Ducks in Disguise or How Stars Hide Their True Colors

Do star clusters form all at once or over several generations? A team of astronomers finds an answer among the spinning stars of an amateur favorite, the Wild Duck Cluster. 

The Wild Duck Cluster or M11 is one of the most enticing telescopic sights in the summer sky. I can't count how many times I've gotten lost in its stellar maze. There are some 2,900 stars here — a rich assemblage of suns that for some resembles a flock of ducks in flight.

Astronomers have long believed that open clusters like M11 form during the gravitational collapse of a large cloud of dust and gas. Once the newborns start kicking, their gusty stellar winds clear the baby-faced cluster of remaining gas. Though their masses may differ and evolutionary pathways diverge, nearly all cluster members are thought to be born in a single generation and close in age.

Most of a star's life is spent on the "main sequence" burning hydrogen in its core, which produces the energy to shine as well as to resist the crushing force of gravity. When its core hydrogen is exhausted, the star switches to other fuels and burning strategies, causing it to expand, redden, and leave the main sequence. Young clusters may begin hot and blue, but their stars redden over time. As a general rule, a star's color is a good indicator of its age.

That's why astronomers couldn't make sense of why stars of similar brightness (and presumably of similar mass) in the Wild Duck Cluster displayed different colors. If open clusters are comprised of a single generation of stars of approximately the same age, why the color spread? Either M11 birthed a second generation of suns or something's missing in our understanding of how clusters form.

Enter Beomdu Lim (Kyung Hee University, South Korea). He led an international team of astronomers who used the Multiple Mirror Telescope in Arizona to examine the spectra of stars in M11 using Hectochelle, an instrument that can take spectra of multiple stars simultaneously. To their surprise, it wasn't the stars' ages that caused the spectral variety but their rotation. The team published their findings November 5th in Nature Astronomy.

As a star rotates, one side spins toward the Earth and the other away. Light coming from the side toward us is compressed and shifted toward the blue end of the spectrum; light from the half rotating away gets stretched out and shifted toward the red. Compression and stretching causes gaps in the star's spectrum to spread across a broader range of wavelengths (colors) instead of registering as a single hue.

"The effects of rotation on stellar evolution were often neglected in the past," says co-author Yaël Nazé (University of Liège, Belgium).

There were more surprises. Spectra revealed that the stars are spinning at different rates. The faster a star spins the better it mixes hydrogen into its core and the longer it can remain on the main sequence — 15% to 62% longer — compared to its slower rotating cousins of similar mass.

Notwithstanding that many stars redden as they evolve off the main sequence, fast rotators get a head start, appearing redder than slow rotators while still basking on the main sequence. Fast rotation also deforms a star's shape into an ellipse — as its equatorial diameter expands, equatorial regions cool and redden.

So we see that a range of stellar spin rates leads to differences in star color and ultimately stellar lifetimes. The Wild Duck Cluster has managed to mimic two stellar populations when only one exists. How does the old saying go? If it looks like a duck, swims like a duck, and quacks like a duck . . . it may not always be a duck. Something like that.

Reference:

B. Lim et al. "Extended Main Sequence Turn-Off Originating from a Broad Range of Stellar Rotational Velocities." Nature Astronomy. November 5, 2018. (free version available at arxiv.org)