StarDate

The clouds here on Earth frequently form arcs and spirals — patterns related to our planet’s rotation on its axis. The clouds on Jupiter are related to its rotation as well: Because Jupiter spins so fast, the clouds are stretched into bands that encircle the entire planet.

The bands show different colors, which means their clouds are made of different materials and hover at different altitudes. The highest clouds are white, and are made of ammonia. Clouds in the next layer contain ammonia mixed with other chemicals, so they look brown or orange. The lowest layers contain water vapor and water ice, so they look blue.

The cloud bands form alternating light and dark stripes. The lighter stripes are decks of clouds that are pushed high into the atmosphere by rising bubbles of warm gas. The darker stripes consist of cooler material that’s dropping back down into the atmosphere.

Each zone has its own weather, which draws much of its energy not from the Sun, as is the case on Earth, but from deep within Jupiter itself. As gravity squeezes the planet, it produces heat that wells up through the atmosphere, then escapes into space. Before it escapes, though, it helps drive giant thunderstorms, and hurricanes that can be bigger than Earth — giant weather for a giant planet.

And Jupiter looks like a brilliant star. It’s low in the east at nightfall, climbs high across the south at midnight and sets around sunrise. More about Jupiter tomorrow.

Script by Damond Benningfield

The king of the planets is putting in an appearance that befits its status right now. It’s at a point known as opposition. It lines up opposite the Sun in our sky, so it rises around sunset and is in view all night. The planet is also closest to Earth for the year, so it shines brightest.

Jupiter is the biggest planet in the entire solar system. It’s about 11 times Earth’s diameter, and more than 300 times its mass. In fact, it’s more massive than all the other planets and moons in the solar system combined.

Jupiter probably grew so big through a two-step formation process.

In the first step, small bits of rock and ice stuck together to form larger and larger clumps. Jupiter was far enough out that solar heat couldn’t vaporize the ices and some other solid materials that encircled the Sun. That allowed the growing clumps to get even bigger, and many of them merged to form what became Jupiter’s core.

The core quickly reached several times the mass of Earth. As it did so, the second step kicked in. The gravity of the big, heavy proto-Jupiter pulled in vast amounts of gas left over from the Sun’s birth. That gave the planet a thick envelope of hydrogen and helium around the core — and helped Jupiter grow to giant proportions.

Look for this massive planet all night long. It’s low in the east as night falls and arcs high across the south around midnight. It looks like a brilliant cream-colored star, so it’s hard to miss.

More about Jupiter tomorrow.

Script by Damond Benningfield

The morning sky is about to lose its guiding light. Venus, the “morning star,” soon will pass behind the Sun. It’s moving closer to the Sun, so it’s getting lost in the Sun’s glare. It won’t return to view until July — this time in the evening sky.

Because of the relative orbits of Venus and Earth, Venus traces the same path across the sky every eight years. In other words, any time you see Venus, you can know that it stood at the same spot in the sky eight years earlier, and will appear in that spot once again in eight years more.

Over those eight years, Venus makes five appearances in both morning and evening skies. And each of those appearances is almost identical to the one eight years before.

Venus’s transition from morning to evening sky takes longer than the other way around, because it happens when Venus is on the opposite side of the Sun. So even though it’s moving at a steady orbital speed, it takes the planet longer to cover a given angle on the sky as seen from Earth. So when it disappears in the morning twilight, it stays out of sight for a long time.

Right now, Venus is quite low in the eastern sky as twilight grows bright. Tomorrow, it’s close to the right of the crescent Moon. You need a clear horizon to spot them. And skywatchers in the far-southern latitudes of the United States will have a better view than those in the north, where Venus is already tough to spot as it closes another cycle as the “morning star.”

Script by Damond Benningfield

Encounters with comets are fairly common these days. Several spacecraft have flown near them, and a European craft has been orbiting one for almost a year and a half.

The first encounter with a comet was an all-out assault. Five probes flew past the comet over a period of a week or so. The first one made its closest approach 30 years ago tomorrow.

Comet Halley was flying through the inner solar system. It’s probably the most famous comet of all. It’s been immortalized in works of art, and it’s inspired fear during many of its encounters.

Among scientists, though, the comet inspired great excitement. And its passage near Earth presented a great opportunity to examine a comet up close.

So several space agencies launched missions to study it. The first to reach the comet was Vega 1, the first of two Soviet craft.

Vega had dropped off a probe to study Venus. Then, on March 6th, 1986, it flew through Halley’s tail, about 6,000 miles from the comet’s nucleus. Vega 2 followed the next day, with twin Japanese craft right behind.

The champ of the Halley fleet was the European mission, Giotto. It flew just a few hundred miles from the nucleus, where it was pelted by grains of comet dust. It snapped pictures of the nucleus, and measured the composition of the gas and dust streaming from its surface.

The combined fleet provided the most detailed look at a comet to that time — the first encounters with an icy visitor from deep space.

Script by Damond Benningfield

A black hole in the center of a small galaxy appears to be pushing gas and dust away from it. That stops the black hole from growing, while potentially giving birth to new stars.

NGC 5195 is a companion to the Whirlpool galaxy, a beautiful spiral near the end of the Big Dipper’s handle. But NGC 5195 itself is a galactic knot — a small, dense clump of stars surrounded by a lot of dust. It has swept past the Whirlpool, perhaps creating waves that sculpted the galaxy’s beautiful appearance.

That encounter also may have sent clouds of gas and dust falling toward the giant black hole at the heart of NGC 5195. As the material collapsed around the black hole, it was heated to many millions of degrees. Radiation and winds from this hot gas then began pushing back at the infalling gas.

Observations by telescopes on the ground and in space recently revealed two “arcs” of gas around the black hole. One arc is quite hot, while the other is cool. The hot arc appears to be pushing at the cool one — in essence, “snowplowing” gas out of the galactic center.

The discovery isn’t all that surprising. Astronomers have long suspected that galaxies and their central black holes affect each other’s evolution. The black hole pulls in gas and dust, triggering the birth of new stars. But as the black hole grows larger, the material around it gets so hot that it pushes away material that’s farther out — shutting off the process of starbirth.

Script by Damond Benningfield

One of the most beautiful spiral galaxies spins high across the north tonight. It’s known as M51 — the Whirlpool. It has nearly perfect spiral arms, punctuated by bright stellar nurseries. And the cause of both stands at the end of one of those arms — a smaller galaxy that appears to have stirred up its bigger companion.

The Whirlpool is about 25 million to 30 million light-years away. It’s a little smaller than our own galaxy, the Milky Way.

Its companion looks like a fuzzy blob. It probably flew past M51, creating waves within the bigger galaxy’s disk. The waves spin around the galactic center, creating the spiral arms.

Although they’re brighter than their surroundings, the arms don’t really contain any more stuff than the rest of the galaxy. Instead, the waves that create the arms compress clouds of gas and dust, causing them to give birth to new stars. Many of these stars are hot and bright, so they make the spiral arms look bright.

These bright young stars don’t last long, though — they explode after a few million years. In fact, three stellar explosions have been seen over the last couple of decades — compared to none in the bigger Milky Way. Even as they die, though, these stars are replaced by new ones — the progeny of a galactic encounter.

M51 stands close to the end of the handle of the Big Dipper, which is low in the northeast in early evening and wheels high overhead later on.

More about M51’s companion tomorrow.

Script by Damond Benningfield

Hundreds of millions of years ago, a galactic bowling ball rolled through the edge of the Milky Way galaxy. It scattered stars like bowling pins, triggered the birth of new stars, and created waves in the galaxy’s disk that are still rippling today.

That’s the picture outlined by Sukanya Chakrabarti, an astronomer at the Rochester Institute of Technology. She used the ripples in the Milky Way to trace their origin. She also predicted where the disrupting galaxy would be today — then found evidence of the galaxy itself.

Chakrabarti was using a new technique, known as galactoseismology. Just as geologists use earthquakes to probe Earth’s interior, astronomers can use “galaxy quakes” to learn about galaxies.

In this case, Chakrabarti traced the possible origin of ripples in the Milky Way’s disk. Her simulations showed that they were caused by a collision with a small but heavy galaxy. The simulations showed that today, that galaxy should be about 300,000 light-years away.

And last year, she discovered a possible culprit, just where it was expected. She found three pulsating stars inside a dwarf galaxy. The way the stars pulse reveals the distance to the galaxy. The galaxy is small and faint, but it appears to contain a lot of “dark matter,” so it’s heavy. Additional observations should reveal more about the galaxy — perhaps confirming that it triggered a “quake” in the Milky Way a long time ago.

We’ll talk about an even bigger galaxy quake tomorrow.

Script by Damond Benningfield

In all the universe, we know of exactly one world with life: our own planet Earth. Yet we’re finding more and more possible homes for life all the time. Many of them are planets in other solar systems. But some of them are right here in our own solar system. In fact, two of them orbit the planet Saturn, which huddles near the Moon late tonight.

Enceladus and Titan are among Saturn’s most intriguing moons. Both appear to have all the ingredients for life.

Enceladus, for example, has a possible ocean of liquid water beneath its icy crust. The water stays liquid because the moon’s interior is heated by Saturn’s gravitational pull. And it contains chemical elements that are important for life. So this cold little moon has everything needed for life as we know it: water, energy, and the right chemistry.

Titan is much bigger, but it has those same ingredients. It may have a subsurface ocean that contains far more water than all the oceans on Earth. Its surface is awash in organic compounds — the chemistry of life. And like Enceladus, its ocean is kept warm by the heat generated by Saturn’s pull.

Of course, no one has spotted life on these moons or anywhere else. And we don’t know if life would form on other worlds even under the best of conditions. All we know is that the conditions for life can be found right here in own solar system.

Look for Saturn close to the right of our moon at first light tomorrow. It looks like a bright golden star.

Script by Damond Benningfield

Today marks an oddity in the calendar that comes around once every 1,461 days: Leap Day, a day that’s thrown in to keep the calendar in sync with the seasons.

The concept of a leap day is as old as the calendar itself. The ancient Roman calendar, which was based on the motions of the Moon, threw in leap days, weeks, and months to make things balance. After centuries of such additions, though, the calendar was a mess. So Julius Caesar ordered the creation of a new one in the year 46 B.C.

The resulting Julian calendar consisted of a 365-day year, with a leap day added to every fourth year. That created an average of 365-and-a-quarter days per year. The true year, however, is about 11 minutes shorter than that, so the Julian calendar gradually drifted out of alignment with the seasons.

To correct that problem, in 1582 Pope Gregory XIII instituted a small update. Under the Gregorian calendar, three leap days were dropped for every 400 years. With this change, the difference between the calendar year and the true year adds up to just one day in 3300 years.

Incidentally, the name “leap year” comes from the fact that the extra day causes succeeding days to leap over a day of the week. For example, Independence Day fell on a Friday in 2014 and a Saturday in 2015. This year, however, the date leaps over Sunday and falls on Monday — a leap that helps keep the calendar aligned with Earth’s motion around the Sun.

Script by Damond Benningfield

Leap Day gets off to a beautiful start tomorrow, with a close encounter between the Moon and Mars. The planet looks like a bright orange star. It’s below the Moon as they climb into good view around 1 or 2 a.m., and closer to the lower left of the Moon at first light.

Mars is growing brighter in a hurry right now. That’s because Earth is closing fast on its neighbor world, thanks to our smaller, faster orbit around the Sun.

The two planets are like a pair of thoroughbreds locked in a perpetual race. Earth is the third planet out from the Sun, while Mars is the fourth. Since Earth has the inside lane, it travels a shorter distance around the Sun. And thanks to the laws of planetary motion, closer planets move faster than those that are farther out. So every couple of years, we overtake Mars.

As we do so, the distance to the Red Planet decreases. We’ll be closest in about three months, at just 47 million miles. And as the range decreases, Mars grows brighter. A month from now, it’ll be twice as bright as it is now. And when Earth and Mars are at their closest, the planet will shine almost 10 times brighter than now — twice as bright as the brightest true star in the night sky.

So keep an eye on Mars as we gallop closer to the planet over the next few months. Not only will it grow brighter, but it’ll rise a little earlier each night — putting in a better and better showing deep into spring.

We’ll talk more about Leap Day tomorrow.

Script by Damond Benningfield