The White Dwarf System

A few days ago, an international space group found that a white dwarf star had lost its shine in half an hour, which generally consumes the time of many days to months. This fluctuation of the shining of white dwarf stars can be called a switching on and switching off event.

The team saw the event using NASA’s Transiting Exoplanet Survey Satellite (TESS). The white dwarf is a part of the binary system called TW Pictoris. In this, a star and a white dwarf orbit around each other. TW Pictoris is located in the Pictoris constellation, and the binary system is about 1400 light-years from the planet earth.

What is a White Dwarf Star?

The white dwarf is a star that is long dead but is still hot and dense. These are the leftover stellar cores of a star after the majority of its gas and dust is finished and its fuel supply is over. They are called the white dwarf stars due to the fact they were found in the same colour for the first time.

Almost 97% of the stars in Earth’s galaxy, the Milky Way, are considered white dwarfs stars by scientists. The white dwarfs consist of Carbon and Oxygen and are scorching hot when it is initially formed at a temperature in excess of 10,000 Kelvin.

However, the heat is gradually cooled down as there is no energy source to keep it alive. After that, the star cools down slowly but steadily in a billion years. However it won’t stop there and it will start to crystallize from the core. The star’s low temperature means that it won’t release any heat, turning into a cold black dwarf star.

Interestingly, it would take billions of years for a white dwarf star to become a black dwarf star. But the present age of the universe is just around 13.8 billion years, so it is assumed that no black dwarf star exists as of now. The oldest white dwarf stars are still emitting light at heat of thousands of kelvin.

Initial Sightings of White Dwarf Stars

The first-ever white dwarf was found because it was a fellow star to Sirius, a bright and shining star in Canis Major’s constellation. Astronomer Friedrich Bessel in 1844 observed that Sirius had a little back and forth motion, to give an image that it was orbiting an unseen object.

The telescope maker, Alvan Clark, spotted this mysterious object in 1863. This fellow star was later believed to be a white dwarf. The duo of stars is called Sirius A and B, with B being known as the white dwarf.

The Atmosphere of White Dwarf Star

The surface gravity of a white dwarf star is 100,000 times that of Earth. Thus, it makes the atmosphere of a white dwarf quite peculiar in its way. While the heavier atoms present in its atmosphere sink, the lighter ones stay at the surface.

 A few white dwarfs consist of the lightest of gases which are pure hydrogen or helium atmospheres. Hence, gravity pulls the atmosphere close around it in a weak layer.

Astronomers and scientists believe that there is a crust around 50 km thick underneath the atmosphere of numerous white dwarfs.

What is the switching on and off event of a white dwarf star?

The assistant professor for extragalactic astronomy at Durham University in the United Kingdom, Dr Simone Scaringi, says to Indian Express about the recent phenomena –“ What generally happens in these types of systems is that the donor star in orbit around the white dwarf keeps feeding the accretion disk. As the accretion disk material slowly sinks closer towards the white dwarf, it generally becomes brighter…In some systems, it is known that the donor stars stop feeding the disk (for yet unclear reasons).

When this happens the disk is still bright as it “drains” material that was previously still there. It then takes the disk about 1-2 months to drain most of the material, something we saw happen in different accreting white dwarfs.

We think what may be happening in TW Pictoris is that instead of the disk being drained out so fast, we are seeing some sort of reconfiguration of the white dwarf magnetic field.

We are running a programme with TESS to observe hundreds of accreting white dwarfs. This will hopefully reveal just how standard (or not!) the features observed in TW Pictoris really are?”

Considering this newly found event regarding the white dwarf stars, we can be sure that a lot of further unexpected information about it would be revealed.

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This post was last updated on November 1st, 2021 at 10:44 am

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