StarDate

Stars are big and heavy. But they aren’t necessarily very dense. In fact, the outer layers of some stars are little more than a vacuum — a few atoms of gas with a lot of space between them.

One example is Aldebaran, the bright orange star that marks the eye of Taurus, the bull. It’s to the upper left of the Moon this evening, and to the lower right of the Moon tomorrow night.

All stars are big balls of gas, so they don’t have a rigid structure. But they’re massive, so they have a strong gravitational pull. Gravity squeezes the star’s core tightly, making temperatures high enough to sustain nuclear fusion.

Aldebaran has completed the first round of its fusion reactions. When that round ended, the core got smaller and denser — hundreds of times denser than water. The star’s outer layers, on the other hand, got puffier. Today, the star is almost 45 times the Sun’s diameter — and 85,000 times its volume.

Recent work says Aldebaran is only a bit more massive than the Sun. So when you compare its “weight” to its volume, you find that Aldebaran’s outer layers are really spread out. On average, they’re less than a hundredth of a percent as dense as Earth’s atmosphere at sea level. And the outermost layers are even thinner.

So much of Aldebaran is pretty insubstantial — not much more than a vacuum. The star is so big, though, that its material adds up — making it a bold presence in the galaxy — and in the night sky.

Script by Damond Benningfield

Children sometimes play leapfrog. We’re often encouraged to take a leap of faith. Technology sometimes takes a quantum leap. And today, the whole world takes a leap — it’s leap day, an extra day added to the calendar every four years.

The calendar is based on Earth’s annual journey around the Sun. But that journey isn’t an even number of days — it’s roughly 365 and a quarter days. So if the calendar was always set to 365 days, it would fall out of sync with the seasons.

To compensate, Julius Caesar ordered that an extra day be added to every fourth February. That made up for the extra few hours it takes Earth to orbit the Sun.

Well, almost. The full year is actually a few minutes less than 365 and a quarter days. Those minutes added up. By the 16th century, the calendar had drifted several days with respect to the seasons.

So Pope Gregory XIII ordered a change to the Leap Year rule. Leap day was dropped from years that are divisible by a hundred, but not by four hundred. So the year 1900 wasn’t a Leap Year, but 2000 was.

With that change, the calendar won’t need another adjustment for thousands of years.

Incidentally, the name “leap” day comes from the fact that a calendar date jumps over a day of the week during Leap Year. Last year, for example, March 1st came on a Friday. In a regular year, the next March 1st would fall on a Saturday. This year, though, it’s on a Sunday — a “leap” forward thanks to Leap Year.

Script by Damond Benningfield

To the people of Cortland, New York, Lewis Swift was a bit of an oddball. Most nights, for a little while after sunset and before sunrise, the hardware merchant used a small telescope to scan the sky around the Sun.

In 1862, though, his status changed from “oddball” to “celebrity.” That summer, Swift discovered a comet — one that went on to stage a great show. And today, the comet is better known as the parent of the Perseid meteor shower.

Swift was born on Leap Day of 1820 — 200 years ago tomorrow. At age 13, he fractured his hip in an accident. So for the next couple of years, he devoted more time to study than farm work. In 1850, he developed an interest in microscopes. And a few years later, he built his first telescope.

Swift became fascinated by comets after seeing a brilliant one in 1858. The comet was the topic of his first scientific paper.

When Swift first saw the comet of 1862, he wasn’t sure if it was new or one that had been seen before. A few days later, though, he heard that Horace Tuttle had reported the discovery of that same comet. Swift quickly reported his own observations, so the comet was named Swift-Tuttle in their honor.

After 1862, the comet disappeared from view for more than a century. Swift discovered several more comets, and eventually became a full-time astronomer. He studied everything from eclipses to galaxies — a career built on the discovery of a comet.

Script by Damond Benningfield

Venus is blanketed by clouds. They cover the entire planet — there’s not a single hole in them to provide a view of the surface below. Clouds at the top of the atmosphere are made of sulfuric acid. And they buzz around the planet at up to 250 miles per hour — carried by super-strong winds.

That’s especially puzzling because Venus rotates on its axis at a relative snail’s pace. It takes 243 Earth days for it to make one full turn. So the high-speed winds are moving much faster than the surface below — an effect called “super-rotation.”

The pattern of those winds is complicated. Observations by a European spacecraft showed that the wind speed picked up over a period of eight years — by about a third.

And a Japanese mission found that the wind speed changes from year to year. It also found a difference in winds between the northern and southern hemispheres, and between the day and night sides of the planet.

Scientists still aren’t sure why Venus’s high-level winds are so fast, or why they’ve changed in recent years. It may take new missions to our neighboring planet to solve the mystery of its super-speedy winds.

And Venus is in great view right now. It’s the brilliant “evening star,” so it’s the brightest object in the night sky other than the Moon. Venus is quite close to the right of the Moon tonight — a brilliant light that’s blanketed by brilliant clouds.

Tomorrow: from store owner to comet-hunting celebrity.

Script by Damond Benningfield

Stars can be artistic. They can sculpt fantastic figures: hourglasses, butterflies, owls, and many others. Sometimes, the figure is created by a single star. Other times, it’s a collaboration between two stars. And in at least one case, it’s three stars.

The trio has created NGC 246, the Skull Nebula. It’s about 1600 light-years away, in the constellation Cetus. As the sky darkens tonight, it’s just above the horizon, to the lower left of the line formed by the Moon and Venus, the “evening star.”

The raw material for the skull was supplied by one of the three stars. About 6600 years ago as seen from Earth, the star began expelling its outer layers into space.

Ultraviolet energy from the star’s now-dead core causes those layers to glow. The core is a bit less massive than the Sun. Models show that the star began life with more than four times the Sun’s mass. Its “normal” lifetime lasted only a quarter of a billion years.

As the star’s expelled outer layers expand, their shape is sculpted in part by its companions. One of them is fairly close, and is about the same mass as the donor star. Its gravity pushes the gas and dust around, helping form the “skull”-like appearance. The third star is small and a long way out, so its influence is small.

The nebula is continuing to expand — at 90,000 miles per hour. And the original star is completing its transition to the next phase in life — a small but hot corpse known as a white dwarf.

Script by Damond Benningfield

Maria Kirch was the first woman to receive credit for discovering a comet. But the credit didn’t come right away. Her husband was concerned that having his wife receive credit for a discovery would damage his career. So he claimed the discovery for his own. He didn’t set the record straight for eight years.

Kirch was born 350 years ago today, as Maria Winkelmann, in Germany. She was educated by her father and uncle. And she learned about the stars from a local astronomer.

The astronomer introduced her to her future husband, Gottfried Kirch. After they were married, he was made Astronomer Royal of Brandenburg. Maria was his assistant. They took turns observing the heavens. That’s how Maria discovered her comet, in 1702.

By 1710, she was publishing under her own name. She wrote about aurorae, and about prominent sky events. That made her one of the best-known astronomers in Europe.

When her husband died, she applied for his position. But the Royal Academy didn\'t want to set a precedent by hiring a woman. So she assisted other astronomers.

Kirch also taught astronomy to her children. And in 1716, her son was appointed to a post with the Berlin Observatory. Kirch and her daughters became his assistants.

But the Royal Berlin Academy of Sciences complained that she was too prominent in observatory life. When she refused to tone it down, she was expelled from her observatory quarters — ending her career in astronomy.

Script by Damond Benningfield

Our galactic neighborhood is a long way from the bright lights of downtown. But the boring location is also a lot safer. We don’t have to worry about exploding stars, neutron stars, black holes, and some of the other threats that fill the Milky Way Galaxy.

The heart of the Milky Way is jam-packed. Stars are separated from each other by light-months or even days. Powerful magnetic fields loop through star clusters and clouds of gas and dust, causing some to collapse and others to stay spread out. And in the past, the galactic center has produced millions of powerful stellar explosions. Today, the massive corpses of those stars orbit the supermassive black hole at the galaxy’s hub.

That’s probably not a great environment for giving birth to planets. And it’s an even worse environment for nurturing life. That doesn’t mean life can’t exist there — only that it seems unlikely.

Our solar system is about 27,000 light-years from that chaos — half way from the center to the edge of the galaxy’s disk.

Stars out here are widely spread. So are the nurseries that give birth to stars. Because of that, we’re hundreds of light-years from any stars that are likely to explode. And there are no known neutron stars or black holes within thousands of light-years.

So our skies are a lot darker than those of many other regions in the Milky Way. But we face fewer dangers from the neighbors — making our part of the galaxy much more comfortable for life.

Script by Damond Benningfield

The proper names of many stars come from ancient Arabic. And many of those names describe a star’s position in the sky. Denebola, for example, means “the tail of the lion,” Betelgeuse is the armpit of Orion, and Hamal is the head of the ram.

Five stars in a dark region of the sky were all named Minazal, which means “belonging to the uninhabited spot.” At the time they were named, they didn’t belong to any constellation. Each one was assigned a number to keep them apart.

The brightest member of the group is Minazal 5. Today, it’s also called Zeta Hydrae because it’s in the modern constellation Hydra, the water snake.

The star is about 160 light-years away. It’s much bigger, heavier, and brighter than the Sun.

It’s only about one-tenth of the Sun’s age — about 400 million years. Because of its greater mass, though, it’s already passed the end of its “normal” lifetime. It’s burned through the hydrogen in its core, creating helium. Now it’s burning the helium to make carbon and oxygen — a phase of life the Sun won’t enter for billions of years.

The five Minazal stars form the head of the water snake. The group is in the east-southeast as night falls. It’s about half way between the bright stars Regulus, well to its lower left, and Procyon, to the upper right. From a light-polluted city, Minazal 5 may be the only member of the group that’s visible to the unaided eye — the brightest of the five stars in “the uninhabited spot.”

Script by Damond Benningfield

Ancient skywatchers drew 48 constellations that are still in use today. Most of them consisted of patterns of bright stars that formed a somewhat recognizable figure. An example is Leo, the lion, which is low in the east at nightfall. Its “heart” is the star Regulus. Its body extends below Regulus, with its head and mane to the left of the bright star.

The dark regions between those patterns were left alone. But a couple of astronomers filled in many of those gaps in the 17th and 18th centuries.

Johannes Hevelius, for example, drew 10 constellations. Seven of them are still in use today. And two of them are cats that align to the upper left of Leo: Lynx and Leo Minor, the little lion. Leo Minor abuts Leo, with Lynx above the little lion.

Hevelius didn’t name Lynx for its resemblance to the cat, though. Its brightest stars form a faint zigzag that looks more like a snake. Instead, the constellation’s name is a bit of a joke. Hevelius wrote that skywatchers would need the eyesight of a lynx to see the constellation’s stars. But he was challenging them to give it a try.

The brightest stars of the two faint cats are similar. Each star is heavier than the Sun, and much farther along in life. That’s caused the stars to puff up to giant proportions. The brightest star of Lynx, in fact, is more than 50 times the Sun’s diameter, and hundreds of times brighter. Even so, it’s so far away that it’s pretty faint — a target for the eyes of a cat.

Script by Damond Benningfield

A faint star near the boundary between Orion and Taurus could have a bright future. The system consists of two stars. They’re so close together that they’re actually touching. Over time, they could merge to form a single star. The merger would produce a brilliant outburst known as a red nova.

Similar fates may await many other star systems. Astronomers have discovered many “contact” binaries — systems where the stars are touching each other. Some of them are known as over-contact binaries. They’re not just touching — they’re sharing their outer layers of gas.

The system in Orion isn’t quite that far along. If we could see the system in profile, it would look a bit like a rounded bowling pin. The bigger, heavier, and brighter star forms the fat end of the pin, with the lesser star forming the neck. A streamer of gas connects them.

Because the stars are exchanging some of their gas, they’re moving closer together. As that process accelerates, they could merge with a brilliant flash — something that’s been seen before.

An especially heavy contact binary in another galaxy could be on the path to a merger as well. If the stars do merge, they might produce a gamma-ray burst — one of the brightest events in the entire universe. But contact binaries don’t always merge. If the two monster stars stay apart, both of them might collapse to form black holes. And in the far-distant future, they might merge — final contact for two giant stars.

Script by Damond Benningfield