StarDate

Summer returns to the northern hemisphere tomorrow. It’s the summer solstice — when the Sun stands farthest north for the entire year. The solstice also brings the year’s longest days.

In centuries long past, the skywatchers of many cultures watched the solstice from special observatories. Some of those observatories were circles of stones. The most famous is Stonehenge in England, but circles of smaller stones were also found in the Americas.

One of the most ancient stone observatories was built in southern Egypt, at a site called Nabta. It was used at least 6500 years ago, and perhaps even earlier — many centuries before Stonehenge.

Anthropologists discovered the site in 1974. Studies in the 1990s confirmed that Nabta probably served an astronomical purpose.

Among other artifacts, the site contains a 12-foot-wide circle of small stones. Two pairs of stones stand across the circle from each other. Looking through the gaps between each pair reveals the point where the Sun rose on the summer solstice thousands of years ago.

This alignment was important to the people of Nabta because monsoons brought a few inches of rain to the region soon after the solstice.

Over the centuries, though, the rains dried up and Nabta was abandoned. But the people of Nabta may have left a legacy. Their culture may have helped stimulate the formation of Egypt’s Old Kingdom — the beginning of the culture that built the great pyramids.

Script by Damond Benningfield

Support McDonald Observatory

Thirty-five years ago this month, a pair of balloons took a high-speed joyride through the clouds of Venus. They zipped along at 150 miles per hour, transmitting details on the Venusian atmosphere back to Earth. By the time their batteries died, they’d skirted a third of the way around our sister planet.

The balloons were part of twin Soviet missions, Vega 1 and 2. They actually had two destinations — Venus and Halley’s Comet. In fact, the name “Vega” comes from the Russian names for Venus and Halley.

As they sped past Venus in June of 1985, each one dropped off a lander and a balloon. Each lander survived on the surface for about an hour. They measured a surface temperature of 865 degrees, and pressure more than 90 times greater than at the surface of Earth. They also measured a sample of the surface, finding that it consisted of volcanic rock.

The balloons deployed about 35 miles high. That’s in the middle of Venus’s three layers of clouds. They measured temperature and pressure, the composition of the atmosphere, and more. Driven by strong winds at that level, each of them covered about 7,000 miles — providing what is still our only long and close look at Venus’s clouds.

And Venus just peeks into view in the dawn sky now. Tomorrow, it’s close to the crescent Moon as the sky brightens, quite low in the east-northeast. It’s the brilliant “morning star.” It’ll move higher into the sky, and into better view, over the coming weeks.

Script by Damond Benningfield

Support McDonald Observatory

The core of a galaxy in Perseus is all churned up. Two supermassive black holes inhabit the galaxy’s heart. Combined, they’re 15 billion times the mass of the Sun. But they’re only 24 light-years apart — far closer than any other pair of giant black holes yet seen.

The black holes neared each other as two galaxies merged. The gravity of the black holes should kick stars out of the galaxy, causing the black holes to spiral closer together.

As they get especially close, though, they might hold their positions — never merging to form a single black hole. That’s known as the “final-parsec” problem. Models suggest that when two supermassive black holes close to within about a parsec of each other — about three light-years — they stop getting closer. They’ve kicked out all of the stars around them, so there’s nothing left to exchange energy with — nothing to push the black holes closer together.

The problem is that black holes at the hearts of merging galaxies sometimes do come together.

That could require a third black hole. In one scenario, one black hole might be kicked out of the system, pushing the other two closer together. But only two black holes have been seen in the galaxy in Perseus. So right now, there’s no way to know if the black holes might someday merge.

Perseus is low in the northeast at first light. The galaxy is quite faint. But you can see its position, roughly half way between the stars Epsilon and Xi Persei.

Script by Damond Benningfield

Support McDonald Observatory

Black holes are just plain weird.

If you were to fall into a black hole a few times the mass of the Sun, for example, your body would be stretched, squeezed, and pulled apart atom by atom. So it seems logical that if you fell into an especially heavy black hole — one that’s billions of times the Sun’s mass — the effects would be even more dire. But that’s not the case. Instead, you could cross into the black hole and never even know it. As long as you stayed away from the center of the black hole, you’d get along just fine.

That’s because a black hole gets less dense as it gets heavier. A black hole that’s twice as massive as the Sun would contain eight times the volume of a black hole equal to the Sun’s mass. That makes the heavier black hole just one-quarter as dense.

And as the mass reaches jumbo numbers, the density drops dramatically. A black hole a hundred million times the Sun’s mass would be less dense than water. And at ten billion times the Sun’s mass, it would be less dense than air.

And as the black hole’s volume increases, its surface gravity goes down. So the tidal gravity of a super-jumbo black hole would be less than that of Earth.

Keep in mind, though, that a black hole is still a black hole. Anything that crosses its event horizon — the boundary between the black hole and the rest of the universe — could never come back out. It would be trapped forever — inside a black hole.

More about black holes tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

For 500 million years, the galaxy known as XMM 2599 churned out the equivalent of a thousand suns per year. Then, about 12 billion years ago, the star factory shut down — the galaxy stopped making stars at all.

It’s possible that the galaxy used up all the gas for making stars. But it’s also possible that the gas was blown out of the galaxy — by a supermassive black hole.

Black holes themselves only pull things in. Anything that falls into a black hole is lost — nothing can come back out. But as gas and dust fall toward a black hole, they form a wide, thin disk around it. As material piles up in the disk it gets hot — up to millions of degrees. At that temperature, it produces a strong “wind” of charged particles.

The black holes in the hearts of galaxies are millions or billions of times the mass of the Sun. So the disks around them can be huge and brilliant. And their winds can be massive. They can “blow” outward at up to a quarter of the speed of light. And like a strong desert breeze blowing sand, a black-hole wind can blow away the gas and dust around it — hundreds of times the mass of the Sun per year. That can shut down starbirth in the surrounding galaxy.

But the picture is complicated. Black-hole winds can also cause the birth of new stars. The winds can squeeze clouds of gas and dust, causing them to collapse and create stars — all in the hot wind from a black hole.

More about black holes tomorrow.

 
Script by Damond Benningfield

Support McDonald Observatory

In 1770, Earth dodged a big bullet. A comet several miles in diameter passed just 1.4 million miles away. It was the closest observed approach by a comet in history.

Lexell’s Comet was discovered 250 years ago today. Comet hunter Charles Messier had just finished looking at Jupiter. He turned his telescope toward the constellation Sagittarius, to study some “fuzzy” objects he’d discovered there earlier. And he saw a new fuzzy object — a comet.

The comet quickly grew larger and brighter. By late June, it was an easy target for the unaided eye. Its coma — a cloud of gas and dust around its icy body — spanned several times the width of the Moon. The comet didn’t show much of a tail, though.

Lexell’s Comet passed closest to Earth on July 1st. Had it hit our planet, it was big enough to cause destruction on a global scale. Instead, though, it passed on by, then swung around the Sun and back into the outer solar system — never to be seen again.

Anders Johan Lexell later calculated its orbit, so the comet was named in his honor. Lexell found that the comet probably was hurled our way by a close encounter with Jupiter, the largest planet in the solar system. A second encounter, in 1779, then pushed it away from Earth.

A study a couple of years ago found that an asteroid discovered in 2010 could be the famous comet. But the evidence isn’t strong enough to be certain. So the search for Lexell’s Comet continues.

Script by Damond Benningfield
 

Support McDonald Observatory

Ten years ago, an interplanetary projectile streaked to Earth. It actually came from Earth, but it carried a cargo from beyond our planet: less than a gram of dust from the surface of an asteroid.

Hayabusa was a test in patience and planning. The Japanese mission was launched in 2003. But a lot went wrong. A solar flare damaged its solar cells, which powered its engines. That delayed its arrival at the asteroid Itokawa.

Once there, it was supposed to drop to the surface and fire some small “bullets” into the asteroid. It then would gather up some of the dust blasted off the surface. But that didn’t work. Instead, the craft hit the asteroid. That knocked off a few dust grains for its collectors.

Hayabusa also carried a small rover. But it was deployed a bit late, so it missed the asteroid entirely. And problems with communications and other systems delayed the mission’s return to Earth.

Flight controllers worked around the problems, though, and got the craft back home on June 13th of 2010. Analysis found that Hayabusa had collected about 1,500 dust grains. They showed that Itokawa probably was a fragment of a larger asteroid. They also showed that the asteroid matches the composition of a common class of meteorites.

Hayabusa 2 has studied another asteroid. It successfully deployed several small rovers, and appears to have nabbed samples of the surface. It’s on its way back to Earth, with arrival scheduled for December.

Script by Damond Benningfield
 

Support McDonald Observatory

Mars is a land of dust devils. Thousands of them sweep across the landscape every day — and perhaps hundreds of thousands. They’re far larger than the ones that twist across the deserts and plains on Earth. And they could be a problem for future Mars explorers.

A dust devil is a twirling column of air that looks like a weak tornado. It forms under clear skies. The Sun warms the land, causing air to rise. As it rises it interacts with cooler air above, causing the column to spin. It can pick up tiny grains of sand and dust and carry them across great distances.

On Earth, most dust devils are no more than a few hundred feet high, with winds equal to a good gust. On Mars, though, they can get much bigger — almost half a mile wide and about five miles high. Their top speeds are greater, too.

Dust devils could be a problem for future space missions. Recent research suggests that it’s not just the wind that picks up the dust. The dust grains may create a static electric charge — like scuffing across the carpet in your socks. That charge causes some of the dust grains to “levitate” off the surface. The electric field could damage or interfere with sensitive equipment on future missions — a real “dust up” for Mars explorers.

Look for Mars near the Moon early tomorrow. The planet looks like a bright orange star. It’s above the Moon as they climb into good view by 2:30 or 3, and to the upper right of the Moon at first light.

Script by Damond Benningfield
 

Support McDonald Observatory

Most of Mars is fairly dark — shades of grey and orange. But it’s topped in pure white — its ice caps. They span hundreds of miles, and contain huge amounts of frozen water. And they preserve a record of the Martian climate that future scientists may plumb.

The northern cap is the bigger one — about 600 miles across. It’s not quite as thick as the southern one, but it contains more ice.

Both caps are dynamic — they wax and wane with the seasons. They consist mainly of vast amounts of frozen water, which pretty much stays in place all year long.

During winter, though, a cap is topped by a layer of frozen carbon dioxide — dry ice. That crust can range from a few feet to a couple of dozen feet thick. It’s deposited by snow-laden clouds, or it freezes directly from the atmosphere. In spring and summer, that layer vaporizes and rushes back into the air. That makes the atmosphere thicker. It also stirs up dust storms, some of which can blanket the entire planet.

The process of freezing and thawing creates layers in the ice. Dust is sandwiched between each layer. So if scientists could drill into the ice caps, they could “read” the planet’s climate history like biologists reading the rings of a tree — a history preserved in ice.

Mars teams up with the Moon the next few mornings. Tomorrow, they climb into good view by 2 or 3, with Mars close to the left of the Moon. They’ll be even closer at first light.

More about Mars and the Moon tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

If you want to find neighbors, you probably start by looking next door, not blocks away. And that’s the strategy astronomers used in looking for neighboring planets. They started by looking around neighboring stars that were close by. In fact, they’ve been looking at the closest star system, Alpha Centauri, for decades.

Alpha Centauri is a bit more than four light-years away. It consists of three stars. Two of them — Alpha Centauri A and B — are close together, and both are fairly similar to the Sun. The third star is far away from the others, but it’s quite small and faint.

That star is known as Proxima Centauri, because it’s closer to Earth than the other two. And there’s pretty clear evidence that it has at least one planet. It’s a rocky world a little heavier than Earth, and it’s in the star’s habitable zone — the region that’s most comfortable for life.

Astronomers thought they’d found a planet around star B, but it turned out to be a glitch in the data.

But they haven’t given up. They’ve increased their efforts this year, in fact, because stars A and B are at their farthest apart as seen from Earth. That makes it a little bit easier to find planets.

Alpha Centauri is the third-brightest star system in the night sky. But it’s so far south that it’s not visible from most of the U.S. From far-southern Texas and Florida, and from Hawaii, it’s low in the south as night falls — a system where we’re still looking for neighbors.

Script by Damond Benningfield

Support McDonald Observatory