The Movement of the Stars from Different Perspectives

Stars have been vital to civilizations throughout the world. Religions have been founded, calendars created, and discoveries made simply based off of observation. The movement of the stars has been tracked since the time of the Egyptians, their measurements calculated by ancient Greek astronomers Ptolemy and Hipparchus. However, what the Egyptians and Greeks had yet to discover, was that the stars were not actually moving, instead, the Earth. Stars are born when there is a gravitational collapse of a gaseous nebula. This nebula must be comprised primarily of hydrogen, along with trace amounts of heavier elements. Once the stellar core is dense enough, the hydrogen is steadily converted into helium via a process called nuclear fusion. This process can take up to ten million years to completely occur. Eventually, the star will use up all of its hydrogen, and then resort to burning helium, swelling into something called a red giant.

Stars are not always red, in fact they come in a variety of colors. Their colors can range from red to yellow to blue. Stars that are red signify they have the coolest temperature, less than 3,500 Kelvin or about 5840 Fahrenheit. Stars that appear to be yellowish white have an average temperature of 6,000 Kelvin (10340 F). The hottest stars are blue, meaning that their temperature is over 12,000 Kelvin (21140 F). Because temperature and color are related, it is very easy to determine the temperature of a star based purely on observation. Furthermore, the temperature of the star is determined by its mass. The more mass, the larger the star and therefore more nuclear fusion occurring at its core. This means that when you are looking at the sky, you can determine the temperature and size of the star based solely on its color. Another thing you can determine is the lifespan, as the most massive stars are also the shortest lived.

The lifespan of a star depends on its mass. The larger the mass, the faster the star will burn through its fuel supply, ending its life. Blue stars will burn out and explode after only a few million years, however yellow stars can generate enough fuel for around ten billion. Stars that are very small, like red stars can keep fusing hydrogen for up to a trillion years. Once a star has exhausted all its fuel supply, they will “explode in a core-collapse supernova, some of the most energetic explosions in the universe” (Temming, Maria.) The left-over part of the star’s core will form a black hole, depending on how large the remaining mass is.

Looking at the stars, they do not appear to be so far away. However, the closest star to earth is Proxima Centauri, which is approximately 4.2 light years away. If we were to try and travel from earth on our fastest spaceship, it would still take over 70,000 years to get there. We can measure the distance to the stars with something called Parallax. This is where we measure how much a star moves during one orbit of the sun. With this information we can calculate the distance to each star.

From our perspective on the Northern Hemisphere, the stars are rising in the east and setting in the west. If we observe carefully, they appear to be rotating around the North Star in counter clockwise circles. Some of these stars will never set below the horizon, and these are called circumpolar stars. As the stars “move” they remain fixed in their same pattern, meaning that the stars rotate in a kind of constant background across the sky. The patterns that they are in are called constellation. There are eighty-eight different constellations, many of them originating from the Greeks. Some of these constellations named by the Greeks are Ursa Major, Draco as well as the constellations that are associated with the astrological signs such as the Twins (Gemini) or the Lion (Leo).

The movement of the stars is constant. They move approximately 15 degrees every hour throughout the day. Over the course of the day, the stars move 361 degrees, just one extra degree past a perfect circle. This means that every day, the stars will rotate a little bit further and further from the original position where they started, eventually ending up in the same exact spot at the same time the next year. If we were to look at The Big Dipper for example, every night it will complete its full rotation throughout the sky. However, if we look at it at on the first day of each month at the same time at night, the Big Dipper will have moved 30 degrees each time. This is why different constellations are visible during different points in the year, and why the night sky in the winter looks very different from that of the night sky in the summer.

The apparent motion of the stars also depends on what point you are standing on the earth and looking at the night sky. The further north you go, the higher in the sky the north star will be, therefore there will be more circumpolar stars. Most stars will be rotating around the North Star, and very few will rise and set. As you move south, the North Star will appear lower in the sky, causing fewer constellations to be circumpolar, and as a result, most stars will rise and set. Stars that are visible in the northern hemisphere are usually not visible from the southern hemisphere and vise versa. If we were to travel to the equator, the stars would appear to be rising directly above us in the east and setting directly behind us in the west. The explanation for all these effects is simply because the surface of the earth is curved. So, when we travel to a different location and the stars appear to be in different positions in the sky, it is because the horizon tilts with respect to the stars. This also suggests that the stars aren’t actually moving and that instead, we are.

When we look up at the night sky, it appears that all of the stars are moving except for one, the North Star. This is because the North Star is closest to the north pole, lying almost directly above it. In fact, the North Star isn’t directly above the north pole, however it is the closest star and because the tiny motions are impossible to notice, it is named as such. Although the North Star is currently Polaris, in the year 3000 BC, it was a star called Thuban. In about 13,000 years, the North Star will be Vega. This is because the axis of the earth is very slowly tilting as a result of the gravitational pull of both the sun and the moon on the equatorial bulge of the earth. The rotation of the earth is not the only reason that the stars are moving. Occasionally, we are observing a binary system, where two stars are orbiting around each other. Stars can also move because of proper motion. Proper motion is essentially a result of gravity, and because gravity makes every object in space move, logically the stars would move as well. Stars also have some random motion as they don’t orbit the galaxy in exact motions. This usually only amounts in a few kilometers in either directions, and in a galaxy as vast as ours, the movement is hardly noticeable.

Much like the Greeks, I first became interested in the movement of the stars on one of my many camping trips. I looked up at the night sky at several different points throughout the night and noticed that the stars I had observed before were no longer in the same place. The entire night sky had shifted in its position. After that experience, I was very motivated to learn everything that I could about the stars, as well as the various constellations. Since then, I have been watching the night sky as much as possible, although observing the stars has become increasingly difficult in New York. While I love the idea that the stars are the ones who are moving, this is not the case. Instead Earth is the one moving in orbit, while the stars remain ever fixed above us.