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News Release 619 of 960

November 24, 1999 02:00 PM (EST)

News Release Number: STScI-1999-46

A Puzzle of Galactic Evolution is Solved — Massive Gas Clouds Seed the Galaxy with the Stuff of Stars

November 24, 1999: Massive clouds of gas, discovered long ago but only recently identified as being within the margins of the Milky Way, play a key role in the galaxy's ability to churn out new stars by raining gas onto the plane of the galaxy. Researchers have chipped away at a three-decade-old mystery about the nature and role of those massive gas clouds, called high-velocity clouds. In the process, they've discovered a mechanism by which the galaxy is seeded with the stuff of stars and solved a long-standing question of galactic evolution.

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Q & A: Understanding the Discovery

  1. 1. What are high-velocity clouds?


  2. Discovered 35 years ago, these speedy clouds behave differently than most galactic objects - coursing through space at high speeds and not neatly rotating along with the rest of the galaxy. Scientists could never pinpoint their exact location, with distance estimates ranging from a few hundred light-years to 10 million light-years, an estimate that would place them well beyond the pale of the Milky Way. But improved instrumentation, such as new large ground-based telescopes and the orbiting Hubble telescope, recently allowed astronomers to determine that one such cloud lies about 20,000 light-years from Earth in the halo of the Milky Way, a region high above the star-studded plane of the galaxy. This composite radio light image and rendition of our galaxy as seen in visible light shows the enigmatic "high-velocity clouds" of gas high above the plane of the Milky Way.

  3. 2. Why are these clouds so important?


  4. After measuring distances to some of these speedy clouds, astronomers decided to probe their chemical composition. They discovered that the clouds contain the simple chemical elements, such as hydrogen and helium, necessary for star creation. The clouds are not keeping these elements to themselves: They're seeding the galaxy with them, which stokes star birth and counteracts a buildup of heavy elements within the stars and gas found in the disk of the Milky Way. In this way, the clouds play a key role in the chemical evolution of the galaxy. Without them, the Milky Way would very slowly use up its batch of hydrogen and helium through star birth.

  5. 3. What is chemical evolution?


  6. When the universe was born, only the lighter, simpler chemical elements existed. Clouds composed of these elements condensed, igniting a spate of star birth. These stars created the heavier elements or metals, such as carbon and oxygen, and released them into space when they died. The heavy elements are important for planet formation, the lighter elements for star creation. So the universe began building up the heavier elements. This scenario suggests that recently formed stars should be richer in metals than old stars. Yet astronomers observe that most stars in the disk of the Milky Way have similar heavy element concentrations no matter how old or young they are. The Hubble observations show that metal production by stars is offset by an influx of hydrogen and helium.

 
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Photo Credit: Image composite by Ingrid Kallick of Possible Designs, Madison Wisconsin. The background Milky Way image is a drawing made at Lund Observatory. High-velocity clouds are from the survey done at Dwingeloo Observatory (Hulsbosch & Wakker, 1988).