Cristina Deptula on Dr. Jason Dexter’s lecture on black holes

Despite my childhood fancies, black holes aren’t about to scoop up the entire universe, or even everything in their paths. Dr. Jason Dexter of UC Berkeley demolished some common myths and provided some information about the mysterious objects during his volunteer enrichment talk at Oakland’s Chabot Space and Science Center.

Deep within the redwood forests of the Oakland Hills, the Chabot Space and Science Center welcomes museum guests and viewers for free Friday and Saturday night viewings through its three large telescopes. Used by professional astronomers, the two refracting and one reflecting scopes search out Mars, Venus, Saturn, and star clusters on the edge of our galaxy.

Chabot volunteers are encouraged to attend free science lectures, such as November 13th’s lecture from Dr. Dexter.

A black hole, according to Dexter, is simply an object in space so dense that even light that hits it cannot move fast enough to escape the pull of its gravity. Hence their name, since light does not reflect back to our eyes from black holes to allow us to see them. If our Sun became denser and exerted more gravity on our solar system, we wouldn’t necessarily get pulled inside it – just drawn closer. So, in the same way, plenty of black holes have stars stably orbiting them, at far enough distances not to be sucked inside.

Black holes result from the collapse of large stars, those greater than 25 times the mass of our sun. Stars 10-25 the sun’s mass become neutron stars, and those under ten times its mass turn into white dwarves. Also, giant black holes exist at the center of many galaxies, up to millions or billions the mass of the Sun. No one knows how or why these formed.

Astronomers and cosmologists can observe black holes by watching for changes in the velocities, orbits and accelerations of nearby stars. Also, material falling into a black hole forms a disc that gets very hot and glows as it crosses the edge, or event horizon. This enables us to ‘see’ black holes.

According to Dexter, “Black holes power the brightest objects in our universe. They’re extremely efficient at generating energy.”

For the rest of his talk, Dr. Dexter discussed current enhancements in our ability to observe black holes’ event horizons. Right now we view the effects of black holes through combined data from a global array of radio telescopes (California’s CARMA and observatories in Arizona and Hawaii). This group of scopes is known together as the Event Horizon Telescope. Researchers are building more telescopes around the world to add to this array, aiming to create sharper, higher-resolution images.

Scientists can locate black holes by noticing the Doppler Effect, where light appears to bend as it moves due to a large object’s gravity. Also, sometimes plasma jets of superheated matter escape from the glowing discs of particles about to enter a black hole. The Event Horizon Telescope has recently detected plasma jets from the area around the black hole at the center of the M87 galaxy, more than 50 million light years away from us. We had to look so far because the black hole at the center of our own galaxy appears too small to incite the formation of these plasma jets.

Dr. Dexter encouraged us to visit the website for the Event Horizon Telescope ( to learn more about how we observe these objects, and to see some unique photos and animations.

This enrichment brought out a larger crowd, and everyone seemed engaged, peppering the speaker with plenty of questions. Afterwards, people stayed to discuss what had been quite a valuable and informative experience for those with all levels of background in astronomy.

Cristina Deptula is a writer from San Leandro, California. She can be reached at