StarDate

A star that’s always two and a half centuries behind the times perches to the lower left of the Moon this evening.

Spica is the leading light of Virgo, and the 16th brightest star in the night sky. It dazzles even though it’s farther than most of the other members of the Top 16. That tells us that Spica doesn’t just look bright, it really is bright.

Spica’s distance has been measured at 250 light-years. That’s the distance covered by a beam of light — which travels at 670 million miles per hour — in 250 years. So the light we see from Spica tonight actually left the star 250 years ago.

There’s a bit of wiggle room in that distance, though — about 10 light-years in either direction.

In part, that’s because of the technique that’s used to measure distances to stars that are fairly close — like Spica. Astronomers compare the star’s location relative to the stars around it when Earth is on opposite sides of the Sun. That change in viewing angle causes the star to appear to shift back and forth a tiny bit compared to the background. The size of the angle reveals the star’s distance. But as the distance increases the angle gets smaller, which makes it harder to measure — hence the margin of error.

A 10-light-year uncertainty isn’t a big problem for astronomy. But it would be a problem for a starship headed to Spica. Without a precise distance, the ship might stop short or travel too far — missing brilliant Spica.

Script by Damond Benningfield

When scientists dispatched Mariner 6 and 7 toward Mars, they carefully planned what portions of the planet the twin craft would study. But no one had seen the planet in enough detail to aim them at its most interesting features. As a result, the probes photographed lots of craters and plains. But they missed the planet’s giant volcanoes and canyons — features that really set Mars apart.

The second of those craft, Mariner 7, flew past Mars 50 years ago today — just five days after Mariner 6. Both of them skimmed about 2100 miles above the surface. One flew over the equator, while the other took aim at the region around the south pole.

In all, the craft snapped almost 60 close-range pictures of the Red Planet. The images revealed craters of various sizes and ages. They also showed some dusty plains.

It wasn’t until the next Mariner, which entered orbit around Mars a couple of years later, that we began to see what an interesting world Mars really is. In particular, that’s when we saw that water had flowed across the planet in the distant past.

The water is long since gone. But Mariner 6 and 7 found one hint about its fate. They measured hydrogen atoms escaping from the top of the atmosphere. Hydrogen is one of the two elements that make up water molecules. That suggests that the molecules were split apart, and the lightweight hydrogen drifted off into space — leaving Mars dryer and a lot less interesting.
 

Script by Damond Benningfield

Billions of years ago, when the solar system was young, two mountain-sized agglomerations of ice and rock staged a gentle merger. Combined, they look a bit like a flat cookie with a small scoop of ice cream along its rim.

The combined object is known formally as 2014 MU69, and informally as Ultima Thule. It’s in the Kuiper Belt — a wide zone beyond the orbit of Neptune, the Sun’s most remote major planet. The belt contains millions of chunks of ice and rock.

Ultima Thule is the second member of the belt studied by the New Horizons spacecraft. The first was Pluto. Ultima Thule is a billion miles farther from the Sun than Pluto. And it’s a lot smaller and much less active.

The object is known as a “contact binary.” It consists of two parts, which were born separately but close together. They soon became linked by gravity, forming a pair. They then moved closer together until they made contact. The contact was gentle enough to not blast them to bits, but strong enough to fuse them together.

The combined object is more than 20 miles long. It’s quite dark, and quite red — perhaps the result of billions of years of exposure to radiation. There aren’t many craters on the surface. But New Horizons did see evidence of frozen water and other compounds.

It’ll take the better part of a year for New Horizons to transmit the rest of its observations to Earth — giving us an even better view of the most distant object ever visited by a spacecraft.
 

Script by Damond Benningfield

Under dark skies, the human eye can see a few planets, a few thousand individual stars, and the combined glow of millions of stars in the band of light known as the Milky Way. Yet there is much more to the universe than the eye can perceive. Telescopes reveal many more wonders, from the remnants of dead stars to colliding galaxies.

One of those wonders is appearing quite near Jupiter right now. The solar system’s largest planet is about a third of the way up the southern sky at nightfall. It looks like a brilliant star — the brightest object in the sky for most of the night.

Quite close to the lower left of Jupiter, there’s a vast cluster of stars. But it’s so far away that you need a good-sized telescope to see it.

NGC 6235 is about 37,000 light-years from Earth. It’s a globular cluster. It contains hundreds of thousands of stars, all packed into a tight ball. The stars are all quite old. In fact, they’re probably some of the oldest stars in the galaxy.

NGC 6235 lines up more than 10,000 light-years beyond the center of the Milky Way Galaxy. It’s also a little above the galactic plane — the plane that divides the galaxy’s disk into northern and southern halves. If the cluster were closer to the plane, it would stand behind thick clouds of dust. That would make it really hard to see. Instead, it’s only pretty hard to see — a target for a good telescope on a dark night, not far from brilliant Jupiter.

Script by Damond Benningfield

The subtle glow of the disk of our home galaxy arcs across the sky on summer nights. It’s anchored in the south by the constellation Sagittarius, which forms the outline of a teapot.

The Milky Way’s glowing band hides many other astronomical wonders from view. That includes one of our closest companion galaxies. It’s a puffball compared to the Milky Way. And it’s being pulled apart by the Milky Way’s gravity. Still, it’s held on to enough of its material that it still looks like a galaxy.

The Sagittarius Dwarf Galaxy is only about 70,000 light-years away. By galactic standards, that’s just down the block.

The galaxy is located a bit to the upper right of Sagittarius’s teapot. But it’s behind some bright star clouds and dark dust clouds in the Milky Way. Combined with the fact that the dwarf galaxy is quite faint, that kept anyone from seeing it until just 25 years ago.

The Sagittarius dwarf is orbiting the center of the Milky Way. It periodically passes through the Milky Way’s disk, stirring up the stars around it. In fact, there’s evidence that a few hundred million years ago, it started a “wave” in the stars near the Milky Way’s heart that’s still rippling.

At the same time, the Milky Way is pulling some stars away from the dwarf galaxy. Some of them form a long streamer behind the galaxy. Others have been absorbed into the Milky Way’s disk. So the Sagittarius dwarf is slowly being consumed by the bigger galaxy: the Milky Way.

Script by Damond Benningfield

The supermassive black hole at the heart of the Milky Way isn’t alone. Astronomers estimate there could be as many as 10 thousand black holes within a few light-years of it. And a recent study found one about 20 light-years away. This one is heavier than the others, though — more than 30,000 times the mass of the Sun.

The new object is a member of a rare class known as intermediate-mass black holes. They’re heavier than most black holes, but not as heavy as ones like Sagittarius A-star, the monster at the heart of the Milky Way Galaxy.

Astronomers in Japan were studying a gas cloud in the galaxy’s heart with ALMA, an array of radio telescopes in Chile. They found that the cloud was unusually compact, and that its gas was moving in an odd way. They simulated what might be responsible, and found that it’s probably the gravitational influence of an intermediate-mass black hole.

The researchers didn’t find a glowing disk of material around the black hole, so they can’t confirm the black hole’s existence. But there’s not much else that could explain the motions of the gas cloud — a cloud that’s probably under the influence of a heavy black hole.

The heart of the galaxy is in Sagittarius, which is low in the south at nightfall at this time of year. The constellation’s brightest stars form the outline of a teapot. The galaxy’s heart is in the “steam” above the spout of the teapot — 27,000 light-years away.

More about Sagittarius tomorrow.

Script by Damond Benningfield

Today, the supermassive black hole in the heart of the Milky Way is quiet. That hasn’t always been the case, though. Several million years ago, it might have shined as a quasar.

The black hole is called Sagittarius A-star. It’s in the center of the Milky Way Galaxy — about 27,000 light-years away. It’s about four million times the mass of the Sun.

We don’t see the black hole itself. But we do see the glow of material around it. As gas and dust spiral into the black hole, they get hot, so they produce energy.

Every few years, the black hole flares up for perhaps a few hours as it ingests something more substantial, such as an asteroid. And about 300 years ago, it flared a million times brighter than normal, perhaps as it gobbled up a star.

There’s evidence of an even bigger eruption six million years ago. The black hole could have shined billions of times its normal brightness as a massive disk formed around it. During that time, it would have been a quasar — one of the brightest objects in the entire universe.

That outburst created a shockwave. It’s still plowing through the galaxy, and should reach Earth in about three million years — reverberations of a quasar.

The black hole is above the spout of the teapot formed by the constellation Sagittarius, which is low in the south at nightfall. Sagittarius A-star is hidden behind thick clouds of dust, so it’s invisible from the eye alone — no matter how brightly it shines.

More tomorrow.

Script by Damond Benningfield

You can’t always tell the relative ages of siblings just by looking at them. Differences in genetics — and in lifestyle — can make a younger one look older, or an older one look younger.

The same thing is true of the star cluster Messier 11. Its stars should all be about the same age. But they don’t look it. That may be the result of some fast living.

Messier 11 is one of the biggest and heaviest clusters of its type. It contains about 3,000 stars, all packed into an area that’s a few dozen light-years across.

All the stars in M11 should have been born from a single giant cloud of gas and dust. And they should have been born at the same time — about 220 million years ago. But not all the individual stars look that age.

Some recent research says the difference is fast living. The stars all rotate at different speeds. Stars that spin faster mix up the gases in their cores, extending their lifetimes. They also look redder, which makes them look older. So while the stars are about the same age, they just don’t look it.

Messier 11 is in the southeast at nightfall. It’s in the constellation Scutum, to the upper left of teapot-shaped Sagittarius. Under dark skies, M11 is just visible to the unaided eye, even though it’s about 6,000 light-years away.

Seen through binoculars or a telescope, its brighter stars form a “V” that resembles a flock of ducks. That earned the cluster a nickname: the Wild Duck.

Script by Damond Benningfield

The surfaces of all stars “jiggle” a bit. For many stars, the jiggles are quite small. For a few stars, though, they’re big enough to see with the eye alone.

An example is in Scutum, the shield. As night falls, the constellation is above the teapot outlined by the bright stars of Sagittarius.

Delta Scuti is a good bit bigger, brighter, and heavier than the Sun. And although it’s much younger than the Sun, it’s much farther along in its life cycle. It’s no longer fusing the hydrogen in its core to make helium. But it’s not yet ready to fuse the helium to make heavier elements.

There’s a helium-rich layer in the star’s thick envelope of gas. This layer heats up, which causes the helium to become more opaque. That traps heat from inside the star, causing its outer layers to expand. As they expand, the extra heat escapes, so the star looks brighter.

As the heat escapes, though, the outer layers cool and fall back inward. That causes the helium layer to heat up once more, restarting the process.

In the case of Delta Scuti, there are several such cycles. The main one lasts about four and a half hours, and causes the star’s brightness to vary by about 20 percent. There are other cycles, too, in which some of the surface moves outward, but some moves inward.

Delta Scuti is the prototype of an entire class of variable stars. The brightest member of the class stands well to the left or upper left of Scutum: Altair, a member of the bright Summer Triangle.

Script by Damond Benningfield

A pretty binary star system climbs across the southern sky on summer nights. Gamma Delphini represents the snout of Delphinus, the dolphin. The little constellation is in the east as night falls, with the snout on the left and the tail on the right.

Under dark skies, Gamma Delphini is visible to the unaided eye as a single point of light. But a small telescope shows that it’s not single at all. Instead, the system consists of two stars. On average, they’re separated by about 30 billion miles — a gap that’s wide enough for us to see the system’s individual members.

One of the stars is white or pale yellow, while the other is yellow-orange. Seen side by side, though, the contrast between the two looks much more pronounced. So to most observers, the pale star looks blue, while the other looks orange.

The colors reveal the surface temperatures of the two stars. The pale star is hotter than the Sun, while the orange star is cooler than the Sun.

Not long ago, the orange star would have looked much like its companion does now. But it used up the hydrogen fuel in its core. That caused the core to shrink and get hotter. The extra radiation pushed on the surrounding layers of gas, causing them to puff outward. As they did so, they got cooler, which changed the star’s color from white to yellow-orange.

Before long, the companion will undergo the same process — adding a bit more color to the celestial dolphin.

Script by Damond Benningfield