How to Measure the Distance from Celestial Objects?

by Carson
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When you analyze profiles of celestial objects online, you may be curious how the distance between us and the object is measured even though a laser altimeter will take years to complete its mission. Therefore, we’ll learn about the more efficient methods to measure the distance from celestial objects today.

1. Using Parallax

A great way to obtain the distance is by using parallax. To understand parallax, place your finger in the middle of your field of sight. Close one eye to see the position of your finger. After that, open the closed eye and immediate close the other eye. Did you see the finger move? That is basically how parallax mathematically works.

To use parallax, you must first locate the object and record its location on the image and the exact angle at which the photo was taken. Then, aim the imager at the same angle and locate the object again six months later. If your imager is sensitive enough, you should find a noticeable difference in the star’s positions over six months in the precise data that it has generated. Then, the distance between the imager and the object can be obtained using a little bit of trigonometry and the data acquired from the measurement.

An illustration of a parallax system
NOTE: The objects of the image are NOT to scale.

In fact, the unit of “parsec” is based on parallax. If the locations of the object differ by one arcsecond (1/3600 of a degree) in both images, the object is measured to be precisely one parsec away. One parsec is approximately equal to 3.26 light-years.

2. Using the Luminosity of the Star

Parallax is indeed an excellent method, but sometimes the star is too far that there is no noticeable difference between the star’s positions in the two images. In that case, we must first obtain the luminosity of a star by figuring out its temperature using the spectrum and then working out the object’s mass so that the diameter of a star can be approximately obtained. After that, the star’s luminosity is calculated, and the absolute magnitude of a star is figured out.

Other than that, Cepheid variables can also be used. They are variable stars whose luminosity goes back and forth. They are exceptional even in this category of variable stars because their periods can be used to determine their brightness. Thus, they are valuable in measuring the distance to nearby galaxies. The stars whose luminosity can be directly measured easily is called standard candles.

What is the absolute magnitude of a star? It’s the apparent brightness of an object if it’s observed from 10 parsecs, or roughly 32.6 light-years, from the object. It is an essential metric as the light from an object fades away because the observer gets farther from it at the speed that obeys the inverse square law. If the brightness seen by an observer from a distance (n) is equal to (m), the apparent brightness will be (m/4) if the distance becomes 2n. Therefore, the distance from the star can be obtained by comparing the actual luminosity of the star and the apparent brightness observed from Earth.

A graph plotting the relationship between distance and the apparent brightness or gravitational pull in the inverse square law.
Image created using Canva

3. Using Redshift

For very faraway galaxies, parallax and Cepheid variables cannot be used because the objects are too far away for any reliable data to be measured from them. However, there is one method to calculate the distance from distant galaxies, and it relies on the Doppler effect and the fact that the Universe is expanding.

The Doppler effect is the effect where if the object that emits light or sound is moving further away from us, its wavelength increases, causing redshift. If the object gets closer to us, a decrease in wavelength will be measured, causing blueshift.

The Universe is constantly expanding, meaning that most objects are getting farther away from us. It also implies that the farther away an object is, the faster it gets farther away from us, even though we are not in the center of the Universe.

Therefore, the amount of redshift measured when continuously measuring the wavelength of the light coming from the galaxy can help us approximate how far the galaxy is from our planet. However, this is not suitable for nearby galaxies such as the Andromeda galaxy because the mutual gravitational effects are substantial over these distances. Therefore, it’s better to use Cepheid variables to measure the distance from these galaxies.

Conclusion

Today, we discussed the ways to measure the distance from celestial objects, ranging from parallax, to using the star’s intrinsic luminosity, and obtaining the amount of redshift that a faraway galaxy exhibits. If you want to learn more about these methods, please visit the webpages in the references below.

References and Credits

  1. Jim Lucas Tereza Pultarova. (2018, December 12). What Is Parallax? – How Astronomers Measure Stellar Distance. Retrieved January 15, 2022, from https://www.space.com/30417-parallax.html
  2. (n.d.). How are astronomers able to measure how far away a star is? Retrieved January 15, 2022, from https://science.howstuffworks.com/question224.htm
  3. (n.d.). Cepheids. Retrieved January 15, 2022, from https://starchild.gsfc.nasa.gov/docs/StarChild/questions/cepheids.html
  4. (n.d.). Standard Candle. Retrieved January 15, 2022, from https://astronomy.swin.edu.au/cosmos/s/Standard+Candle

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