On April 1st, 2017, the 18th session of the "New Worlds, New Horizons" Public Lecture Series, "Understanding our Galaxy", was held in the Auditorium of Tsinghua University. The lecturer, James Binney, Professor of Physics at Oxford University, and the laureate of the Dirk Brouwer Award of the American Astronomical Society and the Eddington Medal of the Royal Astronomical Society, attracted an audience of over 400, which includes students, faculty staff, and the general public, as well as over 3000 online viewers.

In his lecture, Prof. Binney presented to the audience our current view of the Galaxy, methods of exploration and remaining open questions. Our Galaxy is among millions of galaxies in the universe, and astronomers have studied its mass distribution via observations of the positions and motions of stars, gravitational lensing, and other phenomena. Current research shows that stars, along with interstellar medium, gravitate together primarily in the form of a flat disk, which is surrounded by a halo of dark matter. Dark matter contributes 95% of the mass of our Galaxy. Whereas ordinary matter accounts for the majority of the mass at the center of the Galaxy, dark matter gradually increases in proportion as we move to the outer edge of the Galaxy.

A number of mysteries about the Galaxy remain to be explored. For instance, while normal matter only takes up 5% of the mass of our Galaxy, the corresponding ratio is 20% in terms of the whole universe, so where is the missing 15%? How does the bar-like structure at the center of the Galaxy comes into existence and how does it influence the development of the whole Galaxy? Inspiringly, further observations and more advanced theories will help us answer these questions. The Gaia satellite of the European Space Agency (ESA), for example, is monitoring more than one billion stars in the Milky Way Galaxy, and some large telescopes aim at collecting numerous spectra of the stars to determine their mass, age, and chemical composition. Based on these data, astronomers will assemble a complete model of galactic evolution.

Finally, the lecturer addressed questions from the audience. Some people were concerned about how to obtain more information on our Galaxy, some concentrated on the progress of gravitational waves and dark matter, while other students were curious about Prof. Binney's viewpoint on science fiction.

Galactic Astronomy has made giant leaps in the past several decades, and will remain a hot subject for a long period of time. It is expected that the story of our Milky Way galaxy will become more and more complete as time goes on.