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

rod

Oct 22, 2019
1,311
421
1,560
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.
 
Aug 22, 2020
2
2
15
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?
 
Last edited:
  • Like
Reactions: Lovethrust and rod

rod

Oct 22, 2019
1,311
421
1,560
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

 
  • Like
Reactions: Lovethrust
Apr 7, 2020
153
26
110
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.
 
Jun 1, 2020
530
332
760
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. [This would require the WD from becoming a Type 1a SN, admittedly.]

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.
 
Last edited:
Aug 22, 2020
2
2
15
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...
 
Last edited:

ASK THE COMMUNITY