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- The Hertzsprung-Russell diagram was developed by Ejnar Hertzsprung and Henry Norris Russell in the 1900s (Shipman, Wilson, Higgins & Torres, 2016). The chart uses the star’s overall temperatures and size to categorize the many different types of stars in our universe. Every star in the atmosphere goes through evolutionary stages that are based on their inner cores, how it generates its energy and the size of the star. Each step represents a change in the star’s temperature and radiance. These changes cause the stars to move around on the HR diagram over time. Where the star is placed on the chart tells astronomers what the star’s core is made of and where the star is within its developmental stages (Cain, 2009).
Stars begin their life cycles as dust and gas. Then and gravity comes along and pulls the dust and gas together into protostars that eventually form into compacted clouds of gas (Redd, 2019). The smaller stars are unable to reach nuclear fusion and simply turn into brown dwarfs that never light. If the overall size of the star is large enough, the dust and gases will cause the star to burn at a higher temperature that will eventually fuse the hydrogen into helium (Cain, 2009). This allows the star to ignite and is then considered a main-sequence star, powered by hydrogen fusion. The star becomes stable after fusion forces out the gravitational pressure. The typical lifespan of the main sequence star is determined by the size of the star. The bigger the star is, the faster it will burn thanks to core temps and higher gravitational forces. The size and radiance of a star also factor into the color and temperature of its photosphere. The stars that burn the hottest on the sequence are blue in color, while the colder stars burn in red and the stars that are in between the hottest and the coldest burn shades of yellow, white, and orange. The massive bright stars have been named red giants, and the super-bright stars are called red supergiant’s (Shipman, Wilson, Higgins & Torres, 2016). Stars that are not on the sequence, but a little below it, are dimmer colors and called white dwarfs. The stars that are a part of the main sequence span range anywhere in size from about a tenth of the sun, up to 200 times as colossal (Redd, 2019). Over time the main-sequence star will burn through all the hydrogens that sit in its center, and the cycle of the star is over and is no longer on the main sequence. The smaller stars collapse into white dwarfs when it burned the hydrogen from its core but still radiates heat until it cools enough to be a black star and disappears (Cain, 2009). With the more prominent stars, the outside layers start to close into itself until temperatures reach the point where it’s hot enough to merge the helium into carbon. Combining helium into carbon releases a vast amount of pressure that pushes outwards and allows the star to grow until it becomes much more abundant than its original size, becoming a red giant. The stars that were already big to begin with, explode in a violent supernova death at the end of their lifecycle, spewing its pieces throughout the galaxy. All that is left behind is a single neutron star (Shipman, Wilson, Higgins & Torres, 2016).
- A black hole is gravity’s final victory over all forces (Shipman, Wilson, Higgins & Torres, 2016). When a more massive star has reached the end of its lifespan, the hydrogen fusing into helium is almost gone. These stars burn what’s left of the helium, which combines the atoms that are leftover into heavier elements. Once the star reaches the iron element, it no longer provides energy to support the outer layers of the star (Shipman, Wilson, Higgins & Torres, 2016). The top layers of the star start to collapse, then shatters into a bright supernova, leaving behind only a part of the star. Albert Einstein’s theory of general relativity predicted that if the leftover star is enormous in size then the star’s gravitational pull will overpower it and compress it into an infinite density, known as a black hole (Mann, 2018). A black hole is so condensed to the point that light cannot escape because of the powerful gravitational field. Generally, a black hole will do nothing when it is alone. However, when gas and dust surround the black hole, it will suck all the materials around it into itself. The gas and dust spins around and begins to heat up until it creates bright bursts of light (Shipman, Wilson, Higgins & Torres, 2016). The black hole will keep the combined gas and dust within its self and lets it continue to grow in size as it grabs the other materials that the black hole encounters and sucks in until it becomes a singularity (Mann, 2018). The singularity is surrounded by a boundary known as the event horizon that is invisible and cannot be seen. The event horizon determines what the size of the black hole will be by looking at where matter and radiation can go inside but cannot come back out.
- Knowing that galaxies move away from each other at the present time, it makes sense that the galaxies started out close to each other in the past. This conclusion led astronomers to think that the universe started in a small, hot, and very dense state that grew very rapidly and has been named the Big Bang Theory (Shipman, Wilson, Higgins & Torres, 2016). Scientists believe that they can follow the creation of the universe and subatomic particles within 10 -43 seconds of the Big Bang. The universe was so dense and compacted before the Big Bang that our current knowledge of the laws of relativity and quantum physics do not work. Within 10-4 seconds after the Big Bang started, the universe was already filled with photons that had high temperatures and density. A few seconds later, the photons formed neutrons, protons, and electrons. The universe was still growing and still cooling down at this point. After about three minutes, the protons had formed deuterium and helium nuclei. At thirty minutes old, the universe cooled enough to stop the nuclear reactions (Nagaraja, 2002). This left the universe at 25% helium nuclei and 75% hydrogen nuclei. After 500,00 years the universe had cooled down enough to allow the helium and hydrogen nuclei to take electrons and turn them into neutral atoms (Nagaraja, 2002). This allowed the photons to move through space openly while matter was influenced by gravity and started to form the stars and galaxies.
The standard model of the Big Bang theory is accepted because of the evidence that supports it: astronomers have observed galaxies that are continuously expanding but are not actually moving around, space is growing and is carrying the galaxies with it. Cosmic background radiation has been found coming from all different areas in space. This type of radiation is the same kind of radiation that was found 500,000 years after the Big Bang (Shipman, Wilson, Higgins &Torres, 2016). Astronomers have observed a ratio of 3 to 1 of hydrogen to helium within the stars and matter, which was predicted by the Big Bang model. The standard model of the Big Bang left too many questions unanswered. So, it was revised into the new inflationary model. This new model suite the idea of a flat universe that suggests at 10-35 seconds after the Big Bang began, the universe had cooled enough that some of the four fundamental forces joined as a unified force and released a tremendous amount of energy. This release of energy pushed enough pressure outwards to inflate the universe to about 1050 times its original tiny size by the time the universe was only 10-30 second old (Nagaraja, 2002). The universe was the size of an atom when this began and grew to the size of a grape in that minimal amount of time. Both the standard model and the inflationary model is the same after the first 10-30 second. The inflationary model suggests that the universe is infinite in size and has no edges or a center because the Big Bang occurred throughout the whole universe. We are unable to see the entire universe because of the amount of time it takes for light to travel back towards the powerful telescopes that allow us to see in space (Nagaraja, 2002).
- All stars are hot balls of glowing plasma that is held together by its own gravity, which is intense (Shipman, Wilson, Higgins & Torres, 2016). Stars are continuously crushing themselves inward, which causes the gravitational friction of the interiors to heat up. The high pressure and temperature at the core of a star allow nuclear fusion to occur, which fuses atoms of hydrogen into atoms of helium (Cain, 2009). This releases a tremendous amount of energy in the form of gamma rays, which become stuck inside the star and push outward against the gravitational tightening of the star. They’re absorbed by one atom and rereleased. This can occur numerous times a second, but a single photon can take up to 100,000 years to get from the center of the star to its surface (Cain, 2009). By the time the photons reach the surface, they have lost some of their energy allowing the photons to be visible and not the gamma rays they started out as. These photons leap off the surface of the Sun and head out in a straight line into space and can travel forever if they do not run into anything (Shipman, Wilson, Higgins & Torres, 2016). An example of this is when looking at a star that is eight light-years away, we are seeing the photons that left the surface of the star eight years ago and traveled through space without running into anything. Stars shine because they have substantial fusion reactors in their cores, releasing a tremendous amount of energy (Cain, 2009).
- Cain, F. (2009). Star Main Sequence. Universe Today: https://www.universetoday.com/24643/star-main-sequence
- Mann, A. (2018). How Does a Black Hole Form? Space. https://www.livescience.com/63436-11m-how-black-holes-form.html
- Nagaraja, M.P. (2002). The Big Bang. NASA Science. https://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-bang
- Redd, N.T. (2018). Main Sequence Stars: Definition and Life Cycle. Science & Astronomy. https://www.space.com/22437-main-sequence-stars.html
- Shipman, J.T., Wilson, J. D., Higgins Jr., C. A., & Torres, O. (2016). An introduction to physical science (Custom ed. for University of Oklahoma LSTD 3513). Boston, MA: Cengage Learning.
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