StarDate

The stars that outline the head of the big dog are all late in life. But that doesn’t mean they’re the same age. In fact, their ages vary by about 10 billion years.

Canis Major, the big dog, is in the southeast as night falls. It’s home to Sirius, the brightest star in the night sky.

The head forms a triangle to the left of Sirius during the early evening, and above it later on. None of the stars is much to look at. If you lined them up at the same distance as Sirius, though, they’d far outshine it.

The snout is marked by Theta Canis Majoris. It’s about 260 light-years away. The star at the ear is Gamma Canis Majoris. It’s a couple of hundred light-years farther. And the star that connects the head and neck — Iota Canis Majoris — is more than 3,000 light-years away.

All three stars are at or near the ends of their lives. Because they have different masses, though, they’ve taken different lengths of time to reach that point.

Iota is the heaviest, so it burned through its nuclear fuel much faster. It’s only 15 million years old, but its life is almost over. And it’s likely to end with a bang — as a supernova.

Theta is the least massive, so it’s lived the longest — 10 billion years. It will end by expelling its outer layers, leaving its dead core — a white dwarf. Gamma is between the ages of the other two, and it faces the same future as Theta — the same fate that awaits the Sun in five billion years.
 

Script by Damond Benningfield

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The brightest star in the night sky scoots low across the south on winter nights. So do several past and future brightest stars. They’re all in Canis Major, the big dog.

The current champion is Sirius, the Dog Star, which is low in the southeast as night falls.

Sirius appears so bright for a couple of reasons. For one, it really is bright — about 25 times brighter than the Sun. For another, it’s close — less than nine light-years away.

Sirius and the Sun are moving through the galaxy at different rates, though, so they haven’t always been this close — and they won’t stay this close for long. The distances to other stars are changing as well, which means that other bright stars drift in and out of view — giving us different “brightest” stars at different times.

About 4.7 million years ago, the title belonged to Adhara, which today is the second-brightest star in the big dog. It’s much bigger and brighter than Sirius. So while it never got closer than about 34 light-years, it peaked at about 10 times Sirius’s current brightness. That’s as bright as Venus, which currently reigns as the “morning star.”

A few hundred thousand years later, another of the stars of Canis Major took the title. And about three million years from now, the brightest star will be NR Canis Majoris. It won’t be as bright as Sirius is today, but it’ll outshine all the other stars — one more bright light from the big dog.
 

Script by Damond Benningfield

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The planet Jupiter is about to end another long trek across the night sky. The planet will drop from view in the evening twilight in two or three weeks. It then will pass behind the Sun before returning to view in April — in the morning sky.

This evening, though, Jupiter is in great view — it teams up with the crescent Moon. It’s close to the upper right of the Moon as the sky begins to darken, and looks like a bright star. Jupiter and the Moon set a couple of hours after sunset.

Jupiter has two different motions across the starry background. One is caused by Earth’s motion around the Sun. As we orbit the Sun, our viewing angle to the stars and planets changes. So each of them appears to progress across the sky as the nights roll by. Jupiter first appears at dawn, then rises and sets earlier each night. Over the months, it travels across the sky, until it sets with the setting Sun.

The other motion is caused by Jupiter’s own orbit around the Sun. The planet is more than five times farther from the Sun than Earth is. At that distance, Jupiter takes almost 12 years to complete a single orbit. And that’s also how long it takes to move through the constellations and return to the same spot in the sky relative to the stars. So on February 2nd of 2034, Jupiter will stand at almost the same spot in the sky as tonight — getting ready to disappear once more in the evening twilight.
 

Script by Damond Benningfield

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The first three months of the year feel a little like a sandwich — thick slabs of January and March, with a thin layer of February in the middle. But it took a while to get that recipe to its current form — mainly because calendar makers kept changing how “thin” February should be.

The modern calendar has its roots in ancient Rome. Its original calendar had only 10 months, beginning with March and ending with December. At the end of the year, days were added to keep in line with the seasons.

Around 713 B.C., though, king Numa Pompilius added two months to the end of the year — January and February.

The Romans considered even numbers unlucky, so most of the months were either 29 or 31 days long. To keep the calendar in line with the length of 12 lunar months, though, one month had to have an even number of days. And since February came at a time of year associated with the dead, it got the nod — 28 days.

It wasn’t that long every year, though. Some years it was as short as 23 or 24 days, with extra days added after that to keep in line with the seasons.

The calendar was adjusted many times over the following centuries. Finally, Julius Caesar standardized the calendar at its current form. February was set at 28 days, with a leap day every fourth year. The leap-day rule was tweaked in the 16th century, giving us the present-day calendar — with a thin slice of February as the second month of the year.
 

Script by Damond Benningfield
 

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Skywatchers came up with some great names for the stars that outline the body and belt of Orion the hunter: Rigel, Betelgeuse, Bellatrix, Mintaka, and others. When it came to his shield, though, the imagination just pooped out. Six stars in a row are called Pi Orionis. The only thing that sets them apart is that they’re numbered from top to bottom.

Orion is in the southeast at nightfall. Look for his three-star belt inside a box of four bright stars. If you follow the belt to the upper right, the first decently bright star is Pi-3. The stars that share the name line up to its upper left and lower right.

Orion’s more lyrical names date back thousands of years. They came from Greek and Roman mythology, and many were translated to Arabic.

Those proper names were given only to bright stars. In the early 1600s, though, Johann Bayer assigned names to more than 1500 stars. Other astronomers added to the list.

Bayer was looking for a simple way to catalog the stars. So he designated each star with a letter of the Greek alphabet followed by the name of its constellation. In a few cases, two stars appeared so close together that they got the same letter, with each one getting its own number. Pi Orionis, however, is an exception. The stars are spread as wide as your fist held at arm’s length, so there’s no way to get them confused. Even so, they share a name — Pi Orionis: the stars of Orion’s shield.

Script by Damond Benningfield

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If you want a good test of both your eyes and your night sky, look high in the west this evening. If your eyes are sharp and your sky is clear and dark, you might just make out M33, the Triangulum Galaxy. It’s three million light-years away — perhaps the most-distant object visible to the unaided eye.

M33 is the third-largest member of the Local Group — the cluster of galaxies that includes the Milky Way. The Andromeda Galaxy is the largest member, with the Milky Way second.

M33 is roughly 60,000 light-years across — about half the size of the Milky Way. And it contains perhaps 10 percent as many stars as the Milky Way.

Like the Milky Way, M33 is a spiral galaxy. Beautiful spiral arms wrap around its heart. The arms are busy nurseries. They’re giving birth to stars at a faster rate than either the Milky Way or Andromeda.

M33 may be a satellite of Andromeda. And it’s possible that it’s made a close pass by the larger galaxy. If so, then M33 may someday merge with Andromeda, which in turn will merge with the Milky Way — creating a super-galaxy for the Local Group.

Look for M33 high in the west during the early evening. It’s about half way between two bright stars — Hamal to the left, and Mirach to the right. If you can’t see the galaxy with your eyes alone, it’s an easy target for binoculars — three million light-years away.
 

Script by Damond Benningfield

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The interplay of the stars in a binary system can produce some amazing sights. One example is NGC 2346, a colorful cloud of gas and dust. It’s about 4800 light-years away, in the unicorn. It looks a bit like a butterfly with its wings unfurled.

The nebula is the final gasp of a dying star. The star could no longer sustain the nuclear reactions in its core, so the core shut down. The star’s outer layers were expelled into space, forming a giant, bright cloud.

Many nebulas of this type form bubbles — still beautiful, but much simpler. Many of those nebulas were produced by single stars. But NGC 2346 was sculpted by two stars — the dying star and a companion.

As the first star began to die, it expanded to giant proportions. It engulfed its companion. But the companion survived.  Its motion stripped away material from the expanding star’s outer layers, forming a ring around the equator.

As the expanding star died, radiation from its hot core pushed away the gas in its outer layers. The gas was hemmed in by the ring around the equator, though, so it spread out from the poles. That formed two bubbles — one from each pole. Today, the bubbles form the “wings” of this beautiful cosmic butterfly.

NGC 2346 is in Monoceros, the unicorn. The nebula is close to the upper right of the bright star Procyon, which is low in the southeast at nightfall. The butterfly is an easy target for small telescopes.
 

Script by Damond Benningfield

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For most of its life, a star like the Sun is drab. Seen from afar, it’s just a pinpoint of light. When the star dies, though, it’s like a caterpillar leaving its cocoon — as a beautiful butterfly.

As the nuclear engine in its core shuts down, a star sheds its outer layers of hot gas. They form a colorful bubble known as a planetary nebula. The nebula can glow for thousands of years as it expands and cools. The star’s rotation and magnetic field, as well as any companion stars, can sculpt the nebula into incredible shapes.

One of the most impressive planetary nebulas is known as the Spirograph. That’s because it resembles the geometric shapes produced by the old children’s toy. It’s probably more than 4,000 light-years away.

The Spirograph probably began taking shape about 50,000 years ago as seen from Earth, when the star expelled a shell of gas. A second outburst followed about ten thousand years ago. Both of those shells have faded.

About 2600 years ago, however, a third outburst produced the nebula that’s visible today. It spans about a fifth of a light-year, and it’s filled with filaments and bubbles of gas that produce its amazing appearance. The Spirograph should remain visible for thousands of years before it dissipates and vanishes.

The nebula is in Lepus, the hare, which bounds below the feet of Orion, which is in the southern sky on winter evenings. But you need a telescope to see this final act of a star.
 

Script by Damond Benningfield

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When a massive star explodes, it doesn’t just vanish. In some ways, it becomes even more prominent. And it can remain visible for tens of thousands of years.

An example is the Veil Nebula. It’s in Cygnus, which is low in the west and northwest at nightfall. The nebula lines up along the swan’s left wing.

The Veil is the remnant of a supernova — a massive star that blasted itself to bits. The explosion probably took place about 20,000 years ago as seen from Earth.

The star exploded when it could no longer sustain the nuclear reactions in its core. The core collapsed, and the star’s outer layers fell inward, then rebounded. They blasted into space at a few percent of the speed of light.

As the blast ran into nearby clouds of gas and dust, it created shockwaves, heating the material to millions of degrees. The decay of radioactive nickel and other elements forged in the blast added to the light show.

Since then, the Veil has expanded and cooled. Today, it spans about 130 light-years. It’s still visible, though. Colorful ribbons of gas and dust entwine like the strands of a rope. They’ll stay visible for thousands of years.

A lot of the debris isn’t visible to the eye, though. It’s so hot that it produces ultraviolet light and X-rays, which are visible to space telescopes. Combined, the entire complex is known as the Cygnus Loop — the beautiful remnant of an exploded star.

We’ll talk about another kind of stellar remnant tomorrow.
 

Script by Damond Benningfield

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Predicting the future isn’t easy — just ask your local weather forecaster. But scientists recently got some help in predicting the future of supergiant stars.

The help came in the form of a supernova — an exploding supergiant in a pair of merging galaxies about 60 million light-years away. A planet-hunting space telescope was pointing that way at the time of the blast. It was taking pictures every 30 minutes, so it provided a time-lapse view of the star before and after it erupted. That was the first time astronomers got to watch a supernova so close to its actual explosion.

In addition, Hubble Space Telescope and other telescopes in space and on the ground turned toward the galaxy within hours or even minutes. And over the following weeks, astronomers poked through older observations of the galaxy to see what the star looked like before the outburst.

Such a detailed view could tell astronomers what to look for when a supergiant is about to become a supernova — helping them predict the future.

Several supergiants thought to be close to the end are in good view now. There’s Betelgeuse, the bright shoulder of Orion the hunter, which is high in the south at nightfall. And there’s Antares, the heart of the scorpion, which is quite close to the Moon at first light the next couple of days. Both stars could explode any time within the next hundred thousand years — even tonight.

More about supernovas tomorrow.
 

Script by Damond Benningfield

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