Penetrating the Murk of Star Formation

As we peer through the plane of the Milky Way, the center of our galaxy is completely hidden due to interstellar dust.14 The galactic center is 28,000 light-years away and only one in a trillion optical photons can escape—it’s like trying to look through a closed door. Unless this cold and grainy dust is accounted for, intrinsic stellar properties can be badly misjudged, a point first noted by Robert Trumpler in 1930. Star formation takes place in chaotic, dense, swirling regions of gas and dust where the concentration of dust is so high the action is mostly hidden from view. There’s a dramatic difference, for instance, between the optical and infrared views of the Orion nebula. In the optical view the bright nebula contains a dark region that appears to be devoid of stars, while in the in­frared view that penetrates the dust a dense star cluster shows up in that dark region.15 The youngest stars are invisible even to the most powerful optical telescope. However, with Spitzer’s ability to penetrate dust, the broad sweep of star formation in the Orion Nebula (M42) and the Omega Nebula (M17) and throughout the Milky Way has been revealed (plate 15).

Spitzer’s detectors are so sensitive that the telescope has lifted the veil on some of the earliest star formation in the universe. Black holes that inhabit the centers of all galaxies are generally starved of fuel and are quiet. But it was not always this way. When the universe was 7 percent of its present age, nearly 13 billion years ago, it was seven times hotter and 350 times denser than it is now. There was a furious phase of star formation and galactic construction in the first 3 billion years after the big bang, during which many of the largest galaxies were put together. Spitzer has two unique advantages for studying the distant universe. The first is associated with the redshift caused by expanding space. By the time the light from a galaxy 13 billion light-years away reaches us, its light has been stretched and mostly shifted to the infrared. The second advantage is that light from star formation and black hole growth shrouded in dust in the early universe is absorbed and reradiated at longer wavelengths where Spitzer works best. As a result, Spitzer has provided a more reliable measure of the star for­mation history of the entire universe.16 The most distant, infrared- luminous galaxies have a rate of star formation thousands of times larger than the Milky Way. Such a rate was sustained by the ready availability of gas in the denser, primordial universe. Gas was con­sumed so quickly that the fireworks didn’t last long.

One provocative and tantalizing study claimed to use Spitzer to reveal the first objects produced by gravity in the universe, at the end of the cosmic “dark ages.” Alexander Kashlinsky and his group studied the diffuse infrared background in ultra-deep Spitzer images.17 They first carefully removed light from foreground stars and nearby galaxies, leaving only the most ancient diffuse radia­tion. They then analyzed the fluctuations of the faint infrared emis-

sion that remained and matched them to simulations of clustering in the early universe (figure 9.2). “Imagine trying to see fireworks at night from across a crowded city,” explained Kashlinsky. “If you could turn off the city lights, you might get a glimpse of the fire­works. We have shut down the lights of the city to see the outlines of its first fireworks.”18 It’s unclear whether the faint infrared glow in Spitzer’s ultra-deep images actually comes from a first genera­tion of huge stars hundreds of times the mass of the Sun, or from the first wave of supermassive black holes. Either way, this truly ancient radiation carries information about the universe when it was a fraction of its present age.

Closer to home, Spitzer conducted a sky survey of our own gal­axy dubbed GLIMPSE (Galactic Legacy Infrared Mid-Plane Sur­vey Extraordinaire) by making images of swaths spanning 65 de­grees on either side of the galactic center and one degree on either side of the galactic plane.19 A hundred times the sensitivity and ten times the resolution of any previous survey, the GLIMPSE mosaic represents 110,000 different pointings of Spitzer and 840,000 in­dividual images, with the postage stamps painstakingly stitched together to make a seamless map. Spitzer’s GLIMPSE photos are as scientifically meaningful as they are aesthetically stunning. But since Spitzer takes data at invisible wavelengths, there’s no direct correspondence between its images and the colors of the rainbow. The filters Spitzer used for the GLIMPSE mosaic are sensitive to thermal emission from dust at temperatures from 30 K to 1600 K.

In the maps, blue represents radiation typically coming from older stars. Green colors trace stellar embryos by the emission of complex molecules called polycyclic aromatic hydrocarbons. On Earth, these molecules are found in car exhaust and barbeque grills, or anywhere carbon is incompletely burned. Red colors depict ra­diation from the youngest stars, still in their cool placental dust, as well as diffuse emission from graphite grains similar to tiny pieces of pencil lead. While no simple coding scheme can convey all the subtleties of the astrophysics of star formation, this three-color scheme is useful for researchers and armchair connoisseurs alike. Polycyclic aromatic hydrocarbons are too thinly dispersed for any­one to be able to savor their sooty aroma, but they’re excited by ultraviolet radiation so they map out newborn, massive stars.20 New stars reside in bubbles or on the edge of green ridges carved out by stellar winds. Many of these stars die as supernovae, which further excite the diffuse gas. The young stars appear as yellow or red dots, some in tight knots where they formed together; others are more randomly sprinkled throughout the disk of the galaxy. Older stars register as blue in the combined image and they’re so abundant that they merge into a blue-white haze.

The GLIMPSE survey has contributed to new understanding of the morphology of our Milky Way galaxy. Teams working on the GLIMPSE images cataloged over 100 million stars, and used them to trace out the Milky Way’s spiral arms and stellar bar. Spitzer project scientist Michael Werner reports that the survey “strongly confirmed previous evidence that the Milky Way is a barred spi­ral galaxy.”21 Prior to this survey and that of Hipparcos, it was believed that our galaxy had four spiral arms: Norma, Perseus, Sagittarius, and Scutum-Centaurus. Adding its data to the Hip – parcos survey, GLIMPSE confirms that the Milky Way has only two major arms, Scutum-Centaurus and Perseus that trail from the ends of the stellar bar at the galactic core. Additionally, as a result of GLIMPSE, astronomers have determined a new obscuration relationship for the galaxy, allowing more accurate estimates of distance to stars even in the presence of dust. Most fundamentally, they’ve measured the “pulse” of the galaxy by counting how fast new stars are forming. Our galaxy generates, on average, one star like the Sun per year, using mostly recycled gas from the death of older stars. That doesn’t sound like much for a system with several hundred billion stars, but as the Milky Way settles into the steady rhythm of middle age its wild times of forming thousands of stars a year are long over.

Given the volume of GLIMPSE data, the public has been re­cruited via the website Zooniverse. org to assist astronomers in analyzing Spitzer’s views of our galaxy. Launched in 2007, Zooni – verse developed the Milky Way Project, by which citizen scientists scour Spitzer’s GLIMPSE images to identify gaseous nebula and newly forming stars.22 Zooniverse offers a variety of mission data sets to volunteers who assist in identifying new solar system for­mation, track ancient weather patterns, report solar storms, and hunt for massive explosions of stars called supernovae. For the Milky Way Project, volunteers use simple online tools to circle huge bubbles of gas and dust in which stars are currently form­ing. The GLIMPSE survey imaged thousands of these globe-l ike bubbles where high-energy radiation from newborn stars collides with surrounding dust clouds. As the Milky Way Project site ex­plains: “Right now our best understanding is that these are regions around young massive stars that are so bright that their light has caused a shock wave to affect the cloud around them and blown a bubble which we can see in infrared light.”23 Identifying mas­sive bubbles in Spitzer images assists scientists in determining the mechanisms occurring in star birth.

Of course, it was always understood that the day would arrive when the last drop of Spitzer’s 350 liters of liquid helium would evaporate away. This occurred on May 15, 2009. Without liquid helium, the telescope’s long wavelength instruments have lost all their sensitivity. However, the two shortest wavelength channels of one of the cameras can operate at their design sensitivity, so Spitzer was approved for a “warm mission” phase where it is con­tinuing to do important science. Even in its “diminished” state, Spitzer is more sensitive than any ground-based telescope in the infrared. Also, “warm” is a relative term; touching any part of the spacecraft would give you a very serious cold-burn. At the balmier operating temperature of -242°C, Spitzer is continuing work with two channels of one instrument as part of its “warm mission.”24 Given that its near-infrared channels are able to do science from comets to cosmology, more than 10,000 hours of programs were approved in the first two years of the “warm mission.” Exoplanets are “hot” so some of the largest programs were to characterize exoplanets where our line of sight is aligned with the orbital plane of the exoplanet. As the planet passes behind its parent star, the combined infrared emission from the star plus the exoplanet dips, and measurements made at different wavelengths can give the size and temperature of the exoplanet. By the end of the mission, the number of exoplanets characterized will have increased from a dozen to 50 or 60 and we’ll have a much better understanding of the extraordinary diversity of other worlds in space. To set the stage for this exciting Spitzer science, we take a detour into the role of light and darkness in the evolution of life. We can only imagine the flora and fauna that might possibly exist on planets orbiting other stars, but life’s adaption to light here on Earth might offer some scant clues.