THE ORION NEBULA (MESSIER 42)
No celestial bucket list would be complete without the inclusion of this magnificent star forming region in the constellation of Orion the Hunter. It’s one of the most viewed and imaged deep-sky objects in the night sky. I vividly remember that frosty January evening several years ago when I saw it for the first time. I had only recently acquired my first telescope, an 8 inch Dobsonian, and was eager to find this famed nebula. After aiming my scope at what I hoped was the right spot I put my eye to the scope’s eyepiece and took a peek. For a split second my initial thought was, “oh no, there are clouds beginning to move in!”, but then I realized that those clouds were not of earthly origin, they were many light years away and I had indeed found the nebula. My excitement grew as I realized that I was witnessing Nature in the act of creation, and on the grandest of scales.
HOW TO FIND THE ORION NEBULA
To locate the Orion Nebula, we must first find the constellation of Orion. Fortunately, this is an easy thing to do. If you pay much attention to the sky at all then you might have noticed that the stars of winter are exceedingly bright compared to other times of the year. It isn’t just because the air at this time of year is more transparent, it’s also because the stars really are brighter. During the winter months, the planet Earth is in a place within its orbit around the Sun such that our night sky offers us a view along the spiral arm of the galaxy in which we happen to reside. This view is filled with star-rich areas of space and many of these stellar neighbors are indeed close to us.
Looking to the southeast at around 8:00 PM or 9:00 PM on January evenings you will notice several of these bright stars. Halfway up the sky from the horizon you are sure to see three stars forming a straight, if slightly angled, line. This is an asterism (an unofficial star pattern) known as Orion’s Belt. To the lower right of the Belt you will see a bright blue white star known as Rigel, which marks the knee of the hunter. To the upper left, marking his shoulder, is the reddish-orange star Betelgeuse.
Let’s return to the belt. The three stars that make up the belt are (from left to right): Alnitak, Alnilam, and Mintaka. Let your eye go back to the first star in the Belt, Alnitak. Trailing away beneath Alnitak (towards the southwest) are three faint stars. These form another asterism known as Orion’s Sword. If your sky is sufficiently dark, you may notice that the middle “star” in the Sword appears to be rather fuzzy-looking. This is the Orion Nebula and now that you have found it take a look at it with binoculars or a telescope, you won’t be disappointed.
WHAT YOU ARE SEEING
The Orion Nebula is object number 42 in French astronomer Charles Messier’s famous 18th century catalog of deep-sky objects, hence the official designation Messier 42 (or simply M42). It is a classic example of what is known as an “emission nebula”, a huge cloud of molecular hydrogen gas and dust containing newborn stars whose radiation is making the gas cloud glow. In other words, you are looking at a stellar nursery, and this one is filled with thousands of bouncing baby stars. However, you are only going to see three, just emerging from out of the gas and dust, known as the Trapezium (the other stars are hidden from view by denser regions of gas and dust). These three stars are among the youngest that you will ever see, being only around 1 or 2 million years of age. That certainly sounds old to most people but stop to consider that our Sun is some 4.6 billion years of age, that the dinosaurs died out 68 million years ago, and the human lineage began 6 million years ago while modern humans have only been around for some 200,000 years.
Most photographs of M42 show lots of color, notably red. Why is that? Well, the UV radiation from the hot young stars of the Trapezium ionizes the hydrogen atoms. The electron of a hydrogen atom is given an energy kick from the UV radiation and it is separated from the neutral atom. We are then left with a positive hydrogen ion (the atom’s proton) and the free electron. But the electron soon pairs up with another proton to form a neutral atom once again. When an electron returns to its normal energy state it gives off a photon of light, in the case of hydrogen this photon is colored red. While you will not be able to see this red color through a small telescope, some observers say that they can detect a faint greenish hue at times. This is the color emitted by oxygen ions and the nebula does indeed contain traces of that element. Observers using larger apertures (10 inches or more) say that they can sometimes make out a rusty red or pinkish hue around the edges of the nebula. In an ironic twist, the harsh UV light from these infant stars is also clearing out the gas and dust that forms the nursery, the very stuff that gave birth to the stars in the first place. You can see the Trapezium because these stars have eroded away the cocoon of gas and dust they were born within.
Messier 42 is estimated to be some 1,300 light years away, making it one of the closest star forming regions to Earth. The part that you see is some 25 light years across but it is only a small portion of a much larger bit of nebulosity known as the Orion Molecular Cloud Complex, some 240 light years across and containing other famous objects such as Barnard’s Loop, the Flame Nebula, and the Horsehead Nebula.
Using the Hubble Space Telescope, astronomers have been able to view at optical wavelengths embryonic planetary systems forming around some 42 stars within the nebula. These infant solar systems, called “proplyds”, are in the form of spinning discs of gas and dust surrounding these young stars. Maybe one day, in the far future, there may be Earth-like planets orbiting around a few of these stars and who knows, there might even be some form of sentient life that develops on one of them. A life form capable of looking out across the gulf of space and contemplating its own existence in this amazing universe.