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Scientists find evidence for Einstein's general relativity in the cores of dead stars

The white dwarf star discovered in the planetary nebula NGC 2440 may be the hottest one discovered yet.
The white dwarf star discovered in the planetary nebula NGC 2440 may be the hottest one discovered yet.
(Image: © NASA/ESA/K. Noll (STScI))

Scientists have bolstered Albert Einstein's theory of general relativity by exploring the strange mysteries of white dwarf stars

Astronomers have long theorized about the relationship between a white dwarf star's mass and radius but haven't been able to observe the stars’ mass-radius relationship until now, a new study shows. As white dwarf stars gain mass, they shrink in size unlike most known celestial objects.

In this new work, researchers used a novel method that incorporated data from thousands of white dwarfs to observe the strange phenomenon and provide further evidence for the theory of general relativity

Related: Space-time is swirling around a dead star, proving Einstein right again

When stars like our sun run out of fuel, they shed their outer layers and are stripped down to their Earth-sized core. This core is known as a white dwarf star, which is believed to be the final evolutionary state of a stellar object.

But these stellar remnants hold a mystery, as when white dwarfs increase in mass, they shrink in size. White dwarfs therefore will end up with a mass similar to that of the sun, but packed into a body the size of the Earth. 

White dwarfs become so small and compact that they eventually collapse into neutron stars, highly dense stellar corpses with a radius that usually does not extend beyond 18 miles (30 kilometers). 

The odd mass-radius relationship within white dwarf stars has been theorized about since the 1930s. The reason why white dwarfs increase in mass while shrinking at the same time is thought to be caused by the state of its electrons — as a white dwarf star is compressed, the number of its electrons increases.

This mechanism is a combination of quantum mechanics — a fundamental theory in physics on the motion and interaction of subatomic particles — as well as Albert Einstein's theory of general relativity, which deals with gravitational effects.

"The mass-radius relation is a spectacular combination of quantum mechanics and gravity, but it's counterintuitive for us,"" Nadia Zakamska, an associate professor at the Department of Physics and Astronomy at Johns Hopkins University, who supervised the new study, said in a statement. ""We think as an object gains mass, it should get bigger."

In this new study, the team from John Hopkins University developed a method to observe the mass-radius relationship in white dwarfs. Using data collected by the Sloan Digital Sky Survey and the Gaia space observatory, the researchers looked at 3,000 white dwarf stars.

The team of researchers measured the gravitational redshift effect, which is the effect of gravity on light, on the stars. As light moves away from an object, the wavelength of light coming from the object lengthens, causing it to appear redder. By looking at the gravitational redshift effect, they were able to determine radial velocity of the white dwarf stars that share a similar radius. 

Radial velocity is the distance from the Sun to a given star which determines whether a star is moving towards or away from the Sun. By determining the stars’ radial velocity, they were also able to determine the change in the stars’ mass.

"The theory has existed for a long time, but what's notable is that the dataset we used is of unprecedented size and unprecedented accuracy," Zakamska added. "These measurement methods, which in some cases were developed years ago, all of a sudden work so much better and these old theories can finally be probed."

The method used in the study essentially turned a theory into an observational phenomenon. Additionally, it can be used to study more stars in the future, and can help astronomers analyze the chemical composition of white dwarf stars.

"Because the star gets smaller as it gets more massive, the gravitational redshift effect also grows with mass," Zakamska said. "And this is a bit easier to comprehend—it's easier to get out of a less dense, bigger object than it is to get out of a more massive, more compact object. And that's exactly what we saw in the data."

The study was accepted for publication in The Astrophysical Journal and has been posted online to the preprint server arXiv.org.

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  • rod
    The arXiv.org link provided is very useful. From the arxiv report, "Finally we restrict our sample to d <= 500 parsecs - where the distance d = 1/pi - to assume a locally co-moving population. After applying these selection cuts, we have a sample size of 3316 stars for our photometric results in Section 4.1."

    My observation, a very good report and easy to read. Log g values (surface gravity) obtained for the WDs surveyed range 7.6 to 8.6 in the table on page 9. Using a conventional WD of 1.44 solar mass and 0.0124 solar radius, log g = 8.4092. Sirius B white dwarf companion to Sirius A, log g = 8.272. Sirius B plot is shown too in Figure 5 of the paper. Another successful test for Einstein GR :)

    Sirius B could be log g = 8.57 too, NASA ADS Abstracts.
    Reply
  • Jykll13
    Is this guy for real?

    The original scientific article refers to the radius of white dwarfs being dependent on mass, in an inverse fashion. This occurs because the higher mass white dwarfs “squish” electrons closer together, and electron degeneracy pressure is the only thing preventing a white dwarf from collapsing. Passant Rabie (the author of this summary for Space.com) seems to think the original article is about how stars shrink down to white dwarfs (a process where they LOSE mass!) and then shrink down to neutron stars (extremely unlikely, and unproven to ever happen!).

    Not only has Rabie totally missed the point of the original scientific article, but he seems to think that white dwarfs routinely collapse into neutron stars!

    Where did Space.com find this guy?
    Reply
  • rod
    The space.com report stated "White dwarfs become so small and compact that they eventually collapse into neutron stars, highly dense stellar corpses with a radius that usually does not extend beyond 18 miles (30 kilometers)."

    Interesting. We have another report out now that shows white dwarfs slowly evolve into black dwarfs, not neutron stars, https://forums.space.com/threads/scientist-calculates-the-sad-lonely-end-of-the-universe.33019/
    Confusion can occur between defining white dwarfs and neutron stars it seems. WD radii ~ 1E+9 cm, and mean densities ~ 1E+6 to 1E+9 g cm^-3. Neutron stars can have more mass than WDs, some up to 2 solar masses or so, have radii ~1E+6 cm, mean densities ~ 1E+14 g cm^-3. WD surface gravity log g ~ 8.0, Neutron star surface gravity log g ~ 14. The Sun's surface gravity is log 4.4377.

    Some enormous property differences between WDs, neutron, stars, and the Sun.

    Reference - Advanced Stellar Astrophysics, Rose, W. K., Cambridge University Press 1998, p. 325, Chapter 11 White dwarfs, novae and supernovae

    https://en.wikipedia.org/wiki/Neutron_star
    Reply
  • Lovethrust
    I have a feeling this was either poorly edited or something got lost somewhere when the article was being written.
    Obviously most white dwarfs simply cannot gain enough mass flying through the universe unless it is pushing the size limit for white dwarfdom.
    White dwarves in binary systems are another story, With a companion to suck from they can indeed gain significant mass, enough to cross the limit of max size for a white dwarf.
    Here’s the kicker though... as they gain that mass the star will collapse further and can trigger a type 1a supernova blowing it apart.
    No one knows if it’s possible for a white dwarf to avoid that fate and peacefully become a neutron star.
    Reply
  • Helio
    Jykll13 said:
    Passant Rabie (the author of this summary for Space.com) seems to think the original article is about how stars shrink down to white dwarfs (a process where they LOSE mass!)
    He mentioned on at least four occasions that it GAINS mass. How did you miss that?

    "As white dwarf stars gain mass, they shrink in size "

    "But these stellar remnants hold a mystery, as when white dwarfs increase in mass, they shrink in size. White dwarfs therefore will end up with a mass similar to that of the sun, but packed into a body the size of the Earth. "

    "The reason why white dwarfs increase in mass while shrinking at the same time ..."

    '"Because the star gets smaller as it gets more massive, the gravitational redshift effect also grows with mass," Zakamska said. '

    A continuous mass flow onto a WD will shrink it as it gains mass. Given enough mass increase, a WD will become a neutron star. Further mass would cause it to become a BH.
    Just from reading the article, it seems that the reduction in size along with the increase in mass will increase the surface gravity, thus the gravitational redshift will increase. If this redshift can be tickled out of the redshift caused by peculiar motion, then it will be the redshift associated with GR.
    Reply
  • Jykll13
    Yes, Helio, I am aware that Pasant Rabie mentioned white dwarfs get smaller as they gain mass. My point is that the original article this summary is based on was NOT talking about stars becoming white dwarfs or white dwarfs becoming neutron stars. These are examples that not only do not illustrate what the original article was talking about, they are flat wrong. Any star that becomes a white dwarf actually loses mass in the process, so this is a totally fallacious “example”. And while a theoretical white dwarf collapsing into a smaller neutron star would have to gain mass first, this process has never been observed and it is not clear if it is even possible for a white dwarf to collapse into a neutron star. The author writes as if this is a standard evolution for white dwarfs, which it definitely is not!

    The original article, by the way, was all about analysing the current white dwarf population to tease out sizes vs radii. The contention was that the more massive a white dwarf the smaller the radius. The study was attempting to find data to support this contention, based on current white dwarf masses and radii. The original article was NOT about the creation or evolution of white dwarfs, per se, and in fact did not discuss either topic...
    Reply