The Infrared Sky
20 JANUARY 2020 • SKY & TELESCOPE
the bright emission of the atmosphere or the telescope itself,
allows the 33-inch Spitzer to be many times more sensitive
than even a 10-m ground-based telescope operating at the
same wavelengths. Spitzer’s instruments exploited this gain
by fi lling its focal plane with (what were then) large-format
detector arrays. These arrays not only enabled effi cient spec-
troscopy at wavelengths between 5 and 40 microns, but also
allowed Spitzer to achieve both deep and rapid imaging sur-
veys over fi elds of view comparable to or much larger than the
angular size of the full Moon. These capabilities gave astrono-
mers a valuable window on the universe, from star formation
and exoplanets to the evolution of galaxies over cosmic time.Formation of Stars and Planetary Systems
Although our Milky Way Galaxy came together some 13 bil-
lion years ago, stars have been forming
throughout its history, from its fi rst years
through when the Sun and Earth coalesced
some 4.6 billion years ago and to today.
We now understand that, in most cases,
a forming star gives birth to a planetary
system.
Infrared observations can peer through
dense interstellar dust clouds, which are
opaque at visible wavelengths. They can
also record the light emitted by objects
which are too cold (below a few thousand
degrees kelvin) to produce appreciablevisible light. Spitzer’s extensive studies of the formation and
evolution of stars and planetary systems exploit both these
qualities.
Starbirth begins when a portion of a dense interstellar
cloud of gas and dust starts to collapse under its own gravity.
The forming star passes through a number of stages, each of
which has a characteristic appearance in the infrared, driven
initially by the energy released by the infalling material and
later by the onset of nuclear fusion. Even as the core bulks up
and develops into a star, conservation of angular momentum
dictates that some of the collapsing cloud forms a protoplan-
etary disk orbiting the star.
Spitzer’s surveys have measured hundreds to thousands of
young stars in each of these stages. Those observations have
shown that the coagulation process that results in planetsWavelength (microns)Carbon
Water dioxide ice
ice SilicatesMethane
gas
Methyl
alcoholBrightness10 15pTHE GREAT OBSERVATORIES NASA’s four premier space telescopes observed from the far infrared to gamma rays. The Compton Gamma-Ray
Observatory shut down in 2000; the Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope now patrol that spectral range.uCOSMOCHEMISTRY Spitzer observations of
stars cocooned in dusty gas (illustration, center)
have picked up several common compounds.
Silicate minerals show up in spectra of protoplan-
etary disks seen edge on (left, for protostar HH 46
IRS 1), whereas face on we have a clear view of
the warm inner regions around the star (far right,
for AA Tauri).Spectrum of an Edge-on DiskEdge OnSpitzer106 103 100Wavelength (meters)Photon energy (eV )10 –3 10 –6 10 –9 10 –1210 –12 10 –9 10 –6 10 –3 100 103 106Gamma rays X-rays UV Visible Infrared Microwaves Radio wavesCompton Chandra HubbleGrains Insects Humans Buildings
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