Spock’s Sun Has a Magnetic Cycle, Too

Tuesday, September 18, 2018

Life imitates art, sometimes remarkably well... A recent paper has announced the discovery of an exoplanet around the nearby star 40 Eridani, which had previously been identified as the host star of Vulcan, the fictional home world of Star Trek’s cultural icon, Mr. Spock. Remarkably, the newly discovered exoplanet shares some of the qualities attributed to the fictional Vulcan, such as higher-than-Earth gravity and a higher temperature. But the reason 40 Eridani was chosen as the presumed host star for Vulcan was due to the ways that it is similar to our own Sun.

In 1991 three astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA), Sallie Baliunas, Robert Donahue, and George Nassiopoulos, along with Star Trek creator Gene Roddenberry, wrote a letter later published in Sky & Telescope magazine laying out the case for 40 Eridani to be the star that should be considered the Vulcan sun. As their letter reveals, the reasons for selecting 40 Eridani were partly for fictional consistency, as two other Star Trek-themed works had identified real stars in the constellation Eridani as the location of Vulcan. The other candidate star was epsilon Eridani. The three CfA astronomers and Gene Roddenberry argued that 40 Eridani is the better choice for the Vulcan host star because it is older—4 billion as opposed to 1 billion years old—making it more plausible for life to evolve there as it has done on Earth. Finally, the Baliunas et al. letter also mentions that 40 Eridani has a “starspot cycle of roughly 11 years, just like the Sun.”

The three CfA astronomers worked on the Mount Wilson Observatory HK Project, an observation program aimed at revealing the long-term magnetic variability of other Sun-like stars. Our Sun has a sunspot cycle of roughly 11 years, through which magnetic sunspots periodically appear in greater and greater numbers until “solar maximum,” and then fade to hardly producing any sunspots at “solar minimum.” To observe such behavior on other stars, dedicated long-term programs must capture some proxy for stellar magnetism over decades.  In this case, the proxy is the emission in cores the Fraunhofer H & K lines, which originate in the lower chromosphere in the presence of enhanced magnetism. Recent work done at HAO has constructed a record of the Sun-as-a-star activity cycle in H & K emission that stretches back over 100 years. These solar and stellar magnetic activity cycles are presumed to be generated by a large-scale dynamo driven by the star’s rotation and convection, however the detailed physical mechanisms underlying stellar magnetic dynamo cycles are still unknown.

The Mount Wilson HK Project was terminated in 2003, however the Solar-Stellar Spectrograph Built at HAO in the 1980’s under the direction of HAO scientists Dimitri Mihalas and Richard Fisher, among others, the SSS has been in regular operation at Lowell since 1992.  Observations of the Sun over the course of the solar cycle give us the most detailed look into the remarkable patterns produced by a stellar dynamo. But observations of magnetic activity in other Sun-like stars, while more limited, give a glimpse as to how the stellar dynamo behaves in different physical conditions, such as, for example, faster rotation or more vigorous convection due to higher luminosity. In this case, 40 Eridani rotates 40% slower and is only 46% as luminous than the Sun. Taken together, diligent solar and stellar long-term observation will eventually provide scientists the clues needed to fully understand the magnetic dynamo.

Ricky Egeland is an NCAR Advanced Study Program Postoctoral Fellow working at the High Altitude Observatory.  The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Sample template imageH & K emission cycles for the Sun
H & K emission cycles for the Sun (top, source: Egeland et al. 2017, Astrophysical Journal) and the Vulcan host star 40 Eridani (bottom, source: Baliunas et al. 1995, Astrophysical Journal). Data for the Sun come from the Mount Wilson Observatory HK project (red points) and the NSO Sacremento Peak K-line program (gray), while 40 Eridani data are entirely from Mount Wilson Observatory. Both stars have a roughly 10-year cycle.