This illustration shows three possible interiors of the seven rocky exoplanets in the TRAPPIST-1 system, based on precision measurements of the planet densities. Overall the TRAPPIST-1 worlds have remarkably similar densities, which suggests they may share the same ratio of common planet-forming elements. The planet densities are slightly lower than those of Earth or Venus, which could mean they contain fractionally less iron (a highly dense material) or more low-density materials, such as water or oxygen. In the first model (left), the interior of the planet is composed of rock mixed with iron bound to oxygen. There is no solid iron core, which is the case with Earth and the other rocky planets in our own solar system. The second model shows an overall composition similar to Earths, in which the densest materials have settled to the center of the planet, forming an iron-rich core proportionally smaller than Earths core. A variation is shown in the third panel, where a larger, denser core could be balanced by an extensive low-density ocean on the planet's surface. However, this scenario can be applied only to the outer four planets in the TRAPPIST-1 system. On the inner three planets, any oceans would vaporize due to the higher temperatures near their star, and a different composition model is required. Since all seven planets have remarkably similar densities, it is more likely that all the planets share a similar bulk composition, making this fourth scenario unlikely but not impossible. The high-precision mass and diameter measurements of the exoplanets in the TRAPPIST-1 system have allowed astronomers to calculate the overall densities of these worlds with an unprecedented degree of accuracy in exoplanet research. Density measurements are a critical first step in determining the composition and structure of exoplanets, but they must be interpreted through the lens of scientific models of planetary structure.

Detailed measurements of the physical properties of the seven rocky TRAPPIST-1 planets and the four terrestrial planets in our solar system help scientists find similarities and differences between the two planet families.

Measuring the mass and diameter of a planet reveals its density, which can give scientists clues about its composition. Scientists now know the density of the seven TRAPPIST-1 planets with a higher precision than any other planets in the universe, other than those in our own solar system

This graph presents measured properties of the seven TRAPPIST-1 exoplanets (labeled b through h), showing how they stack up with one another as well as with Earth and the other inner rocky worlds in our own solar system. The relative sizes of the planets are indicated by the circles. All of the known TRAPPIST-1 planets are larger than Mars, with five of them within 15% of the diameter of Earth. The vertical axis shows the uncompressed densities of the planets. Density, calculated from a planets mass and volume, is the first important step in understanding its composition. Uncompressed density takes into account that the larger a planet is, the more its own gravity will pack the planets material together and increase its density. Uncompressed density, therefore, usually provides a better means of comparing the composition of planets. The plot shows that the uncompressed densities of the TRAPPIST-1 planets are similar to one another, suggesting they may have all have a similar composition. The four rocky planets in our own solar system show more variation in density compared to the seven TRAPPIST-1 planets. Mercury, for example, contains a much higher percentage of iron than the other three rocky planets and thus has a much higher uncompressed density. The horizontal axis shows the level of illumination that each planet receives from its host star. The TRAPPIST-1 star is a mere 9% the mass of our Sun, and its temperature is much cooler. But because the TRAPPIST-1 planets orbit so closely to their star, they receive comparable levels of light and heat to Earth and its neighboring planets. The corresponding habitable zones regions where an Earth-like planet could potentially support liquid water on its surface of the two planetary systems are indicated near the top of the plot. The the two zones do not line up exactly because the cooler TRAPPIST-1 star emitting more of its light in the form of infrared radiation that is more efficiently absorbed by an Earth-like atmosphere. Since it takes less illumination to reach the same temperatures, the habitable zone shifts farther away from the star. The masses and densities of the TRAPPIST-1 planets were determined by measurements of slight variations in the timings of their orbits using extensive observations made by NASAs Spitzer and Kepler space telescopes, in combination with data from Hubble and a number of ground-based telescopes. The latest analysis, which includes Spitzers complete record of over 1,000 hours of TRAPPIST-1 observations, has reduced the uncertainties of the mass measurements to a mere 3-6%. These are among the most accurate measurements of planetary masses anywhere outside of our solar system.

The nebula known as W51 is one of the most active star-forming regions in the Milky Way galaxy. First identified in 1958 by radio telescopes, it makes a rich cosmic tapestry in this image from NASA's recently retired Spitzer Space Telescope. Located about 17,000 light-years from Earth, in the direction of the constellation Aquila in the night sky, W51 is about 350 light-years - or about 2 quadrillion miles - across. It is almost invisible to telescopes that collect visible light (the kind human eyes detect), because that light is blocked by interstellar dust clouds that lie between W51 and Earth. But longer wavelengths of light, including radio and infrared, can pass unencumbered through the dust. When viewed in infrared by Spitzer, W51 is a spectacular sight: Its total infrared emission is the equivalent of 20 million Suns.

This artwork illustrates the newly-discovered the exoplanet HIP 67522 b, which appears to be the youngest hot Jupiter ever found. It orbits a well-studied star that is about 17 million years old, meaning the hot Jupiter is likely only a few million years younger, whereas most known hot Jupiters are more than a billion years old. The planet takes about seven days to orbit its star, which has a mass similar to the Sun's. Located only about 490 light-years from Earth, HIP 67522 b is about 10 times the diameter of Earth, or close to that of Jupiter. Its size strongly indicates that it is a gas-dominated planet. HIP 67522 b was identified as a planet candidate by NASA's Transiting Exoplanet Survey Satllite (TESS), which detects planets via the transit method: Scientists look for small dips in the brightness of a star, indicating that an orbiting planet has passed between the observer and the star. But young stars tend to have a lot of dark splotches on their surfaces - starspots, also called sunspots when they appear on the Sun - that can look similar to transiting planets. So scientists used data from NASA's recently retired infrared observatory, the Spitzer Space Telescope, to confirm that the transit signal was from a planet and not a starspot. (Other methods of exoplanet detection have yielded hints at the presence of even younger hot Jupiters, but none have been confirmed.)

Two supermassive black holes are locked in an orbital dance at the core of the distant galaxy OJ 287. This diagram shows their sizes relative to the solar system. The larger one, with about 18 billion times the mass of our sun (right), would encompass all the planets in the solar system with room to spare. The smaller one is about 150 million times the mass of our sun (left), which would be large enough to swallow up everything out to the asteroid belt, just inside the orbit of Jupiter.

This artwork shows two massive black holes in the OJ 287 galaxy. The smaller black hole orbits the larger one, which remains stationary and is surrounded by a disk of gas. When the smaller black hole crashes through the disk, it produces a flare brighter than 1 trillion stars. But the smaller black hole's orbit is elongated and moving relative to the disk, causing the flares to occur irregularly.

This series of image taken by NASA's Spitzer Space Telescope on Jan. 25, 2020, shows part of the California Nebula, which is located about 1,000 light-years from Earth. This is the final mosaic taken by the mission before it was decommissioned on Jan. 30, 2020. Spitzer's infrared detectors reveal the presence of warm dust, similar to soot, mixed in with the gas. The dust absorbs visible and ultraviolet light from nearby stars and then re-emits the absorbed energy as infrared light. The image displays Spitzer's observations much the way that research astronomers would view them: From 2009 to 2020, Spitzer operated two detectors simultaneously that imaged adjacent areas of the sky. The detectors captured different wavelengths of infrared light (referred to by their physical wavelength): 3.6 micrometers (shown in cyan) and 4.5 micrometers (shown in red). Different wavelengths of light can reveal different objects or features. Spitzer would scan the sky, taking multiple pictures in a grid pattern, so that both detectors would image the region at the center of the grid. By combining those images into a mosaic, it was possible to see what a given region looked like in multiple wavelengths, such as in the gray-hued part of the image above. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This image shows the section of the nebula captured by Spitzer in the context of a larger, visible-light image of the nebula. This series of image taken by NASA's Spitzer Space Telescope on Jan. 25, 2020, shows part of the California Nebula, which is located about 1,000 light-years from Earth. This is the final mosaic taken by the mission before it was decommissioned on Jan. 30, 2020. Spitzer's infrared detectors reveal the presence of warm dust, similar to soot, mixed in with the gas. The dust absorbs visible and ultraviolet light from nearby stars and then re-emits the absorbed energy as infrared light. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

For the first time, scientists have directly measured wind speed on a brown dwarf, pictured here in an illustration. Brown dwarfs are objects larger than Jupiter (the largest planet in our solar system) but not quite massive enough to become stars. To achieve the finding, they used a new method that could also be applied to learn about the atmospheres of gas-dominated planets outside our solar system. Described in a paper in the journal Science, the work combines observations by a group of radio telescopes with data from NASA's recently retired infrared observatory, the Spitzer Space Telescope, managed by the agency's Jet Propulsion Laboratory in Southern California. Officially named 2MASS J10475385+2124234, the target of the new study was located 32 light-years from Earth - a stone's throw away, cosmically speaking. It is one of the coldest known brown dwarfs.. The researchers detected winds moving around the planet at 1,425 mph (2,293 kph). For comparison, Neptune's atmosphere features the fastest winds in the solar system, which whip through at more than 1,200 mph (about 2,000 kph).

Brown dwarfs are more massive than planets but not quite as massive as stars. Generally speaking, they have between 13 and 80 times the mass of Jupiter. A brown dwarf becomes a star if its core pressure gets high enough to start nuclear fusion.

This artist's concept shows planet KELT-9b orbiting its host star, KELT-9. It is the hottest gas giant planet discovered so far. Now, a team of astronomers using NASA's Spitzer space telescope has found evidence that the heat is too much even for molecules to remain intact. Molecules of hydrogen gas are likely ripped apart on the dayside of KELT-9b, unable to re-form until their disjointed atoms flow around to the planet's nightside. With a dayside temperature of more than 7,800 degrees Fahrenheit (4,600 Kelvin), KELT-9b is a planet that is hotter than most stars. But its star, called KELT-9, is even hotter -- a blue A-type star that is likely unraveling the planet through evaporation. KELT-9b is a gas giant 2.8 times more massive than Jupiter, but only half as dense. Scientists would expect the planet to have a smaller radius, but the extreme radiation from its host star has caused the planet's atmosphere to puff up like a balloon. The planet is also unusual in that it orbits perpendicular to the spin axis of the star. That would be analogous to the planet orbiting perpendicular to the plane of our solar system. One "year" on this planet is less than two days long. The KELT-9 star is only 300 million years old, which is young in star time. It is more than twice as large, and nearly twice as hot, as our sun. Given that the planet's atmosphere is constantly blasted with high levels of ultraviolet radiation, the planet may even be shedding a tail of evaporated planetary material like a comet.

This artist's concept shows planet KELT-9b orbiting its host star, KELT-9. It is the hottest gas giant planet discovered so far. Now, a team of astronomers using NASA's Spitzer space telescope has found evidence that the heat is too much even for molecules to remain intact. Molecules of hydrogen gas are likely ripped apart on the dayside of KELT-9b, unable to re-form until their disjointed atoms flow around to the planet's nightside. With a dayside temperature of more than 7,800 degrees Fahrenheit (4,600 Kelvin), KELT-9b is a planet that is hotter than most stars. But its star, called KELT-9, is even hotter -- a blue A-type star that is likely unraveling the planet through evaporation. KELT-9b is a gas giant 2.8 times more massive than Jupiter, but only half as dense. Scientists would expect the planet to have a smaller radius, but the extreme radiation from its host star has caused the planet's atmosphere to puff up like a balloon. The planet is also unusual in that it orbits perpendicular to the spin axis of the star. That would be analogous to the planet orbiting perpendicular to the plane of our solar system. One "year" on this planet is less than two days long. The KELT-9 star is only 300 million years old, which is young in star time. It is more than twice as large, and nearly twice as hot, as our sun. Given that the planet's atmosphere is constantly blasted with high levels of ultraviolet radiation, the planet may even be shedding a tail of evaporated planetary material like a comet.

This image from NASA's Spitzer Space Telescope shows the Tarantula Nebula in two wavelengths of infrared light, each represented by a different color. The red color at the heart of the nebula shows the presence of particularly hot gas emitting infrared light at a wavelength of 4.5 micrometers. The blue regions are dust composed of molecules called polycyclic aromatic hydrocarbons (PAHs), which are also found in ash from coal, wood and oil fires on Earth. Regions emitting both wavelengths appear white. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This image shows the location of Supernova 1987A and the starburst region R136 where massive stars form at a significantly higher rate than anywhere else in the galaxy. NASA's Spitzer Space Telescope shows the Tarantula Nebula in three wavelengths of infrared light, each represented by a different color. The magenta-colored regions are dust composed of molecules called polycyclic aromatic hydrocarbons (PAHs), which are also found in ash from coal, wood and oil fires on Earth. PAHs emit in multiple wavelengths. The PAHs emit in multiple wavelengths, so the magenta color is a combination of red (corresponding to an infrared wavelength of 8 micrometers) and blue (3.6 micrometers). The green color in this image shows the presence of particularly hot gas emitting infrared light at a wavelength of 4.5 micrometers. The stars in the image are mostly a combination of green and blue. White hues indicate regions that radiate in all three wavelengths. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This image from NASA's Spitzer Space Telescope shows the Tarantula Nebula in three wavelengths of infrared light, each represented by a different color. The magenta-colored regions are dust composed of molecules called polycyclic aromatic hydrocarbons (PAHs), which are also found in ash from coal, wood and oil fires on Earth. PAHs emit in multiple wavelengths. The PAHs emit in multiple wavelengths, so the magenta color is a combination of red (corresponding to an infrared wavelength of 8 micrometers) and blue (3.6 micrometers). The green color in this image shows the presence of particularly hot gas emitting infrared light at a wavelength of 4.5 micrometers. The stars in the image are mostly a combination of green and blue. White hues indicate regions that radiate in all three wavelengths. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This artist's concept depicts NASA's Spitzer Space Telescope in space much as it would appear at the end of its mission on January 30, 2020. The backdrop depicts the sky in infrared light much as Spitzer would have seen it early in its mission. On this date, Spitzer is 1.77 times as far away from the Earth as the Earth is from the sun. Since launch, Spitzer has orbited our sun much as the Earth does, though taking slightly longer to complete a revolution. Over time it will continue to drift farther away from us until it eventually is on the opposite side of the sun. Spitzer has spent over 16 years helping astronomers explore the infrared universe. Its collected data archives will continue to be a valuable resource for decades to come, and will be instrumental in helping astronomers effectively utilize future NASA missions like the James Web Space Telescope (JWST) and the Wide Field Infrared Survey Telescope (WFIRST).

This artist's concept depicts NASA's Spitzer Space Telescope in space much as it would appear to an observer at the end of its mission on January 30, 2020. On this date, Spitzer is 1.77 times as far away from the Earth as the Earth is from the sun. Since launch, Spitzer has orbited our sun much as the Earth does, though taking slightly longer to complete a revolution. Over time it will continue to drift farther away from us until it eventually is on the opposite side of the sun. Spitzer has spent over 16 years helping astronomers explore the infrared universe. Its collected data archives will continue to be a valuable resource for decades to come, and will be instrumental in helping astronomers effectively utilize future NASA missions like the James Web Space Telescope (JWST) and the Wide Field Infrared Survey Telescope (WFIRST).

This artist's concept depicts NASA's Spitzer Space Telescope in space much as it would appear at the end of its mission on January 30, 2020. The backdrop depicts the sky in infrared light much as Spitzer would have seen it early in its mission. On this date, Spitzer is 1.77 times as far away from the Earth as the Earth is from the sun. Since launch, Spitzer has orbited our sun much as the Earth does, though taking slightly longer to complete a revolution. Over time it will continue to drift farther away from us until it eventually is on the opposite side of the sun. Spitzer has spent over 16 years helping astronomers explore the infrared universe. Its collected data archives will continue to be a valuable resource for decades to come, and will be instrumental in helping astronomers effectively utilize future NASA missions like the James Web Space Telescope (JWST) and the Wide Field Infrared Survey Telescope (WFIRST).

This artist's concept depicts NASA's Spitzer Space Telescope in space much as it would appear to an observer at the end of its mission on January 30, 2020. On this date, Spitzer is 1.77 times as far away from the Earth as the Earth is from the sun. Since launch, Spitzer has orbited our sun much as the Earth does, though taking slightly longer to complete a revolution. Over time it will continue to drift farther away from us until it eventually is on the opposite side of the sun. Spitzer has spent over 16 years helping astronomers explore the infrared universe. Its collected data archives will continue to be a valuable resource for decades to come, and will be instrumental in helping astronomers effectively utilize future NASA missions like the James Web Space Telescope (JWST) and the Wide Field Infrared Survey Telescope (WFIRST).

A collection of gas and dust over 500 light-years across, the Perseus Molecular Cloud hosts an abundance of young stars. Located on the edge of the Perseus Constellation, it was imaged by NASA's Spitzer Space Telescope. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This carved-out cloud of gas and dust has been nicknamed the "Jack-o'-lantern Nebula" because it looks like a cosmic hollowed-out pumpkin. Powerful outflows of radiation and particles from a massive star known as an O-type star and about 15 to 20 times heavier than the Sun has likely swept the surrounding dust and gas outward, creating deep gouges in the cloud. The image shows infrared light (which is invisible to the human eye) captured by NASA's Spitzer Space Telescope. This image features three wavelengths of infrared light. Green and red represent wavelengths emitted primarily by dust radiating at different temperatures, though some stars radiate prominently in these wavelengths as well. The combination of green and red in the image creates yellow hues. Blue represents a wavelength emitted, in this image, mostly by stars and some very hot regions of the nebula. White regions are where the objects are bright in all three colors. The O-type star appears as a white spot at the center of a red dust shell near the middle of the scooped-out region. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This carved-out cloud of gas and dust has been nicknamed the "Jack-o'-lantern Nebula" because it looks like a cosmic hollowed-out pumpkin. Powerful outflows of radiation and particles from a massive star known as an O-type star and about 15 to 20 times heavier than the Sun has likely swept the surrounding dust and gas outward, creating deep gouges in the cloud. The image shows infrared light (which is invisible to the human eye) captured by NASA's Spitzer Space Telescope. This image is a high-contrast version in which the red wavelength is more pronounced. Together, the red and green wavelengths create an orange hue. The picture highlights contours in the dust as well as the densest regions of the nebula, which appear brightest. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

This cloud of gas and dust in space is full of bubbles inflated by wind and radiation from massive young stars. Each bubble is about 10 to 30 light-years across and filled with hundreds to thousands of stars. The region lies in the Milky Way galaxy, in the constellation Aquila (aka the Eagle).

This cloud of gas and dust is full of bubbles, which are inflated by wind and radiation from massive young stars. Yellow circles and ovals show the locations of more than 30 bubbles. Squares indicate bow shocks, red arcs of warm dust formed as winds from fast-moving stars push aside dust grains.

These four images show bow shocks, or arcs of warm dust formed as winds from fast-moving stars push aside dust grains scattered sparsely through most of the nebula.

Wispy patterns of dust trace the spiral arms of the nearby galaxy Messier 81 in this image from NASA's Spitzer Space Telescope. Located in the northern constellation of Ursa Major (which also includes the Big Dipper), this galaxy is easily visible through binoculars or a small telescope. M81 is located at a distance of 12 million light-years. The infrared view of M81 at a wavelength of 8 microns (red) has been specially processed in this image to remove most of the glow of starlight to isolate the glow of dust. This image reveals the distribution of dust throughout M81, from its outer spiral arms all the wan into its core. This image shows infrared light at a wavelength of 8 microns (red) that has been specially processed to remove most of the glow of starlight to better highlight the dust. Dust in the galaxy is bathed by ultraviolet and visible light from nearby stars. Upon absorbing an ultraviolet or visible-light photon, a dust grain is heated and re-emits the energy at longer infrared wavelengths. The dust particles are composed of silicates (chemically similar to beach sand), carbonaceous grains and polycyclic aromatic hydrocarbons and trace the gas distribution in the galaxy. The well-mixed gas (which is best detected at radio wavelengths) and dust provide a reservoir of raw materials for future star formation. Since stars have been subtracted from this image, there are a scattering of black dots in M81 that are an artifact of this process. Most of the other red dots outside of the galaxy represent the glow of dust within even more distant background galaxies.

The magnificent spiral arms of the nearby galaxy Messier 81 are highlighted in this image from NASA's Spitzer Space Telescope. Located in the northern constellation of Ursa Major (which also includes the Big Dipper), this galaxy is easily visible through binoculars or a small telescope. M81 is located at a distance of 12 million light-years. M81 was one of the first publicly-released datasets soon after Spitzers launch in August of 2003. On the occasion of Spitzers 16th anniversary this new image revisits this iconic object with extended observations and improved processing. This Spitzer infrared image is a composite mosaic combining data from the Infrared Array Camera (IRAC) at wavelengths of 3.6/4.5 microns (blue/cyan) and 8 microns (green) with data from the Multiband Imaging Photometer (MIPS) at 24 microns (red). The 3.6-micron near-infrared data (blue) traces the distribution of stars, although the Spitzer image is virtually unaffected by obscuring dust and reveals a very smooth stellar mass distribution, with the spiral arms relatively subdued. As one moves to longer wavelengths, the spiral arms become the dominant feature of the galaxy. The 8-micron emission (green) is dominated by infrared light radiated by hot dust that has been heated by nearby luminous stars. Dust in the galaxy is bathed by ultraviolet and visible light from nearby stars. Upon absorbing an ultraviolet or visible-light photon, a dust grain is heated and re-emits the energy at longer infrared wavelengths. The dust particles are composed of silicates (chemically similar to beach sand), carbonaceous grains and polycyclic aromatic hydrocarbons and trace the gas distribution in the galaxy. The well-mixed gas (which is best detected at radio wavelengths) and dust provide a reservoir of raw materials for future star formation. The 24-micron MIPS data (red) shows emission from warm dust heated by the most luminous young stars. The scattering of compact red spots along the spiral arms show where the dust is warmed to high temperatures near massive stars that are being born in giant H II (ionized hydrogen) regions.

The magnificent spiral arms of the nearby galaxy Messier 81 are highlighted in this NASA Spitzer Space Telescope image. Located in the northern constellation of Ursa Major (which also includes the Big Dipper), this galaxy is easily visible through binoculars or a small telescope. M81 is located at a distance of 12 million light-years. M81 was one of the first publicly-released datasets soon after Spitzers launch in August of 2003. On the occasion of Spitzers 16th anniversary this new image revisits this iconic object with extended observations and improved processing. Because of its proximity, M81 provides astronomers with an enticing opportunity to study the anatomy of a spiral galaxy in detail. The unprecedented spatial resolution and sensitivity of Spitzer at infrared wavelengths show a clear separation between the several key constituents of the galaxy: the old stars, the interstellar dust heated by star formation activity, and the embedded sites of massive star formation. The infrared images also permit quantitative measurements of the galaxy's overall dust content, as well as the rate at which new stars are being formed. Winding outward from the bluish-white central bulge of the galaxy, where old stars predominate and there is little dust, the grand spiral arms are dominated by infrared emission from dust. Dust in the galaxy is bathed by ultraviolet and visible light from the surrounding stars. Upon absorbing an ultraviolet or visible-light photon, a dust grain is heated and re-emits the energy at longer infrared wavelengths. The dust particles, composed of silicates (which are chemically similar to beach sand) and polycyclic aromatic hydrocarbons, trace the gas distribution in the galaxy. The well-mixed gas (which is best detected at radio wavelengths) and dust provide a reservoir of raw materials for future star formation. The infrared-bright clumpy knots within the spiral arms denote where massive stars are being born in giant H II (ionized hydrogen) regions. The 8-micron emission traces the regions of active star formation in the galaxy. Studying the locations of these regions with respect to the overall mass distribution and other constituents of the galaxy (e.g., gas) will help identify the conditions and processes needed for star formation. With the Spitzer observations, this information comes to us without complications from absorption by cold dust in the galaxy, which makes interpretation of visible-light features uncertain. The infrared image was obtained by Spitzer's Infrared Array Camera (IRAC) that combines three wavelengths of infrared light: 3.6 microns (blue), 4.5 microns (green), and 8.0 microns (red).