TWINKLE, TWINKLE, WINTER STAR
Hello, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences, welcome to The Night Sky.
I imagine many of you have stepped outside on a cold winter’s night and noticed just how bright the stars look. Part of this is due to the fact that the atmosphere is more transparent; there’s just less humidity and haze to obscure the starlight. But it’s also due to the fact that our winter sky is filled with more bright stars and constellations than any other time of the year. Some of the most prominent stars in our winter sky can be seen while facing south during the early evening hours this month and include Betelgeuse and Rigel in the constellation of Orion, Aldebaran in Taurus, Capella in Auriga, Castor and Pollux in Gemini, Procyon in Canis Minor and Sirius, the brightest star in our night sky within Canis Major. Most of these stars make up a pattern known as the Winter Hexagon. Such patterns are not constellations however; they are called “asterisms” and are made up of the stars from one or more of the official 88 constellations. You already know one of the most famous of asterisms, “The Big Dipper” within the constellation of Ursa Major seen in our northern sky year round. I suggest you spend a little time learning more about these different stars on your own as they offer a good survey of the types of things you can find within the stellar zoo.
For right now however I want to draw your attention to how, on some nights, these stars twinkle like flickering sparks of light in the dark. All stars can be seen to twinkle at anytime of the year but it seems to be particularly noticeable on winter evenings and even more so with brighter stars than with dimmer ones. Oh, and if you want to amaze and impress your friends you can use the more technical term of “astronomical scintillation” rather than “twinkling”.
So, why do stars scintillate? Take a deep breath. There’s your answer: atmosphere. We live on a planet that is insulated by layers of air with differing densities, temperature, and humidity and it’s always churning. It’s turbulence within the atmosphere that makes the stars twinkle. The photons emitted by the stars have been traveling unimpeded, in straight lines, across trillions of miles of vacuum but once they enter our atmosphere their path can become bent, or refracted. Light refracts when it passes from one medium to another. You’ve seen this effect before when a spoon appears to be bent inside a glass of water. Of course it isn’t the spoon that’s bent, it’s just the way light is being refracted as it passes from the water into the air. In the cold vacuum of space there was nothing to bend the starlight but inside our atmosphere there exists multitudes of tiny packets of air known as cells that are only a few dozen centimeters across and these cells are constantly moving around in all directions, more so when there’s lots of atmospheric disturbance. The cells act like tiny lenses that bend the star’s light back and forth. From our vantage point on the ground the star appears to twinkle.
If you are an especially good observer you might notice that stars low along the horizon seem to twinkle much more than do the stars overhead. Not only that, but some of the stars are also changing colors. When you look at any star while it’s low in the sky you are seeing it though a much thicker layer of air than when it’s directly overhead, and as it’s light is having to travel through more air before finally reaching your eye, it’s also getting refracted more and twinkles more conspicuously than does a star overhead. To see the color change I suggest watching Sirius while it‘s low upon the horizon during the early evening. Finding Sirius is easy, it’s our brightest star, and if you draw a line through Orion’s belt stars, and extend it out and to the left, you will run right into it. When Sirius is high upon the sky it’s light appears as an unchanging white, but when it’s near the horizon and there’s a lot of turbulence you can see it rapidly change colors from white to blue to green and red. The white light of stars actually contains every color of the rainbow but if you pass the light through a prism you can split it into its constituent colors. This is what happens as Sirius’ light passes through all of those cells of air in our atmosphere. If the turbulence is especially bad it can refract the star’s light so much that Sirius looks like a disco ball as it changes colors in fractions of a second. It’s not at all unusual for folks to report seeing a UFO when Sirius is so scintillating.
Now, you’ll sometimes hear folks say that you can always distinguish a planet from a star by the fact that stars twinkle and planets do not. This is mostly true but not always. Around 8PM when Sirius is still low in the southeast look to the east to see the planet Jupiter on the rise. In most cases you’ll see that Jupiter’s light remains steady while Sirius’ may be flickering. Planets don’t appear to twinkle because they are closer to us than the background stars. While not apparent to the naked eye a planet is a disc rather than a pinpoint light source like a star is. Remember, the cells of air in our atmosphere are usually only a few dozens of centimeters across and are too small to make the light from the planet’s disc scintillate very much but are just the right size to really distort the starlight. However, on nights with lots of turbulence even Jupiter may appear to twinkle. Last year at this time I recall trying to observe Jupiter through my telescope on a seemingly calm night. The view through the telescope however told a different story. Jupiter’s disc appeared to shimmer wildly and I couldn’t make out any of the planet’s distinctive features. While twinkling stars can look very pretty they are a royal pain to both amateur and professional astronomers. Twinkling stars means the seeing conditions are bad and you won’t be able resolve stars or planetary details through the eyepiece of a telescope.
Astronomers have had to be very creative in overcoming the limitations of trying to study the universe through an uncooperative atmosphere.
This is a photo of me at the Keck Observatory located 14,000 feet above sea level atop the extinct volcano Mauna Kea on the Big Island of Hawaii. There are a number of good reasons to build observatories at such altitudes but one of the most important is that the atmosphere is very thin at these elevations and consequently starlight is not distorted as much as it would be at lower altitudes.
But we still aren’t completely free of atmospheric disturbances even at higher elevations so astronomers have had to become even more creative. One of the most inventive and high tech methods astronomers use to compensate for atmospheric distortion is known as “adaptive optics” and involves the use of lasers, deformable mirrors, and supercomputers. To get clear, sharp images from the telescopes at the Keck Observatory astronomers fire a laser up into the atmosphere. Their target is not a star but tiny atoms of sodium located about 60 miles up. The laser excites the sodium atoms for a brief moment and when the atoms return back to their normal un-energized states they emit light. In effect, astronomers are creating an artificial star, although it’s far from being a real star in any shape or form. The light from this false star is analyzed by a computer to determine how the atmosphere is distorting it. Once this is established the computer then sends instructions to the telescope’s mirror to change its shape in order to correct for the light refraction. Over the past decade many other observatories have employed adaptive adaptive optics with much success.
Of course the ultimate way to get around the distorting effects of our atmosphere is to escape it altogether and send telescopes into outer space itself. For 24 years now the Hubble Space Telescope has allowed astronomers to understand our universe in ways never before dreamed of. It has helped us understand how galaxies evolve over time, given us data that has helped refine the age of the universe itself, shown us that supermassive black holes are a ubiquitous component of galaxies, revealed how planets form, and has helped us figure out the rate of expansion of the universe. And the images acquired by the Hubble Telescope have inspired us, filled us with awe, and revealed to us just how beautiful and wondrous the universe really is.
Until next time, I encourage you to experience some of this beauty for yourself by simple stepping outside and looking up in both awe and wonder.