# If Sunlike stars become a red giant and eventually a white dwarf, what do red dwarfs become?

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The Sun is said to become a red giant at the end of its life (before that it will become an orange subgiant first and then an orange giant or so) and after ejecting its outer layers it should become a white dwarf. If yellow dwarfs like the Sun become red giants, what do red dwarfs become? Even redder giants? How do the red giants of former red dwarfs (say Proxima Centauri) differ from those of former yellow dwarfs (say the Sun) and how would Proxima's white dwarf be different from the Sun's white dwarf? I mean, a red dwarf's eventual white dwarf would be much less massive than that of the Sun, right?

A relevant paper here is Laughlin, Bodenheimer & Adams (1997) "The End of the Main Sequence". From the abstract:

We find that for masses $$M_ast < 0.25 M_odot$$ stars remain fully convective for a significant fraction of the duration of their evolution. The maintenance of full convection precludes the development of large composition gradients and allows the entire star to build up a large helium mass fraction. We find that stars with masses $$M < 0.20 M_odot$$ will never evolve through a red giant stage. After becoming gradually brighter and bluer for trillions of years, these late M dwarfs of today will develop radiative-conductive cores and mild nuclear shell sources; these stars then end their lives as helium white dwarfs.

Section 3 of the paper provides a detailed description of the lifetime of a $$0.1 M_odot$$ star. A brief summary:

1. After approximately 2 Gyr of contraction, the star reaches the zero-age main sequence point with a temperature of 2228 K and a luminosity of $$10^{-3.38} L_odot$$.

2. On the main sequence, the mass fraction of $$^3 m He$$ increases steadily over a trillion years. The completely convective nature of the star ensures that it is mixed throughout the structure of the star. The star slowly increases its temperature and luminosity.

3. The maximum mass fraction of 9.95% $$^3 m He$$ is reached at 1380 Gyr. After this, the mass fraction declines as the rate of consumption exceeds the rate of production.

4. Between 1500 and 4000 Gyr (the text appears to use values that are too small by a factor of 1000 judging by figure 1 and the statement of total lifetime at the start of §3.2) the star starts turning itself into $$^4 m He$$, with this isotope becoming the main component of the star (by mass) around 3050 Gyr.

5. By 5740 Gyr, the star develops a radiative core due to the helium mass fraction lowering the opacity. This causes a small amount of contraction of the star and a decrease in luminosity.

6. After the development of the radiative core, shell burning proceeds outward through the star, increasing the surface temperature to a maximum of 5807 K at 6144 Gyr. The luminosity at this point is about $$10^{-2.3} L_odot$$. This star is called a "blue dwarf".

7. The star becomes cooler and less luminous. Shell burning continues during this time, eventually ending with the star having a hydrogen mass fraction of ~1%. The nuclear burning lifetime ends at 6281 Gyr, producing a helium white dwarf with temperature 1651 K and a luminosity of $$10^{-5.287} L_odot$$.

A discussion of the appearance of the blue dwarf stage and how blue they actually are can be found in this question.

The $$0.16 le M_ast / M_odot le 0.20$$ range is transitional between the stars that become blue dwarfs and the stars that become red giants. From the paper:

In connection with their increased luminosity output, the transitional stars in the mass range $$0.16 le M_ast / M_odot le 0.20$$ are able to produce increasingly larger expansions of the overall stellar radius after the radiative hydrogen-exhausted core has developed.

In the models calculated in the paper, the lowest mass object that unambiguously produced a red giant was $$0.25 M_odot$$, but as noted the transition region is not sharp. Nevertheless, this does mean that the higher-mass M dwarfs will eventually go through a red giant stage.

## Red dwarf

A red dwarf is the smallest and coolest kind of star on the main sequence. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one star that fits the stricter definitions of a red dwarf is visible to the naked eye. [1] Proxima Centauri, the nearest star to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way. [2]

The coolest red dwarfs near the Sun have a surface temperature of

2,000 K and the smallest have radii of

9% that of the Sun, with masses about

7.5% that of the Sun. These red dwarfs have spectral classes of L0 to L2. There is some overlap with the properties of brown dwarfs, since the most massive brown dwarfs at lower metallicity can be as hot as 3,600 K and have late M spectral types.

Definitions and usage of the term "red dwarf" vary on how inclusive they are on the hotter and more massive end. One definition is synonymous with stellar M dwarfs (M-type main sequence stars), yielding a maximum temperature of 3,900 K and 0.6 M . One includes all stellar M-type main-sequence and all K-type main-sequence stars (K dwarf), yielding a maximum temperature of 5,200 K and 0.8 M . Some definitions include any stellar M dwarf and part of the K dwarf classification. Other definitions are also in use (see definition). Many of the coolest, lowest mass M dwarfs are expected to be brown dwarfs, not true stars, and so those would be excluded from any definition of red dwarf.

Stellar models indicate that red dwarfs less than 0.35 M are fully convective. [3] Hence the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding helium buildup at the core, thereby prolonging the period of fusion. Low-mass red dwarfs therefore develop very slowly, maintaining a constant luminosity and spectral type for trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs yet exist at advanced stages of evolution.

## Surprise: Neutron Stars And White Dwarf Stars Aren’t Actually Stars

Sirius A and B, a normal (Sun-like) star and a white dwarf star in a binary system. Many such . [+] systems like this are known to exist, and the accretion of matter from the star onto the white dwarf is what drives the classical novae that create the Universe's lithium. The normal star is an actual star the white dwarf is not.

NASA, ESA and G. Bacon (STScI)

When we think about the objects in our Universe, they fall into two categories:

1. self-luminous objects, like stars, which generate their own light,
2. and non-luminous objects, that require an external energy source to be seen.

The latter category, which includes planets, moons, dust, and gas, will only emit light if it’s either reflected from a luminous source or absorbed and re-emitted from an external energy source.

But does being self-luminous automatically mean that you’re a star? Surprisingly, not only are there many exceptions to that rule, but some of those exceptions even have the word “star” right there in their name, despite not being actual stars. Brown dwarf stars, white dwarf stars, and even neutron stars aren’t actually stars, while red dwarf stars, yellow dwarfs (like our Sun), and all giant stars do turn out to be stars. Here’s what makes all the difference.

Stars form in a wide variety of sizes, colors and masses, including many bright, blue ones that are . [+] tens or even hundreds of times as massive as the Sun. This is demonstrated here in the open star cluster NGC 3766, in the constellation of Centaurus. If the Universe were infinite, even a cluster like this wouldn't display 'gaps' between the stars, as a more distant star would eventually fill those gaps in.

In our colloquial, day-to-day lives, most of us like to think that we know a star when we see it. We conventionally think of a massive ball of matter, giving off its own light, radiating energy out into the Universe. That’s true in a sense: all stars do actually do those things. They are massive clumps of matter, pulled into hydrostatic equilibrium by gravity. They undergo physical processes in their interior, which transfers energy outwards towards their surface. And from their boundaries — known as a star’s photosphere — energy, some of which falls in the visible light range, radiates out into the Universe.

### The Unfiltered Truth Behind Human Magnetism, Vaccines, And COVID-19

All of those things are true of stars, but they’re also true of other objects, some of which aren’t stars at all. To an astronomer, there’s a more stringent threshold that needs to be crossed if you’re going to be a star: you need to ignite nuclear fusion in your core. Not just any type of fusion, mind you, but the fusion of hydrogen (raw protons) into helium, or the products of that reaction into still heavier elements. Without achieving this, astronomers cannot consider an object to be a star.

The evolution of a solar-mass star on the Hertzsprung-Russell (color-magnitude) diagram from its . [+] pre-main-sequence phase to the end of fusion. Every star of every mass will follow a different curve, but the Sun is only a star once it begins hydrogen burning, and ceases to be a star once helium burning is completed.

Wikimedia Commons user Szczureq

This might seem arbitrary, but there’s an important set of reasons for it: reasons that become clear if we begin from a cloud of gas, which is the origin of all stars that we know of in the Universe today. Gas clouds are found throughout the Universe, are primarily made of hydrogen and helium (with only a few percent of other, heavier elements added into the mix), and — if they get cold and massive enough, or have a significant enough instability in them — will begin to collapse.

When this gravitational collapse begins to occur, there will inevitably be regions that begin with greater-than-average densities of matter. These overdense regions will exert a greater attractive force on matter than the other regions, and so will grow denser over time. What then ensues is a race between different regions to draw in as much matter as possible. There’s a problem with this scenario, however: when gas clouds collapse, the particles inside collide and heat up, preventing them from collapsing further.

The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or . [+] dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. While the external environment of these globules may be extremely hot, the interiors can be shielded from radiation and reach very low temperatures indeed.

The only way out is if these collapsing clouds of gas can somehow radiate energy away: they have to cool themselves down. The most efficient way to do that is through those heavier elements, which are far better at radiating energy away than hydrogen or helium atoms alone. As the clouds develop regions of matter that become hotter and hotter, the heated gas begins to not only radiate, but to trap that energy inside, causing the internal temperatures to skyrocket.

This gas might be emitting light, but it isn’t a star, at least not yet. It could be considered a proto-stellar nebula, however, as it’s taking a path that could lead to it becoming a full-blown star. But in order to get there, its temperature needs to continue to rise, and that can only continue so long as matter continues to fall into this overdense region, growing it and trapping even more heat.

When the temperature rises over about 1 million K in the core, the very first fusion reactions begin to occur.

The protostar IM Lup has a protoplanetary disk around it that exhibits not only rings, but a spiral . [+] feature towards the center. There is likely a very massive planet causing these spiral features, but that has yet to be definitively confirmed. In the early stages of a solar system's formation, these protoplanetary disks cause dynamical friction, causing young planets to spiral inwards rather than complete perfect, closed ellipses. The central protostar has not yet ignited nuclear fusion in its core.

S. M. Andrews et al. and the DSHARP collaboration, arXiv:1812.04040

What happens first is that deuterium — an isotope of hydrogen made of one proton and one neutron — can fuse together with a free proton to form a helium-3 nucleus: with two protons and one neutron. When this threshold is crossed, the nebula officially becomes a protostar: a large mass of matter that’s still accruing mass from its molecular surroudings, whose core is supported by pressure. The deuterium fusion reaction that’s occurring provides that pressure, while gravitation counteracts it.

Under most circumstances, there will be many points in this large clouds of gas that race to grow and grow, siphoning mass onto themselves and away from the other protostars. There are winners and losers in this war, as some protostars will gain enough mass to heat up above

4 million K, where they’ll begin the same chain reaction that powers our Sun: the proton-proton chain. If you cross that threshold, you’re a cosmic winner, as you’ll become a true star. But if you don’t, and you remain in this “limbo” where you only fuse deuterium, you’ll become a brown dwarf star: a failed star.

Gliese 229 is a red dwarf star, and is orbited by Gliese 229b, a brown dwarf, that fuses deuterium . [+] only. Although Gliese 229b is about 20 times the mass of Jupiter, it's only about 47% of its radius. Failed stars will become brown dwarfs with between 13 and 80 times the mass of Jupiter.

T. Nakajima and S. Kulkarni (CalTech), S. Durrance and D. Golimowski (JHU), NASA

Brown dwarfs range in mass from about 13 times the mass of Jupiter up to about 80 Jupiter masses: about 7.5% the mass of our Sun. Although they’re often called brown dwarf stars, they’re not truly stars, because they don’t meet that critical threshold: they cannot undergo the fusion reactions that are required to become a full-blown star. If a brown dwarf ever merges with another or accretes enough mass from a companion to cross this mass threshold, it can raise its game to become a red dwarf star: fusing hydrogen into helium and becoming a true star.

These actual stars come in a wide variety of masses, colors, and brightnesses. The ones that range from 7.5% to about 40% of the Sun’s mass are the red dwarf stars: they will burn hydrogen into helium and that’s it they will never reach higher temperatures to do anything else. Stars from 40% to 800% the Sun’s mass will eventually evolve into red giants, fusing helium into carbon when they do, before running out of fuel. And the even more massive stars will become supergiants, eventually going supernova when they reach the end of their lives.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star . [+] class shown above it, in kelvin. Our Sun is a G-class star, producing light with an effective temperature of around 5800 K and a brightness of 1 solar luminosity. Stars can be as low in mass as 8% the mass of our Sun, where they'll burn with

0.01% our Sun's brightness and live for more than 1000 times as long, but they can also rise to hundreds of times our Sun's mass, with millions of times our Sun's luminosity and lifetimes of just a few million years. The first generation of stars should consist of O-type and B-type stars almost exclusively, and may contain stars up to 1,000+ times the mass of our Sun.

Wikimedia Commons user LucasVB, additions by E. Siegel

All the stars that burn hydrogen, helium, carbon, or heavier elements up to iron — whether they’re dwarf-sized, giant-sized or supergiant-sized — are all stars. So long as they’re converting light elements into heavy elements via the energy-releasing process of nuclear fusion, they can be considered stars. Some are stable, others pulse and flare. Some are constant, others are variable. Some are red, others are blue some are extremely faint, others are millions of times as luminous as the Sun.

None of that matters they’re all stars. For as long as nuclear fusion (aside from deuterium burning) occurs in the cores of these objects, they’re stars.

But there’s a finite amount of fuel in each of these stars, and a finite amount of mass that they will convert into energy via Einstein’s most famous equation: E = mc². When the fusion stops, and new fusion doesn’t proceed when the core contracts and heats up further, the star’s life is over. At this point, the only questions is what comes next.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova. At the . [+] end of its life, if the core is massive enough, the formation of a black hole is absolutely unavoidable. If mass is siphoned off, an exotic white dwarf can emerge, and if its mass is too low, a neutron star will be formed instead.

Nicole Rager Fuller for the NSF

As far as we can tell, there are five options, depending on the star’s mass and situation.

1. Red dwarfs will be made entirely of helium, where the entire (former) star contracts down to a white dwarf star, eventually fading away to become a black dwarf.
2. Sun-like stars will blow off their outer layers in a planetary nebula, while the core contracts down to a carbon-oxygen white dwarf star, eventually fading away to become a black dwarf.
3. Heavier stars are destined to go supernova, where the lower-mass supernovae will produce neutron stars in their cores, up to about 2.5-2.75 solar masses.
4. Higher-mass supernovae will still explode, but their cores are too massive to produce neutron stars, and will produce black holes instead.
5. Or, in rare circumstances, the supergiant stars that would give rise to supernovae have their outer envelopes stolen away. In this fashion, “exotic” white dwarfs, like neon or magnesium white dwarfs, can be produced from the mass that’s left behind.

Those general fates, however — white dwarf stars, neutron stars, and black holes — represent what we know is possible.

At the cores of the most massive neutrons stars, the individual nuclei may break down into a . [+] quark-gluon plasma. Theorists presently argue over whether that plasma would exist, and if so, whether it would be composed of up-and-down quarks only, or whether strange quarks would be a part of that mix, too.

Sure, there are more exotic possibilities that can also occur. A neutron star can merge with a giant star, creating a Thorne-Zytkow object. A superluminous supernova or tidal disruption event can rip an entire supergiant star apart, leaving nothing behind at all. Or perhaps there are further degenerate forms of compressed matter — strange stars, quark stars, preon stars, etc. — that we simply have yet to discover and identify. Additionally, all white dwarf stars will cool and fade over time, turning red, then infrared, and eventually fading away to total blackness over nearly a quadrillion year timespan.

Despite the names of these remnants, they are not stars at all. Once they cease fusing elements in their cores, they’re only stellar remnants: what’s left behind by former stars. White dwarf stars aren’t stars the black dwarf stars that they’ll become aren’t stars either. Neutron stars aren’t stars neither are black holes, or (if they exist) any of the exotic stars like strange stars, quark stars, or preon stars. Thorne-Zytkow objects will remain stars so long as the giant star continues to fuse heavy elements once it ceases, it’s a star no more.

A Thorne-Zyktow object should be a red supergiant star that's merged with a neutron star that sank . [+] to its core. Arguably, approximately 1-out-of-70 observed red supergiant stars showed the spectral signature you'd associate with a Thorne-Zytkow object. It's an unusual fate for a supergiant star, but these exceptional cosmic beasts do exist.

Screenshot from Emily Levesque's Perimeter Institute lecture

When you put all of this information together, we can draw a clear line between what is a star and what isn’t. If something has a collapsed core held up by radiation but is still gathering gas from a surrounding molecular cloud, it’s a protostar, not a true star. If something is fusing deuterium but nothing else in its core, it’s a brown dwarf star (i.e., a failed star), not a true star. Only if your core is successfully fusing hydrogen into helium, or helium (or heavier elements) into something more massive, at temperatures of 4 million K or higher, can you be considered a true star.

But once you’re done with that nuclear fusion in your core, you’re also done being a star. Any sort of stellar remnant — white dwarf stars, neutron stars, black dwarf stars, etc. — isn’t a star at all, but the leftover remains of a one-time star that’s now deceased. These remnants may continue to shine and radiate for trillions of years, longer than even the lifetime of the stars that spawned them, but they themselves are not actual stars, despite their names. You can still be brilliant without fusion in your core, but you can no longer be considered a star.

## Surprise: Neutron Stars And White Dwarf Stars Aren’t Actually Stars

Sirius A and B, a normal (Sun-like) star and a white dwarf star in a binary system. Many such . [+] systems like this are known to exist, and the accretion of matter from the star onto the white dwarf is what drives the classical novae that create the Universe's lithium. The normal star is an actual star the white dwarf is not.

NASA, ESA and G. Bacon (STScI)

When we think about the objects in our Universe, they fall into two categories:

1. self-luminous objects, like stars, which generate their own light,
2. and non-luminous objects, that require an external energy source to be seen.

The latter category, which includes planets, moons, dust, and gas, will only emit light if it’s either reflected from a luminous source or absorbed and re-emitted from an external energy source.

But does being self-luminous automatically mean that you’re a star? Surprisingly, not only are there many exceptions to that rule, but some of those exceptions even have the word “star” right there in their name, despite not being actual stars. Brown dwarf stars, white dwarf stars, and even neutron stars aren’t actually stars, while red dwarf stars, yellow dwarfs (like our Sun), and all giant stars do turn out to be stars. Here’s what makes all the difference.

Stars form in a wide variety of sizes, colors and masses, including many bright, blue ones that are . [+] tens or even hundreds of times as massive as the Sun. This is demonstrated here in the open star cluster NGC 3766, in the constellation of Centaurus. If the Universe were infinite, even a cluster like this wouldn't display 'gaps' between the stars, as a more distant star would eventually fill those gaps in.

In our colloquial, day-to-day lives, most of us like to think that we know a star when we see it. We conventionally think of a massive ball of matter, giving off its own light, radiating energy out into the Universe. That’s true in a sense: all stars do actually do those things. They are massive clumps of matter, pulled into hydrostatic equilibrium by gravity. They undergo physical processes in their interior, which transfers energy outwards towards their surface. And from their boundaries — known as a star’s photosphere — energy, some of which falls in the visible light range, radiates out into the Universe.

### The Unfiltered Truth Behind Human Magnetism, Vaccines, And COVID-19

All of those things are true of stars, but they’re also true of other objects, some of which aren’t stars at all. To an astronomer, there’s a more stringent threshold that needs to be crossed if you’re going to be a star: you need to ignite nuclear fusion in your core. Not just any type of fusion, mind you, but the fusion of hydrogen (raw protons) into helium, or the products of that reaction into still heavier elements. Without achieving this, astronomers cannot consider an object to be a star.

The evolution of a solar-mass star on the Hertzsprung-Russell (color-magnitude) diagram from its . [+] pre-main-sequence phase to the end of fusion. Every star of every mass will follow a different curve, but the Sun is only a star once it begins hydrogen burning, and ceases to be a star once helium burning is completed.

Wikimedia Commons user Szczureq

This might seem arbitrary, but there’s an important set of reasons for it: reasons that become clear if we begin from a cloud of gas, which is the origin of all stars that we know of in the Universe today. Gas clouds are found throughout the Universe, are primarily made of hydrogen and helium (with only a few percent of other, heavier elements added into the mix), and — if they get cold and massive enough, or have a significant enough instability in them — will begin to collapse.

When this gravitational collapse begins to occur, there will inevitably be regions that begin with greater-than-average densities of matter. These overdense regions will exert a greater attractive force on matter than the other regions, and so will grow denser over time. What then ensues is a race between different regions to draw in as much matter as possible. There’s a problem with this scenario, however: when gas clouds collapse, the particles inside collide and heat up, preventing them from collapsing further.

The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or . [+] dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. While the external environment of these globules may be extremely hot, the interiors can be shielded from radiation and reach very low temperatures indeed.

The only way out is if these collapsing clouds of gas can somehow radiate energy away: they have to cool themselves down. The most efficient way to do that is through those heavier elements, which are far better at radiating energy away than hydrogen or helium atoms alone. As the clouds develop regions of matter that become hotter and hotter, the heated gas begins to not only radiate, but to trap that energy inside, causing the internal temperatures to skyrocket.

This gas might be emitting light, but it isn’t a star, at least not yet. It could be considered a proto-stellar nebula, however, as it’s taking a path that could lead to it becoming a full-blown star. But in order to get there, its temperature needs to continue to rise, and that can only continue so long as matter continues to fall into this overdense region, growing it and trapping even more heat.

When the temperature rises over about 1 million K in the core, the very first fusion reactions begin to occur.

The protostar IM Lup has a protoplanetary disk around it that exhibits not only rings, but a spiral . [+] feature towards the center. There is likely a very massive planet causing these spiral features, but that has yet to be definitively confirmed. In the early stages of a solar system's formation, these protoplanetary disks cause dynamical friction, causing young planets to spiral inwards rather than complete perfect, closed ellipses. The central protostar has not yet ignited nuclear fusion in its core.

S. M. Andrews et al. and the DSHARP collaboration, arXiv:1812.04040

What happens first is that deuterium — an isotope of hydrogen made of one proton and one neutron — can fuse together with a free proton to form a helium-3 nucleus: with two protons and one neutron. When this threshold is crossed, the nebula officially becomes a protostar: a large mass of matter that’s still accruing mass from its molecular surroudings, whose core is supported by pressure. The deuterium fusion reaction that’s occurring provides that pressure, while gravitation counteracts it.

Under most circumstances, there will be many points in this large clouds of gas that race to grow and grow, siphoning mass onto themselves and away from the other protostars. There are winners and losers in this war, as some protostars will gain enough mass to heat up above

4 million K, where they’ll begin the same chain reaction that powers our Sun: the proton-proton chain. If you cross that threshold, you’re a cosmic winner, as you’ll become a true star. But if you don’t, and you remain in this “limbo” where you only fuse deuterium, you’ll become a brown dwarf star: a failed star.

Gliese 229 is a red dwarf star, and is orbited by Gliese 229b, a brown dwarf, that fuses deuterium . [+] only. Although Gliese 229b is about 20 times the mass of Jupiter, it's only about 47% of its radius. Failed stars will become brown dwarfs with between 13 and 80 times the mass of Jupiter.

T. Nakajima and S. Kulkarni (CalTech), S. Durrance and D. Golimowski (JHU), NASA

Brown dwarfs range in mass from about 13 times the mass of Jupiter up to about 80 Jupiter masses: about 7.5% the mass of our Sun. Although they’re often called brown dwarf stars, they’re not truly stars, because they don’t meet that critical threshold: they cannot undergo the fusion reactions that are required to become a full-blown star. If a brown dwarf ever merges with another or accretes enough mass from a companion to cross this mass threshold, it can raise its game to become a red dwarf star: fusing hydrogen into helium and becoming a true star.

These actual stars come in a wide variety of masses, colors, and brightnesses. The ones that range from 7.5% to about 40% of the Sun’s mass are the red dwarf stars: they will burn hydrogen into helium and that’s it they will never reach higher temperatures to do anything else. Stars from 40% to 800% the Sun’s mass will eventually evolve into red giants, fusing helium into carbon when they do, before running out of fuel. And the even more massive stars will become supergiants, eventually going supernova when they reach the end of their lives.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star . [+] class shown above it, in kelvin. Our Sun is a G-class star, producing light with an effective temperature of around 5800 K and a brightness of 1 solar luminosity. Stars can be as low in mass as 8% the mass of our Sun, where they'll burn with

0.01% our Sun's brightness and live for more than 1000 times as long, but they can also rise to hundreds of times our Sun's mass, with millions of times our Sun's luminosity and lifetimes of just a few million years. The first generation of stars should consist of O-type and B-type stars almost exclusively, and may contain stars up to 1,000+ times the mass of our Sun.

Wikimedia Commons user LucasVB, additions by E. Siegel

All the stars that burn hydrogen, helium, carbon, or heavier elements up to iron — whether they’re dwarf-sized, giant-sized or supergiant-sized — are all stars. So long as they’re converting light elements into heavy elements via the energy-releasing process of nuclear fusion, they can be considered stars. Some are stable, others pulse and flare. Some are constant, others are variable. Some are red, others are blue some are extremely faint, others are millions of times as luminous as the Sun.

None of that matters they’re all stars. For as long as nuclear fusion (aside from deuterium burning) occurs in the cores of these objects, they’re stars.

But there’s a finite amount of fuel in each of these stars, and a finite amount of mass that they will convert into energy via Einstein’s most famous equation: E = mc². When the fusion stops, and new fusion doesn’t proceed when the core contracts and heats up further, the star’s life is over. At this point, the only questions is what comes next.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova. At the . [+] end of its life, if the core is massive enough, the formation of a black hole is absolutely unavoidable. If mass is siphoned off, an exotic white dwarf can emerge, and if its mass is too low, a neutron star will be formed instead.

Nicole Rager Fuller for the NSF

As far as we can tell, there are five options, depending on the star’s mass and situation.

1. Red dwarfs will be made entirely of helium, where the entire (former) star contracts down to a white dwarf star, eventually fading away to become a black dwarf.
2. Sun-like stars will blow off their outer layers in a planetary nebula, while the core contracts down to a carbon-oxygen white dwarf star, eventually fading away to become a black dwarf.
3. Heavier stars are destined to go supernova, where the lower-mass supernovae will produce neutron stars in their cores, up to about 2.5-2.75 solar masses.
4. Higher-mass supernovae will still explode, but their cores are too massive to produce neutron stars, and will produce black holes instead.
5. Or, in rare circumstances, the supergiant stars that would give rise to supernovae have their outer envelopes stolen away. In this fashion, “exotic” white dwarfs, like neon or magnesium white dwarfs, can be produced from the mass that’s left behind.

Those general fates, however — white dwarf stars, neutron stars, and black holes — represent what we know is possible.

At the cores of the most massive neutrons stars, the individual nuclei may break down into a . [+] quark-gluon plasma. Theorists presently argue over whether that plasma would exist, and if so, whether it would be composed of up-and-down quarks only, or whether strange quarks would be a part of that mix, too.

Sure, there are more exotic possibilities that can also occur. A neutron star can merge with a giant star, creating a Thorne-Zytkow object. A superluminous supernova or tidal disruption event can rip an entire supergiant star apart, leaving nothing behind at all. Or perhaps there are further degenerate forms of compressed matter — strange stars, quark stars, preon stars, etc. — that we simply have yet to discover and identify. Additionally, all white dwarf stars will cool and fade over time, turning red, then infrared, and eventually fading away to total blackness over nearly a quadrillion year timespan.

Despite the names of these remnants, they are not stars at all. Once they cease fusing elements in their cores, they’re only stellar remnants: what’s left behind by former stars. White dwarf stars aren’t stars the black dwarf stars that they’ll become aren’t stars either. Neutron stars aren’t stars neither are black holes, or (if they exist) any of the exotic stars like strange stars, quark stars, or preon stars. Thorne-Zytkow objects will remain stars so long as the giant star continues to fuse heavy elements once it ceases, it’s a star no more.

A Thorne-Zyktow object should be a red supergiant star that's merged with a neutron star that sank . [+] to its core. Arguably, approximately 1-out-of-70 observed red supergiant stars showed the spectral signature you'd associate with a Thorne-Zytkow object. It's an unusual fate for a supergiant star, but these exceptional cosmic beasts do exist.

Screenshot from Emily Levesque's Perimeter Institute lecture

When you put all of this information together, we can draw a clear line between what is a star and what isn’t. If something has a collapsed core held up by radiation but is still gathering gas from a surrounding molecular cloud, it’s a protostar, not a true star. If something is fusing deuterium but nothing else in its core, it’s a brown dwarf star (i.e., a failed star), not a true star. Only if your core is successfully fusing hydrogen into helium, or helium (or heavier elements) into something more massive, at temperatures of 4 million K or higher, can you be considered a true star.

But once you’re done with that nuclear fusion in your core, you’re also done being a star. Any sort of stellar remnant — white dwarf stars, neutron stars, black dwarf stars, etc. — isn’t a star at all, but the leftover remains of a one-time star that’s now deceased. These remnants may continue to shine and radiate for trillions of years, longer than even the lifetime of the stars that spawned them, but they themselves are not actual stars, despite their names. You can still be brilliant without fusion in your core, but you can no longer be considered a star.

## Contents

A red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a reddish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun ( L ), spectral types of K or M, have surface temperatures of 3,000–4,000 K, and radii up to about 200 times the Sun ( R ). Stars on the horizontal branch are hotter, with only a small range of luminosities around 75 L . Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red-giant branch, up to several times more luminous at the end of the thermal pulsing phase.

Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to the surface in what is called a dredge-up. [1] The first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge up occurs during helium shell burning on the asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars.

The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations. Rather, due to the very low mass density of the envelope, such stars lack a well-defined photosphere, and the body of the star gradually transitions into a 'corona'. [2] The coolest red giants have complex spectra, with molecular lines, emission features, and sometimes masers, particularly from thermally pulsing AGB stars. [3] Observations have also provided evidence of a hot chromosphere above the photosphere of red giants, [4] [5] [6] where investigating the heating mechanisms for the chromospheres to form requires 3D simulations of red giants. [7]

Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells (solar granules), red-giant photospheres, as well as those of red supergiants, have just a few large cells, the features of which cause the variations of brightness so common on both types of stars. [8]

Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M to around 8 M . [9] When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of "metals" (in stellar structure, this simply refers to any element that is not hydrogen or helium i.e. atomic number greater than 2). These elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million kelvin) and establishes hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium its main-sequence life ends when nearly all the hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars. [10]

When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its own gravity. This brings additional hydrogen into a zone where the temperature and pressure are sufficient to cause fusion to resume in a shell around the core. The hydrogen-burning shell results in a situation that has been described as the mirror principle when the core within the shell contracts, the layers of the star outside the shell must expand. The detailed physical processes that cause this are complex, but the behaviour is necessary to satisfy simultaneous conservation of gravitational and thermal energy in a star with the shell structure. The core contracts and heats up due to the lack of fusion, and so the outer layers of the star expand greatly, absorbing most of the extra energy from shell fusion. This process of cooling and expanding is the subgiant star. When the envelope of the star cools sufficiently it becomes convective, the star stops expanding, its luminosity starts to increase, and the star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram. [10] [11]

The evolutionary path the star takes as it moves along the red-giant branch depends on the mass of the star. For the Sun and stars of less than about 2 M [12] the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 10 8 K, hot enough to begin fusing helium to carbon via the triple-alpha process. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash. In more-massive stars, the collapsing core will reach 10 8 K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash. [10] The core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram. [13]

An analogous process occurs when the central helium is exhausted and the star collapses once again, causing helium in a shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase. [14] The helium fusion results in the build up of a carbon–oxygen core. A star below about 8 M will never start fusion in its degenerate carbon–oxygen core. [12] Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution. [10] The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is spent on the red-giant branch. The horizontal-branch and asymptotic-giant-branch phases proceed tens of times faster.

If the star has about 0.2 to 0.5 M , [12] it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium. [9] These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become a white dwarf.

### Stars that do not become red giants Edit

Very-low-mass stars are fully convective [15] [16] and may continue to fuse hydrogen into helium for up to a trillion years [17] until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and the luminosity by around 10 times. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as the Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs. [9]

Very-high-mass stars develop into supergiants that follow an evolutionary track that takes them back and forth horizontally over the H–R diagram, at the right end constituting red supergiants. These usually end their life as a type II supernova. The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all. [18] [19]

Red giants with known planets: the M-type HD 208527, HD 220074 and, as of February 2014, a few tens [20] of known K-giants including Pollux, Gamma Cephei and Iota Draconis.

### Prospects for habitability Edit

Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a 1 M star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional one billion years. [21] Later studies have refined this scenario, showing how for a 1 M star the habitable zone lasts from 100 million years for a planet with an orbit similar to that of Mars to 210 million years for one that orbits at Saturn's distance to the Sun, the maximum time (370 million years) corresponding for planets orbiting at the distance of Jupiter. However, planets orbiting a 0.5 M star in equivalent orbits to those of Jupiter and Saturn would be in the habitable zone for 5.8 billion years and 2.1 billion years, respectively for stars more massive than the Sun, the times are considerably shorter. [22]

### Enlargement of planets Edit

As of June 2014, fifty giant planets have been discovered around giant stars. However, these giant planets are more massive than the giant planets found around solar-type stars. This could be because giant stars are more massive than the Sun (less massive stars will still be on the main sequence and will not have become giants yet) and more massive stars are expected to have more massive planets. However, the masses of the planets that have been found around giant stars do not correlate with the masses of the stars therefore, the planets could be growing in mass during the stars' red giant phase. The growth in planet mass could be partly due to accretion from stellar wind, although a much larger effect would be Roche lobe overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet. [23]

Many of the well-known bright stars are red giants, because they are luminous and moderately common. The red-giant branch variable star Gamma Crucis is the nearest M-class giant star at 88 light-years. [24] The K0 red-giant branch star Arcturus is 36 light-years away. [25]

## Contents

The revised Yerkes Atlas system (Johnson & Morgan 1953) [14] listed 11 G-type dwarf spectral standard stars however, not all of these still conform to this designation.

The "anchor points" of the MK spectral classification system among the G-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years, are beta CVn (G0V), the Sun (G2V), Kappa1 Ceti (G5V), 61 Ursae Majoris (G8V). [15] Other primary MK standard stars include HD 115043 (G1V) and 16 Cygni B (G3V). [16] The choices of G4 and G6 dwarf standards have changed slightly over the years among expert classifiers, but often-used examples include 70 Virginis (G4V) and 82 Eridani (G8V). There are not yet any generally agreed upon G7V and G9V standards.

Some of the nearest G-type stars known to have planets include the Sun, 61 Virginis, HD 102365, HD 147513, 47 Ursae Majoris, Mu Arae, and Tau Ceti.

## Dwarf stars

Most stars in the sky, except the brightest, appear white or bluish to the naked eye because they are too dim for color vision to work.

Red giant stars are cooler and redder than red dwarf stars.

From the moment a star is born, until the hydrogen in its core is depleted, practically ninety percent of the life of that star passes.

Stars. Evolution of Stars. Credit: youtube.

During this phase of its life, the star shines less than the Sun.

The vast majority of the stars in the universe are dwarf stars and represent “normality” in stellar astrophysics.

### First steps in star rating?

Until 1980, “astronomy” consisted of the study of “position and movements” of the celestial bodies.

Edward Pickering set out to go one step further and come to know the nature of the stars. He set out to discover the physical composition of the stars. That was already “astrophysics.

Edward Pickering astronomer, discoverer of stars and highlighted the worth of women. Credit: Popular Science Monthly.

This visionary professor began by placing a prism on the objective of the telescope, in order to obtain the light spectra of the stars.

This elemental technique had already been devised by William Hershel in 1798. Thus he made the first descriptions of the spectra of two well-known stars: Sirius and Arthur.

Soon after, in 1814, Joseph Fraunhofer began to study the lines that appeared in the spectrum of the Sun.

Each chemical element originates a unique light spectrum from that element. Credit: web slideshare.net

In 1861, Gustav Kirhhoff and Robert Bunsen used the lines discovered by Fraunhofer to identify the chemical elements in the solar atmosphere.

In 1862, Lewis Rutherford had obtained the first spectral plates of starlight.

In 1867, the Jesuit Angelo Secchi, made a classification of the stars, based on the chemical elements that their light spectra showed.

### Star rating at Harvard

In 1881, at the Harvard Observatory, Edward Pickering had accumulated photographic plates with the most detailed stellar spectra captured up to that date.

Professor Pickering decided to offer temporary employment to his young housekeeper, Williamina Fleming, to work on sorting the material.

Williamina Fleming. Credit: Wikipedia

As the astute and clairvoyant professor expected, once her intelligence was at the service of an attractive cause, Williamina Fleming worked tirelessly and efficiently.

In this early stage, she identified and classified the spectra of more than 10,000 stars.

In 1886, the widow of Henry Draper, a pioneer in obtaining photographs of star spectra, decided to fund the work of the Harvard Observatory.

Edward Pickering did not waste a single moment. His first experience with a smart woman couldn’t have been better, so he hired nine other women.

He tasked them with performing routine calculations to analyze the photographs of the stars and classify the spectra recorded on the photographic plates.

Edward Pickering and his team of calculators outside Harvard College’s Building C. Credit: Wikipedia.

This was undoubtedly a more challenging job for these young women than cleaning in a house or working in a factory.

Women were trained in the women’s universities in the area and the team, led by Williamina Fleming, began to stand out for its efficiency and sagacity.

It was a wonderful team, and these young women became known as “Harvard computers.

### New star rating system

Williamina Fleming helped to develop a star assignment system, which basically consisted of assigning the star a letter, which depended on the amount of hydrogen observed in its spectrum.

The stars classified with the letter A were almost entirely made up of hydrogen, those classified with the letter B contained less hydrogen, and so on, 16 types of stars, from A to N.

Professor Pickering did not hesitate to make public recognition of her authorship and it is the basis of the spectral classification in use today: Harvard Classification.

### Improvements to the Harvard ranking system

The system devised by Williamina Fleming served as the basis of work to develop a classification of stars based on the observed temperature.

In 1896, Annie Cannon joined the Harvard Computer team. She was commissioned to continue with the stellar classification of the Southern Hemisphere.

Annie Cannon, the first woman to receive a Ph.D. from Oxford University. Credit: web “on this day.com”.

In an attempt to make improvements and streamline work on the cataloging system, Annie Cannon established classification rules based on the temperature of the stars.

These substantial advances in spectral classification are the basis of the system currently used.

In 1906, the Danish astronomer Ejnar Hertzprung suggested that the reddest stars, assigned as K and M in the Harvard Classification scheme, be divided into two groups:

• Giant stars, those that were much brighter than the Sun
• Dwarf stars, those that shone much fainter than the Sun.

### Classification of dwarf stars.

The group of dwarf stars was later divided into seven subgroups:

• Red dwarfs: they are low-mass stars during their evolution.
• Yellow dwarfs: their masses are comparable to that of the Sun.
• Orange dwarfs: they are stars with a mass slightly greater than that of the Sun.
• Blue dwarf is a hypothetical class of very low mass stars that increase in temperature as soon as they reach the end of their life.
• White dwarfs: they are stars made up of electrons, which are in the final stage of their evolution. They don’t have enough mass to collapse into a neutron star or to explode as a supernova.
• Black dwarfs: these are white dwarfs that have cooled so much that they no longer emit any visible light.
• Brown dwarfs: they have little mass, less than 0.08 solar masses. This small mass is not enough to cause the fusion of hydrogen into helium.

Stars do not remain in their dwarf state for life, but instead become giants, although, in the course of their evolution, they may eventually revert to a white dwarf state.

The Sun, currently a dwarf star, will be a red giant in five billion years, and in another half a billion years it will be a dwarf again, this time a white dwarf.

The group of dwarf stars is technically called “luminosity class V” stars.

### Red dwarfs

Red dwarf is a small and relatively cool star.

This type is formed by most of the stars, their mass and diameter values ​​being less than half those of the Sun and a surface temperature of less than 4,000º K.

According to some estimates, red dwarfs represent three-quarters of the stars in the Milky Way but, due to their low luminosity, they cannot be easily observed.

From Earth, none are visible to the naked eye. Proxima Centauri, the closest star to the Sun, is a red dwarf, as are twenty of the thirty closest stars.

Red dwarfs with less than 0.35 solar masses develop very slowly, harboring a constant luminosity and spectral type, so – in theory – their fuel will take a few billion years to run out.

### White dwarfs

White dwarf is a stellar remnant that is generated when a star of mass less than 10 solar masses has exhausted its nuclear fuel, and has expelled much of this mass in a planetary nebula.

Ancient white dwarf explosión Credit: web “newatlas.com”

White dwarfs are, along with red dwarfs, the most abundant stars in the universe.

97% of the stars that we know, including the Sun, go through this stage of stellar evolution.

### Brown dwarfs

Brown dwarfs are believed to be failed stars, as they contain the same materials as a star like the Sun, but with too little mass to shine.

They are very similar to the gaseous planets They are not quite planets, but neither are they stars.

Artist’s impression of the relative sizes of brown dwarfs compared to stars and gas giant planets. Credit: Carnegie Institution for Science

Using Jupiter as a comparison, the brown dwarf is 10 times more massive, the low-mass star is 100 times more massive, and the Sun is approximately 1,000 times more massive.

The first verified brown dwarf was Teide-1, in 1995, at the Teide Observatory, in the Canary Islands.

The mass of this dwarf star is 25 times that of Jupiter. Canarian researchers referred to it as a superplanet.

María Teresa Ruiz, on March 15, 1997, was able to make a very important contribution to Astronomy.

Her gaze met an object she wasn’t looking for.

She at first she did not know what this object was. It didn’t look like a star It could be a giant planet, a super Jupiter, or a brown dwarf.

Ultimately, it turned out to be a system of two brown dwarfs located in the southern constellation Hydra, approximately 61 light-years from Earth.

This photo shows a small sky area around the newly discovered Brown Dwarf object KELU-1 in the southern constellation of Hydra. It is indicated with tick marks. Credit: ESO.

This object discovered by Dr. María Teresa Ruiz has been called Kelu-1 brown dwarf object.

The image that María Teresa obtained on March 15, 1997, she made through an infrared filter, with the 3.6-meter telescope, at the La Silla Observatory.

Brown dwarfs occupy the mass range between the heaviest gas giant planets and the lightest stars.

The mass of the largest brown stars is between 75 and 80 times the mass of Jupiter.

Nuclear fusion occurs in the youth of the star, but the atomic fuel disappears quickly, and its nuclear reaction cannot withstand the immense gravitational collapse.

Brown dwarfs continue to glow for a time due to residual heat from reactions and the slow contraction of the matter that forms them. But, they cool down until they reach an equilibrium.

Stars are classified by spectral class, with brown dwarfs being designated as M, L, T, and Y types.

Despite their name, brown dwarfs come in different colors: magenta, orange, or red.

## What's Causing the Two White Dwarfs to Collide

Over time, their orbits can deteriorate, eventually causing a collision between the two white dwarfs. What's going to take place next is according to the situation.

Frequently, as shown in the recent Astronomy & Astrophysics paper, the stars can explode either as a nova or supernova, developing a remnant neutron star, although there are times when they can form something more uncommon.

Furthermore, an x-ray source was detected in 2019 that looked the same as a white dwarf, although it was too bright to be caused by the latter.

It was proposed that the object could be an unsteady merger of two white dwarfs. In this new research, a group of researchers used a tool identified as the XMM-Newton X-ray telescope to capture an object's image.

## Surprise: Neutron Stars And White Dwarf Stars Aren’t Actually Stars

When we think about the objects in our Universe, they fall into two categories:

1. self-luminous objects, like stars, which generate their own light,
2. and non-luminous objects, that require an external energy source to be seen.

The latter category, which includes planets, moons, dust, and gas, will only emit light if it’s either reflected from a luminous source or absorbed and re-emitted from an external energy source.

But does being self-luminous automatically mean that you’re a star? Surprisingly, not only are there many exceptions to that rule, but some of those exceptions even have the word “star” right there in their name, despite not being actual stars. Brown dwarf stars, white dwarf stars, and even neutron stars aren’t actually stars, while red dwarf stars, yellow dwarfs (like our Sun), and all giant stars do turn out to be stars. Here’s what makes all the difference.

In our colloquial, day-to-day lives, most of us like to think that we know a star when we see it. We conventionally think of a massive ball of matter, giving off its own light, radiating energy out into the Universe. That’s true in a sense: all stars do actually do those things. They are massive clumps of matter, pulled into hydrostatic equilibrium by gravity. They undergo physical processes in their interior, which transfers energy outwards towards their surface. And from their boundaries — known as a star’s photosphere — energy, some of which falls in the visible light range, radiates out into the Universe.

All of those things are true of stars, but they’re also true of other objects, some of which aren’t stars at all. To an astronomer, there’s a more stringent threshold that needs to be crossed if you’re going to be a star: you need to ignite nuclear fusion in your core. Not just any type of fusion, mind you, but the fusion of hydrogen (raw protons) into helium, or the products of that reaction into still heavier elements. Without achieving this, astronomers cannot consider an object to be a star.

This might seem arbitrary, but there’s an important set of reasons for it: reasons that become clear if we begin from a cloud of gas, which is the origin of all stars that we know of in the Universe today. Gas clouds are found throughout the Universe, are primarily made of hydrogen and helium (with only a few percent of other, heavier elements added into the mix), and — if they get cold and massive enough, or have a significant enough instability in them — will begin to collapse.

When this gravitational collapse begins to occur, there will inevitably be regions that begin with greater-than-average densities of matter. These overdense regions will exert a greater attractive force on matter than the other regions, and so will grow denser over time. What then ensues is a race between different regions to draw in as much matter as possible. There’s a problem with this scenario, however: when gas clouds collapse, the particles inside collide and heat up, preventing them from collapsing further.

The only way out is if these collapsing clouds of gas can somehow radiate energy away: they have to cool themselves down. The most efficient way to do that is through those heavier elements, which are far better at radiating energy away than hydrogen or helium atoms alone. As the clouds develop regions of matter that become hotter and hotter, the heated gas begins to not only radiate, but to trap that energy inside, causing the internal temperatures to skyrocket.

This gas might be emitting light, but it isn’t a star, at least not yet. It could be considered a proto-stellar nebula, however, as it’s taking a path that could lead to it becoming a full-blown star. But in order to get there, its temperature needs to continue to rise, and that can only continue so long as matter continues to fall into this overdense region, growing it and trapping even more heat.

When the temperature rises over about 1 million K in the core, the very first fusion reactions begin to occur.

What happens first is that deuterium — an isotope of hydrogen made of one proton and one neutron — can fuse together with a free proton to form a helium-3 nucleus: with two protons and one neutron. When this threshold is crossed, the nebula officially becomes a protostar: a large mass of matter that’s still accruing mass from its molecular surroudings, whose core is supported by pressure. The deuterium fusion reaction that’s occurring provides that pressure, while gravitation counteracts it.

Under most circumstances, there will be many points in this large clouds of gas that race to grow and grow, siphoning mass onto themselves and away from the other protostars. There are winners and losers in this war, as some protostars will gain enough mass to heat up above

4 million K, where they’ll begin the same chain reaction that powers our Sun: the proton-proton chain. If you cross that threshold, you’re a cosmic winner, as you’ll become a true star. But if you don’t, and you remain in this “limbo” where you only fuse deuterium, you’ll become a brown dwarf star: a failed star.

Brown dwarfs range in mass from about 13 times the mass of Jupiter up to about 80 Jupiter masses: about 7.5% the mass of our Sun. Although they’re often called brown dwarf stars, they’re not truly stars, because they don’t meet that critical threshold: they cannot undergo the fusion reactions that are required to become a full-blown star. If a brown dwarf ever merges with another or accretes enough mass from a companion to cross this mass threshold, it can raise its game to become a red dwarf star: fusing hydrogen into helium and becoming a true star.

These actual stars come in a wide variety of masses, colors, and brightnesses. The ones that range from 7.5% to about 40% of the Sun’s mass are the red dwarf stars: they will burn hydrogen into helium and that’s it they will never reach higher temperatures to do anything else. Stars from 40% to 800% the Sun’s mass will eventually evolve into red giants, fusing helium into carbon when they do, before running out of fuel. And the even more massive stars will become supergiants, eventually going supernova when they reach the end of their lives.

0.01% our Sun’s brightness and live for more than 1000 times as long, but they can also rise to hundreds of times our Sun’s mass, with millions of times our Sun’s luminosity and lifetimes of just a few million years. The first generation of stars should consist of O-type and B-type stars almost exclusively, and may contain stars up to 1,000+ times the mass of our Sun. (WIKIMEDIA COMMONS USER LUCASVB, ADDITIONS BY E. SIEGEL)

All the stars that burn hydrogen, helium, carbon, or heavier elements up to iron — whether they’re dwarf-sized, giant-sized or supergiant-sized — are all stars. So long as they’re converting light elements into heavy elements via the energy-releasing process of nuclear fusion, they can be considered stars. Some are stable, others pulse and flare. Some are constant, others are variable. Some are red, others are blue some are extremely faint, others are millions of times as luminous as the Sun.

None of that matters they’re all stars. For as long as nuclear fusion (aside from deuterium burning) occurs in the cores of these objects, they’re stars.

But there’s a finite amount of fuel in each of these stars, and a finite amount of mass that they will convert into energy via Einstein’s most famous equation: E = mc². When the fusion stops, and new fusion doesn’t proceed when the core contracts and heats up further, the star’s life is over. At this point, the only questions is what comes next.

As far as we can tell, there are five options, depending on the star’s mass and situation.

1. Red dwarfs will be made entirely of helium, where the entire (former) star contracts down to a white dwarf star, eventually fading away to become a black dwarf.
2. Sun-like stars will blow off their outer layers in a planetary nebula, while the core contracts down to a carbon-oxygen white dwarf star, eventually fading away to become a black dwarf.
3. Heavier stars are destined to go supernova, where the lower-mass supernovae will produce neutron stars in their cores, up to about 2.5–2.75 solar masses.
4. Higher-mass supernovae will still explode, but their cores are too massive to produce neutron stars, and will produce black holes instead.
5. Or, in rare circumstances, the supergiant stars that would give rise to supernovae have their outer envelopes stolen away. In this fashion, “exotic” white dwarfs, like neon or magnesium white dwarfs, can be produced from the mass that’s left behind.

Those general fates, however — white dwarf stars, neutron stars, and black holes — represent what we know is possible.

Sure, there are more exotic possibilities that can also occur. A neutron star can merge with a giant star, creating a Thorne-Zytkow object. A superluminous supernova or tidal disruption event can rip an entire supergiant star apart, leaving nothing behind at all. Or perhaps there are further degenerate forms of compressed matter — strange stars, quark stars, preon stars, etc. — that we simply have yet to discover and identify. Additionally, all white dwarf stars will cool and fade over time, turning red, then infrared, and eventually fading away to total blackness over nearly a quadrillion year timespan.

Despite the names of these remnants, they are not stars at all. Once they cease fusing elements in their cores, they’re only stellar remnants: what’s left behind by former stars. White dwarf stars aren’t stars the black dwarf stars that they’ll become aren’t stars either. Neutron stars aren’t stars neither are black holes, or (if they exist) any of the exotic stars like strange stars, quark stars, or preon stars. Thorne-Zytkow objects will remain stars so long as the giant star continues to fuse heavy elements once it ceases, it’s a star no more.

When you put all of this information together, we can draw a clear line between what is a star and what isn’t. If something has a collapsed core held up by radiation but is still gathering gas from a surrounding molecular cloud, it’s a protostar, not a true star. If something is fusing deuterium but nothing else in its core, it’s a brown dwarf star (i.e., a failed star), not a true star. Only if your core is successfully fusing hydrogen into helium, or helium (or heavier elements) into something more massive, at temperatures of 4 million K or higher, can you be considered a true star.

But once you’re done with that nuclear fusion in your core, you’re also done being a star. Any sort of stellar remnant — white dwarf stars, neutron stars, black dwarf stars, etc. — isn’t a star at all, but the leftover remains of a one-time star that’s now deceased. These remnants may continue to shine and radiate for trillions of years, longer than even the lifetime of the stars that spawned them, but they themselves are not actual stars, despite their names. You can still be brilliant without fusion in your core, but you can no longer be considered a star.