An Astronomer's Guide to the Galaxy

Measuring distances in astronomy is notoriously difficult. Indeed, a quick glance at the night's sky reveals only a two-dimensional image, with no depth information. Yet, knowledge of precise distances is crucial, since that's the only way to determine the true, intrinsic luminosity of an object, from the apparent one, measured by our telescopes. The determination of distances therefore constitutes a major component of almost every astronomical study, whether that involves the evolution of stars or the accelerating expansion of the universe.

The most direct and precise method to measure distances is using trigonometric parallaxes (Figure 1). This method is based on the fact that as the Earth orbits the Sun, relatively nearby stars appear to move against the more distant stellar background. By measuring the angle associated with this apparent motion from two positions along the Earth's orbit (say, in January and in July), and knowing the diameter of the Earth's orbit, the distance to the star can be calculated using simple trigonometry. Unfortunately, ground-based telescopes can only perform such distance measurements to stars that are no farther than a few hundred light-years, due to the blurring effects by turbulence in the Earth's atmosphere. Astronomers have therefore come up with a variety of other ingenious (if somewhat less precise) methods to determine distances to more distant objects.

Figure 1. Measuring distances to stars through the method of trigonometric parallax. (Credit: STScI.)

On December 19, 2013, however, the European Space Agency launched its Gaia space observatory (Figure 2), which promises to open a new era in precision measurements of distances. From its orbit at the so-called L2 Lagrangian point (about a million miles from Earth, in the direction opposite to the Sun), Gaia will use its two telescopes and three science instruments to observe each one of its target stars about 70 times during the five-year mission duration. This will allow Gaia to generate a three-dimensional map of about a billion stars in our Milky Way galaxy. In addition to extremely precise positions, Gaia will map the stellar motions, and will determine the luminosities, temperatures and compositions of the observed stars. To appreciate the expected precision, suffice it to note that for stars about 4000 times fainter than those we can see with the naked eye or brighter, Gaia will determine the position to about 24-millionths of an arcsecond (equivalent to the width of a human hair at 600 miles!).

Figure 2. An artist's impression of the Gaia Satellite deploying its sunshield. (Credit: ESA.)

In addition to this extraordinary 3D map of the Galaxy, Gaia is expected to detect thousands of extrasolar planets, about half a million quasars (supermassive black holes that accrete mass at the centers of distant galaxies), and tens of thousands of asteroids and comets within our own solar system.

Shortly after Gaia's launch, the European Space Agency discovered some ice deposits in the observatory that appear to cause a problem with stray light entering the telescope. The current assessment is, however, that this will only degrade the science performance for the faintest stars among Gaia's targets, so that the overall mission goals will not be significantly affected.

Douglas Adams, the author of The Hitchhiker's Guide to the Galaxy, once wrote:

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.

Irrespective of whether it has already happened or not, it appears that once all the results from Gaia are in, we will have such a better understanding of our Galaxy and of the Universe that we may find ourselves much closer to the imaginary cosmic scenario envisioned by Adams.