One of the Splendid sights of space, which form Stars, planets, and even us. Let's explore the different types of Nebulae, which are not mere scenes or gas clouds in the cosmos, but tell us completely different stories. In Today's post, we'll categorize them and find some missing clues. Thanks for searching this blog and reading it. Without wasting any time. Let's clear those distant fogs and find out what they might be hiding.
Introduction:
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| The Nebular Complex showing all 3 effects of Light |
Whenever you see those diffused, sparse, and beautiful things in space photos, you wondered and amazed by their appearance. Truly, they are one of the most splendid things in the cosmos, but equally dangerous.
Before we start, let's know what a nebula is. In ancient times, the vast dispersed structure in night skies excluding Earthly objects was called Nebula, it's a Latin word which means the cloud or fog, and its plural is Nebulae or Nebulas. We'll use these words interchangeably, so don't be confused.
Since these are one of the naked eye objects from old times, ancient cultures have been keeping records of them and gave various names. However, their true nature was revealed after scientific advancements.
The pollution in the Earth's sky has made them extremely fade and requires a sharp eyesight; otherwise, they were easily visible as a piece of faint cloud, not like you see in this post or space photos. Just like every type and class, nebulae are abundant in our cosmos and often need to be distinguished from one another. Not all nebulae are the same. Let's know more about them.
The interstellar medium (ISM) is the empty region between two Stars in space. (for example, outside the solar system and between the Alpha Centauri's boundary will be an ISM) We call this region as common space, which has extremely low density yet is rich in gas, dust, and other things. Before it was considered a vacuum, but it's not; yet it is much less dense than any man-made vacuum.
To understand this, take a bottle and try to remove the gases of the atmosphere, like oxygen, nitrogen, and carbon dioxide. You will need a specific pump, and you will create a vacuum where no gas molecule or atom exists inside your bottle or container, which is not possible, no matter how hard you try. There will still be hundreds to millions of gas particles, even if you seal it tightly. But in space, there are only 10 atoms in 1 cubic meter.
Besides density, there is no sign of a special star meaning in the entire neighborhood of the solar system, including outer regions; you will find the clues of the Sun and its solar winds. This is currently unknown where this exactly ends, but Voyagers have found in the heliopause where solar winds interact with the incoming radiation of other stars. This is where Sun's influence ends and outer influence or common space begins, but this is a billion km wide boundary, not a clear line.
So, the interstellar medium has different compositions and densities depending on the nearby Stars or other objects. Across the galaxy or local neighborhood, these configurations often change over time and form Nebulae sometimes. However, not all nebulae are the same; again, they may have various origins, structures, and natures. We'll talk about them throughout this post.
Just as you see the sand, bricks, cement, concrete, beams, metal planks, and rods collected in a place, you can easily imagine that a building is going to be constructed or is being made. While if you look at these objects in a shattered or destroyed way, you know there is a reconstruction or demolition taking place. So, in space, these bricks, concrete, etc., are like nebulae, and their future products, like Stars, planets, or moons, are buildings.
However, these collected construction ingredients can form different types of products or have their appearances from various events. We'll keep an eye on this, too. This is like a house may form from soil or sand, but if you break it down, it will become soil again, so Stars form in clouds, and after their death, they again dissolve in clouds.
Basic Features:
Almost every nebula has common visual features; some of them emit light due to the Ionization of gas particles. Some nebulae absorb the light of stars and appear dark because of either Ionization or dense dust accumulation. While some nebulae reflect the light and look like sparse fog in space. Sometimes, a single nebula can show various light effects depending on the gas's nature and external factors. Below, in the image, we've shown the example of those 3 kinds of light effects.
If you ever reach those regions, you'll find nothing. Like a fog or cloud,
they only appear from vast distances. If we discuss nebulae and forget the
ISM, then we miss the most important part, and our knowledge will be
incomplete because all nebulae are parts of the Interstellar medium
with different densities and mixtures of gases. Before we start, let's see
how they are classified according to the current system.
- 1) Emission Nebula: which emits its own light through various reactions
- 2) Reflection Nebula: Reflects the light of other nearby Stars.
- 3) Dark Nebula: Dense gas and dust reservoirs that block the light and appear brown to dark.
- 4) Planetary Nebula: Formed after the ejected materials of Stars, Neutron Stars, White dwarfs, or any other objects.
You might have noticed that Nebulae's color, behavior, and other properties
can change due to various reasons. If the one nebula reflects, emits, and
blocks the light in many of its regions, then? That's why we'll understand
it somehow better, yet an intuitive system. We've made a chart after
researching them, and we have assigned 3 groups.
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| The Roadmap of This Post |
In the chart, there are 3 major groups named Generative,
Concurrent, and Remnant. Here's their brief description;
further, we'll know in detail about them.
i. Generative Nebulae: They are capable of creating stars and
their planetary system, which has many other sub-types.
ii. Concurrent Nebulae: This is the largest major group in both
type and size. Meaning, they can be larger than a whole galaxy or may have
several sections, origins, and functions. Their main feature is that they
coexist with other ongoing processes or structures. If you remove their
cause, then they'll no longer form and disappear or dissolve in a few
thousand years.
iii. Remnant Nebula: its self-explanatory. These types of nebulae
form after an event or object ceases to exist in one form. Like, Proplyd
(Ionized protoplanetary disk) forms in later phases of the protostar phase
and disappears when the planets form and reach their common size.
Let's see them in depth.
A. Generative Nebula
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| Zone of Intense Nebular Activities : The Green Represents the M.Cloud, Bright White pink and Purple Regions denote H II Regions and Rest shows Stellar Nurseries |
Please keep in mind that every nebula is a part of the Interstellar
medium and they vary in colors, densities, and shapes, along with other
properties, which are heavily tied to their vicinity.
Moreover, in a galaxy, its common ISM fog and dust composition
will be more or less similar and will vary from galaxy to
galaxy. That's why Clusters, Galactic arms, and other large complexes of
nebulae create a similar kind of stars. Like in our local neighborhood
of Sun, there is a large number of
M-Type Stars, then K
are slightly fewer, and this number declines as the Star groups go
massive.
Therefore, we live in less massive Star zones because it was our ISM's
properties, and it created the stars based on it. So, we're talking
about a region of Interstellar medium cloud or fog that is
capable of creating stars.
As we know, these nebulae can create something, but not all the
Generative types won't start to make stars. They need appropriate
conditions and triggers. In every nebula, their particles are floating
and colliding with each other, which means instead of sticking together,
they move away after collisions with fellow particles and transfer their
energy.
What if their behavior can change by some trigger, and they start to
stick together? If Nebulae can create stars, then why shouldn't they
bear more mass than stars? Yes, they have indeed more mass; the lone
Orion Nebula (M42) can have 2000 Solar masses, far more
than the most massive star. Since they are the creators of stars, they
surely need masses in tremendous amounts. Did we forget gravity? No,
sometimes, if such a region violates the limit called Jeans mass, based
on its Local composition and other factors, it may collapse under its
gravity. Let's see their sub-types so that we can differentiate which
regions will create stars and which are creating them, and which have
created them.
Aa- Molecular Cloud
When we say molecular cloud in Astronomical contexts, we usually mean
the clouds of Hydrogen. Note that atoms form molecules, so this cloud
will be full of Molecular hydrogen, which consists of
2 Hydrogen atoms, which is different than ISM, which consists
mainly of Ionized gas. They have an average temperature of around
10 to 30 K. Molecular clouds are the specific regions of ISM
with densities from 100-300 to 10000 particles per Cubic
centimeter, sometimes.
They are sometimes called stellar nurseries, but they have a slight
difference in terms of creating stars. Molecular clouds are the
inactive regions of Hydrogen molecules which is not currently
creating stars; in the future or past, they did.
These clouds are ready to create Stars; they only need triggers like
(a) Supernova Shockwave, which pushes/tears/compresses or overall
changes the morphology of the cloud. (b) Ionizing Radiations (UV, X-ray, or Gamma Rays) from nearby stars will change their behavior by
electron exchange and other processes. (c) Supermassive Black
Holes' Jets, which change these clouds' chemistry.
Once these cloud particles start to come closer and stick together, they
will no longer be inactive molecules and become a Stellar nursery. The
examples of this type are Cloud complexes of Taurus,
Ophiuchus, and Orion, etc.
Ab- Stellar Nursery
As the name suggests, these types of molecular clouds are actively
creating Stars, but it's not the topic of this post. Instead, we'll
focus on Nebula. If you could see at the atomic level, you would see the
massive particle movement in stellar nurseries' clouds as compared to
Molecular clouds.
Their temperature are slightly higher than the previous type, about
10-50 K with densities from 10 thousand to 10 million particles
per cubic cm. These are again dependent on their local characteristics
and vary over time, as the star formation processes further. As
molecular clouds, they are also dominated by H₂, the molecular hydrogen.
Some notable Stellar Nurseries are: Eagle Nebula (Pillars of
Creation), Lagoon, and Carina Nebula.
In our blog, a special post will come soon, which will describe the
detailed process of Star formation with a comparative version where
the less massive, intermediate, and massive stars evolution and condition
will be shown side by side as they progress. So, stay tuned with us.
Ac- H II Regions
This is another type of hydrogen which consists of 1 proton and 0
electrons, while normal Hydrogen or H I has an electron and a
proton. We call this different configuration to H II.
in some places of the Interstellar medium, this particular hydrogen
dominates. This region is so hot, about 7,000 to 10,000 K,
and average densities of 10 to 10 thousand particles in a cubic cm. This
is because the ionizing radiation of nearby young and massive stars,
which emit tremendous UV radiation. This is a very active region, where
many stars are still forming or have formed. This is usually found
around Newborn O and B stars; their radiation causes cavities and holes
in some parts of the nebula.
Let's see why the radiation is so important and what the ionization is
that we're uttering their names like divine beings. You may know that a
normal atom has 1 proton, electron, and neutron. Electrons orbit the
nucleus (proton+neutron). If Certain Radiation with a certain
wavelength and energy is absorbed by electrons, they get
excited and leave their orbit, they can move freely or join another
matching atom and its electron group. All these processes require a
specific amount of energy, which may not be available in every type of
radiation.
For example, a normal Hydrogen Atom has one electron and 1 proton, we say
this is H I type hydrogen. At least a photon (particle of light) with
energy about 13.6 eV (Electron Volts) would be needed to remove
this electron. The electron will absorb and get enough energy to escape
from the proton's pull and freely float in the Interstellar medium.
Let's say it comes across with a 5 eV photon, then it can absorb
the photon and get excited, but it will not leave the proton; it
will keep orbiting. If it absorbs a photon with 15 eV, an electron
will leave, and not only will it escape but it will also gain a higher
speed than a 13.6 eV photon absorption. If it absorbs the
30 eV photon, the electron will move at rocket speeds and carry
more energy. This electron liberation process from an atom is called
Ionization. There is a lot more of it, but at least you know this
line.
The nebulae NGC 604 from the
Triangulum Galaxy, Orion, and Rosette Nebulae are Examples of H II
Regions.
Since O Stars' photons have much higher energies, they cause rapid movements in
electrons, and their movement will heat the most regions of the cloud
because Heat is the result of the Movement of particles. This can change
the shape of the region, can reform or create cavities in the nebula. This
will have two most probable outcomes, either further Star formation can
stop or increase the atom combination rate than ever, meaning more stars
will be created.
B- Concurrent Nebulae
We have assigned Nebulae that form as a byproduct or result of an
ongoing process. They might have more than one origin, and their size,
density, and chemical composition vastly vary. Let's some of their
types.
Ba- Bow Shock Nebula
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| Bow Shock Formed by the Interactions of Nearby Stars and ISM |
Almost every star emits many types of particles, which contain protons,
electrons, neutrinos, or dust particles. We call it stellar wind, much
like Earth's wind, which also contains the flow of atmospheric gas, but
in stars' case, it may contain a wide variety of gas and dust particles.
It differs from a planet's wind since its gas particles flow around the
planet and go where the pressure is low or high.
If you push the water surface, you'll see that the water immediately
fills in this gap and flows in the other directions. It also depends on
temperature, Planetary tilt, and rotation mainly, as well as external
gravitational influences like nearby planets or moons sometimes.
Whereas, Star's wind and atmosphere are totally different things,
meaning, Atmospheric gas mostly surrounds the star's outer region, while
fusion products like radiation push these atmospheric particles into
outer space, this also contains a little amount of plasma and tremendous
neutrino outflow. We call this outflow Stellar wind or Solar wind if we
talk about the sun. In physics, gases and liquids share various
similarities, so we treat them similarly.
Imagine the river's water flows in a direction if there are other nearby
water reservoirs like a pond, a puddle, or a river, their mixing water
will generate a kind of turbulence like two rivers' water are colliding to
each other.
A similar thing happens in space between Stellar winds and the
Interstellar medium (ISM), since water and water collisions don't make a
change that much, but Stellar winds and ISM have different compositions,
they make a change a lot, on a much higher scale. This may affect many
light-years.
Stellar wind strength on the speed of ejected material and how massive a
star is, because Massive stars show powerful winds due to their
aggressive fuel-burning mechanisms, and mostly they expel their outer
layers into the cosmos. This is mostly done by powerful radiation
pressures, which act like an air blower on those surrounding plasma
particles.
The massive the star, means stronger the outflow. Our Sun-like star
ejects these particles at a speed of 200-700 km/s, while a
powerful Star will do it more than 2000 Km/s. Sometimes, such
massive stars can lose 50% of their outer layers and become
Wolf-Rayet stars or fail to create a supernova if they lose more.
Now the question is, where does all that ejected material go? We need to
keep in mind that in space, there is no friction, meaning if you throw a
ball in space, it can keep moving for hundreds of years unless it crashes
into an asteroid or is pulled by a gravitational field. Because Issac
Newton said once, if anything is at rest, it will keep resting, and if
anything is moving, it will keep moving unless external forces try to stop
or alter its state. So theoretically, thrown objects can keep moving in
space as long as there is no resistance.
Now, coming on Stellar winds, they can also move in space, but there's
something named ISM and its gas, which tries to stop them by colliding,
absorbing, ionizing, or annihilating each other. Both have sources, so
they will form a large zone where it happens, mostly looking like an arc
or pointed, diffused tail-like structures in space.
Like a boat moves in water, we call it Bow Shock, Since Stellar wind
particles feel a kind of shock due to ISM particles, and their speeds
greatly decrease. That's how Bow shock nebulae form. Stars like
Beta Crucis, Zeta Ophiuchi, and many other stars are known
for creating bow shock nebulae.
Bb- Wolf-Rayet and Stellar Ejections:
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| This Massive star has taken vow to Lose its outer layers and create Strong winds to Become a Wolf-rayet Star |
We've talked earlier about Stellar winds, but here we're actually
referring to previous stages of a star's death. Whether it's massive or
weak, both kind of stars do lose their significant part of their
material before they either form a Planetary nebula or
Supernova Remnant (SNR).
In this phase, the Sun, like Stars, feels dramatic events like
Dredge-ups and Fusion-related aftermaths, which cause
massive damage to the star and stars lose material and form, sometimes a
nebula. The same goes for the Wolf-Rayet star, which is a grown-up
version of Massive Stars with more than 16-25 Solar masses.
This kind of nebula mostly forms from a Massive Star's ejections. Note
that this is slightly different from Stellar winds, which consist of a
small amount of plasma, whereas Ejection means the star's material itself
is thrown into space. Like we have the Sun that shoots a massive amount of
plasma in space sometimes, it will do it more frequently in its later
stages, especially when the Sun will have the Helium Burning phase and
Hydrogen shell burning.
The energy of Both Fusion zones will try to tear the sun apart or any star
that possesses it. but it will immediately be managed by gravity. This
won't let the star explode, but it won't let it shine quietly either. It
will make the Star lose its very parts. This ejected material will form a
cloud-like or at least faint smoky structure.
While Massive Stars keep losing their materials since the first second
they formed, their loss rate increases as they progress further. That's
why Stars with 8-10 Solar masses are the bridges between Intermediate and
Massive Stars, because if they lose enough material let's a star was 10
Solar masses loses 3 solar masses over a long time and remains total mass
of around 7 Solar masses, it won't create a supernova because a Supernova
is always a signature of the death of a massive star. Such a star will
either create a mini supernova, or we call it a Nova. If it's 1000 times
more powerful than the Nova, it will be called a Kilonova.
Not all 8-10 Solar masses Stars lose mass like this sometimes if they have
low metallicity, meaning less amounts of Iron. Oxygen,
Carbon, Magnesium, or any other heavier element than
Hydrogen and helium, it won't lose the mass that much and can create a
Regular supernova and the star's core will create a
Neutron Star, not a black hole. Those elements cause to loss the
mass at slightly lower rates.
Just for comparison, let's say we have two stars with
10 Solar masses, one is Metal-rich (Population I) and
another is Metal-poor (Population II).
The Pop I star will every second lose around 6.3 x 10²¹ Kg at a
speed of 1000-3000 Km/s, while the Pop II Star will lose
6.3 x 10²⁰ Kg at 500-1500 Km/s.
This was the data of their Main sequence phase, where they were young and
mostly stable.
When they are about to die, the Pop I Star will lose
6.3 x 10²² Kg at a speed of 2000-5000 Km/s, while the Pop II star
loses 6.3 x 10²¹ Kg at 1000-3000 Km/s every second. Well,
these are the theoretical calculations. In reality, this will show some
variations.
So, More Massive Stars, about 16+ Solar masses, will 90% blow off
their outer layers and show their Radiative layers, and we call them
Wolf-Rayet Stars, which form some new nebulae and shape the pre-existing
Nebulae through their winds and Ejections. They also have ionizing
radiation emission; hence you can imagine they really shape the local
interstellar space.
Bc- Dark Nodules
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| A Stellar Nursery Showing some Dark Knots called Bok Globules |
Many of you may have heard that Stars form in dense regions of Stellar
nurseries, but exactly where? The answer is hidden inside these Dark
Node-like structures. To form a dense region, gravity must bring all
particles very close together so that they can stick together after
their ionization triggers the formation. However, first, the region
becomes denser every year, so it must be opaque to light and block it;
they are called Bok globules.
Remember, these are not dark nebulae; instead, they appear as dark
points or tiny irregular dots. Their size is about
10,000-20,000 AUs and consists of 10-50 Solar masses. They
are the initial stages of Star formation, where it has only accumulated
to a specific region, and after many millions of years, Stars, planets,
and moons will form if their luck is kind enough.
There is another similar structure called Cometary globules, which
are slight variations of Bok Globules, but they are found among massive
stars like O and
B. In these structures, they have a small head and long tail-like
formations, which are shaped by the winds and radiation of those stars.
The LDN 1228, Bernard 68, and CB 26 are examples of Bok globules, while
the CG 30-31 Complex, Knots in helix Nebula, The CG 4 (The hand of god)
are the famous examples of Cometary globules.
Note: In the chart, we have connected a curved line from the Stellar
nurseries to these two entities because they exist in the star's
formation period and disappear when the star is about to be born
Bd- Herbig-Haro objects
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| A Protostar's Jet Interacting with Local Stellar Nursery and Forming an Arc like Structure called Herbig-Haro Object |
They formed with the same mechanisms as the Bow shock nebulae, but when
the Jets of a protostar interact with a molecular cloud or ISM, they
create an arc-like structure, which is called
Herbig-Haro or HH objects. HH objects. Like hatching a chick from
its egg. jets cut clouds and clear the way so that a protostar can
emerge from the stellar nursery.
The Arc is the main reaction point where the gas of the stellar nursery
ionizes, and it gets turbulent, further destroying the cloud and that's
how a protostar can tear its womb in the stellar nursery and freely
float in ISM. Some well-known Herbig-Haro objects are: HH 34, HH 2, HH 46
and HH 47, etc.
Be- Interstellar Medium Nebula
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| The Interstellar medium as a form of Dark grey Diffused Structure Between Stars |
Sometimes, they are also called the Integrated Flux Nebula. These
are not a kind of nebula but host millions of nebulae, and all the
nebulae you see in a galaxy are parts of it. However, Remnants are not
might directly linked since they form after a star's death. ISM is not a
complete vacuum; it has a definite composition that slightly shows
variations due to those factors we've talked about earlier. Now, let's
focus on ISM.
In a low-pollution zone in the Earth, you might have seen this dusty,
whitish, and faint structure between stars. This vast structure is spread
across the galaxy; it's the prime entity that turns into molecular clouds,
stellar nurseries, or other forms, and then stars form in it. Black holes
eat this material.
Whether a star is born, evolves, or dies, every process affects this
medium. Sometimes, an Interstellar medium can be larger than its galaxies
if they are part of local clusters. The main composition of ISM defines
which kind of stars to create; they form nebulae, especially the molecular
clouds.
If you use robust and good ingredients, then you can make good items, so
here the same principle applies.
For example, our Milky Way has the majority of M-type Stars, the Andromeda
galaxy is dominated by G and
K
stars, and the Triangulum galaxy has mostly young and Hot stars. The M82
galaxy has the majority of powerful young O and B Stars with an intense
rate of star formation due to its starburst nature.
The M87 Galaxy is an elliptical galaxy that has low-mass, old stars. Most
of them are red giants and a minority are the Wolf-Rayet stars. The M101
is also called the Pinwheel galaxy, its spiral arms contain young and Hot
stars while its central part is dominated by older stars.
All these stars denote the properties of ISM. Everything we're talking
about, except for stars, galaxies, or anything other than fog or clouds,
is part of the Interstellar medium.
C- Remnant Nebulae
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| The Supernova Remnant Nebula |
As its name suggests, these types of nebulae form when Stars cease to
exist, whether they are intermediate or m stars. In this category, the
supernova Remnant is discussed as Staple food, but we often neglect the
other kinds of remnants. We'll see the supernova Nebula in brief because
it would take another post to fully explain; instead, we will see its
basic details and some examples. We'd focus mainly on the other types.
Ca- Nova and Supernova Remnant
As we saw earlier, the Stars between 5-8 Solar masses' remnants are
called novae unless they explode similarly. because these stars are too
weak to trigger a regular Supernova. These stars suffer enormous mass
loss and explosions from flashes, dredge-ups, and pulses.
such events create the remnant cloud, which is similar to a supernova.
However, if such a star explodes but doesn't generate energy as a
regular supernova, we call it a Nova. If it produces 1000 times more
energy, it will be called a kilonova, and a hypernova is much powerful,
but a Supernova is the most powerful event.
Almost all these remnant clouds are spherical in shape, as if something
has exploded. However, such clouds are not immune to external factors.
There are two types of Supernovae, called Type I and
Type II. They are different on the basis that Type I lacks Hydrogen
or may have an extremely low abundance. It is several times brighter than
type II. It occurs in a Binary system where either member is a White
dwarf. This white dwarf sucks the material of its companion star. When it
accumulates more mass than the Chandrasekhar limit (almost 1.44 Solar
masses) the both Stars explode like giant bombs and create a Type I
supernova. However, there are many variations of Type I, which are called
Ia, Ib or Ic. We'll talk about them in another post in detail.
Type II is the remnant of a single Star whose core collapsed due to
gravity in its later stages, and the star explodes with enormous energy.
The GK Persei, DQ Herculis and T Pyxidis Shell are some Novae, whereas the
Crab Nebula, Cassiopeia A, and Vela Supernova Remnant are SNR (Supernova
Remnant)
Cb- Planetary Nebula
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| AGB Star is losing its outer layers and Forming a Planetary Nebula |
This is the remnant of slightly low-mass stars below
8 Solar masses. This kind of cloud forms in a Star's AGB phase,
where it suffers extreme mass loss like Massive stars around 10-15 Solar
masses. If any planet is orbiting such a star, this outflowing material
will strip off the planet's atmosphere or sometimes vaporize it. When
all these materials are stopped by interstellar medium particles, it
creates a nebula. This is also the Future of our Sun.
When our Sun enters into AGB phase, it will engulf the 3 Inner planets,
including Earth. If nearby Asteroids, planets don't change their orbits,
they will be vaporized by outflowing gas from the sun if they are not even
swallowed by the sun. Its gas and dust will create a planetary Nebula, and
the core of the sun will transform into a white dwarf due to gravitational
compression. Some famous Planetary Nebulae are Cat's Eye, Helix, and Ring
Nebula.
Here, our Nebula classification post ends. We hope you enjoyed reading
it.
Thanks for reaching and reading here.
Have a nice day and night.