How far are the Stars?

The nearest star to our planet is the Sun at a distance of 1500 million kilometers. Next in line is the Proxima Centauri, a red-dwarf star in the constellation of Centaurus, and it is merely 400,000 billion kilometers from us. The word `merely’, used in the sentence is a relative term. Before we get an idea of how far the bright spots in the skies are away from us, and how do we get to measure their distance from our earth, let us first have a quick look at some of the terms that we would find useful in explaining these concepts.

The first term is a light year, abbreviated as ‘ly’, and it is the distance traveled by light traveling continuously for one year. The speed at which light travels is 300,000 km/s. So one light year is the number of seconds in a year times the speed of light, and it turns out to be 9.4605284 × 10¹⁵ m , that is, 9.4 followed by 15 zeros in terms of meters or 12 zeros after 9.4 in kilometers.

Another commonly used unit of length in astronomy is the ‘astronomical unit’, abbreviated as AU, and it is defined as the mean distance between the sun and the earth. It is equal to 149,598,000 kilometers.

Parallax is a phenomenon in which a nearby object viewed from two different positions appears to move with respect to a more distant background.

Measurements:
The parallax method is commonly used to measure the distance of stars from us. This method employs some simple concepts to do this – properties of triangles and the principle of the relative motion of a far-off star (object) compared to that of a near-by star (object). The principle is that a nearer object will appear to have moved more, if observed from two different positions than a distant object. If the distance between the two positions of observations (baseline) is known, and if the shift in the relative position of the object in question is measured, the distance between those two objects can be found. Let us apply this principle to measure the distance of the stars from us:

We know that the earth revolves around the sun, thus it is at different locations at different times during a year. Keeping in mind this fact, any two points with a time difference of six months act as two different observation points with the distance of 2 AU, i.e. the baseline is 2 AU.

Now, the star in question is the bigger one and the diagram demonstrates how it would look like with respect to a much distant background. And a triangle like this one gets formed: the distance AB = 1AU and angle T can be measured using various instruments. Now two angles of the triangle ABQ are known along with the length of one side. So one just needs to use the equation:

Tan a= perpendicular (AB) / base (QB)

The method has got its limitations, and it can be used the measure the distance of not more than 10% stars because only some of the stars are in the range of this method.

 Print This Page