Astronomers have for the first time probed the magnetic fields in the mysterious inner regions of stars, finding they are strongly magnetized. Using a technique called asteroseismology, the scientists were able to calculate the magnetic field strengths in the fusion-powered hearts of dozens of red giants, stars that are evolved versions of our sun.

"In the same way medical ultrasound uses sound waves to image the interior of the human body, asteroseismology uses sound waves generated by turbulence on the surface of stars to probe their inner properties," says Caltech postdoctoral researcher Jim Fuller, who co-led a new study detailing the research.

The findings, published in the October 23 issue of Science, will help astronomers better understand the life and death of stars. Magnetic fields likely determine the interior rotation rates of stars; such rates have dramatic effects on how the stars evolve.

Until now, astronomers have been able to study the magnetic fields of stars only on their surfaces, and have had to use supercomputer models to simulate the fields near the cores, where the nuclear-fusion process takes place. "We still don't know what the center of our own sun looks like," Fuller says.

Red giants have a different physical makeup from so-called main-sequence stars such as our sun--one that makes them ideal for asteroseismology (a field that was born at Caltech in 1962, when the late physicist and astronomer Robert Leighton discovered the solar oscillations using the solar telescopes at Mount Wilson).

The cores of red-giant stars are much denser than those of younger stars. As a consequence, sound waves do not reflect off the cores, as they do in stars like our sun. Instead, the sound waves are transformed into another class of waves, called gravity waves.

"It turns out the gravity waves that we see in the red giants do propagate all the way to the center of these stars," says co-lead author Matteo Cantiello, a specialist in stellar astrophysics from UC Santa Barbara's Kavli Institute for Theoretical Physics (KITP).

This conversion from sound waves to gravity waves has major consequences for the tiny shape changes, or oscillations, that red giants undergo. "Depending on their size and internal structure, stars oscillate in different patterns," Fuller says. In one form of oscillation pattern, known as the dipole mode, one hemisphere of the star becomes brighter while the other becomes dimmer. Astronomers observe these oscillations in a star by measuring how its light varies over time.

When strong magnetic fields are present in a star's core, the fields can disrupt the propagation of gravity waves, causing some of the waves to lose energy and become trapped within the core. Fuller and his coauthors have coined the term "magnetic greenhouse effect" to describe this phenomenon because it works similarly to the greenhouse effect on Earth, in which greenhouse gases in the atmosphere help trap heat from the sun. The trapping of gravity waves inside a red giant causes some of the energy of the star's oscillation to be lost, and the result is a smaller than expected dipole mode.

In 2013, NASA's Kepler space telescope, which can measure stellar brightness variations with incredibly high precision, detected dipole-mode damping in several red giants. Dennis Stello, an astronomer at the University of Sydney, brought the Kepler data to the attention of Fuller and Cantiello. Working in collaboration with KITP director Lars Bildsten and Rafael Garcia of France's Alternative Energies and Atomic Energy Commission, the scientists showed that the magnetic greenhouse effect was the most likely explanation for dipole-mode damping in the red giants. Their calculations revealed that the internal magnetic fields of the red giants were as much as 10 million times stronger than Earth's magnetic field.

"This is exciting, as internal magnetic fields play an important role for the evolution and ultimate fate of stars," says Professor of Theoretical Astrophysics Sterl Phinney, Caltech's executive officer for astronomy, who was not involved in the study.

A better understanding of the interior magnetic fields of stars could also help settle a debate about the origin of powerful magnetic fields on the surfaces of certain neutron stars and white dwarfs, two classes of stellar corpses that form when stars die.

"The magnetic fields that they find in the red-giant cores are comparable to those of the strongly magnetized white dwarfs," Phinney says. "The fact that only some of the red giants show the dipole suppression, which indicates strong core fields, may well be related to why only some stars leave behind remnants with strong magnetic fields after they die."

The asteroseismology technique the team used to probe red giants probably will not work with our sun. "However," Fuller says, "stellar oscillations are our best probe of the interiors of stars, so more surprises are likely."

The image at the top of the page shows the nebula around the bright red supergiant star Betelgeuse that was created from images taken with the VISIR infrared camera on ESO's Very Large Telescope (VLT). This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space.

The Daily Galaxy via Caltech

Image credit: ESO

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"The Suzaku satellite has opened a brand new window on the Universe and shown us that wherever you look, over vast scales, the mix of chemical elements is essentially the same" said Steven Allen, Professor of Physics at Stanford University and co-author of the study. "It's a beautifully simple result, and another step in understanding how the Universe around us came to be."

All of the chemical elements that are heavier than carbon, the oxygen we breathe, the silicon that makes up the sand on the beach, were produced inside stars through nuclear fusion and released by stellar explosions called supernovae. By measuring the chemical composition of the Universe, scientists are trying to reconstruct the history of how, when, and where each of the chemical elements so necessary for the evolution of life were produced.

Very generally speaking, there are two ways that a supernova explosion can take place, and the proportion of chemical elements that are produced depend on the supernova type. Lighter elements, like oxygen and magnesium, originate mainly from the explosions of very massive stars, more than 10 times the size of our Sun, at the end of their lifetimes. These are known as "core-collapse supernovae".

Smaller stars instead usually end their life cycles as "white dwarves", a small fraction of which can explode as a "thermonuclear" or "type Ia" supernova if they later accrete matter from a companion star, causing the white dwarf to become unstable to the pull of its own gravity. Heavier atoms like iron and nickel mostly come from this latter type of supernovae. To make up the chemical composition of our Solar System, for instance, we require a mixture of roughly one thermonuclear for every five core-collapse supernova explosions.

JAXA research fellow Aurora Simionescu wanted to find out whether the average chemical composition of the Universe was similar to that of our Solar System, or whether our local neighborhood was, after all, a special place.

Actually, perhaps counterintuitively, the answer to this question is best found not by looking at the stars themselves, but rather looking at the intergalactic space. That is because most of the normal matter in the universe, and thus also most of the metals, are presently not contained in stars, but rather in a very hot, diffuse gas that fills the space between galaxies, and is so hot that it shines in X-ray light. The brightest X-rays come from so-called clusters of galaxies, the places in the Universe where the galaxies are packed closest together.

"I've found this idea fascinating ever since the first year of my PhD: X-raying the chemical content of our Universe", says Aurora Simionescu. But back then, almost 10 years ago, it was very hard to obtain reliable measurements of the metal abundances except for the very densest, brightest parts of the intergalactic medium, due to a lack of X-ray photons and high background noise. So we could only really probe the chemical composition of roughly the central one-thousandths of the typical volume of any given galaxy cluster.

JAXA's Suzaku X-ray satellite dedicated a great amount of observing time, collecting data over many weeks, to address this problem. The first such deep observations, targeting the brightest system, the Perseus Cluster (shown at top of the page), allowed remarkably detailed measurements of the iron abundance in the intra-cluster medium on large scales. However, information about chemical elements predominantly produced by core collapse supernovae was still missing.

For such measurements, observations of a galaxy cluster with a lower average temperature were needed, in order for the emission from lighter elements to be comparatively stronger than in the Perseus Cluster. Suzaku therefore spent about two weeks looking at the Virgo Cluster, the nearest and second brightest cluster in the X-ray sky, which has such a suitably low temperature. With this new data set, Simionescu and her colleagues at JAXA and Stanford University succeeded to detect not only iron but for the first time also magnesium, silicon and sulphur all the way to the edge of this galaxy cluster. Their results are reported in a study published recently in the Astrophysical Journal.

"What we found was that the ratios between the abundances of iron, silicon, sulphur, and magnesium, are constant throughout the entire volume of the Virgo Cluster, and indeed roughly consistent with the composition of our own Sun and most of the stars in our Galaxy", explains Dr. Norbert Werner from Stanford University, a co-author of the article. Galaxy clusters cover such a large volume that the content of each such object is believed to be representative for the rest of the Universe as well. The new Suzaku finding means that the chemical elements in the cosmos are very well mixed, with a chemical composition that remains the same from scales of the solar radius (hundreds of thousands of kilometers) to the size of a cluster of galaxies (several million light years).

Although there may still be a few special places in the Universe that retain a different chemical make-up, on average, the bulk of the Universe has a very similar composition to our local neighbourhood — the same raw soup of elements that is necessary for life like ours is found, wherever you look.

The Daily Galaxy via JAXA

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