Distances in Astronomy

How do we know the distance of the sun from the earth? Or that of the nearest galaxy to our own? This article introduces four methods that astronomers use to measure distances in space.

Much of our knowledge of astronomy is based on our ability to measure distances in space. Knowing astronomical distances helps us understand, among other things, how stars, star clusters, nebulae, galaxies are distributed in space; or, how bright they truly are as opposed to how bright they appear to us. However, given the scale of this task, estimating distances to objects in outer space is not a trivial task.

Over time, astronomers have come up with some really interesting ways to pin down distances to stars in our own as well as other galaxies. These include:

  1. Trigonometric parallax: this technique is useful in estimating distances to stars within a few 100 light-years from us (see Box 1).
  2. Observations of variable stars: this technique is useful in estimating distances to variable stars, or star clusters with variable stars, in our own as well as neighbouring galaxies (see Box 2).
  3. Observations of standard candles in the dark: this technique is useful in estimating distances to galaxies in which supernova explosions (standard candles) are underway (see Box 3).
  4. Estimation of the velocity with which galaxies are receding from us: this technique is useful in estimating distances of galaxies that are >100 million light-years away (see Box 4).




To conclude

The universe is vast and every technique to measure distances to objects in space is suited to an optimal range. Some techniques work well for the estimation of distances to objects that are very close to us, while others work best for objects that are far away. This range is captured in a sequence that astronomers often refer to as the cosmic distance ladder. Techniques to estimate distances in space may help answer some of our most foundational questions about the universe. For e.g., we estimate the current age of the universe to be around 13.8 billion years. How do we know if this is true? Can we see the universe in its early stages? We can, at present, measure distances to galaxies that are >100 million light-years away from us. This means that we can see these galaxies as they were more than 100 million years ago. Will we one day be able to see even further back into our past than that? Only time will tell. 

Anand Narayanan teaches astrophysics at the Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram. His research is on understanding how baryonic matter is distributed outside of galaxies at large scales. He regularly contributes to astronomy–related educational and public outreach activities. Every so often he likes to travel, exploring the cultural history of South India.


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