March 7, 2007 | David F. Coppedge

Sun as a Star: How Does It Compare?

Not too many years ago, our sun was described as a common, ordinary star.  More recently, the Type G2 Dwarf Main-Sequence class, of which Ol Sol is a member, is believed to comprise only 5% of all stars.  An important paper in Astrophysical Journal is now revealing that the sun is special within its class: it is unusually quiet and steady.
    O. R. White, L. Wallace, and W. Livingston have just published results of a 32-year study of sunlight at the McMath Solar Observatory at Kitt Peak, Arizona.1  Even thought 32 years represents a third of a century, and half a percent of recorded history, it is rare to have a continuous data set for such a long period.  Now ready to retire from this long experiment, they discussed what they found.  Most of the paper describes details of particular spectral lines and how they vary with the 11-year solar cycle: doubly-ionized calcium lines, for instance, show clear oscillations.  Some spectral lines, from deeper within the sun’s photosphere (visible “surface”), show almost no variation.  They conclude that the deeper regions of the sun are immune from being heated by the cyclic rise and fall of magnetic disturbances.  This means the sun’s energy output is remarkably stable.
    How our sun compares with other stars was discussed briefly.  “Some of our results,” they said, “… will be of interest to stellar astronomers.” 

It is again of interest in a stellar context to compare our mean H + K index values for the center disk with that of solar-type stars, particularly stars that may be considered Maunder-minimum candidates.2  Our mean center disk value could be indicative of the values to expect in especially quiescent stars, or even in the Sun during prolonged episodes of relatively reduced activity, as appears to have occurred during the Maunder minimum period.

Astrophysicist M. S. Giampapa compared the integrated results with comparable stars.  The amount of variation in solar output was “about 10% less than the seasonal mean values, as measured over several seasons of observation, for even the most quiet solar-type stars,” he wrote.  “In any event, our mean center disk value in comparison with stellar observations appears to be representative of ‘immaculate photospheres,’ with little in the way of magnetic-field-related nonradiative heating.3
    In plain English, this means our sun is quiet and well behaved.  The cycles of magnetic energy that cause sunspots and flares somehow escape through the photosphere without heating the surface, resulting in a “solar constant” of energy output – constant, that is, compared with other G2 stars.  The discussion summarizes the finding: “Converted to stellar S [a normalized spectral measurement for all stars], the center disk Sun resides at a position of minimum activity, as found for the most quiescent solar-type stars.”
    This has been a career-long project for the researchers.  “In summary, what have 30 years of spectral observations told us about the physical Sun?”  The solar constant varies only 0.06% both on short and long terms.  “That behavior has basically continued over the entire observational period, with no indication of cycle modulation or secular change.  We conclude that the basal quiet photosphere is constant in temperature within our observational error,” they said.

1O. R. White, L. Wallace, W. Livingston, and M. S. Giampapa, “Sun-as-a-Star Spectrum Variations 1974-2006,” Astrophysical Journal, 657:1137-1149, 2007 March 10, 2007.
2The Maunder Minimum was an unusual period of almost no sunspots from 1645-1715.  Maunder-minimum candidate stars would be those in a similar quiescent state.  Since telescopes were first trained on the sun, sunspot number has risen and fallen in a familiar 11-year cycle except for that 70-year period.  (There is a 22-year period superimposed on the 11-year cycle in which the polarity of the magnetic field reverses.)
3Most of the sun’s heat comes from radiation.  An example of non-radiative heating would be the excitation of the sun’s photosphere from magnetic energy.  This apparently is not happening on the sun; the magnetic energy escapes between the granules.

Congratulations to this team for their perseverance and steadfastness in achieving one of the longest data-gathering experiments in astrophysics.  In science, there is no substitute for data.  Speculation is cheap.  Carl Sagan used to yarn on about how ordinary is our neighborhood in the universe: “We live on an insignificant planet of a humdrum star, lost in a galaxy tucked away in some forgotten corner of a universe,” la-de-da-de-da-zzz.  Sounds so scientific till you look at the details.  Yes, there are other solar-type stars, but one in this class that is this quiet and constant now appears to be a rarity.  Combine that with all the other factors that make our world habitable, and it really does look more and more like we live on a Privileged Planet.

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Categories: Physics, Solar System

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