The stellar nursery of Orion
The wonderful winter constellation of Orion is still visible to the west in the early
evening. I described some of the interesting objects visible to the naked eye in and
around Orion, and some of the constellation’s rich cultural history, in Stars’nTides 35.
But Orion is special for another reason, to do with invisible radiation. It was in the
Sword of Orion, close to the fuzzy cloud of hot gas known as the Orion Nebula which
makes up the central ‘star’ of the Sword, that the first really new type of infrared
astronomical source was discovered, in 1967. Gerry Neugebauer and Eric Becklin, from
the California Institute of Technology mapped the area of sky around the Orion Nebula
and found a bright source of infrared radiation in a completely dark area of sky. This
became known as the Becklin-Neugebauer or BN object and turned out to be the first
example of a star in the process of formation. Till then, infrared astronomers had
detected only the Sun (William Herschel 1800), the Moon (Piazzi Smyth1856), planets
and known stars.
Just a few years later, in 1970, Arno Penzias and Bob Wilson, more famous for their
discovery of the cosmic microwave background radiation left over from the Big Bang,
first detected microwave radiation from the carbon monoxide molecule in the vicinity of
the Orion Nebula. Although this was not the first detection of radiation from interstellar
molecules, it triggered an explosion of activity in molecular line astronomy. Carbon
monoxide is the most abundant interstellar molecule after hydrogen, which is much
harder to detect. Astronomers were eventually able to map the whole Milky Way in
carbon monoxide and trace out vast clouds of molecular gas in which new stars are
forming. Much of the Orion constellation is covered with huge clouds of dust and
molecules, especially in the region of Orion’s Belt and Sword. When the Infrared
Astronomical Satellite (IRAS) was launched in 1983 to map the whole sky at infrared
wavelengths, it produced an iconic image of the Orion constellation, showing the dust
clouds lit up by newly forming stars. Only a couple of stars from the visible constellation
are noticeable on the infrared image. Most are just foreground objects seen in front of the
vast stellar nursery that is Orion.
And on its distant orbit a million miles from Earth, the Herschel Space Observatory is
studying star-formation in distant galaxies, taking place at epochs before the Solar
System was formed. The Herschel scientists, including myself, are working on the first
set of scientific papers on the early results and I look forward to reporting on these in a
later issue of the Organ.
Infrared image of the Orion constellation (left), from the Infrared Astronomical Satellite
(IRAS) mission, compared with an optical image on the right. The bright patch in the
lower centre is the Sword of Orion, with the bright nebolusity just above it and to the left
surrounding the Belt star Zeta Orionis. The bright spot surrounded by a large ring is
Lambda Orionis, and the spot just outside this ring on the left is Betelgeuse.
Michael Rowan-Robinson
Stars’n Tides 49, Apr 2010
Southwold Organ