Rather than using tracers such as star counts, which is how its shape was previously determined, the team measured a large sample of Cepheid variable stars which allowed them to construct a 3D model of the galaxy. Cepheids are young, pulsating supergiants with brightness varying with a well-defined period ranging from hours to several dozen days.
“Cepheids are ideal to study the Milky Way for several reasons,” said University of Warsaw astronomer and study co-author Dorota Skowron. “Cepheid variables are bright supergiant stars and they are 100 to 10,000 times more luminous than the sun, so we can detect them on the outskirts of our galaxy. They are relatively young – younger than 400 million years – so we can find them near their birthplaces.”
The Milky Way is thought to be home to more than 100 billion stars, including our sun, and is comprised of four major spiral arms and a bar-shaped core region.
The astronomers tracked over 2,400 Cepheids using the Warsaw Telescope located in the Chilean Andes to build the 3D map. The map showed that the galaxy’s disk is significantly warped and varies in thickness from place to place, with increasing thickness measured further from the galactic centre. The disk is approximately 140,000 light years in diameter.
The map is the first based on direct distances to thousands of individual objects as distant as the expected boundary of the Galactic disk.
“Warping of the Galactic disk has been detected before, but this is the first time we can use individual objects to trace its shape in three dimensions,” said Przemek Mróz, a graduate student at the University of Warsaw.
The Milky Way began to form relatively soon after the Big Bang explosion that marked the beginning of the universe some 13.8 billion years ago. The Sun formed about 4.5 billion years ago, and is located roughly 26,000 light years from the supermassive black hole residing at the centre of the galaxy.