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Can someone please tell me what the mass (or size) of the star at the center of the Helix Nebula is, and how much larger it is than our own star (we call the Sun or Sol). Also, what is the name of this star? I cannot find a website that addresses these questions. I believe I have heard, however, that the star is roughly 100 times larger than our sun.

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The central star is a hot DAO white dwarf. As far as I can tell, the numbers that are the most reliable are those from Napiwotzki et al. (1999). They use a direct spectral fitting technique to estimate the surface gravity and effective temperature of the central white dwarf star. These are $g = 10^{(7.00 \pm 0.22)}$ cm/s$^2$ and $T_{\rm eff} = 103600 \pm 5500$ K.

They use these numbers and theoretical white dwarf evolutionary models (there is little alternative for a star that isn't part of a binary system) to estimate a mass of $0.57 \pm 0.02 M_{\odot}$.

At a distance now known to be around $219 \pm 24$ pc (Harris et al. 2007), this makes the luminosity of the white dwarf around 100 times that of the Sun. A bit of calculation shows that for this gravity and mass, then although the star is small (about $0.04 R_{\odot}$), it is much bigger than most white dwarfs. That is because it is recently formed (planetary nebulae are recently formed objects) and hot; the white dwarf is still contracting towards its final equilibrium size of around $0.015 R_{\odot}$.

The CDS SIMBAD entry for the star is http://simbad.u-strasbg.fr/simbad/sim-id?Ident=WD+2226-210&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id

It is listed in several catalogues, so goes by various names e.g. 2MASS J22293854-2050136, WD 2226-210.

The most recent (and probably most accurate) distance measurement is from the Gaia DR2 parallax survey, giving $201 \pm 4$ pc ($655 \pm 13$ light years).

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  • $\begingroup$ Okay, so its the luminosity of the central declining star (hot white dwarf) that is around 100 times that of our sun, and its size is currently around 90% this mass of our sun. That makes more sense. And the number of this central declining star is WD-226-210. Are, then, white dwarfs, in general, given a number designation rather than a name designation, due to their evolution into a a planet or planetorid sized mass?. $\endgroup$
    – Dirk F.
    Nov 23 '18 at 20:24
  • $\begingroup$ @DirkF. It is much fainter than can be seen with the naked eye. It doesn't have a "name"; like most stars. It is 0.57 times the mass of the Sun. Its mass isn't changing, just its radius $\endgroup$
    – ProfRob
    Nov 23 '18 at 20:38
  • $\begingroup$ You say that (Harris et al 2007) puts Helix nebula at 220 pc or 717 light years away from us. Other websites state the nebula is 650 light years away. That's a big difference! I wonder which one is right. $\endgroup$
    – Dirk F.
    Nov 23 '18 at 20:41
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    $\begingroup$ @DirkF. The Harris et al. result has an error bar too. $\endgroup$
    – ProfRob
    Nov 23 '18 at 20:54
  • $\begingroup$ Surely GAIA can sort out that distance, no ? $\endgroup$
    – Fattie
    Nov 25 '18 at 14:37

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