“Many a night I saw the Pleiads, rising thro’ the mellow shade
Glitter like a swarm of fireflies tangled in a silver braid.”
– Tennyson, from Locksley Hall
The Pleiades, a small dipper-shaped cluster of young stars in the constellation Taurus, is one of a handful of celestial objects, along with the Moon and Saturn, that can turn casual observers into lifelong stargazers. A network of new and mostly icy-blue stars wrapped in faint nebulosity set against a sparse background of black sky, the cluster is visually stunning in binoculars or a wide-field telescope. Even for those who care little for stars, the Pleiades rivet the attention like few other sights in nature.
The Pleiades also rank as a favorite of astrophotographers. These stars are bright compared to most objects of the deep sky, so in the “old days” of film cameras they often overwhelmed the faint reflection nebula in which they are embedded. But digital cameras and processing techniques enable modern imagers to mask the bright stars and tease out an astonishing amount of detail in the surrounding nebulosity. Such is the case with a dazzling new image of the Pleiades by astrophotographers Terry Hancock and Robert Fields who combined multiple exposures collected with two small telescopes into a single, stunning, wide-field portrait of this famous galactic star cluster.
The light for this image of the Pleiades was collected with high-end CCD cameras and a Takahashi FSQ106 f/5 refractor and a nifty little Takahashi Epsilon-180ED astrograph at f/2.8. Robert Fields collected RGB of 6 x 10 min each for a total of 180 minutes, while Terry Hancock did the honors on luminance with 8×5 minute exposures for a total of 40 minutes. This is not a beginner image: these guys know what they’re doing.
The stars of the Pleiades formed about 100 million years ago and lie at a distance of about 440 light years. Most of the cluster’s 1,000 members remain on the main sequence, and the brightest and most massive stars shine with a blue-white light. Some of the largest stars have begun to evolve off the main sequence as they exhaust the hydrogen in their cores and swell into orange and red giants. A few are visible in the image above.
Astronomers once believed the Pleiades were embedded in the remains of a cloud of dust from which they formed. But the cluster is too old to retain such material. Instead, it turns out these stars are passing by chance through a thick cloud of fine interstellar dust which scatters their blue-white light. This nebulosity becomes readily visible in most images, but rarely do you see such intricate detail as in the image above by Hancock and Fields.
With some 220 minutes of light collection and extensive processing, this image reveals not only the blue-white reflection nebulosity of the Pleiades but also a surrounding faint lacework of grey-brown nebulosity. Some of this may be associated with the Pleiades. But much of it looks like the integrated flux nebula (IFN), an extremely faint glow caused by the combined light of the stars of the Milky Way reflected and re-emitted by interstellar gas and dust. Perhaps this smoky lacework, which is unrelated to the Pleiades itself, is the “mellow shade” in the above passage from Tennyson. He had no way of knowing about this phenomenon, which was discovered just 20 years ago, but his work so often resonates with modern readers.
This splendid image of the Pleiades by Robert Fields and Terry Hancock reminds me of another of my favorites of the Pleiades, one taken by Jack Newton many years ago and which has graced many textbooks and websites over the years.
The most fascinating aspect of this image, which Jack himself pointed out to me a few years ago, is not the Pleiades itself but what lies in the background. In the close-up of Jack’s image below, you see galaxies, dozens of them, a million or more times farther than the stars of the Pleiades itself.
As we’ve learned from the famous Hubble Deep Field image, the Ultra Deep Field, and the new Frontier Field images, if you look deep enough in space and far back enough in time, you will see galaxies. Even if you’re just looking at the dark sky between the stars of the Pleiades. These distant galaxies call to mind, perhaps, another prophetic passage from Tennyson’s Locksley Hall:
“For I dipt in to the future, far as human eye could see;
Saw the vision of the world, and all the wonder that would be.”