A Detailed Look at the Cygnus X Star-Forming Complex

Astronomers have long attempted to understand why star-forming regions generate just a few high-mass stars while churning out many low-mass stars. Today’s article investigates a giant molecular cloud complex to understand the connection between dense, dusty cores and the stars that they form.

Scales Large and Small

map of hydrogen column density with thousands of cores marked

Thousands of cores (yellow ellipses) are visible in this hydrogen column density map of Cygnus X derived from Herschel Space Observatory data. Cores selected for later follow-up are indicated in red. [Cao et al. 2021]

When a giant molecular cloud begins to collapse, it forms dense cores. These cores fragment into even more condensed clumps that eventually become stars. But does the number of small and large cores directly predict the number of small and large stars?

Astronomers use the core mass function and the initial mass function to describe the number of cores and stars, respectively, that form in a molecular cloud as a function of mass. Thus far, observations indicate that the core mass function and the initial mass function both show the same strong preference for forming small objects over large ones, leading astronomers to suggest that the functions are closely related. To test this picture and explore the roots of this relationship, a team led by Yue Cao (Nanjing University, China) has now dramatically expanded our sample of star-forming cores by analyzing observations of the enormous Cygnus X giant molecular cloud complex.

An Extensive Search

In order to determine the core mass function precisely, Cao and collaborators needed a large sample of cores. Their search brought them to the Cygnus X star-forming region: a 650-light-year-wide molecular cloud containing three million solar masses of gas and dust and home to thousands of star-forming cores.

core mass function derived in this work versus two existing initial mass functions

Corrected core mass function for Cygnus X (red symbols) compared against the uncorrected core mass function (black symbols) and two initial mass functions (IMFs; dotted lines). [Cao et al. 2021]

Cao and collaborators used a search algorithm to identify cores in far-infrared and submillimeter observations of the Cygnus X cloud complex. In total, they selected 8,431 cores with masses ranging from just a tenth of a solar mass to more than a thousand solar masses, making this the largest sample of star-forming cores in a single cloud complex analyzed to date.

After accounting for biases in their search algorithm, the authors found that while the core mass function generally follows the same shape as the initial mass function, there are discrepancies at both mass extremes; Cygnus X has more low-mass cores and fewer high-mass cores than expected if the core mass function is directly related to the initial mass function. This contradicts findings from smaller studies of star-forming regions, which found greater agreement between the shapes of the two mass functions.

From Cloud to Core to Condensation

greyscale images of four cores with contours of the hydrogen column density and ellipses marking the diameters of the cores

Selected observations of cores. Hydrogen density contours are in yellow, core diameters are in red, and condensations are marked with cyan crosses. [Adapted from Cao et al. 2021]

The authors pushed their investigation to even smaller scales by performing follow-up observations of 48 cores with the Submillimeter Array. Using these highly detailed observations, they searched for substructures in the cores called condensations, finding 180 in total. The authors find no correlation between the core masses and the condensation masses, indicating that there isn’t a simple relation between them.

Overall, these results indicate that the initial mass function doesn’t arise directly from the core mass function, perhaps due to the chaotic influence of turbulence in star-forming regions. As always, star formation is anything but simple!


“Core Mass Function of a Single Giant Molecular Cloud Complex with ∼10,000 Cores,” Yue Cao et al 2021 ApJL 918 L4. doi:10.3847/2041-8213/ac1947