Galactic Fireworks

Image: Gemini North GMOS image of the 'Fireworks Galaxy' NGC 6946 in the Constellation of Cepheus. Credit: Gemini Observatory

"If you could make a time-lapse movie that compressed millions of years into seconds, this galaxy would resemble a sparkler," said Dr Rachel Johnson, UK Gemini Spokesperson from the University of Oxford. "New stars would flare into view as old stars expired in huge explosions of light and energy called supernovae - this galaxy is literally a gigantic multi-coloured light show, but definitely not on human timescales!"

"In order to sustain this rate of supernova activity, massive, quickly evolving stars must form or be born at an equally rapid rate in NGC 6946," said Gemini North Associate Director, Jean-René Roy. "Its stars are exploding like a string of firecrackers!"

It is speculated that if just a million years of this galaxy's history were compressed into a time-lapse movie lasting a few seconds, there would be nearly constant outbursts of light as new stars flare into view, while old ones expire in spectacular explosions. Over the past century, eight supernovae have exploded in the arms of this stellar metropolis, occurring in 1917, 1939, 1948, 1968, 1969, 1980, 2002, and 2004.

"In order to sustain this rate of supernova activity, massive, quickly evolving stars must form or be born at an equally rapid rate. Stars in this galaxy are exploding like popcorn!" added Dr Johnson. But it is this extreme stellar birth-rate, not the supernovae, which lends the galaxy its blazingly colourful appearance.

By comparison, the average rate for such catastrophic stellar outbursts in the Milky Way is about one per century, and only four have been recorded over the last thousand years. The last known supernova went off in our galaxy in the constellation Ophiuchus in 1604.

Yet, it is the ubiquitous occurrence of starbirth throughout NGC 6946 and not its supernovae that lend this galaxy its blazingly colourful appearance. For reasons not completely understood, this galaxy experiences a much higher rate of star formation than all the large galaxies in our local neighbourhood. The prodigious output of stellar nurseries in this galactic neighbour eventually lead to accelerated numbers of supernova explosions.

Starbirth regions exist in most galaxies, particularly in spirals, and are obvious as clouds of predominantly hydrogen gas call H II regions. These areas coalesce over millions of years to form stars. Young, hot, massive stars formed in these regions emit copious amounts of ultraviolet radiation, which strips the electrons from hydrogen atoms in which they are embedded. When these ionized hydrogen atoms re-associate with electrons they radiate in a deep red colour (at a wavelength of 656.3 nanometres) as the electrons transition back to lower energy levels.

This Gemini image of NGC 6946 utilizes a selective filter specifically designed to detect the radiation emanating from the starbirth regions. Additional filters help to distinguish other details in the galaxy, including clusters of massive blue stars, dust lanes, and a yellowish core where older more evolved stars dominate.

NGC 6946 lies between 10 and 20 million light-years away in the constellation Cepheus, and was discovered by Sir William Herschel (1738-1822) on September 9, 1798. It continues to fascinate astronomers, who estimate that it contains about half as many stars as the Milky Way. They often use it to study and characterize the evolution of massive stars and the properties of interstellar gas. As viewed in the new Gemini optical image, we see only the "tip of the iceberg" of this galaxy. Its optical angular diameter is about 13 arcminutes, but viewed at radio wavelength at the frequency of neutral hydrogen (1420 Mhz or 21-cm line), it extends considerably more than the angular diameter of the Moon.

Source: PPARC