The practice of modern astrophysics had begun, and triggered the development of a variety of new fields and techniques to investigate the cosmos. The distance measurements led to the extraordinary revelation that there are countless other galaxies beyond our Milky Way and that the entire Universe is expanding. Combining these observations with the developments in theoretical physics of the time, astronomers could finally understand the physical processes that power stars, outline the shape and size of the Milky Way, and establish that the Sun is not at the centre of our Galaxy, but closer to the periphery. The ability to measure the distance to faraway sources and examine their composition led to breakthrough after breakthrough in the 1920s and 1930s. This happened in 1913, when the Danish astronomer Ejnar Hertzsprung measured the parallax of several Cepheid stars, building the first step in the cosmic distance ladder that would soon allow astronomers to chart the cosmos far beyond our local stellar neighbourhood. Before Cepheid stars could be used to estimate distances, the method needed to be calibrated by measuring the distances of a nearby sample of such stars independently. Her results suggested that these stars could be used to measure cosmic distances because the period of their variations is directly related to their intrinsic luminosity. In 1908, the American astronomer Henrietta Leavitt pioneered the study of Cepheid variability while analysing photographic surveys of the southern sky. This method, which can be applied to star clusters across our Galaxy, is known as main-sequence fitting the term main sequence refers to the core epoch in a star's lifetime, during which hydrogen is burned into helium in the stellar centre.Īnother method to estimate distances, which can be applied also to other galaxies beyond the Milky Way, relies on the observation of specific classes of stars and galaxies – known as standard candles – whose intrinsic luminosity can be evaluated from other properties.Ĭepheids - a type of variable star - are widely used as standard candles. One of these methods can be applied to determine the distance to stellar clusters: when astronomers compare the observed magnitude and colour of stars belonging to a cluster with predictions from stellar evolution models, they can estimate their intrinsic luminosities and, from those, the distance to the cluster. These secondary distance indicators rely on the comparison between a star's observed and intrinsic luminosities, given that the intrinsic luminosity can be estimated on the basis of other observed properties of that star. Seeing fartherĭistances inferred from parallax measurements are limited in reach but are vital for calibrating indirect methods which can be used for even greater cosmic distances, albeit with less precision. With this precision, which corresponds to the diameter of the full Moon divided by 180 000, astronomers could probe stellar distances out to a few dozen light-years from Earth. In 1924, Frank Schlesinger, an American astronomer and pioneer of photographic methods, published a catalogue with the positions of almost 2000 stars to a precision of about one hundredth of an arc second. Spectroscopy enabled astronomers to scrutinise the chemical composition of distant and otherwise inaccessible stars and nebulae, and using photography they could chart the sky with a precision and sensitivity that had not been possible before.īy measuring stellar parallaxes in large photographic surveys, astronomers started to secure distances to a great number of stars close to the Sun. Photography and spectroscopy were game changers in astronomy in the late nineteenth century. The 40-inch (102 cm) refractor telescope at the Yerkes Observatory in Wisconsin, USA, used by F. Schlesinger in the early 20th century. ESA's Gaia mission is astrometry for the twenty-first century, building on the legacy of Hipparcos to chart one billion stars and explore the composition, origin and evolution of our Galaxy. Operating from 1989 to 1993, Hipparcos allowed astronomers to map the positions and velocities of, and distances to, more than 100 000 stars with unprecedented precision. ESA's Hipparcos mission was the first space telescope devoted to measuring stellar positions. The flickering effect caused by Earth's atmosphere prevented further improvements in astrometry until observations could be made from space. Star catalogues from ground-based telescopes grew in size and precision for the first half of the twentieth century, but then they hit their limit. Astrometry moves to space: the mapmakers guide to the Galaxy
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