Hottest Stars in the current stellar generation, Massive Blue hot aged
stars, have lost their outer layers and shine tremendously. Yes, we are
talking about Wolf-Rayet Stars which are fascinating discoveries by
themselves. Hello and welcome, I'm your friend K* and you are reading a post
from CosmicWisdom Blog, where we discuss Astronomical objects and structures
with little to no use of maths and in simple language of course. Today in
this post, we will learn about Wolf-Rayet Stars, what they are. how they
form and much more.
Introduction:
We always start our posts with a brief intro so let's get some knowledge
about them. In 1867, Two Astronomers called Charles Wolf and Georges Rayet
found unusual Spectra of 3 Stars in the Cygnus Constellation. Their name
were
HD 191765 (now called WR 134), HD 192103 (WR 135) and HD 192641 (WR
137). So, the normal Star's spectrum consists only of Absorption lines while
these stars showed Strong emission lines of Ionized Helium, Nitrogen, carbon
and Rarely oxygen. If you don't know what we are talking about just hold on,
we'll tend to it later. These kinds of strange spectra were a mystery at
that time. Later on, it was revealed, these unusual broad emission lines
were caused Doppler effect. Which occurs when the light source has a
powerful, turbulent, high-speed speed dense stellar wind and the star's
outer layer is being peeled by such winds. The result is a Wolf-Rayet
Star.
Spectral Features:
Since WR Stars are known by their spectrum, we all know Spectrum forms when
light is split into all its structural wavelengths, and each wavelength
represents a color. Sometimes, If any chemical element absorbs a photon
(particles of light), it simply does not let it move further, so we see
those dark bars. Since each photon has an appropriate wavelength according
to its frequency and other factors, something is now blocking it; therefore
we will not see anything Instead, we see a dark bar where this photon's
wavelength is missing. keep in mind that Most elements only absorb certain
wavelengths of photons, while some take more than one it depending on the
various factors of both the photon and element. These lines are called
Fraunhofer lines. Let's see them deeply so that we can understand the WR
Stars
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The Generated Spectrum of Sun like Star
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In the above image, you see a fake Spectrum of a Sun-like star.
Since a Real spectroscope reveals real-time data, while it only resembles
the actual one. You see the rainbow due to all kinds of radiations aka light
is emitting from the photosphere. near the outer regions of the Star, there
are so many elements in ionized form that grab the light, scientists analyze
this light and they see these black lines. each represents a certain
chemical that is disturbing the light.
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The Generated Spectrum of Wolf Rayet Star
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Now in this image, you see the generated spectrum of a Wolf-Rayet Star. You
see the dark rainbow due to Dense fog or smoke is blocking
some amount of light, hence it is dark. However, the striking feature
is those white lines instead of Black ones. When Light is released
from the WR star, it usually gets trapped in the surrounding dense cloud of
ejected material from the star while some parts of the cloud where Ionized
Helium, Nitrogen, carbon or sometimes oxygen smoke/fog glows due to the
intense environment. it behaves like a little light source along with the
star.
When these kinds of stars are analyzed by a spectrograph, they show white
lines because light is being released instead of captured. However, light
was captured when it was moving through clouds but now, due to ionization,
it is re-emitting, which is why there are white lines. In actual
spectrum, you may see some wide lines that are the result of high-speed
moving material and light is traveling through it. This is called
Doppler Effect. To understand it, if you use your flashlight in clean
water and shake the water a little bit instead of firm force, you will see
that the light of the flashlight would bend or be disturbed. The same
principle also applies here. Now you know how WR stars are different from
normal stars.
Mass, Temperature and Luminosity:
These stars are evolved versions of Massive O-type stars that eject
their material at high speed but why? To address this question we'll have a
separate post where we discuss why Mass is so important in Star's life.
Look, If you have read our post about Stellar classification or you have
some knowledge about it you know that O-type is a diverse group of stars.
They have shorter lifespans, are extremely hot, and glow in blue and white
colors. Their Masses range between 16 and 90 Solar masses. In this range,
various kinds of Massive Stars Can come. For 20 Solar masses, they do lose
their mass by Powerful Stellar winds, while The Star with 60 Solar masses
will have even Powerful than a 20 Solar mass Star and it can simply remove
outer layers of stars, shining intensely due to rapid escape of photons from
these regions than the photosphere.
This kind of star with Revealed inner regions is called Wolf Wolf-Rayet
star. They are the result of Extensive mass loss and they expose their deep
Regions. which makes them unique among Massive Stars and Their Strong
stellar wind creates an ionized environment containing Nitrogen, Carbon or
Oxygen, which comes out from the CNO Cycle in the fusion Process. which
further can be ionized when it flows around the star by its wind. These
Ionized elements are responsible for unusual Emission lines.
Their Temperature is also Higher than 30,000 K and can reach around 210,000
K, which is higher than any other True Star groups. Perhaps this is because
It doesn't contain any heat blocking or Convection zone like layers that mix
the heat instead of directly emitting into space. Stars have a convection
zone and a photosphere.
In normal O stars, As soon as the Energy reaches to the convection zone it
is absorbed by Radiative Convection zone and Re-emit this energy in the form
of heat and Radiation (light), Then it can again captured by other group of
molecules then Re-emit, this process continues until it goes outer portions
of convection zone. When it arrives photosphere after many thousands of
years, it just escapes in the form of heat and Radiation at 300,000 Km/s,
which can further get blocked by certain elements or molecular clouds in
space.
Since Photosphere is entirely peeled from the star and the Convection zone
is also torn in several places of the star, so Fusion products can release
directly into space. Since there are no blocking layers like the Convection
and Photosphere. WR Star's Luminosity can reach beyond 100,000 Solar
Luminosities which is also a feature of Massive O Stars, they are only
different in the fact that they have light-blocking layers. Imagine if you
wrap a Blanket around a light bulb while WR has little to no such layers. If
a Massive O-type star has higher luminosities, there would be a totally
different mechanism responsible.
Such a star's outer layer explodes into a Supernova Remnant Nebula after
the Great Explosion. The core gets compressed by powerful gravity and
becomes a Black hole. In this event, a Powerful gamma-ray burst also takes
place.
Classification:
Now we know O Stars with more than 20 Solar masses can lose their outer
envelopes and become Wolf-Rayet stars. However, Not all WR Stars would be
the same, especially if we are talking about 100 Ms (mass of the sun) stars
or 50 Ms stars. They wouldn't show the same features either. Therefore,
let's learn about them one by one.
Position in MK System:
Basically, the MK system didn't mention the WR Stars and Brown dwarfs, For
ease of Recognition and study, scientists had to add them in their
appropriate places. Just like the M Class also consists of some Brown
dwarfs, particularly young dwarfs. On the higher specs' end, we have O type,
which consists of 16 - 90 Ms Stars.
Among them, Not all are same, Some are cooling down meaning changing their
color to red side of the spectrum, Some are coming back to their original
colors, Some are newborn Stars, Many are losing their masses due to strong
stellar winds, Stars with 15 Ms will not lose that much to become a WR,
while 25 Ms Star can become a WR star. Their winds would be flowing at a
speed of 1000-3000 Km/s, which would be enough to erode their outer layers
and their wind's composition will further intensify the mass loss process.
This Mass loss phenomenon is almost negligible in Sun-like stars.
You can imagine two people are working out in the gym, one guy just works
out for 1 hour while the second guy comes and goes by running, follows a
strict diet plan, performs intense exercise to create attractive muscles but
he just becomes skinny and weak by losing most of his energy. This is the
same case.
Classification method:
Now let's see how they are classified into Different Groups. One more
thing, this is the evolutionary stage of Massive Blue Stars when they
further evolve and lose their mass. They also eject some of their Fusion
products directly into the local surroundings. These can be Nitrogen or
Carbon but rarely Oxygen. Scientists have made dedicated classes on the
abundance of these elements, which can be confirmed through their Spectral
studies.
Earlier, Wolf-Rayet stars were classified as W stars, But Modern
discoveries have divided them into 3 major classes named WN, WC and WO. Each
class is based on the Spectral Domination of Nitrogen, Carbon and Oxygen. In
Addition, there is sub subclass after the major group which can be E (early)
For Hot stars or L(late) for Cool Stars. Before going on those major groups,
let's know something more about them.
WN or Nitrogen Rich WR Stars:
These are the youngest Wolf-Rayet stars, Rich in Helium and Nitrogen.
Their Temperature ranges between 25,000-100,000 Kelvin. This class is
divided from WN2 to WN11, in which, 2.5 and 4.5 were discontinued. Keep in
mind, unlike the MK system, which uses numbers to indicate Temperatures,
here for WR stars, it indicates fusion product Abundance in its
environment.
WNE or Early Nitrogen Wolf-Rayets:
These WN Stars are older than Late version stars. The Temperature Stays
between 50,000-100,000 K and have little to no Hydrogen left due to mass
loss. These stars are linked with long-period gamma-ray bursts.
WNL or Late Nitrogen Wolf-Rayets:
These stars are losing their outer layers of hydrogen, meaning they are
leaving the O common Stage. Keep in mind, here we are not talking about
giant or subgiant phases. Instead, we talk about mass loss and its
consequences because Massive stars lose more mass than regular stars,
which can alter their life path. That's why it's so important. Their
temperature is similar to Early O common stars around 30,000-50,000
K.
Here you can check in the chart below.
WC or Carbon Rich WR Stars:
As the name suggests, these stars show dominance of Carbon and Helium
emission lines in spectra. This group has been divided into WC4 to WC11
subgroups, each containing variations in Carbon and Helium emission
lines. All WRs with temperatures around 30,000-120,000 K come in this
class. Let's see their Early and Late Versions.
WCE or Early Carbon Wolf-Rayets:
These stars are usually Hotter than WNL Stars at a temperature between
70,000 - 120,000 K, They have very little Nitrogen, meaning they are
more evolved than WN Stars. These stars are often seen in Binary systems
and Transfer mass which means they steal or suck the material of partner
star while sometimes destroying the Companion star through their
powerful winds.
WCL or Late Carbon Wolf-Rayets:
They are cooler versions of WCEs with a Temperature around 30,000-70,000
K. They may have some Oxygen along with Carbon lines. These Stars are
closer to exploding as a Supernova.
WO or Oxygen Rich WR Stars:
Their spectra is dominated by Helium and Oxygen emission lines, They are
divided from WO1 to WO4. The whole group's identical temperature stays
between 80,000 to 210,000 Kelvin, which is the hottest among the current
known Stars. The Early WO Stars have extreme surface temperatures
between 120,000-210,000 K. These stars are progressing to a process
called Core collapse, which will turn their core into a Black hole. Late
WOs have lower temperatures around 80,000-120,000 K and these stars are
often found in Population II or low-metallicity Environments.
The Stellar winds and their significance:
Stellar winds are made of sub-atomic particles like electrons, protons
and neutrons in low mass stars, while it could be a Stream of Ionized
Nitrogen, Oxygen or Carbon and other fusion products in the case of
Massive stars. Stars up to 15 Solar masses have very little effect on
their evolutionary path, while More than 25 Solar masses can eventually
lose their outer zones and become Wolf-Rayet stars. For example, in
1859, the Earth faced a powerful tide of Solar winds (Similar to stellar
wind but originating from the Sun), which was basically emanated from a
powerful solar flare, an ejection of Solar Radiation, and as well as
Solar winds. Now you should also read its effects on Earth. The Aurorae
on Earth came around extremely low latitudes. For example, it could be
seen from Mexico, Colombia, India, China, Saudi Arabia, and sometimes
near the Equator of the Earth too. Radio Coms destroyed, Power lines
threw massive sparks finally breaking down, Some Radio and telegraph
workers electrocuted, Telegraph stations caught in fires, Global
communication at that time was just hit so hard by such a stream of
solar winds.
Now, you may or mayn't know that our Sun's winds are so weak compared to
O-type Stars or WR stars. The sun ejects its particles at a speed of 200
to 700 Km/s, while massive stars do it with speeds of 2000 Km/s. Moreover,
Sun-like stars can't be seen creating fog due to ejection in lesser
amounts than Massive Stars. These massive stars just spread heavy
pollution on a Cosmic Scale that they can sometimes hide within
completely. We can also discuss what happens when Stellar winds interact
with other physical entities but due to the topic of this post will not be
us to do so.
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Wolf Rayet Star creating Large Cloud through its wind
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These events are very dangerous for humans, but they are extremely
beneficial for Cosmic chemical enrichment because, through these powerful
winds the fusion products and other chemicals spread in space. Later, by
Supernova, it disperses over several light years, which is used to make
new stars and planets. Massive Stars and Neutron Stars create Various
elements that ensure the Chemical Richness of our Universe.
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Supernova Remnant Nebula
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That's why they can also create life forms. For example, what is our
universal food on our earth? it is the Carbohydrate. This chemical
compound is made of Carbon, Hydrogen and oxygen, it creates our Cell
boundaries, some organs' outer layers and much more. If there were no
Supernova, these elements could be trapped inside the massive star and
crushed into black holes and never show their miracles by creating those
wonderful life forms on earth or any planet where life can exist.
So That's all for today's post, We hope you enjoy reading it. Thanks again
for coming here. Have a nice day!
