StarDate

Comets are studies in contrast. A comet can be both tiny and huge, dark yet bright, ancient yet new.

A comet started life as a ball of rock and ices left over from the birth of the solar system. It was hurled far from the Sun by the gravity of one of the giant planets. If it gets a nudge from another body — an impact with another iceball, for example, or a gravitational nudge from a passing star — it can fall toward the Sun.

The body of a comet is known as a nucleus, and it’s quite small — most are no more than a few miles in diameter. Its surface is coated with rock, dust, and organic compounds that make it darker than charcoal.

As it approaches the Sun, though, it undergoes a dramatic change. As it warms up, some of its ices vaporize. In turn, that releases grains of solid particles. The gas and dust form a glowing cloud around the nucleus, known as a coma. It can span tens of thousands of miles or more.

Sunlight and solar wind push some of this material away from the nucleus, sculpting a tail that can be millions of miles long.

And that’s when a comet becomes a comet. The name comes from an ancient Greek word that means “wearing long hair” — a reference to the tail. The coma and tail can be quite bright, allowing us to see a comet over a great distance.

And while its nucleus is ancient, a comet can be brand new. Many are making their first trip through the inner solar system — the first appearance of a comet.

More about comets tomorrow.

Script by Damond Benningfield

If you find yourself feeling down over the next few days, it could be caused by the Sun — or the lack of the Sun. That’s because we “fall back” tonight as Daylight Saving Time comes to an end. The Sun will set an hour earlier on our clocks, so we’ll see less evening sunlight. And that could lead to more episodes of depression.

The United States has been using Daylight Saving Time off and on since World War I. Currently, we “spring forward” on the second Sunday of March, and fall back on the first Sunday of November.

The practice has been controversial from the start. Only in the last couple of decades, though, have researchers really probed its effect on human health.

There are good indications that the spring transition — where we lose an hour of sleep — causes problems. Several studies have found a jump in the number of heart attacks, for example. A 2009 study found an increase in the number and severity of injuries among mine workers. And a study in Australia found a jump in the number of male suicides.

“Falling back” seems to cause fewer physical problems, although a Swedish study found jumps in the number of strokes after both spring and fall transitions. But a study in Denmark found that the fall change brings a spike in the number of hospital visits for depression — perhaps as a result of seeing less sunlight in the evening. So experts recommend getting out in the sunlight over the next few days to help adjust to the changing clocks.

Script by Damond Benningfield

A solar eclipse in 1919 made Albert Einstein world famous. So his home country decided to build a scientific instrument worthy of the scientist. Germany created a Sun-watching telescope that was the largest in the world. And it placed the telescope inside a fairy-land structure that’s a masterpiece of architecture.

Einstein’s theory of gravity had predicted that a massive object would bend a passing beam of light. That was confirmed during the eclipse. Astronomers measured stars that appeared near the Sun, and found that they were slightly out of position — their light deflected by the Sun’s gravity.

The new telescope was designed to test other predictions of Einstein’s theory.

The design job was given to Erich Mendelsohn. The young architect produced a tower six stories tall, in a style known as Expressionism. Its contours include curved, inset windows, a flared base, and other delicate touches — topped by a silver dome.

Work on the Einstein Tower began in 1919, and observations began in 1924. A few years later, though, Einstein was out of favor with the new Nazi regime. He left Germany, and his name was stripped from the tower.

The tower was damaged during World War II, but it survived. Einstein’s name was restored. And the telescope went back to work. It didn’t confirm Einstein’s predictions — the measurements were just too tricky. But the telescope still watches the Sun today — work made possible by an eclipse a century ago.

Script by Damond Benningfield

In Greek mythology, few creatures were scarier than the Gorgons. The three sisters had venomous snakes atop their heads, and faces so terrifying that a single glimpse turned the watcher to stone. And it just so happens that they climb high across the sky tonight. There’s no need to avert your eyes, though — the stellar versions are quite beautiful.

They’re in the constellation Perseus, the hero, which is in the northeast at nightfall. The Gorgons are there because Perseus killed one of the three sisters — the mortal Medusa. He later used her head to turn Cetus, the sea monster, to stone.

The constellation’s most famous star is Algol. Its name comes from the Arabic phrase “ras al ghul.” The phrase means “head of the demon,” so Algol is known as the Demon Star. It represents Medusa’s head.

The star got such a fearsome name because for a few hours every three days or so, it gets a good bit fainter — a change that wasn’t understood until the 19th century. Today, we know the dimming is caused by an eclipse between the two stars that make up the Algol system. The eclipse blocks the light of one of the stars.

Three fainter stars to the right of Algol form a small, lopsided box. These stars are the Gorgonea — the Gorgons. Each one represents one of the three sisters. The stars aren’t related, though — they just happen to lie in the same direction in the sky.

So enjoy these four stellar “demons” as you make your way through Halloween night.

Script by Damond Benningfield

The young solar system was like a giant pinball machine. Planets and smaller bodies slammed into each other. Some ricocheted away, some were blasted to bits, and some stuck together, turning two planets into one.

One example of the “sticking together” may be Jupiter, the biggest planet in the solar system. A recent study suggested that it had a head-on collision with another body, resulting in a merger of the two worlds.

The Juno spacecraft has been orbiting Jupiter for three years. It makes precise measurements of Jupiter’s magnetic and gravitational fields. That probes Jupiter’s interior, allowing scientists to learn about the planet’s core.

The observations have been surprising. They suggest that instead of a small, dense ball, the core is wide and fluffy.

An idea for why that’s the case was proposed by a team from China and Rice University. The team ran thousands of simulations of impacts between the newborn Jupiter and another body. The best fit to what Juno has found was a head-on collision with a planet at least 10 times as massive as Earth.

That object mixed with Jupiter’s core, stirring up the core and making it bigger but much less dense. Some of the core material mixed with less-dense material around it. The core is still puffed up today — at the heart of giant Jupiter.

And Jupiter pairs up with the Moon this evening. The planet looks like like a brilliant star. It’s close to the upper left of the Moon as night falls.

Script by Damond Benningfield

Venus is continuing its slow return to view as the “evening star.” It stands to the lower right of the Moon about 30 minutes after sunset. The planet is quite low in the sky, though, so any obstructions along the horizon will block it from view.

The surface of Venus is rugged. Almost all of it consists of volcanic rock. For a while, the leading idea said the entire surface was repaved by a massive outpouring of molten rock several hundred million years ago.

But studies in the last decade or so have presented a more nuanced view of the planet. Different rock formations appear to have been deposited at different times, with some formations lying atop others.

And there’s pretty good evidence that Venus is volcanically active even today. Blocks of rock that are miles wide appear to be shifting around. They look like blocks of ice moving atop an ocean or sea here on Earth. That suggests the blocks are floating atop molten rock not far below the surface.

Orbiting spacecraft have seen “hotspots” on the surface that could be active eruptions. They’ve also “sniffed” elements in the upper atmosphere that probably came from recent volcanic eruptions.

Together, these findings suggest that it took a long time to repave the surface of Venus — perhaps a billion years or longer — and that the process isn’t done yet.

Again, look for Venus close to the Moon shortly after sunset. The brilliant planet will inch its way higher into the evening sky over the coming weeks.

Script by Damond Benningfield

Like Earth, most of the other planets in our solar system have seasons. And also like Earth, the seasons are caused by a planet’s tilt on its axis.

The planet with the oddest tilt is Uranus. It’s the Sun’s third-largest planet — four times the diameter of Earth. When it was young, it was knocked over on its side — perhaps by a collision with a planet as big as Earth. So instead of aiming roughly up and down, Uranus’s poles aim sideways, along the plane of the solar system.

Uranus takes 84 years to orbit the Sun, so each season lasts a quarter of that — 21 years. During northern winter, the north pole aims toward the Sun, so it sees 21 years of sunlight. During southern winter, it’s the south pole that sees constant sunlight. And between those seasons, the Sun passes above the equator, creating spring and fall.

Spring arrived in the planet’s northern hemisphere back in 2007. The season will continue until 2028 — the beginning of the northern winter on this giant planet.

And Uranus is putting in its best showing of the entire year about now. It’s lining up opposite the Sun, so it rises around sunset and remains in view all night. It’s in a fairly empty patch of sky, near the intersection of the constellations Aries, Pisces, and Cetus.

Uranus is brightest for the year, too. Under especially dark skies, in fact, it\'s just visible to the unaided eye. Most of us, though, will need binoculars to find this tilted giant, shining like a tiny blue-green star.

Script by Damond Benningfield

One of the highlights of the autumn sky is M31, the Andromeda Galaxy. It’s the most distant object that’s readily visible to the unaided eye — about two-and-a-half million light-years away.

M31 and our own galaxy, the Milky Way, are near twins.

Both are spiral galaxies. They’re shaped like wide, thin disks. Their hottest, brightest stars outline spiral arms, so from afar, the galaxies look like pinwheels. The Milky Way spans about a hundred thousand light-years, while Andromeda is bigger.

Both galaxies are at least a trillion times as massive as the Sun. Astronomers used to think that Andromeda was the heavier of the two, but recent work seems to give the nod to the Milky Way.

Each galaxy has several hundred billion stars. But most of their mass is in the form of dark matter. It produces no detectable energy. But we know it’s there because it exerts a gravitational pull on the stars and gas around it.

The dark matter is concentrated in vast “halos” that surround the galaxies — ball-shaped volumes of space that contain few stars. Andromeda’s halo may extend a half-million light-years from the galaxy’s heart — a long way toward the Milky Way. Over the eons, the two galaxies will get even closer. Eventually, they’ll merge — one monster galaxy instead of two giant ones.

M31 is high in the east-northeast by the time it gets nice and dark. It looks like a small, fuzzy patch of light, so you need to get away from city lights to see it.

Script by Damond Benningfield

To take an amazing journey through time, look toward the east this evening. The bright orange star Aldebaran is low in the sky around 10 o’clock. It’s about 65 light-years away. That means the light we see from Aldebaran tonight actually left the star about 65 years ago.

The Pleiades star cluster stands above Aldebaran. It’s more than 400 light-years away, so we see it as it looked around the start of the 17th century.

But an object that stands high above Aldebaran and the Pleiades carries us much farther back in time: about two-and-a-half million years — a time when the ancestors of modern humans were developing the first stone tools.

M31, the Andromeda Galaxy, is the most-distant object that’s easily visible to the eye alone — two-and-a-half million light-years away. Under a fairly dark sky, it looks like a faint smudge of light.

M31 is similar to our own galaxy, the Milky Way. It’s a wide, flat disk that contains hundreds of billions of stars. There’s a big bulge of stars in the middle, with two beautiful spiral arms wrapped around it.

The entire disk may span more than 200,000 light-years. But the outer regions of the disk are quite faint, so the part that’s visible to the eye alone is about half that size. So we see the far side of the disk as it looked a hundred-thousand years farther in the past than the near side — a sort of time machine within a time machine for this giant city of stars.

More about M31 tomorrow.

Script by Damond Benningfield

Three billion years ago, a giant space rock rammed into Mars with a “splat.” It plunged into a shallow ocean. That gouged a 75-mile-wide crater. It also created a tsunami that raced hundreds of miles in every direction, leaving an imprint on the coastline.

That’s the conclusion of a new study of a region known as Arabia Terra, a smooth plain in the northern hemisphere. It’s one of several studies in recent months that reinforce the idea that Mars was once much warmer and wetter than it is today.

One of the studies found there was intense runoff from rains all across the planet for its first billion and a half years. After that, the planet cooled enough for the water to freeze. Yet there were warmer periods as recently as a billion years ago. Each of those periods lasted millions of years, and produced more big runoff events across the whole planet. That produced periods where life could have existed on the Martian surface.

Scientists have a tough time explaining why Mars was so warm and wet — especially early on. The young Sun was much cooler than it is today, so there was less energy to warm Mars and the other planets. So the Red Planet’s history is keeping scientists a little befuddled.

And Mars is slowly climbing into view in the early morning. It’s quite low in the east as twilight begins to paint the sky, and looks like a moderately bright orange star. It’ll be easier to pick out tomorrow because it’ll stand just below the crescent Moon.

Script by Damond Benningfield