SOFIA Discovers a Massive Stellar Surprise
On April 29, the European Space Agency announced that its premier infrared space observatory had run out of coolant and the mission had come to an end. Observing the cosmos in far-infrared wavelengths, the space telescope has given us some of the most striking views of cool nebulae, star forming regions, comets being pulverized around nearby stars, even asteroids buzzing around our own solar system. As we say goodbye to the historic mission, and astronomers continue to analyze the huge wealth of data Herschel has left us with, it's time to have a look back at some of the mission's most spectacular observations.
In this picture, embryonic stars feed on the gas and dust clouds deep inside the Orion Nebula. This image combines far-infrared data by Herschel and mid-infrared data by NASA's Spitzer space telescope.
ESA/Herschel/PACS & SPIRE Consortium, O. Krause, HSC, H. Linz
The Andromeda galaxy in infrared -- Herschel took this portrait of the famous spiral galaxy, picking out the fine detail from gas and dust running through its structure.
ESA/PACS & SPIRE consortia, A. Rivera-Ingraham & P.G. Martin, Univ. Toronto, HOBYS Key Programme (F. Motte)
This three-color image of the W3 giant molecular cloud combines Herschel's 70 μm (blue), 160 μm (green) and 250 μm (red) filters. W3 is located about 6200 light-years away and is a hub of intense star formation. Filaments of gas and dust cocooning protostars (yellow dots) can be seen.
ESA/Herschel/PACS/L. Decin et al
The star Betelgeuse is observed in infrared by Herschel as it rapidly approaches a "barrier" of interstellar gas. The bow shock of the star's stellar winds can easily be seen.
ESA/Bonsor et al (2013)
The star Kappa Coronae Borealis is captured in this infrared observation by Herschel. The star itself is blocked out whereas the ring of debris (likely from asteroid/comet impacts) glows bright.
ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium
The infrared emissions from dust produced by a huge number of cometary collisions surrounding the famous star Fomalhaut glows in bright blue in Herschel's eye. At least one exoplanet is known to orbit within this ring of dust.
Herschel: Q. Nguyen Luong & F. Motte, HOBYS Key Program consortium, Herschel SPIRE/PACS/ESA consortia. XMM-Newton: ESA/XMM-Newton
Supernova remnant W44 is the focus of this observation created by combining data from ESA's Herschel and XMM-Newton space observatories.
ESA and SPIRE & PACS consortia, Ph. André (CEA Saclay) for Gould’s Belt Key Programme Consortia
Herschel picks out 600 newly forming stars inside the W40 nebula cradle of stars -- located 1,000 light-years away in the constellation Aquila.
ESA/Herschel/PACS/MACH-11/MPE/B.Altieri (ESAC) and C. Kiss (Konkoly Observatory)
Herschel could also study solar system objects with ease. In this observation, asteroid Apophis was captured during its approach to Earth on 5/6 January 2013. This image shows the asteroid in Herschel’s three PACS wavelengths: 70, 100 and 160 microns, respectively.
ESA/Herschel/PACS, SPIRE/N. Schneider, Ph. André, V. Könyves (CEA Saclay, France) for the “Gould Belt survey” Key Programme
This striking image complemented Hubble's 23rd anniversary optical view of the Horsehead Nebula. Herschel's infrared observation of the Orion Molecular Cloud complex (including the Horsehead Nebula -- visible far right of image) provided a unique perspective on this astronomical favorite.
It turns out that massive stars form in exactly the same way small stars (like the sun) do. This was the conclusion released last month when a team led by Yichen Zhang at the University of Florida published the most detailed observations ever, taken with the SOFIA telescope, of a massive protostar — a hot bundle of gas that is still collapsing to form a star.
Massive stars are rare beasts that are, in many ways, not very much like the sun at all. Just over 0.1 percent of all stars accumulate enough hydrogen to shine the brightest, and they don’t live long. Termed O-stars and B-stars, these are the bright, blue, and incredibly hot stars that will likely end their lives as dramatic supernovae, scattering their ashes across the cosmos.
Massive stars tend to be found grouped together in herds, known as OB associations, often at the center of larger open clusters of stars. It was long thought that the turbulent conditions in these star clusters may make the formation process for these huge behemoths a rather complicated process. If the protostar G35.20-0.74 (called G35 for short), studied by Zhang and his colleagues is anything to go by, this is probably not the case.
The images at the top of this article are mid-infrared images of G35, showing a hot clouds of gas and dust collapsing to form this star. All stars are swathed in gassy cocoons like this when they form. It seems, when they’re being born, stellar hulks like G35 look exactly like scaled up versions of the innumerable smaller stars in the night sky.
Protostars are difficult to find. They’re only visible in infrared light, being buried deep inside the giant molecular clouds in interstellar space from which they form like oversized raindrops. The optical telescopes with which many of us are more familiar are no good for spotting protostars, and infrared light is absorbed strongly by Earth’s atmosphere (particularly by water vapor), making this kind of object rather troublesome to look for.
This is why the team used SOFIA (the Stratospheric Observatory For Infrared Astronomy) telescope. Being mounted on a modified Boeing 747, SOFIA avoids the problems of the atmosphere by flying above most of it. From an altitude higher than most passenger aircraft, the skies are much clearer for SOFIA, giving it the best possible view in the infrared — the only way to get a better view is to actually be in space, like NASA’s Spitzer space telescope.
In particular, they chose G35 because it shines so brightly in infrared. The only possible cause for an object this bright, deep inside an interstellar cloud, would be a massive protostar.
Massive stars are hard to find, but they’re valuable objects to study. As James De Buizer, a SOFIA staff scientist at the Universities Space Research Association (USRA), noted in a press release, “Massive stars, although rare, are important because there is evidence they foster the formation of smaller stars like our sun, and because at the ends of their lives they create and distribute chemical elements that are the basic building blocks of Earth-like planets.”
G35 has given astronomers some valuable information into how the Galaxy’s largest stars come to be. “The focus of our study has been to determine how massive stars actually form,” explained Yichen Zhang. Using infrared light to find out exactly what a forming star may look like is easily the best way to get decipher a puzzle like this. “We thought that the G35 protostar’s structure would be quite complicated, but instead we found it is simple, like the cocoons of protostars with the sun’s mass.”
Image credit: Zhang et al. 2013, Astrophysical Journal