StarDate

Astronomy is a science of numbers. And none is more important than the Hubble constant — a measurement of how fast the universe is expanding. Astronomers have been refining that number for decades. But they don’t yet agree on just what it should be.

The number is named for Edwin Hubble, who discovered that the universe is expanding. He also found that there’s a relationship between the distance to a remote galaxy and how fast the galaxy is moving away from us as a result of that expansion. He used that relationship to calculate the rate of expansion.

After Hubble, other astronomers used several types of stars to get better measurements of the distances to other galaxies — and how fast the galaxies were moving away from us.

In the language of astronomy, that yielded a value for the Hubble constant of about 74 kilometers per second per megaparsec. In other words, for every million light-years you travel away from Earth, the rate of expansion increases by about 50,000 miles per hour.

But a technique that measures the “afterglow” of the Big Bang came up with a smaller number. And a third method, which measures dying stars, recently came up with a number that’s between the other two. So there’s still no agreement on the Hubble constant — one of the most important numbers in astronomy.

It’s especially important because the expansion rate helps astronomers determine the age of the universe. More about that tomorrow.

Script by Damond Benningfield

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There aren’t any road signs to tell us the distances to remote galaxies. So astronomers look for the next-best thing: stars that flicker. The way they flicker helps reveal how far away they are. Astronomers use those distances to help learn such details as the age and fate of the universe.

Lots of stars flicker.

Cepheid variables, for example, pulse in and out like beating hearts. They get brighter and fainter with each beat. And there’s a relationship between the length of the beats and a star’s true brightness. So by measuring the beats and how bright the star looks, astronomers can determine how bright it really is.

It’s like seeing a faint light down a dark highway. If you know it’s a porchlight, then it must be fairly close. But if you know it’s a searchlight, then it must be far away. So knowing how bright a star is tells astronomers its distance as well.

Another type of star flickers just once: a Type 1a supernova. That’s because it marks the star’s total destruction. The way it brightens and fades reveals its peak brightness — and, therefore, its distance: possibly billions of light-years.

Finding the distances to these stars reveals the scale of the universe. Astronomers combine that with measurements of how fast the stars’ home galaxies are moving away from us to determine how the universe is expanding. That helps tell us how old the universe is — and what will happen to it in the distant future.

More tomorrow.

Script by Damond Benningfield

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Comet Thatcher is pretty ordinary. It’s a ball of ice and rock that takes about 415 years to orbit the Sun. It follows a long, skinny orbit that takes it deep into the outer solar system — more than five billion miles from the Sun.

The comet reminds us of its presence every year, though, with a meteor shower. And the shower should be at its best tomorrow night. It’s called the Lyrid shower because its meteors all appear to “rain” into the atmosphere from the constellation Lyra.

Comet Thatcher was discovered 160 years ago this month. Amateur astronomer A.E. Thatcher was scanning the skies above his home in New York with a telescope when he saw a small, fuzzy blob. The comet soon became bright enough to see with the naked eye. A few years later, astronomers figured out that the comet was the source of the Lyrid meteors.

As the comet moves close to the Sun, heat vaporizes some of the ice at its surface. That releases bits of rock and dirt into space. They spread out along the comet’s orbit. Earth passes through that path every April. So some of the bits of debris plunge into our atmosphere at speeds of 30 miles per second. At that rate, they quickly vaporize — forming the streaks of light known as meteors.

This year’s Lyrids should be at their best late tomorrow night. Unfortunately, the gibbous Moon will be in the way during the peak hours. So only a few Lyrids are likely to shine through — the offspring of a comet.

Script by Damond Benningfield

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The residents of Punjab, a region of India, got a good scare 400 years ago today. A bright meteor streaked across the dawn sky, accompanied by a loud rumble. It then plunged to the ground, burying itself in the dirt. A local official ordered villagers to dig up the remains: a chunk of iron that weighed several pounds.

The official sent the prize to the emperor of the Mughal Empire, which incorporated much of present-day India and part of Afghanistan. And the emperor had it made into swords and daggers.

The piece of iron — a meteorite — probably was forged in the heart of an asteroid. The asteroid was big enough that it separated into layers, with iron and nickel at its center. A collision with another body split it apart, sending pieces flying through the solar system. One of those pieces landed in Punjab in 1621.

The emperor, Jahangir, was interested in many subjects, including astronomy and works of art. And any rock that fell from the sky was thought to have mystical powers. So it was an easy call to have the meteorite converted to weapons — symbols of royal power. The emperor named one of the swords “lightning iron.”

In the 1950s, one of the daggers found its way to the Smithsonian. Eventually, an X-ray showed there was a bit of nickel mixed with the iron in the blade. That’s good confirmation that the blade was forged from a meteorite — a bit of “lightning” that struck India 400 years ago.

Script by Damond Benningfield

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One of the oldest pieces of the International Space Station has been in place for more than 20 years. The Russian-built module is a descendant of the first space station, which was launched 50 years ago today.

Salyut 1 was based on a design for a military station. As the station was being developed, though, the Soviet Union transferred it to another agency.

It was given the name “Salyut” just days before launch. It’s the Russian word for “salute.” The name honored cosmonaut Yuri Gagarin, who became the first person in space a decade before Salyut was launched.

The space station was about 50 feet long, and weighed 40,000 pounds at launch. It was divided into several compartments for cosmonauts to live and work in.

The first cosmonauts arrived at Salyut five days after its launch. But they couldn’t link up with the station, so they had to return home. A second crew arrived a couple of months later. The three cosmonauts operated several experiments, including an ultraviolet telescope that looked at hot stars. But the cosmonauts died during the return to Earth when a valve opened early and their air escaped into space.

The Soviets launched eight more Salyut space stations, and six of them were successful. They hosted dozens of cosmonauts, who eventually spent as long as a year aboard them. Their success led to the Mir space station — and, eventually, to one of the longest-serving pieces on the International Space Station.

Script by Damond Benningfield

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The Moon and Mars drop down the western sky this evening. They’re high in the sky at nightfall, with Mars to the lower right of the Moon. They set by 1 or 2 in the morning.

The Sun lights up only about a quarter of the lunar hemisphere that faces our way, but you can still make out some of the Moon’s dark features. They formed when giant asteroids or comets slammed into the Moon. The impacts blasted out wide, shallow basins that filled with molten rock.

Samples from the Apollo missions have suggested the bombardment was especially heavy about four billion years ago — about a half billion years after the birth of the planets.

By then, there shouldn’t have been many big asteroids or comets still around. But the giant outer planets may have been changing positions, moving toward the Sun, then back out again. That could have stirred up any leftovers, sending them toward the inner planets. If so, then not just the Moon got smacked; so did Earth, Mercury, Venus, and Mars.

The idea of a late bombardment has faced some challenges. Some researchers have suggested that many of the Apollo samples came from the formation of just one large basin, not many. In that case, the volcanic rocks from the Apollo missions should be the same age.

The issue won’t be settled until new missions bring back more samples from all across the Moon — telling us whether the worlds of the inner solar system took a heavy pounding four billion years ago.

Script by Damond Benningfield

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If science fiction has its way, future travelers to Mars might arrive on a giant volcano. Pavonis Mons is miles high, and it lies along the equator. That makes it a perfect spot for a space elevator — a massive chord connecting Mars and a platform thousands of miles out in space. People and cargo could arrive at the platform, then enter elevator cars for the long ride down.

Pavonis Mons is the middle peak in a chain of three giant volcanoes. It’s about nine miles high and a couple of hundred miles wide. It’s topped by two large craters from which lava once flowed. The bigger of the two is also older — it’s mostly filled with dust. The other is younger and sharper.

Pavonis Mons and the other volcanoes built up as thick lava flowed down their flanks. The lava could have flowed for hundreds of millions of years or longer, building the mountains to giant proportions. There’s no evidence that Pavonis Mons is active today. But it could have been active within the last few million years.

If people ever do arrive at Pavonis Mons from an elevator, the mountain even offers a few places to stay. Spacecraft have photographed the entrances to deep caves that tunnel into the mountain. If they could be sealed up, they might provide a comfortable habitat — an arrival spot on a Martian volcano.

Mars follows the Moon down the sky this evening. The planet looks like a fairly bright orange star.

Script by Damond Benningfield

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Aldebaran, the bright orange eye of the bull, accompanies the Moon down the western sky this evening. It’s to the left of the Moon, and it’s easy to pick out.

Thousands of years ago, Aldebaran wasn’t dropping from sight in the evening sky at this time of year. Instead, it was just climbing into view in the morning sky. And some cultures might have paid close attention to that first appearance.

About 6,000 years ago, the cultures of central Portugal built many stone monuments. Some of them consisted of a central chamber made from tall slabs of granite, with a long, narrow entryway. Researchers have concluded that many of those entryways aligned with the sunrise during late spring. That was when the tribes were moving their sheep and goats to higher ground for the summer.

But some of the passageways have different alignments. Instead of the sunrise, they face the rising points of Aldebaran and Betelgeuse, the bright orange shoulder of Orion. Aldebaran rose first, in late April — a more likely time to be heading for the summer grounds. So one researcher has concluded that those monuments could have served as observatories. An observer sat inside the main chamber and looked out through the entryway. When he first saw Aldebaran climb into view in the bright dawn twilight, it was time to move — nudged to higher ground by the eye of the bull.

We’ll talk about the Moon and another orange companion tomorrow.

Script by Damond Benningfield

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A star in the system known as T Pyxidis isn’t a good neighbor. It’s taking gas from a companion star. And every few decades, it erupts, blasting away more of the companion. So the companion could soon disappear.

T Pyxidis is a recurrent nova. That means it sometimes flares thousands of times brighter than normal. The first known outburst took place as early as 1866. And the most recent was first seen 10 years ago today.

The system’s main star is a white dwarf — the dead core of a once-normal star. It appears to be more massive than the Sun, but only about as big as Earth.

The companion star is larger but less massive. The white dwarf pulls gas from its surface, which then settles on the white dwarf. This gas is squeezed tightly, so it gets hotter and hotter. When it gets hot enough, it explodes like a giant H-bomb. A new blast takes place roughly every two to four decades.

A study a few years ago found that the system gets a little fainter after each outburst. That could mean that the white dwarf is blasting more material into space than it’s taking from the companion. The strong outbursts could be eroding the companion. So in a couple of hundred thousand years, the companion star could vanish.

T Pyxidis is in Pyxis, the compass, which is low in the south at nightfall. The star is too faint to see without a telescope. It may be biding its time — building up to another eruption in a decade or so.

Script by Damond Benningfield

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A star can be old and new at the same time. In fact, it can even be dead and new. The dead star can gain a bit of new life from a companion. That turns the star into a nova — a Latin word that means “new.”

The term was first used more than four centuries ago. At first, it applied to stars that suddenly became visible where no star had been seen before. Such stars were “new” to the night sky.

As astronomers developed the tools to study the stars in detail, though, they discovered that the stars weren’t new at all. Instead, they were dead stars that gained a brief bit of glory from a close companion.

The dead star in such a system is a white dwarf — the hot, dense core of a once-normal star. The white dwarf’s powerful gravity pulls hydrogen off the surface of its companion — usually a bloated star that soon will become a white dwarf as well.

Some of the gas piles up on the surface of the white dwarf. This new layer gets hotter and hotter. Eventually, it gets hot enough to explode like a giant H-bomb. That blasts the new material out into space. In a few hours, the system can flare to thousands of times its normal brightness — creating a nova.

The blast interrupts the flow of hydrogen from the companion. Before long, though, it starts again. And that can produce another explosion — usually thousands of years later. But some novas can pop off again in just a few decades. And we’ll talk about the best-known example tomorrow.

Script by Damond Benningfield

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