The Whirlpool Galaxy


The spectacular spiral galaxy M51 is shown in its glorious infrared colours. Two huge waves of star formation encircle its central nucleus, making beautiful spiral arms. Each one shines brightly with its dust being warmed by the young stars.

This image shows the Whirlpool Galaxy, 23 million light years away in the constellation Canes Venatici. It was obtained with the Photodetector Array Camera and Spectrometer (PACS) aboard ESA’s Herschel Space Telescope.

This galaxy was first catalogued by Charles Messier in 1773, who placed it as item 51 on his list of cloud-like celestial objects. However, Messier’s telescope only saw the galaxy’s central portion as a faint smudge. It was not until 1845 that the great spiral pattern was seen. This discovery was made using the Leviathan 72-inch (1.82 metre) mirror telescope at Birr Castle, Ireland. At the time Leviathan was the largest telescope in the world. Herschel has a mirror measuring 3.5 metres across, about twice the size of Leviathan and it is in space.
This image shows the giant waves of star formation that circle the Whirlpool Galaxy, creating the spiral arms. Spiral galaxies are the star forming factories of the Universe, with the stars born in their spiral arms then heating up the surrounding dust causing it to release infrared radiation. That dust shows up in these images, with the blue spots indicating the sites of the most concentrated star formation.

The image is false-coloured with the shortest wavelengths of infrared being coloured blue, and the longest red. The shortest wavelength is 70 micrometres (one hundred times longer than the longest visible wavelength of light). The longest wavelength in this image is 160 micrometres.

The bright blue dot at the top of the image is the centre of a smaller galaxy that is passing by the larger. Its gravitational field has slightly deformed the spiral shape of the Whirlpool Galaxy. Had this smaller galaxy passed any closer to the centre of M51, it would have destroyed the delicate spiral shape of the larger galaxy altogether.

This image was taken on 14 June 2009, when Herschel opened its infrared ‘eyes’ to the Universe for the very first time. Even before the instruments were finely tuned, it revealed that Herschel was working extremely well, and gave a hint of the extraordinary images to come.

Credits: ESA and the PACS consortium


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