StarDate

Putting together a profile of a star is hard, even for one of the brightest stars in the sky. It takes careful observations and good models of how stars work. And as technology and theory improve, so does the profile.

As an example of that process, consider Canopus, the second-brightest star in the night sky. At this time of year, it just peeks above the southern horizon in early evening from the southern third of the United States. It’s well below Sirius, the night’s brightest star.

One key is a star’s distance. Until about 25 years ago, estimates of the distance to Canopus varied by hundreds of light-years. But a space telescope locked it down: 310 light-years.

Knowing the distance reveals the star’s true brightness. That’s combined with other details, then plugged into models of how stars evolve. That provides a good estimate of the star’s age, its stage in life, and more.

One other key detail is the star’s size, which is hard to measure — Earth’s atmosphere blurs the starlight into a fuzzy blob.

In a recent study, though, astronomers got the best measurement of Canopus’s diameter to date — about 74 times the Sun’s diameter. Adding that to the other details refined the star’s profile. It varied a little depending on which model was used. But the models agreed that Canopus is almost 10 times the mass of the Sun, and anywhere from 24 million to 34 million years old.

Of course, that could change — with more hard work.

Script by Damond Benningfield

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Every once in a while, the planets go into hiding. They pass behind the Sun as seen from Earth, so they’re lost in the Sun’s glare. They can remain hidden for weeks.

Right now, it’s Jupiter’s turn to hide. The solar system’s largest planet is at conjunction — on the opposite side of the Sun from Earth. It’s farthest from Earth now, too — about 555 million miles away.

Solar conjunction gives the teams that operate spacecraft a bit of a break. There’s so much radio interference from the Sun that controllers can’t stay in touch with their craft.

In Jupiter’s case, that’s the Juno mission. It’s been orbiting Jupiter since July of 2016. It’s provided the first good look at Jupiter’s insides. It’s done so by measuring Jupiter’s magnetic and gravitational fields from different altitudes and different locations above the planet. That’s provided a three-dimensional view of the interior.

Those observations have revealed that Jupiter’s core isn’t what scientists expected. Instead of a solid ball of rock and metal, it’s more mushy — it just kind of melts into the layers around the core.

Juno is expected to keep working for at least a few more years — except for some short breaks at solar conjunction.

Jupiter will start to peek into view in the dawn sky by the end of March. But it won’t be an easy target until April — shining like a brilliant star in the eastern sky at first light.

Tomorrow: a brilliant star in the south.

Script by Damond Benningfield

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One of nature’s most beautiful events should get started over the next few weeks. Monarch butterflies will leave their winter homes in central Mexico and head north. They’ll fan out over the eastern two-thirds of the United States and into Canada. And they’ll use the brightest of all lights to guide them: the Sun.

Many animals navigate with the help of the Sun and stars, from birds to sea turtles. Monarchs do so using special cells in their brains. The cells process details about the Sun’s location in the sky. If the butterfly knows the time of day, then the cells can act as a solar compass.

A recent study found that the cells don’t switch on until the butterflies take to the air. Researchers studied the behavior of several monarch butterflies inside a flight simulator. They tied the butterflies to a rotating rod inside the simulator, and used tiny electrodes to monitor the monarchs’ brain activity.

They found that when the butterflies were on the ground, those regions of their brains were turned off. When they took to the air, though, the special cells turned on. The butterflies maintained their direction by keeping an eye on the simulated Sun in the sky.

The research team isn’t through with the study, though. The next step is to examine monarch behavior under more realistic sky conditions — in open-air simulators. The additional work will tell us even more about how these beautiful but delicate butterflies migrate across thousands of miles.

Script by Damond Benningfield

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The solstices and equinoxes have been important calendar markers over the centuries. To track them, many cultures built special observing sites. Perhaps the most impressive was in the desert along the coast of Peru. Known as Chankillo, it was designated as a UNESCO World Heritage site last year.

Chankillo was a complex of three sites: a hilltop fortress, a plaza, and a row of blocks known as the Thirteen Towers. The complex was built more than 2300 years ago, and was abandoned a couple of centuries later — perhaps as the result of a war.

The towers were the centerpiece of a solar observatory. They were built along the ridge of a small hill, making the site look like the spiny back of a dinosaur. The towers are up to 20 feet high, and are spaced about 15 feet apart.

Viewing sites flank the towers — one to the east, the other to the west. From these sites, observers could watch the Sun rise and set above or between the towers across the year. The northernmost tower marked sunrise at the winter solstice, in June. The southernmost tower marked sunrise at the summer solstice, in December. So the Sun would spend half the year moving in one direction across the towers, and the other half moving the opposite direction.

The site would have allowed its users to know the date with an accuracy of two or three days. That makes Chankillo unique in the ancient world — a year-long calendar that tracked the motions of the Sun.

Script by Damond Benningfield

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The Big Dipper wheels across the north on March nights. Tonight, it’s low in the northeast at nightfall, with the bowl above the handle. It passes high overhead at midnight, and is low in the northwest at first light.

This prominent star pattern has been recognized by many cultures, although with different names and descriptions. In ancient Egypt, for example, it was Meskhetyu — the leg of a bull. It’s recorded on one of the oldest astronomical charts on the planet — created 3500 years ago.

The chart decorates the ceiling of a tomb, which was being built for Senenmut. He was the top advisor to Hatshepsut, the most successful of Egypt’s female “kings.” The tomb was never finished, and Senenmut wasn’t buried there. But it remained sealed until 1927, so the ceiling is well preserved.

It’s divided into two panels — one for the northern half of the sky, the other for the southern half. The northern panel shows the constellations, although only one is easy to recognize: the Big Dipper. Its stars were called “imperishable” because they never set; at the time, they always remained above the horizon as seen from Egypt.

The southern panel shows four of the five planets visible to the eye alone — only Mars is missing. The panel may record a conjunction of the four planets. The panel also indicates that the Egyptian day was divided into 24 hours, with a different star for each hour — tracking time by the stars.

Script by Damond Benningfield

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The stars have guided human actions for thousands of years. Farmers used the stars to tell them when to plant and harvest crops. And kings and queens used them to pick the best times for wars, coronations, weddings, and other big events. So it was important to find the right spot for watching the stars. Often, that meant building an observatory — a structure with a clear view of the night sky.

An example is Cheomseongdae — a name that means “star-gazing platform,” in South Korea. It’s the oldest-known surviving observatory in eastern Asia, dating back almost 1400 years.

The observatory is a 30-foot tower made from granite blocks. The tower is shaped like an old milk bottle — wider at the base than at the top. A wider tier sits atop it. Seen from above, that tier forms an Asian character for the word “well.”

The tower consists of 365 blocks — one for each day of the year. The entire structure consists of 30 tiers — the number of days in a lunar month. And its base consists of 12 stones — perhaps representing the number of months in a year.

No one is sure just how Cheomseongdae was used to watch the stars. There’s a window half way up the tower. An observer might have climbed to the window on a ladder, then climbed a ladder on the inside to reach the top. From that high perch, the observer could watch the stars, Sun, Moon, and planets — perhaps using them to help guide the people of ancient Korea.

Script by Damond Benningfield

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You can look at something from afar all you want, but there’s nothing like being there. For the scientists who study the Sun, that means plunging into its hot outer atmosphere. And they’ve done that several times in the last year with Parker Solar Probe — a spacecraft that’s flown close to the Sun’s visible surface.

Nicola Fox, the leader of NASA’s program to study the Sun, announced the achievement late last year:

   Humanity has touched the Sun. Parker Solar Probe has allowed us to dive beneath the surface to experience, for the first time, the atmosphere of a star.

The outer atmosphere is the corona. It’s visible during solar eclipses as faint wisps around the Sun. It extends several million miles into space. Scientists weren’t sure of its extent until last April, when Parker made its first pass through the corona — the first passage by any mission.

The corona is heated to millions of degrees by the Sun’s magnetic field, but astronomers aren’t sure how. These encounters will help them explain it. They’ll also show how the corona feeds the solar wind — a flow of charged particles that races through the solar system at a million miles per hour or faster.

Parker Solar Probe follows long, looping orbits around the Sun. It comes close every few months. Heavy shielding protects it from the heat, and it moves faster than any other spacecraft ever. So far, that’s kept it safe during its dives into the atmosphere of the Sun.
 

Script by Damond Benningfield

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There’s much more to the Sun than the brilliant ball we see in the sky. Eruptions on the surface produce towers of hot gas, and a million-degree outer atmosphere extends far into space. Those structures are impossible to see in the glare of the solar disk, which is a million times brighter. But an astronomer born 125 years ago today invented a device to block out the disk, revealing the extended Sun.

Bernard Lyot was born in Paris. He trained as an engineer, then taught at a Paris high school. While there, he became interested in astronomy, and even built his own telescopes.

In 1920, he joined France’s Meudon Observatory. He worked there until his death, during an eclipse expedition, in 1952. He was especially interested in the Sun, and in developing new tools for studying it.

In the 1930s, he perfected the coronagraph. It uses a series of lenses and filters to block out the Sun. That reveals the eruptions, known as prominences, and the outer atmosphere — the corona. Before then, these features were visible only during solar eclipses.

Lyot used his invention to snap the first pictures of the prominences and corona not taken during an eclipse. And at a conference in 1938, he wowed his fellow astronomers with movies of those features.

Today, astronomers around the world use the invention to keep a close eye on the space near the Sun.

A spacecraft is flying into that region, and we’ll have more about that tomorrow.
 

Script by Damond Benningfield

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There’s a beautiful conjunction at first light tomorrow — a line-up of the Moon and the planets Venus and Mars. Venus is the “morning star.” Mars is much fainter, but it’s almost directly between Venus and the Moon, so you shouldn’t have any trouble finding it.

The Moon, Venus, and Mars are the closest worlds to Earth. Not surprisingly, they’re also the most often explored. To date, space agencies have dispatched more than 200 missions to them. And the success rate is about 50-50.

The Moon has been the target of the most missions — more than a hundred, depending on how you count them. That’s included fly-bys, orbiters, landers, and rovers, plus the astronaut-carrying Apollo missions.

Mars is in second place. Again, it depends on how you count them, but roughly 30 missions have been fully or partially successful, with a similar number of failures.

Venus has been targeted fewer times, but with a higher success rate — roughly two dozen successes, and fewer than 20 failures. In addition, several craft have studied Venus while using its gravity to boost them to other planets.

Most of the failures came in the early years of space exploration. The success rate has been far higher since then. That should come as encouraging news for the folks who send out missions today. Space agencies are planning to send dozens of craft to the Moon, Mars, and Venus through the end of this decade — emissaries to our close neighbors.
 

Script by Damond Benningfield

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The planets of the solar system are all different. But most of the planets with atmospheres have something in common: seasons. Today, in fact, marks the start of autumn in the northern hemisphere of Mars.

The seasons are caused by a planet’s tilt on its axis. Mars is tilted by about 23 degrees — almost exactly the same as Earth. Because of that, the Martian poles dip toward or away from the Sun at different points in the planet’s orbit. When a pole dips toward the Sun, its hemisphere sees more sunlight, so it gets hotter.

Right now, at the equinox, the poles receive equal amounts of sunlight. But the south pole will tip toward the Sun over the coming months, heading toward southern summer.

One planet with an atmosphere that doesn’t have seasons is Venus. It’s tilted by only three degrees, so its poles don’t really dip toward the Sun — they barely nod, in fact. So the northern and southern hemispheres don’t see much change in either the amount of daylight or the temperature.

And Mars’s orbit is lopsided, so the planet’s distance to the Sun changes by a good bit. That amplifies the seasonal changes. But Venus’s orbit is almost circular. So conditions stay just about the same all across Venus, all year long.

Look for these worlds in the southeast at first light. Venus is the “morning star,” with Mars close to its lower right. The Moon slides past them the next couple of mornings, forming a bright trio in the dawn sky.
 

Script by Damond Benningfield

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