Sky and Telescope - November 2017 - 29

N ASA / ESA / C. E VA NS / N. WA LBOR N / ESO

Chris Evans (Royal Observatory, Edinburgh) and colleagues found that VFTS 016 emits one of the strongest stellar
winds known for an O-type star, implying that it's roughly 90
solar masses. VFTS spectra later ruled out that VFTS 016 has a
massive companion.
How could such enormous stars reside in isolation? One
idea is if a massive star in a binary system within the cluster
explodes as a core-collapse (Type II) supernova, its surviving
sibling might gain enough of a "kick" to be ejected from the
cluster. Alternately, if a loosely bound binary encounters a
third star at close range, that chance meeting could eject either
the interloper or one of the binary's members from the cluster.
Danny Lennon (Space Telescope Science Institute) notes
that R136 is young enough that the cluster's most massive
stars have not yet exploded as supernovae. "This implies that
the star must have been ejected through dynamical interaction," he explains. Walborn adds, "This is the ﬁrst direct
observation of the process in such a region."
To further investigate runaway stars, proper-motion studies
provide the direction of escape in addition to the line-of-sight
velocity. By combining a runaway's proper motion and lineof-sight velocity, astronomers can begin to deduce its probable
origin. Determining the proper motion of stars 160,000 lightyears away might seem impossibly difﬁcult, but Hubble can
measure the plane-of-sky motion of runaway stars typically
moving at speeds of 100 km per second (200,000 mph) over a
baseline of two decades. That's how Imants Platais (Johns Hopkins University) and colleagues identiﬁed two runaway stars,
one of which (VFTS 285) seems to have been ejected from the
cluster R136. Conﬁrmation of their points of origin will come
with further Hubble observations.
Binary system interactions can give stars more than just
a kick. VFTS 102 is the fastest rotating star known: It spins
at more than 600 km per second, nearly fast enough to be
torn apart. Lurking nearby is the pulsar PSR J0537-6910 and
an associated supernova remnant. Philip Dufton (Queen's
University Belfast) and colleagues argue that VFTS102 was
initially half of a two-star system so close together that gas
from the companion spilled onto VFTS 102, causing it to spin
up. After about 10 million years, the companion exploded as
a supernova, leaving behind PSR J0537-6910 and giving VFTS
102 the kick needed to escape.
"This is a compelling story because it explains each of the
unusual features that we've seen," says Dufton. "This star
is certainly showing us unexpected sides of the short, but dramatic, lives of the heaviest stars."
Another study has found 17 other rapidly rotating O-type
stars. All but four of them lie near the edges of the clusters
NGC 2060 and NGC 2070, suggesting that these stars are in
the process of escaping.

VFTS 016

p RUNAWAY STAR VFTS 016, a blue-hot star 90 times more massive
than our Sun, has escaped the R136 cluster and is hurtling across
space at more than 100 km per second.

ing stars. As for the Tarantula itself, Walborn notes that it
should evolve into a giant "shell" H II region: The most massive stars within the core will go supernova and disappear,
leaving behind the ionized region they created. But before violently ending their fast and furious lives, these stars will clear
out the region's gas and dust with their powerful ultraviolet
radiation, creating a completely evacuated cavity. All that will
remain is an association of evolved, less-massive suns.
While this is happening, the prominent ﬁlaments now seen
in the outer parts of 30 Doradus are "star-formation fronts."
There energetic outﬂows from the core are colliding with the
surrounding dense clouds, triggering a new round of starbirth.
Astronomers in the far future will ﬁnd the region's most massive stars on its periphery rather than in the present core.
In a sense, say some researchers, we are witnessing the
birth of a globular cluster. While open star clusters typically
have tens to hundreds of stars, NGC 2070's number in the
hundreds of thousands - as do globulars. These concentrated
star-balls are ancient, typically as old as the galaxies they live
in, and many questions remain regarding their formation.
Whether NGC 2070 makes this transformation over the
next 100 million years depends on whether it contains enough
low-mass stars to keep the cluster bound once the massive
stars have gone and the Tarantula's wispy ﬁlaments have
dissipated. Walborn thinks the probable outcome is a lowmass globular. Given that eventual fate, 30 Doradus provides
astronomers with a unique observational opportunity to comprehend what the early universe must have been like and how
those ﬁrst galaxies evolved through the life cycles of their stars.

¢ Contributing Editor GREG BRYANT has been observing the
Tarantula Nebula ever since the adjacent Supernova 1987A
burst on the scene. He was Editor of Australian Sky & Telescope from 2006 to 2014.

The Spider's Future
The LMC is a busy place, and 30 Doradus lies at the northern
edge of a cold, dense, and massive interstellar cloud some
6,000 light-years long that seems destined to keep on produc-

For a video featuring discussion of the Tarantula's massive
stars by researcher Paul Crowther, go to deepskyvideos.
com/videos/other/tarantula_nebula.html.
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

http://deepskyvideos.com/videos/other/tarantula_nebula.html http://deepskyvideos.com/videos/other/tarantula_nebula.html http://www.skyandtelescope.com

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017