to look at Orion to see the meteors, though – they can blaze across any part of the sky.

The meteors are bits of debris from Comet Halley. The comet sheds grains of dust as it orbits the Sun. When Earth crosses the comet’s path, some of those grains plunge into the atmosphere. They instantly vaporize, creating the streaks of light known as meteors.

Most of the dust grains are no bigger than pebbles. But a few are larger. They form brilliant streaks that are visible even in a somewhat light-polluted sky. And some of them can leave glowing trails that remain visible for a couple of minutes.

The shower has been declining in recent years. Halley’s Comet is near its greatest distance from Earth, so there aren’t as many bits of comet dust in this part of its orbital path. Even so, the shower could produce 20 or more meteors per hour at its peak.

To watch the Orionids, find a dark but safe site away from city lights. Bundle up against the autumn chill, then sit back and watch the sparks from Halley’s Comet.

Script by Damond Benningfield

Venus doesn’t have any moons. But it does share its orbit around the Sun. Astronomers have discovered 20 asteroids known as “co-orbitals,” but there could be many more. These big space rocks follow roughly the same path as Venus. But they won’t stay in that lane forever. And when they leave it, they could threaten Earth.

These objects are nudged along by the gravity of Venus and the Sun. They generally stay well ahead of or behind Venus. Only one follows exactly the same orbit as the planet. The others move in and out a bit, getting closer to the Sun, then moving farther away.

Over the long term, though, their orbits aren’t stable, so they can break free and head elsewhere. A recent study found that of the 20 known objects, six could threaten Earth within the next 12,000 years. And three of them are especially dangerous. All three are at least a thousand feet in diameter, so they could cause major damage if they hit our planet.

A study also found that there could be many more of these Venus groupies. They stay so close to the Sun in our sky that they’re hard to see through the solar glare. And they move quickly, making them even harder to find. But a new telescope in Chile might pick out some of them – helping us find potential threats far in advance.

Look for Venus near the Moon in the dawn sky tomorrow. It’s the brilliant “morning star,” so you can’t miss it.

Tomorrow: an autumn meteor shower.

Script by Damond Benningfield

California is the land of the stars. It’s also in the stars as the California Nebula – a cloud of gas and dust that looks like the outline of the state. It’s more than a thousand light-years away, in Perseus.

The nebula belongs to a giant star-forming complex – the Perseus O-B-2 association. The region has given birth to many class O and B stars – the biggest and brightest of all stars.

The California Nebula probably is energized by one of those stars, known as Xi Persei. The star is more than 30 times the mass of the Sun, and tens of thousands of degrees hotter. At that temperature, it produces huge amounts of ultraviolet energy.

When that radiation zaps hydrogen atoms, it splits them apart. When they link back up, the atoms produce red light – the main color of the nebula. Oxygen and other elements produce their own colors, but they’re not nearly as common as hydrogen.

The California Nebula probably is about a hundred light-years long. It’s likely to split into smaller clumps that will collapse to form even more stars. But radiation and winds from Xi Persei and other big stars will blow away much of the nebula’s material – limiting the number of new stars for this cosmic California.

Perseus climbs into good view, in the northeast, in early evening. Xi Persei is visible to the naked eye, near the bottom of the constellation. But you need a telescope to see the faint outline of the California Nebula.

Script by Damond Benningfield