StarDate

The crescent Moon starts the week as the centerpiece of a beautiful morning lineup tomorrow. It’s flanked by four pinpoints of light — two above, and two below. The ones below it are a bit tough to find, but the ones above should be easy targets.

The brightest of the Moon’s companions is Venus, the dazzling “morning star.” It rises a few hours before the Sun, so it’s in good view by dawn’s early light.

Although it looks like a star, Venus is really a planet — our closest planetary neighbor. Thanks to the combination of its distance and our viewing angle, it’s at its faintest — it’s only half as bright as it is at its peak. It won’t start brightening until the end of the year.

The true star Regulus stands below Venus. It’s only about one percent as bright as Venus. But it’s one of the night sky’s leading lights, so you won’t have any trouble finding it.

Things get trickier when you look below the Moon. The planet Mercury is to the lower left of the Moon as twilight begins to paint the sky. And it’s pretty bright — only the Moon and Venus outshine it. But it’s so low in the sky, and so immersed in the waxing twilight, that you need a clear horizon to pick it out.

And Mercury has a close companion that’s especially hard to find. Mars is only about a tenth as bright as Mercury, so it’s a tough target. But it lines up between Mercury and the Moon, which can help you find it — the faintest member of a beautiful morning lineup.

Script by Damond Benningfield

Venus is a world you probably wouldn’t want to visit. When the solar system was young, though, it could have been the most hospitable of all the planets — a world with oceans and a comfortable atmosphere.

Today, Venus’s atmosphere is made mainly of carbon dioxide, with surface pressure more than 90 times greater than on Earth. And the surface temperature is more than 850 degrees Fahrenheit.

But a study released last year found that conditions on Venus could have been relatively comfortable for a long time.

Researchers looked at Venus’s atmosphere, its landscape, and its rotation on its axis. And they used models of climate behavior and other factors.

When they ran the numbers, they found that shallow oceans could have covered much of the surface for up to two billion years. And air temperatures could have remained comfortable through most of that period.

Eventually, though, all that came to an end. As the Sun aged, it got brighter, which bombarded Venus with more energy. The oceans evaporated, and water molecules were split apart by solar radiation. The water’s hydrogen escaped into space, while the oxygen combined with carbon to make carbon dioxide. That created a runaway greenhouse effect — transforming Venus from paradise to purgatory.

Even so, Venus is still beautiful to look at. It’s the brilliant “morning star.” Tomorrow, it’s close to the lower left of the Moon at dawn — a beautiful world that might have had a beautiful past.

Script by Damond Benningfield

While one European spacecraft steps closer to Mars, scientists are narrowing down the list of landing sites for a craft that will step across Mars.

ExoMars Trace Gas Orbiter arrived at Mars last October. But the trip from Earth was just the start of its journey. As planned, it entered a lopsided orbit that’s too high to do its work. So engineers are lowering its orbit. They’re doing so by having the craft drop deeper into Mars’s atmosphere on each lap. That creates drag that pulls the high point of its orbit closer to Mars.

Trace Gas Orbiter will complete the maneuvers in about six months. After that, it’ll start “sniffing” the atmosphere for methane and other compounds that could be produced by living organisms.

In the meantime, scientists are studying landing sites for a rover that will arrive at Mars in 2021. They’ve narrowed it down to two possibilities. One is in a low plain that was filled with water billions of years ago. The other is near the mouth of a valley that may have been filled with water for hundreds of millions of years. Scientists will pick the winning site a year before launch.

And Mars is climbing into view in the dawn twilight right now. It’s quite low in the east, and it’s not very bright yet. But it’s near some brighter bodies. Venus, the “morning star,” is high above it, with the star Regulus a little below Venus. The bright planet Mercury is well below Regulus, with fainter Mars just a whisker away from Mercury.

Script by Damond Benningfield

The end is near for the Cassini spacecraft. After 13 years of exploring Saturn and its moons and rings, the craft will end its mission tomorrow with a fatal plunge into Saturn’s atmosphere.

Cassini arrived at Saturn in the summer of 2004. Since then, it’s orbited the planet almost 300 times. It’s discovered lightning crackling inside giant storms, found lakes of liquid methane and ethane on the big moon Titan, and seen geysers of water and ice erupting from the moon Enceladus.

But it’s about out of propellants for its rocket engine, so engineers won’t be able to control where it’s going. Scientists don’t want it to crash into Titan or Enceladus, though, because the moons are considered possible homes for life. No one wants want to run the risk — however minuscule — that Cassini could carry microbes from Earth that could contaminate those worlds.

So for the last few months, Cassini has been setting up for its demise. It’s been following a path that carried it especially close to Saturn — between its clouds and the inner edge of its rings.

And tomorrow, its final orbit will carry it into Saturn’s atmosphere. Cassini will transmit its observations for as long as possible. Eventually, though, the plunge will rip the craft apart. And its remains will burn up, forming bright meteors in Saturn’s sky.

That will end one of the most impressive space missions in history — a grand tour of Saturn and its entourage of moons and rings.

Script by Damond Benningfield

 

Lakes and seas filled with liquid hydrocarbons dot the surface of Titan, the largest moon of Saturn. Geysers of water squirt from the south pole of a smaller moon, Enceladus. And lightning sometimes crackles through the clouds of Saturn itself.

Those are among the many discoveries made by Cassini, a spacecraft that entered orbit around Saturn more than 13 years ago. When it arrived, Saturn was like a neighbor you’ve never met — you see them in the front yard or on the street, but you don’t know much about them. But Cassini gave us quite an introduction — it turned that giant but distant neighbor into a well-known friend.

Cassini watched as small moons created waves in Saturn’s rings, like the wake of a boat speeding across a still lake. In 2011, it kept an eye on a giant storm that eventually spread all the way around Saturn’s northern hemisphere. And it discovered a ridge on Iapetus that makes the moon look a bit like a walnut.

Cassini also dropped a robotic probe into the cold, dense atmosphere of Titan. The atmosphere is topped by an orange haze, so Titan’s surface had been hidden from view. The probe photographed dry river channels and flat pebbles of ice on the surface. And Cassini itself used radar and other instruments to map the surface, revealing not only the lakes and seas, but giant dunes, an ice volcano, and many other amazing features.

Cassini’s time is up, though, and we’ll talk about its final act tomorrow.

Script by Damond Benningfield

The Sun outshines most other stars, but that won’t always be the case. Billions of years from now the nuclear reactions in its core will shut down. The Sun will then expel its outer layers into space. All that will remain will be the Sun’s dead core, known as a white dwarf — a faint cosmic cinder.

We can get a glimpse of that future by looking at a white dwarf that was discovered 100 years ago. It’s the closest single white dwarf yet seen.

The star is named for the astronomer who discovered it: Adriaan van Maanen, who worked at Mount Wilson Observatory in California. Yet the discovery was an accident. Van Maanen was actually examining another star. He was comparing photographic plates taken in 1917 with some taken three years earlier. He noticed a faint star in the constellation Pisces that had moved quite a bit during the interval — a sign that the star was close by.

To confirm that, Van Maanen and other astronomers measured its distance. And they found that, as expected, it’s one of our nearest neighbors — just 14 light-years from Earth. But the star is so faint you that need a telescope to see it; the Sun emits more light in 24 hours than this feeble star does in 15 years.

Only two other white dwarfs are closer to Earth. But both of them are companions to much brighter stars. In contrast, Van Maanen’s Star is all alone and fading from view — just as the Sun will billions of years from now.

Tomorrow: a final look at a giant planet.

Script by Ken Croswell, Copyright 2017

 

You can’t tell it just by looking, but our solar system is orbiting the center of the Milky Way galaxy at about half a million miles per hour. And the stars around us are moving at similar speeds. In fact, one of the first stars to have its motion measured stands quite close to the Moon tonight.

Aldebaran is the leading light of Taurus, the bull. It’s just an eyelash away from the Moon as they climb into view around midnight, and even closer to the Moon at first light. In fact, as seen from parts of the western United States, the Moon will pass in front of Aldebaran, blocking its light — an event known as an occultation.

Three centuries ago, Edmond Halley studied an occultation of Aldebaran that happened in the year 509. And he got a surprise: He found that, over the centuries, Aldebaran had shifted position relative to the other stars. Studies of two other stars showed that they, too, had moved.

That change in a star’s position is known as proper motion. It’s caused by a combination of motions — ours, and the other star’s.

Aldebaran is fairly close — just 65 light-years away — so its motion is more pronounced than that of many other visible stars, which are farther away. Since the occultation of 509, in fact, Aldebaran has moved roughly a quarter the diameter of the Moon in our sky. It’s a tiny shift. But it adds up. Over the millennia, Aldebaran will drift slowly southward — depriving the bull of his bright eye.

Script by Damond Benningfield

 

Hubble Space Telescope may get all the attention, but it’s not an only child. It’s one of four space telescopes built to study the universe at different wavelengths: the Great Observatories. One of them was named for a physicist who was born 125 years ago today.

Arthur Holly Compton was born in Ohio in 1892. He developed an early interest in science, and earned his PhD in physics from Princeton.

Compton joined the faculty of the University of Chicago, where he studied X-rays. His experiments proved that X-rays and other forms of light can behave like solid particles. It was an astounding discovery — and it earned him a Nobel Prize.

Compton also studied cosmic rays — mysterious radiation from outside Earth. His work showed that the “rays” are actually solid particles with an electric charge, so they interact with Earth’s magnetic field.

During World War II, Compton was a key player in the development of the atomic bomb. He oversaw the construction of the first nuclear reactor, and led the creation of plutonium-fueled bombs.

Decades later, when NASA built a Great Observatory to study gamma rays, its detectors took advantage of Compton’s Nobel-winning discovery. So the agency named the telescope in his honor. And for a decade, scientists studied some of the most powerful objects in the universe with Compton Gamma-Ray Observatory.

Script by Damond Benningfield

 

In the last couple of decades, astronomers have discovered more than 50 moons of Jupiter. All of them were found in pictures of the giant planet. In fact, the last moon of Jupiter or any other planet discovered without the aid of photography was found 125 years ago tonight.

Edward Emerson Barnard was one of the leading observers of his age. And he was observing the sky with one of the leading tools of the age: a 36-inch telescope at Lick Observatory in California.

Barnard was a junior member of the staff, so he hadn’t been allowed to use the telescope until just a couple of months earlier. And even then, he had it for just one night a week. But he made the most of that time. In August of 1892, for example, he discovered a cloud of gas around a bright “new” star known as a nova. From that, he became the first astronomer to deduce that a nova is a type of stellar explosion.

On the night of September 9th, Barnard saw a small “star” near Jupiter. He thought it was a moon — the first discovered since 1610. The following night, he borrowed the telescope from a colleague to confirm the discovery. The odd little moon was named Amalthea after a character from Greek mythology. It was the last moon discovered just by looking through a telescope.

About that time, Barnard started taking photographs through the telescope. And just weeks after he found Amalthea, he discovered a comet in one of his pictures — the first comet ever discovered with photography.

Script by Damond Benningfield

 

Stars are born when giant clouds of gas and dust break apart and collapse. And if that’s all there was to it, the Milky Way galaxy would give birth to a couple of hundred stars every year. Instead, thanks to feedback from the stars themselves, it makes only a few.

Feedback is a process that clears away the material for making more stars, but can also trigger the birth of more stars.

Very young stars, for example, produce strong winds and jets that blow away the gas and dust around them. Since stars are born in clusters, a lot of young stars can be sweeping away the star-making material at the same time. That greatly reduces the number of stars that can be born in a cluster.

Mature stars add to the feedback — not only with winds, but also with radiation. Hot stars generate a lot of ultraviolet radiation, which vaporizes tiny particles of dust — eliminating possible building blocks for new stars.

The heaviest stars explode as supernovae. These blasts can clear out the space for light-years around a supernova, creating big, empty bubbles. And supernovae also accelerate subatomic particles around them to almost the speed of light. These “cosmic rays” help to sweep away the raw materials for making more stars.

But supernovae can also enhance the stellar birth rate. Their blast waves can cause distant clouds of gas and dust to collapse to form stars. So feedback is a complex process — one that both aids and hinders the birth of new stars.

Script by Damond Benningfield