FINDING OUR PLACE IN THE UNIVERSE

By Ronald D. Ferdie, Tim Hunter, and James McGaha

Where are we in the Universe?

Observational results from the Hubble Space Telescope (HST) and advances in other satellite and ground-based astronomy have greatly enhanced our ability for determining distances in the Universe compared to prior times when the largest functional telescope in the world was the Mount Palomar 200-inch. One hundred years ago, the known Universe was the Milky Way. Thirty years age, the known Universe stretched around us some five billion light years with cautious suspicion it stretched perhaps thirteen to fifteen billion light years in space as well as time. Twenty years ago, quasars were considered enigmatic super energy structures. Today, we feel they formed early in the history of the Universe and are energetic galaxy nuclei.

This article summarizes how we determine our place in the Universe by building upon different overlapping yardsticks to measure distances. However, these yardsticks are still built upon a "house of cards" wherein parallax methods used to directly and precisely determine “close” distances to earth are then in turn used to support other increasingly less precise yardsticks for determining distances to far away Milky Way stars and nearby galaxies (Cepheid variable stars, supernova explosions, and planetary nebulae brightness) which in turn are used to support other yardsticks (spiral galaxy surface brightness fluctuations, elliptical galaxy fundamental plane and red shift determinations) for determining distances to remote galaxy clusters and quasars (Sky & Telescope December 1983, pages 516; Sky &Telescope February 2002, pages 18-19).

This house of cards technique for overlapping distance scales allows us to literally take a ruler to the Universe. However, it is fraught with uncertainty and must constantly be re-evaluated. If you change or modify a parameter anywhere in one method of determining distances, then the downstream distance scales are changed, and our view of the Universe can radically change, particularly at its distant fringes.

Also, each method is limited to a certain scale; for example, if the stars of a particular galaxy cannot be individually resolved, the technique for measuring distance by using the spectral classification or absolute magnitude of selected stars in the galaxy cannot be used for this galaxy, and its distance must be inferred by the next, more indirect and less precise method in the chain of distance scales.

The methods for finding our place in the Universe are summarized in an overlapping set of scales:

Figure one. Diagram showing the overlapping scales used to measure the Universe.

The cited modern distances to celestial objects are based on a number of sources, such as the Hipparcos and Tyco Catalogs. From approximately 1600 to 1850, the distances to the Moon, the Sun, the planets, and the nearby stars were determined with reasonable accuracy. During this time, astronomers began to realize the vast size of the Universe far exceeded the distance scales used by mankind on the Earth. The mile and kilometer became too limiting to describe the vast distances to nearby stars and beyond. For the planets, miles and kilometers are still used for their distances. For a larger view of the Solar System, the Astronomical Unit (AU), the mean distance of the Earth from the Sun (1.496 x 10E13 cm or 92.92 x 10E6 miles), is used . It describes distances to the outer planets and the Kuiper Belt of asteroids beyond Pluto and the Oort Cloud of comets beyond at the outer fringes of our solar system.

Even the AU is, however, too small to easily describe distances to even the closest of the stars. Here, the AU measurement unit is supplanted by the light year (5.865 billion miles), the parsec (3.26 light years), the kiloparsec (3260 light years) and Mpc (3.26 million light years). However, it is the light year that has emerged as the most popular measurement to use since it accents how far back in time we are seeing the images of remote objects.

DIRECT OBSERVATION FROM THE EARTH'S SURFACE

The distances to the Moon, Sun, and planets in our solar system can be measured using simple trigonometry. Simultaneous observations of these objects made by observers at two points sufficiently spread apart on the Earth’s surface allow for trigonometric calculation of the distance to the object. Since the 1960’s, direct observation from the Earth also includes the use of laser and radar measurements. Highly accurate distances to the Moon to the fraction of an inch have been obtained by using laser beams going from ground-borne telescopes to optical reflectors left on the Moon’s surface by U.S. astronauts. Additionally, radar has been used from the Earth to obtain return signals from the Moon, Venus, near Earth passing asteroids, and other nearby solar system bodies. The radio telescope at the Arecibo Observatory in Puerto Rico can transmit and receive radio signals as far out as 9.6 AU (one hour and twenty minute travel time for the speed of light). This is the distance to Saturn, and the telescope has been used to observe and measure distances to Saturn and Titan.

TRIGONOMETRIC PARALLAX

Trigonometric parallax is a direct measurement technique to determine close star distances using the Earth’s orbit around the sun as a baseline. A nearby star appears displaced relative to more distant reference stars when photographed or imaged six months apart (Parallax - Wikipedia, the free encyclopedia). The displacement relative to the background star images can be equated to a parallax angle, and the mean distance to the star solved by trigonometry. Alan Hirshfeld in his wonderful book Parallax presents a detailed history of “the race to measure the cosmos.” A shortened version of this history to measure the parallax of the nearest stars is also found in his article in the November 2001 issue of Sky & Telescope, pages 38 –45. The brief historical overview of parallax measurements below is derived from these and other sources.

Soon after the Polish astronomer Nicholas Copernicus suggested in the latter part of the 1500’s that the Earth and planets circled the Sun rather than circling the Earth, it became recognized that distances to the stars might be measured by using the Earth’s orbit as a baseline. While this concept became apparent in the 1600’s, there were many obstacles to overcome before it was first successfully employed in the mid 1800’s. First, it took quite some while for the concept that stars were other suns to be recognized. Second, no one appreciated how vast the distances to the stars were and the resultant difficulty that would entail to obtain accurate parallax measurements. Moreover, it was unknown whether stars were spread out at different distances from the Earth and whether they had differing brightness and mass.

Early attempts at parallax measurements were unsuccessful because the available equipment was not precise enough. In the late 1500’s the Danish astronomer nobleman Tycho Brahe was the first to try to determine the parallax of a star, but, at best, his pre-telescopic measuring equipment could only determine star positions to one arc minute. In 1669 and again in the 1720’s, Englishmen Robert Hooke and James Bradley, respectively, attempted to determine the parallax displacement of Gamma Draconis, a second magnitude star. They compared its position to a nearby dimmer star they assumed to be much farther away. Both of them made measurements over many months. Gamma Draconis was chosen, since it is bright star, and Hooke and Bradley assumed it is close by the Earth relative to other stars. They liked it because it had a dim “background” star in the same telescopic field of view convenient for their measurements. Also, Gamma Draconis passed nearly overhead at the latitude of London where they made their measurements.

Bradley’s results showed that the star displayed an unmistakable shift, but the shift was opposite to what was expected! Eventually Bradley realized that this shift was a natural phenomenon, the aberration of light caused by the Earth’s motion through space. This was the first direct proof the Earth moves through space rather than being a fixed body about which all other solar system objects revolve. Thus, while Copernicus first proposed that the solar system objects, including the Earth, revolve around the Sun in the late 1500’s, and Galileo was punished in part for supporting the Copernican hypothesis, proof of the Earth’s motion through space was not obtained until the 1720’s! The aberration of light is common to all stars, and it must be reduced out of the calculation for determining the trigonometric parallax of a star.

In the late 1700’s and early 1800’s, William Herschel also attempted to find parallax motion of a star using his “double star” method wherein the brighter star of a double star pair was assumed to be much closer than the dimmer one. He assumed all stars were equally intrinsically bright.

Herschel observed and measured the very precise motion of several hundred close pairs of bright and dim stars over more than twenty years, but he found that most of these “double stars” were not random alignments that could be used to determine the parallax of the brighter star. Instead, Herschel discovered that many were actually real double stars, which were bound together by mutual gravitation orbiting a common center of mass. For truly randomly aligned optical double stars, Herschel’s observations still lacked the accuracy to successfully determine the parallax of the brighter star. However, his failure clearly indicated that indeed most stars must be very far away.  

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