Sunday, July 5, 2020

How scientists are using sound to study stubborn stars

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is,” said the great Douglas Adams, author of one of my favorite series, The Hitchhikers Guide to the Galaxy. He was absolutely correct in that statement that the universe, as we far as we can observe, is absolutely massive. It’s no surprise that every day new observations come to light that helps to colorize the complex nature of our reality, but sometimes things out there don’t seem to want to be studied.

Recently, NASA’s Goddard Space Flight Center unveiled a series of studies by which the data has the potential to change the way we understand the great architects of the universe, the stars you look up at every night. Utilizing a technique very similar to that which seismologists use here on Earth to map the interior of the planet, asteroseismologists study patterned shifts in the star’s luminosity, called pulsation, generated by sound waves as they pass through the interior of a star. Sound waves change in speed and shape depending on the material through which they pass, and we can use the observed variations in the star’s pulsation to help determine things such as density and composition as well as the star’s age. 

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Delta Scuti stars are a special class of star and are so named for the first of its kind identified as a variable in 1900 within the Scutum constellation. Since then, there have been thousands more discovered. They are known to be around two times the mass of our sun and can complete two full rotations within a 24-hour cycle, which is at least 12 times faster than our own resident star. This rapid rotation muddles the pulsation pattern and makes it extremely difficult to accurately study these behemoths. That is, until now. A recent conglomerate of studies may describe what could only be seen as a breakthrough in the study of these mysterious giants. NASA’s Transiting Exoplanet Survey Satellite (TESS), along with several other observational studies from astronomic installations across the globe, has produced data that indicates a pattern in some of these stars long thought to be random. 

Tim Bedding, a professor at the University of Sydney, studied the compiled observations of TESS and other installations with a team of colleagues, which led to the discovery. “Delta Scuti stars clearly pulsate in interesting ways, but the patterns of pulsation have so far defied understanding,” says Bedding. “To use a musical analogy, many stars pulsate along simple chords, but Delta Scuti stars are complex, with notes that seem to be jumbled. TESS has shown us that’s not true for all of them.” Their findings have been published in a paper within the May 14th edition of the scientific journal Nature, and are now available to view online. 

Capable of observing massive spreads of the sky at one time, TESS monitors the selected portion of space for nearly one month. Four individual cameras mounted on TESS work in unison, using all apertures at once to take a mosaic photo every thirty minutes. This allows scientists to observe shifts in the pulsation of a star as objects with gravitational mass such as planets and asteroids pass in front of, or behind them. TESS’s primary mission is, of course, to identify exoplanets; however, its thirty-minute exposure window is much too slow to capture the fluctuations from Delta Scuti stars, which can occur within just two minutes. So TESS is directed to observe pre-selected stars, some of which are Delta Scuti, and take pictures of these stars every two minutes.

Professor Bedding and his team soon began to notice that some of these Delta Scuti stars seemed to exhibit a regular and traceable pattern, despite their rapid pulsations. As soon as they had their M.O., the professor and team went straight to work reviewing data from TESS. They also studied data compiled from the Kepler mission, which studies the stars in much the same way as TESS, as well those from the W.M. Keck Observatory in Hawaii and two distinct studies of information provided by the Las Cumbres Observatory, which is a global network of stellar study. So far, at least 60 stars have been observed to have clear patterns in pulsation despite their classification as Delta Scuti. Scientists expect that number to increase exponentially.

“This really is a breakthrough. Now we have a regular series of pulsations for these stars that we can understand and compare with models,” said Simon Murphy, who is a postdoctoral researcher at the University of Sydney as well as co-author of the paper along with Bedding.

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Their findings have already proved useful. A star labeled HD 31901, part of a system of stars that was recently discovered within the Milky Way, has been the subject of some debate. This cluster was originally dated to be about one billion years old based on the observations and suspected age of a red giant star believed to be resident to the system. However, the rotational intensity of other stars in the system would indicate a much younger age, more in the realm of just 120 million years. Bedding and his team used their data to build the asteroseismic model of HD 31901 that seems to support the argument that the system is much younger than previously thought. This also indicates that the observed “well-behaved” batch of Delta Scuti stars is relatively young and that their pulsation patterns will grow increasingly complex and indecipherable as they age. 

This observation alone grants us invaluable insight, as we now have the means to determine the relative age of some Delta Scuti stars. This will allow scientists to identify “young” Delta Scuti stars in order to observe and study their descent into chaos, which may help us to understand how these once mysterious stars begin and how they may end.

Scientists are continuing to modify and improve upon TESS’s study algorithm, and the installation will begin taking photos every ten minutes instead of every thirty minutes. The hope is that this change will help identify even more Delta Scuti stars for future study. 

TESS’s creators knew at the start that their system would go beyond the call of duty. “We knew when we designed TESS that, in addition to finding many new exoplanets, the satellite would also advance the field of asteroseismology,” said George Ricker, the Principal Investigator for TESS at the Kavli Institute for Astrophysics and Space Research, a Cambridge arm of the Massachusetts Institute of Technology.

It looks as though TESS has a lot of work ahead, as the initial two-year mission has been completed and approved for extension, but I believe we can all be excited for the imminent and invaluable discoveries it will provide for years to come.

Source: ScienceDaily, NASA/Goddard Space Flight Center, Nature

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