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Panning over a VLT image of the cometary globule CG4
VLT images cometary globule CG4
Like the gaping mouth of a gigantic
celestial creature, the cometary globule CG4 glows menacingly in this
new image from ESO’s Very Large Telescope. Although it appears to be big
and bright in this picture, this is actually a faint nebula, which
makes it very hard for amateur astronomers to spot. The exact nature of
CG4 remains a mystery.
In 1976 several elongated comet-like objects were discovered on
pictures taken with the UK Schmidt Telescope in Australia. Because of
their appearance, they became known as cometary globules even though
they have nothing in common with comets. They were all located in a huge
patch of glowing gas called the Gum Nebula. They had dense, dark, dusty heads and long, faint tails, which were generally pointing away from the Vela supernova remnant
located at the centre of the Gum Nebula. Although these objects are
relatively close by, it took astronomers a long time to find them as
they glow very dimly and are therefore hard to detect.
The object shown in this new picture, CG4,
which is also sometimes referred to as God’s Hand, is one of these
cometary globules. It is located about 1300 light-years from Earth in
the constellation of Puppis (The Poop, or Stern).
The head of CG4, which is the part visible on this image and
resembles the head of the gigantic beast, has a diameter of 1.5
light-years. The tail of the globule — which extends downwards and is
not visible in the image — is about eight light-years long. By
astronomical standards this makes it a comparatively small cloud.
The relatively small size is a general feature of cometary globules.
All of the cometary globules found so far are isolated, relatively small
clouds of neutral gas and dust within the Milky Way, which are
surrounded by hot ionised material.
The head part of CG4 is a thick cloud of gas and dust, which is only
visible because it is illuminated by the light from nearby stars. The
radiation emitted by these stars is gradually destroying the head of the
globule and eroding away the tiny particles that scatter the starlight.
However, the dusty cloud of CG4 still contains enough gas to make
several Sun-sized stars and indeed, CG4 is actively forming new stars,
perhaps triggered as radiation from the stars powering the Gum Nebula
Why CG4 and other cometary globules have their distinct form is still
a matter of debate among astronomers and two theories have developed.
Cometary globules, and therefore also CG4, could originally have been
spherical nebulae, which were disrupted and acquired their new, unusual
form because of the effects of a nearby supernova explosion. Other
astronomers suggest, that cometary globules are shaped by stellar winds
and ionising radiation from hot, massive OB stars. These effects could first lead to the bizarrely (but appropriately!) named formations known as elephant trunks and then eventually cometary globules.
To find out more, astronomers need to find out the mass, density,
temperature, and velocities of the material in the globules. These can
be determined by the measurements of molecular spectral lines which are
most easily accessible at millimetre wavelengths — wavelengths at which
telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA) operate.
This picture comes from the ESO Cosmic Gems programme,
an outreach initiative to produce images of interesting, intriguing or
visually attractive objects using ESO telescopes, for the purposes of
education and public outreach. The programme makes use of telescope time
that cannot be used for science observations. All data collected may
also be suitable for scientific purposes, and are made available to
astronomers through ESO’s science archive.
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 15 countries: Austria, Belgium, Brazil, the
Czech Republic, Denmark, France, Finland, Germany, Italy, the
Netherlands, Portugal, Spain, Sweden, Switzerland and the United
Kingdom. 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”.