NGC 2440
"Very bright" (Curtis). His drawing (on the left), a composite
from a series of photographic exposures, nicely matches the Hubble
view on the right if you rotate the latter about 20 degrees to the
right. Like NGC 7027, the visual
appearance of this planetary
nebula seems to be affected by local circumnebular dust. The
distance is not well known. If at an estimated distance of 4500
light years, the maximum angular diameter of 74 seconds of arc
gives a physical diameter of 1.6 or so light years.
Again like NGC 7027, NGC 2440 is one of the most highly excited
nebulae known. It is so bright from the exceedingly hot (219,000
Kelvin, a near record) central star that the star becomes very hard
to see against the nebular background. Even though radiating with
a power of more than 500 Suns, most of the
light comes out in the ultraviolet. Indeed, Curtis states "Has no
central star," though in the Hubble view it pops out nicely. The
star has probably passed its maximum temperature and has begun the
cooling and dimming phase that will take it into the realm of the
white dwarfs. The nebula
is enriched in helium, nitrogen, and carbon, which it is donating
back to interstellar space. NGC 2440 also exhibits twin opposing
flows of gas from the center that are seen in the radio radiation
given off by carbon monoxide and that lie nicely along the long
axis as drawn by Curtis.
Left: Image and quotes by H. D. Curtis from Publications of the
Lick Observatory, Volume 13, Part III, 1918. Right: NASA and the
Hubble Heritage Team (AURA/STScI).
