Astronomy is a science full of large measurements of space and time. In this article, we address the intriguing subject of how distances are measured in space. In order to calculate distances in space, there are four different techniques:
- Radar
- Parallax
- Standard Candles and Their Brightness Comparisons
- Redshift
1. Measuring Distance in Space Using Radar
Nowadays, the distances to nearby planets, asteroids, and comets are often gauged using radar. By sending a powerful radar signal to a target (like Venus) and measuring the time it takes for the signal to bounce back, astronomers can directly determine the distance to that object.
2. Parallax Method of Measuring Space Distances
By observing an object (like a planet, asteroid, or comet) from two well-separated locations on the Earth (or from opposite points in the Earth’s orbit around the Sun), astronomers can measure the apparent shift in the object’s position against the background of distant stars. Using simple trigonometry, this angular shift can be converted into a physical distance. The principle is similar to how our eyes perceive depth due to their slight separation.
For measuring the distance to nearby stars, astronomers turn to stellar parallax. This technique leverages the Earth’s orbit around the Sun. By observing a nearby star from opposite points in our orbit (six months apart), astronomers notice a slight shift in the star’s position against the backdrop of more distant stars. This angular shift, or parallax angle, is inversely proportional to the star’s distance. The smaller the shift, the farther away the star is. Using basic trigonometry, the distance to most stars in our immediate neighborhood of a few hundred light-years can be determined with extremely high precision.
The art of measuring parallax angles has been a fundamental component of astronomical research – especially in the pre-photography era, when it was the prime area of all astronomy studies. The European Space Agency’s Gaia space telescope represents a monumental leap in the field of stellar parallax measurements. Launched in December 2013, Gaia’s primary objective is to chart a three-dimensional map of our galaxy by accurately determining the positions, distances, and motions of over a billion stars with unprecedented accuracy. Its astrometric observations are so meticulous that, if Gaia were on Earth, it could measure the thumbnails of a person standing on the Moon!
3. Using Standard Candles and Brightness Comparisons
Comparing apparent and absolute brightness is a cornerstone of astronomical distance measurements. The basic principle is rooted in the inverse square law for light: as you move away from a light source, its brightness decreases inversely with the square of the distance. In more practical terms, if you know how bright an object truly is (its absolute brightness) and you measure how bright it appears (its apparent brightness), you can calculate its distance from you. This principle can be applied to objects both within our Solar System and far beyond.
For faint objects in our Solar System, such as asteroids or Kuiper Belt objects, the process is a bit more nuanced than with distant stars or galaxies. First, you would need to know or estimate the object’s intrinsic brightness, which could be determined by understanding its size, albedo (reflectivity), and how much sunlight it receives. Once its expected absolute brightness is known or estimated, this can be compared with its apparent brightness as observed from Earth. The difference between these two values can be used to estimate the object’s distance in space. This technique is often combined with other methods, like parallax, to improve accuracy.
Type Ia Supernovae as Standard Candle
Type Ia supernovae serve as one of the most reliable “standard candles” in space distance measurement. These exploding stars have a consistent and well-known peak absolute brightness due to the mechanism that triggers them. When we observe a Type Ia supernova, we can measure its apparent brightness. The difference between its known absolute brightness and its apparent brightness directly gives us the distance to the supernova and, by extension, to its host galaxy. This method allows astronomers to determine distances to galaxies billions of light-years away, making it a powerful tool in cosmology.
Originating from binary star systems, Type Ia supernovae have a distinct genesis. These start when one star in the system is hotter, brighter, and more massive than the other. The hotter star has a shorter lifespan, which means it runs out of hydrogen to burn and ends its life, turning into a red giant and fizzling into a white dwarf before the other one even begins to evolve off the main sequence. The formerly more massive star sheds much of its material into a planetary nebula, and the remnant white dwarf star ends up having a fraction of the mass it previously did. This often results in the two stars going their separate ways, but in some cases, the loss of mass results in the white dwarf being pulled closer to its companion on account of the companion’s now-greater mass. So close, in fact, that the gravity of the dense white dwarf begins to siphon matter off the other. When the material of the companion star hits the surface of the white dwarf, it can explode like a hydrogen bomb and briefly outshine both stars by thousands of times. Such systems are known as cataclysmic variable stars during these phenomena.
Eventually, the white dwarf star draws material from its companion until it hits a critical mass, approximately 1.4 times that of the Sun, known as the Chandrasekhar limit. This accumulation leads to a thermonuclear explosion, giving birth to the supernova. The consistent peak luminosity of Type Ia supernovae is a consequence of this standard critical mass. Nevertheless, slight variations do exist. To account for these, astronomers use the “brightness-decline rate relation,” a correlation between a supernova’s decreasing brightness post-peak and its absolute brightness. This allows for enhanced calibration, solidifying the reliability of Type Ia supernovae as standard candles. Their study, especially in the late 20th century, unveiled the universe’s accelerated expansion, hinting at the presence of enigmatic dark energy.
Cepheid Variable Stars as Standard Candle
Before the nature of Type Ia supernovae – and thus their usefulness as standard candles – was fully understood in the 1980s, the primary standard candle used by astronomers was the Cepheid variable star. Characterized by their pulsating outer layers – and thus regular shifts in apparent brightness – the frequency of their pulses directly correlates with their intrinsic average brightness. This relationship provides a consistent yardstick. By juxtaposing a Cepheid’s intrinsic brightness with its perceived brightness on Earth, its distance in space can be deduced. This was how the distance to nearby galaxies was first measured in the 1920s.
Building on our understanding of Cepheid variable stars as reliable “standard candles,” their role in one of the most profound astronomical discoveries of the 20th century becomes all the more evident. The period was marked by fervent discussions regarding the true nature of “spiral nebulae,” epitomized by the Andromeda “Nebula” (M31). A question loomed: Were these nebulae merely clusters of gas and stars within our Milky Way or entirely distinct galaxies? Prevailing theories suggested that our Milky Way encapsulated the entire universe and that these nebulae were just protoplanetary disks of nascent stars, albeit a flawed assumption given their prominence in the night sky.
The sheer abundance of these nebulae also raised doubts. Edwin Hubble, echoing the audacity of Copernicus from centuries prior, posited a revolutionary idea: just as Earth is not the center of the Solar System, the Milky Way might be one among numerous “spiral nebulae,” or as some proponents aptly named them, “island universes.”
After years of speculation, Hubble managed to find the answer. Using the 100-inch Hooker Telescope at the Mount Wilson Observatory, Hubble identified Cepheid variable stars within Andromeda in 1923-24. Leveraging the known relationship between a Cepheid’s pulsation period and its intrinsic brightness, Hubble was able to calculate the distance to Andromeda. His conclusion was groundbreaking: Andromeda was not a nearby nebula but a distant galaxy, situated over two million light-years away. This revelation not only confirmed the existence of galaxies beyond our own but also stretched our understanding of the vastness of the universe. In recognition of Edwin Hubble’s transformative contributions, including his subsequent insights into the expanding nature of the universe, the Hubble Space Telescope was aptly christened in his honor decades later.
4. By Making Use of Redshift
While Cepheids are bright compared to many other individual stars, their brightness pales in comparison to an entire galaxy or a supernova. At vast cosmic distances, even the brightest Cepheids become too dim to be detected with current telescopes, making them unsuitable for measurements beyond a range of about 100 million light-years. If you look at a planetarium program like SkySafari or Stellarium, you’ll notice a sudden drop-off in measured space distances, especially precise ones, to galaxies around this range, which is partly due to the dimness of Cepheids beyond such distances.
Type Ia supernovae, in contrast, are extraordinarily bright—so much so that they can outshine their host galaxies for a brief period. This brightness makes them excellent standard candles for much greater distances than Cepheids. However, they come with their own set of challenges. Firstly, such supernova events are infrequent, occurring only once every few centuries in a typical galaxy. Secondly, it wasn’t until the 1980s that astronomers realized the potential of Type Ia supernovae as standard candles. Their consistency in luminosity, linked to the mechanism of their explosion, was a revelation, but their sporadic nature means that astronomers often have to wait or be lucky to catch one in the act. The precision of measurement one can get with a supernova explosion is also more limited than that of Cepheids.
Given the limitations of Cepheids and Type Ia supernovae as standard candles, astronomers sought other methods to probe the vast distances of the universe, leading to the utilization of redshift. The concept of redshift is rooted in the Doppler effect, where light from objects moving away from us is stretched to longer (redder) wavelengths. This redshift of galaxies was first observed by Vesto Slipher in the early 20th century, and its implications were profoundly interpreted by Edwin Hubble.
Hubble, in the 1920s, noticed a correlation between the redshift of galaxies and their distance, as measured by other methods. This relationship, now known as Hubble’s Law, posits that the more distant a galaxy is, the faster it’s receding from us. The redshift thus becomes a measure of distance in space. This discovery laid the groundwork for the idea of an expanding universe, a foundational pillar of modern cosmology.
The beauty of using redshift is that it applies to any galaxy, no matter how distant, as long as its light can be collected by our telescopes. It doesn’t rely on the sporadic occurrence of supernovae or the detectability of individual stars, making it a more universally applicable method for gauging cosmic distances.
