Going Back to the Twins

In a past conceptual objective, I have discussed the the existence of two stars that make up the constellation Gemini, Castor and Pollux. Just as a recap: these two stars, well actually one dying red giant and the other the product of three binary systems (you hear that, 6 stars for the price of one!), display different qualities that are explained thoroughly in the article. However, information shared in class about luminosity and temperature lead me down the path of deeper understanding. Today I will be revisiting this article within a different focus, distance, size, and mass.

During lecture, we discussed how scientists study the properties of stars including the distance, size, and mass. When accounting for the distance of stars, we learned, in the Lecture-Tutorial on the Parsec, that scientists use telescopes to determine the parallax angle, which is the smallest angle created in the right triangle between the Earth, Sun, and the star whose distance you are measuring. First, scientists observe a star in the night sky which goes through parallax in our night sky. Parallax is defined as the apparent motion of nearby objects relative to distant objects. On Earth, objects also experience parallax. For example, in class we mentioned how if we place a finger in front of our nose, close one eye, and then switch which eye is closed, we can observe parallax. The finger’s position has not actually changed; However, when observing it one eye at a time and relating it to our background, we can determine how close it is to our face because of how dramatically different each side sees it “change” distance. The further away from our face we place our finger, the smaller the parallax, until it is so far away that it just blends in with the background (maybe if we had detachable fingers, but I digress). Similarly, scientists determine the distance a star is from Earth. A “close” star is observed in relation to the “fixed” stars in the background and the two are compared when the Earth on opposite sides of its orbit. This way scientists see the two extremes of the star’s parallax motion in the night sky. They then use geometry to come up with the parallax angle.

Image result for parallax angle

Knowing this parallax angle, scientists use some more geometry to figure out the distance to the star. This distance is is described in the unit of length called the parsec. One parsec is the distance to a star that has a parallax angle of 1 arcsecond. Also, parsecs and parallax angles are reciprocally related; The smaller the parallax angle, the larger the distance to the star. For example, if a star has a parallax angle of 1/6 of an arcsecond, the distance to that star is 6 parsecs. However, no star is actually that close, so the act of figuring out the distance is a little harder on an even smaller-angle and larger-distance scale. In relation to distance, these two stars in Gemini are actually relatively close, one being at 50 light years and the other at 34.  Since these stars are so “close”, it can be assumed that the parallax they display every 6 months is greater than other constellations that are farther away.

When it came to measuring the size and mass of other stars, scientists use distance in conjunction with its luminosity to determine those traits. When comparing two equidistant to us but near each other stars and comparing their luminosities, the larger star will appear to be more luminous, therefore knowing the distance and luminosity to a star can determine its size. A more massive star will appear to be more luminous at a distance as well. Relating to the article, while both Pollux and Castor seem to be about the same apparent luminosity, Pollux is about 16 light years closer than Castor, meaning that its actual luminosity is much dimmer than that of Castor.

Upon constant reflection on how stars are are studied and characterized, I feel as though I am more comfortable with sharing my knowledge with others. Besides being a thoroughly useful topic in understanding the world beyond us, astronomy is a field in which many take interest into but do not know where to start in gaining understanding. Attending these lectures and taking this class encourages me to inspire others to pursue an education in the sky, even if it is just for fun.


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