Press releases – April 2016

Astrophysicists from Goethe-University Frankfurt have found a simple formula for the maximum mass of a rotating neutron star and hence answered a question that had been open for decades. A young women did the analysis during her bachelor thesis.

Neutron stars are the most extreme and fascinating objects known to exist in our universe: Such a star has a mass that is up to twice that of the sun but a radius of only a dozen kilometres: hence it has an enormous density, thousands of billions of times that of the densest element on Earth. An important property of neutron stars, distinguishing them from normal stars, is that their mass cannot grow without bound. Indeed, if a nonrotating star increases its mass, also its density will increase. Normally this will lead to a new equilibrium and the star can live stably in this state for thousands of years. This process, however, cannot repeat indefinitely and the accreting star will reach a mass above which no physical pressure will prevent it from collapsing to a black hole. The critical mass when this happens is called the "maximum mass" and represents an upper limit to the mass that a nonrotating neutron star can be.

However, once the maximum mass is reached, the star also has an alternative to the collapse: it can rotate. A rotating star, in fact, can support a mass larger than if it was nonrotating, simply because the additional centrifugal force can help balance the gravitational force. Also in this case, however, the star cannot be arbitrarily massive because an increase in mass must be accompanied by an increase in rotation and there is a limit to how fast a star can rotate before breaking apart. Hence, for any neutron star there is an absolute maximum mass and is given by the largest mass of the fastest-spinning model.

Determining this value from first principles is difficult because it depends on the equation of state of the matter composing the star and this is still essentially unknown. Because of this, the determination of the maximum rotating mass of a neutron star has been an unsolved problem for decades. This has changed with a recent work published on Monthly Notices of the Royal Astronomical Society, where it has been found that it is indeed possible to predict the maximum mass a rapidly rotating neutron star can attain by simply considering what is maximum mass of corresponding the nonrotating configuration.

"It is quite remarkable that a system as complex as a rotating neutron star can be described by such a simple relation", declares Prof. Luciano Rezzolla, one of the authors of the publication and Chair of Theoretical Astrophysics at the Goethe University in Frankfurt. "Surprisingly, we now know that even the fastest rotation can at most increase the maximum mass of 20% at most", remarks Rezzolla.

Although a very large number of stellar models have been computed to obtain this result, what was essential in this discovery was to look at this data in proper way. More specifically, it was necessary to realise that if represented with a proper normalisation, the data behaves in a universal manner, that is, in a way that is essentially independent of the equation of state.

"This result has always been in front of our eyes, but we needed to look at it from the right perspective to actually see it", says Cosima Breu, a Master student at the University of Frankfurt, who has performed the analysis of the data during her Bachelor thesis.

The universal behaviour found for the maximum mass is part of a larger class of universal relations found recently for neutron stars. Within this context, Breu and Rezzolla have also proposed an improved way to express the moment of inertia of these rotating stars in terms of their compactness. Once observations of the moment of inertia will be possible through the measurement of binary pulsars, the new method will allow us to measure the stellar radius with a precision of 10% or less.

This simple but powerful result opens the prospects for more universal relations to be found in rotating stars. "We hope to find more equally exciting results when studying the largely unexplored grounds of differentially rotating neutron stars", concludes Rezzolla.

Publication: Cosima Breu, Luciano Rezzolla: Maximum mass, moment of inertia and compactness of relativistic, in: Monthly Notices of the Royal Astronomical Society http://mnras.oxfordjournals.org/content/early/2016/03/14/mnras.stw575;doi: 10.1093/mnras/stw575

Interview with Cosima Breu und Luciano Rezzolla on Goethe-Uni online (in German): http://tinygu.de/Neutronensterne

Images for download: www.uni-frankfurt.de/60794123

Contact: Prof. Luciano Rezzolla, Institute for Theoretical Physics, Goethe University Frankfurt, Tel,: + 49 69 798 47871, rezzolla@th.physik.uni-frankfurt.de.

FRANKFURT.The number of children in Europe and the USA with type 1 diabetes is growing by four percent each year. A group of European researchers has now joined forces under the leadership of the Goethe University, with the goal of sparing affected people from lifelong insulin therapy. They plan to develop three-dimensional cellular structures of insulin-producing cells (organoids) in the laboratory and to work with pharmaceutical industry partners to develop a process for their mass production. The European Union is providing over five million Euro over the next four years to support the project. The first clinical studies on transplantation of organoids are planned after that.

Patients with type 1 diabetes are unable to produce insulin due to a genetic defect or an autoimmune disorder. They could be cured by transplanting a functional pancreas, but there are not nearly enough donor organs available. This is why researchers had the idea of growing intact insulin-producing cells from donor organs in the laboratory to form organoids, which they would then transplant into the pancreas of diabetes patients. "The method has already been shown to work in mice", explains Dr Francesco Pampaloni, who coordinated the first project together with Prof. Ernst Stelzer at the Buchmann Institute for Molecular Life Sciences at the Goethe University.

Researchers have only recently discovered how to produce organoids. Adult stem cells, which develop into cells for wound healing or tissue regeneration in the body, are the starting point. These cells can be grown in the laboratory through cell division and then allowed to differentiate into the desired cell type. The key is now to embed them in a matrix so that they grow into three-dimensional structures. The organoids are typically spherical, hollow on the inside and have a diameter of approx. 20 micrometres – about half as thick as the diameter of a human hair – to hundreds of micrometres. "If the structure were compact, then there would be a risk of the inner cells dying off after transplantation because they wouldn't be supplied by the host organ's cellular tissue", Pampaloni explains.

The task of the Frankfurt group under Stelzer and Pampaloni is to control the growth and differentiation of the filigree organoids under a microscope. To do so, they use a light microscopy method developed by Stelzer with which the growth of biological objects can be followed cell for cell in three dimensions. The project is called LSFM4Life, because light sheet fluorescence microscopy (LSFM) plays a key role in the project.

The Frankfurt group is also responsible for developing quality assurance protocols, because of the cooperation with industrial partners in Germany, France, the Netherlands and Switzerland, the original goal of the project is the large-scale production of organoids in accordance with good manufacturing practices for pharmaceuticals. Two research groups in Cambridge specialise in isolating insulin-producing cells from donor organs and growing organoids, while a group of clinicians in Milan is developing methods for transplanting organoids.

As is the case for all organ transplants, care will have to be taken with organoids as well so that rejection responses by the recipient's immune system are avoided. However, over time the researchers plan to build cell banks from which immunologically compatible cell types can be selected for every recipient.

Video: https://youtu.be/L3xjCEBHYZg

Caption: Mouse Pancreas Organoid imaged with a Digitally Scanned Light Sheet-based Fluorescence Microscope (LSFM, mDSLM). Left: actin cytoskeleton (staining Phalloidin-Alexa488). Right: cell nuclei (staining Draq5). Illumination objective lens Carl Zeiss Epiplan Neofluar 2.5x, NA 0.05. Detection objective lens Carl Zeiss W N-Achroplan 10x, NA 0.3. Imaging and visualization by Francesco Pampaloni, Goethe University Frankfurt, BMLS. Pancreas organoids from Meritxell Huch and Christopher Hindley, Gurdon Institute, Cambridge, UK

Information: Dr. Francesco Pampaloni, Buchmann Institute for Molecular Life Sciences, Campus Riedberg, Tel,: (069) 798 42544, francesco.pampaloni@physikalischebiologie.de

http://lsfm4life.eu