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Dark Matters

Julia Kennefick, Assistant Professor, Department of Physics

The year 2009 has been designated the International Year of Astronomy to celebrate the 400th anniversary of Galileo’s first viewing of the heavens with a telescope. He was the first to notice that the Milky Way, the streaky band across the night sky, contains thousands of individual stars and suggested that the stars could be further away than anyone had yet imagined.

By the early 20th century, scientists determined the composition of stars and found how stars produce their energy. They also explored how stars form and die, and are recycled into new stars in the Milky Way galaxy.

The pressing question in early 1920s astronomy, however, was: How big is our galaxy and are there others like it? Once astronomers began using telescopes, they saw small “cloudy” objects dotted around the heavens. They named these objects “nebulae.” But were these objects in our own galaxy, or did galaxies outside our own?

In 1920, astronomers debated this. Harlow Shapley argued that our galaxy is large, and any nebulae observed could be contained within it. Heber Curtis argued that the Milky Way is small and the nebulae could lie beyond its boundaries.

Edwin Hubble later ended the debate by determining the distance to the Andromeda nebula using a special class of star that he could resolve in the outer reaches of that galaxy. The distance proved to be far greater than the large size suggested, correctly, by Shapley for our own galaxy. Shapley and Curtis both proved to be correct: the Milky Way galaxy is large, and more galaxies exist beyond it.

While we now know that there are over 100 billion galaxies in the observable universe, we still lack a deep understanding of how galaxies form, evolve with time and end their lives. The situation bears an analogy with the state of astronomy at the beginning of the 20th century. Just as the 20th century saw the birth of a complete and sophisticated science of the star, so the 21st century is shaping up to be the century in which we come to know and understand galaxies.

The first clue that galaxies, like stars, evolve with time came in the 1960s with the discovery of quasars, the energetic centers of some galaxies. Theorists began to study the stages of advanced gravitational collapse, which gave birth to the notion of black holes, now thought to power quasars through matter falling into them. Astronomers have evidence that most well formed galaxies have a black hole in their centers.

Astronomers also have observed correlations between the size of the supermassive black hole and certain features of their host galaxies. One possible link between them is the dark matter halo, which astronomers believe exists in all galaxies and which probably played a role in their evolution.

In the 21st century, astronomy has gone beyond starlight to study another type of mass in the universe -- black holes and dark matter. This mass seems more plentiful than the shining mass, and may dominate how galaxies have formed throughout the lifetime of the universe.

Julia Kennefick
Assistant Professor, Department of Physics