Orion Nebula. Exposure 15 s. Camera CANON EOS 400D, Telescope Konusky Motor 150. Credit: Giovanni La Mura.
By Giovanni La Mura – Department of Astronomy, University of Padova (Italy)
The first observational experiences of the night sky, with a small instrument, are always a source of surprise and wonder. The huge landscape, that we see in a starry night, indeed, conceals a number of fascinating details, which we can explore with the help of a small telescope. A few hours should be enough to realize that the Universe is characterized by many colorful views. Mountains and craters draw spectacular frames on the surface of the Moon. The atmosphere of Jupiter strikes with the amazing contrast among its stripes. Mars is unique in its red and desert soil. And Saturn may never be forgotten, for the spectacular effects of light and shadows which involve its rings. Stars, nebulae and galaxies are not an exception, in this sense, although more powerful instruments could be required to fully enjoy their beauty.
But the colors of sky have a fundamental scientific role, as well. Actually, the impression which we call “color” is a signature of the energy carried by light and, therefore, it is an indication of the physical properties in the light source. In the case of stars, the light color tells us the approximate temperature of the stellar surface, from which astronomers are able to understand many other details, concerning the activity of stars. Stars that emit blue or white radiation are very hot at their surface, a property which is characteristic of typically young objects. If, on the contrary, their light turns toward orange and red colors, it means that the stellar surface is much colder, which is commonly the signature of very old or quite small mass stars.
The situation that we observe in gaseous nebulae, on the other hand, is quite different. In normal conditions, indeed, the gas particles can only emit over a well selected range of pre-determined energies, corresponding to specific colors, and the environment temperature can only influence the strength of light emission. For this reason, the colors of a gaseous nebula are a more effective indicator of chemical composition rather than of temperature, because different chemical elements produce their own specific colors.
Another important actor in the most spectacular landscapes of the space is dust. This component generally has not such a high temperature to shine in optical light by itself and we can often infer its presence only by means of the shadows that dusty clouds cast on the surrounding space. However, when it happens that dust lies close to a bright source of light, it can still reflect some of its radiation into our line of sight and shine with approximately the same colors of the original source. Such effects of cosmic mirroring turned out to be fundamental for the interests of ground based astronomers, who could study the properties of many hidden objects in the reflected light, thus understanding their nature.
With these ingredients, we are able to understand the physical scenario illustrated in this picture of the Orion Nebula. The image shows a small group of young and very hot stars (notice the white and blue colors), which are getting rid of the gaseous envelope where they were born in a quite recent time. The color of gas is red, a characteristic feature of hydrogen rich materials, according to the expectation of modern astrophysics, which states that hydrogen is the fundamental fuel of new stars. On the upper left region, we may notice the presence of an obscuring structure of dust. This “cold” and dark component tells us that the original environment was probably an obscure cloud of dusty gas, which collapsed under its own weight, until the stars at the centre were formed. After the star formation event was over, the light of the new stars heated the surrounding gas, blowing away the dark cover.
Other properties of the Universe are summarized in the colors of its light. The motion and the distance of galaxies is related to the colors of their light, which appear to be shifted toward red or blue, according to whether they are receding or approaching our point of view. This is a general property of the light propagation across the Universe which does not involve only the familiar concepts of “red” and “blue”, but it rather affects the whole electro-magnetic spectrum of radiation, including those “colors” that we are not able to see, such as gamma and X rays, ultraviolet and infrared radiation, and radio waves, but that we nonetheless observe with our most advanced scientific instruments.
Web Astronomical Album by Giovanni La Mura is available at: http://picasaweb.google.com/dkkf16/Astrofotografia#5308665379291867282 .
Many thanks, Giovanni. Sabrina