January 30, 2007 | David F. Coppedge

Dating a Star is Glamorous Only in Theory

Hollywood stars may be fickle, but so are great balls of fire in outer space when it comes to understanding them.  Some recent examples:

  1. Taking the pulse:  The Chandra X-ray Observatory wrote a glowing report about a “textbook supernova,” which is a nice pairing of observation and theory.  It added this caveat, though, about dating stars:

    By combining X-ray and radio observations, astronomers have evidence that G11.2-0.3 is likely the result of the explosive death of such a massive star, perhaps witnessed in 386 A.D.  Radio observations measure the remnant’s expansion rate, which, in turn, can be used to calculate how long ago the star exploded.  The radio data is consistent with association of the supernova remnant with the “guest star” reported by Chinese astronomers nearly 2,000 years ago.  Chandra’s ability to pinpoint the pulsar at nearly the very center of G11.2-0.3 also supports the idea that this debris field could have been created around the time of the Chinese observations.  Surprisingly, the age of the pulsar determined from the X-ray and radio data differs from the standard pulsar age estimate, usually determined from how fast it is spinning.  In this case, the so-called spin parameters suggest the G11.2-0.3 is 10 times older than the remnant age.  This argues strongly that young pulsar spin ages can be very misleading and should be considered with caution.

    Previously, pulsar ages determined from spindown rates were thought to be well constrained.

  2. Standard (flickering) candles:  An article on EurekAlert described another supernova remnant observed by Chandra.  The goal was to determine if Kepler’s supernova, observed by Johannes Kepler 400 years ago, was the “Type Ia” variety.  “Astronomers have studied Kepler intensively over the past three decades with radio, optical and X-ray telescopes,” the article states, “but its origin has remained a puzzle.”  In theory, the white dwarf companion of a neutron star pulls in iron-rich material that produces a Type Ia.  But the material in the surrounding nebula is rich in nitrogen, more characteristic of a Type II.  To explain the unusual mix, the astronomers speculate that this event was a rare “prompt Type Ia” explosion that took place in a young progenitor (100 million years, not several billion).
        Why is it important to tease out the oddballs among Type Ia supernovas?  The ramifications are simply astronomical.  “This information is essential to improve the reliability of the use of Type Ia stars as “standard candles” for cosmological studies of dark energy as well as to understand their role as the source of most of the iron in the universe.”  All the hubbub over the last decade about an accelerating universe of 73% dark energy depends, in large measure, on distance measurements made using Type Ia supernovas.  This case shows that not all members of the type are cooperative.
  3. Anorexic black hole:  The black hole at the center of our Milky Way had a snack recently, reported Space.com.  Maybe it was a Mars bar or Starburst.  Anyway, Ker Than wrote that for a supermassive black hole in a galaxy’s core, ours doesn’t eat much.  “Why our black hole is so dim is not entirely understood,” he said.  Quoting an astronomer, “The huge appetite is there, but it’s not being satisfied.” 
  4. Supernova blasts theory:  Supernova 1987A was caught in the act 20 years ago.  Finally, astronomers had a fairly nearby supernova to watch develop with modern telescopes.  And watch they did, with shock and awe.  A press release from UC Berkeley contained some interesting glimpses into astronomer reactions to uncooperative data.  SN1987A “provided important tests for theories of how stars die, but it also raised some new questions,” the article begins.  In theory, blue supergiants become red supergiants before exploding.  Three analogues to 1987A, however, never went through such a phase.  Also, the rings are supposed to form after the explosion, not before.  Why are similar rings found around some stars that haven’t exploded yet?
        The article confesses, “This makes a pretty solid case,” for what?  for confirmation of a theory?  No: “that we should rethink models for how the rings around SN1987A were formed.”  Nathan Smith remarked that this “would be a bit of a shock” to what? to the interior of a star?  No; “to our understanding of stellar evolution.”  In addition, the triple-ring nebula formed around SN1987A has been “difficult to understand”.  Astronomers are modeling a complex interaction between two cannibalizing stars and a supergiant to fit the data.  Other problems between theory and observation are noted in the article.
  5. Faux pas de trois:  A rare triple-quasar system was described in Science (Jan 27, p. 454).  How could this form?  Dr. Frederic Rasio [Northwestern U] believes he has the program notes.  At a recent meeting of the American Astronomical Society, he told a story of colliding galaxies, their central black holes waltzing happily till a third galaxy collided and its black hole intervened, leading to a violent reaction.  The three then split apart at up to tens of thousands of kilometers per second.  Testing this “partner-swapping dance” theory, however, might take some time.  The black holes are only in Act 1.  The peroration of the denouement, when all is understood, won’t happen for 100 million years.  One critic said,

    “The process that Dr. Rasio has modeled is very, very far in the future,” said astronomer Virginia Trimble of the University of California, Irvine.  “So in some sense, the prediction has been verified by the observation, and the observation has been explained by the theory.” 

    Writer Tom Siegried seems to have sensed a non-sequitur here.  How can something be considered verified in the present if the verification data lies in the future?  “100 million years is a long time to wait to see whether the future behavior of the triplet really matches the theoretical forecast,” he remarked.  Rasio obviously will not be concerned about defending his story then.

  6. Midlife crisis:  Two old interacting galaxies are producing stars like newlyweds, reported East Tennessee State University.  The Arp 82 pair looks middle-aged, the article states, but apparently never got reproductively active till now.  “The puzzle is: why didn’t Arp 82 form many stars earlier, like most galaxies of that mass range?  Scientifically, it is an oddball and provides a relatively nearby lab for studying the age of intermediate-mass galaxies.”  They call this a case of “arrested development,” that needed a “kick-in-the-pants to get the stars forming recently.”

This model only works, of course, if the pants are kicked at the correct angle.

It’s kind of sadistic watching astronomers try to fit their observations to their theories.  Science operates only by the constant interplay of modeling and observing.  Astronomy wouldn’t be fun if all the ideas were locked up.  It’s important to remember, though, as these examples illustrate, that one must always be prepared to shed assumptions and chuck theories in the light of new evidence.  The one needing a kick-in-the-pants is the cocky astronomer.

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Categories: Astronomy

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