Each new year brings the great hunter Orion into prominent view in the mid-evenings of northern skies. Orion is one of the most easily-recognized constellations, as it is centered upon and framed by very bright and relatively near-by stars. It is also one of the most ancient patterns of stars to be associated with a variety of mythologies. The Babylonians identified the pattern as “The Great Shepard”, the Egyptians saw a representation of their great god Osiris, a major symbol of rebirth in their religion, the texts of ancient India describe the pattern as a deer; the Lakota Native Americans saw a bison.
The constellation is easily found by looking to the southeast after sunset (in January, progressively farther to the west as we move through February and March) and finding three bright stars, evenly spaced in a line. These form Orion’s belt, pointing from southeast to northwest. Framing the belt and covering a vast region of sky are four stars forming a rough rectangle – Betelgeuse to the upper left, Rigel to the lower right, Bellatrix on the upper right, and finally Saiph on the lower left. Each of these stars is physically gargantuan in size, all classified as supergiants, with masses from 8-30 times that of the Sun, and all lying within 1000 light years from Earth.
It is in this area of the sky that one of the richest collection of astronomical objects is found. Below the first and second (from right) stars of Orion’s belt, the eye can easily see a patch of light that appears as a fuzzy star. In fact, this patch is a brightest section of a truly colossal cloud of interstellar gas, covering hundreds of light years, centered 1500 light years from Earth.
What can be seen with the unaided eye – The Orion Nebula – becomes an astonishing sight in a small telescope, increasingly fascinating using larger instruments. At the center of this brightest section of the gas cloud lie four very young stars, each less than a million years old, forming the “Trapezium”. In fact a total of six stars, two of the four being binary, these extremely hot stars emit most of their energy in invisible ultraviolet light, causing the surrounding gas in the cloud to fluoresce and give off a very strong glow in visible light, allowing us to view the detailed structure of the cloud.
Within this structure (see the accompanying photo) we can see bubbles of gas caused by shockwaves of star formation, various colors – green, blue and red, caused by the reaction of different gas elements and different states of the atoms within the gas to various wavelengths of light coming from the Trapezium. A large cloud of dust – very fine microscopic grains of sand – forms a dark cloud lying in front of sections of the glowing gas cloud.
The majority of the stars illuminating the Orion Nebula are, however, invisible to the observer, hidden deep within the cloud, still surrounded by the dense gas from which they recently formed. Approximately 700 stars have been detected using infrared observations, which allow us to see within the obscuring gas and dust clouds.
Beyond this brightest region of the cloud, several other areas of this vast nursery of stars shine with fainter glows. Near the easternmost star in Orion’s belt lay two fascinating and beautiful regions of the cloud. This star, Alnitak, is again a giant, with a mass of at least 20 times that of our sun. A very young star, only a few million years old, it produces about 20 times the energy of our sun in visible light, but an amazing 100000 times the energy of the sun in ultraviolet wavelengths. As in the case of the Trapezium, this causes a nearby gas cloud to fluoresce and glow as the Flame Nebula.
Very close to Alnitak within the same massive cloud lies another hot young star, Sigma Orionis, which again causes a region of the gas cloud to fluoresce. What is particularly fascinating about this object is not the glowing gas cloud, but an immense dark cloud of dust and molecular gas that lies between the glowing gas and Earth. This cloud has the distinctive shape of the head of a horse, and the object is known as the Horsehead Nebula.
This nebula has been studied extensively to determine its chemical composition, resulting in the surprising discovery of large concentrations of organic compounds. The Horsehead is a particularly dense section of the entire Orion gas complex with a mass of about 100,000 stars. Observations in long infrared and radio wavelengths show the presence of as many as 10,000 star systems in formation. With the presence of large amounts of dust in the cloud, it is highly likely that many if not most of these systems will contain rocky planets similar in composition to the Earth.
So, on any clear evening this month or next, take the opportunity to look to the southeast to find this unique constellation, and consider the sheer magnitude of the cloud of gas that fills this region of space, and the innumerable worlds that are only now forming deep within this structure. Over the next several billion years, these worlds will settle into solar systems, some within a range of temperature that will allow water (which is prevalent throughout the cloud) to be in liquid form. On many of these planets we may expect life to arise long after life here on Earth has perished with the demise of our own sun.