Sky & Telescope - February 2023 - 14

HOW DUSTY SPIRALS FORM: GREGG DINDERMAN / S&T; WR 140: JWST / MIRI / JUDY SCHMIDT
Stripped-down Stars
Astronomers disagree about how exactly a star goes from
this stage to being a Wolf-Rayet. Some think that as the
core's hydrogen reservoir approaches empty, just before the
helium becomes the star's fuel, there is a split in the star's
evolution depending on its mass. For O stars below roughly
30 solar masses, the star proceeds through the red supergiant
phase and on to the Wolf-Rayet stage. But if the star's mass is
greater than 30 solar masses, then it may instead experience
an intermediate stage as it approaches the helium-burning
process: The star becomes somewhat unstable, ejects material,
and transforms into a luminous blue variable (LBV). Eta Carinae
and P Cygni in the Milky Way and S Doradus in the Large
Magellanic Cloud are the most famous LBV examples.
LBVs are rare: Only a dozen or so LBVs have been catalogued
in the Milky Way thus far, due in part to their
spending less than 100,000 years in this phase of evolution.
Eventually, they lose so much mass that essentially all
that remains is their helium core. At this point, the star has
become a Wolf-Rayet, typically weighing between 10 and
25 solar masses but with a few pushing 100. The star is now
in its final stage of life; within a million years, it will head
towards a violent end in a supernova.
In 2017, however, a study by Nathan Smith (University of
Arizona) drew attention to some possible problems regarding
the categorization of LBVs as a step in the evolutionary chain
between O-type stars and Wolf-Rayets. Whereas Wolf-Rayets
occupy the same spatial distribution throughout the Milky
Way's galactic disk as O-type stars do, LBVs are much more
isolated from their presumed O-type brethren. This disconHow
Dusty Spirals Form
3. Hot dust forms
in the tail.
Such interactions can influence how
much mass a star loses and direct
the star down evolutionary routes
that a single star would not follow.
nect would suggest that LBVs are instead the result of smaller
stars, which have become unstable after interacting with each
other in a binary system, Smith posited.
Others, such as Crowther, suspect the reality lies somewhere
between these extremes: Perhaps the most luminous
LBVs become Wolf-Rayet stars, while the least luminous don't.
We do think that a significant fraction of massive stars,
including Wolf-Rayets, have close binary companions. Given
that, binary interactions should be common for massive
stars. Such interactions can influence how much mass a star
loses and direct the star down evolutionary routes that a
single star would not follow. For Wolf-Rayets, their presence
in low-metallicity galaxies would be difficult to explain if
they were single stars, because they lack the heavy elements
that drive excessive mass loss and shouldn't easily expel
their hydrogen envelopes. A metal-poor star in a close binary
system, however, can lose enough gas to its companion to
become a Wolf-Rayet.
ENDLESS SPIRAL Astronomers used JWST to capture this mesmerizing
image of WR 140's spiral, processed by amateur astronomer Judy
Schmidt. The rings correspond to episodes of dust formation when the
WR star and its companion approach periastron and their winds collide.
2. The entangled
winds are swept
back behind the
companion star.
1. The WR star's wind collides with the
companion's wind and overwhelms it.
4. As the stars orbit, the head of the plume
revolves like the nozzle of a lawn sprinkler.
The WR star's wind pushes the dusty plume
away, creating an expanding spiral.
14 FEBRUARY 2023 * SKY & TELESCOPE
Sky & Telescope - February 2023

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