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Take a Tour of the Fabulous Stars of Winter!

Tonight is the night you should brave the cold and check out some bright & beautiful celestial treats in the night sky!

Happy New Year!

Now that Christmas & New Year’s have passed, as well as the December solstice (12-21-21), the nights are long (it is getting dark here at 4:30 PM… excellent time for astronomy!) and the days are short (especially if you are off from school and spend most of the day watching sci-fi and eating chocolate goodies, as I do).

The winter is a wonderful time to gaze at the stars. There are several bright and interesting stars to choose from. If you live where I do however, you may be loath to go outside in -26 degrees (F) weather (current average here), walking amongst the recent snow (10+ inches lately!).

For those willing to brave the chilly nights, assuming clear weather, they will be rewarded with a crisp view of several easy-to-spot constellations (such as Orion) that contain some real luminous gems. Let us take some time to explore a few…

Betelgeuse

I begin, appropriately, with the head of Orion, Alpha Orionis, also known as Betelgeuse (not to be confused with the film).

In astronomy, we tend to label stars in a sequence of Greek letters, beginning with the brightest star in a constellation first. This is where the designation ‘Alpha’ in Alpha Orionis comes from. The ‘Orionis’ part simply designates the constellation name. For example, the brightest star in the constellation  Orion (Betelgeuse) is labeled as Alpha Orionis. The second brightest star in the constellation Orion is Beta Orionis, also known as Rigel (this is one of my favorite stars, and one I’ll come back to later). The third brightest star in Orion is Gamma Orionis, also known as Bellatrix. You get the idea…

Betelgeuse is a type M1 red supergiant, meaning it is a cooler star, as compared to the Sun (Betelgeuse has an effective (surface) temperature of ~3,600 K whereas our Sun has an effective temperature of ~5800 K). It is relatively close to Earth, at a distance of 197 ± 45 pc or ~642.5 light years (Harper et al., 2008), which is nothing in cosmic terms.

Betelgeuse is a truly immense star… with a radius at least 887 times larger than the Sun, if it were exchanged with the Sun it would extend outward to the orbit of Jupiter. Only Saturn and planets beyond would be outside its surface. Its atmosphere is very tenuous however, and not spherically symmetric, with such low density that our own Sun and even normal air are far denser (Betelgeuse has less than one ten-thousandth the density of Earth’s atmospheric air).

Betelgeuse is a variable star, and a long period one at that. The star has been found to have multiple cycles of dimming: a 5.9-year main cycle and, within that, several smaller ones. One of its cycles is ~ 420 days long. A third cycle is shorter; about 100 to 180 days. Most of its fluctuations are predictable and follow these cycles. Some are not, such as the recent observed dimming event of 2019. For more information on that strange dimming event:

https://earthsky.org/space/betelgeuse-dimming

Aldebaran

Located roughly 65 light years away, Aldebaran is a giant, cool star (surface temperature of 3,910 K, a cool type K star). It is the brightest star in the constellation Taurus, the Bull (its astronomical designation, Alpha Tauri, is not to be confused with the F1 racing team).

With a radius roughly 45 times that of the Sun, Aldebaran is much smaller than Betelgeuse, yet so much larger than our Sun (the Sun would just be a tiny dot next to a huge, reddish orange sphere if they were placed next to each other). The NASA space probe Pioneer 10 is currently heading in the direction of Aldebaran, and should at least pass close to it in about 2 million years (be patient). If you can find Orion’s Belt (three stars – Alnitak, Alnilam, and Mintaka), then draw a line straight up connecting them and out, through Orion’s Bow, and just underneath (a little to the SW of) the Pleiades. You should see a bright orangish star, that is Aldebaran.

Rigel (Beta Orionis)

Rigel is one of my favorite stars, perhaps second only to Deneb (See below link for more info).

https://sbsramesh.blog/2021/06/02/take-a-tour-of-the-brightest-stars-of-summer/
Check out my past blog for more on Deneb, my favorite star! (Take a Tour of the Stars of Summer)

Rigel is an amazingly beautiful type B star, appearing blueish-white to the eye, and comes in at a solid #7 on the brightest stars in the night sky list.

Why is Rigel, a brilliant blue supergiant with a much hotter temperature than Betelgeuse (~12,000 K surface temperature vs. 3,600 K for Betelgeuse) labeled as only the second brightest star (beta designation) in the constellation Orion?

stars on night sky
Photo by Skiartist on Pexels.com – The beautiful Orion Nebula is very close to Rigel in the night sky! Find Rigel and look slightly up and to the left…

The truth lies in variability. Since antiquity, Betelgeuse, a variable star, has been known to occasionally outshine Rigel in terms of apparent brightness (the brightness the star appears to have from Earth) at its peak magnitude.

Rigel can be seen with the naked eye as the left foot (or leg) of Orion, perhaps a little low in the sky during early winter until the entire constellation shifts upward near the zenith (top of sky, point directly above). Hurry and see it while it is still around, its fate is to go supernova (well, a million or two years from now, but that’s a short time on the cosmic scale) most likely leaving behind a black hole.

Castor & Pollux

The Twins (Constellation Gemini) make quite an interesting pair. To begin with, Castor, the second brightest of the two, is not one but actually six stars (a triple binary system) that only appears as a single star to the unaided eye.

The Castor system is 51 light years away from Earth. The two brightest of the six (Castor A and B) are spectral type A stars, each with fainter red dwarf companions (Castor Ab and Bb, both less massive than the Sun, type M). The final pair of stars (Castor Ca and Cb) are both red dwarfs (type M) and almost identical, with masses around half that of the Sun. In addition, Castor C is a variable type of star, specifically a BY Draconis variable (cool type K or M stars that are typically on the main sequence, with variations in brightness typically up to .5 magnitude that are usually due to starspots rotating in and out of view). We see Castor as a bright blue star (with the unaided eye), giving the appearance of a higher surface temperature (Type A stars do indeed glow a hot blue color).

IMAGE OF CASTOR STAR SYSTEM w/ SUN FOR COMPARISON – CREDIT: NASA/JPL

Pollux is a yellow-orange giant star, larger and somewhat cooler than our own Sun (Type K, surface temp 4,600 K, 9 times the Sun’s radius and approximately 2 times its mass). In 2006, astronomers discovered a planet orbiting around it that was roughly 2.3 times the mass of Jupiter!

Pollux was once a type A star, much hotter than we observe at present, but stopped fusing hydrogen and now is fusing lighter elements, having expanded into its cooler giant phase. There is evidence for a weak magnetic field, one of the weakest ever detected around a star, and so perhaps this field was much stronger during Pollux’s earlier type A hydrogen-burning phase (Auriere, 2018).

Castor and Pollux are not gravitationally bound to each other, just close to each other along our line of sight (appearances can be deceiving). Look for the pair off the shoulder of Orion in the night sky… in fact you can draw a nearly straight line starting from Rigel up through Betelgeuse and out toward Castor, the top member of the pair (with Pollux below).

Capella

Capella (Alpha Aurigae) is the sixth brightest star in the night sky and member of one of my favorite constellations, Auriga (The Charioteer). I love this constellation because it contains three top-tier sparkling open star clusters: M36, M37 (my favorite), and M38. These picturesque open clusters are worth a look if you have a pair of binoculars or a telescope.

Like many stars on this list, Capella is actually a multiple (quadruple) system! It appears as a single star to the naked eye, but the system is actually composed of two type G stars (both roughly 2.5 times as massive as the Sun) and two smaller stars (cooler red dwarfs, type M). The two larger stars (Aa and Ab) are in a very tight orbit, circling each other every 104 days.

At roughly 43 light years distant, Capella is in the same galactic arm as our Sun, that of Orion-Cygnus.

Sirius (AKA The Dog Star)

A friend of mine once mistook Sirius for a UFO!

When Sirius first rises, coming up over the horizon (where the air currents tend to be strong), it can twinkle and even shift colors… red, green, white… appearing to sparkle as if it were some magical floating object hovering over (or perhaps surveilling) the countryside. If you have ever witnessed this, it is easy to see how my friend (who was not an astronomer and not very experienced with the night sky) could mistake this blazing white splendid star for an alien invasion.

Sirius is nicknamed “the Dog Star” because it is the brightest member of the constellation Canis Major (the Greater Dog). Although it appears as a single bright white star to the naked eye, it is actually a double star, a binary system that consists of a type A star roughly twice the mass of the Sun and a much hotter (25,000 K vs. 10,000 K for Sirius A), much smaller white dwarf companion (Sirius A is roughly 1.7 times the Sun’s radius, Sirius B (the white dwarf) has roughly the same mass as the Sun yet has roughly the same volume as the Earth). Whatever star Sirius B was before (most likely a bright blue star perhaps even more massive than Sirius A), it became a red giant, sloughed off its outer layers, and collapsed into its white dwarf present state over 100 million years ago.

At approximately 8.7 light years away, Sirius is a relatively close star (system) to Earth. It always rises after Orion, as the faithful hunting hound (Canis Major) chases its master across the sky in pursuit of fabulous game (sadly, Orion seems to only have an eye out for the Pleiades (The Seven Sisters)).

These are only a few of the amazing, fascinating, and brilliantly beautiful stars shining right now in the winter night sky. I encourage you to use your eyes or a pair of binoculars and catch a quick glimpse of them, in all their glory, perhaps bundled up with some hot tea or chocolate! Happy viewing and as always, clear skies!

Winter Sky (January 2022)

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