Farewell Orion / See a Dragon / Aurora update

Orion's Belt (left) is reflected in calm waters in  a lake north of Duluth as the constellation tilts toward the western horizon last night. Credit: Bob King

Orion the Hunter and his three belt stars (left) are reflected in a lake north of Duluth last night. Venus, along with the Hyades and Pleiades star clusters, shine at right. Credit: Bob King

Last night while watching the northern lights, we saw the Great Hunter head for the hills. Orion has been the center of attention since December, as recognizable in the southern sky as the Big Dipper is in the north. Now he’s trotting off toward the western horizon, replaced by Jupiter, Leo and the constellations of spring.

Like a furniture clearance commercial that airs during the nightly news, ALL WINTER CONSTELLATIONS MUST GO! With new inventory coming in, the sky’s gotta make room.

Like it or not, Orion will toddle out of the picture soon. The rising and setting of the stars is a reflection of our planet’s rotation; the Earth spins toward the east, pushing up new stars from the eastern horizon and leaving those in the west behind. We see this grand parade every clear night.

View of Earth's orbit around the sun seen from above the northern hemisphere. As our planet moves to the left, the background constellations appear to drift to the right or westward. Credit: Bob King

View of Earth’s orbit around the sun seen from above the northern hemisphere. As our planet moves to the left during its yearly swing around the Sun, the background constellations appear to drift to the right or westward. Credit: Bob King

But there’s a more subtle shift happening at the same time. As Earth travels in its orbit around the sun, we peer out into different sectors of the sky at night as the weeks and months pass. Like a runner facing a different set of fan-packed bleachers while circling the track in a 1000-meter race, Earth faces Orion in winter, Leo in spring, Scorpius in summer and Pegasus in the fall.

They constellations appear to drift westward very gradually at the rate of about 1° per day. That’s small enough we don’t notice night to night. But the degrees add up. In two weeks, a constellation will drift about 14° to the west — more than the length of your fist held at arm’s length against the sky.

To see this for yourself, find a bright star like Betelgeuse in Orion or Arcturus in Bootes (located below the Big Dipper’s Handle) and note its position in relation to a nearby landmark like a church steeple, mountaintop or Pizza Hut at a particular time. Wait a few nights and then return to the exact same spot at the same time, and you’ll see that the star has slid westward.

Just the way the rising and setting of the stars is an illusion caused by Earth’s rotation, the seasonal drift of the stars and constellation is a sleight of hand caused by Earth’s revolution around the Sun. Now you know why the stars can sit still — it’s YOU who’s doing the moving.

Earth zips around the Sun at a speed of 18.5 miles per second (30 km/sec), covering 26,640 miles each day. In the time it takes for you to notice your reference star has moved – we’ll say 3 nights – you and the home planet have traveled nearly 80,000 miles!

Falcon 9 was carrying the Dragon spacecraft, which is loaded with about 4,300 pounds of supplies and payloads bound for the International Space Station (ISS). Credit: SpaceX

Falcon 9 lifts off April 14 carrying the 9,300-pound Dragon spacecraft, which was loaded with about 4,300 pounds of supplies and payloads for the International Space Station (ISS). Credit: SpaceX

Being outside at night can be a wonderful thing because you inevitably see something unexpected. Two nights ago, I was out with my astro class and we saw the weirdest pair of satellites in the northeastern sky – a bright one and a faint companion just a few degrees apart. Both moved quickly and faded out in less than a minute.

I later discovered we saw the Dragon cargo ship en route to deliver supplies to the International Space Station and the upper stage of the Falcon 9 rocket that lofted the ship into orbit.

You can see either or both in the next few nights by going to Heavens Above and selecting your city and then clicking on the Dragon CRS-6 link just below the ISS link under the Satellites heading. Passes are only a minute or two long and occur in evening twilight. The brighter object will be the rocket stage.

The Falcon rocket's second stage engine burns for about 7 minutes to deliver the Dragon ship to its initial orbit. Credit: SpaceX

The Falcon rocket’s second stage engine burns for about 7 minutes to deliver the Dragon ship to its initial orbit. Credit: SpaceX

Bring binoculars and you might still be able to see the solar panel covers that were ejected from the Dragon as separate fainter objects nearby. Dragon will arrive at the space station around 6 a.m. CDT tomorrow (April 17), when astronauts will use the robotic arm to secure it. I expect that tonight, Dragon will follow the ISS by only a couple minutes.

Last night’s aurora glowed until the wee hours. I hope you were able to see at least some of the show. Tonight, forecasters predict another chance for more northern lights though they’re expected to be a little “quieter” than yesterday.

Happy Pi Day! Find Pi in the Sky

A lovely lemon angel meringue pie in honor of Pi Day. Who knew math could make you hungry? Credit: Bob King

A lovely lemon angel meringue pie in honor of Pi Day. Pi, denoted by the symbol in the photo, has the unique power of stirring up an appetite. Credit: Bob King

Happy Pi Day! Pi is one of the few mathematical constants that immediately conjures up thoughts of food. Pies in particular. My wife Linda, inspired by this important day, prepared a lemon angel meringue pie I can’t wait to taste.

The parts of a circle – the encompassing circumference, the radius and diameter, equal to 2x the radius. Pi is the ratio between the circumference and diameter of a circle

The parts of a circle – the encompassing circumference, the radius and diameter, equal to 2x the radius. Pi is the ratio between the circumference and diameter of a circle

Exactly what is pi? It’s the ratio of the circumference of a circle to its diameter or the number you get when you divide the circumference of any circle by its diameter. It starts with 3.1415 and goes on forever in a never-repeating pattern. Mathematicians call it an infinite decimal. Unlike 3.57 or 7.5, which have a finite number of numbers after the decimal point, pi continues on into infinity. Divide C by D and you’ll never get to the end.

Not that math geeks with computers haven’t tried. By October 2011 two Japanese guys calculated 10 trillion digits of pi, a world record. Nice work, but still far from infinity.

Pi Day happens every March 14 because the calendar date 3/14 is the same as pi’s first three digits. But this year’s pi celebration is an exceptional one. When you add on the last two digits of the year you get 3.1415. As you might guess, this date alignment happens just once a century.

Let’s go further. When the clocks strikes  9:26:53 a.m. and 9:26:53 p.m. today we can add an additional five digits to make 3.141592653. If you find yourself in a bar or pub this evening, see if you can convince the crowd to celebrate the world of mathematics with a toast to the moment.

How about 6 slices of pi? The brighter stars in constellations are named for the letters of the Greek alphabet with Alpha typically denoting the brightest star. Much of Orion's shield is composed of similarly bright stars neatly lined up, so each received the "pi" designation with a number. Created with Stellarium

Help yourself to six slices of Orion pi if you’re out tonight. The brighter stars in constellations are named for the letters of the Greek alphabet with Alpha typically denoting the brightest. Most of the stars in Orion’s shield are of similar brightness and neatly lined up, so each received the “pi” designation with an individual number. Created with Stellarium

If you’re not out sipping suds but find yourself instead at the telescope, consider celebrating the special moment with a hefty helping of Orion’s “pi stars” and the very attractive double star Pi Bootis. After proper names, the brighter stars in the constellations are labeled with Greek letters, meaning most groups have a “pi star”.

Pi Bootis, a beautiful double star comes up in the east around 11 p.m. local time. You'll find the magnitude 4.5 star not far below brilliant Arcturus, the brightest star in Bootes. Created with Stellarium

Pi Bootis, a beautiful double star comes up in the east around 11 p.m. local time. You’ll find the magnitude 4.5 star not far below brilliant Arcturus, the brightest star in Bootes. Created with Stellarium

Pi Bootis is a striking, close double star that looks like pair of headlights approaching from interstellar space. The brighter star is magnitude 5 with a mag. 5.8 companion star just 5.5″ due east. Even a small telescope will split this beauty so long as you use a magnification around 60x or higher.

Pi shows up in more places than your oven or neighborhood greasy spoon. Anything involving circles, spheres and ellipses feature pi front and center which is why astronomy and architecture require healthy servings of pi for sustenance. Galileo, Copernicus and Kepler used pi in their calculations of the sizes, distances from Earth, and orbits of the planets. It pops up in statistics, mechanics, cosmology and even in Einstein’s Theory of Relativity equations.

Coincidentally it’s also the wild-haired Einstein’s birthday today. Happy E=mc² Day, too!

PS. I have to admit I ate my pie as I wrote this entry. Truly delicious. I like Pi Day.

It’s double duo week: Moon-Jupiter and Venus-Uranus

The nearly full moon is in conjunction with Jupiter tonight. Their minimum separation of 5 happens around 10 p.m. (CST). Source: Stellarium

The nearly full moon is in conjunction with Jupiter tonight. Their minimum separation of 5° happens around 10 p.m. (CST). Source: Stellarium

I looked until around 11 p.m. last night but moonlight diluted any aurora that may have out. But the predicted storm did hit between about 2 a.m. and dawn this morning. While some readers might think I stay up all night, I really did sleep through this aurora. I know at least a few of you saw it. Tonight, there’s a chance for more minor storming.

There’s also an even better chance you’ll be struck by two very bright objects in the eastern sky at nightfall: a plump gibbous moon and the jolly giant planet Jupiter. They’ll be in conjunction tonight just ahead of Leo’s brightest star Regulus. Pairings like these make for great company and contemplation while walking the dog at night.

Venus and Uranus will be very close together on March 4th, an ideal time to find the fainter planet in binoculars. Source: Stellarium

Venus and Uranus will be very close together on March 4th, an ideal time to find the fainter planet in binoculars. Source: Stellarium

A planet-to-planet pairing occurs on Wednesday evening the 4th when Venus and Uranus will be just 1/3° apart. Like last month’s close graze with the crescent moon, this will provide yet another easy opportunity to see a planet that is too dim for most to see with the naked eye. Just point your binoculars at brilliant Venus in late twilight in the western sky and look for a tiny speck of light immediately below it.

I love how planets can appear so close and yet be so far from one another. Venus is a quick jaunt at 128 million miles from Earth compared to Uranus’ 1.9 billion miles, nearly 15 times farther away.

The stars in the constellation of Orion all look like they are at the same distance. Turn the constellation through 90 degrees and you can see the stars are actually at different distances. Two of the Belt stars plus the two bottom stars in the constellation are far from the sun but relatively near one another in space. Betelgeuse is much closer to us. Credit: ESA

The stars of Orion might be easily dismissed as all being at the same distance from us. That’s how they appear on the 2-D “surface” of the sky. Butturn the constellation through 90 degrees (look at it from the side) and you can see the true distances of each star. Notice that Betelgeuse is much closer to us than the Belt stars. Credit: ESA

When we see conjunctions and appreciate the real distances between objects in the sky, it’s helpful to remember the same applies to the constellations. We see familiar rectangular outline of Orion and the neat arrangement of his three belt stars by lucky chance. Looking back toward Orion’s stars from a different direction in space (a couple hundred light years beyond the solar system) Orion would be unrecognizable.

Japan launches imaginative mission to a carbon-rich asteroid

Japan’s Hayabusa 2 asteroid mission rockets into space Tuesday. Credit: JAXA

On Tuesday at 10:22 p.m. (CST) the Hayabusa 2 mission launched from Tanegashima Space Center in southern Japan. Destination: 1999 JU3, a C-type carbon-rich asteroid nearly 3,000 feet (900 meters) across.

C-type space rocks are the most common and may have delivered some of the essential chemical building blocks important to the origin of life on Earth during the formation of the planets 4.5 billion years ago. Scientists hope to detect water and organic molecules on this never-before-explored world-let.

Anybody want a peanut? This close up photo of 25143 Itokawa was taken by Hayabusa 1 in 2005. The Mars-crossing asteroid spans just 1,755 feet (535-meters). Credit: JAXA

This is Japan’s second mission to an asteroid. The first – Hayabusa 1 – made a round trip journey to 25143 Itokawa, a rocky asteroid just a third of a mile across some 123 million miles from the Sun, from 2003 to 2010. Even though that mission encountered numerous technical problems including a pointing system failure, a fuel leak and a malfunction of the device used to collect rock samples, engineers babied it back to Earth.

At the time, no one was knew for certain whether any samples had been gathered at all, but upon re-entry, scientists discovered a tiny fraction of material inside the craft landing vehicle. Success!

Artist depiction of Hayabusa 2 in orbit around the dark, carbon-rich asteroid 1999 JU3. Credit: JAXA

“We changed a lot of parts on Hayabusa 2,” said Hitoshi Kuninaka, JAXA’s Hayabusa 2 program manager. “We installed four reaction wheels, and Hayabusa 1 had only three. The sampling system also has some improvements. Our operations software was upgraded for better proximity operations around the asteroid.”

Like the first probe, Hayabusa 2 will propel itself with an ion engine, where ionized (electrified) xenon gas is accelerated through a strong electric field and expelled at high speed to produce a steady thrust. This time around, the engines were upgraded to produce more thrust. A new antenna system will beam back data at four times the previous rate.

One of the four MINERVA II robotic landers that will hop around the asteroid taking pictures and measurements. Credit: JAXA

The spacecraft will arrive at 1999 JU3, which orbits between Earth and Mars, in June 2018. Before dispatching four landing robots, the main spacecraft will study and map the surface from a distance of about 12 miles (20 km).The landers are mobile and able to hop across the asteroid to study its environment from several locations.

Warning – don’t hop too hard! 1999 JU3 is even smaller than Rosetta’s Comet with a gravitational pull 60,000 times weaker than Earth. It wouldn’t be difficult to bounce off the asteroid and not return to the surface for a long time much as what happened to the Philae lander.

Hayabusa 2 will remain at the asteroid for a year and a half, long enough to move in close and use its collection tube to gather rocks from three different locations. In an audacious move, the spacecraft will fire a 2-kilogram (4.4-pound) copper disk into the asteroid to blast out an artificial crater about 10 feet (several meters) across and 3 feet deep. Why? To sample more pristine rocks not exposed to direct solar radiation.

Hayabusa 2 samples the crater floor after the blast, gathering fresh rocks shielded from the damaging effects of cosmic and solar radiation. Credit: Akihiro Ikeshita / JAXA

After the explosion, the spacecraft will swing by and use its sampler arm to fire tiny “bullets” made of the element tantalum into the crater and funnel debris that ricochets up from the surface into its collection tube. Mission planners hope to harvest at least 1/10 of a gram of asteroid dust.

As the spacecraft returns to Earth’s vicinity, it will eject a container with the dust that will drop through the atmosphere and land by parachute in the Australian outback in December 2020.

This is a big week for rocket launches. NASA’s new Orion space capsule is scheduled to launch into orbit on an unmanned test flight at 6:05 a.m. (CST) tomorrow. If all goes well, this could be our first step toward a manned mission to Mars. You can watch the launch live on NASA TV.

Daylight saving time puts spring stars on ice / ‘Cosmos’ premieres tonight!

1918 post card that citizens could mail to their congressman to show support for the daylight saving time law, which was enacted on March 19, 1918 in the U.S. Click to read the history of DST. Credit: Library of Congress

It happens every year. The seasonal westward drift of the stars is temporarily put on hold when we “spring forward” an hour on the second Sunday in March.

With daylight saving time or DST we gain an hour of evening daylight and lose an hour of early morning light. After a long winter, who doesn’t welcome more light at the dinner hour?

A one-hour later sunset naturally means an additional hour for night to begin. Yesterday, twilight ended and true night began around 7:45 p.m. Darkness descends tonight around 8:45 p.m. Since we’ll have to wait that extra hour for stars to come up in the east, the time change has the effect of retarding every star’s rising by an hour.

Sirius, the brightest star, heads up the constellation Canis Major the Big Dog and is very prominent in the southern sky this month. With the start of daylight saving time, it now appears one hour east (to the left) of where it was last night at the same clock time. Stellarium

For instance, last night around 8:30 p.m. I watched brilliant Sirius twinkle west of due south. When the clock glows 8:30 p.m. tonight, Sirius will sparkle east of due south. The same holds true for Orion and the other winter stars. Compared to last night at the same clock time, the winter constellations will be higher up in the sky. Meanwhile, the spring constellations of Leo, Virgo and Bootes, which have been steadily gaining ground in the east, will be shoved back an hour.

While it’s not a great hardship, it does mean that to see the cool spring stuff, we have to go out an hour later compared to a night ago. I’ve been watching Mars finally make its way into the evening sky. To observe it now means getting out at midnight instead of 11. Oh well.

All will take care of itself in due time. We’ll come to accept the change, just as our bodies will finally figure out to do with the hour we lost. And since the westward drift of stars with the seasons never stops, come late-April, Sirius and friends will be memories and bright Arcturus and Mars will gleam in the southern sky.

View of Earth’s orbit around the sun seen from above the north pole. As we zip along at 18.5 miles per second, we see a different set of constellation in the night sky depending upon where we are in our orbit. From this moving perspective, the background constellations appear to drift to the right or westward. Credit: Bob King

As Earth travels in its orbit around the sun at 18.5 miles per second, we peer into a different direction in space as the weeks and months pass. Think of going for a ride on one of those merry-go-round horses at a carnival. As the merry-go-round turns, we look out to see a different part of the fairground as the seconds pass. After one spin, the view repeats until the ride is over.

If you substitute the Earth for the horse and our orbit for the merry-go-round, the very same thing happens during a year’s stargazing with the view repeating once every year. During Earth’s “little spin” around the sun, we see the stars and constellations drift from east to west across the sky as we pass them by.

This isn’t the same as the nightly rising and setting of stars – that’s due to Earth’s rotation. Every star you see makes a complete circle around the sky in 24 hours. The much more leisurely seasonal drift is superimposed on that pattern and reveals itself to sky watchers who spend time regularly under the stars.


Cosmos trailer on Fox-TV. The series will air at 8 p.m. Central Time Sundays starting tonight 

A final note. The new 13-part “Cosmos” TV series, hosted by astrophysicist and astronomy popularizer extraordinaire Neil deGrasse Tyson, starts tonight on Fox-TV at 8 p.m. CDT (Central time) and 9 p.m. Monday nights on the National Geographic Channel. Don’t miss it!

 

What’s on YOUR astronomical bucket list?

Orion rising toward the meridian. Credit: Bob King

8:15 p.m. I’m starring at Orion the Hunter from the driveway thinking about my observing buddy who was recently diagnosed with cancer. The doctors told him that if treatments and surgery go well, he might live for several years.

I can’t imagine being given a deadline on my life, though I suppose we all live under one. Only difference is some of us know it and the rest of us pretend it’s not there. We always live as if.

My friend’s a positive person with an easy sense of humor and passion for astronomy. You’ll find him out on the coldest nights, star chart and red flashlight in hand, mapping a route to a new galaxy or star cluster he’s never seen before.

In the middle of his new reality, he decided to set goals. One of his first is to do additional public outreach astronomy, to bring his telescope to more places to share his love of the sky with more people. Next, he wants to travel to the southern hemisphere to finally see the stars forever blocked from view by his southern horizon.

His courage and optimism in the face of a grim prognosis filled me with positive emotion. It may even have been the reason I stayed out in the cold last night as long as I did.

As Orion tipped to the west, I thought about my own bucket list. For me it would be to return to the southern hemisphere and set up a big scope somewhere in the deserts of Australia to explore the Milky Way’s satellite galaxies, the Large and Small Magellanic Clouds.

I’d also like to see the planets at high magnification from southern Florida, where atmospheric turbulence is at a minimum and images are rock solid. Canoeing in moonlight (as often as possible), enjoying a total solar eclipse without a camera and spending lots more time showing school-age children the moon and planets with my telescope. These top my list.

Funny hobby this astronomy. The long-lived stars and their unfathomable distances constantly remind us of the brevity of our lives, though we often don’t pay attention. We’re only human after all; it takes work to always be aware of how sweet life is. For some, that sweetness is made purer by knowledge of life’s end.

Like the stars, my friend helped me appreciate this ever-so-short dance in the light.

Once upon a midnight clearing

Jupiter shines brightly over tall spruce trees after the snow finally stopped late Wednesday night. The constellation Orion with his three belt stars is at upper right. Credit: Bob King

The sight of stars can heal. We know this. Last week after three days of heavy snow, hard work moving it and hours spent driving through it, the sky miraculously cleared overnight. At the time, my neighbor had come over to help me fix my broken snowblower. When we finished and turned to walk back down the driveway, both of us looked up at the same time and could hardly believe what we saw. Stars. Brilliant stars. After nights and days of clouds, the sky cracked open to reveal there was something beyond the grayness.

Maybe it was the way the snow-clad spruces towered against the night, but the sky looked pitch black and the stars popped with extra sparkle.

I often write about what we know about stars – the facts as it were – but lovers of the night know their light shines on our emotions, too. That night their appearance brought us both a wonderful sense of relief.

Comet ISON and I meet again on a cold September morning

Comet ISON shows a small head or coma and short tail in this photo taken on Aug. 31, 2013. Credit: Krisztián Sárneczky / Konkoly Observatory

Months ago, when temperatures still dropped into the teens at night, I got my last look at Comet ISON. The comet glowed dimly then at the limit of my largest scope. This morning I’m happy to report that ISON has returned wearing a brighter face.

Sketch of Comet ISON from this morning seen through my telescope around 4:35 a.m. CDT just before the start of morning twilight. South at top, east to the right. Field of view is about 1/4 degree. Credit: Bob King

“Bright” of course is a relative term. Through a 15-inch telescope the comet was a faint, 12th magnitude 12.7 magnitude hazy spot that looked like a puff of smoke among the stars, but compared to last March and April, it was MUCH easier to see. And while the tail stands out clearly in the photo, I could only tell the comet was “stretched” a little with its bright head off to one side.

Orion the Hunter rises out of the forest this morning. He’s well on his way up by the start of dawn in early September just as air temperatures begin their decline. This morning it was 36 degrees (2 C).  Credit: Bob King

Face to face with ISON, I’ll be honest and say this bit of fluff’s got a long way to go before I ask my neighbors to pile out for a look. But seeing the comet finally kick it up notch gives me hope that we’ll have a nice show come early November.

The fat cone of light, brighter at the bottom and fading towards the tip, is the zodiacal light. It’s composed mostly of comet dust and gathers into a thick disk in the plane of the solar system. This morning it pointed straight to Jupiter, the bright object near top. Credit: Bob King

Getting up before dawn also brings other treats besides a faint comet that may one day achieve greatness. There’s Orion the Hunter and the sprawling Orion Nebula, home to hundreds of infant stars still swaddled in their nebular blankets. Jupiter glares right at you from Gemini in the east and it’s all topped off by the return of that eerie hump of comet dust glowing in the still dark sky – the zodiacal light.

A very thin crescent moon rose during twilight this morning September 3. Tomorrow morning will probably be the last time you’ll see it before it moves into the evening sky. Watch for the moon starting about 45 minutes before sunrise very low in the eastern sky. Credit: Bob King

Now that fall is near, the zodiacal light is tipped up into view again just before dawn. Look for a large, diffuse glow that you might first mistake for dawn. It spans the sky from Cancer, near the horizon, all the way past brilliant Jupiter nearly to Taurus the Bull.

And yes, with dawn came the crescent moon, a gentle reminder it was time for bed.

For more on Comet ISON including detailed maps on how to find it, check out my post on Universe Today.

Move over Orion, it’s the Big Dog’s turn

The sky facing south around 9-9:30 p.m. local time in mid-February. Canis Major lies to the right and below the constellation Orion and represents a large dog. Created with Stellarium

Orion the Hunter claims the southern sky as his own during evening hours in early February, but the times are changing. Nipping at his heels is the Dog Star Sirius and the rest of the stars that comprise the constellation Canis Major the Greater Dog. ‘Greater’ because there’s also a ‘lesser’ dog in the neighborhood called Canis Minor.

Truthfully, this constellation’s so small, it looks more like a dog bone treat for the bigger Canis.

Open your door and look outside at 8 o’clock and you can’t miss Orion’s three belt stars. If you shoot a line downward through the belt toward the southern horizon, you’ll soon arrive at scintillating Sirius, flashing like a silent firecracker in the turbulent air common at lower elevations.

Dangling below Sirius are some half dozen stars in the shape of a musical triangle. These form the legs, tail and head of Canis Major. Connect the dots the next clear night and you’ll see they do form the likeness of a dog jumping at your feet.

Mythological depiction of Canis Major and nearby Lepus the Hare. That rabbit better watch out. Credit: Urania’s Mirror atlas

Canis Major has been described in myth as Orion’s hunting companion and also as Laelops, “a dog so swift that no prey could outrun it”, according to Ian Ridpath, author of Star Tales. The Big Dog doesn’t have to look far for a snack –  just beneath Orion, Lepus the Hare munches contentedly in the stellar garden.

It wasn’t too many weeks ago that you had to stay up late to catch the canine constellation, but now I see it’s nudging Orion from its top spot by 9:30 p.m. As the Earth spins, stars appear to rise in the east, reach their greatest height when due south and set in the west. By month’s end, Canis Major will be due south around 8 p.m. and Orion will have taken a hike to the west.

The Greater Dog constellation topped by Sirius, nicknamed the “Dog Star” for obvious reasons. The star cluster M41, located just a short distance below Sirius, is a very pretty sight in binoculars and very easy to find. Photo: Bob King

If the nightly east-west drift of stars is due to Earth’s rotation, what causes the seasonal drift of the stars? Why doesn’t Orion always stay in the same place week after week, month after month? Blame it on Copernicus. He made the claim, outrageous for his time, that the Earth moved around the sun. Throughout antiquity and into the Middle Ages people thought that if the Earth moved, every time you jumped off the ground, the planet would rush away and leave you behind. Since that didn’t happen, it was obvious the Earth must stand still.

Few understood that EVERYTHING – the Earth, the jumper, birds and atmosphere – were all moving at the same speed and so appeared to be at rest relative to each other. The same thing happens when you’re flying at 550 mph in a plane. Once the plane has reached a constant speed, you’re hardly aware you’re moving. And since you, your laptop and that ginger ale are all traveling at 550 mph, they don’t go flying around the plane. This habit of things to stay put as long as they’re all moving at the same speed is called inertia.

Because our planet orbits the sun, we see into different directions in space over the weeks and months of a year. In January (right), Orion dominates the southern sky; in April it’s Leo and in June, Scorpius. The whole cycle repeats every year. Illustration: Bob King

OK, back to the Big Dog. As the Earth orbits the sun, our perspective on the nighttime sky changes over the weeks and months. At 10 p.m. in mid-January Orion stands straight up in the southern sky, but at 10 p.m in February, he’s been replaced by Canis Major. Come 10 p.m. in April, Leo the Lion will be high in the south and Orion will have set in the west.

As the weeks and months go by, we peer into a different direction of sky just as a runner sees different groups of fans as she runs the 1500-meter on a race track. Like many good things in skywatching, the cycle repeats anew every year.

Meet Rigel in Orion, a star with supernova potential

Both Betelgeuse and Rigel are potential supernova candidates. The view shows the sky facing southeast around 8 p.m. local time in early January.  Maps created with Stellarium

Everybody’s always worried about Betelgeuse in Orion blowing up as a supernova. There’s a good chance that may happen one day, but no need to panic. The star’s too far away to trouble earthlings with its future fireworks. Opposite Betelgeuse and below the winsome triad of stars that form Orion’s Belt, another potential supernova star sparks and sputters on winter nights – Rigel. The name comes from ancient Arabic and refers to the foot or leg of Orion.

Like Betelgeuse, Rigel (RYE-jel) is also a supergiant star but one of a different color and temperature. Astronomers classify it as a blue supergiant with a surface temperature of over 20,000 degrees, twice that of the sun and 3 1/2 times hotter than Betelgeuse.

Rigel, a blue supergiant star, is 18 times more massive than the sun and 74 times its size. Credit: CWitte with minor alterations by Bob King

At a distance of 860 light years, Rigel is big enough and close enough to have its diameter measured directly. As you might guess, it’s huge – 74 times the size of our sun.

Placed where the sun is now, this stellar beast would extend nearly to the orbit of Mercury. From Earth, Rigel would span 35 degrees of sky and shine at a blinding -38 magnitude. We’re talking a powerful sunburn in a minute or two.

Great distance tames Rigel’s true ferocity as a young, energy gobbling star into a pretty blue-white twinkle reminiscent of sunlight on snowflakes. Rigel shines at 0.1 magnitude or about as bright as Capella in Auriga and Vega in Lyra.

Being extremely hot, blue supergiants burn up their energy stores quickly. At the tender age of 10 million years (young for a star), Rigel has already depleted its core of hydrogen fuel and has moved on to burning hydrogen in a surrounding shell.

If put in place of the sun 93 million miles from Earth, Rigel would cover 35 degrees or sky or about twice the area of the constellation Orion.

Helium “ash” created from hydrogen burning will one day ignite and serve as fuel as will progressively heavier elements like oxygen, neon and silicon over time. Rigel will puff up and redden just like Betelgeuse in those far-off days.

Just before a supergiant star blows it has a core made of iron that cannot “burn” to create energy to push back the force of gravity. Gravity takes hold and the star collapses.

Unfortunately, supergiant stars reach the end of the line once all the remaining silicon fuel has undergone nuclear fusion to create a core of iron. Iron requires more energy to fuse than the energy it releases, so it won’t burn like the other elemental fuels. With no burning to push back against the crushing force of gravity in so large a star, the core collapses and sends out shock waves that rip it apart in a supernova explosion.

Rigel is a close double star in a small telescope. Use 100x and up to split it cleanly. Credit: Fresno State University Observatory

When will this happen? Probably millions of years down the road. Since Rigel’s 300 light years farther from Earth than Betelgeuse, we needn’t worry about it either. Instead, our future descendants should prepare for a wondrous light show. Jim Kaler, professor emeritus at the University of Illinois, estimates that Rigel will become as bright as the half-moon when it finally blows up. Picture all that light concentrated in a tiny point of light. We’d easily see our shadows at night by supernova light!

If you have a small telescope 4.5 inches or larger, point it at Rigel some night. It’s one of the finest, if challenging, double stars in the sky. The 7th magnitude companion peaks out from under the glare of the main star a very short distance (9 arc seconds) to its south. On a night with steady air and good seeing, this pair is a beautiful sight.