Life Cycles of Typical Stars
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| The Milky Way over Lake Meke. 4burakfe via Wikimedia Commons. |
Imagine a time that you went stargazing: What did you see? Chances are, you saw a few–or a few thousand–glimmering stars, some shining brighter than others; some orange, some red; some massive, some small; some luminous, some dim. In the winter, the night sky bedazzles us with its coruscant and pulchritudinous glitter, as it does in the summer; Sirius, Regulus, Adhara, Betelgeuse, Rigel, Aldebaran, Procyon, Capella, Bellatrix, Alnitak, Alnilam, and Mintaka, all pierce our eyes–and our hearts–with their characteristic luminosities. And in the summer, the Milky Way, that dense band of stars appearing to us as a glimmering line of milk, conveys its own peculiar brilliance; Deneb, Vega, Altair, Antares, Arcturus, Sadr, and the great stars of Sagittarius and Ursa Major also dominate our vision.
Observing this prodigious assemblage of nuclear fusion, our brains fail to understand the immensity of the universe around us. Even watching those few thousand stars (among a billion-trillion) in our night sky, we are humbled by the formidable collection which exists above our heads. Few of us realize that there are yet many more stars–and types of stars–lurking in the shadows, surrounding those stellar immensities above.
Let me ask you one thing, assuming you are reading this entry on a clear night: Can you see stars? If so, what do they look like? Do they vary in brightness? Color? Questions like these can help classify a star. Based on these questions, we can determine, among others, several different general stellar classifications (all of which will be covered in this entry): main sequence (spectral classes, from left-to-right on the HR Diagram, O through M), non-main sequence giants, non-main sequence bright giants and supergiants, and non-main sequence hypergiants.*
The main sequence (O-M spectral type)
The term “main sequence” is one of the most general terms in the universe, describing hydrogen-burning stars that form a distinctive band on the Hertzsprung-Russell diagram–a graph of stars that is obtained by plotting stars’ surface temperatures against their luminosity. The main sequence is composed of a prolific variety of stars, from red dwarfs to blue hypergiants. The characteristic that likens these two apparent opposites is the fusion in their cores: Every main sequence star, whether a red dwarf or a blue hypergiant, is fusing hydrogen into helium. Per the condition, a star thereby leaves the main sequence as it begins fusing helium to carbon and other elements.
Defining Characteristics
As the main sequence is so all-encompassing, there are many differences between the smallest and largest stars within it. Indeed, these characteristics depend largely on where they are in the main sequence. The sun, for example, is a G2V spectral class star–in other words, a slightly-yellow dwarf. With a surface temperature of approximately 5,778 Kelvin and an absolute magnitude of 4.83, the sun is near the middle of the downward-sloping main sequence curve. The sun is used as a reference point for luminosity (so, +4.83 is the central absolute magnitude), with other stars in the diagram measured in terms of their luminosity relative to that of the sun.
Stars on the uppermost branch of the main sequence are the blue giants, newly formed and unstable stars that formed following the collapse of a massive molecular cloud. Blue giants are significantly larger, brighter, and younger than the sun, and often will spend only a few million years on the main sequence. Blue giants are anywhere from ten to one-million times brighter than the sun, and with surface temperatures of the largest stars reaching upwards of 30,000 Kelvin, they are among the hottest stars in the universe. On the other end of the spectrum are the red dwarfs, which constitute 90% of the stars in the universe. Red dwarfs are far cooler than their blue giant main sequence cousins, and can often remain in the main sequence for even trillions of years. Red dwarf stars have such long lifespans that there has not yet been a single documented case of a red dwarf burning itself out.
Life Cycles
As with its characteristics, the life cycle of a main sequence star depends entirely on its position in the main sequence; for smaller stars, their lifespans will be longer, whereas for larger stars, their lifespans will be shorter.
Stars with greater than eight solar masses will undergo nuclear fusion until their core, once full of an “iron ash”, implodes and results in a core-collapse supernova; in the largest stars, however, there is no supernova, and the core collapse is simply an implosion straight to a black hole.
Following hydrogen fusion, stars with less than eight solar masses will undergo helium fusion and eventually exhaust all their helium, but that will be it; at this point, with cores composed largely of carbon and oxygen, the solid husks of their stars–all that remain now that most of the star beyond its core has been ripped away from it–stand alone in the universe; they are now white dwarves–dead stars’ slowly decomposing cores–and a vibrant, though fleeting, planetary nebula.
Notable Main Sequence Stars
Considering a vast majority–90%, that is–of stars are main sequence stars, there are quite a few notable examples in the universe; some notable examples of main sequence stars include the sun, Alpha Centauri, Toliman, Rigel, Canopus, and the three stars of Orion’s Belt–Alnitak, Alnilam, and Mintaka.
Non-main sequence giants
The non-main sequence giant is a common type of star that has exhausted its core-borne hydrogen fuel and thereby left the main sequence. The classes of stars considered in this section are giants, bright giants, supergiants, and hypergiants. The first such class–the giant–is the smallest of the non-main sequence giant branches. The giant originates from main sequence stars of greater mass than red dwarfs. This particular giant has exhausted all of the hydrogen fuel in its core (as all non-main sequence giants have), and has the ability to fuse helium into heavier elements. Giants, however, will not fusion further than helium fusion, and as a result will experience different deaths than their supergiant and hypergiant counterparts. Giant stars constitute the red giant branch.
Life Cycle
The life cycle of a typical giant is similar to that of an intermediate main sequence star; the star begins as a protostar and eventually undergoes hydrogen fusion. The star continues fusing hydrogen to helium until the core of the star is entirely helium, at which point the core of the star experiences gravitational collapse and develops sufficient pressure for the triple-alpha process–helium fusion. When this occurs, a region beyond the core of the star begins performing hydrogen fusion itself, increasing the total energy (and, therefore, force) produced by the star. As the mass remains unchanged due to the conservation of matter, the increased radiation force yields a net force, which accelerates much of the star’s outer mass further away from the core and vastly increases the aggregate size of the star. Once again, the star is removed from the main sequence and added to the giant branch. If the giant is less than eight solar masses, it will continue fusing helium into heavier elements until those heavier elements occupy the entire core. Once this occurs, the star is no longer able to perform fusion in its core, and after a series of helium flashes that occur throughout its giant branch stage, it will eject the entirety of its non-core material to form a planetary nebula; the star’s dead core–a white dwarf–will remain for several trillion years until it, too, eventually loses most of its energy to heat and becomes, to an extent, a mere husk.
As was considered earlier, the typical, intermediate mass giant star will experience a death significantly different from the high-mass supergiants, and even more different than the hypergiant. The death of an intermediate-mass giant star is far more spread out and far less instantaneous and violent than with bright giants, supergiants, and hypergiants. The intermediate star undergoes a series of helium flashes, brief intervals of runaway helium-to-carbon nuclear fusion occurring in the core of stars of .8 to 2 solar masses. These helium flashes cause significant amounts of star matter to be blown away from the star, forming a planetary nebula such as the Dumbbell Nebula. The core, on the other hand, eventually runs out of helium, and as it is unable to produce sufficient pressure required for further fusion, it ultimately dies, leaving in its wake a dead husk, the white dwarf. In some rare cases, though, the white dwarf can reach the Chandrasekhar limit of 1.4 solar masses, at which point a Type 1a supernova will occur and transform the white dwarf into a neutron star–the more extreme core of a dead star. The only notable possibility of this event is with Sirius B, the white dwarf companion of Sirius A, which is around .3 solar masses away from the critical limit.
Notable Giants
Some notable giant branch stars include Aldebaran, the brightest star in Taurus, and Arcturus, the brightest star and object in the constellation Bootes–as well as the second brightest star in the Northern night sky. 42 Draconis, γ Aquilae, δ (delta) Eridani are also notable examples.
Non-main sequence bright giants and supergiants
As with giant stars, bright giants and supergiants are in the process of fusing higher-order elements (that is, helium or further) into larger such elements. A notable difference, however, is that bright giants and supergiants transition from hydrogen fusion to helium fusion more smoothly than their lesser mass companions. Bright giants and supergiants, unlike their giant counterparts, do not experience severe helium flashes resulting in the loss of stellar mass. In addition, once these stars have fused all the helium in their cores into carbon, they have masses that can provide the pressure sufficient for fusion of higher elements–all the way to an iron ash that cannot be fused. Once this star reaches a full iron-ash core, it is unable to fuse further elements (iron fusion requires more energy than it releases), thereby contracting the core further. As mass “seeps” into the core and is consequently fused, the star’s core eventually reaches the Chandrasekhar limit–the mass value at which electron degeneracy pressure can no longer solely balance the force of gravity–the result is a core collapse and a subsequent type II supernova–the most common supernova subtype. Its life is, of course, significantly different and often significantly shorter than a red giant, and it experiences quite a more entertaining demise than its giant counterparts.
A red bright giant or supergiant is currently fusing at least helium. As with the typical giant stars, bright giants and supergiants have left the main sequence and are instead near the top of the Hertzsprung-Russell diagram. The larger a star is, the faster it fuses elements into heavier elements; as a result, a sun-sized star has a longer lifespan than its supergiant counterparts.
Notable Bright Giants and Supergiants
There are, indeed, many notable bright giants and supergiants in the night sky. Among the most obvious is Betelgeuse–one of the brightest stars in the night sky–which is most well known for its unusual dimming in 2019 and early 2020. Another few notable supergiant stars include Antares, Epsilon Pegasi, among numerous others. Red supergiant stars are quite rare, but their luminosity makes them very obvious in the night sky, even from significant distances, thus they contain some of the most notable and brightest stars in our night sky.
Non-main sequence hypergiant
The hypergiant has only one characteristic that contrasts it from supergiants, bright giants, and typical giant: For stars that are exceptionally massive and whose cores are as well, their cores not only reach the Chandrasekhar limit throughout their life cycle, but the Oppenheimer-Volkoff limit–the maximum mass a neutron star (neutron core of a massive star) can have before collapsing into a black hole, a value agreed to be around 2.17 solar masses. These stars, upon collapsing into a black hole, do not produce any supernova whatsoever; they leave not a trace of their existence, besides a pitch black husk. These hypergiants are exceptionally rare, and there are no notable hypergiants that are easily visible in our night sky. The largest, such as UY Scuti and VY Canis Majoris, are still too dim to be visible–with the naked eye, that is–from Earth.
Wrapping it up
The stars have, for as long as we’ve known them, been intrinsic to our everything; they were not only the harbingers of the seasons, the growing seasons and the calendars, but the guardians of all our greatest triumphs, fears, and aspirations. For hundreds of thousands of years, our ancestors survived off the night sky’s familiar coordinate system; navigation relative to Polaris allowed ancient sailors, setting sail without compasses, a sense of direction. The North Star to Freedom guided enslaved peoples of the United States to safe havens in the north and in Canada. The constellations of Eridanus, Aquarius and Pisces were the harbingers of the wet season in Greece. The intricate grid of our night sky holds not only intrinsic beauty, but practicality as a means of location and guidance–whether directional or moral.
For humanity, the night sky is a sanctuary; it is a view intrinsically connected with the deepest realms of our collective souls; “it is not restricted to any one nation or ethnic group”, Carl Sagan once said, referring to a penchant for wandering, “but it is an endowment that all members of the human species hold in common.” We all live under the same sky, the same stars. As always, take care and stay curious, everyone.
* Please note that the classification system used in this entry is not that used by scientists. It is, instead, a brief overview of stellar life cycles (which I’ve classified in this way so as to avoid some of the technicalities of current scientific research).
References
“Asymptotic giant branch.” n.d. Wikipedia. Accessed March 5, 2023. https://en.wikipedia.org/wiki/Asymptotic_giant_branch.
“Hypergiant.” n.d. Wikipedia. Accessed March 5, 2023. https://en.wikipedia.org/wiki/Hypergiant.
“Main sequence.” n.d. Wikipedia. Accessed March 5, 2023. https://en.wikipedia.org/wiki/Main_sequence.
Tillman, Nola T. 2022. “Main Sequence Stars: Definition & Life Cycle.” Space.com. https://www.space.com/22437-main-sequence-stars.html.
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