Gaia's View Of Dark Interstellar Clouds

While charting the positions of more than a billion stars, ESA's Gaia mission provides all-important information even about the dark patches of the sky where fewer stars are observed. These images, based on Gaia's first data release, are an appetizer to the astronomical riches that will be unleashed with the mission's second release on 25 April.

The constellation of Orion, the Hunter, is not only home to the bright stars that make it an iconic portion of the sky, but also to a vast star-forming complex of cosmic gas and dust: the Orion Molecular Cloud, located some 1500 light years away. This massive stellar nursery is largely hidden to the naked eye with only one bright patch visible - the Orion Nebula, or M42.

These two images show the structure of a dark cloud in the Orion A portion of this large star-forming region. They are based on the first release of Gaia's data, which were collected by the satellite during its first 14 months of operations and published in 2016.

The view on the left was compiled by mapping the total density of stars detected by Gaia in each pixel of the image. It reveals the distribution of all stars in the area, and clearly outlines the silhouette of a dark cloud of gas and dust hiding background stars from view.

Various southern hemisphere civilisations identify shapes of creatures from the animal world in the dark lanes crossing the bright background of the Milky Way. Echoing this time-honoured tradition, astronomers have spotted the shape of a cat - or that of a fox - in this cloud, depending on whether the bright spot just right of centre is viewed as a nose or an eye. In fact, the cat's nose (or fox's eye) corresponds to the Orion Nebula Cluster, a young open cluster near M42. Another open cluster - NGC 1981 - and the reflection nebula NGC 1977 lie to its upper right.

The image on the right provides a complementary view of the same region. Obtained by mapping the total amount of radiation, or flux, recorded by Gaia in each pixel of the image, it is dominated by the brightest, most massive stars. In some spots, these stars outshine their less bright, lower-mass counterparts.

The Orion A nebula can be seen towards the right, just below the bright horizontal band of the Galactic plane, in this all-sky view of the total flux based on Gaia's first data release.

A team of astronomers using Gaia data have studied the three-dimensional distribution of stars in the Orion complex, including the area depicted in these images, revealing distinct groupings of stars with different ages.

The first data release, published in 2016, contained the position on the sky of more than one billion stars, as well as the distance and proper motion of about two million stars. Gaia's second release, planned for 25 April, will include the distance and proper motion for all catalogued stars. This will enable astronomers to explore much farther away and to investigate in great detail star-forming regions like the Orion Molecular Cloud.

Besides studying the distribution of stars, the data will also help astronomers to reconstruct the three-dimensional structure of the dusty dark clouds where stars are born.

Another dark cloud in our Milky Way, part of the Rho Ophiuchi complex, is portrayed in the two images above. This large stellar nursery is located about 440 light-years from us, in the constellation Ophiuchus, the Serpent Bearer, and can be seen just above the Galactic Centre in the all-sky view.

These images are also based on data from the first 14 months of Gaia science operations, and display the total density of stars (left) and the total flux (right) measured by the satellite. Five bright stellar clusters stand out in both views: the brightest one, towards the right of the frame, is the globular cluster M4.

Not far from Rho Ophiuchi in the sky, on the opposite side of the same constellation, is another interesting dark cloud known as Barnard 68.

These images are also based on the first Gaia data release and display the density of stars (left) and the flux (right).

There are no stars observed in this dense nebula, which is thought to be a stellar nursery cooking up stars. Data from Gaia's future releases might help astronomers study the three-dimensional structure of this cloud by measuring distances to the stars near its edges.

Related Links
Gaia at ESA
Stellar Chemistry, The Universe And All Within It

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