The Life Cycles of Stars

1. PowerPoint Created by: Alexander J. Hawkins Information documented from DK Smithsonian UNIVERSE Definitive Visual Guide The Life Cycles of Stars

2. Stellar Life • As human beings go through cyclic lives, maturing from birth to maturity to old age, stars also follow a series of stages from their creation until death. Stars follow varying sequences of change depending greatly on their solar mass, or their gravitational weight (measured in 1.9891 × 10 (30th) kilograms per 1 solar mass). Regardless of whether a star is a low-mass, moderate-mass, or high-mass star, they are all born from interstellar clouds of gas that collapse under the pressure of gravity, enter a long period of stability called the main sequence, and die off to become another celestial body forever changed. However, what happens during a star’s existence dictates what unique paths it follows. As a result, our night sky is filled with distinct bodies of light, all stars that have experienced a different story in their lives.

3. Star Formation

4. Dense Clouds of Gas Collapse • Stars are born in cold interstellar clouds of gas that drift through space. Depending on how cool the cloud is in temperature, the gaseous clump, composed mainly of hydrogen, is less protected from a gravitational collapse. At lower temperatures, the cloud’s hydrogen atoms collect together to form hydrogen molecules. Once, the cloud grows to surpass a certain mass, and experiences a gravitational disturbance, sometimes caused by supernovae, it will begin to collapse into itself. As this occurs, fragmented pieces of the cloud of varying mass and sizes separate to form protostars, the earliest form of a star.

6. Protostar Development • Once protostars are formed out of an interstellar cloud, the stellar objects continue to collapse, causing central temperatures and internal pressure to build up. Depending on the mass of a protostar, temperature and pressure levels increase, so it can be stated that temperature and pressure are higher with higher mass protostars. If a protostar has a size less than 0.08 solar masses, then the temperature and pressure at its core will not reach high enough levels for nuclear reactions to begin, allowing it to reach adolescence, and the star will become a brown dwarf star. However, if a protostar surpasses 0.08 solar masses, the gas that had clustered to form the protostar begins to rotate around the star, increasing in speed as it draws near the stellar body, being pulled slowly in, until a ring of stellar material is formed around the protostar. Until entering its main sequence, the protostar demonstrates unstable movements and reactions, ex. Rapid rotations, strong stellar winds, etc.

8. Entering Main Sequence/ Planet Formation • With protostars with a mass over 0.08 solar masses, the internal pressure and temperature of the protostar will meet the requirements needed for nuclear reactions within the star to start. With this, the pressure of the stellar body will stabilize to balance gravity, classifying the protostar as a official star. After entering the main sequence, with the new star in a stable condition, the rings of extra material rotating around the star will begin to cool in temperature. As this happens elements within the disks will begin to condense, sticking to one another. Small pieces of material then join larger clumps, and the process continues until the balls of matter are the size of a planet. Of course, planet formation may take time, for while the clumps of material are still warm, other fragments impacting them may cause the piece to split apart again, making it so that planets are not permanently formed until they have cooled enough. Any excess, loose material from the star’s formation, after cooling without becoming a planet, become comets, asteroids, or trails of gas.

9. Forming Planets Revolving Around a Parent Star

10. The Main Sequence

11. A Long Stability • For 90% of a star’s life it exists in a period of stability called the main sequence. 90% of all stars in the night sky are currently in their main sequence, since the time frame of such calmness makes for most of a star’s life. During this time, stars expand and contract, but at very small levels, not changing dramatically in activity. Temperature and pressure levels remain mostly constant, with little differentiations over the course of the billions of years a star stays in the main sequence. However, depending on the initial mass of a star, the time a star follows the main sequence varies (more massive stars exit the main sequence sooner due to their faster burning of fuel in comparison to small stars). At the end of a star’s main sequence, their solar mass dictates what path they will follow in the last leg of their lives, and even the outcome of their death.

13. The Final Stretch

14. Different Sizes of Stars • Low-Mass Stars: Any star half or less the mass of our sun is considered a low-mass star. • Sun-Like Stars: Any star with equal or approximately equal mass as our sun is considered a sun-like star. • High-Mass Stars: Any star with a much greater mass than our sun is considered a high-mass star.

15. Low-Mass Stars • As most stars follow, low-mass stars eventually burn, or deplete, their hydrogen fuel in their cores. Once this happens, a low-mass star will convert its atmosphere slowly to helium instead of hydrogen, causing it to collapse; a similar trait among low-mass, sun-like, and high-mass stars. However, due to low-mass stars’ inferior mass, their internal pressure and temperature levels in its core can not reach the point of helium burning. This then causes the star to slowly cool down and loose luminance until the star fades into a black dwarf.

17. Timeline After Main SequenceLow-Mass Stars • Star grows in size as its hydrogen layer is burned away. • Star begins to collapse and shrink as its hydrogen fuel dissipates. • Star continues collapsing due to its inability to produce helium burning. • Star grows so small and cold that only a gaseous pressure contradicts gravity. • Minuscule, dark star progressively fades away. • After losing most of its regular pressure and temperature, having decreased in size tremendously, the low-mass star turns into a dim black dwarf star.

18. Sun-Like Stars • Sun-like stars, as can be concluded, have a similar mass as our solar system’s sun. After exiting the main sequence, such stars begin to use up all of their remaining hydrogen in their cores until the quantity of hydrogen available becomes depleted. Upon the occurrence of this, sun-like stars begin their process of hydrogen shell burning, where the hydrogen in their atmosphere begins to burn away, increasing their size until they become a red giant star. Red giants are massive stars that a sun-like star transforms into nearing the end of its life, which eventually sheds its outermost layers, becoming a planetary nebula. Over time, the planetary nebula builds up pressure and temperature at its core, causing helium burning to reactivate, and the star to expand once more. Soon after, the planetary nebula collapses into a white dwarf (slightly more illuminant and hot than a black dwarf), and then a black dwarf after it cools furthermore. • Note: Scientists have predicted that this is the likely path of our current sun, which is relative in size to other stars that have had similar timelines.

19. Timeline After Main SequenceSun-Like Stars • Star grows to become a red giant as hydrogen burning causes an increase in the size, of the star. • Red giant star’s outer layers of hydrogen and helium are released from the star, forming a planetary nebula. • Star within planetary nebula begins to expand due to helium burning, triggered by high temperature and pressure levels in the core of the star. • Star collapses inside planetary nebula after its helium shell is burned away, causing it to cool down into a white dwarf star. • New white dwarf star gradually fades and cools until it becomes a black dwarf star.

21. High-Mass Stars • It is a common misnomer that our sun is large enough for a supernova (an extreme release of stellar material and heat) to occur in its future. In truth, the only type of star that can undertake such an explosion is a high-mass star, a star with a greater solar mass than that of our sun. In astronomy, the higher a star is in mass, the more times it will go into a period of expansion and contraction. The mass of a star, in addition, decides the temperature of a star’s core each time it goes into a period of flexion. Depending on the stage in a star’s development, various elements are formed within the core of the star, with the heaviest sustainable material being iron, when a massive star forms an iron core. However, any elements heavier and denser than iron cannot be produced internally by stellar bodies (stars). The only way to create such substances is through a supernova explosion, which can create elements such as gold in the process. Once most high mass stars expand due to hydrogen burning, they become supergiant stars, with a mass greater than any other form of star, which produces heavy elements such as iron (some result in changing into red giant stars). However, instead of calmly settling into becoming a black or white dwarf star, high-mass red giant or supergiant stars collapse violently resulting in supernovae. After ejecting new, heavy elements into space, supernovae create either a neutron star or black hole that create extreme gravitational pull.

23. Timeline After Main SequenceHigh-Mass Stars • Star begins hydrogen burning, growing in size as its hydrogen reserves are lowered. • High pressure and heat cause the high-mass star to turn into either a red giant star or supergiant star. • Red giant star/supergiant star creates heavy elements, such as iron, inside of itself through nuclear reactions at its core. • After reaching the point in which the red giant/ supergiant star has made the heaviest element it can form, iron, the star collapses and explodes into a supernova, producing even denser elements with more weight in the process. • In the aftermath of the supernova, the collapsed star could turn into either a neutron star or a black hole depending on its solar mass before the supernova. • If the high-mass star remnant is over 1.4 solar masses, it will collapse to form a neutron star. • If the high-mass star remnant is over 3.0 solar masses, it will collapse to form a black hole.

24. Aftermath of a Supernova

25. Neutron Stars and Black Holes • Once a high-mass star reaches its stellar end point, the ultimate stage of development in a star, it either turns into a neutron star or black hole varying on the remnant of a supernova. If this remnant is 1.4 solar masses or larger (otherwise known as the Chandrasekhar limit), then the destroyed high-mass star will become a neutron star. However, if the remnant surpasses 3.0 solar masses, then the collapsed high-mass star will become a black hole. • Neutron Stars- Neutron stars are one of the two resulting bodies created by a supernova (especially said of type II supernovae explosions). A neutron star is an incredibly dense, compact star with a internal body made primarily of neutrons. Much different than their parent stars, neutron stars have a crystalline outer crust, a much stronger gravitational pull despite their small mass (usually between 0.1 and 3.0 solar masses), and their rapid rotation. This rotation slows over time due to the loss of energy, but will occasionally spike up again due to “starquakes”, small tremors that occur beneath the solid, thin surface of neutron stars. Some neutron stars eject beams of radiation regularly, commonly classified as pulsars. • Black Holes- Black holes are another resulting body created after a supernova, usually having to be greater than at least 3.0 solar masses. When a star collapses at such a size, the stellar object becomes incredibly small and dense, resulting in a gravitational pull so powerful that radiation (heat) or visible light can’t even escape. Such celestial forces are classified as stellar-mass black holes, which are only able to be detected through the effect and alterations they make to nearby objects in space; such as the light of distant objects in space being bended by their gravitational pull, the matter sucked onto its accretion disks (rings), and the changes they make to object movement in space. Stellar-mass black holes can be pinpointed by their high radiation levels created by the material they suck in, allowing astronomers to carefully observe them, despite the fact that they can hardly be seen by even telescope lenses. The most symbolic area of all black holes, the dark middle section, is called the event horizon, where light, radiation, or any matter can no longer escape from the black hole’s immense gravity. This area has troubled astronomers for years, for it is unknown what is on the other side of a black hole, or even if there is one. Like neutron stars, black holes are one of the many rare and spectacular phenomena produced by the death of a star, demonstrating that a star’s life continues on even after its primary period of activity.

26. Example of Neutron Star

27. Example of Black Hole

28. The Beating Hearts of Space • As if nature created all things, living and non-living, alike, the stars in our nighttime skies are not much different than the people of Earth. Stars go through a constant cycle, being formed in a tremendous display of growth, existing in a long period of stability, and finally being extinguished in a remarkable eruption of gas and fire. Then, as with the human race, stars are then born through the death of others, as the remnants of all supernovae result in the birth of a nebula. From there, the systematic pattern of stellar life repeats itself, producing new stars in place of the old. Stars are an important piece of our vast universe, giving life to planetary systems like ours, and demonstrating the basis of known space, upon which we can study the workings and particulars of the final frontier. All stars that reach the main sequence are formed in a similar manner, through the formation and activation of a protostar, but what lies between the main sequence and their stellar end points can vary. Some stars, with lower mass dissipate quickly due to rapid cooling, and turn into white and black dwarfs. Others with higher mass grow through hydrogen burning, becoming supergiant stars and, eventually, supernovae. So whenever you look up at the night sky, peering at the twinkling stars drifting endlessly, it is important to remember those distant, beating hearts of space.

29. PowerPoint Created by: Alexander J. Hawkins Information documented from DK Smithsonian UNIVERSE Definitive Visual Guide The Life Cycles of Stars