D
ifferent Stars birth, shine, and die differently. Hello and welcome to our blog called CosmicWisdom, where we talk about the Cosmos and its different aspects. In today's post, we will be exploring the lives of different kinds of stars and their destinies, in brief. Don't be disappointed if you didn't get sufficient information because this is a post that connects several of the stellar life posts that have been published or will be published in the future. Without wasting any time, let's begin.
Introduction:
Have you ever thought, when scientists say this star is going to die as a supernova or become a white dwarf, or it lies in this particular phase of its life, how they get those things? If you ask your grandpa, " Do you know when the sun was born? He might say My great-grandpa also asked this question to his grandparents; they will say we see the sun and moon as they look now.
Scientists are indeed humans with an average lifespan of 70-100 years; no single star has ever been born, shone, and died on a human timescale. They actually study the different stars together in a cluster because stars in a stellar cluster form and evolve almost simultaneously.
It's a well-known and proven fact that a star's life is mainly the story
of its birth mass, meaning its destiny is tied to the mass it was born
with, or it has been on the zero-age main-sequence phase ever since. In
the early 1910s, two scientists, Ejnar Hertzprung and
Henry Norris Russell, developed a diagram independently called the
Hertzsprung-Russell Diagram or HRD, which looks like
this:
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| The Hertzsprung-Russell Diagram |
Credit:
“Adapted from Hertzsprung–Russell diagram illustration by R.J. Hall,
licensed under CC BY‑SA 3.0.”
Hertzsprung-Russell Diagram:
If you used to hate or fear graphs and diagrams in your school exams,
because this one is also adorned with wings, horns, and tails like a
dragon. No problem, stick with us, and we'll eliminate this monster so
that you can know what this is all about. Let's see its four sides.
- A) On the bottom side, or the X-axis, you can see there are words like Color (B-V) and some numbers. It's the measurement of the star's photosphere (where the light emanates from), whether it appears blue (0.0) or red (+2.0); numbers denote intermediate colors. This generally means that as you move from left to right, stars get cooler because blue means hot and red means cool.
- B) On the left side, or the Y-axis, you can see the word Luminosity and some numbers. Luminosity means how many photons (particles of light) are emitted from the photosphere. Note this also includes all the types of radiation, like radio, infrared, visible, UV, or Gamma rays. It is measured on a solar scale, meaning the luminosity of the Sun and its multiples for brighter stars or decimal fractions for the faint ones.
- C) On the top side, spectral classes and temperature are plotted. You can check this article if you wanna know its details. O is the hottest, and M is the coolest; not so cool that you can walk on its surface; it's still around 3000 kelvins. Also, the stellar class colors like Yellow (G), Orange(K), or White(A) are the colors that are connected with spectroscopic observations, not the ones you see with your eyes.
- D) On the right vertical side, a term called absolute magnitude with its numbers is located. There are basically two kinds of magnitudes: apparent and absolute. Apparent magnitude means the brightness of a light source (a star, for instance), depending on its distance. For example, the Sun would appear brighter on Mercury and fainter on Neptune; in this case, the Sun's apparent magnitude would be -27 from Mercury, -26 from Earth, and -22 from Neptune. The brighter the object, the smaller the number, while faint light sources would be in positive numbers.
Now, let's see the absolute magnitude. It simply means the
apparent magnitude from
10 parsecs (~32 light years) without external light-blocking
factors like interstellar medium clouds or others.
You now have the idea of this diagram: it's just plotted thousands of
stars along with those physical properties of the four sides. You may
ask how the hell it will tell the star's life because it only accounts
for stars in the current observed time?
Indeed, a lone Hertzsprung-Russell diagram (HRD) can't tell anyone
anything magically about the stars and their lives. It requires the
study of stellar physics, which is the study of stellar interiors,
energy transport, plasma dynamics, virial theorems, state equations,
subatomic particle physics, quantum mechanics, and much more, many of
which we often discuss in our blog too.
You could say the HRD is basically an observational stellar toolkit or
map of the observed region. The image above shows the stars in the solar
neighborhood. Of course, you can create such a diagram of any observed
region, like the Orion Spur, the Pleades cluster, or
anything where stars are found. Therefore, an HRD has a similar role as
the periodic table in chemistry or the circle of 5ths in music; say, one
term or curious about an entity and its basic info could be known
easily, if you know how to interpret.
Luminosity classes:
You can see there are some horizontal lines with labels like main
sequence, subgiant, giant, bright giant, and supergiant. These are the
names for luminosity classes from V to I or 0. Actually, there are VII
for white dwarfs and VI for sub dwarfs, but for this stellar topic,
we'll mainly look for those 5 classes. Let's know them briefly.
V-Main sequence:
When a star is burning hydrogen as a fuel and producing helium in
its core, we call such a star a main-sequence star. 90% of a
star's life is spent in this stage. In this phase, the star is almost
balanced between its fusion energy output and crushing gravity.
Depending on mass, we divide stars into three categories:
massive (10+ Solar masses), intermediate (2-8 Solar
masses), and low mass (1.5-0.08 Solar masses). Further, we'll
talk about their life cycle in brief.
IV-Subgiant:
When the main-sequence stars are about to exhaust their hydrogen supply
in their cores, gravity starts to crush it, and the star's outer layers
(envelope) expand and cool in response. At this time, Fusion in the core
is at a minimum and sometimes totally zero fusion activity, and it
remains a degenerate helium core, which is very hard to fuse for most of
the stars.
That's why hydrogen fusion ignites just outside of the dead core, where
it reaches its minimum threshold of about 13 million kelvin and
100 gm/cc of density; it's called shell burning. This way, it
adds more helium, and the core grows in mass. An expanding subgiant
star's envelope keeps expanding and cooling until it reaches about 5000K
or below. When it arrives at this stage, the star settles into the
initial periods of the giant phase.
III-Giant:
Expansion of the envelope and contraction of the core halt for some
time, in its initial period. The star continues to expand and cool while
Hydrogen fusion in a shell keeps adding mass to the core. Low-mass stars
spend here too long, while intermediate ones may have a short time. It's
a very important crossroad for stellar life, and we'll treat this part
specially because many stars die here, some regain their previous
colors, or some also become supernovae.
II-Bright giant:
This is a different kind of star that is born massive and shines much
brighter. In their case, brightness always remains high. Luminosity
classes denote their evolutionary phases rather than luminosities in
general.
0 or Ia-Supergiant:
Like II, they are more massive star groups that are born massive, and
they usually fuse all the way to iron, die as supernovae, and leave a
neutron star or black hole depending on their masses.
Specific portions of the diagram:
We'll look closely at the diagram that contains the 22,000 stars from
the Hipparcos and 1000 from the Gliese catalogs (the HRD, which is shown
in this article). The main-sequence stars are highly abundant, and its
reason we talked earlier.
Hertzsprung Gap
The subgiants are relatively rarer since main-sequence stars like F to K
are long-living, while hot stars O, B, and A quickly go through this
phase and often overlap, so they have relatively lower numbers in our
neighborhood. This gap is called the Hertzsprung gap.
Red Giant Branch
You can see a dragon or fowl-like shape if you are imaginative enough;
let's call the large being a dragon. The trunk and abdomen host the
main-sequence stars, while there is a wing- or branch-like
structure dominated by red giant stars.
Its reason: stars less than 40 solar masses lose their photospheric
temperatures at some point in their lives, whether its blue loop, a
permanent red giant, or just a long-lasting stage. Another reason is
that low- and intermediate-mass stars are far more common than 30-40
solar mass stars.
That's why observers have plotted those red giant stars, and they are
arranged in a branch or wing, automatically as the stars get plotted in
their places. That's why the zone in the diagram is called the
red clump or red giant branch (RGB).
Asymptotic Giant Branch
There is another portion on the right side of this wing around the
solar luminosities 1 to 10 in the diagram. It is dominated by less
observed and rarer stars in our neighborhood. They are larger than
usual RGB stars, losing their plasma at enormous rates and speeds.
It feels as if they are dying or destroying their vicinities; they are
indeed dying and form planetary nebulae and leave the white dwarf.
They bear the final flames of fusion, which are extinguishing day by
day; gravity is actively killing them. We call this region the
Asymptotic Giant Branch, or AGB; hence, its members are
called AGB stars. The Sun will engulf the Earth in this phase,
not in the RGB, when it enters this phase.
Instability strip
You will see stars that leave the main sequence, are less stable, and
sometimes pulsate; this anomaly usually happens when the stars develop
the conditions to ignite helium and face the consequences due to their
lower system properties. Another similar instability occurs when Helium
has produced enough Carbon and oxygen, and Helium burns in the shell.
However, their pulsation can last some hours to years or thousands of
years. During this time, they lose mass more and more, which causes the
AGB phase to evolve further.
The instability strip can span from subgiant, RGB, and AGB stars to the
massive stars' counterpart phases like Wolf-Rayet (WR) or
luminous blue variables (LBV). In general, whether a star is
massive or intermediate, it must suffer mass loss by either gravity
(shell burning, RGB, AGB, dredge-up, or helium/carbon flash) or by
intense fusion (multiple fusion layers, enormous energy output, weak
gravity, such as WR and LBV).
While the low-mass stars or M dwarfs, about 0.45 solar masses,
last extremely long thanks to their efficient helium mixing abilities
and fuse the hydrogen until the last hydrogen atoms. Below we have shown
an animated diagram of multiple shell burning of hydrogen, helium, and
carbon.
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| Diagrammatic Animation of three element fusion with convective overshooting and star's shape |
You can see those layers aren't stable and are pushing or pulling each
other, and the star has to manage itself accordingly. Also, you see some
leaks or offsets; hydrogen sometimes goes into the inter-shell region;
likewise, helium is fusing in the inter-shell or carbon zone. We call it
Convective overshooting, which redistributes the fuel and its
products and doesn't let the evolution be too much expected as theories.
Life cycle of stars:
Here, we'll provide a brief overview and focus on key features. Each
of their phases can be documented in a separate article; if we create
one, we'll definitely drop a link in their sections here. Let's be
clear that the life of a star depends on the interplay of gravity,
energy transport, plasma currents, nearby environments, and a little
bit of metallicity. HRD only arranges the stars from the observed
region into classes, like we categorize the human population into
children, adults, the elderly, etc.
Here are some points to notice: Stars less than 0.45 Solar masses never
become giants, and stars less than 40 solar masses never become red,
yellow, or white; they live and die as blue supergiant stars and leave a
black hole.
When gravity crushes the non-fusing or partially fusing core, the
envelope expands and cools; if fusion is strong, the core expands, and
the envelope shrinks. We call it the mirror principle. Now, you can
expect the pulsating stars like Betelgeuse, VV Cephei, and
others that are struggling to balance the energy of fusion and
gravitational compression.
We'll focus on stars with Solar masses: 1, 5, 20, 30, and 40 because
they differ a lot from each other, while other similar-mass stars can
have similar phases. Also consider that the massive star is constantly
losing its material in the form of a stellar wind, which is the effect
of intense fusion, radiation pressure, and internal mechanisms.
Look at these colorful noodles in this image, it represents the life
paths of different stars based on their mass and point out some key
events like shell burning, AGB, or WR stages, while the noodle tells
us which color its envelope might be at that phase. We've also divided
the top portion into 3 parts, which denote their final destinies.
1 solar mass star:
These stars face the gravitational compression moderately enough that
the star doesn't have to shine immensely, nor at cool temperatures or
red colors. They start in the main sequence, which is their
longest-lasting phase. For the Sun, it's about 10 billion years. Since
the Sun's age is 4.58 billion years, you can calculate how much time
it has left to fuse hydrogen.
The secret of its long youth and childhood lies in the hydrogen
fusion method
called the Proton-Proton chain or PP chain, which takes about a
billion years and releases almost
26.7 MeV (million electron volts) of energy
per reaction. Our sun fuses around 4 billion kg of
hydrogen every second.
Don't be afraid, the sun won't disappear any second, because its total
mass is 2 × 10³⁰ kg or 2000 octillion kilograms. Even in a
period of 10 billion years, it would have fused a mass equivalent to
that of Jupiter.
Such stars usually grow in brightness when their core heats up due to
billions of years of fusion, which adds helium in the center. The star
now also grows in size. When the sun takes a step in the last stages of
10th billion years, it will explode or become a white dwarf, you could
expect. But no, no, no.
The sun would only leave its youth; there are more stages and events to
take place. Because you'll miss the sun's adulthood, the 50-60s and
70-80s, just like humans don't die right after their youth, they suffer
declining body and metabolism.
In the shell burning of hydrogen, it would be a
CNO cycle-powered fusion. The sun's envelope would expand in
response to the collapsing core. Note that Helium in the core is
degenerate and too low temperature to ignite helium as a fuel.
Degenerate matter is a superconductor of heat and electricity thanks
to its quantum configuration; the weak interactions between electrons
would cause neutrino losses, and it cools down despite the hydrogen
shell adding heat and mass.
The sun would find a little stop after 2 billion years of gradual
expansion until its photosphere falls to 5000K, and the sun would be
classified as a III star or a red giant star of an earlier phase.
Until then, it will be denoted by IV due to the gradual expansion.
After crying and screaming, the sun would eventually ignite the Helium
in the inert core, which has come into condition at about
100 million kelvin and 100,000 grams/cc, or gravity packs
100 kg of helium in just a centimeter cube. This would
take a long time after a series of cooling, burning, exploding, and
losing mass because red giants often lose mass like massive stars. It's
a complicated story of how that'll happen, but when it starts, the
massive eruptions and explosions would take place, and the sun would
often keep them hidden and rarely let them reach its outer surface.
When the sun would burn Helium, it will produce carbon and Oxygen. The
fusion of helium wouldn't be a one-off deal because it would require
constant heating of the core and lifting the degeneracy; often, it would
start in periodic episodes of helium flashes until the entire
core is hot enough. You can also expect the sun to keep expanding, but
not as it expanded in the subgiant phase. This will happen when the sun
would be its 13-14th billion years old.
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| Artist Impression: Planet and debris disk orbiting a white dwarf inside planetary nebula |
The intense energy of helium fusion would wreak havoc on the sun, and
the sun would become a pulsating red giant. The helium fusion would
undermine the solar material, stability, and composition. Hence, in
its later phases, when Helium would ignite in a shell along with a
hydrogen-burning shell, we call it an asymptotic giant branch or AGB
star.
To start Carbon fusion, a star must be more than 4 solar masses,
500M kelvin in temperature, and have a density of about
200,000 gm/cc. That's impossible for the Sun. So it will
keep losing the outer envelope and keep cooling; this time, the Sun
would totally engulf the Earth. The lost material in space would form
a planetary nebula, and the core would further collapse as
Carbon-Oxygen degenerate matter. Eventually, it'll become a white
dwarf. The planetary nebula would disperse in the local cosmos after a
few thousand years. The brief life cycle of sun like stars:
5 Solar mass star:
Almost all stars heavier than 1.5 solar masses fuse their
hydrogen with the CNO cycle, which means carbon, nitrogen, and oxygen
act as a catalyst and produce helium. Unlike the slow PP
chain that gives about 26 MeV per reaction after a billion years, this
process is significantly faster because the same MeV per reaction can
be extracted in just 10-15 minutes. Also, it works even
better with higher temperatures, so you can say it increases
the temperature by fusion and intensifies later on if the core wasn't
relatively hot earlier.
That sounds great, but here's the disadvantage: CNO-powered stars
don't even finish with hydrogen properly, as the low-mass stars do, yet
gravity reaches the critical point from where it starts to
compress the helium core. Also, the helium core forms much
earlier than PP chain-powered stars. Everything happens just like
low-mass stars, but in shorter time spans. Also, they don't
suffer as much as our Sun would to burn helium; they are totally
optimized for helium fusion, and they easily burn it.
Their photospheres also cool down more quickly compared to the Sun in a
subgiant, which was about 2 billion years. Because things tend to move
from high to low physical quantities like temperature, potential, and so
on. Considering the Sun has a photospheric temperature of about 5800 K,
it will take too long to cool down to 5000 K, whereas a star with a
temperature of 10,000 K loses temperature relatively faster. Because the
high-to-low gradients are steep.
The blue loop
Once the intermediate stars reach the Red giant stage, they are ready to
burn helium, and it doesn't show eruptions or explosions like low-mass
stars. The energy of helium fusion is sufficient to heat the
photosphere, and stars appear again yellow, white, and whitish blue, but
not like their main-sequence stage. This way, the star gets hotter
temperatures again, but for short periods; we call it the first blue
loop; it takes a few thousand years in this process.
The second blue loop
As the helium keeps adding Carbon and Oxygen, gravity becomes dominant again and starts to compress the C-O core; the star's envelope again cools down in response, in its very late phases. A similar event happens as we saw in the Sun's late stages, while it was trying to burn helium, but the intermediate stars struggle with carbon, and it gives them sufficient energy to glow again in hotter wavelengths, but carbon fusion is destabilizing just as helium is to low-mass stars.
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| A landscape with AGB star |
The star appears in orange, yellow, or even white, but for a really
short time, and carbon fusion blows away outer layers, and the stellar
envelope again cools and appears red. This is their AGB stage, and as
the sun-like stars, they lose the plasma of their outer layers and
create a planetary nebula. Since Carbon fusion produces the product mix
of Oxygen and Neon, the core becomes an O-Ne white dwarf.
The brief path of this kind of star:
10 Solar mass star:
When we talk of massive stars, we must bear this fact in our minds:
massive stars lose their plasma at higher rates all their
lives, and this intensifies as they evolve further, so a 10 solar mass
star could turn into a 6-8 solar mass star, making it difficult to
track the evolution path and other details.
These kinds of stars' lives depend on both their
initial mass and final mass, unlike low-mass stars,
which depend on their initial mass. So if an 11 solar mass star loses
mass and has a mass of 7 solar masses during its death, it would end
as a white dwarf instead of a neutron star.
Everything after reaching the main sequence happens the same as a 1- and
5-solar-mass star, but in even shorter timescales and mass losses. Such
stars already have strong CNO cycle-powered fusion, and it burns fuel
much faster, releasing enormous energies.
Some stars already start burning hydrogen in a shell when they haven't
left the main sequence properly and enter subgiant expansion (IV)
when helium fusion is about to ignite. The subgiant term or IV class is
misleading, and often such stars are classified as both; it's a very
brief period.
With later stages when the carbon is about to ignite, they go through
some energy crises, lose temperature, and become red giants by becoming
blue, white, yellow, orange, and finally red. In its final stages, if it
fails to retain a mass of over 8 solar masses, it would become a
mini-nova, leaving a white dwarf with an oxygen-neon composition.
If it maintains more than 10 solar masses, its outer layer would explode
as a supernova, and the core compresses into a neutron star. A supernova
from 10 solar masses is highly unlikely because of the mass loss
process, but metal-poor or population II stars can become supernovae
because lower metallicities cause less mass loss than population I or
metal-rich stars.
In brief:
20 Solar mass star:
Now we have stepped into the territories of bright giants that were
born massive, like Betelgeuse, VY Canis Majoris, and others. They
suffer severe mass loss, but their huge solar masses suppress the
suffering. Instead, massive stars suffer more from intense fusion than
strong gravity. Everything mentioned earlier happens at a much shorter
timespan; they leave the main sequence relatively quickly, in about a
few million years.
They ignite helium, then helium-shell fusion and carbon easily, and
almost three fuel types, and fusion makes the photosphere and outer
layers expand dramatically. Gravity is only there to bind the system
sufficiently, and it's silent in action, unlike what we saw in the case
of stars that form white dwarfs.
The star becomes too large, and we call it a Red Supergiant or
RSG. Since fusion's energy is immense but not ferocious enough to
distribute the heat energy to return the star to a blue color.
Therefore, no matter how they burn 6 fuels (Hydrogen, Helium, Carbon,
Neon, Oxygen, and Silicon), their relatively unstable outer layer mixes
the energy poorly, so they die as RSGs, not blue stars, despite starting
their lives as blue stars.
Also consider the gravity, which is weaker somehow and struggles to keep
the outer layers together; that's why such stars contract and expand
many times; we call them variable stars.
Supernova
When silicon fusion creates iron, then suddenly the game flips
entirely. Iron absorbs energy rather than radiating outward. It's
impossible to fuse. 6 fusion shells are still active and blowing the
outer envelope while gravity suddenly takes control over that iron
ball.
Silicon keeps adding more iron, hence more compression and gravity
become stronger, but only in the region where the iron has enough
mass. Such an iron ball would be 2 solar masses, which is beyond the
Chandrasekhar limit (1.44 solar masses). This traitorous iron
core causes the death of such stars. Core collapse, neutrino losses,
intense fusion, and other forces wreak havoc on stars; it takes about
7-8 hours before the explosion of a supernova.
So outer layers explode in immense energy, forming a beautiful supernova
remnant nebula (SNR). The core collapses into a neutron star.
This is only supported by quantum laws, not by thermodynamic rules.
Remember, gravity hasn't done yet; it keeps pressing the dead core;
hence the degenerate neutrons resist the compression, and the result is
a neutron star.
In brief:
30 solar mass star:
Their evolutionary stages are similar to those of 20 solar mass stars,
but they differ in that they go through a blue loop. Because their
mass is still higher than 20 solar masses but lower than 40 solar
masses, they borrow both kinds of stars' properties. After Carbon
ignition, their fusion is much stronger.
Don't forget that massive stars are powered by the CNO cycle of
hydrogen fusion, which is highly efficient at high mass and
temperatures. So the power of three fusions and already higher
temperatures around outer regions help the star to come back to a blue
color, and they become a blue supergiant (BSG).
After returning to blue, they ignite further fusions smoothly and suffer
intense mass losses, eventually removing the outer layers and most of
the radiative mid-layers. Thus, the star expels the energy more directly
than ever and appears blindingly blue white; we call it the
Wolf-Rayet (WR) phase.
About less than 25 solar masses don't enter this stage and higher than
this mass mostly produce a black hole after supernova, But here's the
key thing, they suffer intense mass losses, then the black hole,
supernova or WR phase are the talks of if the stars retains that minimum
required mass, otherwise, it'll behave like lower masses like 20 or 10
solar mass stars.
Life path:
40 solar mass star:
All the fusions initiate smoothly, without any halts or troubles. Gravity is also strong enough to compress the outer layers to match the ferocious fusion and its energies. Such stars never become red, and their lifespan is much shorter, so the outer layers don't get the time to respond to the internal situations.
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| Artist Impression: a Wolf-Rayet star ready explode into Supernova and leave a black hole |
Some stars of higher masses go into a brief stage called Luminous Blue Variable or LBV, where their brightness varies immensely, not because of any weak specs, but because they actively erode their outer layers to become a Wolf-Rayet star. They always produce black holes after the Wolf-Rayet phase. Also, remember they always represent the highest portions of the HRD, meaning they always shine like supergiants, regardless of their type of fusion.
Their brief path:
In the end
Here, now you know the basics of stellar evolutionary stages within mass ranges. We know that we've skipped many things, but we can go into details in other dedicated articles. This post was created mainly to let you know, when discussing WR, AGB, RGB, RSG, or Supernova, which stage of which star we are talking about. This post will also act as a table of contents to link together all the stages of stars after the main-sequence phase.