# Information on the star at the centre of the Helix "Eye Of God" Nebula

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.

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 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).