StarDate

For the light from the star Spica, it’s been a long journey. It began about 250 years ago — around the year 1771. By then, the American colonists were getting angry at their rulers in England, and were only a few years away from declaring independence. A few skirmishes even broke out that year in some of the colonies.

Since then, the light from Spica has traveled about one-and-a-half million billion miles. To put it another way, if you were flying in a jet airliner, it would take you more than 300 million years to cover that distance.

Spica’s light doesn’t take nearly that long to cross the interstellar gulf because it moves at, well, the speed of light — 670 million miles per hour. According to Albert Einstein’s theories of relativity, nothing can move faster than that.

As the light races toward Earth, some of it gets absorbed by gas and dust between the stars. There’s not a lot between here and Spica, though, so most of the light is uninterrupted — it just keeps on cruising.

Spica stands well to the lower left of the Moon at first light tomorrow, and closer to the Moon on Wednesday morning. And if you happen to look in that direction, some of the star’s light will end its journey across the galaxy by striking your eye — a journey that began on the surface of a brilliant star, and ended on a winter’s night on a planet 250 light-years away.

Tomorrow: seeing the future in a shadow.

Script by Damond Benningfield

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Galaxies clump together in many ways. Our galaxy, the Milky Way, belongs to a group — a collection of a few dozen galaxies. Others belong to clusters, which can contain hundreds of galaxies. And the clusters can clump together to make superclusters of thousands of galaxies.

On the other end of the scale are the Hickson Compact Groups — handfuls of galaxies that are packed especially close together.

The groups are named for Paul Hickson, who cataloged a hundred of them almost 40 years ago. The galaxies in many of the groups are so close that they’re interacting with each other. Their gravity may pull out ribbons of stars that stretch across thousands of light-years. Or it may squeeze clouds of gas and dust, causing them to collapse and give birth to millions of new stars.

Because of their tightness, such groups shouldn’t last long. Their galaxies should merge quickly, forming a single super galaxy. The fact that we see so many of them suggests that there’s a lot of work to do to explain these tiny but interesting groups of galaxies.

A couple of fairly bright groups are in good view at this time of year. Group 16, which consists of half a dozen galaxies, is in the south-southwest at nightfall, in Cetus, the sea monster. And the most famous of them, Group 92, is in the west-northwest, in Pegasus. Under dark skies, each of them is visible through fairly strong binoculars, and an easy target for a small telescope.

Script by Damond Benningfield

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Four star clusters hang out above the “head” of Canis Major, the big dog. As night falls, they’re to the left or upper left of Sirius, the brightest star in the night sky. They wheel above the Dog Star as the night goes on.

The clusters are known as NGC 2323, -35, -43, and -53. Each one is a family of dozens to hundreds of stars. And all of them are fairly young — a hundred million years old or so. That may not sound young until you compare it to the age of the Sun — four and a half billion years.

In fact, that entire region of the sky is awash in young stars — and in stars that are still being born. Big clouds of gas and dust in the region are collapsing. Some of them are compressed by exploding stars, others by radiation and winds from hot young stars. As the clouds collapse, they break into smaller and smaller fragments. Those fragments, in turn, collapse to make stars.

NGC 2343 is the brightest of the four clusters near the big dog’s head. Under dark skies, good binoculars reveal two or three dozen individual stars. The cluster is more than 3,000 light-years away. It’s on the edge of one of the star-forming clouds, and may be the result of an earlier wave of starbirth from the same complex.

NGC 2335 is also on the edge of that complex, and it’s also fairly bright. Even so, you need dark skies and binoculars or a telescope to see this family of young stars above the head of the big dog.

Script by Damond Benningfield

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Astronauts who spend a lot of time on the Moon will face many potential dangers. There’s no atmosphere, and temperatures can range from 250 degrees Fahrenheit to minus two-fifty. The jagged particles of lunar dust could clog machinery or get into the lungs, causing major health problems. And the lunar surface is blasted by radiation — from the Sun during the day, and from the rest of the universe both day and night.

Scientists have been measuring that radiation with an instrument on a Chinese lander. The probe touched down more than two years ago, on the lunar farside — the side we never see from Earth.

The instrument is enclosed inside a box that provides about the same protection as a spacesuit would for a person. Its measurements show that, over the course of a year, an astronaut would be exposed to about 200 times the average annual dosage here on Earth.

Because the dose would be spread out over such a long time, though, it’s not clear how much it would increase the risk of cancer or other health problems. And there are ways to limit the radiation, such as covering habitats with lunar dirt. Besides, it’ll be a long time before anyone is spending a year on the Moon. So while life there will be risky, it may not be impossible.

Look for the just-past-full Moon climbing into good view by about 7 o’clock this evening. A bright companion will stand close by: Regulus, the leading light of Leo, the lion.

Script by Damond Benningfield

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Not many planets are bigger than their stars. And not many have spiraled close to a dead star known as a white dwarf and survived. But astronomers recently discovered a planet that’s defied the odds in both of those cases.

WD 1856b orbits a white dwarf — the corpse of a star that was similar to the Sun. It’s about half as heavy as the Sun, but only about as big as Earth. It became a white dwarf at the end of its normal lifetime, almost six billion years ago.

1856b, on the other hand, is a giant planet. It’s about as big as Jupiter, the giant of our own solar system. That makes it several times wider than the white dwarf.

Astronomers discovered the planet with space telescopes, which saw the system’s light fade as the planet passed in front of the star. They confirmed it with telescopes on the ground, including the Hobby-Eberly Telescope at McDonald Observatory.

Astronomers have discovered the pulverized remains of planets orbiting white dwarfs. They’ve even seen the remains of dead planets polluting the surfaces of such stars. But this is the first time they’ve seen an intact planet this close to a white dwarf — just two million miles away.

The planet probably was born a lot farther out. As the star died, the planet’s orbit changed — it became extremely lopsided. Over time, the strong gravity of the white dwarf molded the orbit to its present configuration — just beyond the surface of a dead star.

Script by Damond Benningfield

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A couple of our stellar neighbors are ancient. They were born more than 11 billion years ago. And for most of that time they’ve been “dead” — they’re corpses known as white dwarfs.

A white dwarf is the final stage for a star like the Sun. When the star can no longer produce nuclear reactions, it ejects its outer layers, and its core collapses to about the size of Earth.

The tiny white dwarf is still quite hot. In fact, astronomers determine the age of a white dwarf by measuring its temperature and mass. They know the rate at which a white dwarf of a given mass cools, so taking its temperature tells them how long it’s been a white dwarf.

The nearby white dwarfs — they’re roughly a hundred light-years away — are much cooler than the Sun. One of them is about two-thirds the mass of the Sun, while the other is about three-quarters the Sun’s mass.

Those numbers tell us that the smaller white dwarf began life at about twice the mass of the Sun, about 12 billion years ago. It became a white dwarf roughly a billion years later. So it’s spent the last 11 billion years as a corpse.

The heavier white dwarf started at three times the Sun’s mass, at about the same time as the other. It shed its outer layers in just a quarter of a billion years, leaving a stellar corpse: a white dwarf.

One of those neighbors lines up in front of the Pleiades, which is high in the sky at nightfall. The other one is below the bowl of the Big Dipper.

Script by Damond Benningfield

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In the far distant future, the universe may be packed with diamonds — the remnants of stars like the Sun. Today, though, there are only a few of them. That’s because it takes a long time to make one.

When a star like the Sun dies, it flings its outer layers into space. That leaves only its hot but dead core — a white dwarf. Such an object is as heavy as a star, but only as big as Earth, which is much smaller than a star.

At first, a white dwarf is quite hot. So even though it’s small, it does produce light. It slowly cools off, though, which makes it dimmer. So over hundreds of billions of years, it will fade from view entirely — forming a black dwarf. But the universe isn’t old enough for even a single black dwarf to have formed.

Along the way, though, other things happen. Most white dwarfs are made mainly of carbon and oxygen. As they cool, the carbon crystallizes — turning to diamond. And the heavier the white dwarf, the faster that happens. For a white dwarf as massive as the one the Sun will form, it takes several billion years. But for heavier stars it can happen in as little as one billion years.

Astronomers have seen white dwarfs that have turned to diamond. But such stars are rare. That’s because they had to form from heavy stars, which are also rare. But as more stars become white dwarfs, the number of crystallized ones will grow — sprinkling more diamonds across the universe.

More about white dwarfs tomorrow.

Script by Damond Benningfield

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The Sun will stay bright and strong for a long time — but not forever. Billions of years from now, it will shed its outer layers. That will leave only its dead core — a cosmic ember known as a white dwarf.

In the prime of life, the Sun shines by producing nuclear reactions in its core. Right now, it’s “fusing” atoms of hydrogen to make helium. When the hydrogen is gone, the core will get smaller and hotter, allowing it to fuse the helium to make oxygen and carbon.

But the Sun isn’t massive enough to fuse those elements to make heavier ones. Without the radiation produced by the nuclear reactions, the core will collapse — but only so far. The electrons in the core will smash together far more tightly than in ordinary matter. But they can be squeezed only so much. They generate a pressure that stops the collapse.

As the core collapses, the Sun will cast off its outer layers, leaving only the white dwarf. It’ll contain about 60 percent of the Sun’s original mass, but squeezed into a ball the size of Earth. By comparison, today’s Sun is more than a hundred times wider than Earth.

The white dwarf will be so dense that a chunk the size of a sugar cube would weigh tons. So its surface gravity will be a hundred thousand times greater than Earth’s gravity.

The white dwarf will be extremely hot. It’ll take hundreds of billions of years to cool off — turning the white dwarf into a black dwarf.

More about white dwarfs tomorrow.

Script by Damond Benningfield

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How do you hide two giant planets? You don’t need black holes, aliens, or even Harry Potter’s cloak. Instead, all you need is the Sun. And right now, it’s hiding its two biggest planets with its brilliant light.

Jupiter is the largest planet in the solar system, with Saturn just behind it. And both of them are at a point in their orbits known as conjunction. They’re passing behind the Sun as seen from Earth, so they’re completely hidden in the Sun’s glare. Saturn had its conjunction yesterday. And Jupiter will follow on Thursday night. But both planets will remain hidden for several weeks.

The “hidden” period is stretched out because the planets are at their farthest point from Earth. When something is far away, it appears to move more slowly than when it’s close by. If you see cars moving on a distant highway, for example, they seem to be just crawling along. But if you pull up to the highway and try to cross it, the cars are moving at a scary clip. Yet the speed on the road hasn’t changed at all — only your distance from it has.

Jupiter and Saturn will begin to pull into the dawn sky next month. Depending on your latitude and other factors, Saturn may climb into reasonable view as early as the 20th. Jupiter trails behind it, but it’s also brighter, so you should be able to pick it out just a few days later. Both planets will be in better view by the middle of March.

Tomorrow: Tiny futures for big stars.

Script by Damond Benningfield

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The bull keeps a close eye on the Moon tonight. That’s because Aldebaran, the star that marks the eye of Taurus, will be quite close to the Moon throughout the night. They’ll be high in the sky at nightfall, and will set in the wee hours of the morning.

The name “Moon” comes from words in old English that mean “measures time.” They indicate that ancient cultures used the Moon’s phases as a calendar. The period of those phases — almost 30 days — was named for the Moon: a month.

Other languages, of course, have used other names. Several use “Luna,” from the Latin name for our satellite world. And several words have evolved from that, such as lunatic — an indication that people who acted “crazy” were considered to be under the influence of the Moon.

According to some sources, “Luna” has become a popular name for pets. Companies that insure pets or manage networks of pet sitters release annual lists of the most popular names for cats and dogs. And Luna has been on several of them. A 2019 list, for example, said it was the second-most-popular name for female dogs, and the most popular for cats. And the name seems to be just about as common in Australia and the United Kingdom as in the U.S.

It’s not just popular for pets, though. A 2018 ranking said Luna was the 31st most popular name for baby girls in the U.S. And in 2019, it climbed to 16th — a name shared with the brightest light in the night sky.

Script by Damond Benningfield

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