An admirable aspect of this image is the very fine star colors, which look perfectly "RGB" rather than "mapped".
It is extremely obvioius from Michael Joner, David Laney and Robert Gendler's image that HDE 227018 is the ionizing source of this nebula. The blue arc to the left of this star is mapped as blue because it represents the most highly ionized gas of this nebula, and the fact that it shapes itself into an arc around HDE 227018 proves that the gas and the star are intimately associated.
Fascinatingly, my software says that the two bright red stars seen to the lower right and upper right of HDE 227018 are probably as far away from us as HDE 227018. Possibly, they too might be inside the nebula. If so, they must be intrinsically very bright, and they must also be older than HDE 227018. If they, too, are products of the star formation of the Tulip Nebula, then HDE 227018 would represent the second wave of star formation in this nebula.
But I don't think so. There is no sign of a cluster in the Tulip Nebula, and that suggests that HDE 227018 itself was not born inside the nebula. (Any star formation that actually produces an O star ought to produce a number of smaller stars at the same time.)
My software says that HDE 227018 seems to move through space quite speedily. It might therefore possibly be a runaway star. If so, it has left its "birth cluster" far behind, which is why we don't see it. The blue arc to the left of it in today's APOD might therefore be a bow shock, showing us which way HDE 227018 moves.
And if HDE 227018 is a runaway star, it might just be "lighting up" and ionizing this gas cloud as it is passing through, in the same way as the even speedier and somewhat more nearby star
AE Aurigae is just "lighting up" the Flaming Star Nebula as it is passing through.
And the two bright red stars in the image may well be as far away as HDE 227018, and they may even be physically rather close to it, but they are in all probability quite unrelated to it.
Ann
The Tulip in the Swan
