A black hole is a region in space from which nothing, not even light, can escape. It is the result of the deformation of space time caused by a very compact mass. Around a black hole is an event horizon, a surface that marks the point of no return. Until recent times, black holes were a mystery. Scientists knew they were there, but little else.
In the last two decades, researchers have come to understand that black holes influence what happens in the universe. When matter begins its descent into a black hole, it swirls like a tornado. That movement releases a huge amount of energy, both as radiation and as material outflows known as black hole winds.
“Our research shows that the black hole winds are powerful enough to stop the growth of galaxies,” Arav said. “At the same time, this process starves the black hole by stopping its own feeding, so the black hole doesn’t become too powerful and totally destroy the galaxy.”
Arav’s group is studying the ultraviolet and X-ray spectra of these winds. An example of a spectrum is a rainbow, which separates light into different colors. A scientific spectrum shows the intensity of light at each color, or more accurately, at every wavelength.
“Imprinted on the spectrum as dark bands, are the chemical fingerprints of the material we observe as well as its speed, temperature, and other physical information,” Arav said. “Studying the spectrum allows us to determine all these parameters for a wind emanating from a black hole billions of light years away from us.”
Arav employs undergraduate students to do data mining of quasar spectra produced by the Sloan Digital Sky Survey, the largest imaging and spectroscopic survey of galaxies and quasars. Students are given an enriching experience and learn how science is done while making crucial contributions to science by identifying objects for follow-up reserach. This frees Arav and his team to pursue other higher-level aspects of their research.
Analyzing spectral observations from the Hubble Space Telescope occupies most of the research time of Arav and his group, which includes Benoit Borguet of Saint-Remy, Belgium, who is doing postdoctoral work in physics, and Doug Edmonds, a graduate student in physics from Richmond, Va.
To observe on the Hubble is a coveted thing among scientists, Arav said, and the proposals for observing time are competitive.
Arav estimates each observing day on the Hubble is worth $1 million in total resources and $100,000 in research money to the university. “So the stakes are very high in this game.”
In the classroom, Arav is an activist of sorts, eager to take the geekiness out of the field of physics.
Arav teaches an Introduction to Astronomy class that underwent a significant revamping last fall. It was launched as the university’s first undergraduate signature experience, a unique learning environment that integrates a broad base of disciplines to enhance creative teaching and comprehension. The lecture course was expanded from 200 to 600 students.
“We have many students on campus who are fascinated by astronomy,” Arav said. “The desire for knowledge about it is very broad.”
As part of the class, a team of advanced undergraduates with an interest in astronomy serve as learning assistants, leading small study groups outside of lecture time, and students can provide immediate feedback during with “clickers,” an electronic assessment device that students use to submit answers to questions.
The small study groups increased the test scores over those of previous semesters. “An important aspect was that we offer a course the students want to take rather than a course they have to take,” Arav said. “ Many of them went outside of class, searched media, visited the university’s public observatory, and became engaged in astronomy.”
Nahum Arav discusses some of the celestial wonders he covers in his Introduction to Astronomy class.
There is no limit to how large a black hole can be. However, the largest black holes researchers think are in existence are at the centers of many galaxies and have masses equivalent to about a billion suns. Their radii would be a considerable fraction of the radius of our solar system.
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