How to Measure Asstronomical Distances: An Introduction
What are the distances we come across in our daily lives? One km, 10 km, 100 km, 1000 km, or even 50,000 km. But the distance between the sun and earth is approximately 150 million kilometers (i.e. 150,000,000 km). So how to scientists make such large astronomical measurements? Up to a certain level (i.e. within our solar system) we can follow some direct precise measurement methods. But beyond that we have to rely upon photometry and spectroscopy.
The extragalactic distance scale is known as cosmic distance ladder (a series of methods used to determine the distances of celestial objects with each step building upon the results of the previous step). By extending the ladder, we can measure larger and larger distances which implies that the base of the ladder refers to direct measurement techniques like RADAR, LiDAR etc.
RADAR (Radio Detection and Ranging) is not limited to applications within our earth. It can be used to measure the distances up to certain planets in our solar system. The principle of RADAR basically involves sending out radio waves and receiving them. The distance of the target is calculated by the time taken for travelling to and fro.
LiDAR is similar to RADAR with the difference being that the light source is Laser pulses instead of radio waves. LiDAR provides accurate measurements but it can be used only within a short astronomical range (say to find the distance between Earth and its moon).
The distance to nearby stars can be studied by parallaxes (a.k.a triangulation). A simple analogy would be to place a pencil in front of your nose and to see it through your left eye and then through the right eye while closing the other eye. You would see the shift in the position of the pencil. This is because the distance of the pencil from your left eye is different than from the right eye. This difference in distance forms the base of an isosceles triangle while the distance to the star forms the lengthier sides of the triangle. The base is taken to be the earth's diameter (measurements taken across two extremes of the earth). As time evolves, the stars appear to shift in position leading a different angle. The difference in angles is used to calculate the distance of the star. The greater the distance, the smaller the parallax (angle). Thus it cannot be used to measure distances larger than few thousands of light years.
To measure the distances of stars at large distances we rely on photometry i.e. measurements are made in terms of their brightnesses. According to the inverse-square law, brightness decreases proportional to the square of the distance. If we know the absolute brightness of a star, then by measuring the apparent brightness of star, we can calculate the distance of the star. The problem here is knowing the absolute brightness of the star whose distance which we don't know. But this problem can be somewhat solved with the help of known neighboring stars.
Apart from a few of the popular methods above, there are many other methods such as observing shift in colour spectrum, Hertzsprung-Russell diagram etc. The major units of astronomical distances are au and parsec although light year is very common.
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