StarDate

The star cluster M79 is messy. It’s shedding some of its stars, creating a “tail.” Over the eons, in fact, the cluster might have lost most of the stars it was born with.

Messier 79 is a globular cluster – a ball-shaped family of about 150,000 stars. The cluster is more than 11 billion years old, so its stars are among the oldest in the entire Milky Way Galaxy.

There’s a trail of stars behind the cluster. The stars probably were stripped away by the gravity of the rest of the galaxy – especially its dense core.

Today, M79 is about 42,000 light-years from the Milky Way’s heart. But its orbit might bring it within just a few hundred light-years of the center. At that distance, the gravity of the galaxy’s core overpowers the gravity of the cluster. So stars in M79’s outskirts are pulled away. Eventually, they move away, and follow their own paths across the galaxy.

Some simulations have suggested that M79 has lost up to 85 percent of its original population of a million stars or so. And every future passage through the heart of the galaxy will pull away more stars – leaving only a glimmer of M79’s original glory.

M79 is in Lepus, the hare. The constellation is close to the lower right of bright Orion, in the southeast at nightfall. M79 is below the outline of the rabbit. You need binoculars to pick it out.

Script by Damond Benningfield

In 1845, British astronomer J.R. Hind saw an amazing star in the constellation Lepus, the rabbit. He wrote that the star looked “like a drop of blood on a black field.” Officially, the star is called R Leporis. But it’s also known as Hind’s Crimson Star – a star that looks redder than almost any other star in the galaxy.

R Leporis is a little heavier than the Sun. But it’s much later in life, which makes it a lot more interesting. It’s “fused” the original fuel in its core to make oxygen and carbon. Today, it’s producing energy in shells of hydrogen and helium around the core.

Those changes have caused the star’s outer layers to puff up, so R Leporis is hundreds of times the Sun’s diameter. But those layers are unstable. They pulse in and out like a beating heart. Each “beat” lasts about 14 and a half months. During that cycle, the star’s brightness varies dramatically; at its peak, it’s hundreds of times brighter than at its faintest.

As the star pulses, its temperature changes. At its largest, it’s a bit cooler, so it looks redder. And that color is amped up by the material in its outer layers. Carbon is pulled up from deep inside the star. It absorbs blue wavelengths of light, allowing the red to shine through – enhancing the “bloody” look of Hind’s Crimson Star.

Lepus is in the southeast in early evening, to the lower right of Orion. But you need a telescope to see Hind’s Crimson Star.

Script by Damond Benningfield

The brightest star of the rabbit is a member of a rare class. It’s a yellow supergiant – a star that’s about the same color as the Sun, but much bigger and brighter. It won’t stay in that class for long, though. It’ll quickly get hotter and bluer, then blast itself to bits as a supernova.

Arneb is the leading light of the constellation Lepus, the hare. It’s in the southeast at nightfall, to the lower right of brilliant Orion. Its name is Arabic for hare – a name that also represented the whole constellation.

Arneb is about a dozen times the mass of the Sun, perhaps a hundred times its diameter, and tens of thousands of times its brightness.

The star is about 13 million years old – compared to four and a half billion years for the Sun. But because of its great mass, Arneb has already completed the main phase of life. Changes in its core caused it to puff up to become a red supergiant.

Now, it’s getting smaller, which is making its surface hotter. As part of that transition, it’s turned yellow. But it won’t stay that color for long. As it continues to contract, it’ll get even hotter, so its surface will turn blue. And within a couple of million years, Arneb will explode. That will leave only a small, superdense core – a neutron star. It’ll be surrounded by an expanding cloud of debris that will shine for millennia – the final act of a rare and mighty star.

We’ll have more about the rabbit tomorrow.

Script by Damond Benningfield

From parts of the U.S., the Moon will briefly cover the heart of the lion tonight. The Moon will pass directly between Earth and the bright star Regulus, creating an occultation.

The Moon can occult Regulus because the star lies almost atop the ecliptic – the Sun’s path across the sky. The Moon stays close to the ecliptic, but it does move a little to either side. So occultations of Regulus come in groups. This one is part of a cycle that began last July and will continue through the end of this year. The occultations are separated by about 27 days, which is how long it takes the Moon to circle through the background of stars.

Each occultation is visible from a different part of Earth. In part, that’s because the Moon and Regulus are below the horizon as seen from much of the world. And the Moon is so close to us that there’s a big difference in the viewing angle across the globe. So from any specific location, sometimes the angle is just right, but more often it’s a little off.

This month, the angle is right for skywatchers in the eastern United States. For most of the rest of the country, the Moon will just miss the star. So all of us will see an amazingly close encounter between the Moon and the heart of the lion.

Only one more occultation in this sequence will be visible from anywhere in the contiguous United States – on April 25th. After that, we won’t see another one until 2044.

Script by Damond Benningfield

Odd little February is the shortest month of the year. Historians aren’t exactly sure just why that’s the case. But tracing its evolution gives us a capsule history of the evolution of the calendar.

The modern western calendar is a descendant of the earliest Roman calendar. It included only 10 months, beginning with March. The months were followed by about 60 days that weren’t part of any month.

That system didn’t work very well, though, so two months were added to the end of the year – January and February. Eventually, they were shifted to the start of the year.

The lengths of the 10 original months were changed to leave 56 days for the newcomers. But the Romans feared even numbers, so they added a day to January to give it 29. February was the month for festivals of repentance and for honoring the dead, so it stayed an unlucky even number.

But this version of the calendar contained only 355 days. So an extra month was added every other year. In those years, the last five days of February were dropped.

After that, February remained unchanged until 46 B.C., when Julius Caesar introduced the basic calendar that’s in use today. He named the seventh month for himself: July. And he might have lengthened February to 29 days. If so, it was cut back to 28 by Augustus Caesar, who took the extra day for the month that bears his name: August.

Script by Damond Benningfield

Powerful cold fronts move across North America at this time of year. These blankets of dry, cold air push away the clouds and haze, providing some amazingly beautiful blue skies. That color is produced by the interaction of sunlight with Earth’s atmosphere.

The Sun is classified as a yellow star because its energy output peaks at yellow wavelengths. And if we could see the Sun from a distance of a few light-years, where it would appear as only a pinpoint of light, it would have a yellow hue.

But from close range, the Sun is so intensely bright that we see its light as a mixture of all the colors of the rainbow. As a result, it looks white.

As the Sun’s light enters Earth’s atmosphere, it’s subjected to a number of effects. Most of the time, the most important effect is Rayleigh scattering. It’s named for a British scientist who studied the effect in the late 19th century.

Blue light waves are shorter than waves of red light. That makes them the right size to bounce off molecules of nitrogen and oxygen in the atmosphere. That scatters them in random directions. Since the blue wavelengths are scattered across the entire sky, the sky looks blue.

Molecules in the air actually scatter a lot of violet light as well. But our eyes are more sensitive to blue wavelengths, so we see the sky as distinctly blue – the frosty color of clear winter days.

Tomorrow: the oddball month of February.

Script by Damond Benningfield

The constellation Gemini consists of two long lines of stars capped by two of the brighter stars in the night sky. Many cultures have seen these stars as two men. But the legend that endures is the Greek story of Castor and Pollux. The two bright stars bear their names.

In the story, the twins had the same mother – Leda, the queen of Sparta. But they had different fathers. Castor was the son of the king – a mortal – while Pollux was the son of Zeus, the king of the gods. The boys were inseparable. They had many adventures together. They joined Jason and the other Argonauts in the search for the golden fleece, and saved their legendary boat during a nasty storm.

But during a later battle, Castor was killed. Pollux was inconsolable. He begged Zeus to let him die so he could join Castor in the underworld. Moved by Pollux’s love for his brother, Zeus agreed to keep them together for all time. They would spend half of their time in the heavens, and the other half in the underworld – just like the stars of Gemini.

The twins appear near the Moon the next couple of nights. The Moon aligns along the body of the twins tonight. Castor, the fainter twin, is to the left of the Moon at nightfall, with Pollux to the lower left. The giant planet Jupiter is passing through the constellation as well. It looks like an especially brilliant star.

Tonight, it’s a little closer to the Moon than the twins are.

Script by Damond Benningfield

Gene Cernan was the last American to walk on the Moon. As he prepared to leave it, he expressed optimism that his colleagues would return soon.

CERNAN: As I take man’s last step from the surface for some time to come, but we believe not too long into the future…

Well, it’s probably been a little longer into the future than Cernan expected, but NASA is preparing to send astronauts back to the Moon. The Artemis II mission is scheduled to launch in the coming weeks. It will carry four astronauts to the Moon. They won’t land, or even go into orbit. But it will be the first time anyone has come close to the Moon in more than half a century.

The astronauts will follow a looping path to the Moon. They’ll fly behind it, coming within about 6500 miles of the surface. The Moon’s gravity will sling them back toward Earth. They’ll splash down in the Pacific Ocean.

During the 10-day mission, the astronauts will check out all of the systems on the Orion spacecraft. They’ll also conduct a few experiments, and make some observations of the Moon.

Artemis II has been delayed by several years. Among other problems, during the unmanned Artemis I mission, in 2022, the life support system and heat shield didn’t work as planned.

Astronauts are supposed to land on the Moon during the next mission. Issues with the lander and other problems may delay that until 2028 or beyond – adding to the gap between moonwalks.

Script by Damond Benningfield

The Moon is a tale of two faces. The side we see – the nearside – features giant volcanic plains and a fairly thin crust. The far side features more mountains and craters and much thicker crust. And the differences might go even deeper. The layer below the crust – the mantle – might be cooler on the farside – or was cooler billions of years ago.

That difference is suggested by samples returned to Earth by a Chinese lander – the first samples from the farside. Some of the samples formed from molten rock. It cooled and solidified 2.8 billion years ago, deep inside the Moon. Details about the samples suggest the molten rock was much cooler than the same layer on the nearside – by about 200 degrees Fahrenheit.

That’s probably because the far side has fewer radioactive elements, which heat the interior as they decay. Just why that’s the case isn’t clear. A smaller moon might have splatted into the lunar farside when the Moon was young. Or a giant asteroid impact might have moved things around. The pull of Earth’s gravity might have played a role as well.

Whatever the cause, there’s a big difference in the lunar hemispheres – which may be more than skin deep.

The gibbous Moon is passing through the constellation Taurus tonight. Aldebaran, the bull’s eye, is to the right of the Moon at nightfall. And Elnath, at the tip of one of the bull’s horns, is closer to the lower left of the Moon.

Script by Damond Benningfield

In Greek mythology, Chiron was the wisest of the centaurs – creatures who were half human and half horse. He taught other centaurs about medicine, botany, and other sciences.

Today, the astronomical Chiron is teaching scientists about the formation and evolution of ring systems.

Chiron is one of about a thousand known centaurs – chunks of ice and rock between the orbits of Jupiter and Neptune. It’s one of the larger ones, at an average diameter of about 125 miles. Even so, it’s so far away that it’s tough to study. But it sometimes passes in front of a distant star. Such a passage allows scientists to measure its size.

It also allows them to study the space around Chiron. Rings cause the light of the background star to flicker. Observing that effect from different locations, and at different times, provides a profile of the rings.

A study last year reported some changes. Scientists already knew of three rings. The new study reported evidence of a fourth ring. It’s so far out that Chiron’s weak gravity might not be able to hold it. The scientists also found a wide disk of dust.

The rings and disk might be debris from a small moon, or the result of an outburst from Chiron itself. Chiron is moving closer to the Sun. As it warms up, it could produce more outbursts. So the system could undergo more big changes in the years ahead – teaching us much more about the evolution of rings around the small bodies of the solar system.

Script by Damond Benningfield