This is the Whirlpool Galaxy, or M51, a famous and spectacular example of a spiral galaxy. It’s probably most well known for its picture taken by the Hubble telescope, but it has another claim to fame: it was the first ever galaxy identified as a spiral, in 1845.
Lord Rosse's 1845 sketch of the Whirlpool Galaxy
That may sound surprisingly late, but it’s not really: looked at from far away - the Whirlpool is about 31 million light years distant - and with a small telescope, even the most complicated structure is smeared out into an ellipsoidal blur. It takes a high resolution to see the kind of pretty images we’re used to from Hubble, even with the advantage of not having to look through the turbulent atmosphere that putting your cameras in space brings!
The first telescope to resolve the spiral structure of the Whirlpool was impressive in its own right: the ‘Leviathan of Parsontown‘, built and operated by William Parsons, 3rd Earl of Rosse. Its tube was over 50 feet long and made it the largest telescope in the world until 1917. That’s only a little bigger than Hubble - which, as NASA helpfully says, is about the size of a bus. Rosse used it to identify the spiral structure of the Whirlpool in 1845, and made the above sketch of his findings. Back then galaxies were believed to be much smaller clouds within our own galaxy rather than structures of comparable size. It wasn’t until 1917 that the “island universe” hypothesis was proposed - the idea that galaxies are concentrated clumps of stars and matter, separated by empty tracts, and that we live in just one of them.
The 'Leviathan of Parsontown', largest telescope in the world until 1917
As well as the spiral structure, Rosse’s sketch shows a small object to the right of M51. That’s M51a, a companion galaxy to the Whirlpool. It’s believed that this much smaller galaxy is in the process of merging with its neighbour: it’s already made a couple of passes through the spiral disc over the last half a billion years or so, and probably has several more to go before it settles down as an intrinsic part of M51. These passes are responsible for drawing out the long spiral arms of the larger galaxy: you can see the rightmost arm reaching out to the companion that’s in the process of forming it.
Nowadays we have much more powerful and accurate telescopes, thanks largely to the electronics which process the data and the machine-aided construction of mirrors and other components to a high degree of precision.
The Whirlpool and its companion, taken by Hubble. Credit: NASA, ESA. Click to enlarge.
But we also have the ability to observe different wavelengths of light, some of them well outside the visible range which Rosse and other astronomers of his day were limited to. Here’s a comparison between three views of M51 in three different wavelengths. The first is Hubble’s visible light image again, while the other two are in the infrared: one from the Spitzer telescope and one from Herschel - in fact, this was the first picture Herschel took after its launch last week!
Whirlpool Galaxy in visible and IR light
The reason I’ve included two IR images (apart from to please my office-mate who’s been bouncing around with excitement about getting new data from Herschel) is that they’re from different parts of the infrared spectrum. Spitzer looks at near infrared light, just beyond the end of the visible spectrum, while Herschel is most sensitive to the far infrared futher down the spectrum.
Herschel's 'first light' image of M51 in the far-infrared. Credit: ESA & PACS Consortium.
So what’s the explanation for the differences between these pictures? The answer lies in what types of object emits most at each wavelength. In visible light you’re seeing mostly stars, which are hot enough to glow visibly. The far IR on the other hand is dominated by light from warm dust clouds much cooler than stars (which makes sense since even people ‘glow’ in the infrared, but it takes a lot of heat for something to start glowing red- or white-hot!). These dust clouds are often associated with nurseries where new stars are being born. So it looks like M51 is undergoing a period of star formation, while its companion (which almost disappears in the Herschel image) is much more sedentary and has mainly old stars formed at some time in the past.
Finally here’s the galaxy in X-rays, showing a very different picture indeed. X-rays are emitted wherever clouds of extremely hot (tens or hundreds of millions of degrees) gas are found, and you can see patches of it in the centre of both galaxies. In M51 this is partly due to the shockwave of a supernova which was detected there in 1994: the explosive death of a massive star released a blast of gas which is still spreading through the galaxy, heating up gas clouds as it hits them and stimulating X-ray emission.
Another source of X-ray light is ‘X-ray binaries’: these are neutron stars or black holes in the process of consuming a nearby star, and spitting out vast amounts of energy released as X-rays. A lot of the little points of light you can see in the outer parts of the X-ray image are from these.
The Whirlpool in X-rays, with the same orientation as the above picture. Credit: NASA/CXC/UMd./A.Wilson et al.
So that’s the Whirpool: from a cloudy blob somewhere in our galaxy to a new kind of spiral structure and then an ‘island universe’ of stars, dust, gas and black holes - thanks to pictures from five telescopes spanning a hundred a seventy years.
Further reading
The Wikipedia page has, among other things, information on the size of the Whirlpool and how to find it yourself with a pair of binoculars or small telescope.
The Chandra site has a gallery of some images of the galaxy in other wavelengths here.
Here’s an article on the development of the ‘island universe’ idea of other galaxies.
And here’s a page on X-ray binaries.
Edited on the 27th of June to add Herschel image.
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