My first Magazine Sky & Telescope - 03.2019

Star Sleuthing and so on and so forth, changing methods along the way until we reach the conﬁ nes Apparent of the observable universe. Much of mod- shift = 2p ern astronomical knowledge, from stellar physics to the structure and evolution of the universe, depends on a good calibra- tion of this distance ladder — and thus on knowing the distances to nearby stars to a T. Controversy over the Pleiades’ distance was therefore disconcerting. instrumental or systematic error astrono- Parallax Suspicion quickly fell on Hipparcos, as mers had overlooked? Was there a problem angle (p) additional measurements made with other with just the Pleiades, or also with other Distance instruments and methods contradicted measurements? Or were the Pleiades really to star its result. In 2004, observations of three closer and thus didn’t ﬁt into our models Pleiads done by the Hubble Space Tele- of stellar formation and evolution? scope gave a distance of 435 light-years. In Astronomers now have an answer. 1 a.u. 2014, an extremely precise measurement made by combining data from radio tele- The Importance of Being scopes all over the world gave a result of Clumped 444 light-years. Measurement after measurement agreed with Open clusters play a crucial role in astronomy. Because a the greater distance, making the one from Hipparcos look cluster’s stars formed together from the same interstellar anomalously small, even though successive reassessments of cloud, we know that they are the same age. As such, they the spacecraft’s data only narrowed in on the smaller value. are excellent laboratories to test physical models of stellar It took many years to solve the mystery. The issue is appar- evolution. What’s more, by knowing their distance and using ently a matter of instrument calibration, due to Hipparcos’s it to derive their intrinsic brightness, astronomers can then intricate observing method. Instead of looking at a ﬁ xed spot use these models to calculate the distances to farther stars, in the sky, the telescope rotated about itself and changed the those that are removed from the reach of direct geometrical orientation of its rotation axis over time, a common strategy methods like parallax. for all-sky surveys (see page 20). Using this method, Hip- In this regard, the Pleiades play a keystone role in the cali- bration of the cosmic distances ladder, which proceeds step by parcos built up a map of the celestial sphere by determining the relative angular distances between stars and how those step, from the Sun to the nearest stars, then to farther stars, distances changed with time. Calculating a particular star’s parallax required distinguishing the star’s motion from those around it. But closely packed stars — like those in a cluster — Trigonometric Parallax Gaia DR2 gave tightly correlated measurements. This called for differ- Hipparcos Gaia DR1 ent calibrations at different spatial scales and resulted in an VLBI unexpected source of error for the important and compact open clusters. Spectroscopic twins But even when they knew what the problem likely was, astronomers had trouble correcting Hipparcos’s data to produce a distance that agreed with the others. It could be Moving cluster that there were multiple sources of error. So, what scientists wanted was conﬁ rmation of the larger distance from an Fit of isochrone instrument that worked as Hipparcos did. Putting the Pleiades in Their Place Binary orbits 350 400 450 500 Distance (light-years) p HIPPARCOS’S ANOMALY Shown are distances to the Pleiades ac- cording to a variety of methods. Note the anomalous distance measured by Hipparcos (purple), the precise result of radio interferometry (green), and the early result given by the first Gaia data release (DR1, yellow). The article uses Gaia’s second data release (light blue). 28 M A RCH 2 019 • SK Y & TELESCOPE Hipparcos’s successor, the hat-shaped Gaia, provided the opportunity astronomers needed. Like Hipparcos, Gaia is an ESA satellite of unusual design: It also looks sideways as it spins, its two telescopes scanning the same strip of sky one after the other as the spacecraft slowly rotates its view. Its second data release contains the positions, parallaxes, and proper motions of more than 1.3 billion sources and reaches as faint as magnitude 21. The uncertainty of its parallaxes is around 40 microarcseconds (40 μas) for objects brighter than A u WHAT IS PARALLAX? Parallax is the shift in an object’s position against the background scene when viewed from two different locations. Nearby stars have measurable parallaxes due to Earth’s motion around the Sun, which astronomers can use to calculate the stars’ distances. However, calculating the true distance using the parallax re- quires eliminating all sources of error in the angle measurement.

My first Magazine Sky & Telescope - 03.2019 | Page 30