May 13 - The History and Future of Black Holes

Last Friday, NSF Fellow and future NASA Einstein Fellow Dan D' Orazio gave us an engaging tour to the long history of the most exciting and mysterious astrophysical objects. Black holes are objects with very strong gravitational pull, that nothing can escape from them. John Michell was the first to suggest the idea of such a "dark star", in the late 18th century, although this idea went almost unnoticed at the time.

The story continues in the beginning of the 20th century, when Albert Einstein conceived the general theory of relativity, a revolutionary theory to describe gravity. Dan described the early advances of the theory, including the first solution of Einstein's equations for a spherical non-spinning black hole by Karl Schwarzschild, and the skepticism that followed, mainly driven by the famous British astronomer Sir Arthur Eddington. Dan later discussed General Relativity's golden era (the 60s), with several significant contributions by many great physicists, among which the solution of Einstein's equations for a spinning black hole by Roy Kerr and the suggestion that under specific circumstances black hole can emit radiation, the so-called Hawking radiation. Dan also mentioned the history of the term black hole, which was coined in one of NASA's centers, above Tom's Restaurant in our own neighborhood.

By the early 70s, astronomers acquired the first observational evidence for astrophysical black holes with X-ray telescopes. For instance, the X-ray binary Cygnus X-1, in the constellation of Cygnus hosts one of the most nearby black holes. Earlier in the 1950s, astronomers had discovered radio galaxies with very extended jets, which later realized that can only be powered by supermassive black holes, i.e. black holes with a million to a billion times the mass of the sun. A supermassive black hole resides in the center of our galaxy as well and was discovered by detailed observations of the motion of stars in the close vicinity of the galactic center. Dan finished this exciting journey of discovery by describing the most recent breakthroughs, such as the numerical solutions of Einstein's equations 10 years ago and the first direct detection of gravitational waves of two merging black holes by Laser Interferometer Gravitational Observatory (LIGO), just 3 months ago. Even though, so far we had strong evidence for the existence of black holes, the LIGO event serves as the first direct confirmation of black holes.

Unfortunately, due to bad weather stargazing was not possible. However, the audience had a chance to join tours of the Rutherfurd Observatory on the roof of Pupin, lead by graduate students Steven Mohammed and Steph Douglas. Additionally, undergraduate student Richard Netherlander, guided the audience to the majesty of the cosmos through the projections of 3D movies, while, at the lecture hall, Andrew Emerick presented a mini-lecture on reionization and showed a mesmerizing movie about the end of Dark Ages.

-- Maria Charisi (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) 

November 8: Astronomer vs Astronomer

Scientists do not always agree with each other, and many times, scientific progress is made through debates. On Friday, November 8th, graduated student Jennifer Weston gave a public lecture about a few of the greatest arguments in astronomy in the past 150 years, and how they were resolved. One of the biggest debates in the history of astronomy and astrophysics, the 'Great Debate', was about whether the Milky Way, our own galaxy, was the entire universe, or there were other galaxies out there, seen as the "spiral nebulae". The debate was eventually resolved by the work of Edwin Hubble, which shows that the spiral nebulae are not only galaxies outside of our own, but they are also moving away from us at high speeds. 

After the lecture, many people went to the roof where graduate student Adrian Price-Whelan showed off the Moon and Albiero along with Andrew Emerick, Susan Clark, Aleksey Generozov, and David Jaimes. On the 13th floor, visitors enjoyed 3D astronomy movies with Barnard student Gladys Velez-Caicedo. In the lecture hall, graduate student Yuan Li presented a slide show on planets outside the solar system found by NASA's Kepler telescope. 

Join us on November 22 for a lecture by Jeremiah Murphy on supernova explosions.

-- Yuan Li (graduate student)