Dying White Dwarfs –“Seeding Carbon and Life in the Cosmos”

Caroline's Rose Open Star Cluster


The oldest planet ever detected in the universe, The PSR B1620-26 system lies around 5,600 light years away in galaxy cluster M4, directly west of red supergiant star Antares is estimated to be around 13 billion years—almost three times as old as the Solar System.orbiting a binary system of a white dwarf and a pulsar.

“In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs,” said Avi Loeb, at the Harvard-Smithsonian Center for Astrophysics (CfA) about the phenomena of the dying Sun-like stars that puff offs their outer layers, leaving behind a hot core called a white dwarf that is typically about the size of Earth, slowly cooling and fading over time, yet retaining heat long enough to warm its alien planetary worlds for billions of years.

Fate of 90% of All Stars

Using data captured in 2018 by the W. M. Keck Observatory in Hawaii, an international team of astronomers discovered and analyzed white dwarfs in open star clusters in the Milky Way, shedding light on the origin of the carbon in our galaxy. Approximately 90 percent of all stars end their lives as white dwarfs — spreading their ashes into the surrounding space through stellar winds enriched with chemical elements, including carbon, crucial to all life, in the Milky Way that are synthesized in the star’s deep interior during the last stages before its death.

“From the analysis of the observed Keck spectra, it was possible to measure the masses of the white dwarfs. Using the theory of stellar evolution, we were able to trace back to the progenitor stars and derive their masses at birth,” Ramirez-Ruiz explained, about .the relationship between the initial masses of stars and their final masses of white dwarfs known as the initial-final mass relation, a fundamental diagnostic in astrophysics that integrates information from the entire life cycles of stars, linking birth to death.

Links Birth to Death

In general, the more massive the star at birth, the more massive the white dwarf left at its death, and this trend has been supported on both observational and theoretical grounds. But analysis of the newly discovered white dwarfs in old open clusters gave a surprising result: the masses of these white dwarfs were notably larger than expected, putting a “kink” in the initial-final mass relation for stars with initial masses in a certain range.

“Our study interprets this kink in the initial-final mass relationship as the signature of the synthesis of carbon made by low-mass stars in the Milky Way,” said lead author Paola Marigo at the University of Padua.

NGC 7789, also known as Caroline’s Rose (above) , is an old open star cluster of the Milky Way, which lies about 8,000 light-years away toward the constellation Cassiopeia. It hosts a few white dwarfs of unusually high mass that were analyzed in this study. (Image credit: Guillaume Seigneuret and NASA)

In the last phases of their lives, stars twice as massive as our Sun produced new carbon atoms in their hot interiors, transported them to the surface, and finally spread them into the interstellar medium through gentle stellar winds. The team’s detailed stellar models indicate that the stripping of the carbon-rich outer mantle occurred slowly enough to allow the central cores of these stars, the future white dwarfs, to grow appreciably in mass.

Analyzing the initial-final mass relation around the kink, the researchers concluded that stars bigger than 2 solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not. In other words, 1.5 solar masses represents the minimum mass for a star to spread carbon-enriched ashes upon its death.

Alien Cluster of Young Stars Invading the Milky Way

Prolific Carbon Machines

These findings place strict constraints on how and when carbon, the element essential to life on Earth, was produced by the stars of our galaxy, eventually ending up trapped in the raw material from which the Sun and its planetary system were formed 4.6 billion years ago. “Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses,” said Marigo.

“One of most exciting aspects of this research is that it impacts the age of known white dwarfs, which are essential cosmic probes to understand the formation history of the Milky Way,” said coauthor Pier-Emmanuel Tremblay at University of Warwick. “The initial-to-final mass relation is also what sets the lower mass limit for supernovae, the gigantic explosions seen at large distances and that are really important to understand the nature of the universe.”

“Fate of Our Solar System” –Giant Planet Found Orbiting Dying White Dwarf Star

By combining the theories of cosmology and stellar evolution, the researchers concluded that bright carbon-rich stars close to their death, quite similar to the progenitors of the white dwarfs analyzed in this study, are presently contributing to a vast amount of the light emitted by very distant galaxies. This light, carrying the signature of newly produced carbon, is routinely collected by large telescopes to probe the evolution of cosmic structures. A reliable interpretation of this light depends on understanding the synthesis of carbon in stars.

The Daily Galaxy, Max Goldberg, via UC Santa Cruz

Image credit: NASA APOD