Oct 27 - Cosmic Mergers & Acquisitions: Mergers and Black Holes and the Growth of Galaxies

Our speaker this week was Dr. Jenny Greene, a Professor of Astrophysics at Princeton University.

Much of Jenny’s research concerns some of the most mind-boggling objects in the Universe: the supermassive black holes that exit at the centers of most galaxies. Millions to billions of times more massive than the Sun, these huge black holes are thought to play a crucial role in the development of galaxies.

Jenny began by reviewing what a black hole is: an incredibly dense object. She used the visual of compressing the Sun to the size of Jupiter, then the Earth, then Manhattan; as it gets smaller, the speed you need to escape its gravity gets larger and larger until it exceeds the speed of light. Since nothing moves faster than that, nothing can escape from a black hole.

Even though we can’t see black holes directly, Jenny showed us evidence of the supermassive black hole in the center of our Galaxy. Careful observations of the Galactic center reveal stars moving at phenomenal speeds. Measuring their positions over decades, astronomers can reconstruct their motions and show that they must be orbiting an incredibly dense but invisible object: a black hole.

Astronomers can’t directly see stars moving in other galaxies but their collective motions can be measured very precisely and it is clear that almost all galaxies require a supermassive black hole in their centers to explain these observations. Interestingly, it seems like the black holes’ sizes ‘know about’ the galaxy that they live in: the mass of the black hole can be predicted if the mass of the galaxy is known. How is this possible? Galaxies grow mostly by merging with other galaxies. This process brings in lots of new stars and gas. The gas both forms new stars and feeds the black hole, but as the black hole eats it forms an ‘accretion disk’ of very hot gas that heats up the remaining gas, eventually causing both star formation and its own growth to stop. In this way the size of the galaxy and the size of the black hole can become related.

If this picture is correct we should also see evidence of pairs of black holes, since each galaxy should have brought one along with it. Jenny described one way she and her students have been looking for these binary black holes. When they get close together, they will be orbiting each other at very high speeds — probably thousands of kilometers per second. The light from their accretion disks will be shifted to redder and bluer colors periodically due to the Doppler shift; if we can observe these color-changing black holes it would be good evidence in favor of our cosmological picture. So far the observations haven’t found anything but astronomers are still looking!





-- David Hendel (graduate student)

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)

Feb 19 - Ripples in Spacetime

A record crowd packed into the lecture hall tonight to hear Jillian Bellovary talk about gravitational waves and how to detect them. Jillian's research focuses on supercomputer simulations of supermassive black hole binaries, but her presentation focused on much less massive binaries like those whose merger was detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in September. Gravitational waves are very small distortions in spacetime caused when massive objects are accelerated to high speeds. The distortions measured by LIGO were smaller than the radius of a proton -- and those were caused by two ~30 solar mass black holes merging together. Jillian showed simulations of how these ripples in spacetime are created by the two merging black holes.

Jillian also described how LIGO was able to detect these tiny distortions. LIGO is an interferometer, meaning it measures how light waves interfere with each other (either adding together or canceling each other out) after they travel a very long distance in two perpendicular directions. How the interference pattern changes with time tells LIGO scientists how the distance is changing in each direction. If the "arms" of the interferometer lengthen and compress in a particular pattern, then they know they've detected a gravitational wave!

The sky was cloudy, but graduate students Adrian Price-Whelan and Lauren Corlies, along with undergraduates Amanda Quirk and Cierra Coughlin, showed off the Rutherfurd Observatory. Meanwhile, undergraduates Richard Nederlander and Tze Goh screened short films on the 3D Wall. In the lecture hall, volunteers Stephanie Douglas, Maria Charisi, and Danielle Rowland played videos from PhD comics and from members of the LIGO team at CalTech, and answered audience questions about gravitational waves.

--Stephanie Douglas (graduate student)

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)

Oct 2: The Glorious Past of Our Monster Black Hole

Last week, physics graduate student and NASA fellow, Shuo Zhang, introduced us to the monstrous black hole in the center of our galaxy. It is well established that every massive galaxy hosts a supermassive black hole in its center and Milky Way is not an exception. Astronomers analyzing the orbits of stars in the galactic center, enabled by advancements in observing technology, realized that the only feasible scenario for the dark object in the center is the galaxy is a black hole with mass of a few million times the mass of the sun.

The black hole is known as Sagittarius A-star (SgrA*), since it is located near the border of the constellations Sagittarius and Scorpius. Being the closest supermassive black hole we can observe, SgrA* is of great astronomical interest. But is it a typical black hole? SgrA*, is one of the quietest supermassive black holes among all the nearby galaxies. However, that wasn't always the case. Recently, the Fermi Gamma-ray Space Telescope discovered a bubble of gamma-rays surrounding our galaxy, which indicate that 2 million years ago, the black hole was very active, swallowing gas and expelling in at large distances. 

Shuo also mentioned her own work observing the black hole at the center of the galaxy with an X-ray telescope called NuSTAR (a NASA mission, in which Shuo's research group here at Columbia was a major contributor). Shuo observes sporadic X-ray flares, which last for a few seconds. During these flares the black hole becomes hundreds of times brighter and then returns to its quiet state. The origin of the flares is still unknown and it has been suggested that we observe the heated remnants of an asteroid that was ripped apart by SgrA*.

Unfortunately, the weather did not cooperate to allow stargazing. Whoever stayed after the lecture had a chance to see 3D movies of the galaxy lead by undergraduate student Richard Nederlander, or hear a short presentation by graduate student Emily Sanford, about the exciting discovery of water on the surface of Mars.

-- Maria Charisi (graduate student)

Nov 7: Juggling with Black Holes

This week, Hubble Postdoctoral Fellow Andreas Kupper described the dynamics of multiple massive objects in the same system.  When two massive objects are near each other, they exert gravitational forces on each other and their motions can be described analytically - that is, the motions at all times can be calculated by hand from basic theories.  When three or more massive objects interact gravitationally, the interactions become complicated and can only be modeled using computer simulations. Andreas showed the results of a few of his simulations - after the objects orbit chaotically around each other for a while, one object (usually the one with the lowest mass) is thrown out of the system with a very high velocity.  The remaining objects end up closer together, and this process, called scattering, is thought to be responsible for bringing massive objects like black holes and neutron stars close enough together that they can merge into an even more massive object. 

After the lecture, audiences were treated to a showing of LIGO: A Passion for Understanding, a 20 minute film about a project to detect gravitational waves coming from closely interacting massive objects.  After the film, graduate student Aleksey Generozov and I answered questions about gravitational waves and LIGO.   Graduate student Yong Zheng was the roof captain, sharing glimpses of the Moon, Albireo and the Ring Nebula while undergraduate Pratishta Yerakala took attendees on a 3D tour of the Universe with the 3D wall.

-- Steph Douglas (graduate student)

Oct 24: Stripping Stars

This week's Astronomy public lecture was given by post-doc fellow, Nick Stone, from Columbia Astronomy.  His talk "Stripping Stars: The Exciting Lives and Untimely Demise of Stars Near Supermassive Black Holes" led the audience to discover the dynamic processes associated with black holes.  Starting with an introduction to basic dynamics predicted by Newton's laws of motion and then general relativity, Nick familiarized the audience with the concepts of four dimension time-space and interactions between objects via gravity. Then he explained in detail the physics of black hole accretion, and broadened the case to dual-black hole interactions with the help of simulations. Finally, Nick showed what phenomena when we can expect to see as observers on Earth and how we can observe these energetic events in the Universe. 

About 150 people attended the lecture, after which they headed up to the Rutherfurd Observatory for to observe the Pleides open cluster and the spiral galaxy M31 (aka our nearest neighbor, Andromeda). Those who didn't go to the roof were treated to a slideshow on 'Gas Beyond the Milky Way Disk', by graduate student Yong Zheng, which introduced the very diffuse hot gas with more than 10 thousand degrees in the Milky Way potential well.

-- Yong Zheng (graduate student) 

March 28: Our Hungry Black Hole

This Friday relaunched our lecture series after both spring break and daylight savings time so we now begin at 8pm.  Graduate student Yuan Li gave a lecture on an upcoming galactic event: the closet passing of a gas cloud (called G2) to the supermassive black hole in the center of our galaxy.  After explaining the basics of black holes, Yuan showed proof for the existence of our own, called Sagittarius A*.  She then explained that with our own supermassive black hole being so long dormant, watching the feeding of a black hole up close has implications for most areas of astrophysics.

(Almost) April showers kept us from observing on the roof but roof tours were still available along with our 3D wall.  David Jaimes also gave a presentation on the exoplanets discovered by the Kepler telescope, highlighting a number of resources available on the web for those at home.

Check out this New York Times interactive graphic on different planetary systems.You can toggle between orbital size and discovery as well as hover your mouse over the orbits for various information.

-- Lauren Corlies (graduate student) 

April 13: History of Astronomy and Navigation

On April 13, 260 people crowded a lecture hall in Pupin Physics Laboratories to hear Brandon Horn, graduate student in the Department of Astronomy, gave a lecture on the history of astronomy, navigation, geodesy, and photometry. Wowing the crowd with a zooming presentation, Brandon gave us a tour of major astronomical discoveries starting with the size of the Earth and moving all the way up to the expanding universe. People stayed after the talk to ask questions and explore the universe of Astronomy Picture of the Day in the lecture hall while others headed up the roof for a beautiful, clear view of the heavens.

On the roof, five volunteers helped visitors look at Mars, the Moon, the globular cluster M3, and the Orion Nebula. We were very fortunate to have such a clear view of the night skies and the huge number of visitors seemed to attest to how exciting it was to be able to use the observatories to their fullest capacity. Additionally, visitors who wanted to avoid the crowds could catch a show in our 3D theater where graduate student Yuan Li showed some videos about black holes and pulsars that the audience enjoyed. In all, a fun time was had by all.

--Josh

February 3: Top 11 Astronomy Stories of 2011

Last night, graduating grad student, Jana Grcevich, kicked off our Spring lecture series by recapping the "Top 11 Astronomy Stories of 2011." Some of the highlights included: the most massive galaxy cluster found, a possible detection of the Higgs boson, a possible detection of a supermassive black hole devouring a star, the 2011 Nobel Prize in Physics going to three astronomers, and the discovery of over 3000 new exoplanets.

Jana ranked the new planet discoveries as number one on her list and many people would agree. Understanding and determining the available real estate for life to grab a foothold in our galaxy, let alone the rest of the Universe, ranks as one of the top 3 questions asked by humanity since humanity could ponder its own existence. Jana explained how the discoveries were made, by the Kepler space telescope, by stating that it measures the dimming of light from a star to determine the nature of the object causing the dimming due to occultation (the object passing in front of the star). She then focused on some of the gems from this exciting database of planets which included the discovery of a super earth-massed planet (roughly 2.5x) orbiting around its star in the habitable zone - an object known as Kepler 22b. Afterwards, she pointed out some planets orbit around binary stars, giving a shout out to the fans of Star Wars who recall Tatooine’s planetary system. Last, but not least, she displayed the best determination for many planetary systems’ orbits and configurations which showed us that planetary systems similar to our own are actually common place and not rare as previous thought based on the planetary systems discovered before the Kepler mission.

Many good questions were asked from the 140+ attendees last night, proving that a recap of the LAST year’s exciting discoveries was a great way to start off THIS year’s lecture series.

--Duane

November 18: The Truth About Black Holes

On Friday, November 18th, over 200 astro enthusiasts gathered in Pupin to hear "The Truth About Black Holes." Dan D'Orazio shed some light on these enigmatic photon traps, discussing gravity, the equivalence principal and time as a malleable entity. Dan cut past the hype and speculation surrounding these theoretical predictions, and instead focused attention on the astrophysical contexts in which black holes exist, and how we can spot them.

Afterwards folks ascended to the roof, where Hugh Crawl, Jeff Andrews, Lauren Corlies, Sam Gordon and Lia Corralles showed visitors Jupiter, the Pleiades and Andromeda. For those who stayed downstairs, Munier Salem and Jenna Lemonias led a short quiz on Planet Trivia, which was entirely too easy for the stalwart enthusiasts.

--Munier