White dwarfs are celestial bodies with exceptional characteristics, from their formation to their disintegration. These interstellar objects, observable through sophisticated telescopes like the Hubble Space and Transiting Exoplanet Survey Satellite (TESS), have intrigued astronomers for years. Notably, an international team recently observed a white dwarf dimming its brightness within 30 minutes – a process that typically spans several days to months.
Understanding White Dwarfs
White dwarfs are stars that have consumed all the hydrogen they once relied upon as nuclear fuel. These high-density stars usually have half the size of our Sun, and their surface gravity is 100,000 times that of Earth. Stars similar to our Sun transform hydrogen in their cores into helium through nuclear fusion reactions, which generate heat and outward pressure. However, this pressure is counterbalanced by the gravitational force exerted by the star’s mass. Once the hydrogen fuel is depleted and fusion slows down, gravity causes the star to implode into a white dwarf.
The Concept of Black Dwarfs
After tens or even hundreds of billions of years, a white dwarf gradually cools down to become a black dwarf, a star that emits no energy. No known black dwarfs exist, given that even the oldest stars in the universe only range from 10 billion to 20 billion years old. It’s crucial to recognize that not all white dwarfs cool down and evolve into black dwarfs.
The Chandrasekhar Limit
White dwarfs with sufficient mass can reach a threshold known as the Chandrasekhar Limit. Once the star reaches this limit, the immense pressure at its core triggers a colossal thermonuclear supernova explosion. The Chandrasekhar Limit refers to the maximum possible mass for a stable white dwarf star, roughly 1.4 times the Sun’s mass. Anything more substantial would unavoidably collapse into a neutron star or black hole. The limit is named after Nobel laureate Subrahmanyan Chandrasekhar, who proposed it in 1931 and won the Physics Nobel Prize in 1983 for his contribution to understanding stellar structure and evolution.
The ‘Switch On and Off’ Phenomenon
The recently observed white dwarf, part of a binary system called TW Pictoris, orbits another star. These bodies are so close that the star transfers material to the white dwarf, forming an accretion disk around the latter. Typically, the accretion disk material gradually brightens as it sinks towards the white dwarf, but the transfer of matter can unexpectedly stop, the reasons for which remain unclear. Even when this occurs, the disk remains bright as it depletes the remaining material, usually taking about 1-2 months. However, the sudden 30-minute brightness drop in TW Pictoris was likely due to a process termed ‘magnetic gating,’ where a rapidly spinning magnetic field around the white dwarf creates a barrier that disrupts matter intake.
Significance of the Discovery
This new finding will shed light on the physics behind accretion – that is, how black holes and neutron stars acquire material from nearby stars. The insights gained from this discovery could significantly advance our understanding of the universe’s functioning and the fascinating life cycle of stars.
