Dec 4 - On the Care and Feeding of Black Holes

Aleksey Generozov, a 4th year PhD student in the astronomy department, introduced us to the majesty of black holes. Black holes are regions in space-time, where gravity is so strong that nothing (particles or even light) can escape from inside; we could make a black hole if we squeezed the entire earth into the size of our fingernails.

Aleksey next described the discovery of a supermassive black hole at the center of our galaxy, the Milky Way. Observing the orbits of stars very close to the galactic center, with very high precision, astronomers concluded that an object very massive (with mass a few million times the mass of the sun) and very compact (that would fit in a sphere with radius smaller than the radius of our solar system) must be hidden in the galactic center. The only feasible explanation was a huge black hole. These days, we are actually on the verge of seeing the radius of the black hole (the event horizon), with a network of radio telescopes positioned around the globe.

And although the black hole at the center of our galaxy is relatively quiet, this is not the case for every galaxy. In the '60s, astronomers discovered some very bright radio sources, equally bright as some nearby stars, which they named quasars (quasi-stars). Later, when the size and distance of these objects were measured, it was realized that the enormous amount of energy is produced when gas falls onto a supermassive black holes at the center of a very distant galaxy. The gas is brought to the central black holes, when two galaxies collide. This also explains why our own supermassive black hole is quiet; It's starving! However, there is some evidence from observations of very high energy photons, that our own black hole had a more spectacular past. The talk concluded with Aleksey and the audience singing along "twinkle-twinkle quasi-star".

After the lecture, the audience had the chance to hear about the K2 mission, which is the second phase of NASA's Kepler space telescope, lead by graduate student Stephanie Douglas. Moreover, they had a chance to further explore galaxies and black holes by watching 3D movies, guided by undergraduate Briley Lewis. The clear sky and the mild temperature gave us a wonderful opportunity for stargazing on Pupin's roof with the help of Adrian Price-Whelan, Emily Stanford, Daniel De Felippis, Rasmi Elasmar, and Richard H Nederlander. Attendees were treated to views of the Andromeda Galaxy, the Pleiades star cluster, and the Red Giant Betelgeuse.

-- Maria Charisi (graduate student)

Apr 24: Galactic Synthesizers

Tonight Adrian Price-Whelan, Columbia graduate student and National Science Foundation fellow, showed us how he has used simulated galaxies to compose original music. First, he had to describe the basic shapes of galaxies an how stars move inside them since the motions of stars are the basis of his method. Starting with spiral galaxies, we heard about their three main components: a flattened disk of stars moving in orderly, almost circular orbits around the galactic center, a spheroidal bulge at the center of the disk where stars move in every direction with nearly random orientation, and an outer halo which surrounds the disk and spheroid, spanning an enormous volume but containing only 1% of the stars. The shapes of these spiral galaxies were contrasted with that of ellipticals, which are generally more "bloby" and whose stars move mostly with random orientations which are quite unlike the most prominent largest components of spiral galaxies, the disks themselves. Despite their relative featurelessness, ellipticals are typically triaxial - that is, they have different extents in different directions, which leads to a variety of orbits with different shapes including long-axis tubes, short-axis tubes, and box orbits. 

To turn galaxies into synthesizers, Adrian built a computer model of a galaxy of each type and populated them with stars. Each star moves with specific frequencies, both in and out from the center of the galaxy and around it; these frequencies were mapped onto scales, with low notes corresponding to short-axis tube orbits of the elliptical galaxies and halo stars of spiral galaxy, mid-tones made by the disk and long-axis tube stars, and high notes produced by box orbits and stars in the spheroid. As the simulation advances the stars ring their notes in a manner similar to plucking a guitar string, while encoding actual physical information about the shape of the galaxy they orbit in to sound. You can listen to his compositions over on Soundcloud: spiral galaxies, elliptical galaxies.

Following Adrian's talk, Columbia graduate student Emily Sandford presented some slides describing variety of Earth-Sun-Moon arrangements and how they lead to solar and lunar eclipses, which then smoothly transitioned into a discussion of the Kepler space telescope since it uses a very similar principle to find planets around other stars - it precisely measures the dimming caused by an extra-solar planet passing between its host star and the Earth. Upstairs, undergraduate Gladys Velez-Caidedo and post-doctorate researcher Shy Genel ran the 3D wall, illustrating a wide variety of astronomical phenomena.

-- David Hendel (graduate student)

Oct 10: Explosive Lighthouses

This week, Columbia Astronomy's 4th year graduate student, Maria Charisi, gave a talk on gamma-ray bursts, or GRB for short. Gamma-rays are a kind of electromagnetic wave, like optical light, but with very short wavelengths, even shorter that of X-rays.

Maria started by telling us the history of how GRBs were discovered. The first GRBs were found by detectors which were built to detect nuclear explosion on the Earth during the Cold War. However, scientists found no correlation between these events and any nuclear explosions on the Earth. So this problem was passed assigned to astronomers to find out if they were related to any astronomical events. At first theorists proposed many mechanisms for GRBs, from neutron star collisions to alien space wars. Astronomers also argued about the location of these events. Some believed they were galactic; others believed they were cosmological. The varieties of theories and beliefs were due to the fact that the location distribution and distance of the GRB events were still unknown. With the launch of the Compton Gamma-Ray Observatory in the mid 90's and the Swift Gamma-Ray Burst Mission in the early 21st century, and with the red-shifts measured from spectra, astronomers finally concluded that GRBs are extremely luminous events that happen all over the Universe. 

Maria then described findings from more recent studies on GRBs. GRBs only last for a short time in the gamma-ray band, but astronomers believed there should be afterglows in other wavelengths, just like charcoals will glow as a fire burns out. They did follow-up observations of GRBs in the optical and infrared bands and found evidence of these afterglows. From the duration distribution of the bursts, astronomers split the GRBs into two different categories: short bursts which lasts less than 100 seconds, and long bursts which last a few hundreds seconds. 

The physical image of GRBs is still not clear today. One of the most widely believed theories is that when two compact objects collide, for example a neutron star and a black hole, tremendous energy is produced and ejected from two jets. If we happened to be in the line of sight of these jets, we would detect a GRB.

After the lecture, graduate student Jingjing Chen showed the first half of a PBS movie called 'Alien Planets Revealed' which discussed transit method of detecting exoplanets used by the Kepler mission. Unfortunately, the weather was not good for stargazing, so graduate students Jeff Andrews and Aleksey Generozov gave tours of the Rutherfurd Observatory instead while undergraduate Erin Flowers and post-doc Robyn Sanderson presented a variety of astronomical phenomena with the 3D wall. 

-- Jingjing Chen (graduate student) 

April 25: The Exoplanet Revolution

Less than ten years ago, astronomers only knew about a handful of planets orbiting stars besides our Sun, and all of those are nearby.  But the Kepler spacecraft has changed all that; Kepler scientist Lucianne Walkowicz told us how the telescope has revolutionized exoplanet studies.  Kepler spent 4 years staring at one specific patch of the sky, looking for planets around stars near and far.  Kepler exponentially increased the number of known exoplanets, and found many that are thousands of light years away.

Kepler searches for planets by watching for slight brightness changes caused by a planet traveling in front of its host star. This method easily detects large planets that are close to their host star, but Kepler has also found a bunch of smaller planets (between Earth-size and Neptune size) orbiting at larger distances. Although the larger planets are easier to find, it looks like planets a little bigger than Earth may actually be the most common type of planet in the galaxy! Lucianne told us that although Kepler's main mission has ended due to a mechanical failure, the telescope will likely keep operating in a new observing mode to search for exoplanets around even more stars.

Clouds prevented any roof activities, but volunteers Jennifer Weston and David Hendel provided tours of the observatory and volunteer Claire Ding showed movies on the 3D wall.  Volunteer Alejandro Nunez also gave a short presentation in the lecture hall about the electromagnetic spectrum.

-- Steph Douglas (graduate student)