VLT image of the surroundings of VY Canis Majoris seen with SPHERE
The red hypergiant star VY Canis Majoris
Wide-field view of the sky around VY Canis Majoris
Video
Giant star caught in the act of slimming down
A team of astronomers using ESO’s Very
Large Telescope (VLT) has captured the most detailed images ever of the
hypergiant star VY Canis Majoris. These observations show how the
unexpectedly large size of the particles of dust surrounding the star
enable it to lose an enormous amount of mass as it begins to die. This
process, understood now for the first time, is necessary to prepare such
gigantic stars to meet explosive demises as supernovae.
VY Canis Majoris is a stellar goliath, a red hypergiant,
one of the largest known stars in the Milky Way. It is 30–40 times the
mass of the Sun and 300 000 times more luminous. In its current state,
the star would encompass the orbit of Jupiter, having expanded
tremendously as it enters the final stages of its life.
The new observations of the star used the SPHERE instrument on the VLT. The adaptive optics system of this instrument corrects images to a higher degree than earlier adaptive optics systems. This allows features very close to bright sources of light to be seen in great detail [1]. SPHERE clearly revealed how the brilliant light of VY Canis Majoris was lighting up clouds of material surrounding it.
And by using the ZIMPOL mode of SPHERE, the team could not only peer
deeper into the heart of this cloud of gas and dust around the star, but
they could also see how the starlight was scattered and polarised by
the surrounding material. These measurements were key to discovering the
elusive properties of the dust.
Careful analysis of the polarisation results revealed these grains of dust to be comparatively large particles, 0.5 micrometres
across, which may seem small, but grains of this size are about 50
times larger than the dust normally found in interstellar space.
Throughout their expansion, massive stars shed large amounts of
material — every year, VY Canis Majoris sees 30 times the mass of the
Earth expelled from its surface in the form of dust and gas. This cloud
of material is pushed outwards before the star explodes, at which point
some of the dust is destroyed, and the rest cast out into interstellar
space. This material is then used, along with the heavier elements
created during the supernova explosion, by the next generation of stars,
which may make use of the material for planets.
Until now, it had remained mysterious how the material in these giant stars’ upper atmospheres is pushed away into space before the host explodes. The most likely driver has always seemed to be radiation pressure, the force that starlight exerts. As this pressure is very weak, the process relies on large grains of dust, to ensure a broad enough surface area to have an appreciable effect [2].
“Massive stars live short lives,” says lead author of the paper, Peter Scicluna, of the Academia Sinica Institute for Astronomy and Astrophysics, Taiwan. “When
they near their final days, they lose a lot of mass. In the past, we
could only theorise about how this happened. But now, with the new
SPHERE data, we have found large grains of dust around this hypergiant.
These are big enough to be pushed away by the star’s intense radiation
pressure, which explains the star’s rapid mass loss.”
The large grains of dust observed so close to the star mean that the
cloud can effectively scatter the star’s visible light and be pushed by
the radiation pressure from the star. The size of the dust grains also
means much of it is likely to survive the radiation produced by VY Canis
Majoris’ inevitable dramatic demise as a supernova [3].
This dust then contributes to the surrounding interstellar medium,
feeding future generations of stars and encouraging them to form
planets.
Notes
[1] SPHERE/ZIMPOL uses extreme adaptive optics to create diffraction-limited
images, which come a lot closer than previous adaptive optics
instruments to achieving the theoretical limit of the telescope if there
were no atmosphere. Extreme adaptive optics also allows much fainter
objects to be seen very close to a bright star.
The images in the new study are also taken in visible light — shorter
wavelengths than the near-infrared regime, where most earlier adaptive
optics imaging was performed. These two factors result in significantly
sharper images than earlier VLT images. Even higher spatial resolution
has been achieved with the VLTI, but the interferometer does not create images directly.
[2] The dust particles must be large
enough to ensure the starlight can push it, but not so large that it
simply sinks. Too small and the starlight would effectively pass through
the dust; too large and the dust would be too heavy to push. The dust
the team observed about VY Canis Majoris was precisely the right size to
be most effectively propelled outwards by the starlight.
[3] The explosion will be soon by
astronomical standards, but there is no cause for alarm, as this
dramatic event is not likely for hundreds of thousands of years. It will
be spectacular as seen from Earth — perhaps as bright as the Moon — but
not a hazard to life here.
More Information
This research was presented in a paper entitled “Large dust grains in
the wind of VY Canis Majoris”, by P. Scicluna et al., to appear in the
journal Astronomy & Astrophysics.
The team is composed of P. Scicluna (Academia Sinica Institute for
Astronomy and Astrophysics, Taiwan), R. Siebenmorgen (ESO, Garching,
Germany), J. Blommaert (Vrije Universiteit, Brussels, Belgium), M.
Kasper (ESO, Garching, Germany), N.V. Voshchinnikov (St. Petersburg
University, St. Petersburg, Russia), R. Wesson (ESO, Santiago, Chile)
and S. Wolf (Kiel University, Kiel, Germany).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 16 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the
United Kingdom, along with the host state of Chile. ESO carries out an
ambitious programme focused on the design, construction and operation of
powerful ground-based observing facilities enabling astronomers to make
important scientific discoveries. ESO also plays a leading role in
promoting and organising cooperation in astronomical research. ESO
operates three unique world-class observing sites in Chile: La Silla,
Paranal and Chajnantor. At Paranal, ESO operates the Very Large
Telescope, the world’s most advanced visible-light astronomical
observatory and two survey telescopes. VISTA works in the infrared and
is the world’s largest survey telescope and the VLT Survey Telescope is
the largest telescope designed to exclusively survey the skies in
visible light. ESO is a major partner in ALMA, the largest astronomical
project in existence. And on Cerro Armazones, close to Paranal, ESO is
building the 39-metre European Extremely Large Telescope, the E-ELT,
which will become “the world’s biggest eye on the sky”.
Links
Contacts
Peter Scicluna
Academia Sinica Institute for Astronomy and Astrophysics
Taiwan
Tel: +886 (02) 2366 5420
Email: peterscicluna@asiaa.sinica.edu.tw
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
