The song “twinkle, twinkle little star”, is a children’s classic; which describes how a star changes its brightness. This description of a star has a connotation that, from the point of view of astrophysics and astronomy, is a big problem.
Most likely this song might be familiar for you…
TWINKLE TWINKLE LITTLE STAR
OH I WANDER WHAT YOU ARE!
UP ABOVE THE WORLD SO HIGH
LIKE A DIAMOND IN THE SKY
Certainly, a classic children’s lullaby song; it romantically describes the dazzling twinkle that the stars exhibit in the night sky. Hypnotic and fascinating effect that in fact has nothing to do with the stars.
I’m sorry to spoil this song’s illusion; but this effect is in fact caused by a distortion on the light we receive from the stars due to the earth’s atmosphere, and although it makes the night sky something probably more interesting and romantic to some, it is a real problem for astronomy. This condition affects the light we receive from objects in space, distorting their image and with this, limiting to some extent the sharpness and detail in the captured images.
Light from stars and other objects in space suffers a distortion when it enters the earth due to the atmosphere’s effect, given its constant movement and different composition. This impacts the telescope’s ability to clearly resolve the image of these objects in space. This effect is similar to that observed when an object such as a rock or a coin is underwater; When observing it from outside the water, we can notice how the shape of this rock or coin is distorted due to the refraction that the light undergoes when crossing the irregular and changing surface of the water; This is the same atmospheric effect that produces the fluctuations of light intensity that we notice as twinkling in the stars.
Added to this problem are other difficulties such as absorption and dispersion.
Absorption: The atmosphere’s composition provokes that certain light frequencies never reach the surface of the earth, as they are absorbed by certain compounds in it, a typical example (and beneficial to us), is the absorption of ultraviolet rays by the atmosphere’s ozone layer.
Dispersion: light is scattered by dust and water vapor in the atmosphere, this causes that certain light frequencies that are susceptible to these elements do not reach the ground (at sea level at least). This effect can be noticed on car lights when traveling in a foggy area, in these conditions we can see how the light illuminates the whole area in front of us, in all directions; not just on the direction of the road where we are driving.
Obviously, this has been one of the problems that have caused difficulties in astronomy’s history, and even when the instrument’s resolution and quality have been continuously improved, these effects still cause difficulties.
One of the most important and successful methods to avoid this problem was to consider the use of space telescopes. Where the best-known example is definitely the Hubble Space Telescope (which in my opinion should be called the Hubble space observatory, because this is not only a telescope, as it has different instruments and means of observation and analysis, coupled with their ground equipment).
These spatial instruments completely eliminate the problems caused by atmospheric distortion, and although it is a difficult option to implement, their results are something literally “out of this world,” with examples such as Hubble’s with its deep-field observations; the Chandra X-ray observatory, the flagship system of high-energy radiation observations; the Spitzer infrared space telescope, with which multiple stars with planets have been discovered, including the case of the Trappist-1 system, with seven planets detected; and also the example of the WMAP (Wilkinson Microwave Anisotropy Probe), a probe that generated the detailed map of the cosmic microwave radiation, the afterglow of the Big Bang; we can notice that their advantages are indisputable.
But the teams from land observatories weren’t frustrated with their luck to live on a planet with an atmosphere. Returning to the theme of star’s scintillation, this effect is accentuated when trying to make observations of smaller and smaller areas and objects, where the limitation is imposed by the observation itself, and not by the instrument, this is called “sight limitation”.
The solution to this sight limitation problem is, first, to build observatories in areas where atmospheric effects are diminished; places such as Mauna Kea in Hawaii and the Atacama Desert in the Andes region in Chile are two of the favorite locations. Given their height and location, these sites are above the climatic effects and above from a large portion of the atmosphere; these places, due their location, which are generally in the middle of the ocean or near western coasts, receive an air flow that keeps these zones free of pollution and humidity.
The advantage of space telescopes over terrestrial ones is also being reduced as techniques and systems such as adaptive optics and interferometry are improved; the first being a technique to optically compensate for the distortion caused by the atmosphere, calculating this distortion and compensating it with deforming mirrors in order to counteract its effect. Interferometry in the other hand, is the combination of two or more telescopes with mirrors of a smaller diameter to have the resolution of a much larger telescope, with a diameter equivalent to the total distance that separates these telescopes.
Currently the VLT (Very Large Telescope Array), is the largest land observatory, it uses all the above mentioned aids; it is located on the Paranal mountain in Chile, this is a group of four telescopes with 8.2 meter mirrors, which can be used individually or combined, and makes use of a state of the art adaptive optics system.
Other telescopes with larger diameter mirrors and compensation techniques are in operation or under construction, such as the SALT (Southern African Large Telescope), with an 11 meter diameter mirror (mirror comprised with 91 individual hexagonal mirrors of 1 meter in diameter), and located near Cape Town in South Africa; or the E-ELT (Extremely Large European Telescope), which, when completed around 2024, will have a composite mirror equivalent to 39 meters in diameter, this one being built on the Armazones mountain, also in the Atacama desert in Chile.
Astronomy and astrophysics’ future
Comparing these new telescopes with the next generation of space telescopes; the James Webb space telescope, which can be said to be the replacement of the Hubble space telescope, will have a primary mirror of 6.5 meters, which will be about one fifth of that of the E-ELT.
Astronomy’s future is promising, and even with stars’ twinkling issues, we can count on images with incredible detail, both in telescopes on land and in space. So, it seems that the little star will no longer be able to make use of the earth’s atmosphere to hide from us.