Sky and Telescope - November 2017 - 28

Inside 30 Doradus

HTTP's principal investigator, suggests it formed 25 to 30
million years ago. Compared to nearby clusters, the level of
reddening in Hodge 301 due to dust is relatively low and more
uniform - probably because stellar winds and supernovae
blasts have had time to blow through the cluster, cleansing the
region of dust. How many supernovae? Perhaps 40 to 60, says
Sabbi's team. Given the cluster's relatively old age, Cignoni
and his colleagues believe that Hodge 301, and in particular
its many supernovae, likely contributed energetically and
chemically to the subsequent development of NGC 2070.

Monsters in the Lair
Massive stars play a major role in the evolution of the galactic environment. They create H II regions by ionizing large
regions of gas and, after exploding as supernovae, enrich
the interstellar medium with heavy elements. Because they
radiate prodigiously at ultraviolet wavelengths, they're also
thought to have been a major factor in the early universe's
epoch of reionization (S&T: June 2017, p. 30).
Yet it's still unclear how big they can become. Stars with
more than 100 solar masses usually hide deep within young,
heavily congested star clusters. With lifetimes of only 2 to 3
million years, they're not around long enough for us to observe
a lot of them. Suitable clusters for study in the Milky Way tend
to be highly obscured by dust along our galaxy's midplane.
For decades astronomers thought R136 was a single star
with a mass 2,000 to 3,000 times that of the Sun - but that's
20 to 30 times greater than stellar-evolution theory allows!
By 1980, they'd resolved it into three components (denoted a,
b, and c), but R136a was still well over the stellar-mass limit.
Observations in the mid-1980s suggested that R136a was
itself a dense cluster of at least eight stars, a notion conﬁrmed

EARLY BLOOMER
º Cataloged by Nicolas Louis de Lacaille as a diffuse
nebula in 1751, 30 Doradus received its appellation 50
years later when Johann Bode included it in his
Uranographia star atlas and accompanying catalog,
in which he listed it as number 30 in Dorado.

in 1990 with Hubble's ﬁrst scientiﬁcally signiﬁcant image.
Today, with a stellar population in the many hundreds
of thousands, an estimated 3,000 stars of at least 3 solar
masses, and perhaps 20,000 solar masses concentrated in its
core, the R136 cluster is the densest concentration of stars
in the Local Group. Nine of its stars weigh in at 100 Suns or
more; at 250 solar masses, R136a1 leads the pack as the most
massive star known. Where did all these behemoths come
from? Could mergers of two or more smaller stars be responsible? Probably not, says Saida Caballero-Nieves (Florida
Institute of Technology). "From what we know about the frequency of massive mergers," she explains, "this scenario can't
account for all the really massive stars that we see in R136."

Fast Fliers
Not all massive stars in 30 Doradus reside in clusters - in
fact, several have escaped their web. The ﬁrst to be found,
VFTS 016, came to light in 2006 when a team led by Ian Howarth (University College London) observed it with the 3.9-m
Anglo-Australian Telescope. Particularly intriguing was its
isolated location, more than 200 light-years from any cluster
where such monsters tend to form and reside.

Stellar Mergers in 30 Doradus
◗ Studies of massive stars suggest that some

With a combined 57 solar masses, VFTS 352
is the most massive known overcontact binary
and has the hottest components. Stars like this,
sharing perhaps 30% of their material, are rare
to catch because this phase in their lives is very
short. If they eventually merge, the result might
be a gigantic, faster-rotating superstar.

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

Artist's impression of VFTS 352

ESO / LUÍS CA LÇA DA

40% of all O-type stars may interact with a
nearby companion, and over half of those will
lead to a "deep contact phase." An example in
30 Doradus is VFTS 352, comprising two very
hot stars with their centers just 12 million kilometers apart - their surfaces overlap. Known
as an "overcontact binary," VFTS 352 is one of
just four O-type examples of such systems.

Sky and Telescope - November 2017

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