Old at heart: A solution to red giants' age paradox

Old at heart: solution to red giants' age paradox
Toward the end of their lifetime, main sequence stars, such as the Sun, turn into red giants. The layer in which massive plasma flows transport hot material to surface then expands to the core. In this way, nuclear material can reach the surface of the star—and allows for an indirect view into the interior. Toward the end of their lifetime, main sequence stars, such as the Sun, turn into red giants. The layer in which massive plasma flows transport hot material to surface then expands to the core. In this way, nuclear material can reach the surface of the star—and allows for an indirect view into the interior. Credit: SAGE-group/MPS

Four years ago, several red giant stars were discovered to pose a paradox: even though they are built from very old stellar material, their large masses indicate a clearly younger age. Scientists from the Max Planck Institute for Solar System Research (Germany), Aarhus University (Denmark), and The Ohio State University (U.S.) have now solved the apparent contradiction. For the first time, they investigated the abundances of carbon, nitrogen, and oxygen swirled from the nuclei of these stars to their surfaces. This allows for an indirect look at processes deep within. Several of the investigated red giants must have merged with others during an advanced stage of stellar evolution, the researchers conclude. In these cases, mass is not a suitable criterion for age determination; the stars are indeed old.

Main sequence star, red giant, white dwarf—in the course of their lifespan covering millions or even billions of years, stars pass through different stages of stellar evolution—all differing greatly in appearance. Yet, stars do not reveal their ages easily, at least not at first glance. The duration of each phase differs too greatly from star to star. With deeper look, however, researchers can reconstruct the star's life story. Various methods now make it possible to reliably determine the age of a star.

But there are tricky cases: Four years ago, two groups of researchers led by the Leibniz Institute for Astrophysics and the Max Planck Institute for Astronomy discovered confusing red giant stars. The results of different age measurements diverged by a full four billion years. "The stars seemed to be old and young at the same time," Dr. Saskia Hekker from MPS and the University of Aarhus in Denmark, who was part of both discovery teams at the time and is now the first author of the new study, recalls. "This apparent paradox has intrigued me ever since," she adds. Together with her colleague Dr. Jennifer A. Johnson from The Ohio State University, she has now solved the mystery of some of these stars. Both researchers are convinced that the strange stars only feign youthfulness.

The ' building material points to an ancient age of more than 10 billion years. The stars contain comparatively little iron, an element that in the course of galactic evolution was produced only slowly. Old stars therefore contain little iron compared to other substances such as magnesium, silicon, and calcium, while young stars contain more. In order to determine these elements' ratios scientists split the light from the respective star into its individual wavelengths. In this so-called spectrum, each element found within the star leaves a characteristic fingerprint. Another method of age determination looks at the oscillations of a star. With methods of asteroseismology it is possible to then deduce the star's mass.

Since particularly prevail inside heavy stars, their fuel burns comparatively quickly. Heavy stars therefore have a much shorter life expectancy than low-mass ones. The red giants in question proved to be true heavyweights. The asteroseismic method therefore yields ages of less than 6 billion years.

The new investigation now solves this contradiction. The researchers were able to show that some of the stars look back on an extremely eventful past. "Some of the mysterious stars must have merged with others during or after their transformation into red giants," Dr. Saskia Hekker summarizes the results. "Their large mass is not an original property and therefore not suitable for age determination," she adds. "The stars are indeed old."

Key to these results were the amounts of carbon, nitrogen, and oxygen found at the surface of the stars. These elements allow for an indirect look into the stellar interior. When so-called main sequence stars, i.e. those in the same stage of development as the Sun, turn into red giants toward the end of their life, their inner workings change: carbon, nitrogen, and oxygen, which are formed in the nucleus, can be dredged up to the surface in huge plasma currents and can then be detected. Depending on how hot—and thus massive—the star in question is, the elements can be found in different ratios.

In some of their measurements, the researchers found values typical for low-mass stars. "Before they became red giants, these stars must have been comparatively light," concludes Dr. Jennifer Johnson from the Ohio State University. "Their current high mass can be explained by the fact that as red giants they have merged with other stars," she argues.

The explanation does not apply to all the stars studied. For some, the high mass determined years ago by means of asteroseismology coincides well with the presence of carbon, nitrogen, and oxygen at their surface. "These stars could have merged with others at an earlier stage of development before nuclear material was swirled to the surface," says Hekker. A final explanation is still pending.

The new study also offers a new approach to the question of how often collide and merge as a result. Red giants with such a turbulent past could now be tracked down via the detour of age determination.


Explore further

Stellar wind of old stars reveals existence of a partner

More information: Saskia Hekker et al. Origin of α-rich young stars: clues from C, N, and O, Monthly Notices of the Royal Astronomical Society (2019). DOI: 10.1093/mnras/stz1554

C. Chiappini et al. Young [α/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?, Astronomy & Astrophysics (2015). DOI: 10.1051/0004-6361/201525865

Provided by Max Planck Society
Citation: Old at heart: A solution to red giants' age paradox (2019, June 27) retrieved 22 July 2019 from https://phys.org/news/2019-06-heart-solution-red-giants-age.html
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Jun 27, 2019
So, may we call these 'Red Stragglers', kin to the long-mysterious 'Blue Stragglers', now known as merger-created ?

Jun 27, 2019
"...can be dredged up to the surface in huge  p l a s m a   currents..."

Someone (who reads these articles by means of a 'find' function) is going to be very excited.
'They laughed... but Science has vindicated me, fools.'

Jun 27, 2019
"...can be dredged up to the surface in huge  p l a s m a   currents..."

Someone (who reads these articles by means of a 'find' function) is going to be very excited.
'They laughed... but Science has vindicated me, fools.'


That would be odd since most of the matter in the universe is in the plasma state, yet the universe as a system is guided by gravity (general relativity). If it wasn't, there would be no physics laws (which general relativity guarantees).

What "fools" obsessed with 'plasma' would implicitly dispute that we see laws, or even that we (
are (life is) consequences of laws?

Jun 27, 2019
however much we are enjoying observing the pretty lights?

we need to keep in mind,
that those cosmic sparklers.
all the "plasma" glister observable across the universe?

are just temporary events.
once those burn out of fuel?
all there will be is gravity.

Jun 27, 2019
Star formation is one of the least comprehended phenomenon in astrophysics. Is there a general theory of star formation? No, there is not. There is a number of models based on computer simulations which include supersonic hydrodynamics with non-ideal MHD turbulence influenced by gravity. They include the line and continuum radiative processes of the energy transfer; a number of chemical processes with dissociation, recombination and ionization, with uncertain nomenclature of atoms and molecules, unknown magnetic fields and formation and destruction of dust particles. In addition there is macrophysics that is an environment in the molecular clouds, clumps and cores; inclusion in the multiple systems, collisions among stellar systems; jets and outflows; radiation pressure.
https://www.acade...ormation

Jun 28, 2019
I read the article and how it blames all these anomalies on mergers. Just a few stars and a whole lot of mergers.
Why are we not seeing more of this elsewhere? Even during galactic mergers (of which there are many) the individual stars do not collide. That type of merger has been specifically looked for and not found.
That these giants suffered not only one, but two or more mergers by this theory puts it solidly over the shortstop and into left field.

Jun 28, 2019
I'm sorry, I didn't see where multiple star mergers (i.e. more than one merger) were mentioned in this article. Perhaps you can quote something that says that and I can tell the editors. But I kinda doubt it.

Jun 28, 2019
I'm sorry, I didn't see where multiple star mergers (i.e. more than one merger) were mentioned in this article. Perhaps you can quote something that says that and I can tell the editors. But I kinda doubt it.


Indeed. The Hekker paper is free access, and is talking about stars that have either merged with binaries, or accreted mass from them. They ain't zooming around the galaxy bumping into other stars!

Jun 28, 2019
I'm sorry, I didn't see where multiple star mergers (i.e. more than one merger) were mentioned in this article. Perhaps you can quote something that says that and I can tell the editors. But I kinda doubt it.

"These stars could have merged with others at an earlier stage of development before nuclear material was swirled to the surface," If this is referring to a binary eaten by its partner then it could have been made much more clear.
Not to mention binaries tend to remain binaries. They seldom merge until long after they are cold and dead. Odd that red giants would only eat their partners early in the universe.

Jun 28, 2019
The mythical lifecycles of stars, brought to us by the purveyors of irrelevant maths gymnastics. If the plasma ignoramuses ever figure out that stars are driven by electricity and the type of star is determined by the current density of the Birkeland current feeding the star, it will be the end of their gravity only nonsense.

Jun 29, 2019
I'm sorry, but I didn't see where multiple star mergers were mentioned in this article.

You've failed to present any evidence to support your view. Have you anything more to say?

Jun 29, 2019
The mythical lifecycles of stars, brought to us by the purveyors of irrelevant maths gymnastics. If the plasma ignoramuses ever figure out that stars are driven by electricity and the type of star is determined by the current density of the Birkeland current feeding the star, it will be the end of their gravity only nonsense.


Hahahahaha! Another classic heap of unscientific woo-woo from the Earth-used-to-orbit-Saturn brigade!

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