Traversing the Cosmos and Methods of Measuring Galactic Distances

By Adhriti Raghuram

Distance is a fundamental aspect of our life. It’s simple to find the length of a table, or how far away your favorite ice cream shop is since the objects that are being measured are tangible. But how do you find the distance of something you can barely see, much less touch? Astronomers throughout history have asked the same question. In fact, it wasn't until fairly recently that the parallax method was established and widely used to calculate galactic distances.

The parallax is the apparent displacement of an object when seen from two different vantage points. Hold your thumb out, and close your right eye while keeping your left one still open. Now switch. Notice how your thumb appeared to move? This is the displacement that is referred to as the parallax. Astronomers have extended this concept to beyond the barriers of our atmosphere. Instead of the space in between your eyes, scientists use a six-month period where the Earth is on opposite sides of the Sun as their vantage points. By utilizing the parallax from both points, astronomers can accurately calculate the distance measured in parsecs (3.26 light-years).

The story of measuring cosmic distances is one that spans centuries. The first individual to make a stride in this effort, is Hipparchus (also spelled as Hipparchos). Born in 120 BCE, Hipparchus was an Ancient Greek astronomer and mathematician. One of his greatest works includes the first-ever astronomical catalog, which contains extensive observations about the positions of stars. Although what we know about him is limited to sources of his predecessors, his surprisingly accurate calculation of the distance separating the Earth and its natural satellite is prominent. During a lunar eclipse, Hipparchus documented the relation between Earth’s shadow and the glowing discs of the Sun and Moon. This way, he was able to determine the distance to the Moon with simple trigonometry. He initially estimated it to be approximately sixty-three times the length of Earth's radius. Historical records later confirmed that the true measurement is equivalent to sixty times Earth's radius, meaning Hipparchus was only slightly off.

From 190 BCE to the 19th century, astronomers’ attempts to solidify a method for calculating cosmic distances was ineffective. Credited as the first person to successfully use the stellar parallax, Friedrich Bessel was an incredibly accomplished German astronomer, who used a special telescope known as the heliometer to measure and publish the distance to Cygni 61 (a binary star system) from the Sun in 1838. This marked the first-ever successful calculation using the parallax.

While the stellar parallax is extremely accurate to our immediate cosmic neighborhood, its limitations are apparent as we venture through the Milky Way. Spectroscopic parallax emerges as more effective in calculating distances to stellar objects that are tucked away. This method uses a star’s apparent and absolute magnitude to determine the distance between the object and the viewer. Apparent magnitude is how bright the star appears to an observer on Earth, while absolute magnitude is how bright the star actually is. The absolute magnitude can be estimated using the luminosity class, a common stellar classification used by astronomers. By acquiring both magnitudes and inputting them into the following mathematical formula, m-M=5 log(d/10), scientists are able to calculate the star’s distance from Earth. While it is valid, it is fairly inaccurate due to interstellar extinction (clouds of stellar dust) which can obscure the true brightness of a star. Such inaccuracies lead to error bars of up to 25% in either direction. Despite the variation, the spectroscopic parallax has had a key role in mapping the vast universe.

In the grand cosmic journey to understand the universe, these methods of measuring distance are a result of persistence and innovative efforts built by generations of astronomers. While challenges and uncertainties still persist, these techniques have allowed humanity to transcend the boundaries of our grounded perspective and explore the corners of space, expanding our knowledge of the universe that surrounds us.