One of the first questions that a child wonders is ‘what is the name of our galaxy?’ Our galaxy is known as the Milky Way galaxy. The Milky Way is a barred spiral galaxy and is part of the Local Group of galaxies. Ours is merely is one of billions of galaxies in the observable universe. Our sun is 25,000 light years from the center of the Milky Way. The oldest star in the galaxy is 13.2 billion years old.
The stellar disk of the Milky Way Galaxy is approximately 100,000 light years in diameter and is about 1,000 light years thick. It is estimated to contain up to 400 billion stars. The exact figure depends on the number of very low-mass stars, which is difficult to determine. The stellar disc does not have a sharp edge, a radius beyond which there are no stars; however, the number of stars drops slowly with distance from the center. Beyond a radius of roughly 40,000 light years the number of stars drops much faster the farther you get from the center. Beyond the stellar disk is a much thicker disk of gas that is estimated to be 12,000 light years thick. twice the previously accepted value.
In 2007, the star HE 1523-0901 was estimated to be about 13.2 billion years old. The galaxy is thought to be just slightly older. The estimate of this star’s age was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes from which an estimate of the age of the star can be derived using nucleocosmochronology.
The galactic disc has a diameter of between 70,000 and 100,000 light-years. The distance from the Sun to the galactic center is now estimated at 25,000 light years, but older estimates put the Sun as far as 35,000 light years out. At the galactic center is a compact object of very large mass. The intense radio source, named Sagittarius A, is thought to mark the center of the Milky Way. This anomaly has been confirmed to be a supermassive black hole. It is thought that most galaxies have a supermassive black hole at their center. The galaxy’s bar is thought to be about 27,000 light-years long. It is composed primarily of red stars, all of which are thought to be ancient. The bar is surrounded by a ring called the ‘5-kpc ring’ that contains a large percentage of the molecular hydrogen present in the galaxy, as well as most of the star formation in our galaxy.
Simply getting the answer to ‘what is the name of our galaxy’ is not enough. Above are many facts about the Milky Way, but there is much more to discover.
We’ve also recorded an episode of Astronomy Cast about the Milky Way. Listen here, Episode 99: The Milky Way.