StarDate

The giant planets of the outer solar system have giant families of moons. Saturn, for example, has more than 80 known moons. The planets protect the moons by enfolding them in their own magnetic fields — some of the time.

Saturn’s magnetic field is generated deep within the planet. Saturn’s core is made of rock and metal. It’s surrounded by a thick layer of hydrogen. That layer is squeezed so tightly that it acts like a metal. The core and the metallic hydrogen rotate at different rates. That produces an electric dynamo that generates the magnetic field.

Saturn’s magnetic field is hundreds of times stronger than Earth’s. It deflects much of the solar wind — a flow of charged particles from the Sun — as well as cosmic rays, which are particles from outside the solar system.

But the solar wind sculpts the magnetic field into a teardrop shape. The region that faces the Sun is squeezed in toward Saturn. The region away from the Sun forms a tapered tail that extends millions of miles into space.

So depending on how far it is from Saturn, a moon may be protected when it’s on one side of the planet, but unprotected on the other — and subject to the full force of the solar wind.

Look for Saturn near our own moon tonight. It looks like a bright star, just above the Moon at nightfall. They’re flanked by two brighter planets — Venus to the lower right, and Jupiter to the upper left. More about the Moon and Jupiter tomorrow.
 

Script by Damond Benningfield

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The Moon and the planet Venus put on a great show early tonight. Venus is the brilliant “evening star.” It stands just above the Moon.

The Moon is a thin crescent — the Sun lights up only about one-tenth of the lunar hemisphere that faces our way. As the twilight begins to fade, you can make out the entire lunar disk. It’s bathed in sunlight reflected from Earth.

Since the invention of the telescope, many sharp-eyed observers have noticed a similar glow on Venus. It has nothing to do with Earth, though — the two planets are just too far apart. And it might not even be real.

The glow on Venus is most often seen when the planet is an evening crescent, as it is now. In fact, it’s never been recorded when the planet was more than about 30 percent “full.”

It was first reported in 1643. It was called “ashen light” because it looks faint and grayish. And it’s been observed many times since then.

Scientists have proposed many explanations for the glow. Some have said it’s produced by oxygen atoms in the upper atmosphere that are “energized” by the Sun. Others have suggested it’s caused by big lightning storms. And still others say it’s only an illusion; the glare of the bright crescent, combined with poor viewing conditions, can convince the eye it’s seeing something that’s not really there.

If you have a small telescope, you can check it out for yourself — scanning the night on Venus for the possible glow of the ashen light.
 

Script by Damond Benningfield

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The Sun isn’t a very good clock. The time it takes to move from one “noon” to the next can vary by almost half a minute per day. And that creates some interesting effects. One example is that the year’s earliest sunsets come days or weeks before the shortest day of the year.

There are a couple of reasons for that. One is Earth’s tilt on its axis. It causes the angle and location of sunrise along the horizon to change day by day. The other is Earth’s lopsided orbit. That causes the Sun to return to the same point in the sky a little earlier or later in the day, depending on whether we’re close to the Sun or far away.

Earth is close to the Sun in December, so it moves faster than average. As Earth rotates, it catches up to the “noontime” Sun a little earlier each day—by as much as 30 seconds.

Because of that, sunrise and sunset occur a little earlier, too. From here in the United States, the earliest sunsets of the year come a few days or weeks before the shortest day of the year — the winter solstice, on December 21st.

The exact date of earliest sunset varies by latitude. From the southernmost latitudes, it happens in late November or early December. From the latitude of Denver or Philadelphia, it happens about December 10th. And from Minneapolis or Seattle, it comes closer to the solstice.

The date of the latest sunrise comes about the same gap after the solstice — another result of the imperfect solar clock.
 

Script by Damond Benningfield

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A star that was kicked out of its birthplace is really lighting things up. The supergiant star illuminates a cloud of gas and dust, creating a colorful display known as the Flaming Star Nebula.

The nebula is in Auriga the charioteer, which is in the northeast at nightfall, marked by the bright star Capella. The nebula is to the lower right of Capella, and is an easy target for small telescopes.

The star that’s responsible for the nebula is AE Aurigae, and it’s pretty amazing. It’s more than 20 times as massive as the Sun, and about 60,000 times brighter.

It probably was expelled from a stellar nursery in Orion as part of a ballet involving four stars. It got such a big “kick” that it’s a runaway — it’s moving far faster than most of the stars in the Milky Way. And right now, it’s moving through a cloud of gas and dust that’s several light-years across — the material that makes up the nebula.

The surface of AE Aurigae is tens of thousands of degrees hotter than the Sun’s. That’s one of the keys to making the nebula shine, in shades of red and blue.

Stars that are so hot produce a lot of ultraviolet light. It rips apart atoms of hydrogen. When the atoms come back together, they emit red light. And the blue light is a reflection — visible light from the star reflecting off small grains of carbon and other molecules.

Combined, the red and blue make the nebula look like it’s on fire — the Flaming Star Nebula.
 

Script by Damond Benningfield

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A dead star in Aquila, the eagle, is a double record holder. It’s the heaviest star of its type yet discovered — and also the smallest. It’s only about 130 light-years away. But it’s so tiny and faint that you need a big telescope to see it.

The star is a white dwarf — the dead core of a once-normal star — or in this case, two stars. It probably formed when two white dwarfs merged. The combined star is 1.35 times the mass of the Sun. That’s almost as heavy as a white dwarf can get. Yet it’s only about as big as the Moon.

The core of a star up to about eight times the mass of the Sun is kept “puffed up” by the radiation from its nuclear reactions. When they end, gravity wins out, and the core collapses to form a white dwarf.

But it only collapses so far. That’s because the charged particles known as electrons exert a pressure that keeps them apart.

The heavier the white dwarf is, though, the tighter it’s squeezed by gravity. Its electrons have to move faster and faster to limit the star’s collapse.

Eventually, though, the electrons just can’t move any faster. They combine with protons to form neutrons. The neutrons are perfectly happy to jam together, so at about 1.44 times the mass of the Sun, a core collapses to form a neutron star — a body about the size of a small city.

The star in Aquila isn’t massive enough to become a neutron star. Instead, it’ll remain a small but heavy white dwarf — the remnant of two dead stars.
 

Script by Damond Benningfield

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In 30 million years or so, the Milky Way Galaxy may stage a spectacular fireworks display. It could give birth to hundreds of thousands of stars in just a few million years — the result of a massive collision.

Smith’s Cloud is falling toward the Milky Way’s disk. The cloud is about 10,000 light-years long. To get a sense of how big that is, consider that it spans about 20 times the width of the full Moon as seen from Earth, even though it’s maybe 40,000 light-years away.

Astronomers are still trying to figure out where the cloud came from. A study a few years ago used Hubble Space Telescope to examine its chemistry. From that, scientists suggested that it came from the outer regions of the Milky Way’s disk — perhaps blasted away by a cluster of exploding stars about 70 million years ago.

Today, it’s falling back toward the disk — toward the outer edge of the Perseus spiral arm. When it hits, it’ll slam into other clouds of gas and dust. That will squeeze all the clouds, causing them to split into clumps. The clumps then will collapse to form new stars — the equivalent of perhaps a million stars the mass of the Sun.

For now, Smith’s Cloud is in the constellation Aquila, the eagle, which is low in the west-southwest at nightfall. Its brightest star, Altair, marks the left point of the bright Summer Triangle. Smith’s Cloud is so cold and dark that it’s visible only to radio telescopes — to the lower left of Altair.
 

Script by Damond Benningfield

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The bright star Deneb stands high in the west-northwest at nightfall. It represents both the tail of Cygnus, the swan, and the top of the Northern Cross. And if you have a dark sky, you can see that it’s immersed in the glow of the Milky Way — the light of millions of stars that outline the disk of our home galaxy.

Many of those stars are in the same feature of the galaxy as the solar system — the Local Arm. It’s one of several spiral arms that make the Milky Way look like a pinwheel spinning through space.

For a long time, it wasn’t considered an “arm” at all. Instead, it was classified as a “spur” — a short spike sticking off one of the major spiral arms. But over the last couple of years, several studies have found that it’s a lot longer than thought.

One study plotted the locations of hundreds of thousands of stars, plus hundreds of star clusters. The positions came from Gaia, a space telescope that’s mapping more than a billion stars. Another study plotted the positions of “masers” — bright beams of microwaves energized by hot stars.

Both studies found that the Local Arm extends for at least 25,000 light-years. That’s not nearly long enough to loop all the way around the galaxy. And it may not “loop” at all — it may extend straight across the galaxy. Yet it’s clear that the Local Arm is bigger than expected — an impressive neighborhood for the Sun, Deneb, and most of the stars we see in the night sky.
 

Script by Damond Benningfield

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Nine billion years ago, another galaxy rammed into the Milky Way. It was only a tiny fraction the mass of the Milky Way, but it might have had a major impact on our home galaxy.

Remnants of the galaxy were discovered within the last few years. It was given a couple of names: Gaia-Enceladus, and the Sausage, for the shape of its remains.

According to simulations, the galaxy was only about one-and-a-half percent the mass of the Milky Way. As it plunged through the Milky Way’s disk, though, its gravity stirred up the disk. That made the disk a lot “puffier” — it more than doubled in thickness.

At the same time, clouds of gas from the interloper rammed into similar clouds in the Milky Way. The clouds collapsed to give birth to new stars — many millions of them. Those stars congregated along the plane of the disk, while many of the older stars stayed away from that plane. That gave the Milky Way two disks — a thin one within a thicker one — a structure it maintains even today.

Finally, the gravity of the Milky Way ripped Gaia-Enceladus-Sausage apart. Many of the stars of the smaller galaxy formed long ribbons that loop around the center of the Milky Way. The stars belong to the Milky Way’s halo — a thinly populated region that extends hundreds of thousands of light-years in every direction. In fact, they make up a large fraction of the halo’s stars — stars that now belong to the Milky Way.

More about the Milky Way tomorrow.
 

Script by Damond Benningfield

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Our home galaxy, the Milky Way, is ancient. It was born perhaps 13.6 billion years ago — just a couple of hundred million years after the Big Bang. And it’s been growing and changing ever since.

The Milky Way doesn’t have a birth certificate, so astronomers have to estimate its age. They do so mainly by measuring the ages of its stars.

Stars of different ages have slightly different chemical compositions. The oldest stars have relatively small amounts of anything other than hydrogen and helium — elements created in the Big Bang. The amounts of those heavier elements act as a sort of clock, allowing astronomers to figure out when a star was born.

Astronomers also use the “dead” stars known as white dwarfs — the bare cores of once-normal stars. A white dwarf is quite hot when it’s born, but it cools off at a known rate. So measuring a white dwarf’s temperature reveals how long it’s been cooling off — and hence its age.

Using these and other techniques, astronomers have found that the oldest stars in the Milky Way are more than 13 billion years old. But not all parts of the Milky Way are the same age; more about that tomorrow.

In the meantime, if you have dark skies, look for the glowing outline of the Milky Way standing high in the sky this evening. After night falls, it arcs from Aquila, the eagle, in the west-southwest; to Cygnus, the swan, high in the west; to W-shaped Cassiopeia in the northeast.
 

Script by Damond Benningfield

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The heart of the Sun is a toasty 27 million degrees Fahrenheit. At that temperature, atoms of hydrogen ram into each other and stick together to make helium. And they’ll continue to do so for another five billion years.

The cores of some stars are much hotter than the Sun’s, so the nuclear reactions happen at a much faster rate.

The key difference in stars is their mass. A heavier star has a more powerful gravitational pull. That squeezes its core more tightly. And the tighter the squeeze, the higher the temperature.

An example is Spica, the brightest star of Virgo. It rises well below the Moon in the wee hours of tomorrow, but will be closer to the Moon the following couple of days.

Spica consists of two giant stars. The heavier one — Spica A — is 11 or 12 times the mass of the Sun. With all those extra pounds, its core is tens of millions of degrees hotter than the Sun’s. As a result, its fusion reactions take place far more quickly. So Spica A will use up its hydrogen in about 30 million years.

After that, the core will shrink and get even hotter. That will trigger reactions with heavier elements. The star will step through that process several times, until its core consists of iron. At that point, despite a temperature of more than a billion degrees, the reactions will stop. Spica’s core will collapse. Its outer layers will fall in, then rebound, creating a titanic explosion — bringing Spica A to a spectacular end.
 

Script by Damond Benningfield

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