Astronomical Discovery: White Dwarfs and Cosmic Collisions

In a move that enhances our understanding of the universe, astronomers have made a remarkable discovery about white dwarfs, celestial bodies that represent the final stage of stars that lack the mass to explode as giant stars. This is the first discovery of its kind, identifying a white dwarf resulting from star collision through its ultraviolet spectrum using the Hubble Space Telescope.

What are White Dwarfs?

White dwarfs are dense objects with a diameter similar to Earth’s and are the final stage of stars that do not reach the mass needed to explode as giant stars. It is known that the Sun will become a white dwarf in about 5 billion years. Theoretically, a white dwarf can reach up to 1.4 times the mass of the Sun, but those exceeding the Sun’s mass are rare and referred to as supermassive white dwarfs.

These dwarfs form either through the evolution of a single massive star or through the merger of a white dwarf with another star, such as a binary companion. Previous studies using the European Space Agency’s Gaia mission have shown that some white dwarfs are bluer than expected based on their mass and age, suggesting possible mergers in their history.

The New Discovery Using the Hubble Telescope

Astronomers used the Hubble’s Cosmic Origins Spectrograph to investigate the white dwarf named WD 0525+526, located 128 light-years from Earth and 20% heavier than the Sun. In visible light, the atmospheric spectrum of this dwarf resembled a typical white dwarf, but the ultraviolet spectrum revealed the presence of carbon in the atmosphere, indicating a more violent origin than the typical single-star scenario.

Atmospheres formed by the evolution of a single star usually consist of hydrogen and helium, while the core of a white dwarf primarily contains carbon and oxygen or oxygen and neon. However, the thick atmosphere usually prevents these elements from appearing in the white dwarf’s spectrum.

The Importance of Carbon in the Spectrum

When carbon appears in the white dwarf’s spectrum, it can indicate a more violent origin: a collision between two white dwarf stars or between a white dwarf and a subgiant star. Such a collision can strip away the hydrogen and helium atmosphere of the colliding stars, leaving a thin layer that allows carbon from the white dwarf’s core to rise to the surface where it can be detected.

WD 0525+526 is notable even among the small group of known white dwarfs resulting from star mergers. With a temperature of about 21,000 Kelvin and a mass 1.2 times that of the Sun, this dwarf is hotter and more massive than other white dwarfs in this group.

Challenges and Future Prospects

The high temperature and low carbon abundance mean that identifying this white dwarf as a merger product would have been impossible without Hubble’s sensitivity to ultraviolet light. Spectral lines of elements heavier than helium, such as carbon, become less distinct in the visible wavelengths of hotter white dwarfs, but these spectral signals remain bright in the ultraviolet spectrum, where Hubble excels in detecting them.

Conclusion

This discovery suggests a significant possibility that some white dwarfs that appear “normal” are actually the result of cosmic collisions. This opens the door to further research to understand how common carbon-rich white dwarfs are among similar white dwarfs and how many stellar mergers are hidden within the family of typical white dwarfs. This will greatly contribute to our understanding of white dwarf binaries and the pathways leading to stellar explosions.