Hydrogen-burning white dwarfs enjoy slow aging

Date:
September 6, 2021
Source:
ESA/Hubble Information Centre
Summary:
Could dying stars hold the secret to looking younger? New evidence
suggests that white dwarfs could continue to burn hydrogen in the
final stages of their lives, causing them to appear more youthful
than they actually are. This discovery could have consequences
for how astronomers measure the ages of star clusters.



FULL STORY ==========================================================================
The prevalent 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 discovered the first evidence
that white dwarfs can slow down their rate of ageing by burning hydrogen
on their surface.


==========================================================================
"We have found the first observational evidence that white dwarfs can
still undergo stable thermonuclear activity," explained Jianxing Chen
of the Alma Mater Studiorum Universita` di Bologna and the Italian
National Institute for Astrophysics, who led this research. "This was
quite a surprise, as it is at odds with what is commonly believed."
White dwarfs are the slowly cooling stars which have cast off their
outer layers during the last stages of their lives. They are common
objects in the cosmos; roughly 98% of all the stars in the Universe will ultimately end up as white dwarfs, including our own Sun [1]. Studying
these cooling stages helps astronomers understand not only white dwarfs,
but also their earlier stages as well.

To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars:
the globular clusters M3 and M13 [2]. These two clusters share many
physical properties such as age and metallicity [3] but the populations of stars which will eventually give rise to white dwarfs are different. In particular, the overall colour of stars at an evolutionary stage known
as the Horizontal Branch are bluer in M13, indicating a population of
hotter stars. This makes M3 and M13 together a perfect natural laboratory
in which to test how different populations of white dwarfs cool.

"The superb quality of our Hubble observations provided us with a full
view of the stellar populations of the two globular clusters," continued
Chen. "This allowed us to really contrast how stars evolve 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 clusters. They found that M3 contains standard white
dwarfs which are simply cooling stellar cores. M13, on the other hand,
contains two populations of white dwarfs: standard white dwarfs and
those which have managed to hold on to an outer envelope of hydrogen,
allowing them to burn for longer and hence cool more slowly.



========================================================================== Comparing their results with computer simulations of stellar evolution
in M13, the researchers were able to show that roughly 70% of the white
dwarfs in M13 are burning hydrogen on their surfaces, slowing down the
rate at which they are cooling.

This discovery could have consequences for how astronomers measure
the ages of stars in the Milky Way. The evolution of white dwarfs
has previously been modelled as a predictable cooling process. This
relatively straightforward relationship between age and temperature has
led astronomers to use the white dwarf cooling rate as a natural clock
to determine the ages of star clusters, particularly globular and open clusters. However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years.

"Our discovery challenges the definition of white dwarfs as we consider
a new perspective on the way in which stars get old," added Francesco
Ferraro of the Alma Mater Studiorum Universita` di Bologna and the Italian National Institute for Astrophysics, who coordinated the study. "We are
now investigating other clusters similar to M13 to further constrain
the conditions which drive stars to maintain the thin hydrogen envelope
which allows them to age slowly".

Notes [1] The Sun is only 4.6 billion years through its roughly
10-billion-year lifetime. Once it exhausts hydrogen in its core,
the Sun will swell into a red giant, engulfing the inner planets and
searing the Earth's surface. It will then throw off its outer layers,
and the exposed core of the Sun will be left as a slowly cooling white
dwarf. This stellar ember will be incredibly dense, packing a large
fraction of the mass of the Sun into a roughly Earth-sized sphere.

[2] M3 contains roughly half a million stars and lies in the constellation Canes Venatici. M13 -- occasionally known as the Great Globular
Cluster in Hercules -- contains slightly fewer stars, only several
hundred thousand. White dwarfs are often used to estimate the ages of
globular clusters, and so a significant amount of Hubble time has been dedicated to exploring white dwarfs in old and densely populated globular clusters. Hubble directly observed white dwarfs in globular star clusters
for the first time in 2006.

[3] Astronomers use the word "metallicity" to describe the proportion of
a star which is composed of elements other than hydrogen and helium. The
vast majority of matter in the Universe is either hydrogen or helium --
to take the Sun as an example, 74.9% of its mass is hydrogen, 23.8% is
helium, and the remaining 1.3% is a mixture of all the other elements,
which astronomers refer to as "metals." The Hubble Space Telescope is
a project of international cooperation between ESA and NASA.

========================================================================== Story Source: Materials provided by ESA/Hubble_Information_Centre. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Chen, J., Ferraro, F.R., Cadelano, M. et al. Slowly cooling
white dwarfs
in M13 from stable hydrogen burning. Nat Astron, 2021 DOI: 10.1038/
s41550-021-01445-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210906111316.htm

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