Cosmic rays from the heart of Cygnus

A study using data from NASA’s Fermi Gamma-ray Space Telescope finds that the tumult of star birth and death in the direction of the constellation Cygnus – in a star-forming region known as Cygnus X – has managed to corral fast-moving particles called cosmic rays. The study was published in the November 25, 2011 edition of the journal Science.

Located in the vicinity of the second-magnitude star Gamma Cygni, the star-forming region was named Cygnus X when it was discovered as a diffuse radio source by surveys in the 1950s.

Cosmic rays are subatomic particles – mainly protons – that move through space at nearly the speed of light. In their journey across the galaxy, the particles are deflected by magnetic fields, which scramble their paths and make it impossible to backtrack the particles to their sources.

Yet when cosmic rays collide with interstellar gas, they produce gamma rays – the most energetic and penetrating form of light – that travel to us straight from the source. Astronomers are now using the Large Area Telescope on the Fermi Gamma-Ray Satellite to trace gamma-ray signals throughout the galaxy. This work is helping astronomers understand the sources of cosmic rays and how they’re accelerated to such high speeds.

The galaxy’s best candidate sites for cosmic-ray acceleration are the rapidly expanding shells of ionized gas and magnetic field associated with supernova explosions. For stars, mass is destiny, and the most massive ones – known as types O and B – live fast and die young.

They’re also relatively rare because such extreme stars, with masses more than 40 times that of our sun and surface temperatures eight times hotter, exert tremendous influence on their surroundings. With intense ultraviolet radiation and powerful outflows known as stellar winds, the most massive stars rapidly disperse their natal gas clouds, naturally limiting the number of massive stars in any given region.

Which brings us back to Cygnus X. Located about 4,500 light-years away, this star factory is believed to contain enough raw material to make two million stars like our sun. Within it are many young star clusters and several sprawling groups of related O- and B-type stars, called OB associations. One, called Cygnus OB2, contains 65 O stars – the most massive, luminous and hottest type – and nearly 500 B stars.

Astronomers estimate that the association’s total stellar mass is 30,000 times that of our sun, making Cygnus OB2 the largest object of its type within 6,500 light-years. And with ages of less than 5 million years, few of its most massive stars have lived long enough to exhaust their fuel and explode as supernovae.

Intense light and outflows from the monster stars in Cygnus OB2 and from several other nearby associations and star clusters have excavated vast amounts of gas from their vicinities. The stars reside within cavities filled with hot, thin gas surrounded by ridges of cool, dense gas where stars are now forming. It’s within the hollowed-out zones that the Fermi satellite’s Large Area Telescope detects intense gamma-ray emission. Study co-author Luigi Tibaldo, a physicist at Padova University and the Italian National Institute of Nuclear Physics, said:

We are seeing young cosmic rays, with energies comparable to those produced by the most powerful particle accelerators on Earth. They have just started their galactic voyage, zig-zagging away from their accelerator and producing gamma rays when striking gas or starlight in the cavities.

The energy of the gamma-ray emission, which is measured up to 100 billion electron volts by the Large Area Telescope and even higher by ground-based gamma-ray detectors, indicates the extreme nature of the accelerated particles. (For comparison, the energy of visible light is between 2 and 3 electron volts.) The environment holds onto its cosmic rays despite their high energies by entangling them in turbulent magnetic fields created by the combined outflows of the region’s numerous high-mass stars. co-author Isabelle Grenier, an astrophysicist at Paris Diderot University and the Atomic Energy Commission in Saclay, France, said:

These shockwaves stir the gas and twist and tangle the magnetic field in a cosmic-scale jacuzzi so the young cosmic rays, freshly ejected from their accelerators, remain trapped in this turmoil until they can leak into quieter interstellar regions, where they can stream more freely.

Astronomers know of a dozen stellar clusters at least as young and rich as Cygnus OB2. Energetic gamma rays are detected in the vicinity of several of them, so perhaps they also corral cosmic rays in their own high-energy cocoons.

The constellation Cygnus – whose bright star Deneb is part of the famous Summer Triangle asterism – is now in the western sky as twilight deepens after sunset. Astronomers looking in the direction of the constellation Cygnus at visible wavelengths see only hints of this spectacular activity thanks to a veil of nearby dust clouds forming the Great Rift, a dark lane that splits the starry band of the Milky Way on our sky’s dome. But advancing technologies have let us get above the obscuring blanket of Earth’s atmosphere – to “see” the universe not just in visible light, but in a whole range of energies within the electromagnetic spectrum. Without that ability, this discovery would not have been possible.

Bottom line: Astronomers have used the Large Area Telescope on NASA’s Fermi Gamma-ray Satellite to peer at a star-forming region known as Cygnus X – and learn that it is a source of fast-moving particles called cosmic rays. The study was published in the November 25, 2011 edition of the journal Science.

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