The Cosmic Distance Ladder: How to Measure Distance in Space
How do astronomers determine how far away something is in space? People often take for granted the ability to tell how far away something is. If it is not clear to the plain eye, one can use rulers or other tools as reference points for measurement. In space, however, it is much more difficult to tell. Knowing distance is incredibly important since it is impossible to interpret data to see if, for example, a star appears brighter because it is more luminous (the actual amount of light it is emitting) or is just closer to the planet from which it is being observed. However, despite the minuscule amount of information astronomers can collect about objects in space, amazingly they do have a method of determining their distance. This method, or rather, a set of methods is called the cosmic distance ladder. It is a ladder in that the technique used changes depending on the object's distance which is split into multiple rungs.
The first rung on this ladder is the parallax method. To understand this method,hold up a finger in front of you and close one eye, then switch which eye is closed. Your finger probably appeared to move a little. This happens due to the different angles from each eye to your finger shifting where the background appears relative to your finger. This can be applied by astronomers on a larger scale by using the movement of the Earth around the sun. Astronomers can measure the change in angle from the earth to an observed star between two opposite days of the year. With this data astronomers can make a triangle with the distance between Earth’s position A and B serving as the base, the measured angle being the opposite angle, and then can solve for height, which is the distance. Yet this method does have a major limitation: angles measured this way even for close stars are measured in fractions of arcseconds (1/3600 of a degree), and for farther stars that need measurements more precise than 0.01 arcsecond this method becomes ineffective. For objects where the effects of parallax become unreadable due to distance, astronomers must move to a higher rung in the ladder.
The next two rungs consist of standard candles, objects of which the distance can be determined by observing their properties and then can be used as a landmark to measure the distance of other objects around them. Cepheid variable stars, for example, have a strange pulsating effect by which they appear to become brighter and dimmer. Prominently, the period between their brightest and dimmest points can be used to accurately determine their luminosity, or how much light they actually emit. With their luminosity and how bright they appear from Earth measured, their distance can be quite easily calculated with the equation b=l/4d2. Once the distance of the cepheid variable star is determined, the relative distance of other objects in formations with them can be inferred. Unfortunately, this method becomes unusable if the variable stars are too far to see, at that point, a brighter standard candle is needed.
Type IA supernovae are caused when a star that died but was not massive enough to become a supernova gains enough mass after its death (ex. by absorbing another star) to have a supernova anyway. These supernovae occur with very similar luminosity which can be more precisely determined by measuring the wavelengths of light emitted. The luminosity can then be used to determine distance in the same manner as before, so the supernova can be used as a standard candle. However, some objects are so far away, such as distant galaxies or galaxy clusters, that even the supernova explosions cannot be seen.
For the farthest objects that astronomers can observe, the final rung is used: redshift in relation to the expansion of the universe. Redshift is the phenomenon of light waves becoming stretched out as the universe expands between them causing them to have a higher wavelength and appear more red. Since the vast majority of the mass in galaxies, and the universe, is hydrogen, the wavelength of light coming from far away galaxies can be compared to the wavelength of light observed when light passes through hydrogen on Earth, and the redshift of the galaxies can be determined. This can be compared with the Hubble constant, the rate of expansion of the universe, to determine distance. However this method is not only on the highest rung because it can be used for the farthest objects, but also because it is used only if none of the others are applicable due to it being the least accurate. This is not to say that is not accurate, however, it lacks the degree of precision of the others due to several factors, most prominently the debated value of the Hubble constant, with estimates ranging from 68 km/s/Mpc to 72 km/s/Mpc. Other galaxies can also cause minor interference due to their gravitational pull; however, for father objects, this affects the redshift to a lesser extent.
Because the ability to determine how far away things are relative to each other is often taken for granted, in the great cosmic void it takes creativity to find ways to find where objects in space actually are. The methodologies contained in the cosmic distance ladder manage to help overcome this challenge in astronomy and allow astronomers to more deeply understand what they are looking at.
References
The American Association of Variable Star Observation. The Cosmic Distance Ladder. Aavso. https://www.aavso.org/cosmic-distance-ladder
Strand, K. A. (2024, December 6). parallax. Encyclopedia Britannica. https://www.britannica.com/science/parallax
https://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html
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Preuss, P. (2015, June 1). Standard-candle supernovae are still standard, but why?. Berkeley Lab News Center. https://newscenter.lbl.gov/2014/03/03/standard-candle-supernovae/
Impey, C. (2012, September). Relating redshift and distance. Teach Astronomy. https://www.teachastronomy.com/textbook/The-Expanding-Universe/Relating-Redshift-and-Distance/
Warren, S. The Hubble constant, explained. University of Chicago News. https://news.uchicago.edu/explainer/hubble-constant-explained