To investigate the physics that support the evolution of white dwarfs, astronomers have compared cold white dwarfs in two massive star clusters: the globular clusters M3 and M13. These two groups share many physical properties such as age and mineralization, but the star clusters that will eventually give rise to white dwarfs are different. Together, this makes M3 and M13 an ideal natural laboratory for testing how cold different groups of white dwarfs are. Credit: ESA/Hubble & NASA, G. Piotto et al.
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Can a dying star make them look younger? New guide from
The prevailing view of white dwarfs as inert, slowly cooling stars has been challenged by observations from the NASA/ESA Hubble Space Telescope. An international group of astronomers have found the first evidence that white dwarfs can slow the rate of aging by burning hydrogen on their surface.
“We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,” explains Jianxing Chen of the Alma Mater Studiorum Universita di Bologna and the Italian National Institute of Astrophysics, who led the research. “This is a big surprise, because it goes against what is common.”
White dwarfs are cold stars that slowly shed their outer layers during the last stages of their life. They are common things in the universe. About 98% of all stars in the universe will eventually end up as white dwarfs, including our sun.[1] Studying this cooling stage helps astronomers understand not only white dwarfs, but also their early stages.
To investigate the underlying physics
This image shows how the M13 looks at scale. Image Credit: ESA/Hubble, Digital Sky Survey 2. Acknowledgments: D. De Martin
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“The extraordinary quality of our Hubble observations has given us a complete view of the star clusters of the two globular clusters,” continued Chen. “This allowed us to compare how stars develop in M3 and M13.”
Using Hubble’s Wide Field Camera 3, the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two groups. They found that M3 contains standard white dwarfs that only cool the star’s core. On the other hand, the M13 has two sets of white dwarfs: standard white dwarfs and white dwarfs that manage to stick to the hydrogen’s outer shell, allowing them to burn longer and thus cool slowly.
By comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that nearly 70% of M13’s white dwarf burns hydrogen on its surface, slowing the rate of cooling.
This photo shows a bird’s-eye view of the M3. Image Credit: ESA/Hubble, Digital Sky Survey 2. Acknowledgments: D. De Martin
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This discovery may have consequences for how astronomers measure the ages of the world’s stars
“Our findings challenge the definition of white dwarfs when we look at new perspectives on how stars age,” added Francesco Ferraro of the Alma Mater Studiorum Universita di Bologna and the Italian National Institute of Astrophysics, which coordinated the research. “We are now studying other clusters similar to M13 to limit the conditions that encourage stars to maintain the thin hydrogen envelope that allows them to age slowly.”
Notes
- The sun is only 4.6 billion years old during its 10 billion year age. Once the Sun depletes the hydrogen in its core, it will swell into a red giant, swallowing up the inner planets and scorching the Earth’s surface. It will then shed its outer layer, leaving the core exposed to sunlight as a white dwarf that slowly cools down. The embers of this star would be very dense, gathering most of the Sun’s mass into a sphere roughly the size of Earth.
- M3 contains nearly half a million stars and is located in the constellation Canes Venatici. M13 – sometimes known as the Great Spherical Cluster of Hercules – contains slightly fewer stars, only a few hundred thousand. White dwarfs are often used to estimate the age of globular clusters, and therefore a significant amount of Hubble’s time has been devoted to exploring white dwarfs in densely populated ancient globular clusters. Hubble directly observed the presence of white dwarfs in globular star clusters for the first time in 2006.
- Astronomers use the word “metallic” to describe the proportion of stars that are composed of elements other than hydrogen and helium. Most of the matter in the universe is hydrogen or helium – take the Sun for example, 74.9% of its mass is hydrogen, 23.8% is helium, and the remaining 1.3% is a mixture of all other elements, which astronomers refer to. as “metal”.
further information
The Hubble Space Telescope is an international collaborative project between the European Space Agency and NASA.
The international team of astronomers in this study consisted of Jiancheng Chen (Alma Mater Studiorum Universita in Bologna and the Astrophysics and Space Observatory in Bologna), Francesco R Ferraro (Alma Mater Studiorum Universita in Bologna and the Astrophysics and Astronomy Observatory in Bologna), Mario Cadialano (Astrophysics Observatory). and Bologna Space Sciences), Maurizio Salaris (Liverpool John Moores University), Barbara Lanzoni (Alma Mater Studiorum Universita di Bologna and Observatory for Astrophysics and Space Sciences Bologna), Christina Palanca (Alma Mater Studiorum Universita di Bologna) and Leandro G. Althaus ( Universidad Nacional de La Plata and CCT – CONICET Centro Cientifico Tecnologico La Plata) and Emanuele Dalessandro (Observatory of Astrophysics and Space Sciences in Bologna).
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