More than 250 million stars in the bulge at the heart of the Milky Way have been surveyed in near-ultraviolet, optical and near-infrared light for the first time, offering the chance to re-examine questions about the galaxy’s formation and history.
The work was carried out by a team of astronomers using the Dark Energy Camera (DECam) mounted on the Víctor M Blanco 4-metre Telescope at the Cerro Tololo Inter-American Observatory in Chile.
By detecting the ultraviolet light from what are known as Red Clump stars, they were able to analyse the chemical composition of over 70,000 stars over an area of sky 1000 times as large as the full Moon (an area larger than 20 x 10 degrees stretching over the constellations Sagittarius and Scorpius).
There were more than 7000 DECam exposures in all, comprising more than 3.5 trillion pixels.
A colour-composite showing a main part of these data is shown above, and can be explored in all its whopping 50,000 x 25,000 pixels in a zoomable version. In this image, interstellar dust and gas seemingly acts like a red “filter” in front of the background stars, scattering the blue light away.
The image below, also taken with DECam, shows a region near the centre of the Milky Way that covers 0.5 by 0.25 degrees on the sky (an area about twice as wide as the full Moon) and contains over 180,000 stars.
It shows that the stars near the very centre of the Milky Way have a similar composition, which suggests, the authors say, that they formed at around the same time.
Normally composition is measured with a spectrograph, targeting a relatively small number of stars at a time. The Blanco DECam Bulge Survey took a different approach, precisely measuring the stars’ brightness differences from ultraviolet to infrared wavelengths.
These differences in brightness at different wavelengths, called photometric colours, can reveal the composition of stars when the dataset is calibrated with stars measured spectroscopically.
Related reading: Rethinking the Milky Way’s evolution
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