In recent space news, the stars are at it again. In an article from badastronomyblog.com, two major stars that mark the constellation Gemini are baffling people and scientists alike. Ironically enough, these two stars, Castor and Pollux, that make up a constellation for the zodiac sign Gemini (which are supposed to be twins), are anything far from similar. These two stars, we 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 knowing the material that was covered in class made me understand what the author was referring to much clearer. Without further ado, I will explain what helped me put this article together to make it sound better than a pile of space jargon.
In class we studied how astronomers determine the luminosity, temperature, and size of stars. Luminosity, or absolute brightness, refers to the total amount of power given off by a star into space. What we see on Earth refers to the apparent brightness, which is the amount of starlight that reaches the Earth and can appear brighter or dimmer than the luminosity depending on distance to that star. The closer the star is to Earth, the brighter it appears and vice versa. In the case of this article, the two stars appear to be of equal brightness, but in reality, Pollux is a lot closer to the Earth than Castor, meaning that it is in fact less luminous if they were at the same distance. When it comes to measuring this luminosity, scientists first use an instrument to calculate the stars apparent brightness and obtain its distance to Earth using stellar parallax. Then, scientists use the inverse square law for light to determine the absolute brightness, or luminosity.
When it comes to determining the surface temperature, scientists once again study the spectral lines given off by that star. The hotter the star, the more blue light waves it gives off, giving it a more blue color. Stars that are cooler appear to be more red in color because they give off more red light. Stars in the middle, like our sun, emit light waves from the middle of the spectrum and appear to be white or yellow in color. To be more exact, by studying the spectral lines in comparison to a stars chemical make up, it can be inferred that stars that have more ionized particles are hotter because they need more energy to ionize atoms. Furthermore, stars on the red side generally are less hot and the spectral lines show a composition of molecules in its atmosphere. This promotes the notion that these stars are cooler; Molecules need less heat otherwise they will break apart into atoms, so stars made of molecules must be cool.
Scientists use surface temperature data along with spectral lines to classify stars from hottest to coolest with the letters OBAFGKM. In class we were asked to come up with our own mnemonic other than “Oh Be A Fine Girl, Kiss Me”, in which I memorized the sequence of letters with “Or Buy A Few Great Khakis Maybe?” (I was trying to make up a scenario where I was telling my friend that pants were on sale. Space pants, with stars). The star Pollux is mentioned in the article to be classified as a K star, meaning its temperature is on the lower side of the spectrum. As for Castor, it is a lot more complicated being a combination of 6 stars. The largest 2 stars are type A stars, which are on the hotter end, followed by 2 red dwarfs, and the whole system being followed by 2 cool M type stars.
Finally, when it comes to measuring the size of a star, scientists use other stars to calculate its massiveness. The easiest way to calculate mass is to use stars that is part of a binary system. These stars occasionally pass over each other, causing eclipses that can be measured by a doppler shift. This shift can be used to calculate the orbital period and velocity, giving the orbital distance. With the orbital distance and period, scientists use Newton’s version of Kepler’s third law in order to come up with the mass of the star.
Learning about the luminosity of stars helped me further expand my knowledge about the universe. I never realized how much information one little scientific discovery can lead up to. In further objectives, the importance of luminosity will shine through as we discuss the classifications of stars and other qualities. For now, just keep shining.