Probing the Chemistry of the Early Universe
13 January 1992Using a unique capability of NASA's Hubble Space Telescope (HST) astronomers announced today that they have detected the rare element boron in an ancient star. This element may be "fossil" evidence of energetic events which accompanied the birth of our Milky Way galaxy. An alternative possibility is that this rare element may be even older, dating from the birth of our universe. If so, then the HST findings may force some modification in theories of the Big Bang itself.
Dr. Douglas Duncan, of the Space Telescope Science Institute in Baltimore, MD, and Drs. David Lambert and Michael Lemke, of the University of Texas at Austin, are announcing their results today to a press conference at the 179th meeting of the American Astronomical Society in Atlanta.
The light from boron only appears in the ultraviolet part of the spectrum and so does not penetrate our atmosphere. "That's why no one was able to make this discovery before," says Dr. Duncan, "Having a powerful telescope high above the Earth's absorbing atmosphere has given us a new window on the universe. This was always considered to be one of the most important reasons for building the Space Telescope."
Using HST's Goddard High Resolution Spectrograph, the researchers detected traces of boron in a yellow 7th magnitude star called HD 140283, located 100 light-years away in the constellation Libra. At an estimated age of 15 billion years, the star is one of the oldest known. Because it was among the first stars to form in our Milky Way galaxy, HD 140283 should contain elements which were incorporated into the star long ago. Preserved in the star for billions of years, such material offers clues to conditions of the early universe when the star formed. Predictably, HD 140283 contains mostly primordial elements synthesized in the Big Bang: hydrogen, helium, and traces of lithium. (Heavier elements such as carbon, nitrogen, oxygen, and others which are found in the Sun, the Earth and Solar System planets are thought to have been built up during the lifetime of the galaxy by nuclear reactions in successive generations of stars). The discovery of boron comes as a surprise, however. Previously, another rare element, beryllium, had been detected in the star with ground-based telescopes.
The key question is: where did the beryllium and boron come from? Scientists know that today, beryllium and boron are produced by cosmic rays. Cosmic rays are high-speed and extremely energetic particles which occasionally collide with atoms in interstellar space and split them apart into lighter elements. If substantial amounts of beryllium and boron (the fourth and fifth lightest elements) were formed very rapidly early in the history of the Milky Way, swarms of energetic particles may have been present at the birth of the galaxy. The cosmic rays could have been produced by supernovas or other highly energetic events which occurred early in the life of the Milky Way.
However, the astronomers found slightly different relative proportions of beryllium and boron than what is expected from cosmic ray production. This offers the alternative possibility that beryllium and boron were synthesized in the first moments of the universe's creation.
The currently accepted version of the Big Bang says that the early universe was uniformly hot and dense. However, more recent theories suggest that the Big Bang developed some structure even during the first few minutes. These new theories differ from the traditional one in predicting that small but detectable amounts of beryllium and boron might be created.
To confirm these results, the astronomers plan additional HST observations of an even older star later this year. If the boron was produced by cosmic rays within the young Milky Way it should diminish the farther back in time the astronomers look (hence closer the birth of the Galaxy). If, instead, they find the same amount of boron in the older star, rather than less, the finding will support the alternative explanation that boron was produced in the Big Bang.
"Either way, this will be an exciting test to show which of the possible explanations is correct," concludes Duncan, "We know that our picture of the beginning of the galaxy and the beginning of the universe is undoubtedly oversimplified, and it is satisfying to be able to add a little more detail."