StarDate

Harry Potter fans may boo when they hear its name, but Bellatrix, one of the stars of Orion, is nothing to sneer at. It’s one of the hottest, brightest stars in the neighborhood. It forms one of Orion’s shoulders. As night falls this evening, it stands to the upper left of Orion’s Belt and to the upper right of the hunter’s other shoulder, orange Betelgeuse.

The Harry Potter universe is filled with characters named for stars and constellations. The best known are Bellatrix Lestrange and her cousin, Sirius Black, who was Harry’s godfather. Other members of the family include Orion, Cygnus, and Regulus, the brightest star of Leo.

Sirius is the brightest star in the night sky. It’s low as night falls, below Orion’s Belt. Sirius looks so bright in part because it’s about 25 times brighter than the Sun, and in part because it’s one of our closest neighbors — just 8.6 light-years away.

Bellatrix is much more impressive. It’s tens of thousands of degrees hotter than the Sun, and thousands of times brighter. The key to that power is the star’s mass — about eight or nine times the mass of the Sun. Heavier stars burn through the nuclear fuel in their cores in a hurry, so they produce far more energy than stars like the Sun.

Such stars pay a price for their showiness, though — they live much shorter lives than less-massive stars. In fact, Bellatrix is probably nearing the end of its life — the end of one of Harry Potter’s greatest enemies.
 

Script by Damond Benningfield

Support McDonald Observatory

The “twin” stars of Gemini lead the Moon across the sky tonight. They stack up above the Moon at nightfall. Pollux, the brighter twin, is closer to the Moon.

Although Pollux and Castor are known as the twins, they actually represent only the heads of the two figures. The bodies of the twins are parallel to the horizon in early evening, stretching to the right of Pollux and Castor.

None of the other stars in Gemini’s classical outline is quite as bright as Pollux or Castor, but some of them are interesting anyway.

Several are giants — stars that have puffed up as they neared the ends of their lives. Others are binaries — two stars held close together by their mutual gravitational pull. And some are even more dramatic.

Delta Geminorum, which represents the pelvis of Pollux, is an example. It’s a triple star — three stars in a single system. The main star is beginning the transition from the prime of life to the giant stage. And the system has a place in astronomy history: In 1930, young astronomer Clyde Tombaugh discovered Pluto lurking near it.

Zeta Geminorum, the knee of Pollux, is a Cepheid variable. Once every 10 days or so, it puffs in and out like a beating heart. It’s huge and blindingly bright, and near the end of its life. It’s probably not quite massive enough to explode as a supernova, but no one knows for sure. Either way, its end is nigh — no more than a few million years away.
 

Script by Damond Benningfield

Support McDonald Observatory

The Milky Way arcs high across the sky this evening. Unfortunately, the light of the full Moon makes it almost impossible to see. That’s both good news and bad news for the dung beetle. It navigates by the Milky Way, so that glowing band won’t do it any good. But it also relies on the quality of moonlight, so it should still be able to find its way around the dung piles — if it’s not near a city.

Many nighttime animals navigate with the help of the sky — birds, butterflies, sea turtles, and many others. Most appear to be guided by the stars, although some also rely on the Moon.

But the dung beetle is the only one that’s known to navigate by the Milky Way. Lab studies have shown that it can sense subtle differences in light levels. Since the Milky Way is uneven, the beetle can follow the Milky Way’s bright and dark patches to roll its dung ball to a good hiding place.

Studies have also told us that the beetle can follow the moonlight. In particular, it follows polarized light — light waves that are all reflected in the same direction. The beetle is one of the few creatures that does so.

But a study in 2021 found the dung beetle is facing a challenge: light pollution. Researchers compared the behavior of dung beetles near a bright city with those under dark skies. The brighter skies block the Milky Way and interfere with the moonlight. So the city beetles start using city lights as beacons — replacing nature as their guiding lights.
 

Script by Damond Benningfield

Support McDonald Observatory

Messier 79 is a bit of an oddball. Unlike most star clusters of its type, it lines up “behind” us as seen from the center of the Milky Way Galaxy. And astronomers are having a hard time figuring out just where it came from.

M79 is a globular cluster — a tightly packed ball of tens of thousands of stars. It’s about 42,000 light-years away, in Lepus, the rabbit.

The Milky Way hosts about 150 globulars. Most of them were born with the Milky Way itself. That means they contain some of the oldest stars in the entire galaxy.

But a few dozen globulars don’t appear to be home grown. Instead, they were born in other galaxies. Some were snatched from the outskirts of those galaxies as the Milky Way gobbled them up. Others may have started as the cores of other galaxies, but they, too, were grabbed by the Milky Way.

M79 might be a refugee from the Canis Major Dwarf — a possible small galaxy that the Milky Way may be ingesting even now. A second globular may come from that galaxy as well.

Another idea says the Milky Way grabbed M79 billions of years ago, when it merged with a mid-sized galaxy. And yet other ideas suggest other birthplaces for M79 — an oddball star cluster.

Lepus is close to the lower right of bright Orion, low in the southeast at nightfall. It remains below Orion’s feet as they wheel across the south later on. M79 is below the outline of the rabbit. You need binoculars to pick it out.
 

Script by Damond Benningfield

Support McDonald Observatory

You might not need to break out the T-shirts, but Earth is getting its maximum dose of sunlight today. That’s because we’re at perihelion — our closest point to the Sun for the entire year.

Earth’s average distance from the Sun is about 93 million miles. But our planet’s orbit is lopsided. So today, we’re about one and a half million miles closer than average. That means we receive a little bit more sunlight than average. We don’t actually feel the difference, though, because the oceans and atmosphere spread the heat around.

The date of perihelion isn’t fixed. Instead, it drifts through the calendar. Around the year 1246, it occurred on the winter solstice, in December. It moves a day later every 58 years or so. About 4,000 years from now, it’ll take place on the spring equinox, in March.

Even now, the date can wobble a day in either direction from year to year. Part of that is the result of Leap Year. The perihelion moves a few hours later on the calendar each year, on average, until Leap Year, when it drops back. But the change is also influenced by the Moon’s position in its orbit around Earth. At some points, the Moon’s gravity is moving us a little closer to the Sun, while at others it’s moving us a little farther.

One impact of the close approach is a difference in the length of the seasons. Earth moves fastest when it’s closest to the Sun, so winter is the shortest season in the northern hemisphere.
 

Script by Damond Benningfield

Support McDonald Observatory

A meteor shower that may have a complex heritage should be at its best tonight. Unfortunately, the gibbous Moon is in the way for most of the night. Its glare will overpower all but the brightest meteors.

The shower was first seen a couple of centuries ago. It occurs when Earth flies through the orbital path of a comet or asteroid. The path is littered with small bits of dust and rock, which vaporize as they slam into Earth’s atmosphere.

Just which object is responsible for the Quadrantids remained a mystery for a long time. And even today, the subject isn’t necessarily settled.

Astronomers first associated the shower with a dead comet known as 2003 EH1. Later, they added Comet 96/P Machholz to the list.

A study a few years ago said that both bodies could be involved. In fact, they could share a parent — a larger comet that broke up about 9500 years ago. Over the millennia, the orbits of the two fragments moved apart — influenced in part by the gravity of the planets.

Today, the fragments follow different paths. One of them could account for the short, intense peak of the Quadrantids, which lasts for only about a day. The other could account for a more widely spread “drizzle” of meteors, which lasts for weeks.

The shower peaks before midnight tonight, but the best view may come in the hour or so before dawn, after the Moon sets — a brief window for a meteor shower with a complicated heritage.
 

Script by Damond Benningfield

Support McDonald Observatory

The Moon forms a shifting triangle with two bright orange points of light the next few nights. The brighter dot is the planet Mars. The other is the star Aldebaran, the eye of the bull.

Tonight, Mars stands to the lower left of the Moon at nightfall, with Aldebaran farther below the Moon. The triangle will be tighter as the trio sets in the wee hours of the morning.

On the scale that’s been used for centuries, Mars is at a magnitude of about minus 1.2, while Aldebaran is plus point 85. The lower the number, the brighter the object.

When the magnitude scale was devised, it was based on observations with the unaided eye. The brightest stars and planets were magnitude zero. It was hard to judge precise differences in brightness, especially since you can’t see all the stars at the same time.

Telescopes and photographic plates made it easier to measure brightness. So each star and planet could be assigned a precise number. The brightest objects were given negative numbers.

The magnitude of a planet changes as its distance from Earth changes. Earth and Mars are moving apart now, so Mars is fading week by week. If a star’s magnitude changes, it’s because the star itself is changing, or because it has a companion and they stage eclipses. Aldebaran may vary by a tiny bit — but not nearly enough to notice with the eye alone.

The Moon, Mars, and Aldebaran will bunch closer tomorrow night, then move apart by Wednesday.
 

Script by Damond Benningfield

Support McDonald Observatory

A passel of planets will cap off this New Year’s Day. All five planets that are visible to the unaided eye will be in view shortly after sunset. A sixth planet is up there as well, but you need binoculars or a telescope to pick it out. But its location is easy to spot because it’s next to the Moon.

As twilight begins to fade, look for two of the planets low in the southwest. Venus is the “evening star,” with much fainter Mercury close to its lower right. You need a clear horizon to spot them, and binoculars will help you pick out Mercury.

Slide to the upper left and you’ll come to Saturn, the second-largest planet in the solar system. Then keep going and you’ll come to the largest planet, Jupiter. It’s the brightest object in the night after the Moon and Venus, so there’s no mistaking it.

And high in the east you’ll find bright orange Mars.

The Moon lines up between Mars and Jupiter. It’s in its waxing gibbous phase. That means the Sun lights up most of the hemisphere that faces our way, and the fraction is getting larger by the day. The Moon will be full on Friday.

The planet Uranus lines up close to the upper right of the Moon. It’s the Sun’s third-largest planet. But it’s about 800 million miles farther than Saturn, and it doesn’t reflect as much sunlight. It actually can be visible to the eye alone, but only under especially dark skies. The moonlight overpowers it, so you need help to see this faint giant.
 

Script by Damond Benningfield

Support McDonald Observatory

New Year’s Eve brings loud, boisterous celebrations, with fireworks, music, and lots of champagne. And when you join those celebrations, you’re following a tradition that dates back at least 4,000 years — to ancient Babylon.

But the Babylonians celebrated the start of the year at a different time — at the first new Moon after the spring equinox, in March. In fact, many cultures marked the turning of the year at about that same time. The equinox is a period of new life and new beginnings, so it seemed like a logical time to reset the calendar.

Ancient Rome was among those cultures. But in 46 BC, Julius Caesar reformed the calendar. He set New Year’s Day as January 1st. January honored the god Janus, who had two faces — one to look into the past, the other into the future. The people of Rome made sacrifices, held big parties, and exchanged gifts.

After the fall of the Roman Empire, the date of New Year’s drifted. In the late 1500s, thanks to another calendar reform, it began to return. January 1st was adopted in England and its American colonies in 1753, where it’s remained ever since — setting up big celebrations for New Year’s Eve.

And the evening sky offers some decorations for tonight’s festivities. The Moon is high in the sky during twilight, with Venus, the “evening star,” quite low in the southwest. The planets Jupiter and Saturn line up between them, with bright orange Mars in the east.
 

Script by Damond Benningfield

Support McDonald Observatory

The end of the celestial river really isn’t the end of the river. That’s because the river has two stars with almost the same name — and both names mean “the end of the river.”

Eridanus is in the south and southeast as night falls. It begins near Rigel, the bright star at the foot of Orion. It then makes as many twists and turns as most earthly rivers before reaching its first “end,” Acamar. After that, it keeps going until it reaches the final end, Achernar. From the United States, you have to be in Hawaii or far southern Texas or Florida to see that second ending.

In fact, that’s exactly why Eridanus has two endpoints. From the latitudes of ancient Greece, where the constellation was drawn, skywatchers could see only down to Acamar — everything else remained below the horizon. So Eridanus ended right there.

Centuries later, though, European ships cruised into the southern hemisphere. Crew members saw a much brighter star down to the lower right of Acamar, so they added it to the outline of Eridanus. And they gave it a nearly identical name — giving the river a brighter end.

Acamar is far to the lower right of Orion. It’s only third magnitude — a fairly faint star that’s hard to find without a chart. Achernar is several times brighter, so it’s hard to miss — but only if you’re south of about San Antonio or Daytona Beach. It stands due south, just a skosh above the horizon, a couple of hours after sunset.
 

Script by Damond Benningfield

Support McDonald Observatory