StarDate

On average, our home galaxy, the Milky Way, gives birth to only a few stars per year. But that’s a mere pittance compared to some galaxies. The most prolific churn out thousands of stars. At that rate, they could double their total number of stars in a few hundred million years.

Starburst galaxies don’t look that much brighter than more sedate galaxies like the Milky Way. But they shine much brighter in the infrared — the glow of warm clouds of dust around the newborn stars.

Such galaxies are quite massive. And they have huge amounts of gas — the raw material for stars. The Milky Way has already used up most of its gas, so it doesn’t have a lot left for making more stars.

Something squeezes big clouds of gas in starburst galaxies, causing them to collapse. The clouds then break apart into smaller clumps, which in turn collapse to form stars.

Extreme starburst galaxies are giving birth to a number of stars that’s equal to thousands of times the mass of the Sun every year. And one galaxy appears to be birthing as many as 30,000 solar masses of stars a year. Stars come in a wide variety of sizes, with most stars less massive than the Sun. So the actual number of stars could be even higher.

So far, astronomers aren’t quite sure what’s causing such pyrotechnics. Most models of how stars form can’t produce such impressive numbers. So theorists still have some work to do to explain these prolific galaxies.

Script by Damond Benningfield

The beautiful Pleiades climbs high across the sky on November evenings. It’s in good view in the east by a couple of hours after sunset. The star cluster forms a tiny dipper.

Because the stars of the Pleiades are spread out, it’s known as an open cluster. Like most of its peers, it’s young, because open clusters normally get torn apart soon after their birth. But there’s a star cluster in the western sky that’s broken all the rules: It has survived for eight billion years.

NGC 6791 is more than 13,000 light-years away, in the constellation Lyra. It’s one of the oldest open clusters in the galaxy.

No one knows the secrets to its longevity. As soon as an open star cluster is born, it faces a hostile galaxy that tries to rip it apart. Massive clouds of gas and dust roam interstellar space, for example, and their gravity pulls stars away from the cluster. Other stars that pass near the cluster also lure away its stars. And so does the overall gravitational pull of the entire galaxy.

Part of the secret of NGC 6791 may be that it was born more massive than most clusters. That extra mass could have given it strong enough gravity to hold itself together. And its orbit carries it thousands of light-years outside the galaxy’s disk, safely away from the clouds of gas and dust that try to pull the cluster apart.

Whatever the reasons, NGC 6791 has managed to cheat death for longer than most other clusters in the galaxy.

Script by Ken Croswell

A brilliant comet streaked across the sky four centuries ago. It brought consternation to many — but a poetic plea to not worry about it from the king of England.

Neither the king nor anyone else saw the comet coming. It skirted the Sun on November 8th, 1618, passing about as close to the Sun as the planet Mercury does. But it wasn’t spotted for another eight days, when observers saw its tail in the pre-dawn sky. Eventually, the tail stretched all the way from Libra, in the southern sky, to the Big Dipper, in the far-northern sky.

The red-hued comet passed by Earth in early December. It grew so bright that it was visible even during the day. Because of its color, the length of its tail, and its long run in the sky, it was called “the Angry Star.”

Today, we know that comets are blobs of frozen water and gases mixed with rock and dirt. As the Great Comet of 1618 approached the Sun, some of its ice vaporized, producing the glorious tail.

400 years ago, though, no one knew how comets worked. At the time, many thought of them as omens of evil. But King James I penned a poem to reassure his people. Part of it went like this:

“Misinterpret not with vain conceit, / The character you see of Heaven’s height: / Which, though it bring the World some news from fate, / The letter is such as none can it translate.”

In other words, we don’t really know what it means, so don’t worry about it — just enjoy the show.

Script by Ken Croswell and Damond Benningfield

With a little imagination, you can find geometric figures just about anywhere in the night sky. An example is a right triangle of stars in Perseus and Andromeda. The constellations are high in the east and northeast as darkness falls.

At the top of the triangle is Almach, Andromeda’s third-brightest star. Through a small telescope, it looks like two stars — one of the most beautiful double stars in the night sky. One of them is yellow-orange, while the other is blue.

The blue star is a system of three stars on its own. Two of them orbit each other, while the pair of them share an orbit with star number three.

The second point of the triangle is directly below Almach in early evening. And it, too, is complicated.

Algol consists of three stars. Two of them line up in such a way that they eclipse each other. The fainter star passes in front of the brighter one every three days or so. When that happens, Algol fades to about a third of its normal brightness. Early skywatchers found that a bit spooky, so they gave the star a spooky name: Algol means “the demon.”

The third and brightest member of the triangle is Mirfak, to the left of Algol. As far as anyone knows, it’s a single star. But it’s an impressive one. It’s about eight times the mass of the Sun, and thousands of times the Sun’s brightness. So it’s an easy target, even though it’s farther than the other members of the triangle — about 500 light-years from Earth.

Script by Damond Benningfield

A quiet but steady meteor shower should be at its peak over the next night or two. And the gibbous Moon sets by around 1 or 2 a.m., so it won’t spoil the display. At its best, you might see a dozen or more “shooting stars” per hour.

The shower is known as the Leonids. That’s because its meteors appear to “rain” into the sky from Leo, the lion, which climbs into view in the wee hours of the morning.

There’s no relation between the meteors and the constellation, though. Instead, the meteors are the offspring of a comet.

Comet Tempel-Tuttle was named for the two men who discovered it, in 1865. Not long after the discovery, several astronomers noted a correlation between the orbit of the comet and the Leonid meteor shower. They realized that the comet must somehow spawn the shower.

And they were right. A comet is a ball of frozen water and gases mixed with bits of rock and dirt. When it gets close to the Sun, some of the ice vaporizes, releasing solid particles. When Earth crosses the comet’s path, some of the particles ram into the atmosphere, forming the glowing streaks known as meteors.

The debris in the comet’s path is clumpy, though, so the meteors aren’t evenly distributed. Some experts say we could pass through several clumps over the next few nights, keeping the Leonids going for a while. So if you have a safe, dark viewing spot, it’s worth looking for the Leonid meteors — the offspring of a comet.

Script by Damond Benningfield

The most prominent features on Mars are its polar ice caps. They’re easily visible through any telescope — small white splotches against the orange landscape.

The ice caps undergo major changes over the course of a Martian year. And as they change, so does the Martian atmosphere.

Both ice caps are made primarily of frozen water. The northern cap is a lot wider than the southern one, but it’s not as thick. So they contain about the same amount of water — each one holds roughly half as much water as the ice cap that covers Greenland.

During winter, the caps grow thicker. That’s because they get so cold that carbon dioxide in the atmosphere freezes atop them — a quarter or more of the total atmosphere. During spring and summer, the CO2 vaporizes and returns to the atmosphere. That can stir up big dust storms, like the one that covered much of Mars earlier this year.

All of the carbon dioxide vanishes from the northern cap. But the southern cap is at a much higher elevation, so it stays a lot colder. So it retains a thin layer of carbon dioxide all year.

Both ice caps consist of layers of ice mixed with dust. Probing those layers can reveal the planet’s geologic history. Radar aboard a Mars orbiter, in fact, confirmed that an ice age gripped the planet about 400,000 years ago — making the ice caps even more prominent.

Look for Mars quite close to the upper left of the Moon this evening. It looks like a bright orange star.

Script by Damond Benningfield

When a Sun-like star dies, it stages its own wake — a bright, colorful display that can last for tens of thousands of years.

A star that’s in the middle of that end-of-life display is in Aquarius, which is in the south at nightfall. The star has created the Helix Nebula, a double ring that looks like a giant eye. It’s been going for more than 10,000 years.

The star that created the Helix originally was a little bigger and more massive than the Sun. When it consumed all the nuclear fuel in its core, the nuclear reactions in the core shut down. The star then blew its outer layers of gas into space, creating the nebula. Radiation from the exposed core causes the nebula’s gas to glow.

The Helix is actually shaped like a barrel that’s tilted a bit, and we’re looking down through the center of it. So one of the two rings we see is the top of the barrel, while the other is the bottom.

The barrel spans two or three light-years. And its “wall” contains thousands of dense knots of dust. They’re being eroded by energy from the star’s core, so they look like long tadpoles, with the tails facing outward.

The core soon will settle down into its final phase of life: a hot but faint white dwarf, shining feebly into the cosmic night.

The Helix is visible through binoculars or a telescope. Look for it about half way between the orange planet Mars, which is in the south at nightfall, and the bright star Fomalhaut, to the lower left of Mars.

Script by Damond Benningfield

The brilliant “morning star” has a bright companion for the next few days. Tomorrow, they’ll be separated by only about one degree — less than the width of your finger held at arm’s length. And they’ll stay within a finger-width of each other for about a week.

The morning star is Venus, our closest planetary neighbor. It’s low in the east-southeast at first light. It’s the brightest object in the night sky other than the Moon, so you can’t miss it.

Its companion is the star Spica, the leading light of the constellation Virgo. It’s among the 20 brightest stars visible from Earth. Yet it’s only about a half a percent as bright as Venus.

The difference is a matter of distance. Venus is our closest planetary neighbor — just a few light-minutes away. It shines by reflecting sunlight into space.

Spica, on the other hand, is a true star — and a truly impressive one at that. It’s far bigger, heavier, and brighter than the Sun. That makes it easy to see even though it’s about 250 light-years from Earth.

Millions of years from now, Spica will shine even brighter. It’ll explode as a supernova. For a few weeks, it’ll outshine billions of normal stars. As seen from Earth, it’ll even outshine Venus — making it visible even in the daytime sky.

For now, look for Spica to the upper right of Venus beginning a couple of hours before sunrise. They’ll stay close for a while, before Venus begins to slide away from its bright companion.

Script by Damond Benningfield

Pluto isn’t an easy target for skywatching. It’s so small, remote, and faint that it’s visible only through a good-sized telescope. Today, though, you can get a good idea of its location in the sky because the Moon will pass quite close to it. In fact, as seen from some parts of the globe, the Moon will pass in front of Pluto, briefly blocking it from view.

Pluto is a fascinating world. Although it’s only about two-thirds the diameter of the Moon, it has a dynamic surface. Massive glaciers flow around ice mountains, and a possible volcano may belch ice. And Pluto has a thin atmosphere that supports thin clouds.

Some of the nitrogen from Pluto’s atmosphere may have escaped and coated the north polar region of Pluto’s largest moon, Charon, giving it a red “beanie cap.” Charon and Pluto’s four smaller moons probably formed when Pluto was smacked by another large body, blasting debris into space.

Pluto is passing through Sagittarius, which crosses the daytime sky at this time of year. And today, the crescent Moon is there, too.

The Moon will climb into good view in the southeast in early afternoon. It’s so faint, though, that you’ll need to look carefully to find it. For most American skywatchers, Pluto will stand just to the upper right of the Moon as they rise, and a little farther to the lower right of the Moon after night falls. But you’ll need a good telescope to pluck the little world from the starry background.

Script by Damond Benningfield

We don’t know whether anyone will ever establish an outpost on the Moon. If they do, though, the residents will need a source of supplies, including water, oxygen, rocket fuel, and many others. And they may be able to get some of those things from the Moon itself. They’re warehoused at the Moon’s poles — in big deposits of frozen water.

The first evidence of that ice was obtained a quarter century ago, by a spacecraft in lunar orbit. And the strongest evidence yet was obtained by another orbiter, and reported earlier this year.

Scientists watched the poles with a NASA instrument aboard an Indian spacecraft. The craft entered orbit around the Moon 10 years ago this week, and operated for about nine months.

A recent analysis of its observations found strong evidence of water ice at the poles. Most of it sits inside craters that never see the Sun. Temperatures in the bottoms of craters studied by the mission stay at minus-250 degrees Fahrenheit or colder. So the ice could have been there for billions of years.

Colonists could melt the ice to provide drinking water. And they could separate its components to provide oxygen and rocket fuel — “living off the land” on the surface of the Moon.

The Moon is a beautiful crescent this evening. It’s low in the southwest at nightfall. The planet Saturn stands close to the lower right of the Moon, and looks like a bright golden star. They set a few hours after sunset.

Script by Damond Benningfield