Star formation does not happen as easily as one might suppose from the abundance of stars in a galaxy like the Milky Way, in which around 1000 billion times the mass of the Sun (M⊙) exists in the form of stars. Stars condense out of interstellar gas within the galaxy, but the process is so difficult and complex that in one year, on average, only about 3 M⊙ of new stars form. (That could be more than 3 individual stars, since a typical star is less massive than the Sun.)
The process is difficult because most of the available gas is too warm and/or too diffuse for the process to even get started. Astrophysicists have determined that star formation occurs only in gas that is extremely cold, just a few tens of degrees above absolute zero. Because such gas is so cold, much of it exists in the form of molecules such as hydrogen (H2) and carbon monoxide (CO).
The gas needs to be that cold so gravity can overcome the kinetic energy of atoms and molecules in the gas. That allows the gas to become dense enough for the most dense pockets of it to collapse under their own weight and begin the star formation process. In warmer gas the atoms and molecules simply have too much kinetic energy to overcome gravity.
Because of these requirements, most star formation occurs in molecular clouds of gas, in particular the type known as “giant molecular clouds” (GMCs), which contain enough gas to form into stars. Such GMCs are rather large – 30 to 300 light-years in diameter, with masses of 105 to 107 M⊙. The average density of interstellar gas is only about 1 particle (atom or molecule) per cc, but in a GMC the average is 102 to 103 times as high, and can reach 104 to 106 particles per cc in the densest parts (excluding nascent stars).
So the obvious question is: What leads to the formation of a GMC in the first place? New research just published, based on detailed study of the nearby spiral galaxy M33, suggests that galactic magnetic fields may play an important role.
The formation of molecular clouds, which serve as stellar nurseries in galaxies, is poorly understood. A class of cloud formation models suggests that a large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field. Alternatively, galactic fields could be strong enough to impose their direction upon individual clouds, thereby regulating cloud accumulation and fragmentation, and affecting the rate and efficiency of star formation. Our location in the disk of the Galaxy makes an assessment of the situation difficult. Here we report observations of the magnetic field orientation of six giant molecular cloud complexes in the nearby, almost face-on, galaxy M33. The fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds.
M33 is a classic spiral galaxy in the Local Group, so it is very nearby – only about 3 million light-years away. It’s oriented almost face on to our line of sight, which makes it easy to distinguish discrete features such as GMCs. Although M33 is only about 5% as massive as out galaxy, it is very similar in general shape.
A GMC, of course, is much too cold to emit visible light. Fortunately, carbon monoxide does have a strong emission line in the very short microwave band, at the 230 GHz frequency. This is a frequency that the Submillimeter Array (SMA) radio telescope facility on Mauna Kea is optimized to detect. One good thing about making observations at microwave frequencies is that radio telescopes can use interferometry to achieve high resolution for small details.
In the present research, the hypothesis was that magnetic fields could help promote formation of GMCs. However, there was some doubt that a galaxy’s magnetic fields are strong enough to overcome the natural turbulence present in intergalactic gas. The polarization of microwave photons, if any, could show the orientation of magnetic fields within GMCs. It turned out that these magnetic fields were relatively orderly, and even aligned with the magnetic fields in the galaxy’s arms:
They found that the magnetic fields associated with the galaxy’s six most massive giant molecular clouds were orderly, and well aligned with the galaxy’s spiral arms.
If turbulence played a more important role in these clouds than the ordering influence of the galaxy’s magnetic field, the magnetic field associated with the cloud would be random and disordered.
Thus, Li and Henning’s observations are a strong indication that magnetic fields indeed play an important role when it comes to the formation of dense molecular clouds.
|Li, H., & Henning, T. (2011). The alignment of molecular cloud magnetic fields with the spiral arms in M33 Nature, 479 (7374), 499-501 DOI: 10.1038/nature10551|