Sky and Telescope - February 2018 - 25

T. A . RECTOR / UNIV ERSIT Y OF A L ASK A A NCHOR AG E, H. SCHWEIK ER / W YIN, NOAO / AUR A / NSF

t NOW YOU SEE IT, NOW YOU DON'T Hind's Nebula was first discovered in 1852, when astronomers were still debating the very nature
of this class of object. It's the subject of much discussion, since it has a
tendency to "disappear" from time to time. It's currently visible, so take
out your telescope, point it toward Taurus, and look for this nebula while
you still can.

4h 40m

4h 20m

4h 00m
Pleiades
(M45)

+20°

1554-5

¡

_

Aldebaran

a

TAURUS

Star magnitudes

has since been identified as the prototypical cometary nebula, a
class of fan-shaped reflection nebulae that are often associated
with T Tauri stars (very young, variable, low-mass stars often
in binary systems; some have circumstellar disks and are likely
progenitors of planetary systems). It is also an example of a
cocoon nebula, a term that describes the dusty envelope that surrounds some newborn stars in the process of forming planets.
Our evolving understanding of this system began in earnest
in 1861 when J. F. Julius Schmidt discovered the variability of R
Mon using the 6-inch refractor at the National Observatory of
Athens in Greece. The bright knot that forms the southern tip
of the nebula varies in brightness from about magnitude 10 to
magnitude 13. William Lassell was one of the earliest observers to propose that R Mon was composed of a luminous knot
rather than a star. Edward Emerson Barnard agreed with this
assessment, and, even with the 40-inch refractor at Yerkes, the
famed observer could not resolve R Mon into a star.
In 1916, Edwin Hubble discovered that the nebula itself
varied in shape and brightness. His pronouncement, arrived at
by studying photographs of the object taken between 1900 and
1916, is the source of its popular moniker. Hubble's discovery prompted the research of Lowell Observatory's Carl Otto
Lampland, who produced some 1,000 photos of the nebula
between 1916 and 1951. It was from these important images
that astronomers concluded that variations in the nebula did
not correspond to the star's fluctuations. Using the 42-inch
reflector at Lowell, Lampland recorded variabilities that showed
features displaced by as much as 1 arcsecond in as little as 4
days. He concluded that these displacements could not be the
result of physical movement since that would require superluminal velocities. Instead, he postulated that dark shadows
from dust clouds close to the light source were sweeping across
the nebula. The intense interest in this object within the astronomical community is evidenced by the fact that NGC 2261
was the first official target (first light) photographed by Edwin
Hubble using the 200-inch Hale telescope at Palomar in 1949.
George Herbig noted R Mon to be a tiny triangular nebula
about 15″ long that increased in brightness at its southern tip.
Using the 120-inch reflector at the Lick Observatory in 1959,
Herbig, too, was unable to resolve R Mon as a star. Some five
years later, in 1964, Richard C. Hall (Goethe Link Observatory) measured the polarization of the nebula and found
that it too was variable, with the eastern part more highly
polarized than the western part, and the overall polarization
increasing with distance from R Mon. Subsequently, in 1966,
Frank Low and Bruce J. Smith recorded large amounts of
infrared radiation relative to R Mon's visible light output and
announced that the object was a cocoon nebula, the first such
identified, about 200 light-years across.

2
3
4
5
6
7

+15°

By the 1980s, the prevailing view was that R Mon was a
star surrounded by a dust disk containing about five Earth
masses of coarse dust grains. According to this model, most
of the visible light from the star is absorbed by these grains
and re-emitted at infrared wavelengths. Reporting on Hubble
Space Telescope observations in an Astrophysical Journal article in 1997, Laird M. Close and collaborators confirmed that
dust enshrouds the star, shielding it from detection at visible
wavelengths. With the Canada-France-Hawaii Telescope on
Mauna Kea, Close's team found R Mon to be a binary star
2,500 light-years away. A B-type primary of 10 solar masses
is orbited by a secondary star that's 200 times fainter, with a
separation of 0.7″. That secondary is a very young T Tauri star
about 1.5 times the mass of the Sun.

Variable nebulae can be challenging to observe, but the effort is immensely rewarding.
The current picture of the surrounding nebula is best
described as a hollow cone, shaped by energetic jets of hot
gas from the primary star, which is surrounded by a dusty
disk inclined 20° from edge-on. Darker fi laments, following
twisted magnetic field lines, spiral around the cone, creating
the shadows that cause the nebula's brightness fluctuations. A
second, similar cone probably extends in the opposite direction, but it is obscured from our view.
In the eyepiece, Hubble's Variable Nebula looks like a
celestial badminton shuttlecock with a very dense knot on
its southern end and a wide, irregular fan extending north.
Walter Scott Houston noted that sometimes the nebula can
be seen with a 3-inch telescope, but at other times it requires
a 10-inch. Variations in brightness are apparent to the careful
visual observer who checks it on a regular basis but, unlike
most of the other known variable nebulae, it never completely
fades and is always detectable in mid-size scopes. Its popularity as an observing target is no doubt enhanced by that fact
sk yandtelescope.com * FE B RUA RY 2 018

25


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Sky and Telescope - February 2018

Table of Contents for the Digital Edition of Sky and Telescope - February 2018

Contents
Sky and Telescope - February 2018 - Cover1
Sky and Telescope - February 2018 - Cover2
Sky and Telescope - February 2018 - 1
Sky and Telescope - February 2018 - Contents
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