Into the heart of the Whirlpool galaxy

The majestic swirling arms of a cosmic whirlpool

By BBC News Online science editor Dr David Whitehouse

New images taken by the Hubble Space Telescope (HST) have revealed remarkable new details in one of the most spectacular galaxies known, the so-called "Whirlpool" galaxy, also called M51.

Without doubt the Whirlpool galaxy is one of the most photogenic galaxies in the sky. This celestial beauty has been studied extensively by a range of large ground- and space-based observatories.

The HST images have allowed researchers to view in unprecedented detail the spiral arms and dust clouds of M51, which are the birth sites of massive and luminous stars.

This Hst image, a composite of several observations, shows starlight as well as light from glowing hydrogen, which is associated with the brightest young stars in the spiral arms.

Dust 'spurs'

M51, also known as NGC 5194, is having a close encounter with a nearby companion galaxy, NGC 5195, just off the edge of the latest HST image.

The companion's gravitational pull is causing turbulence in M51's gas and dust clouds triggering star formation in them, as can be seen by the many luminous clusters of bright, young, energetic stars. These bright clusters are highlighted in red by the light from associated glowing hydrogen gas.

Astronomers can see for the first time intricate structure in the dust clouds. Along the spiral arms, dust "spurs" can be discerned that branch out almost perpendicular to the main spiral arms. The regularity and large number of these features suggests to astronomers that discarded ideas about spiral galaxies may have some merit after all.

Dust disc

The new images also reveal a dust disc in the very core of the galaxy. This dust, astronomers speculate, may provide fuel for a central black hole.

Several groups of astronomers are also studying M51 at infrared wavelengths which can be observed by Hubble using special detectors.

At these wavelengths, the dusty clouds are more transparent and the true distribution of stars in the galaxy is seen more easily. In addition, regions of star formation that are obscured in the optical images are revealed.

The galaxy's massive core, the bright ball of light in the centre of the image, is about 80 light-years across (a light-year is 9.6 million million km or six million million miles) and has a brightness of about 100 million Suns.

Astronomers estimate that it is about 400 million years old and has a mass 40 million times greater than our Sun. This concentration of stars is about 5,000 times higher than in our solar neighbourhood.