The Orion Nebula


In many ways the Orion nebula is a visual essay that elegantly and beautifully summarizes all of the concepts that you have encountered in this chapter. To help provide a broader context we will first consider the constellation Orion and the Orion Star-forming region.

Orion - Hunter in the Winter Sky

Orion is possibly the most easily recognized constellation. It hangs majestically in the southern Canadian sky during the winter months and its setting in the west in March also heralds the approach of spring. Beyond its familiarity, however, Orion is home to a huge star forming region. In the sword of Orion, visible to the un-aided eye, is a fuzzy patch - the Orion nebula. Less obvious to the unaided eye and stretching across the entire constellation are numerous clues that we are looking into a vast cloud of gas and dust that has in the recent past and continues to spawn new stars.

Look at this region of the sky with the applet shown below in Figure 9.28. You can select visible light, infrared light or the light produced by hot, hydrogen gas ("Hydrogen-alpha view").

Figure 9.28 The Orion region of the sky as seen in different wavelengths.

Example 9.11 Discuss how the three different wavelength views of the Orion region provide us with clues that this is an active star forming region

Solution: Consider each view:

Visible: One of the most striking things is the number of bright, young stars that make up Orion. Click on the thumbnail shown on the right to see an enlarged image with the spectral type and distance for the 7 brightest stars that make up Orion. O and B stars are very short lived and Betelgeuse is a highly evolved star indicating it is near the end of its life. All of this suggests that intense star formation has occurred in this region.

Infrared: The infrared image shows the presence of warm dust - suggesting that the material needed for star formation exists in this region

Hydrogen - Alpha: Perhaps more revealing is the image produced by hot, glowing hydrogen gas. This shows a shell or bubble of hydrogen radiating outward from the sword region of Orion.

There is ample evidence to suggest that the star formation is actively occurring in the Orion region. As well as the evidence cited above there is also a very large concentration of T-Tauri stars in this region. These are very young stars making the transition from prot-star to star (see section 9.1). Probably about 12 million years ago bright stars in the west shoulder of Orion "turned on". This created a traveling shock that compressed gases in the central part of Orion and triggered ore star formation. The belt stars (Alnilam, Alnitak and Mintaka) probably formed less than 8 million years ago and they in turn helped spawn the stars that we see in the center of the Orion nebula.

Example 9.12 How can you justify the comment that the belt stars in Orion formed approximately 8 million years ago?

Solution: In Example 9.11 you learned that the belt stars are all either late O-type or very early B-type stars. This suggests that they must have masses on the order of 18 Mo. Use the applet "AgeCalc" provided in section 9.4 you can see that the main-sequence lifetime of such a star is only about 8 million years. These stars have already evolved off the main sequence which suggests that they cannot be much older than 8 million years.


The Orion Nebula

Through even a modest amateur telescope the "Great Nebula in Orion" is an awesome sight. Against a dark sky it is a glowing green cloven by dark, sooty lanes of dust. In larger telescopes there are distinct rose and bluish tinges visible. Situated in the middle of Orion's sword, the nebula is about 1500 light years away and is a "stellar nursery" where new stars are forming as you read this. All of the ideas that have been discussed in the previous sections play out here. Please explore the nebula for yourself by using the applet provided below. You can "tour" the nebula through a spectacular Hubble Space Telescope image.

Figure 9.29 Interactive image of the Orion Nebula. You can zoom in either by using the slider on the bottom of the image or by clicking the mouse. Pan the image by dragging the mouse or using the arrow keys. Image from Hubble Space Telescope.



  1. Explore Figure 9.29 and try to find examples of the following:
    1. dust lanes
    2. regions of ionized gas
    3. bright new stars
    4. proplyds
  2. Locate the bright grouping of stars in the center of the nebula. This is the "Trapezium" and consists of late O-type, main-sequence stars. How old is this region? Provide an age estimation and explain how you arrived at this.
  3. Provide comments or brief explanations about the following images that we taken from the larger image of the Orion nebula:

    a. What are the tiny "red stars" in this image? Are they cool, low mass stars or is there a more probable explanation?
    b. What's going on here? Speculate on what the arc shaped structure could be.
    c. What's the dark rift in this image? What are the bright reddish areas?


To understand how the Orion nebula illustrates key ideas of star formation and evolution

Chp 11.4


Many different peoples around the world have their own version of the constellation Orion. The First Nations Ojibwe saw Orion as the "Winter Maker" and the belt stars as the "Three Canoes".