StarDate

The search for aliens keeps coming up empty. Wide scans for signs of intelligent life have heard nothing. And so far, even the best prospects have had natural causes.

A project called Breakthrough Listen has been scanning the skies for alien radio traffic for a couple of years. It uses radio telescopes in the United States and Australia. The telescopes are listening for signals from a million nearby stars, plus the center of the Milky Way galaxy and a hundred other galaxies.

Results reported to date have showed a few odd signals, but none is expected to have been created by aliens.

Back in December, the project scanned an asteroid from deep space that was passing through the solar system. Some had speculated that it was an alien probe. But the study revealed no radio waves at all. So if the asteroid was dispatched by ET, it was either dead or in stealth mode.

And what some thought could be “alien megastructures” encircling a distant star is probably just dust or comets. Known as Tabby’s Star, its light flickers. Some suggested the flickers were giant artificial structures passing in front of the star.

But astronomers examined the star at all wavelengths of light. They found that it flickers at some wavelengths, but not all. That means that whatever is passing in front of the star isn’t solid, as big structures would be. Instead, it’s probably clumpy clouds of dust and comets. So one more possible sign of alien life has turned up empty.

Script by Damond Benningfield

The stars that are visible to the unaided eye aren’t representative of the galaxy as a whole -- far from it. We see many giants and supergiants, for example. Yet those types of stars make up only a tiny fraction of the overall stellar population. Most of the stars in the Milky Way are small and feeble -- so feeble that we can’t see a single one, even though several are among our closest neighbors.

Astronomers call this a selection effect -- some things are easier to see than others. A spiffy European sports car stands out among the usual sedans and SUVs, for example.

For scientists, though, that can be a problem. If astronomers studied only the bright stars -- the equivalent of racy sports cars -- they’d get a completely wrong idea about how stars form, evolve, and die.

Selection effects are a problem in other ways, too.

When astronomers began discovering planets in other star systems, they found almost exclusively one type: giant worlds in close orbits around their stars. The techniques used to find exoplanets work best on those types of worlds.

In the decades since, though, they’ve discovered thousands of planets that are smaller, farther out, or both. Only by studying all of these planets can we get a full picture of the lives of planets and planetary systems.

So astronomers can’t settle for looking at the easy targets. To fully understand the workings of the universe, they need to study the hard ones, too.

Script by Damond Benningfield

Our solar system inhabits the bright disk of a giant spiral galaxy, the Milky Way. The disk is surrounded by a halo of much older stars. There’s also a halo of dark matter. The dark matter emits no light, but scientists know it’s there because of its gravitational pull on the visible matter that’s close to it. From that pull, we know that dark matter outweighs all the rest of the galaxy put together.

An ancient star cluster is helping reveal the shape of the Milky Ways’ dark halo.

The star cluster is Palomar 5, and it’s in serious trouble: It’s losing most of its stars. A stream of escaping stars stretches tens of thousands of light-years across the halo.

Palomar 5 is about 75,000 light-years from Earth, in the constellation Serpens. The cluster hovers high above the galaxy’s wide, flat disk.

The stars that are escaping from it feel the gravitational pull of the dark matter in the halo. So their exact path reveals the shape of the dark halo. The path suggests that the halo is roughly spherical. That’s unlike the galaxy’s disk, which is shaped like a pancake.

Astronomers estimate the cluster has lost more than 90 percent of its stars. So today, the cluster harbors only about 4300 times as much mass as the Sun.

Still, the disintegrating star cluster is helping astronomers figure out the shape of one of the most mysterious parts of the Milky Way.

Script by Ken Croswell

The constellation Sagittarius climbs low across the south on summer evenings. Its brightest stars form the outline of a teapot. The center of the Milky Way galaxy lies just above the spout of the teapot, about 27,000 light-years away. That region is home to the Milky Way’s biggest black hole -- and also a big red star that looks like a comet.

Unfortunately, we see almost no visible light from the galactic center because the Milky Way is quite dusty. But infrared light, whose waves are longer than those of visible light, penetrates the dust.

Of all the stars near the black hole, the brightest at infrared wavelengths is IRS 7, with IRS standing for “infrared source.” The star is big, red, and cool. It’s a red supergiant, like beautiful Antares in Scorpius, the constellation to the west of Sagittarius. And it’s only about one light-year from the black hole.

Recent observations indicate the star is 20 to 25 times as massive as the Sun, and about a thousand times larger. If put in place of the Sun, it would engulf Mercury, Venus, Earth, and Mars, and almost reach Jupiter.

Although IRS 7 is a star, it has a tail that resembles a comet. The tail is at least half a light-year long. Like the Sun, IRS 7 emits a “wind” of charged particles. It comes off the star in all directions. But winds from the stars around it push IRS 7’s own wind, sculpting it into a tail that makes the star resemble an overgrown comet.

Script by Ken Croswell

As seen from most of the United States, the beautiful crescent Moon will just squeak past the bright star Aldebaran early tomorrow. They’ll be separated by no more than a degree or two as they climb into view a couple of hours before sunrise. And from northern Minnesota and parts of Canada, the Moon will pass in front of the star, briefly blocking it from view.

Such a passage is known as an occultation. And over the last century or two, astronomers have used occultations to learn more about the stars the Moon covers up, and about the Moon’s orbit around Earth.

Precise timing of an occultation reveals the Moon’s exact position in the sky. That can be used to refine models of the Moon’s orbit. That helps get spacecraft to the Moon, and helps refine our understanding of gravity.

Occultations also can help astronomers measure the size of a star. Timing just how long it takes a star to disappear behind the Moon reveals its angular diameter -- how big it looks in the sky. When that’s combined with the star’s distance, that tells us how big the star really is.

Occultations also can tell us if a star has a companion. If there’s more than one star in a system, then the light fades away in steps. If there’s only one star, it fades smoothly.

For the most part, other techniques have replaced occultation measurements. Even so, astronomers still keep an eye on most occultations -- trying to learn just a little bit more about the Moon and stars.

Script by Damond Benningfield

Moon-free summer evenings are the best times of year to catch the splendor of the Milky Way. This hazy band of light represents the combined glow of millions of stars. They outline the disk of the Milky Way galaxy.

The disk spans 100,000 light-years or more. And out where we are, more than half way from the center of the disk to its rim, it’s a few thousand light-years thick.

These fantastic distances also represent fantastic amounts of time. If the galaxy is a hundred thousand light-years across, then it takes light a hundred thousand years to cross from one side to the other. So as we look into the Milky Way, we’re seeing different parts of the galaxy as they looked at different times in the past.

That means we don’t know what the galaxy really looks like right now -- we have to wait for light to reach us. So one of those millions of stars in the Milky Way could have exploded thousands of years ago, but we might not know it for thousands of years more.

To see the splendor of the Milky Way -- with or without exploding stars -- you need to get away from city lights. After twilight fades away, it stretches from teapot-shaped Sagittarius, in the southeast, through the Summer Triangle in the east, and down to W-shaped Cassiopeia in the north. And it climbs high overhead by the wee hours of the morning. It’s a sight you need to see at least once. And when you do, you’re likely to want to see it again -- the outline of our galactic home.

Script by Damond Benningfield

The evening star has a date with the lion the next couple of nights. They’re due west as the color of twilight begins to fade away, and set a couple of hours later.

The “evening star” is Venus, our closest planetary neighbor. It’ll snuggle close to the star Regulus, the heart of the constellation Leo, the lion.

Venus and Regulus line up together because they both lie near the ecliptic, which is the Sun’s path across the sky. Regulus, in fact, is only about half a degree from the ecliptic -- a position that it maintains all the time. From that fixed location, it encounters the Moon about once a month, and the planets at varying intervals.

Venus stays close to the ecliptic, too. Its orbital path is tilted a bit with respect to the ecliptic, though, so Venus moves from one side of the ecliptic to the other. But it’s never more than a few degrees away from it.

On rare occasions, Venus can actually cross between Earth and Regulus, briefly blocking the star from view. The last time that happened was 59 years ago today -- July 7th of 1959. And it won’t happen again until October 1st of 2044.

These eclipses are so rare for a couple of reasons. One is that Venus and Regulus generally pass each other no more than once or twice a year. Another is that Venus moves back and forth across the ecliptic. And finally, Venus covers a small area of the sky, so the alignment has to be perfect for Venus to cover up the lion’s mighty heart.

Script by Damond Benningfield

With the Sun beating down on the northern hemisphere, it’s hard to realize that we’re farthest from the Sun for the entire year today. We’re about a million and a half miles farther than the average distance of 93 million miles.

That distance forms one of the basic “yardsticks” for all of astronomy. It’s called the AU -- the astronomical unit. Instead of expressing distances within the solar system in miles or kilometers, astronomers use the AU. So Earth is one AU from the Sun, Mars is about one and a half, mighty Jupiter is more than five, and Pluto averages almost 40 AU.

Determining the length of the astronomical unit took a lot of work. Some of the earliest estimates were made using the size of Earth’s shadow during a lunar eclipse, the angle between the Sun and Moon in the sky, or the length of shadows cast at different locations at the same moment. These measurements were so hard to get right, though, that estimates were off by tens of millions of miles.

Starting in the 1600s, astronomers used transits of Venus across the face of the Sun. Recording the time and angle of a transit from different places on Earth made it possible to calculate the Sun’s distance. But that was tough to do as well, so the measurements still weren’t quite right.

It wasn’t until the 20th century -- using radar and signals from spacecraft -- that astronomers finally zeroed in on the true length of the AU -- the yardstick of the solar system.

Script by Damond Benningfield

The seldom-seen planet Mercury is putting in a pretty decent appearance in the early evening sky right now. It’s quite low as twilight begins to fade away, a little to the north of due west. It looks like a moderately bright star, well to the lower right of brilliant Venus, the “evening star.”

Mercury is closer to the Sun than any other planet. From our perspective here on Earth, that means it never moves far away from the Sun. At best, it’s visible for a little while after sunset or before sunrise. So while it’s usually pretty bright, it’s so low in the sky, and so often immersed in the glow of twilight, that it’s easy to miss or overlook.

Because Mercury is so close to the Sun, it’s quite hot. But a few spots are icy cold. In fact, they may be filled with ice. And a recent study found some icy spots that no one had noticed before.

The spots are at the bottoms of craters at Mercury’s poles. They never see the Sun, so they stay quite cold, allowing ice to build up. And using observations from a spacecraft that orbited Mercury, scientists recently found three more ice-filled craters. The ice in these craters alone could cover more than a thousand square miles.

The study also found smaller amounts of ice outside those craters, in small wrinkles and depressions where sunlight is also rare. If people ever live on Mercury, that ice -- big deposits and small -- could provide a handy resource on the Sun’s closest planet.

Script by Damond Benningfield

In the 1920s, Edwin Hubble confirmed that there are galaxies of stars far beyond the Milky Way, vastly expanding the size of the known universe. Later, he also discovered that the universe itself is expanding.

Those discoveries were made possible by another discovery a few years earlier. Henrietta Swan Leavitt found that a certain type of star makes a perfect “yardstick” for measuring astronomical distances.

Leavitt was born 150 years ago today. She earned a college degree, although little of her schoolwork involved astronomy. She then started taking classes at Harvard College Observatory, but soon got a job there instead. She was a “computer” -- one of several women hired to do the tedious calculations now performed by digital computers.

Leavitt was bright and hard working, so she was given a complex assignment -- to study a class of stars that brighten and fade over time.

She found that there was a direct relationship between the amount of time it took such a star to brighten and fade and the star’s true brightness. And by knowing a star’s true brightness, astronomers can calculate its distance.

Edwin Hubble studied some of these variable stars in what was called the Andromeda Nebula. Many thought the nebula was a mote of matter inside the Milky Way. Others thought it was another galaxy. Hubble’s observations showed that it was so far away that it had to be a galaxy -- a discovery made possible by the pioneering work of Henrietta Leavitt.
 

Script by Damond Benningfield