StarDate

An embryonic star may be about to vanish – perhaps for a century. It’s not going anywhere. Instead, it’ll be cloaked by a dense cloud that encircles two companions.

T Tauri is the prototype for a class of proto-stars. The gravity of such a star is causing it to collapse, making it hot and bright. But its core isn’t hot enough to ignite the fires of nuclear fusion, so it’s not yet a true star.

The star we see as T Tauri is about twice as massive as the Sun. It’s encircled by a disk of gas and dust – the raw materials for making planets. And it might already have given birth to at least one planet.

T Tauri is a member of a triple-star system. Its companion stars are close together, encircled by their own disk. It’s so thick that it hides the stars at visible wavelengths – we see them only in the infrared.

Now, the companions and their disk are starting to slide between us and the brighter star. The star has faded a good bit in recent years. Eventually, it may be hidden behind the disk as well. And it could take a century for the disk to move out of the way – allowing the brightest star of the T Tauri system to shine through once more.

Taurus is low in the east and southeast by late evening. T Tauri is just above Aldebaran, the bull’s brightest star, far to the lower left of the bright Moon. The young star is visible through a telescope – for now.

More about the Moon and the bull tomorrow.

Script by Damond Benningfield

The bull is charging into the evening sky. Taurus is in full view by about 11 o’clock, low in the east. He stands high in the south before dawn. He’s rising earlier each night, and will be in view all night long by about Thanksgiving.

All the stars rise four minutes earlier each night – a result of Earth’s motion around the Sun. Earth makes one full turn on its axis against the background of distant stars every 23 hours and 56 minutes. So, if you looked at the sky every 23 hours and 56 minutes, and you could see through the daytime glare, you’d always see the same stars in the same position.

But during that period, Earth moves along its orbit around the Sun. The distance it covers means the planet has to turn four extra minutes for the Sun to reach the same position in the sky. That makes a day 24 hours long. And it also means that the background stars rise and set four minutes earlier on our 24-hour clock.

As a result, every star and constellation is in prime evening view at different times of the year. For Taurus, it’s fall and early winter – the time the bull charges across the evening sky.

For now, look for Taurus beginning in late evening. Its brightest star is Aldebaran, the bull’s eye. His face is outlined by a V-shaped pattern of stars to the upper right of Aldebaran. And his shoulder is the sparkly little Pleiades star cluster, well above Aldebaran.

More about Taurus tomorrow.

Script by Damond Benningfield

The Moon is full tonight, and it’s especially bright as well. And to top things off, it’s the most famous full Moon of them all – the Harvest Moon.

Harvest Moon is the full Moon closest to the fall equinox, so most years it falls in September. But once every five years or so it skips into October. This year, September’s full Moon came 15 days and 10 minutes before the equinox, which took place on the 22nd. This month’s full Moon comes 14 days, 9 hours, 29 minutes after the equinox, so it barely takes Harvest Moon honors.

The Harvest Moon was important in earlier times because it shined over the fields when crops were ready to be brought in. Its light allowed farmers to work into the night. And because of the angle of the Moon’s path at this time of year, the full Moon rises only a few minutes later each night as seen from more northerly latitudes. So it’s almost like having a full Moon for several nights in a row.

People often think that the Harvest Moon must be especially bright, but that isn’t usually the case. This year, however, it is. That’s because it comes less than a day and a half before the Moon is closest to Earth for its current orbit – roughly 15,000 miles closer than average. That provides some especially bright nights for farmers – and the rest of us, too.

Tomorrow: the bull charges into the evening sky.

Script by Damond Benningfield

It’s pretty easy to measure the length of a day on Mars or most other solid bodies. Just pick a feature on the surface and see how long it takes to spin back into view.

It’s not so easy for planets that don’t have a solid surface. We can track bands of clouds, but different bands can move at different speeds.

That’s been an especially tough problem for Saturn, the second-largest planet in the solar system. Scientists have been trying to pin down its rotation rate – the length of its day – for centuries.

When the twin Voyager spacecraft flew past Saturn in the 1980s, they measured the planet’s magnetic field to reveal the rotation rate of its interior. But when the Cassini spacecraft orbited Saturn decades later, its observations showed the day was about six minutes longer.

At the end of its mission, Cassini flew between Saturn and the inner edge of its rings. Measuring waves in the rings and tiny changes in the planet’s gravitation field produced yet another length: 10 hours, 33 minutes, and 38 seconds.

That’s not necessarily the final answer. Scientists continue to study the giant planet to know how to set their Saturn clocks.

And Saturn is in great view tonight. It looks like a bright star quite close to the lower right of the Moon at nightfall, and below the Moon as they set, before dawn.

Script by Damond Benningfield

Saturn and Venus bracket the pre-dawn sky now. As Saturn drops from view in the west, Venus nudges into view in the east. Saturn looks like a bright star, while Venus is the brilliant morning star.

The planets are both sliding eastward against the background of distant stars.

Saturn lined up opposite the Sun a couple of weeks ago. For a few months around that point, the planet looks like it’s “backing up” against the background of stars – a result of the relative motions of Saturn and Earth.

Earth is closer to the Sun than Saturn is, so our planet moves faster. It overtakes Saturn every 13 months, making Saturn appear to shift into reverse. It’s actually still moving in its usual direction – only our viewing angle is changing. It’s like passing another car on the highway. For a while, the other vehicle looks like it’s moving backward against the background of buildings and trees. When you move far enough past it, though, it appears to resume its normal forward motion.

Saturn will end its backward motion and shift back into forward at the end of November.

Venus, on the other hand, is about to pass behind the Sun as seen from Earth, so it’s dropping closer to the Sun every day. That’s also a result of the orbital motions of the two planets. Venus will disappear in the twilight in December, and cross behind the Sun in January – depriving us of the “morning star.”

More about Saturn tomorrow.

Script by Damond Benningfield

Scientists don’t know what dark matter is. But they have some ideas of what it isn’t. And they took a big step in ruling out some possibilities with the release of a study last year.

Dark matter produces no energy – the reason it’s described as “dark.” But we know it’s there because its gravity pulls on the visible matter around it. In fact, it appears to make up about 85 percent of all the matter in the universe.

The leading idea says dark matter consists of some kind of subatomic particle. A top candidate is called a WIMP – a weakly interacting massive particle.

One experiment is LUX-ZEPLIN. It’s in a former gold mine, almost a mile below the town of Lead, South Dakota. The rock above it blocks other types of particles from reaching the experiment.

Its detectors are inside a vat filled with about 8,000 tons of liquid xenon. The hope is that a WIMP will hit a xenon molecule and trigger that spark of light.

Project scientists conducted 280 days of observations. And they didn’t find any indication of WIMPs. But their test was the most sensitive yet for certain types of WIMPs. So the experiment rules out some candidate particles – narrowing the possibilities for dark matter.

Script by Damond Benningfield

At first glance, the dwarf planet Ceres doesn’t seem like a friendly home for life. It’s small, dark, and scarred by impact craters. Yet a deeper look presents a more optimistic picture. It has more water than any body in the inner solar system besides Earth. It has an abundance of organic compounds – the chemical building blocks of life. And it should be warm enough below the surface to sustain microscopic life.

Ceres is the largest member of the asteroid belt – a wide band of debris between the orbits of Mars and Jupiter. It’s about a quarter the diameter of the Moon. It probably consists of a dense core and mantle surrounded by an icy crust.

The Dawn spacecraft studied Ceres from orbit a decade ago. It saw big patches of bright, salty minerals. It also saw mountains, including one that’s three miles high; if you scaled Ceres to the size of Earth, the mountain would be 40 miles high. And the craft discovered that much of the surface consists of minerals that formed in a wet environment. So Ceres has water, heat, and organic compounds – the basic ingredients for life in what looks like an unfriendly world.

Ceres is at a point called opposition – it lines up opposite the Sun in our sky. That means it rises around sunset and is in view all night. It’s also closest to us at opposition, so it shines at its brightest. Even so, you need binoculars or a telescope to pick it out, in the constellation Cetus.

Script by Damond Benningfield

The constellations are well armed. Several of the star patterns that depict people or gods are carrying weapons. And some of them are in good view at this time of year.

As darkness falls, look low in the west for the brilliant star Arcturus. It stands at the base of Boštes the herdsman. Like many of the ancient star figures,

Boštes has different stories, and is drawn in different ways. In most depictions, he’s holding something long and straight against his right side. In some cases, it’s a staff. But in others, it’s a spear.

Well above Boštes is Hercules, marked by a lopsided box of four stars. He’s wrestling the multi-headed hydra. And in some depictions, he’s holding up a club.

In the south, look for Sagittarius. To modern eyes, it forms the outline of a teapot. But to the ancients, those stars formed an archer. The star at the outer edge of the spout is the point where he’s gripping both bow and arrow.

And low in the northeast there’s a figure with a unique weapon. Perseus the hero is holding the head of Medusa. In mythology, anyone looking at Medusa was turned to stone. Perseus managed to sever the head, then used it to save the princess Andromeda from a monster.

And if you’re stargazing before dawn, there’s another armed figure, well up in the south: Orion the hunter. He has two weapons. He’s holding a club in an upraised arm, with a sword strapped to his belt – a heavily armed figure in the stars.

Script by Damond Benningfield

The star Fomalhaut is a bit of a disappointment. Almost two decades ago, astronomers announced the discovery of a giant planet orbiting the star – the first exoplanet actually seen at visible wavelengths of light. Almost from the beginning, though, other astronomers questioned the discovery. And they were right. It wasn’t a planet at all, but a big clump of dust – the aftermath of a giant collision.

Fomalhaut is about twice as big and heavy as the Sun, and quite a bit brighter. It’s encircled by wide bands of dust. Most of the dust is at least a hundred times the distance from Earth to the Sun.

Fomalhaut is only about one-tenth the age of the Sun. Even so, it’s old enough that it should have blown away most of the dust. The fact that the belts are so prominent – especially the outer belt – means that they’re being renewed. The most likely source is collisions between large comets or asteroids. As those bodies are destroyed, they spew dust out into space.

One estimate says it would take the destruction of 2,000 comets that are one kilometer in diameter every day to keep the belts going. The would-be planet was the result of a collision between two even larger objects – briefly creating the illusion of a giant planet around this bright star.

Fomalhaut is low in the southeast at nightfall, and climbs across the south later on.

Script by Damond Benningfield

The southern evening sky is pretty bare at this time of year – lots of dark, empty spaces, but few bright stars. The one notable exception is Fomalhaut. It’s the brightest star of Piscis Austrinus, the southern fish. It’s low in the southeast at nightfall, and arcs across the south later on.

The star we see as Fomalhaut is 25 light-years away. It’s about twice as big and heavy as the Sun, and more than 15 times brighter. It’s young – about 10 percent the age of the Sun. And it’s encircled by wide bands of dust, which may contain planets; more about that tomorrow.

Fomalhaut has two companion stars – bound to it by their mutual gravitational pull. Both stars are smaller, cooler, and fainter than the Sun. One of them is barely visible to the eye alone, but you need a telescope to see the other.

Both stars are a long way from Fomalhaut itself. One is almost a light-year away, while the other is two and a half light-years. Astronomers know they’re bound to Fomalhaut because they’re moving in the same direction and at the same speed. Their composition is similar to Fomalhaut’s as well, and so is their age.

Fomalhaut itself will shine for another few hundred million years. But the companions will last much longer – billions of years for the larger one, and hundreds of billions of years for the other. So they’ll still be shining across the galaxy long after the demise of their showy companion.

Script by Damond Benningfield