By the 1950s, astronomers had developed several methods of astrometry to determine the distances to and motions of stars in the Milky Way. One method involved using Cepheid variable stars, which vary in brightness in a way that reveals their true or intrinsic brightness. By determining the intrinsic brightness of a Cepheid, astronomers can calculate its distance and position within the Milky Way.
Astronomers also looked for clouds of hydrogen gas and measured their speed and direction of motion. This information helped create a dynamic model of the structure of the Milky Way. Additionally, the distances to open star clusters were determined by measuring the motions of individual stars within the cluster.
Astronomical events, such as recurring nova explosions, were used to gauge the actual brightness of stars and calculate their distances. Furthermore, a star’s intrinsic brightness and distance could be inferred from its type, age, and stage of development.
Combining these techniques, astronomers concluded that the Milky Way has spiral arms emanating from a central hub. The most reliable method for determining stellar distance is parallax, developed by the ancient Greek astronomer Hipparchus. By measuring the apparent change in position of a star from opposite sides of Earth’s orbit, astronomers can calculate the star’s distance using trigonometry.
However, parallax is limited by Earth’s atmospheric turbulence and can only be used to measure stars a few hundred light-years away. To overcome this limitation, the European Space Agency launched the Hipparcos satellite in 1989. It provided stellar catalogs, including the Hipparcos Catalogue with 118,218 entries and the Tycho 2 Catalog with data for over two million stars.
In 2013, the ESA launched Gaia, an astrometric satellite that significantly improved upon Hipparcos. Gaia collected more light, measured distance and motion more accurately, and could detect celestial objects a million times fainter than the human eye. Additionally, Gaia is located at the L2 Lagrange point, outside Earth’s orbit, enabling it to plot stellar positions over the entire sky more easily.
Using various methods, Gaia has mapped the positions and motions of 1.8 billion stars during its 11 years of operation, representing an impressive one percent of the stars in the Milky Way. The data from Gaia has allowed for the creation of detailed maps of our galaxy.