Yes, that's a nice APOD! I particularly like the faint blue "outer halo or nebula". Blue gas does not appear naturally in space, so we have to assume that the blue gas is green OIII emission that has been mapped as blue.
The ionizing star HD 148937 figured prominently in Sky & Telescope a week ago with some exciting information about this star, information that is absent from the APOD caption. HD 148937 appears to be a "collisional product", a stellar merger! It might be a later version of V838 Monocerotis!
Wikipedia wrote about V838 Monocerotis:
V838 Monocerotis (Nova Monocerotis 2002) is a spectroscopic binary star system in the constellation Monoceros about 19,000 light years (6 kpc) from the Sun. The previously unremarked star was observed in early 2002 experiencing a major outburst, and was one of the largest known stars for a short period following the outburst. Originally believed to be a typical nova eruption, it was then identified as the first of a new class of eruptive variables known as
luminous red novae.
The reason for the outburst is still uncertain, but is thought to have been a merger of two stars within a triple system.
Sky & Telescope wrote about HD 148937:
In stellar astronomy, sometimes 1 + 1 = 1.
Astronomers haven’t lost their grip on math. The latest evidence for this calculus comes from the nebula NGC 6164/6165.
At its heart lies the binary HD 148937, which comprises a pair of hot, massive O-type stars.
But not too long ago, there were likely three stars.
Under the right circumstances, these stars can actually spiral in and merge with each other in a violent splash. When they do, they throw off gas that cools and creates dust, temporarily shrouding the new star. This event is called a red nova (red because of the dust’s effect).
Massive stars like these aren’t supposed to have strong surface magnetic fields, because they don’t have the same churning in their interiors that sustains magnetic fields in stars like the Sun. Yet approximately 7% of massive stars have observable fields.
A stellar merger, on the other hand, would stir up the inside of a newly created star, creating a strong magnetic field. The new star would spin abnormally fast, too, and it would look “too young” for its mass — and both of these are properties of HD 148937’s primary...
So, to summarize: Both HD 148937 and V838 Monocerotis are believed to have started out as
3 body systems, where two of the components merged. But in the case of V 838 Monocerotis, the merger happened "just yesterday" - well, we saw it happen in 2002 - but in the case of HD 148937, the merger took place "several thousand years ago", according to Sky & Telescope. That would give the merger product time to settle down and become a hot O-type star like HD 148937, not an M (or even L!) type newborn cool supergiant like V838 Monocerotis.
(And if you object and say that the merger that created V838 also happened several thousand years ago, some 19,000 years ago to be more exact, astronomers don't date nearby novas and supernovas that way. These novas and supernovas are said to have happened on the date when we first observed them.)
So anyway: When three bodies are involved, you don't know what will happen.
And if you dare,
open this 54 MB gif, which shows, according to
Wikipedia, 20 examples of periodic solutions to the three-body problem.
I guess the orbits of the stars that created HD 148937 and V838 Monocerotis were not so regular and periodic.
Ann
Dragon's Egg Bipolar Emission Nebula
