FEATURE ARTICLE
Spitzer’s Cold Look at Space
To get a clear view of infrared emissions from celestial objects, the Spitzer Space Telescope has been cryogenically cooled—and what sights it has seen
Michael Werner
Stargazing
Spitzer is particularly well suited to carrying out surveys of expansive areas that can cover one percent or more of the sky. The large angular coverage of Spitzer’s arrays and their high sensitivity, along with the maneuverability of the spacecraft, make it possible to explore regions of the sky as large as the constellation Orion to unprecedented sensitivity in just a few days’ observation.
A number of surveys of various types have been executed during Spitzer’s lifetime; some are unbiased looks at regions of the sky that have been extensively studied with other instruments, whereas others are more targeted to certain regions with specific science goals in mind.
The most luminous common stars in the galaxy are called K giants; they are the penultimate phase in the evolution of stars of roughly the same mass as the Sun. K giants produce large amounts of infrared radiation, so much so that Spitzer can detect an individual K giant across the galaxy. In addition, infrared radiation penetrates the dense dust clouds that block our view of the central regions of our galaxy in visible light. Therefore, an infrared survey is the best means of determining the distribution of stars throughout the galaxy.
Spitzer carried out such a survey, imaging large areas in the plane of the galaxy as well as the center, because these are the lines of sight where the density of stars should be highest. Analysis of these data revealed two important facts about the stellar distribution in our galaxy. As had been previously conjectured from other studies, Spitzer dramatically confirmed that our Milky Way galaxy is a barred spiral galaxy, a type not uncommon in our area of the universe. In other words, the arms of the Milky Way do not spiral all the way into the center but originate at the ends of a bar-like distribution of stars that extends about 30 percent of the way from the center of the galaxy to the Sun.
The second fact (not necessarily related to the first) is contrary to previous indications. Our galaxy apparently has only two spiral arms, one leaving each end of the bar, as opposed to the four that previously had been identified based on other measurements. Note that because we live in our galaxy we cannot readily get outside of it to determine what it looks like on a larger scale and how it compares with other galaxies, but the Spitzer insights provide a good start.
Spitzer’s observations of stars have not been limited to K giants. Calculations show that stars with a mass only slightly less than 0.08 times the Sun’s will never become hot and dense enough at their centers for the burning of their nuclear fuel to be self-sustaining. “Stars” in this mass range are called substellar objects or brown dwarfs.
When they are young, brown dwarfs can be observed in the infrared as the heat generated during their gravitational collapse diffuses away. With time, they become successively cooler and fainter. For example, an object of 0.05 solar mass with an age of about 5 billion years, like the Sun, is predicted to have a temperature of less than 1,000 kelvins and a luminosity of only a few millionths that of the Sun. Even a star with 0.1 solar mass of the same age—although a very puny star indeed—would be about three times hotter and more than 100 times more luminous.
The atmospheres of brown dwarfs are expected to be rich in molecules. The chemical equilibrium of the molecular gas, and hence the appearance of the objects’ spectra, will vary systematically as the brown dwarf cools. These variations have been translated into three phases that all brown dwarfs should go through. The youngest objects are referred to as L dwarfs; as they cool, their atmospheric chemistry changes and they evolve into T dwarfs. Finally, the as-yet-undiscovered endpoint of brown dwarf evolution, with temperatures under 500 kelvins, will be occupied by the so-called Y dwarfs.
Brown dwarfs have been of intense interest to astronomers for many reasons. The stars could represent a significant reservoir of mass in the solar neighborhood, and they bridge the gap between stars and planets. The identification of the first brown dwarf in 1995 triggered a flood of discoveries not unlike the one in connection with exoplanets. At present, approximately 1,000 brown dwarfs have been identified, and it appears that brown dwarfs may be about as common locally as all other types of stars combined, but not so common as to constitute much of the total mass of all stars.
Spitzer observations of young stellar clusters show that objects destined to become brown dwarfs are as readily detected as those destined to become low-mass stars, because at this early phase the luminosity from the accretion of matter dominates that of nuclear burning. Intriguingly, many of the brown dwarfs forming in these young clusters show evidence of circumstellar disks within which planets might grow.
A search for “field” brown dwarfs—older, fainter and more uniformly distributed than the young-cluster brown dwarfs—is an important objective of Spitzer’s large-area surveys. Analysis of a recent survey focused on a search for T dwarfs in an area of the sky about 50 times the size of the full Moon. It identified almost 1 million sources, from which a total of about 18 of the coolest types of T dwarfs have been identified.
Because of the extreme faintness of the T dwarfs, even Spitzer could see them only out to about 100 light-years, which confines the search to a very local region. (By comparison, the center of the galaxy is about 25,000 light-years away.) Nonetheless, this is a major increase in the total number of true, cool field T dwarfs known, as most of the others have been discovered as companions to other low-mass stars.
The results also yield an estimate of the abundance of T dwarfs. The upcoming Wide-field Infrared Survey Explorer (WISE), which is scheduled to launch at the end of 2009 and will make an infrared survey of the entire sky, can now be expected to detect perhaps several thousand T dwarfs. WISE is certain to discover the T dwarf closest to the Earth. Either this T dwarf, or perhaps a Y dwarf, could be closer to Earth than even the nearest known star, Proxima Centauri, which lies around 4 light-years distant. Because of the growing evidence that brown dwarfs may host planet-forming disks, this raises the possibility that the nearest exoplanets to Earth may be orbiting not a star, but a brown dwarf.
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