Read Time: 4 minutes

The Age and Fate of the Orion Nebular Cloud The initial mass function may have given us the age of Orion, but it still doesn't tell the whole story...

Stars and planets are most often categorized by their mass. In astronomy, the “initial mass function” is used for this categorization. The initial mass function catalogues the number of planets and stars of a certain mass, and creates a diagram of the stars and planets ordered by their size.

This initial mass function, abbreviated IMF, gives us clues to many properties about a star system. It can determine the age, the chemistry, and the future destiny of the star system. It can help determine if a star like our sun could exist, and whether planets could exist around it that would have the conditions for life. There is much argument in the astronomy field whether the IMF alone can determine what the future of a star system will look like, or if there are more complex dynamics that determine the fate of the star and the bodies that orbit it.

The Orion Nebular Cloud is a young cluster of stars, planets, dust, and gases. Scientists have decided it is young because of it’s high density of bright, giant stars. Big, bright stars like this are young, burn hot, burn their fuel faster, and die sooner. Therefore, because these stars are so big and bright and have not died yet, the cluster must be new. Another clue that the Orion Nebula is young is that it’s stars are close together, whereas an older cluster’s stars will be further apart.

The galaxy rotates and turns and tends to separate stars that were formed together. Because the Orion Nebular Cloud still has this high density of high mass stars, it means that the Orion Nebular Cloud hasn’t been around long enough for these stars to burn out, nor long enough that the churn of the Milky Way Galaxy has caused them to rotate away from each other. This makes the Orion Nebular Cloud a good candidate for IMF analysis because it still has its high mass stars, so it more truly represents the distribution of stars and planets.

Credit: Sciworthy

This IMF analysis turned up some interesting results. Curiously, the IMF turned out to be bimodal, meaning it produced two distinct results — two types of celestial bodies. The two peaks represented bodies that were 0.25 solar masses and 0.025 solar masses. Stars are measured in “solar masses,” or how they compare to our sun. These numbers likely represent small stars and brown dwarfs. A “small star” in this example is about a quarter of the size of our sun. The brown dwarf is a star that would be 2.5% the size of our sun, or about 10x the size of Jupiter.

Because of this, researchers assumed the IMF was not sufficient to classify the Orion Nebular Cloud. If the IMF was enough, then the distribution of of star masses would have been expected to have one peak instead of two. Based upon astrophysics and our understanding of how the stars and planets form in this region, we would expect a normal distribution of masses across different items.  Therefore, because we see two favored peaks, there must be some other physics occurring that is causing these two sizes of stars to be created, that cannot be captured in the IMF.

Lastly, the IMF analysis found the age of the Orion Nebular Cloud to be about 3 million years old! The dinosaur extinction event was 65 million years ago, meaning that dinosaurs who would look up at the sky, would not have seen this Nebula! This is quite young by astronomical standards.

This has not only put an age on the Orion Nebular Cloud, but also has told us that this nebula is not going to be producing many things that look like our sun. Furthermore, it has shown scientists the limits of the IMF analysis as a technique.

Article Information

Crystal Riley
Edited By: Gina Riggio
Source: The bimodal initial mass function in the Orion Nebula Cloud
Publication Date: May 2016

Featured Image Source:
NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team. (URL: