StarDate

The Andromeda galaxy is our nearest big galactic neighbor. In fact, it’s bigger than our own galaxy, the Milky Way. Despite its vastness, though, the last time anyone saw a supernova explode there was more than a century ago. But astronomers recently discovered a star in Andromeda that seems destined to explode — in a million years.

The star is a white dwarf — the small, dead core of a once-normal star. A white dwarf explodes if it gets too heavy — in particular, when it gets more than 1.4 times the mass of the Sun.

This white dwarf already becomes a nova at least once a year. A nova is a much less dramatic flare-up than a supernova. It doesn’t get as bright as a supernova, and it doesn’t destroy the star. But the white dwarf is already close to the critical mass of 1.4 Suns. As a companion star dumps more material onto it, the white dwarf is getting heavier. It should exceed the critical weight limit in less than a million years, and this periodic nova will become a supernova.

Actually, if this estimate is right, then the star has already exceeded the weight limit and exploded. We haven’t seen it yet because the Andromeda galaxy is two-and-a-half million light-years away, so its light takes two-and-a-half million years to reach us. So it’s possible that the star blasted itself apart more than a million years ago. Now, we just have to wait for its light to reach Earth, allowing us to see a supernova flare up in the Andromeda galaxy.

Script by Ken Croswell, Copyright 2017

Andromeda is best known for a single object — the Andromeda galaxy, M31. But that’s not the constellation’s only good “deep-sky” object. One of them is inside our own galaxy: a dying star known as the Blue Snowball Nebula.

Even though it’s much closer than the Andromeda galaxy, the Snowball is still a good jaunt from Earth — about 4,000 light-years. It’s a bubble of gas and dust expelled by a dying star.

The star’s nuclear reactor — its hot core — is switching off. The core is blowing away the star’s outer layers. That bubble spans about a quarter of a light-year, and it’s expanding at about 60,000 miles per hour. The gas is “energized” by ultraviolet light from the core, causing it to glow. Seen through a telescope, the object looks like a blue snowball.

With the outer layers barreling away from it, the core is exposed to view. It’s tiny — only about one-tenth the diameter of the Sun. But it’s much, much hotter than the surface of the Sun, so it shines more than a thousand times brighter.

Before long, though, the core’s nuclear fire will be extinguished. The core will continue to shine, but at a tiny fraction of the Sun’s brilliance. Without a strong energy source to light it up, the blue snowball will melt away.

Andromeda is low in the east and northeast as night falls. The nebula is at the right edge of the constellation, not far above the Great Square of Pegasus.

We’ll have more about Andromeda tomorrow.

Script by Damond Benningfield

If not for the writings of a single Greek astronomer, many of the modern-day constellations might not exist. But about 1900 years ago, Ptolemy compiled a famous text known as the Almagest. Its pages included details on more than a thousand stars. And the stars were all listed by their constellation — 48 constellations in total, all created long before the time of Ptolemy himself.

The list includes the 12 constellations of the zodiac, as well as most of the other well-known constellations of today. Most of them were first drawn thousands of years earlier, although their exact origins are unknown.

The signs of the zodiac probably were codified in ancient Mesopotamia, as far back as 5,000 years ago. Some of the other well-known star patterns may have come from sea-faring cultures of the Mediterranean.

All of these constellations were adopted by the Greeks. They tweaked some of the pictures, and wrote down the myths associated with each one. Some of the myths came from the original cultures, but others were created by the Greeks themselves. And the Greeks may even have drawn some original constellations.

That list includes Andromeda, the princess, which is in the east and northeast at nightfall. It’s pretty faint, so you need dark skies to see it.

Andromeda is associated with several other constellations that share a common story — a tale of tragedy, heroism, and betrayal that’s told in the stars. More about Andromeda tomorrow.

Script by Damond Benningfield

Astronomers have discovered thousands of planets in star systems other than our own. They’ve also found evidence of planets around many other stars — including planets that no longer exist.

The first possible example of such an “ex”-planet was recorded a century ago. But nobody realized what the discovery really was until recently.

On October 24th, 1917, Walter Adams, an astronomer at Mount Wilson Observatory, took the spectrum of a newly discovered star, known as Van Maanen’s Star. The star is a white dwarf — the hot, dead core of a once-normal star like the Sun.

A spectrum shows the intensity of the star’s light at different wavelengths. Each chemical element imprints a unique “barcode” in the spectrum, revealing its presence at or above the star’s surface.

Van Maanen’s Star showed the presence of iron, calcium, and other “heavy” elements — elements that should have sunk deep into the star. Their presence at the surface was odd, and astronomers of the day couldn’t explain it.

A couple of years ago, another astronomer looked at the spectrum, which was recorded on a glass plate. He recognized the heavy elements as the likely remnants of a planet. As the star was becoming a white dwarf, its inner planets should have been pulverized. Their rocky debris then could have fallen onto the star — “polluting” its surface with heavy elements.

So a century-old glass plate contains what may be the first recorded evidence of planets beyond our own solar system.

Script by Damond Benningfield

All of the Sun’s major planets are a bit flattened — they’re bigger through the equator than through the poles. For Mercury and Venus, the difference is only a few miles. For Earth, it’s a few dozen miles. But for Saturn, it’s thousands of miles. Such a big difference is obvious even to the eye alone: The giant planet looks like a squashed beachball.

The planets are flattened because they’re rotating. The motion pushes material outward. Solid bodies like Earth are stiff, and they don’t rotate all that fast, so they’re not squeezed outward by much. But Saturn and the other giant worlds don’t have solid surfaces. That makes it easier to push stuff away from the center as the planet spins.

Saturn is flatter than the other giants, though. That’s largely because it’s less dense than the others. That means its surface gravity isn’t as strong, so it doesn’t grip material at the surface as tightly.

And Saturn spins in a hurry — roughly once every 10 and a half hours, although the exact timing is still uncertain. That means the planet’s equator rotates at almost 23,000 miles per hour — 20 times faster than Earth’s equator. That slings material outward with great force — making Saturn the most “squashed” planet in the solar system.

Look for Saturn near the crescent Moon early this evening. The planet looks like a bright star to the left of the Moon as twilight fades away. They set about three hours after the Sun.

Script by Damond Benningfield

Just about every star varies a bit — it gets brighter and fainter over a period of time. And in most cases, astronomers can tell you what’s going on. Some stars pulse in and out like a beating heart. Others produce huge dark “starspots,” or brilliant eruptions known as flares. Even the Sun varies a tiny bit, over an 11-year magnetic cycle.

One of the stars of the Little Dipper varies, too, but astronomers aren’t sure why. They’ve come up with several ideas, but none of them seems to work.

Pherkad forms the lower outer corner of the dipper’s bowl. It’s about 500 light-years away. It’s a good bit bigger and heavier than the Sun, and about a thousand times brighter. Over a period of a couple of hours, though, that brightness varies by a few percent.

Astronomers suspect the difference is caused by the star’s stage in life. It’s nearing the end of its “normal” lifetime, which is triggering a series of changes in the star. In stars of similar mass and life stage, those changes can cause the stars’ outer layers to pulse in and out. In fact, that’s happening to one of Pherkad’s neighbors — Polaris, the North Star, at the end of the dipper’s handle.

Such stars don’t change brightness so quickly, though. Flares and starspots don’t cause such regular changes. And there’s no evidence of another object passing in front of the star, which would change the system’s brightness as well. So the cause of Pherkad’s changing brightness remains a mystery.

Script by Damond Benningfield

If you’ve never seen the Little Dipper, this is a good time of year to look for it. And if you have seen it, you might want to look again, because you might not have seen what you thought you saw.

This is a good time to find it because it stands almost directly above the Big Dipper, which is low in the northwest at nightfall. The Little Dipper’s bowl hangs upside down, like it’s pouring its water into the other dipper.

The Little Dipper’s brightest star marks the end of its handle. And it’s one of the most famous stars of all: Polaris, the North Star. It serves as the hub of the northern sky — all the other stars appear to wheel around it.

Overall, though, the Little Dipper is faint and obscure. You need pretty dark skies to see most of its stars. That means you can’t make out the pattern of the dipper at all unless you have dark skies, away from city lights.

Quite a few people who think they’ve seen the Little Dipper have really seen the Pleiades — the star cluster that’s known as the Seven Sisters. Its stars do form a dipper. But it’s tiny. If you hold your hand out at arm’s length, you can cover the whole thing with the tip of your little finger. The Little Dipper is much bigger — you’d need your entire outstretched hand to cover it up.

So if you can escape the glow of city lights, look above the Big Dipper for its fainter and lesser-known relative: the Little Dipper.

We’ll talk about one of the stars in the dipper tomorrow.

Script by Damond Benningfield

The Orionid meteor shower should be at its most active late tonight. And there’s no Moon in the sky then, so it should be a pretty good show.

The shower is known as the Orionids because all of 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.

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 heat up and 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 moderately 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. But the number of meteors appears to vary over a period of a decade or so. There’s evidence that the number is heading upwards, 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

Like peanut butter, planetary rings come in different textures. The rings of Jupiter and Neptune are smooth because the ring particles are tiny. But the rings of Saturn and Uranus are more chunky. They contain plenty of small particles, but they have big chunks mixed in. These different textures can reveal how the rings were made.

The particles in the rings of Jupiter and Neptune probably flake off the moons of these planets when the moons are hit by big space rocks. The bits of debris then spread out to form rings.

The small particles in the rings of Uranus probably form in the same way. But the rings of Uranus also contain chunks as big as pickup trucks. These pieces may be debris from a moon that was pulverized by a collision with a comet or asteroid.

The rings of Uranus are darker than charcoal, so they were discovered just 40 years ago. Even then, they were found only because they blocked the light of a star that was passing behind them.

There are several tight bands of material within the rings. These bands are no more than a few miles wide. They probably are “penned in” by small moons, which act like shepherds — their gravity keeps their flocks of ring materials from drifting away.

And Uranus is putting in its best appearance of the year. It rises at sunset, in the constellation Pisces, and remains in view all night. It’s also closest to us for the year, so it shines brightest. Even so, you need binoculars to find it.

Script by Damond Benningfield

The planet Uranus is putting in its best appearance of the year this week. It rises at sunset and remains in the sky all night. It’s also closest to us for the year, at less than 1.8 billion miles, so it shines brightest. In fact, under especially dark skies, for those with very good vision, it’s barely visible to the unaided eye. Most of us, though, need binoculars to find it.

Uranus is one of the giants of the solar system — only Jupiter and Saturn are bigger. But Uranus may be put together differently from its larger siblings.

Jupiter and Saturn are balls of hydrogen and helium wrapped around solid cores. Uranus probably has a solid core, too, but it has a lot less hydrogen and helium.

All of those elements that it does have are contained in an atmosphere that’s perhaps 3,000 miles thick — about one-fifth of the distance down from the tops of the planet’s clouds.

Below that may be a liquid or partially frozen mixture of water, methane, and rocky minerals. Unlike the boundary between the oceans and air here on Earth, though, there probably isn’t a sharp boundary between the layers of Uranus. The pressure is so great that the liquid and gas merge in a slushy transition zone.

Uranus climbs into view in the southeast an hour or two after sunset, near the eastern edge of the constellation Pisces. Through binoculars, it looks like a faint blue-green star — a giant world far from the Sun.

We’ll have more about Uranus tomorrow.

Script by Damond Benningfield