The Rock Stars of The Universe

Chris DePreeLook up in the sky on a clear night, and Agnes Scott astronomer Christopher De Pree will point out a few bright stars and many smaller and less vivid ones. Many of the brightest stars are called high mass stars. They are important in the universe, he will tell you, as they help form smaller stars like the sun and shape the evolution of galaxies. Their creation was thought to be a neat, orderly process, but as De Pree recently discovered, the formation of high mass stars is more complicated than scientists first thought.

Stars are formed from collapsing clouds of gas, and when a high mass star starts producing energy, it ionizes surrounding hydrogen, and the area around the star begins to shine. Astronomers previously believed that the regions of ionized gas around high mass stars, once formed, would not change rapidly in brightness. The hydrogen gas was thought to form a sphere around the star, quickly expand, and then maintain a relatively constant level of brightness for thousands of years.

“High mass stars are the rock stars of space,” says De Pree, the Charles A. Dana Professor of Astronomy and director of Agnes Scott’s Bradley Observatory. “They live fast and hard. Their short life span of only 10 million years compared to smaller stars that can live for up to 100 billion years makes them harder to study. They have a bigger ‘fuel tank,’ but they burn through it more quickly. When they die, they blow up and throw material out into space. But astronomers thought that as massive stars were created, the clouds of ionized gas around them would not change in brightness. We thought if we looked at these same stars over decades that they would look the same.”

But De Pree and an international team of scientists found that some massive stars continue to gravitationally attract material as they ionize gas. As a result, hot gas can flicker in brightness as material falls toward the star. Some of the regions grow brighter, others fainter, but eventually the ionized gas fades away and leaves behind a high mass star.  

Of the 50 high mass stars with small regions of ionized gas that were observed, four flickered. “In the past, everyone would have thought that zero out of 50 would have changed,” De Pree says.

The changes were observed over 23 years, and the researchers’ findings were published in The Astrophysical Journal Letters in January. De Pree observed the regions in 1989 and 2012 with the Karl Jansky Very Large Array, a set of 27 telescopes in New Mexico that can function as a single telescope.


Christopher De Pree’s new book, Idiot’s Guides: The Cosmos,
is an easy-to-understand exploration of the solar system, the sun,
the asteroid belt, stars, the Milky Way, black holes and more.
The Cosmos is his sixth book on astronomy written for the general reader.