Pacing-off the cosmos

Originally published 10 March 1986

An old proverb says that a jour­ney of a thou­sand miles begins with a sin­gle step. It is lit­er­al­ly true that a sin­gle step was the begin­ning of our jour­ney into the uni­verse of the galaxies.

Some­time in the 3rd cen­tu­ry B.C., a sur­vey­or in the ser­vice of Pharaoh walked with a mea­sured stride from the city of Alexan­dria at the mouth of the Nile south­ward along the banks of the riv­er to the vil­lage of Syene, count­ing his paces along the way.

The geo­g­ra­ph­er Eratos­thenes used the known dis­tance from Alexan­dria to Syene, togeth­er with dif­fer­ences in shad­ows cast by the Sun in the two places, to deter­mine the size of the Earth.

When the size of the Earth was known, Aristarchus of Samos was able to cal­cu­late the dis­tance to the Sun and the Moon from cer­tain obser­va­tions of the Moon at half phase and in eclipse.

In 1838, the astronomer Friedrich Bessel suc­cess­ful­ly tri­an­gu­lat­ed the dis­tance to a near­by star by using the Earth-Sun dis­tance as a baseline.

And so the jour­ney has con­tin­ued, step by step, tak­ing us ever deep­er into the realm of the stars and galaxies.

Measuring the galaxy

This past week [March 1986] the Nation­al Sci­ence Foun­da­tion announced the results of a new effort to mea­sure the cos­mos. An inter­na­tion­al team of radio astronomers, includ­ing Mark Reid, Matthew Schneps, James Moran, and Carl Guinn of the Har­vard & Smith­son­ian Cen­ter for Astro­physics, have made the first direct mea­sure­ment of the dis­tance from the Earth to the cen­ter of the Milky Way Galaxy.

On the basis of their find­ings — made with radio tele­scopes that mon­i­tored radi­a­tion — the galaxy appears to be about 30 per­cent small­er than had been pre­vi­ous­ly believed.

The Milky Way Galaxy is a great spi­ral sys­tem of a tril­lion stars, that turns about a cen­tral hub like a pin­wheel. The text­book val­ue for the diam­e­ter of the pin­wheel is 100,000 light years, which means it takes light 100,000 years to pass from one side of the galaxy to the oth­er. Our Solar Sys­tem is locat­ed two-thirds of the way out from the cen­ter of the spi­ral, or (accord­ing to the text­book) 33,000 light years. The new mea­sure­ment puts the Sun only 23,000 light years from the galac­tic center.

All pre­vi­ous meth­ods of esti­mat­ing the size of the Milky Way Galaxy have involved indi­rect obser­va­tions of one sort or anoth­er. The prob­lem is like try­ing to deter­mine the size of a huge crowd of peo­ple from some­where with­in the crowd. With opti­cal tele­scopes it is not pos­si­ble to see very far into the galaxy because of absorp­tion by gas and dust that pop­u­lates the space between the stars. Radio tele­scopes let us to see deep­er into the galaxy because radio waves pass through the obscur­ing gas and dust.

Cosmic masers

The Cam­bridge astronomers and their col­leagues used radio tele­scopes to mon­i­tor radi­a­tion from sources called masers at the cen­ter of the galaxy. A maser is an intense and high­ly direc­tion­al beam of radio ener­gy emit­ted at a very exact fre­quen­cy. It is the radio coun­ter­part of the light laser.

For a nat­ur­al maser to form, a large num­ber of mol­e­cules must be excit­ed to a high ener­gy and then stim­u­lat­ed to release their ener­gy simul­ta­ne­ous­ly, in a kind of chain reac­tion. The masers near the cen­ter of the galaxy occur in dense clouds of gas and dust that are active star-form­ing regions. The mol­e­cules in the clouds are excit­ed and then stim­u­lat­ed to radi­ate intense radio beams by the ener­gy of new stars.

The astronomers stud­ied a group of the cos­mic masers for a peri­od of many months. By com­par­ing the speed of masers trav­el­ing direct­ly toward the Earth with oth­ers trav­el­ing trans­verse­ly across the sky, they were able to use a trigono­met­ric method to cal­cu­late a dis­tance to the masers. The speed of the masers mov­ing toward the Earth could be deduced from a slight change in the fre­quen­cy of the radio waves caused by the rel­a­tive motion.

The radio data filled 500 com­put­er tapes and the cal­cu­la­tion of the dis­tance to the masers required a full year of com­put­er time.

The beau­ty of the method is that it mea­sures direct­ly a dis­tance to objects that are believed to lie close to the heart of the galaxy. It avoids many of the assump­tions of oth­er Milky Way dis­tance measurements.

Continued steps

So it seems that the Milky Way may not be as large as we sup­posed. A revised esti­mate of the size of the galaxy will affect our under­stand­ing of oth­er prop­er­ties of the galaxy, such as the mass of the galaxy, and the dis­tance and bright­ness of objects with­in the galaxy.

Radio astronomers have found masers in sev­er­al oth­er galax­ies beyond the Milky Way, so the pos­si­bil­i­ty exists of using the same method to refine our scale of inter­galac­tic dis­tance, and ulti­mate­ly to know more exact­ly the size and age of the universe.

The first humans who looked into the star­ry night must have won­dered what was the size of the uni­verse they lived in. The obser­va­tion of cos­mic masers is one more way to extend the human yard­stick into the dome of night.

We have come a long way from that first mea­sured step on the road from Alexan­dria to Syene.