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Crystal clusters

Dozens of stars from a formation in the orbit of the Milky Way are made of diamonds

Nasa/Esa/H. Richer (University of British Columbia)Globular cluster NGC 6397: 42 diamond stars at 6.000 degrees KelvinNasa/Esa/H. Richer (University of British Columbia)

Twelve years ago, astrophysicist Kepler de Souza Oliveira Filho and his American colleague Don Winget wrote in a scientific article that a rather old and cold star, the BPM 37093, in the Centaurus constellation, and technically classified as a white dwarf, had an almost entirely crystal core. The size of the star is comparable to that of the Earth and its mass is comparable to that of the Sun. The star had been described as a diamond star, a term used by the researchers, respectively from the Federal University of Rio Grande do Sul/UFRGS) and from the University of Texas. The core of a diamond star, which is extremely dense, is comprised of carbon and some oxygen, which is very similar to the valuable terrestrial crystal. Now, in a new study conducted by Winget and other colleagues from abroad, Kepler reports that the group has identified another 42 white dwarfs on their way to becoming diamond stars and discovered a characteristic that is unique to this kind of heavenly body. “The temperature of the diamond stars remains constant while the process of the crystallization of the nucleus is on course,” says Kepler, who published the paper is March this year in the Astrophysical Journal. “When this phase is over, the stars get cold again.”

It is interesting to note that the same occurs in another process observed every day on our Planet: the solidification of water. The temperature of the water remains at 0º C until the water is entirely frozen. Only when 100% of the water turns into ice and the change from one phase to another is entirely concluded does the temperature of the solid state drop. In the case of the recently identified diamond stars, the total crystallization is expected to take 1 billion years, a period during which the thermometer will always remain at 6,000 degrees Kelvin (K), a low temperature for this kind of celestial body. The 42 white dwarfs in the process of solidification are located in a globular cluster, the NGC 6397, a dense, spherical-shaped cluster of matter which, due to gravity, congregates 400 thousand stars in orbit around the galactic core of the Milky Way. Lying at a distance of 7,2 thousand light years from the Earth (1 light-year corresponds to 9.5 trillion kilometers)and with an estimated age of 12 billion years, the NGC 6397 is one of the 160 known globular clusters orbiting the core of our galaxy, as if they were satellites, but in fact they are part of the Milky Way. This cluster is the second closest cluster to the Earth, and is part of the Ara constellation.

Dying stars
As is frequently the case with scientific findings, the researchers were not exactly looking for what they discovered. The initial idea was to establish the age of the older white dwarfs in the cluster, the ones whose solidification process had already been concluded and whose temperatures corresponded to 4.500 K, which is still lower than the temperatures measured in stars undergoing crystallization. The plans became more ambitious when the scientists realized that there was a significant number of white dwarfs at a constant, low temperature; nonetheless, this temperature was higher than the temperature of the cluster’s older stars.

That was a hint that reminded the astrophysicists of the first star about to transform itself into a diamond, which had been identified by them in 1997, and drove them to start looking for white dwarfs in the solidification phase in the middle of the cluster. They used data from the Hubble telescope to carry out this task. This telescope had observed the stars of the NGC 6397 for 95 hours, an extremely long period of time for such a valuable piece of equipment eagerly disputed by astrophysicists. “The white dwarfs are stars in the final phase of their lives and their light is approximately 100 million times weaker than the human eye can see,” explains the astrophysicist from UFRGS.

Becoming familiar with a typical star’s evolutionary phases helps understand the researchers’ work. Approximately 98% of the stars in the Milky Way are small or medium sized and have little mass, as is the case of the Sun, and one day they will turn into white dwarfs. This is a relentless destiny. By the time they come to the end of their existence, they will have consumed all their hydrogen and will no longer produce the thermonuclear reactions that supply them with energy. They will become colder, even smaller and extremely dense. Of course there are no means to directly measure the composition of the inside of a distant dying star such as a white dwarf, or to prove with 100% certainty the existence of a core crystallizing itself inside the star. Nonetheless, the astrophysicists have to build their theories based on real parameters, on objective observational data that allows them to defend an idea scientifically.

Measuring the luminosity and radius of stars at a known distance from the Earth – that was the case of the given white dwarfs – is an indirect way of determining their mass and the temperature. This is why the Hubble lens registered the luminosity of approximately 280 white dwarfs in the NGC 6397 globular cluster and indicated that the magnitude of 42 of those stars corresponded to 26.5. For this group of stars, such level of luminosity is equivalent to a temperature of 6.000 K, compatible with the hypothesis that their core was in the process of solidification, that is, it was transforming itself into a giant diamond. “We demonstrated that the data is consistent with the ion crystallization theory in the inner part of the white dwarfs, whose temperature actually does not change when they crystallize,” states Kepler.
The words diamond star might sound sensationalistic coming from the mouth of scientists, but the astrophysicist from UFRGS explains that there is no other alternative. “It is not correct to think that the white dwarfs are the same as the diamonds we know,” he explains. “Their carbon crystal is much more compact.” The average distance between the atoms that comprise a diamond found on Earth corresponds to 3,08 angstroms. The white star’s diamond is highly dense and only 0,01 angstrom separates the elementary particles that comprise it. One angstrom is equivalent to 1 tenth of a billionth of a meter.

Scientific article
WINGET, D.E. et al. The physics of crystallization from globular cluster white dwarf stars in NGC 6397. The Astrophysical Journal. v. 693. p. 6-10. March 2009.