Cycle of the universe: What is a star and how do stars form?
- Mar 20
- 18 min read
Updated: Mar 21
How much do we really know about stars, those balls of fusion found all over the universe? Understanding the fundamentals of stars that organize planets plays a crucial role in our understanding of the rules of the universe. So, let's learn together what a star is, how it forms and dies, and what its characteristics and types are!
Contents
What is a star?
Astronomical celestial bodies that have sufficiently strong internal gravity to ignite the fusion of elements in their core are called stars. They also emit light and energy due to the extreme temperatures in their cores. But if you want a simpler answer to the question "what is a star?", you can summarize it in a single sentence: " The disco balls of the universe that emit immense nuclear energy by burning hydrogen !"
What are the characteristics of stars?
They emit their own light.
They produce thermonuclear fusion.
They are mostly spherical in shape.
Their masses are always greater than those of planets .
They rotate around their own axis and usually have an orbit.
They emit normal light, infrared, ultraviolet, X-rays, and gamma rays.
They have strong magnetic fields.
It may have rings.
They can have temperatures ranging from 2,226°C to 49,726°C.
They can be red, orange, yellow, white, and blue.
They are mostly composed of hydrogen (73-90%), helium (9-25%), and other elements.
They complete their formation in an average of 10 million years.
They have a lifespan of between 50 million and 20 billion years.
They died in a massive explosion (supernova), a neutron star, or a black hole.
How do stars form?
Stars are born within clouds of dust and gas scattered throughout the universe. A gas cloud is “disturbed” by the impact of a passing comet or supernova, and the gases gradually collapse inward . The collapsing gases begin to rotate , and as a result , the gas cloud flattens into a disk . Over time, the disk begins to rotate faster, pulling material towards the center, where a hot, dense core forms.
This core is called a protostar, or primordial star . The protostar continues to heat up due to gravity, and when the core is hot enough (99 million degrees), hydrogen atoms undergo fusion to produce helium and energy . Materials continue to flow into the core, increasing mass and heat. After millions of years, this center reaches a turning point: if enough mass collapses into the core , a two-way burst of material flows through the protostar, completing its transformation into a mature star. This is how stars form in the universe.
The Life Cycle of Stars
The evolutionary process of stars actually begins the moment they start forming in a gas cloud. When they gather at the point of gravity in the gas cloud, they have a baby core. Over time, the mass of this core increases, and if it reaches a sufficient temperature, it becomes a successful star; if not, it fails.
As the star forms, it gradually depletes the hydrogen fuel in its core over time, and in larger stars, helium begins to transform into iron, slowing down nuclear fusion. In this process, the force exerted by the core on the outer layers decreases, and it begins to succumb to gravity, meaning the core begins to collapse. The core, now subdued by gravity, gradually heats up , and helium transforms into carbon and oxygen . Due to the heat and transformations , the star's outer layers expand, and its brightness decreases. The star becomes a red giant. Eventually , the helium in the star runs out, everything collapses towards the core, and the star's evolutionary process ends.
How long do stars live?
The lifespan of stars depends entirely on their size. Large stars quickly deplete their hydrogen fuel because they need more to maintain the temperature in their cores. The opposite is true for small stars. Therefore, the lifespan of large stars is 10-100 million years, while that of small stars is several billion years.
How do stars die?
Stars die when they exhaust the hydrogen fuel in their cores, meaning they can no longer continue nuclear fusion. When a star reaches this stage, it begins to collapse inward due to gravity. Most stars in the universe silently transform into white dwarfs in this way. However, if the dying star has 8-15 times the mass of the Sun, the material collapsing to the center cannot withstand the pressure, and a massive explosion (supernova) occurs due to the resulting energy accumulation. If the exploding star has a mass of 20-60 solar masses, its death may create a black hole; if it has a mass of 8 or more, it may create a neutron star.
The Structure of Stars
Stars have various layers, like an onion. All stars, including the Sun, are composed of 5 different layers:
Core: It is the region where nuclear fusion takes place, with temperatures ranging from 10 to 100 million degrees Celsius depending on the star's type.
Radiation zone: It is the region that carries energy (photons) and surrounds the nucleus.
Convection zone: The region that releases energy from the radiation zone to the surface, causing the release of light and other forms of energy. This is the region that creates explosions and spots.
Photosphere: It is the name given to the surface of a star. Spots are also seen in this area.
Atmosphere: The outermost layer of a star, composed mostly of hydrogen and helium.
How many stars are there in the universe?
There are approximately 200 sextillion (200,000,000,000,000,000,000,000) stars in the universe. For comparison, if we counted all the grains of sand on Earth, it would amount to approximately 7.5 sextillion. So we can say that the universe tirelessly "gives birth" to stars!
How big or small can stars be?
The smallest stars need to be at least 8.7% the size of the Sun and 0.00125% the luminosity . The largest stars, theoretically, could average up to 300 million solar masses.
How much bigger are the stars than Earth?
The Sun, an average- sized star , is 109 times larger in diameter and 330,000 times more massive than Earth. The Sun has a diameter of 1,391,000 kilometers, while Earth's diameter is only 12,742 kilometers. The Sun's mass is 1.989 × 10^30 kg, while Earth's mass is 5.972 × 10^24 kg.
How much larger is the smallest star than Earth?
Considered the smallest star discovered to date, EBLM J0555-57Ab has a diameter of 59,000 km, 4.6 times the size of Earth , and a mass of 1.611 × 10^29, only 2.7 × 10^4 times larger than Earth. With a mass equivalent to approximately 85 Jupiters, EBLM J0555-57Ab is very small compared to other stars, as it is thought to be a baby star.
How are stars classified?
Stars are classified according to their spectral type, that is, their temperature and the lines in their spectra. This classification is called Morgan-Keenan (MK). The basic stellar classification is done with the letters O, B, A, F, G, K, M from hottest to coldest, but there are also classes L, T, C, R, N, Y, S, D, P and Q.
Image source: Wikipedia
Classification of Stars
O-Class: These are the hottest and brightest stars in the universe. They have shades of blue. They are rare and have short lifespans.
Class B stars: Bright blue stars. They have short lifespans. They possess helium and hydrogen lines.
Class A: These stars are white and bluish in color. They have strong hydrogen lines.
Class F: These are white stars. They have weak hydrogen lines and ionized metal lines.
G-class stars: These are white and yellowish stars, with the Sun being the best example.
K-class stars: These are orange-colored stars. They have weak hydrogen lines.
M-Class: This is the most common class of stars and they have low luminosity. They have metallic streaks, but hydrogen streaks are mostly absent.
L-class stars: These are dark red stars. They are cooler than M-class stars.
Class T: These are cold brown dwarfs.
Class Y: These are brown dwarfs that are cooler than Class T dwarfs.
CRN Class: These are carbon stars that have reached the end of their lives.
S-class stars: These are stars that fall between M-class and carbonaceous stars.
Class D: These are white dwarfs whose nuclear fusion has ceased.
Q-Class: A star that theoretically surpasses the mass of a typical neutron star.
Stars can also receive additional letters depending on their type . For example, blue dwarfs are in the O class, while blue supergiant stars are in the OB class.
Classification of Stars According to Their Brightness
Stars are also classified according to the brightness they emit. This classification is known as the Yerkes Spectral Classification.
D or VII: White dwarfs
sd or VI: Lower dwarfs
V: Main Sequence Stars (Dwarfs)
IV: Sub-states
III: Giants
II: The Bright Giants
Ib: Low-light supergiants
Iab: Medium-brightness supergiants
Ia: Luminous superpowers
0 or la+: Supergiants or brilliant supergiants
Image source: Prof. Dr. Bilsen Beşergil
Living Stars
Stars that have not exhausted their hydrogen fuel in their cores and are still undergoing nuclear fusion are considered living stars. We classify these stars according to their mass, temperature, and luminosity. Here are all the types of living stars you need to know:
Brown Dwarfs
Main Sequence Stars
Yellow Dwarfs
Orange Dwarves
Red Dwarfs
Blue Giants
Blue Supergiants
Red Giants
Red Supergiants
Intergalactic Stars
Brown Dwarfs
They are T and Y class stars. Brown dwarfs are not actually stars because the gravity in their cores is not high enough to initiate nuclear fusion. Therefore, they are also known as "failed stars" .
Main Sequence Stars
Main-sequence stars are a general name given to stars that convert hydrogen into helium, and they comprise 90% of the stars in the universe. These stars, including the Sun, can have masses up to 200 solar masses. Their colors range from white to red, encompassing the O, B, A, F, G, K, and M star classes.
Yellow Dwarfs
Yellow dwarfs are G-class stars. They have a mass between 0.8 and 1.2 times that of the Sun and a temperature of 5,000-5,700°C. When yellow dwarfs die, they silently fade into white dwarfs because their mass is not large enough to create an explosive burst. The Sun is the best-known G-class star.
Orange Dwarves
These are K-class stars. Their size falls between M and G-class stars, but their mass is 0.6 to 0.9 times greater than the Sun. Their temperatures range from 3,600 to 4,900°C. These stars are thought to potentially create suitable environments for life, especially on terrestrial planets.
Red Dwarfs
Red dwarf stars are either K or M class stars. Most stars in the universe are red dwarfs, with masses less than half that of the Sun. Their temperatures are below 3,200°C, and the light they emit is low. Some emit only about 1/10,000th of the Sun's light. Because of their small size, red dwarfs can have lifespans ranging from tens of billions to trillions of years.
Blue/White Giants
O, A, and B class stars. Most stars seen from Earth at night are blue giants because they are very bright. O-type stars have an average temperature of 35,000°C, A-type stars 10,000°C, and B-type stars 25,000°C. O-class stars have approximately 15 to 90 solar masses, A-class stars 1.4 to 2.1 solar masses, and B-class stars 2 to 16 solar masses.
Based on their mass, O-class stars become either black holes or neutron stars when they die, A-class stars become white dwarfs, and B-class stars, depending on their mass, become either neutron stars or white dwarfs.
Blue Supergiants
These are OB class stars. They have a mass 10 to 20 times greater than the Sun and temperatures of 9,700-50,000°C. Their luminosity is 10,000 to one million times greater than the Sun. When they die, a massive supernova explosion occurs.
Red Giants
Red giants are classified as K, M, S, C, R, and N stars. Their mass can range from 0.3 to 8 solar masses, and their temperatures can vary from 2,200 to 4,700°C. Red giants are generally stars nearing the end of their lives. Although their temperatures are lower than the Sun's, their enormous size makes them brighter than the Sun.
The reason they are so large is that nuclear fusion in their cores has stopped, and the core is contracting due to gravity. The core, whose temperature increases due to gravity, burns the hydrogen in the surrounding shell, causing it to expand. This expansion is so dramatic that it increases the star's original size by several hundred times.
Red Supergiants
Red giants are K and M class stars. They are the largest class of stars in the universe. They are so large that most red giants have radii 200 to 800 times that of the Sun (278.58 million km to 1.114 billion km), and the four largest known red supergiant stars have radii 1500 times that of the Sun (2 billion km). However, due to their immense size, their surface temperatures are less than 3,800°C. Their masses range from 10 to 40 solar masses. When they die, a supernova occurs, and their cores can transform into neutron stars. If their mass is large enough, they can also become black holes after the explosion.
Intergalactic Stars
Intergalactic stars are the rogue stars of the universe. They wander through the universe without being bound to the center of any galaxy. It is thought that this happens due to the interactions and mergers of different galaxies. Close encounters are believed to tear stars away from their own galaxies, launching them into space and causing them to drift, freeing them from the gravitational influence of any galaxy.
Intergalactic stars were first discovered in the Virgo Cluster by the Hubble Space Telescope in 1997. Located 60 million light-years from Earth, approximately 10% of the Virgo Cluster consists of solitary stars.
Dead Stars
Dead stars are stars whose nuclear fusion has ended and which no longer "burn." There are three known dead stars:
White Dwarfs
Neutron Stars
Black Holes
White Dwarfs
White dwarfs are D-class stars. They are dead stars that remain after the death of red giants, having completed their nuclear fusion. Because nuclear fusion does not occur in the cores of white dwarfs, they should normally be unable to overcome their own gravity and should explode, but they do not collapse thanks to degenerate electron pressure. Degenerate electron pressure is created by the change in the energy field around the electrons and prevents matter from being compressed into smaller volumes. Therefore, white dwarfs are extremely dense.
White dwarfs can have masses ranging from 0.17 to 1.33 solar masses. Because they form from the collapse of the core, their initial temperatures reach 100,000°C and above, but then drop to between 8,000 and 40,000°C. It takes tens or even hundreds of billions of years for white dwarfs to cool completely. As they cool, they begin to crystallize, losing their brightness entirely and transforming into black dwarfs.
Neutron Stars
Neutron stars are one of the last forms that living stars can take, consisting mainly of neutrons, with trace amounts of electrons and protons. Neutrons are unstable and disintegrate into protons and electrons through radiation. However, due to gravity, protons and electrons in the star constantly combine to form neutrons.
Neutron stars can have masses ranging from 1.18 to 3 solar masses, but because they are approximately 20 km in diameter, they are affected by angular momentum . According to this rule, the smaller the volume of a star, the faster it should rotate. They can rotate around their own axis approximately 600-700 times per second. They have a temperature of approximately 1,000,000°C. Theoretically, they could exist forever.
Black Holes
Black holes form when massive stars explode and their cores collapse in on themselves. Black holes are vacuums in space and can have masses ranging from at least 3 to as much as 50 billion solar masses. While their event horizon, from which even light cannot escape, can be millions of degrees Celsius as they pull in gases, their centers are -273°C or colder. The centers of black holes are actually the core of the exploded star and are known as singularities. Black holes can shrink and disappear over time due to Hawking radiation.
Theoretical Stars
Theoretical stars are stars that are possible to form but are considered theoretical because they have not yet been observed. Some of these stars are based entirely on assumptions, while others cannot be found because the universe is not yet old enough. Theoretical stars in the universe:
Blue Dwarfs
Dark Dwarves
Black Stars
Dark Energy Stars
Dark Matter Stars
Blitzars
Electroweak stars
Frozen Stars
Iron Stars
Planck Stars
Nemesis
Blue Dwarfs
Blue dwarf stars are theoretical stars that are thought to have formed after a red dwarf has exhausted all its fuel. They are theoretical because the universe is not yet old enough to contain blue dwarfs. However, based on current observations and information, the formation of blue dwarfs in the future is highly probable.
Dark Dwarves
Black dwarfs are stars that emerge after a star's nuclear fusion ends, transforming into a white dwarf, which in turn will completely cool down. However, this process requires billions of years for that star to evolve, and since we know the Big Bang occurred 13.8 billion years ago, black dwarfs do not yet exist in the universe.
Black Stars
These are theoretical alternative celestial objects to black holes, hypothesized to exist somewhere between a black hole and the core of a collapsed star. They have no event horizons, but their gravitational forces would be so strong that the dead center would resemble a singularity. Therefore, light would not be able to escape, and even Hawking radiation would occur. Because of this level of gravity, particles trapped at the center could virtually split in two, with one able to escape while the other remains trapped.
Dark Energy Stars
Dark energy stars are based on the principle that energy falling into a black hole reaches the event horizon of the vacuum or dark energy. The falling matter then accumulates there, counteracting gravity.
Dark Matter Stars
Dark stars, like modern stars, are mostly composed of normal matter. However, the high density of neutralino dark matter within them generates heat through annihilation reactions between dark matter particles. This heat prevents these stars from collapsing to the relatively compact and dense sizes of modern stars, thus preventing nuclear fusion from initiating between “normal” matter atoms.
According to this theory, dark matter stars are between 1 and 960 astronomical units in size. They are thought to be massive clouds of hydrogen and helium with surface temperatures too low to be observed due to the radiation they emit.
Blitzars
Blitzars are thought to be stars that begin their journey as neutron stars but transform into black holes because they don't rotate fast enough. Neutron stars normally defy gravity with centrifugal force during rotation. A slowing or slow-rotating neutron star will typically have a lower centrifugal force, making it unable to overcome gravity, and it will collapse into a black hole.
Electroweak stars
Electroweak stars are a type of theoretical star that is thought to form when the collapse of the star is prevented by radiation pressure. The event takes place in the star's core, in an area about the size of an apple with twice the mass of Earth, at a temperature of 10^15°C (1,000,000,000,000,000°C).
Frozen Stars
Theoretical stars that could form in the distant future, with very low masses and surface temperatures of only around 26°C.
Iron Stars
Iron stars are a hypothetical type of star that could form in the universe after 10^1500 years. The hypothesis is that cold fusion, caused by quantum tunneling, transforms atomic nuclei into iron-56. Fusion and alpha decay also cause heavier nuclei to decay into iron. As a result, the star transforms into a cold, iron sphere.
Planck Stars
Planck stars are stars that form at the event horizon of a black hole. They are thought to form when the energy density of collapsing stars at the event horizon reaches the Planck energy unit . Under these conditions, gravity and spacetime also have limited values, and a "repulsive" force emerges. The mass and energy accumulated within the star can no longer collapse because it has overcome the uncertainty principle of spacetime.
Nemesis
Nemesis is a red giant or brown dwarf thought to orbit the Sun 1.5 light-years away, beyond the Oort Cloud. In the 1980s, the Infrared Astronomical Satellite (IRAS) failed to locate Nemesis. In 1986, the University of California's Leuschner Observatory similarly failed. Nemesis was specifically searched for using infrared telescopes because cooler stars are relatively brighter in infrared light.
The Two Micron All-Sky Survey (2MASS), in its observations from 1997 to 2001, found no stars or brown dwarfs in the Solar System. In 2017, Sarah Sadavoy and Steven Stahler proposed that the Sun is actually a binary system and that Nemesis moved away from the Sun 4 billion years ago. However, since we have no evidence, it is unknown whether Nemesis actually exists.
Star Systems
A star system is a small number of stars orbiting each other, bound by gravity. A large group of stars bound by gravity is often called a star cluster or galaxy, but generally they are also star systems. Star systems should not be confused with planetary systems, which contain planets and similar objects. Systems that stars can form:
Binary star system: A system formed when two stars orbit each other.
Triple star system: A system consisting of two stars orbiting each other, and a third star orbiting around it.
Multiple Systems: Systems consisting of more than three stars. This includes systems with four to nine stars.
How to Observe Stars
Many techniques and tools are used to observe stars from Earth:
Telescopes: Optical, radio, and space telescopes can provide information about the properties of stars.
Spectroscopy: A technique used to separate the light emitted by stars into its constituent colors.
Photometry: A technique used to measure the brightness of stars.
Space probes and satellites: Instruments that observe Earth's atmosphere in order to directly observe space.
Interferometry: Observing a celestial body from different locations to obtain more detailed and higher-quality results.
Astroseismology: A technique used to determine the internal structure, composition, and age of a star by observing the natural oscillations within the star's interior.
Neutrino Detectors: Instruments used to detect neutrinos produced in nuclear reactions in stars and to understand energy production.
How are the distances of stars measured?
There are several techniques for measuring the distance of stars from Earth:
Parallax Method: This is the most basic technique for measuring the distance of nearby stars. It relies on the apparent shift in a star's position when observed from different points in Earth's orbit. The amount of angular shift is used to calculate the star's distance.
Star Lumen and Apparent Brightness: If a star's intrinsic luminosity is known and its apparent brightness from Earth can be measured, the inverse square law is used to calculate its distance.
Main Sequence Fitting: Used to determine a star's absolute magnitude using its apparent magnitude, class, and luminosity. Once the absolute magnitude is known, its distance can be calculated using the inverse square law.
Cepheid Variable Stars: These are bright stars that flicker with a distinct period. The period-luminosity relationship of Cepheids helps astronomers determine their absolute luminosities, which allows them to calculate their distances.
Supernova Lumen: Knowing that what are called Type Ia supernovae have a known peak brightness helps to estimate the distance of such a supernova from its host galaxy.
Hertzsprung-Russell Diagram: By plotting a star on a Hertzsprung-Russell diagram and comparing its absolute magnitude to its spectral type, you can calculate the distance of a star. This method is commonly used for stars within clusters and galaxies.
Spectroscopic Parallax: This technique involves analyzing a star's spectrum to determine its luminosity, and then comparing this to its brightness to estimate its distance.
Redshift: In extremely distant galaxies and quasars, astronomers use the redshift of these galaxies to calculate the expansion of the universe and estimate the distances of stars.
Tully-Fisher Relationship: Used to determine the distance to spiral galaxies based on their rotation speed and total brightness. The brighter and more massive the galaxy, the faster it rotates.
What is the closest star to Earth?
Image source: Wikipedia
Of course, the closest star to Earth is the Sun, at 149.14 million km. However, excluding the Sun, the closest star to both the Sun and Earth is the red dwarf Proxima Centauri, discovered by Robert Innes in 1915, which is 4.24 light-years away.
What is the largest star in the universe?
Image source: Wikipedia
The largest star in the universe is Stephenson 2-18, a red supergiant star in the Stephenson 2 cluster, discovered in 1990 by American astronomer Charles Bruce Stephenson through deep infrared observations. With a diameter of 2.9915 billion kilometers, this giant star is large enough to contain 8 million Suns. Stephenson's mass is not yet known, but with a luminosity equivalent to 440,000 Suns, it also holds the title of the brightest star in the universe. Stephenson 2-18 is also located 19,570 light-years from Earth.
What is the smallest star in the universe?
The smallest star in the universe is EBLM J0555-57Ab, located 670 million light-years from Earth. With a diameter of 59,000 km, it is smaller than Saturn, but its mass of 1.611 × 10^29 is 250 times greater than Saturn's. Due to its extremely low mass and diameter, scientists believe this star is still in the proto-star stage.
What is the coldest star in the universe?
The coldest star in the universe is a Y-class brown dwarf with a temperature of 25°C, colder than the human body. Discovered in August 2011, this brown dwarf, named WISE 1828+2650, is located in the Lyra constellation and is 32.5 light-years from Earth. Before the discovery of WISE 1828, the coldest star in the universe was CFBDSIR 1458+10B with a temperature of 97°C.
What is the hottest star in the universe?
The hottest star in the universe, among Wolf-Rayet stars, is WR 102, with a surface temperature of 209,727°C. Twenty times hotter and 300,000 times brighter than the Sun, this star is located 8,480 light-years from Earth in the constellation Sagittarius. WR 102 is so hot because it is close to exploding. Stars begin burning heavier elements after exhausting their hydrogen fuel. In WR 102, this temperature is so high that it ionizes the surrounding interstellar material, creating a Wolf-Rayet nebula.
What is the oldest star in the universe?
Image source: Wikipedia
The oldest star in the universe, Methuselah, is thought to be even older than the universe itself, with an estimated age of 14 billion years. Methuselah's age was initially calculated as 16 billion years, but recalculations in 2014 found it to be 14.27 ± 0.8 billion years old .
Could a star be older than the universe?
No, they can't be. Stars like Muthuselah, measured at 14.27 ± 8 billion years old, or BD+17°3248, measuring 13.8 ± 4 billion years old, were once thought to be older than the universe and disprove the Big Bang theory. However, evolving techniques and observational approaches have proven that the oldest stars in the universe are 12-13 billion years old , and that nothing in the observable universe can be older than the Cosmic Microwave Background Radiation.
What is the youngest star in the universe?
(The pink one) Image source: Wikipedia
The youngest star in the universe is the neutron star Swift J1818.0-1607, estimated to be between 240 and 500 years old . Swift was discovered by NASA on March 12, 2020, thanks to an X-ray burst.
Why do stars appear small in the sky?
The only reason stars appear small in the sky is due to their distance from Earth. For example, the Sun, which could easily burn us if we went outside, is actually 149.14 million kilometers away. And yet, the Sun appears about as large as a plastic bottle cap to us.
And with that, we've come to the end of this blog post. Now that we've answered the question "What is a star?" in detail and learned some information about stars, you can brag to your friends. Also, try not to stress too much in your daily life so you don't explode and turn into a black hole! Sending you all my love from Galaxy Explorer.
