December -The Orion Nebula
Finding the Orion Nebula in the Sky
The Orion Nebula is one of the brightest "deep sky" objects visible to the unaided eye under moderately good sky conditions. It appears as a fuzzy "star" in the middle of the sword of Orion the Hunter, south of its prominent belt. Located in the same spiral arm of our Galaxy as the Sun, and about 1500 light years from us, it is relatively nearby considering our home Galaxy’s diameter is about 100,000 light years. You can find observing tips and resources in the December 2009 IYA Discovery Guide, and in the Exploring Further section of this article.
A Glowing Birthplace of Stars
Four centuries ago, when Galileo used the little telescope he built to look at a region of the Orion constellation, he discovered many more stars in it than were visible with the naked eye. Later, he also found that the middle "star" in Orion's sword was actually a triple system, but did not mention any nebula surrounding it. While other European observers around that time may have discovered the diffuse nebulosity using small telescopes similar to Galileo's, a good drawing of the Orion Nebula was made only in the mid-Eighteenth century by Charles Messier for his catalog (thus giving it the name Messier 42 or M42).
Over the last 150 years, improvements in telescopes (which are currently millions of times more sensitive than the ones Galileo built) and their instruments have given us insight into the real nature of this nebula. From spectral analysis of its light, we know that the Orion Nebula is a vast, turbulent cloud of predominantly hydrogen and helium, covering more than 20,000 times the length of our Solar System. Deep, panoramic images have provided evidence of huge arcs and pillars of soot-like interstellar dust banding through the diffuse gas.
Shrouded in this immense expanse are thousands of stars and star clusters old and young, including the dazzling celebrity: the Trapezium Cluster, a veritable infant at barely a million years old. The Hubble Space Telescope and the Spitzer Space Telescope, whose infrared cameras can peer through the thick dust, have unveiled an exquisite array of star birth occurring amidst the turbulent gas of the Orion Nebula. These images show the nebula is host to knots of gas and dust that will condense into stars, as well as nascent stars emerging from their dusty cocoons. There are also very faint brown-dwarfs in the Orion Nebula, the so-called "failed stars" which do not have enough mass to sustain nuclear fusion in the cores the way "real" stars like the Sun do.
Much of the gas and dust in the Orion Nebula will eventually form stars, lighting up the nebula and keeping it in its prime for a good long time. Indeed, our middle-aged Sun is thought to have been born in a cluster within a gas cloud about 4.5 billion years ago.
Stars and Planets: An Eventful Childhood
The Orion Nebula also contains numerous massive, hot, blue stars - merely a few million years old - that profoundly influence their environs and may even trigger the birth of fresh generations of stars. The hot stars' energetic and high-speed outbursts and powerful ultraviolet radiation are carving out huge cavities in the gas and dust, through which we can see the birth of their less massive and fainter siblings. The hefty stars also emit powerful X-rays from their extremely hot (multimillion degree) upper atmospheres, as seen in the Chandra X-ray Observatory images. They will course through their lives in a hurry, ending in a blaze of glory as supernovae, perhaps even before the little stars in the neighborhood emerge fully formed.
Even in this hectic vicinity, many embryonic solar systems may exist and thrive. Newborn stars in the Orion Nebula, in close-up views taken by the Hubble Space Telescope, appear like ethereal Frisbees with disks silhouetted against the background of the bright gas. The disks, composed of gas and tiny amounts of dust (that makes them opaque), span several times the size of our Solar System. They have long been hypothesized to be the kick-off to planet formation. While the existence of numerous disks enhances the possibility of planetary systems, there is no guarantee that planets will indeed form in the Orion Nebula. After all, the harsh X-ray radiation from their own atmospheres or their stellar brethren may wreak havoc on these seeds of planet birth. On the other hand, the X-rays could also be helpful, creating turbulence that prevents planets from migrating inward through the disks and plunging into the parent star.
Thus, observations of the Orion Nebula and many other gaseous nebulae have given astronomers direct insight into the formative years of stars and planetary systems - something Galileo could never have dreamed of.
The Adventure of Science
From detailed studies of newly minted stars, as well as observable trends among the properties of mature stars sorted according to their stellar mass and age, astronomers can weave testable theories of how stars form and evolve. This contributes in turn to the pool of knowledge required to answer what NASA has identified as among the Big Questions in astrophysics: how do planets, stars, galaxies and cosmic structure come into being, and what is the origin of heavy elements in the Universe such as carbon, oxygen, iron, etc., that make up the planets and living beings? Astronomers have not only discovered planetary systems in the making within giant gaseous nebulae, but also recently captured what may be the first direct images of mature planets around other stars. We are on the scientific voyage of discovering how we - our Solar System, our planet, and life - came to be, and ultimately, finding out if there is life elsewhere.
Galileo's drawing of the Orion constellation. 400 years ago, Galileo recorded many more stars through a telescope than were visible with the unaided eye.
An exquisitely sharp view from the Hubble Space Telescope of the immense gas-and-dust landscape of the Orion Nebula and thousands of stars, many seen here for the first time.
The Spitzer Space Telescope has revealed disks of planet-forming dust around some stars in the Orion Nebula.
Observing the Orion Nebula:
- What’s Up for December
- December IYA Discovery Guide: Orion Nebula
- Highlights of the December Sky
- Online Telescopes: Be Galileo for a night!
- Night Sky Network: Find an amateur astronomy club near you