Changes

The outer part of the nebula has an apparent size of 12x22 arcseconds, an apparent size around half that of the [[Moon]] and a physical size of some 5.87 [[light year]]s.<ref name=constellation-guide/> Its inner ring has a size of 8x19 arcseconds. Like most planetary nebulae, the nebula is expanding. Interestingly, the inner and outer regions of the Helix nebula are expanding at different rates, with the inner disk expanding at 32 km/s and the outer ring expanding at roughly 40 km/s.<ref name=constellation-guide/> The [[star]] in the centre of the nebula has an apparent [[star magnitude|magnitude]] of +13.4.<ref name=freestarcharts/>

The outer part of the nebula has an apparent size of 12x22 arcseconds, an apparent size around half that of the [[Moon]] and a physical size of some 5.87 [[light year]]s.<ref name=constellation-guide/> Its inner ring has a size of 8x19 arcseconds. Like most planetary nebulae, the nebula is expanding. Interestingly, the inner and outer regions of the Helix nebula are expanding at different rates, with the inner disk expanding at 32 km/s and the outer ring expanding at roughly 40 km/s.<ref name=constellation-guide/> The [[star]] in the centre of the nebula has an apparent [[star magnitude|magnitude]] of +13.4.<ref name=freestarcharts/>

The nebula was the first nebula discovered to contain "cometary knots" and is believed to have on the order of 20,000 such knots.<ref name=nasa>[https://apod.nasa.gov/apod/ap080413.html Curious Cometary Knots in the Helix Nebula ] from apod.nasa.gov</ref> These knots comprise of a bright globules of material with tails that extend radially out from the nebula (point away from the nebula. These knots, which have masses similar to that of [[Earth]] but have radii several times the distance between [[Pluto]] and the [[Sun]] (some 100 [[Astronomical Unit|AU]]), have been observed in other planetary nebulae such as the [[Ring nebula]], the [[Dumbbell nebula]] and the [[Eskimo nebula]].<ref name=nasa/> It has been suggested they are formed by a strong stellar wind produced by the nebula's central star.<ref name=nasa/>

The nebula was the first nebula discovered to contain "cometary knots" and is believed to have on the order of 20,000 such knots.<ref name=nasa>[https://apod.nasa.gov/apod/ap080413.html Curious Cometary Knots in the Helix Nebula ] from apod.nasa.gov</ref> These knots comprise of a bright globules of material with tails that extend radially out from the nebula (point away from the nebula. These knots, which have masses similar to that of [[Earth]] but have radii several times the distance between [[Pluto]] and the [[Sun]] (some 100 [[Astronomical unit|AU]]), have been observed in other planetary nebulae such as the [[Ring nebula]], the [[Dumbbell nebula]] and the [[Eskimo nebula]].<ref name=nasa/> It has been suggested they are formed by a strong stellar wind produced by the nebula's central star.<ref name=nasa/>

==References==

==References==