How It All Star(t)s!
The Beginning of the End: The Story of Stars
Have you ever stared directly at the sun and wondered what makes it different now than it was 1,000 years ago? First off, you should never look at the sun directly to protect your eyes, however, if you are looking at the sun directly; a solar lens is always a fantastic idea. Many people talk about the sun being unstable and the future of the human race, however, how many can back up their fears with facts? Freight not, after reading this article you'll be able to! In this article, we will be discussing the life cycles of all stars both low/normal and high mass stars and how they are similar and different. Stars are incredible objects that can sustain themselves for billions of years through fusion and work with gravity throughout their life cycle. Many astronomers of the past and present have worked towards determining how and why stars are the way they are. Using strategies like spectroscopy and examining the luminosities of stars have allowed these various astronomers to build upon each other to determine the makeup of stars throughout their life cycle. Continue reading below to enjoy the wondrous story of stars!
Molecular Clouds: The Building Blocks of the Universe
All stars are made from clouds of “star stuff” also known as Molecular Clouds. Many factors come into play when looking into the shapes and mystifying colors of molecular clouds.
Fun Fact! Molecular clouds come in all shapes and sizes some even ranging up to a billion times the size of the Sun!
Temperature has a high impact as colder clouds tend to clump together making them higher in density. As clouds begin to clump and move they will rub against each other heating up. This heat is due to the friction created between the two clouds and will aid in the next step to the stars forming: the creation of the protostar. The gravitational pull on the molecular clumps is another
outside factor that helps in the creation of stars. Each clump in a molecular cloud will eventually collapse due to gravitational forces causing the clump to form a stellar system. This means that each clump will turn into a solar system similar to our own! As the cloud condenses it collapses into a disk shape which initiates the process of star formation, this disk is the protostar...
Protostars: The Star Before the Star
The collapse of the molecular cloud causes the dust to begin to spin and flatten into a protostellar disk spinning around the protostar. The protostellar disk begins to spin because of the angular acceleration of the stellar dust. The rotation of the protostar causes the generation of a magnetic field which in turn creates an outward flow of particles also known as a protostellar wind.
Let's Talk About The Image!
The image depicted to the left is an artist rendition of a protostar. We can see in the center the protostellar disk surrounding the protostar. But you may be wondering what those yellow beams are? You're in luck! We'll dive into that next....
Alongside the protostellar wind protostars also have jets along their rotation axis that get rid of extra dust and debris around the protostar. The larger the star the quicker it completes its phase as a protostar because of the greater amount of gravity acting on the star allows it to progress quicker. The increased amount of gravity allows for the disk to spin faster, the wind to move debris faster, and for the overall process to progress faster. Unfortunately, not all protostars get to experience the glory of eventually becoming a bright and beaming star.
A protostar must be greater than 0.08 solar masses to reach the 10 million Kelvin temperature to begin efficiently performing hydrogen fusion. We'll dive into the importance of hydrogen fusion later on. However, any star that does not reach this threshold becomes a brown dwarf star, brown dwarfs are small and hard to detect. Many astronomers have concluded that brown dwarfs are likely to outnumber all other stars in the universe, due to the difficulties protostars face reaching this temperature. If a protostar is lucky enough to reach the necessary temperature it can begin hydrogen fusion and become either a high or low/normal mass star depending on the amount of mass in its initial molecular cloud.
The Life of Low/Normal-Mass Stars
Star Formation: Creating Low-Mass Stars
Protostars form inside these massive molecular clouds which allow them to gain mass and develop. Slowly, the protostellar wind and radiation blows away the gas and dust, and once cleared the young star becomes visible. These stars are called T Tauri stars, which are pre-main sequence stars. Over time this star will collapse until its temperature and pressure at its core is able to begin hydrogen fusion. The star converts the hydrogen in its core into helium, and eventually becomes a main sequence star.
Red Giants: An Aging Star
When hydrogen fusion can no longer happen in a main sequence star’s core, gravity begins to collapse the core. The star brightens as its core shrinks and its outer layers expand. When the core reaches 100 million K, the helium nuclei fuse into carbon. This continues until a sudden burst of energy called a helium flash occurs. This helium flash heats the core to the point where it can no longer degenerate. The core now expands which lowers its temperature and reduces its energy output. After about 100 million years the star's core will become entirely carbon. The red giant will keep expanding and remain like this for a few million years. During a red giant’s life it can span between a wildly varying 62 million and 620 million miles in diameter.
Fun Fact: During its lifespan a red giant will be about 100 to 1,000 times wider than our sun and about half our sun's surface temperature. In about 5 billion years our sun will become one of these red giants.
Planetary Nebulas: Mysterious Art in Space
Once gravity can no longer contain the outer layers of the red giant, the red giant will eject the layers into space. Its carbon core remains and emits ultraviolet radiation that ionizes the gas of the layers. The gas becomes charged and begins to emit light. This is what causes a nebula to glow with bright color. These ejected outer layers now become the planetary nebula. These nebulae only last for about 20,000 years, which is relatively short when it comes to the other stages of stars.
Fun Fact: Despite the name, planetary nebulae actually have nothing to do with planets. The name comes from a misclassification, 250 years ago, when astronomers thought they were observing gas planets.
White Dwarfs: A Star's Core Revealed
The extremely dense carbon core that is left after the nebula is formed, is a white dwarf. White dwarfs possess immense gravitational forces that are about 100,000 times that of Earth. Its temperature starts at around 100,000 K and the dwarf fades as it slowly cools. Once it can no longer be seen it is considered a black dwarf. This process takes over tens or hundreds of billions of years. We have yet to discover a black dwarf because the universe’s oldest stars are only known to be 10 to 20 billion years old.
Let's Talk About The Image!
The predominant star in this image is actually NOT a white dwarf. This is an image of Sirius A and Sirius B. Sirius B, the white dwarf, pales in comparison to Sirius A. The white dwarf is much less massive and much less luminous.
The Life of High-Mass Stars
Star Formation: Creating High-Mass Stars
Similar to the formation of a protostar high and low mass stars do not form incredibly differently, however, due to the greater amount of mass high mass stars form much quicker. Another reason for the quicker exhaustion of hydrogen is that hydrogen fusion begins before the end of the protostar phase. Hydrogen fusion is able to begin sooner due to the higher temperatures high-mass protostars can reach. The majority of the hydrogen fusion in a high mass star occurs through the CNO Cycle.
What is the CNO Cycle?
The CNO Cycle is the use of other elements (Carbon, Nitrogen, and Oxygen) as catalysts in hydrogen to helium fusion.
The CNO Cycle is only present in high-mass stars due to the higher temperatures these stars foster.
Once a high mass star has exhausted its helium fusion it continues to contract due to the higher gravitational forces which allows it to progress further than a low mass star in element fusion. The star will continue with fusion in the order of carbon, neon, oxygen, silicon, and iron. Each element will form a shell within the star as the core temperature of the star rises causing it to expand and continually heat up. This brings us to the image depicted above, which shows each of the layers in a high-mass star. Once the star exhausts all of its different element fusions up to iron it will inevitably become what we consider a red supergiant.
Red Supergiants: The Celestial Behemoths
Imagine a star so vast that if placed at the center of our solar system, it would engulf planets up to Mars! These are the red supergiants, stars that have lived fast and grown colossal. As stars much more massive than our Sun exhaust their hydrogen fuel, they enter a transformative phase, becoming red supergiants. In this expansive state, their cores shrink, but their outer layers swell immensely, glowing with a deep red as they cool. The sheer scale of a red supergiant is hard to comprehend: they can be over a thousand times wider than our Sun.
Fun Fact: The largest known red supergiant, UY Scuti, could fit more than a billion Suns inside it!
Red supergiants are also among the shortest-lived stars, existing for merely millions of years — a blink of an eye in cosmic terms. These stars burn their fuel at an extraordinary rate due to their massive size. It's fascinating to note that despite their vast size, they are not as hot as smaller stars like our Sun. Additionally, Betelgeuse, one of the most famous red supergiants, is expected to go supernova in the next 100,000 years, an event that will be visible from Earth even during the day!
Supernovas: Cosmic Spectacles
A supernova is the ultimate fate of these red giants, and it is a spectacle that outshines entire galaxies for brief moments. This stellar explosion occurs when the core can no longer sustain its own weight and collapses under the force of gravity. In mere seconds, the core's collapse triggers a rebound effect, blasting the star's outer layers into space at millions of miles per hour. Supernovas are not just stunning; they are crucial for life as we know it. They scatter elements like carbon, nitrogen, and oxygen into space—elements that make up every living thing on Earth
Did you know? All gold and platinum on Earth were formed in such stellar explosions billions of years ago?
Supernovas play a crucial role in the creation of solar systems. The shock waves from these explosions can trigger the formation of new stars and planets by compressing nearby interstellar gas and dust. This process essentially recycles stellar material into new celestial objects. In 1054, Chinese astronomers recorded a supernova so bright that it could be seen during the daytime for over three weeks. This supernova created the Crab Nebula, an object studied extensively by astronomers as a key source of insight into cosmic rays.
Neutron Stars: Lighthouses of the Cosmos
After a supernova, what remains can be a neutron star, an object so dense that a sugar-cube-sized amount of its material would weigh as much as all the humans on Earth combined! These stars may be small, around 20 kilometers in diameter, but they spin incredibly fast, some as many as 600 times per second. These remnants spin at incredible speeds and emit beams of electromagnetic radiation from their magnetic poles. When these beams sweep across Earth, they appear as pulses, leading to some neutron stars being observed as pulsars. These dizzying speeds make neutron stars some of the universe's most powerful magnets and ticking clocks, allowing astronomers to test the limits of physics.
Neutron stars are sometimes called the lighthouses of the cosmos because of their pulsar variant. These beams are so regular that they have been used to confirm the presence of gravitational waves predicted by Einstein's theory of relativity.
Fun Fact: In 2019, astronomers detected the heaviest neutron star known, which is about twice the mass of the Sun but only 30 kilometers in diameter, challenging existing theories about the structure of such stars.
Black Holes: Echoes of Gravity
The most massive stars end their lives by collapsing into black holes, regions of space where gravity is so strong that not even light can escape. The boundary around them, known as the event horizon, marks the point of no return. These mysterious features of the universe influence the formation of galaxies and are thought to be at the center of every massive galaxy, including our own Milky Way. The concept of a black hole is so bizarre that even Albert Einstein initially doubted their existence. Around these mysterious entities, space and time behave in ways that defy our everyday understanding. Interestingly, the nearest known black hole to Earth is only about 1,000 light-years away, in a system visible with the naked eye under dark skies.
Black holes are not just fascinating for their extreme gravity—they also play a significant role in the dynamics of galaxies. They can emit powerful jets of particles that travel at nearly the speed of light, influencing galaxy formation and evolution. In 2019, astronomers captured the first image of a black hole's event horizon, a groundbreaking achievement that offered the first direct visual evidence of a black hole's existence. Moreover, black holes are suspected to be the engines behind quasars, the brightest objects in the universe, which are visible even from billions of light years away.
Simulation
Click down below to animate the different stages in a star's life and see how its brightness, size, and mass changes with time.
Take a Short Quiz to Test Your Knowledge!
Click down below the take a short quiz about the life cycle of stars.