How Fast Do Stars Form?

An image of the Whirlpool Galaxy, also known as M51, at 24 microns, with green, 21-cm emission contours from atomic hydrogen gas superimposed. The small offset between the atomic gas and the star formation traced by the mid-infrared image suggests a short time scale for star formation. The image was taken from the Spitzer Infrared Nearby Galaxies Survey (SINGS) using NASA's Spitzer Space Telescope. The 21-cm hydrogen gas emission was measured by The HI Nearby Galaxy Survey (THINGS) team at the Very Large Array radio telescope, an NSF supported facility.

The speed of star formation from diffuse hydrogen gas has been a subject of strong controversy in the last decade. On one side, it is argued that magnetic fields act to support ionized interstellar gas against gravitational collapse. Then, almost every ion in the gas would need to find an electron to neutralize it before the gas could decouple from the magnetic field and collapse. This would suggest that it takes more than 10 million years to form a star. On the other side, it is argued that gas can slide along the magnetic field lines, allowing gas to accumulate and reach densities high enough for gravity to take hold and cause collapse, dragging the field lines with it. This suggests that a cloud will collapse into a star in only a few million years.

Young stars are deeply embedded within the collapsing clouds of gas and dust from which they form. While dust absorbs the visible light from those young stars, it emits strongly at mid-infrared wavelengths. By studying infrared images of galaxies, the star-forming regions are easily located within a galaxy's spiral arms.

In this study, the rotation of spiral arms through a galaxy is used to directly measure the time scale for star formation. The spiral arms rotate through the galaxy with a measurable speed. Therefore, a measurement of the distance between the peak of the gas emission (green contours) and the peak of the star formation emission (orange), will yield the time it took for the spiral arm to rotate through, and thus the time it took for the stars to form from the gas. The gas emission is observed using the hydrogen 21 centimeter radio emission.

Young stars are deeply embedded within the collapsing clouds of gas and dust from which they form. While dust absorbs the visible light from those young stars, it emits strongly at mid-infrared wavelengths. By studying infrared images of galaxies, the star-forming regions are easily located within a galaxy's spiral arms.

Measuring the relative positions of the star-forming emission versus the hydrogen gas emission in 14 galaxies resulted in time scales ranging from 1 to 4 million years, supporting the argument for fast collapse during star formation.