Monday, April 16, 2018

April 6 - Signal to Noise

We have a special joint event with the Wallach Art Gallery on May 6th. The event, titled “Signal to Noise”, is an interdisciplinary salon that discuss and exhibit the significance and nuisance of sounds and noise in our daily lives.

Andrea Derdzinski, a fourth-year NSF graduate fellow, gives her lecture on gravitational waves. She first introduces the electromagnetic spectra and explains how astronomers use them to study various celestials objects in the universe. Then she focuses on the recently discovered gravitational-wave events, including the first finding of the 30-solar-mass black hole mergers and the recently discovered neutron-star merger.

After Andrea’s lecture, Ariana van Gelder, who is a Ph.D. candidate at CUNY and an experimental musician, make her performance by improvising experimental rhythm as inspired by the random sounds in the lecture hall. Then, Ariana together with her artistic fellows, Emmy Cathedral, Constance DeJong, Ray Ferreira, Dominika Ksel, and Sarada Rauch, exhibit a light and sound salon in the library and stairwell on the 14th floor of Pupin. The salon features how signals (i.e., light and sound) intervene and enhance our daily experiences with the nature and society.

-- Yong Zheng (graduate student)

Friday, March 16, 2018

March 9 - Alien Weather

Our speaker this week was Statia Cook, a Columbia Teaching Fellow and a Research Associate at the American Museum of Natural History. She is an observational astrophysicist whose research focuses on studies of the weather and climate of other planets, especially the giant planets in the outer Solar System.

Statia started by orienting us to some of the key differences between weather on the Earth and on the "gas giants" (Jupiter and Saturn) and "ice giants" (Uranus and Neptune). On the Earth, the atmosphere is very thin compared the diameter of the planet -- similar to a few layers of cling wrap on a basketball. The giant planets, by contrast, are mostly (or entirely) atmosphere, although their density and temperatures vary with depth to such a degree that the same gasses may behave differently at different layers. In addition, the Earth's weather is driven almost entirely by energy from the Sun, whereas the giant planets still retain a large amount of heat from their formation. The release of that energy can be as important as the Sun to their climate. Finally, on Earth almost all clouds are water vapor, while the various colors seen in the giant planets trace clouds with different compositions including methane and ammonia.

Next, we heard about perhaps the best know extraterrestrial weather pattern: Jupiter' Great Red Spot, a persistent storm larger than the Earth that has existed for at least 180 and possibly more than 350 years. This vortex is accompanied by many other short-lived systems in Jupiter's atmosphere, with colors varying from white to pink to red. Neptune also had a giant storm, dubbed the Great Dark Spot, although it vanished in the five years between its discovery by the Voyager 2 spacecraft and observations by the Hubble Space Telescope five years later.

Finally, Statia told us about some of her own research which include detailed maps of Neptune using submillimeter radio interferometry, a pair of storms near Neptune's south pole that seemed to circle and merge together, amateur-inspired observations of a new Dark Spot on Neptune, and seasonal climate variation of the Ice Giants.

After the talk, Statia fielded questions, followed by a presentation by yours truly on the history of evidence for dark matter.

-- David Hendel (graduate student)

Sunday, February 25, 2018

February 23 - The LSST Revolution

Our speaker this week was Federica Bianco, a research scientist at both New York University's Center for Urban Science and Progress and their Center for Cosmology and Particle Physics. Her work focuses on transients - temporary changes in the sky. She is also the chair of the Large Synoptic Survey Telescope (LSST)'s Transients and Variable Stars Collaboration.

Federica's talk revolved around how the survey program of LSST will revolutionize our understanding of the changing universe, also known as time-domain astronomy. In particular, current studies tend to be at most two of wide (covering a large area of the sky), deep (observing faint objects) and fast (repeatedly observing the same place in quick succession). LSST is the first survey to attempt all three of these simultaneously by observing the entire Southern sky from its mountaintop in Chile, cataloging objects about 100 times fainter than current surveys with such a wide area, and imaging each patch of sky about once every three days -- with five different color filters.

LSST can achieve this due to its unique optical design that gives it an enormous field of view -- equal to the size of about 40 full moons -- and a 3,200 megapixel camera. This giant sensor will generate 15-20 terabytes of data a night, which will be immediately piped up to University of Illinois National Center for Supercomputing Applications in Urbana-Champaign, Illinois. There, the goal is to process each image and release in less than 60 seconds, including comparisons to previous data and generating an estimated 10 million science alerts per night. Just storing the 50-100 petabytes of data generated during its 10-year survey will be an immense challenge! Another amazing property of the LSST data is that it will be public immediately to all people in the US, so anyone interested can see what's happening in the sky almost in real time.

Federica also described the diverse science goals of LSST, which include understanding the nature of dark matter and characterization of dark energy; mapping the Milky Way; cataloging the Solar System; and exploring the changing sky. Many more details about LSST and its goals are available on its website.

After her talk, Federica answered questions in the lecture hall. Later, American Museum of Natural History fellow Betsy Hernandez gave a presentation on Active Galactic Nuclei while Columbia undergraduate Richard showed presentations on the 3D Wall and graduate students Daniel, Aleksey, and Adam gave tours of the observatory.

- David Hendel (graduate student)

Tuesday, February 20, 2018

Feb 9th - The Zoomable Universe

Our speaker this week was Caleb Sharf, a research scientist at Columbia University and Director of its Astrobiology Center. Caleb is a prolific writer with contributions in publications such as The New Yorker, The New York Times, The Atlantic, Wired, and Scientific American in addition to highly regarded scientific journals. He has also written a textbook on exoplanets and three popular science books on various astronomical topics.

In his talk Caleb gave an overview of his latest book, The Zoomable Universe. In it, he explores phenomena that cross the vast range of physical scales, from the very largest we can observe (the entire diameter of the observable Universe, about 10^27 meters) to the smallest (the Planck scale, 10^-35 meters, where the fabric of spacetime stops obeying known laws of physics).

Caleb uses this vast range - a factor of one hundred trillion trillion trillion trillion trillion, of which the human scale is conveniently close to the middle - to illustrate the incredible diversity of phenomenon in the Universe. Starting with the mysterious dark energy and the large scale structure of dark matter that makes up the skeleton of galaxy formation, we zoomed in repeatedly to examine the Local Group of galaxies, the birth of a solar system, the surface of a planet, and continued down to our own DNA and eventually the structure of spacetime. An illustrated version of this journey can be found here.

After his talk, Caleb fielded questions in the lecture hall while Columbia undergraduate Richard showed presentations on the 3D Wall while graduate students Steven and Aaron gave tours of the observatory. 

- David Hendel (graduate student)

Friday, November 17, 2017

Nov 10 - Going out in style: Nebulae at the end of a sun-like star’s life

Our speaker this week was Rudy Montez, an astrophysicist at the Chandra X-ray Center of the Smithsonian Astrophysical Observatory in Cambridge, MA. Rudy studies "planetary nebulae," the ghostly shells sloughed off by dying stars.

Rudy began by showing a gallery of beautiful Hubble Space Telescope photographs of planetary nebulae, explaining how the various colors tell us about the composition of the layers of each nebula. He noted that astronomers always expected planetary nebulae to be spherical, because they're believed to form as the outermost layers of a giant star expand away in all directions from the hot core.

But that's not what we see! Real-life nebulae display amazing, complex structure, including dumbbell-shaped bi-polar lobes. Rudy explained that the leading hypothesis to explain the formation of these lobes is that a donut-shaped ring of dust surrounding the dying star blocks the star's outer layers from expanding in certain directions, forcing them out at the opposite openings of the donut hole. The origins of this ring of dust are not quite clear, but they may be leftover debris from a pair of binary stars orbiting each other at the center of what will eventually be the nebula.

Rudy then discussed his area of particular expertise: X-ray observations of planetary nebulae from the Chandra space telescope, which give us a window onto the hottest, densest central regions. He showed beautiful composite images of Chandra, Hubble, and Spitzer data, which illuminate, respectively, the innermost, middle, and outermost regions of planetary nebulae. Three windows onto these extraordinary objects.

Friday, November 3, 2017

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)

Friday, October 20, 2017

Sept 29 - Galactic Archeology

Our speaker this week was Keith Hawkins, a Simons Postdoctoral Fellow based in the astronomy department here at Columbia. Keith is a Galactic Archeologist. He searches for clues to the past of our galaxy, the Milky Way, similar to the way an archeologist seeks to learn about ancient civilizations and cultures. 

Keith started by telling us about the "fossils" he uses to study the galaxy - stars! Stars are fossils in two ways. First, at the great distances involved on the scale of the Milky Way, we see stars not how they are but rather how they were up to hundreds of thousands of years ago, before humans even evolved on the Earth. This is because light moves at finite speed and needs time to reach us from the distant galaxy. Second, many types of stars live for hundreds of millions or billions of years and so their composition provides clues to what was going on at the time when they formed.

Next we learned about the tools used in galactic archeology. We heard about the methods used to measure distances, chemistries, and velocities of stars: parallax and spectra. Keith demonstrated parallax by having the audience hold up a finger and close each eye in sequence; the finger appears to move relative to the background. This is analogous to how astronomers measure distances, except observations taken on opposite sides of the Earth's orbit, 6 months apart, replace winking. The amount the star appears to move relates to its distance from us. Spectra, obtained by splitting a star's light and measuring how bright it is at different colors, contain a wealth of information. Dark bands in the rainbow are often visible. These bands correspond to light being absorbed by different elements in the star's atmosphere, so examining their pattern can tell an astronomer which and how much of the elements are in the star. The specific chemical signature of a star can pinpoint its place of birth or prove association with other stars. The bands in the spectrum may also be shifted to redder or bluer colors than normal; the direction and amount of shift is due to the Doppler effect and indicates the velocity of the star relative to us.

Keith finished his talk by describing his research's goal: a complete map of the Galaxy containing information on the positions, motions, and chemical content of millions of stars. He believes this "chemical cartography" will be key to deciphering the history of the Milky Way. 

After the lecture and a lively question and answer session, undergraduate students Briley and Harrison showed 3D astronomy animations on the 13th floor while graduate students Steven, Aleksey and Haley pointed the Rutherford Observatory's telescopes at the Moon, Ring Nebula, and the double star system Albireo.  

-- David Hendel (graduate student)