Hotter White Dwarfs Get Puffier

When our Sun dies, it will turn into a white dwarf. They are a common aspect of stellar evolution and a team of researchers have now turned their attention onto them. They have just completed a survey of 26,000 white dwarfs and confirmed a long-predicted theory that the hotter the star, the puffier it is! This new study will help us to understand white dwarfs and the processes that drive them. 

All stars age. Our Sun is a giant ball of electrically charged gas and, during the majority of its life will be fusing hydrogen to helium in its core. During this process, the fusion will generate an outward pushing force known as thermonuclear pressure which will for the most part, balance the inward pull of gravity. Eventually, the thermonuclear force will overcome the force of gravity and the star will shed its outer layers, leaving behind a dense, hot core. The core is known as a white dwarf and it is this which, despite its small size and incredibly high density, has captivated astronomers. 

One of the more fascinating aspects of white dwarf stars is their relationship between temperature and density. Theory suggests that the hotter a white dwarf star becomes, the less dense and more puffy its outer layers become. The lower density is thought to be driven by an increase in energy pushing outward which comes from an increased core temperature. Typically the core of a white dwarf can reach between 5,000 to 10,000 Kelvin. 

The team of astronomers led by Nicole Crumpler from the John Hopkins University published the results of their findings in the Astrophysical Journal. They hope that their work will take us a step closer to being able to exploit white dwarfs as natural stellar laboratories to unravel the mysteries of dark matter! The secret, the team believe, is in the puffy nature of white dwarfs. “If you want to look for dark matter, quantum gravity, or other exotic things, you better understand normal physics,” said Crumpler, “otherwise, something that seems novel might be just a new manifestation of an effect that we already know.”

At its core is the fact that these stellar corpses are composed of material far heavier than normal matter.  A teaspoon of their material weighs around a ton, clearly far more than ordinary matter. With all that mass packed so tightly into the small stellar corpse, the gravitational pull is far higher than here on Earth. 

The study focussed on measuring how these high material densities influence light waves travelling away from the star. The waves will lose energy, stretching the radiation and ‘red-shifting’ it so telescopes can measure it. By averaging the measurements of white dwarf stars and their motions relative to Earth, the team were able to isolate the redshift from the affect of gravity to calculate how high the temperatures are and therefore influence the gas density in outer layers. 

To conclude their study, the team used data from the Solan Digital Sky Survey and the ESA Gaia mission. Together these observation programs have recorded positions of millions of stellar objects. By studying tens of thousands of white dwarfs the team hope that probing the nature of the matter will help to understand more about its nature, about the nature of dark matter and the nature of the structure of the white dwarf stars that pervade our Galaxy. 

Source : Survey of 26,000 dead stars confirms key details of extreme stellar behavior