Spitzer will provide new and important insights into the formation and evolution of stars, and their immediate ("circumstellar") environment. The discovery and characterization of circumstellar disks of gas and dust, the signatures of extra-solar planetary systems, is one of the core scientific goals of the Spitzer mission.
NASA/HST/ J. Hester & P. Scowen
Giant molecular clouds, comprised primarily of hydrogen, provide the reservoir of raw materials from which stars are born. These clouds, scattered throughout the interstellar medium of our Milky Way Galaxy, contain enough dense gas and dust to form hundreds of thousands of Sun-like stars. Spitzer will study the temperature and density of molecular clouds in order to characterize the physical conditions and chemical compositions from which protostars are formed.
Stars are born within cocoons of dust and dense molecular gas, and are mostly
hidden from view at visible wavelengths. Near-infrared light, at wavelengths of a few microns, can pierce through the dusty veil to provide astronomers with a peek at a newborn star. Spitzer will use its short-wavelength camera to probe the formation and early evolution of young stellar objects in the first million years of their life. (In astronomical terms, a million years is a bried period of time. Many stars live for billions of years.) Spitzer observations will also reveal the extent to which new stars are formed in clusters, rather than in isolation.
NASA/HST/C.R. O'Dell (WFPC2), R. Thompson (NICMOS)
A substantial amount of Spitzer observing time will be devoted to the study of
circumstellar ("surrounding star") dust disks. These flat disks enshrouding
young stars are thought to be a common feature of stellar evolution and of
planetary system formation. Primordial protoplanetary disks contain gas and
dust and provide the raw materials from which future planetary systems can form. Planetary debris disks represent a later stage of evolution, with most of the gas having been dissipated. These disks are comprised mostly of small dust
grains presumably formed from collisions between small planetesimals and larger
Spitzer will be able to detect and characterize circumstellar disks around nearby
stars, providing key information about the formation of planetary systems.
Attempting to see the faint disk at visible wavelengths is extremely difficult
because of the bright light from the parent star. However, the relative
difference is reduced in the infrared, where Spitzer will conduct its measurements. The Observatory will study hundreds of nearby stars in order determine the frequency of these disks. It will also use imaging and spectroscopy to search to characterize the spatial structure and composition of the disks. These data will prove invaluable in addressing the frequency and nature of planetary systems beyond our own.
|Protoplanetary Disk Seen in Silhouette
|Planetary Debris Disk Around HD 141569
|Artist Depiction of a Brown Dwarf
Dwarfs & Low-Mass Stars
While bright and massive stars may dominate the night sky, most of the stellar
mass in galaxies is found in low-luminosity, low-mass stars. These stars live
for billions of years, but are dimmer and cooler than our Sun, and hence can be
difficult to detect at visible wavelengths. Spitzer will use its infrared
capabilities to detect and characterize these objects. Particular attention
will be devoted to the discovery and characterization of brown dwarfs. These
objects are too small to ignite the thermonuclear reactions that define a star,
and hence radiate primarily in the infrared. Brown dwarfs were a theoretical
concept only when Spitzer was first proposed. Since the mid-1990s, various
telescopes and surveys such as
2MASS have identified a few hundred of these objects, with temperatures extending below 2000 K. Spitzer will detect thousands of brown dwarfs, including those only slightly larger than Jupiter, thereby providing a sufficiently large sample for statistical analyses.
Spitzer observations in this realm will focus primarily on open (or galactic)
clusters, gravitationally bound systems of thousands of young stars typically
found within the plane of our Galaxy. Clusters are thought to be a natural
location for discovering faint brown dwarfs. Spitzer will conduct detailed
imaging studies of the Pleiades and Hyades clusters, in part to search for
previously unseen cluster members as small as ten Jupiter masses.
Spitzer will carry out various programs of research into the late evolutionary
phases of stars. Once it has exhausted most of its thermonuclear fuel over tens of millions to billions of years, a star like our Sun will enter a rapidly
changing stage of its life, with its behavior and ultimate fate depending on the initial mass of the star. During the late stages of its life, a star typically ejects gaseous material from its outer layers, either through gentle periodic processes (such as a
nova), or through a violent cataclysmic explosion
(supernova). Spitzer will study the stellar ejecta in
providing information about the temperature and chemical composition of
the ejected material, and on the mass loss rates of the parent star. The gas
and dust thrown off dying stars is an important constituent of the interstellar
medium, and its study is essential for understanding not only stellar death,
but the birth of the next generation of stars.
|Cat's Eye Nebula
NASA/HST/R. Sahai & J. Trauger
NASA/HST/W. B. Latter
|Water Vapor in Orion
Interspersed between stars is a tenuous interstellar medium (ISM) comprised of
dust grains and atomic and molecular gas. Dust absorbs visible and ultraviolet
light, increases in temperature, and re-radiates in the infrared. Moreover,
many of the most important spectral lines produced by gas in the ISM falls
within infrared wavelengths. Spitzer will exploit these fundamental physical
traits by conducting spectroscopic studies of the ISM. Among these research
investigations will be the important study of water, ices and organic molecules
throughout the ISM. Spitzer observations at near-infrared wavelengths will map
the inner regions of our Milky Way, yielding important new information that is
otherwise obscured by heavy concentrations of dust at visible wavelengths.