An exhausted star still has some punches to deliver. Astronomers have found that a white dwarf is pummeling a companion object — either a lightweight star or a planet — with incessant blasts of heat and radiation plus a relentless gravitational pull tearing it apart.
Most stars, including the Sun, will become "white dwarfs" after they begin to run out of fuel, expand and cool into a red giant, and then lose their outer layers. This evolution leaves behind a stellar nub that slowly fades for billions of years.
A team of scientists used NASA's Chandra X-ray Observatory and ESA's XMM-Newton to investigate some unusual X-ray activity in three white dwarf stars. Typically, white dwarfs give off low-energy X-rays, which researchers saw in their sample. However, these white dwarfs also had surprisingly bright X-ray emission at higher energies.
One of the white dwarfs stood out among this group. The white dwarf KPD 0005+5106 had high-energy X-ray emission that was regularly increasing and decreasing in X-ray brightness every 4.7 hours. This recurring ebb and flow of X-rays indicates that KPD 0005+5106 has an object in orbit around it — either a very low mass star or a planet.
Material from the low mass star or planet may be slamming into the north and south poles of the white dwarf, creating a bright spot of high-energy X-ray emission. As the white dwarf and its companion orbit around each other, the hot spot facing more towards us would go in and out of view, causing the high-energy X-rays to regularly increase and decrease.
KPD 0005+5106, located in our galaxy about 1,300 light-years from Earth, is one of the hottest known white dwarf stars with a temperature of about 360,000 degrees Fahrenheit. By comparison, the surface of the Sun is about 10,000 degrees Fahrenheit.
Not only is the white dwarf much hotter than our Sun, this companion object is much closer than Earth is to our star. In fact, this companion object is about 30 times closer to the Sun than Mercury. At a distance of just 500,000 miles away from the white dwarf, this companion would be getting blasted by heat and radiation at point blank range. Scientists will likely need to do more theoretical modeling of the evolution of double stars to understand how the planet or low-mass star might end up so close to the white dwarf.
Revised: March 24, 2023