1. How far away are massive stars?
It’s so simple a question, it seems extraordinary we don’t yet know the answer. After all, astronomers have accurate distances to a host of low-mass and midsize stars. The best method for getting reliable distances involves measuring the position of a nearby star relative to more distant objects from opposite sides of Earth’s orbit. Simple trigonometry then converts the star’s observed angular displacement into a distance.
Unfortunately, these angles are so small, the method can be applied only to the Sun’s close neighbors. Before 1989, this technique had yielded just a few hundred precise distances. Things changed that year with the launch of the European Space Agency’s (ESA) Hipparcos satellite. Thanks to its 3.5-year mission, astronomers had reliable distances — meaning an uncertainty of less than 10 percent — to nearly 120,000 stars out to some 300 light-years.
Yet, this seemingly large sample of solar neighbors does not contain a single massive star. So rare are these heavyweights that the nearest one lies well over 300 light-years away. Astronomers can only estimate the distances to these stars: The closest O-type star, Zeta (ζ) Ophiuchi, lies roughly 370 light-years away; the nearest Wolf-Rayet star belongs to the binary system Gamma² (γ²) Velorum, and it checks in at a distance of more than 1,000 light-years.
Without a precise distance, it’s impossible to know a star’s real properties, like luminosity, and this leads to uncertainty in theoretical models. Massive stars easily rank as the most luminous in the cosmos — the brightest outshine the Sun by a million times. And these are the only stars bright enough for us to see in distant galaxies. So, understanding massive stars better will improve our knowledge of extragalactic astronomy.
This first problem should be solved quickest. ESA launched Gaia, a successor to Hipparcos, in 2013. This satellite, which lies nearly a million miles beyond the Moon, should get accurate distances to a billion stars out to a distance of 30,000 light-years. That’s far enough to pinpoint a significant fraction of all the Milky Way’s massive stars.