Galaxies have been "green" since the universe existed. They continuously recycle immense volumes of hydrogen gas and heavy elements to build successive generations of stars that stretch over billions of years. Their recycling allows them to maintain their star-forming abillity for over 10 billion years. However, galaxies that ignite a rapid firestorm of star birth can blow away their remaining "fuel," essentially turning off further star-creating activity.
This conclusion is based on a series of Hubble Space Telescope observations that flexed the special capabilities of its comparatively new Cosmic Origins Spectrograph (COS) to detect otherwise invisible mass in the halo of our Milky Way and a sample of more than 40 other galaxies. Data from large ground-based telescopes in Hawaii, Arizona, and Chile also contributed to the studies by measuring the properties of the galaxies.
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