Galaxies Are the Ultimate Recyclers
Galaxies learned to "go green" early in the history of the universe, continuously recycling immense volumes of hydrogen gas and heavy elements to build successive generations of stars stretching over billions of years.
This ongoing recycling keeps galaxies from emptying their "fuel tanks" and therefore stretches out their star-forming epoch to 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-birth 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.
This invisible mass is made up of normal matter--hydrogen, helium, and heavier elements such as carbon, oxygen, nitrogen, and neon--as opposed to dark matter that is an unknown exotic particle pervading space. The results were published in three papers in Science magazine.
The color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. …