Measuring distances across the vast expanse of the universe presents a fundamental challenge for astronomers. Direct measurement methods are often impractical or impossible due to the immense scales involved. For this reason, astronomers rely on a sophisticated hierarchy of indirect techniques, often referred to as the cosmic distance ladder. These methods build upon one another, with techniques for closer objects calibrating those for more distant ones.
Measuring Within Our Solar System
For objects within our solar system, such as planets and asteroids, astronomers employ radar ranging. This method involves sending radio wave pulses toward a celestial body and precisely measuring the time it takes for them to reflect back to Earth. Since radio waves travel at the speed of light, the distance can be calculated by multiplying the travel time by the speed of light and dividing by two (to account for the round trip).
Radar ranging provides accurate distance measurements for objects relatively close to Earth, enabling the tracking of planetary orbits and mapping of surfaces. While effective for our immediate cosmic neighborhood, the strength of the radar signal diminishes significantly over larger distances, making it impractical for measuring stars or galaxies beyond our solar system.
Measuring Nearby Stars
For relatively nearby stars, astronomers use stellar parallax, a geometric method. This technique relies on the apparent shift in a star’s position against the background of more distant stars as Earth orbits the Sun. Observations are typically taken six months apart, when Earth is on opposite sides of its orbit, creating a baseline of approximately two astronomical units (AU).
The observed angular shift, known as the parallax angle, is inversely proportional to the star’s distance. This relationship forms the basis for the parsec, a unit of distance where one parsec corresponds to a parallax angle of one arcsecond. Earth-based telescopes can accurately measure parallax for stars up to about 100 parsecs away, but space-based observatories have extended this range significantly.
Measuring Distances to Other Galaxies
For distances beyond the reach of parallax, astronomers turn to “standard candles”—objects with a known intrinsic brightness. By comparing an object’s known intrinsic brightness to its observed apparent brightness, its distance can be determined.
One type of standard candle is the Cepheid variable star, which pulsates with a direct relationship between its pulsation period and true luminosity. Henrietta Swan Leavitt discovered this period-luminosity relationship, often called Leavitt’s Law, in the early 1900s by studying Cepheids in the Magellanic Clouds. By measuring a Cepheid’s pulsation period, astronomers can determine its intrinsic brightness. Comparing this to its apparent brightness from Earth allows for distance calculation. Cepheid variables can be used to measure distances ranging from about 1 kiloparsec (kpc) to 50 megaparsecs (Mpc).
Another standard candle is the Type Ia supernova. These occur when white dwarf stars in binary systems accumulate matter from a companion star until they reach a critical mass, approximately 1.4 times the mass of our Sun (the Chandrasekhar limit). At this point, they undergo a thermonuclear runaway explosion. This consistent trigger results in Type Ia supernovae having a uniform peak luminosity, making them bright and reliable standard candles.
Type Ia supernovae typically reach an absolute magnitude of about -19.3, making them about 5 billion times brighter than the Sun. Their brightness allows them to be observed across intergalactic distances, beyond the reach of Cepheids. They are particularly useful for measuring distances to galaxies from about 1 Mpc to over 1000 Mpc, helping to map the large-scale structure of the universe.
Measuring the Farthest Objects
For the most distant objects in the universe, where even Type Ia supernovae become too faint to observe reliably, astronomers rely on the phenomenon of cosmological redshift. As the universe expands, light traveling through it has its wavelengths stretched. This stretching shifts the light toward the red end of the electromagnetic spectrum, a phenomenon known as redshift.
The more distant a galaxy is, the more its light has been stretched during its journey, and thus the greater its observed redshift. This relationship is encapsulated in Hubble’s Law, formulated by Edwin Hubble, which states that a galaxy’s recessional velocity (its speed away from us) is proportional to its distance.
The constant of proportionality in this relationship is known as the Hubble constant (H0). Current estimates for the Hubble constant fall within the range of 67 to 74 kilometers per second per megaparsec. By measuring the redshift of a distant galaxy, astronomers can infer its recessional velocity and, using Hubble’s Law, calculate its approximate distance. This method helps probe the most remote corners of the observable universe and understand its expansion history.