Lifecycle of a Star

A star is born in a cloud of gas and dust, lives for millions or billions of years, and then dies in a supernova explosion or collapses into a black hole. Mass determines the lifecycle of a star. 

The larger the star’s mass, the shorter its life. And that’s because the hugest stars use their fuel faster than smaller ones that can shine for billions of years. Let’s delve into the lifecycle of stars to understand better what fuels them and what happens when that fuel runs out. But first, some background.

What Is A Star?

A star is a giant ball of hot gas that shines because of nuclear fusion in its core. Stars are the building blocks of space and the most easily recognized astronomical objects. They are busy making and distributing nitrogen, carbon, and oxygen for the nearby planetary systems depending on those building blocks. 

Moreover, the composition and lifecycle of stars within a galaxy help astronomers determine the evolution and dynamics of that galaxy. And all of these reasons make understanding the lifecycle of a star so essential.

Astrophysicists define a star as a massive hydrogen and helium sphere, a giant gas ball. It has enough mass that it can ignite its core elements through nuclear fusion. Stars stay balanced between their gravity’s inward pressure and their internal heat’s outward pressure for most of their lives.

Luminosity

A star’s absolute magnitude is a way to describe how luminous the star is. Luminosity is a way to measure the total electromagnetic energy amount radiated by the star. In other words, luminosity is a star’s total power output. The star’s size and temperature determine its intrinsic property of luminosity.

Now don’t confuse luminosity and apparent brightness. The luminosity comes from inside the star and is a part of it. But the apparent brightness changes depending on the observer’s location. So if you and I were on opposite sides of the planet, we might see the same star’s apparent brightness at different levels since we are at different distances away from it.

Apparent brightness works like a car coming toward you at night. As it draws closer, the lights appear brighter. And as your car passes the oncoming one, its headlights appear almost blindingly bright. The reason is that light spreads out as it gets closer to you.

So a star’s luminosity (absolute brightness) comes from within itself, while a star’s apparent brightness (or magnitude) comes from the observer’s location.

Temperature

A star’s temperature helps determine its luminosity. Think about a black body like a light bulb that gets brighter as it gets hotter, emitting more energy. In very simplistic terms, the same holds true for a star, even though it’s not strictly linear, and stars aren’t perfect black bodies.

For example, a slight increase in a star’s effective temperature may appreciably increase energy emissions per second from every square foot of the star’s surface. So essentially, hotter stars are more luminous.

Scientists use the Kelvin scale to measure space temperatures. It is based on Celsius temperatures, does not use negative numbers, and works well for very cold or hot temperature extremes. For example, zero degrees Celsius is equal to 273 degrees Kelvin.

WR 102 is the hottest star in our universe, with a surface temperature 35 times more scorching than the Sun. This intense star reaches about 28033.15 K (50,000℉ or 27,760°C.)

Spectral Type

Determining their spectral type is a way of classifying stars based on their temperature.  The strength of certain absorption lines in the star’s spectrum determines its spectral type. The different spectral types are O, B, A, F, G, K, and M. So, the hotter the star, the earlier its spectral type.

A star’s spectral type has a lot to do with its lifecycle. The most massive stars, O and B-type, have very short lifespans. They will only live for a few million years before they explode as supernovae. The less massive stars, which are M-type stars, have much longer lifespans. They can live for trillions of years before eventually running out of fuel and dying.

The spectral type of a star also affects its color. The hottest stars, which are O and B-type stars, are blue. The cooler stars, which are M-type stars, are red.

The spectral type of a star is an essential clue to its age, mass, and composition. Scientists use it to predict how the star will evolve.

Size or Mass

Another indicator of a star’s luminosity is its size. A star’s initial mass, how it produces energy, and its internal structure dictate which specific evolutionary changes it goes through. And that evolutionary path corresponds to changes in the star’s luminosity and temperature.

We’ll discuss the H-R Diagram below, an astronomical powerhouse showing scientists a star’s evolutionary stage and internal structure by determining its position on the diagram.

Larger stars run hotter and use their fuel quicker, so they live shorter lives than smaller stars. And the mass of a star depends on the amounts of matter available within its stellar nursery. How much gas and dust exists within a star’s nebula at birth determines its size. And that eventually determines the lifecycle of the star.

Color

A star’s color, size, and temperature are also heavily intertwined. Even though most stars look white to the naked eye, their size and temperature are determining factors in their color. 

For example, Rigel appears bluish-white and is a blue supergiant star. However, Betelgeuse looks reddish-orange as a red supergiant star. Both stars in the Orion Constellation are excellent examples of the differences in color depending on the star’s temperature and size.

The star’s surface temperature is one of the most significant color-determining factors. And we know that star temperature relates to star size. Here’s a list of approximate temperatures and corresponding star colors.

Approximate Star Surface Temperature (Kelvin)	Approximate Star Color	Spectral Type
＜3,700	Red	M
3,700 – 5,200	Orange	K
5,200 – 6,000	Yellow	G (The Sun)
6,000 – 7,500	Yellow-White	F
7,500 -10,000	White	A
10,000 – 30,000	Blue-White	B
＞30,000	Blue	O

H-R Diagram

The Hertzsprung-Russell diagram is essentially a periodic table of stars. It is a star scatter graph that plots luminosity versus temperature. And it helps astronomers understand star evolution.

Two scientists in the early 1900s independently determined that stars’ temperature and luminosity are not randomly distributed when plotted. But instead, like elements in the periodic table, regions of stars share common characteristics.

U.S. astronomer Henry Norris Russell and Danish scientist Ejnar Hertzsprung created one of astronomy’s most essential diagrams and gave us a way to trace which evolutionary paths different types of stars follow (depending on their mass.)

Of course, the lifecycle of a star determines its changing physical characteristics over time, so the star’s position on the H-R diagram can also change. Scientists use the diagram to classify a star and learn vast information about it. The star’s position on the graph help determine the following.

• Luminosity
• Temperature
• Mass
• Chemical Composition
• Spectral Type
• Color
• Age
• Evolutionary History
• Projected Evolutionary Path

Lifecycle of A Star Depends Partly On Its Sequence

The majority of stars, about 90%, are main sequence stars. That means they fuse hydrogen to form helium in their cores. Even the Sun is a main sequence star, but others can range in size from a tenth of the Sun’s mass to 200 times its size.

Tiny bodies without enough mass to reach nuclear fusion are called brown dwarfs. Essentially they are small celestial bodies under 0.08 the Sun’s mass that can’t ignite into stars. But when the body does have enough mass, the nebula’s collapsing dust and gas start burning hotter until, over hundreds of thousands of years, they reach high enough temperatures to fuse hydrogen into helium. 

And that’s when the celestial body becomes a main sequence star. It gets stabilization from the hydrogen fusion that creates outward pressure to balance with gravity’s inward pressure.

Red and yellow dwarfs, like our Sun, live billions of years. The Sun is about 4.6 billion years old, and scientists predict it is about halfway through its main sequence. So it should live about another 5 billion years.

It formed from the solar nebula, an enormous spinning cloud of celestial dust and gases. As its own gravity collapsed the nebula, it spun and flattened. Then most of its material (about 99.8%) pulled toward the center and formed the Sun. The remaining dust and gas cloud materials formed planets and moons. Or it got blown away by the intense solar winds of the new Sun.

Composition and The Lifecycle of A Star

Galaxies consist of 90% dark matter, 7% cosmic gases and dust, and about 3% of the universe’s universe contains stars.

What Are Stars Made Of?

Stellar dust and gas cloud nurseries, or nebulae, bring stars to life, and here’s how.

• Cloud turbulence clumps the dust and gas together, sometimes creating a large mass that collapses under its own gravity.
• The matter at the clump’s center heats.
• The hot center core starts nuclear fusion and becomes a protostar before developing into a star.
• Star groups form when the spinning clumps break into portions.
• The nebula’s remaining dust and gasses either stay in their current form or later become the building blocks for asteroids, comets, and planets.

Stars form in nebulae, which are clouds of gas and dust. The gravity of the nebula pulls the gas and dust together, and as it does, it heats up. Eventually, the temperature and pressure at the center of the nebula become so high that nuclear fusion begins, and a star is born.

How Long Do Stars Live?

The lifecycle of a star depends on its mass and the rate at which it burns its fuel. For example, the more massive a star is, the more fuel it has to burn and the faster it will burn through it. On the other hand, the least massive stars have so little fuel that they burn it very slowly and can live for billions or trillions of years.

Lifecycle Of A Star: How Do Stars Die?

There are no further nuclear reactions once a star fuses all remaining hydrogen atoms within its core. And with no more energy production maintaining gravity’s push and pull, the star’s core starts collapsing. Then it really heats up!

Since hydrogen still exists outside a collapsing star’s core, hydrogen fusion takes place in a shell around the center. The core continues heating up and pushes the star’s outer layers, expanding, cooling, and transforming the collapsing star into a red giant.

For massive stars, the heating and collapsing core might create nuclear reactions that use up all the helium to produce heavy elements like iron. And thus, the reason stars are building blocks of the universe.

However, even the production of heavier elements doesn’t stop the lifecycle of a star’s process. Instead, its internal nuclear fusion grows more unstable and alternates between dying down and burning wildly. The extreme variations cause pulsating, and the star sheds its outer layers, throwing off more gas and dust.

The Lifecycle Of A Star By Type

The dying star’s next steps depend on how large its core is. Next, let’s delve into what happens to different types of stars.

Main Sequence Stars: White Dwarfs, Red Giants, and Planetary Nebulae

Main sequence stars less than eight times the Sun’s mass start collapsing when they run out of hydrogen in their cores. When the star loses the fusion’s energy, gravity forces it to start collapsing in on itself. But the collapsing also raises the star’s temperature and pressure until helium fuses into carbon. Then the carbon releases energy and expands the star’s outer layers, turning the star into a red giant. 

Eventually, the red giant grows more unstable and starts pulsating, alternately expanding and expelling parts of its outer layers until it creates an enormous dust and gas cloud called a planetary nebula.

Average, main sequence stars like the Sun throw off their outer layers until the hot stellar core is all that’s left. The star is actually dead but still incredibly hot. And scientists call the star’s remnant a White Dwarf, a stellar cinder around the size of Earth.

Small Red Dwarfs

The most petite main sequence stars are red dwarfs, having only a fraction of the Sun’s mass. These tiny stars don’t shine very brightly since they have only about 0.01% of the Sun’s energy. Plus, their temperature is relatively low for a star, between 3000-4000 Kelvin.

Red dwarfs are tiny but numerous, found throughout the universe. The lifecycle of a low-mass red dwarf star could have a lifecycle longer than the universe’s current age, from billions up to 14 trillion years.

Some White Dwarfs Transform Into Novae

When a White Dwarf is close to a binary or multiple-star grouping, the lifecycle of the star may end as a nova. And this usually happens only with stars 1.4 times the Sun’s mass and bigger.

Scientists once thought these were new stars, and so gave them the Latin name for “new.” However, they now know that novae are extremely old White Dwarfs. A nova is the short-lived brightening that happens when a companion star’s hydrogen streams onto a White Dwarf’s surface.

Here’s what happens.

When the White Dwarf is close to a grouping or companion star, its gravity pulls hydrogen from the companion to build up its own surface. Once the White Dwarf accumulates enough surface hydrogen, a nuclear fusion bursts. And that causes the White Dwarf to brighten and throw off its remaining matter.

But after a few days, the bright glow dims, and the White Dwarf starts the cycle of pulling hydrogen from nearby stars again. The White Dwarf grows and accumulates mass from this cycle until it eventually collapses in on itself to completely explode (sometimes into a supernova.)

Super and Hypergiants

The most massive stars are hypergiants; some are more than 100 times the Sun’s mass. They also emit tons more energy than the Sun, hundreds of thousands of times more. These massive stars are super hot, with 30,000 Kelvin surface temperatures.

But because they use their fuel so much faster than smaller red dwarfs, hypergiants only have a lifecycle of a few million years. Scientists think many more hypergiants existed in our universe’s early days. But now, there are likely less than ten hypergiants in the Milky Way galaxy.

Lifecycle Of A Star: Supernova

What happens when an enormous main sequence star reaches the end of its lifespan? Stars over about eight solar masses die in a colossal explosion that scientists call a supernova.

And these are not just more significant novas because what happens when a star dies in a nova is that its surface explodes. But when a supernova occurs, the star’s entire core collapses in on itself and then explodes.

Supernovae happen after a massive star experiences a series of nuclear reactions that lead to iron production in its core. Iron is the heaviest element the star can form without consuming energy instead of producing it. But once iron is present, the star has essentially used all the energy possible from nuclear fusion. 

So the star loses its eternal struggle for balance between internal and external gravity pressures. It can no longer support its own mass, so the iron core collapses. In an explosion taking only seconds, the core shrinks to about a dozen miles across from a previous vastness of up to 5,000 miles across.

Time to Shed a Bit of Energy

In addition, temperature spikes reach more than a hundred billion degrees. And that causes the star’s outer layers to begin collapsing with its core. But suddenly, it rebounds with the explosion’s energy release, and the outer layers are thrown into space. 

Supernovae release astronomical amounts of energy, making them shine brightly for days or weeks. Keep reading below to learn more about SN 1006, an enormous supernova that was even visible during the daytime, shining brighter than Venus. In 2013, NASA’s Chandra X-ray Observatory showed the 1006 A.D. supernova in a new light.

Supernovae also release an enormous amount of naturally occurring elements and subatomic particles into the surrounding atmosphere. So once again, you can see how the lifecycle of a star contributes to the galaxy’s building blocks. They occur about once every hundred years in most galaxies. And scientists discover 25 to 50 supernovae every year in other galaxies.

Neutron Stars

When a collapsing star’s core in a supernova’s center is between 1.4 and three times the Sun’s mass, a neutron star forms.

Very simply, a neutron star forms when a star collapses at the end of its life. It is not a black hole. But instead, a neutron star is a very dense star that has collapsed under its own gravity. It is so dense that the protons and electrons in its atoms have been squeezed together to form neutrons. Whereas a black hole is an area of space where gravity is so strong that nothing, not even light, can escape.

Like an atomic nucleus, neutron stars are extraordinarily dense. And because the small neutron star contains so much mass, it has immense surface gravitation. So it can pull and accrete gases from nearby companions and star clusters.

NASA’s Rossi X-Ray Timing Explorer (RXTE) captured X-ray gas emissions within miles T5X2 during observations of the neutron star in 2010. The graph below shows the rising X-ray emissions, with an increasing burst amount and decreasing brightness. Scientists believe that understanding the X-ray emissions from neutron stars can help them learn more about the star’s evolution and future.

In addition, neutron stars have formidable magnetic fields that gather atomic particles near the star’s magnetic poles. They produce potent radiation beams that sweep around like searchlights when the star rotates.

Scientists call neutron stars “pulsars” when that radiation beam points toward Earth. Astronomers observe the beams as regular radiation pulses as it sweeps past our line of sight.

Lifecycle Of A Star: Black Holes

When a dying star’s collapsed core is more extensive than three solar masses, it completely collapses in on itself to create a black hole.

A black hole is an infinitely dense object packed into a small volume with such strong gravity that not even light can escape it. Furthermore, scientists can only detect black holes indirectly. Here’s another way to describe a black hole: it is made of such tightly packed matter that gravity engulfs all other forces.

Indirect black hole observations are possible because the gravitational fields are so influential. Any close material gets caught up by its gravity and dragged in. So the outer layers of companion stars often get sucked into the black hole. Scientists observe a heated disc of vast temperatures that emits X-rays and Gamma-rays as an indication of the hidden black hole.

In 2019, using the Event Horizon Telescope, scientists took an image of a black hole’s outline. The hole is at the center of the M87 Galaxy and shows hot gas swirling around the event horizon.

Lifecycle Of A Star: New Birth

The dust clouds and remaining debris from novae and supernovae eventually become part of the neighboring interstellar clouds of dust and gases. The debris enriches the surrounding clouds with chemical compounds and heavy elements like iron produced by the star’s death.

Eventually, the universe recycles those materials, and they become the building blocks for new star generations. And since there is no new matter, only changing forms, particles from a stellar death are likely within each of us.

A 2018 Nova Explosion

Three satellites, two of which were NASA missions, captured a 2018 Nova explosion and provided the first evidence that the visible light came from shock waves. NASA’s NuSTAR and Fermi space telescopes and Canada’s BRITE-Toronto satellite studied the nova.

In addition, scientists from over 40 institutions and ground-based facilities observed the star system’s outburst in the constellation Carina, about 13,000 light-years from Earth. They later named the star system V906 Carinae.

Observers believe it takes tens of thousands of years for a White Dwarf to accrete a deep enough hydrogen layer to reach critical pressure and temperature. But once it does, it erupts and blows off all its accumulated layers. 

Scientists measured V906 Carinae’s nova in shock waves, or abrupt temperature and pressure changes that formed within the explosion’s debris fields.

Nova Explosion Scientific Summary

When the scientific team compiled all the observations, here is what they think occurred.

• In the outburst’s beginning days, the stars’ orbital motion pulled a thick debris and gas-shell cloud together into a doughnut shape.
• The cloud pressed outward with speeds around 1.3 million miles per hour (2.2 million kph), which is about the speed of the Sun’s outflowing solar winds.
• Then, a fast-moving outflow of twice the speed slammed into the doughnut’s denser structures. This motion created the gamma-ray and visible light-emitting shock waves scientists observed.
• Finally, about 20 days after the initial explosion, another nova outflow arose from the White Dwarf’s surface and nuclear fusion reactions. This outflow slammed into the slower debris at speeds around 5.6 million miles per hour (9 million kph.) New shock waves resulted in more optical and gamma-ray flares.

You can see the circled V906 Carinae below, in March 2018, just three days after scientists discovered the nova. It shined at near-peak brightness within the cloud of dust and gas of the Carina Nebula.

Lupus Constellation Supernova Remnant: SN 1006

SN 1006 in the Lupus constellation resides about 7,000 light-years from Earth. People from all over Earth could see the supernova explosion, first observed on May 1, 1006, A.D. Astronomers worldwide documented the fantastic sight as the White Dwarf star exploded and sent its outer layers hurtling into space.

In 2013, astronomers compiled ten overlapping views from the Chandra X-ray Observatory. In doing so, you can see an astonishing view of the Type Ia supernova, SN 1006’s debris field.

Scientists found different layers of elements among the debris, including oxygen, magnesium, and silicon. And studying these elements helps them see how the star might have looked before the explosion, which helps determine the order of the star’s ejected layers. And from that data, researchers create theoretical explosion models.

Now why you may wonder why researchers show interest in a supernova from over a thousand years ago. But they use distant explosions as mileposts that mark the Universe’s expansion.

SN 2023ixf: A Newly Discovered Supernova

Japan’s Koichi Itagaki discovered a new supernova in the Pinwheel Galaxy on May 19, 2023. The Zwicky Transient Facility confirmed the supernova discovery, dubbed SN 2023ixf. It is about 21 million light-years from Earth, making it the closest one seen in the last five years.

As telescopes around Earth hurried to observe SN 2023ixf, astronomers noted that it is a Type II, meaning an explosion occurred once a massive star used up all its nuclear fuel and collapsed. Researchers expect the new supernova to brighten so that you can view it through your telescope in the coming months.

Conclusion: Lifecycle Of A Star

Stars are born in stellar nurseries or nebulae. They are giant balls of hot gas that shine due to nuclear fusion in their cores. The fusion’s energy helps the star maintain its balance against the constant gravitational forces wanting it to collapse.

As a star ages, it moves through different lifecycles portrayed on the H-R Diagram. And its positioning helps scientists determine the age and probable evolution of the star. This powerful astronomy tool aids researchers in tracking a star’s progress through life.

And finally, when smaller main sequence stars die, they become red giants, who eventually eject more and more of their accreted gas outer layers to become sizzling white dwarfs. At the same time, the ejected matter becomes the clouds of interstellar gas and dust, which eventually become the stellar nurseries that give life to the next generation of stars.

So the continual lifecycle of a star ensures a balance in the universe.