In Galaxies Not So Far Away

Stars begin to move in elliptical paths

The audience gathered in the basement auditorium of the Jawaharlal Nehru Planetarium in the afternoon of January 21, 2017. The crowd wasn’t numerically comparable to those who would be found in a political rally or outside a film star’s house. But what they lacked in strength, they made up in excitement. The occasion was a popular lecture on black holes by Ramesh Narayan, Thomas Dudley Cabot Professor of the Natural Sciences at Harvard University and Senior Astronomer at the Smithsonian Astrophysical Observatory. The audience included children, students and working professionals, and also eminent names such as former ISRO chairman, AS Kiran Kumar, and Rajesh Gopakumar, Convenor and Centre director, ICTS-TIFR. Ramesh Narayan immediately had the audience’s attention, with his humorous and upbeat nature. His is an astrophysicist and his love for his subject was obvious, along with his enthusiasm in sharing his learning with others. What are Black Holes? Mr Narayan says that black holes are “weird, but real”. A normal surface, he explained, has a surface and a density. But in a black hole, matter is squeezed into a geometric point, called a singularity, which is of infinite density and thus has a huge gravitational pull. And it only has a pseudo surface, called an event horizon. It is theorised that any matter or energy that enters the event horizon, never gets out. The gas, dust and other space debris that has trailed close to the event horizon, but not yet fallen in, forms a visible disk around it, called the accretion disk. Every galaxy has one supermassive black hole, which is 106 to 1010 times the mass of the sun. The super massive black hole in our galaxy (Milky Way) is called Sagitarius A*, which Mr Narayan describes as “weak and wimpy” compared to others. Galaxies will also have thousands of stellar mass black holes, which are 5 to 30 times the mass of the sun. How do Black Holes affect their galaxies? Black holes exist in binaries with stars, from whom they pull in gases and energy. All black holes can be explained in astrophysics by their mass and angular momentum. They feed on the energy from the stars, and then their potential energy is translated into “thermodynamically free energy” in the perpendicular direction, called jet streams, at velocities comparable to the speed of light. Stars should move in straight lines. But it has been observed that at the centre of the galaxy, stars begin to move in elliptical paths, due to the pull of black holes. It was also found that the mass of the black hole is correlated to the mass, luminosity, etc. of its host galaxy. Smaller galaxies have small black hole, and larger galaxies have large black holes. A galaxy once formed does not retain its identity. It may merge with other galaxies and grow. But there seems to be a “communication between the galaxy and the black hole” which tells the galaxy how to adjust during mergers. Mr Narayan explained this phenomenon with an analogy. He said that it is equal to a grape, governing the behaviour of the earth. Image of Black Holes Mr Narayan concluded his talk by addressing the potential of black hole imaging. He says that one would need a telescope as big as the earth to be able to get the proper image resolution to view a black hole. But the field of interferometry makes it possible to get images of black holes, using smaller telescopes places at strategic points around the world, including California, Arizona, Mexico, Greenland, Hawaii, France, Spain, Chile and the South Pole. Mr is currently working on the Event Horizon Telescope (EHT) team that does just this. The first observations of the team where prepared in April 2017, and the first results are expected later in 2018. This was just one of a series of talks that are hosted by the International Centre for Theoretical Sciences (TIFR) in partnership with the planetarium, called ‘Kaapi with Kuriosity’, on the third Sunday of every month. ICTS also recently hosted a lecture by Nobel Prize laureate, Kip Thorne, in its campus. Thorne won the Nobel Prize for his work in LIGO and gravitational waves. Report by Tanvi Shenoy, NSoJ Bureau