T
his post will give you general ideas about Galaxies, Stars, Planets, Moons, Nebulae and other Celestial objects. In addition, we'll clarify or refine some of the common misconceptions related to these objects and structures. Let's begin, hello and welcome to our Blog called CosmicWisdom, where we discuss celestial objects in an interesting way.
Introduction:
|
| Artist impression : A planet orbiting its Star with its Two moons |
We always start our posts with a brief introduction. Here every section will act as an intro; if you want to read more about them and if we have created related posts in the past or future, we'll definitely give a link there as the word and discussion come in.
Celestial objects are impossible to ignore; they are our very close
entities. We live on a planet. A star called the sun sustains life here.
Our moon helps stabilize our planet and all these stars, planets and other
astronomical objects have been writing stock and content in our wonderful
poetry, literature and culture. Apart from that, the governments of major
countries spend a vast amount of money on space exploration
programs.
One more thing, we'll divide our post into two major sections called
Celestial objects and Celestial structures for better clarity, then put
all those famous words in them. We'll provide definitions in a very
understandable way, along with some facts and info. Let's begin with the
smallest, then we'll reach the largest.
Therefore, we will have sections named after well-known entities like
Planet, Asteroid, Star and Galaxy, etc.
Celestial Objects:
These entities are single bodies in their own; they may be microscopic in
size or Super large that even light could take weeks to reach one side to
another. We can call them individual objects. They do have compositional
variations depending on where they are located or formed. Let's explore
some Celestial Individual objects. Keep in mind, these individual objects
may form different structures if there are several similar objects
grouped.
Asteroid:
|
| Artist impression : Asteroid field |
These are spacious rocks of varying sizes, from a few millimeters to
kilometers, in the solar system. Scientists have divided them into many
classes and types based on their location, orbit, composition, and
interaction with others. They are basically the same, only differing in
some respects.
Such objects form first when gas reservoirs collapse under their gravity
and attract more nearby particles, hence they get mass. Usually, they form
after a newly born star finishes its Gas and dust gathering, so unused
surrounding clouds create some pebble-sized objects after thousands of
years. They also attract more similarly sized particles and grow together.
When they reach a size of a few kilometers or hundreds of kilometers, they
acquire a spherical shape due to gravity's compression and finally become
a dwarf planet. If they are lucky enough, they may become planets.
Note this process is violent and several rocks can be broken, fragmented,
or even cease to exist and be molten by the heat of the collision.
Asteroids are pieces of those extinct planets and dwarf planets, or they
may be fragments of our Moon and Earth, just floating in nearby space.
There are some specific terms for the solar system. while in other
systems, we can call them comets or asteroids. Let's see those terms and
conditions.
I) Meteoroid:
These are small particles with sizes from millimeters to some meters and
rarely hundreds of meters. In space, they look like a mini asteroid or
just a rock made of dust, iron, calcium or any other element or mixture.
When they are pulled by the gravity of planets, especially if they are
orbiting the star (sun in our system) around such a planet, they start to
fall at a very high speed. In the planet's atmosphere, they experience
friction with atmospheric particles, and they burn we call it a meteor.
This leads to a long, fiery trail which only glows for a few milliseconds
to seconds.
II) Trojan:
We know that such small rocks behave like mini planets and they orbit the
sun from a certain distance, but their orbit are highly unstable due to
their small sizes. If such a rock orbits while staying in the orbit of a
major planet, we call this rock to planet's trojan. Meaning, if a rock
orbits the sun from a distance that is a usual path or orbit of the Earth,
we call this rock an Earth Trojan. There are several trojans named after
the planet's orbit, like the Trojans of Venus, Mars, Jupiter, Saturn, etc.
III) Asteroid belt:
This is a collection of rocks orbiting the sun between the orbits of Mars
and Jupiter.
IV) Centaur:
These rocks are scattered between Jupiter and Neptune's orbits. They are
mostly made of ice, whereas previous bodies were a mix of rock, dust and
ice.
V) Kuiper belt:
This is a collection of comets and trans Neptunian icy bodies, which are
usually found beyond Neptune's orbit.
VI) Comet:
These objects have various sizes, but their orbit is the key to
differentiating them from other rocky-icy celestial objects. All those
previous rocks were orbiting the sun in almost circular orbits, while
comets have hyperbolic orbits. Comets also show evaporation when they get
too close to the sun. Meaning, if they stay for long periods around the
sun, they may cease to exist and completely vanish in space. Thanks to
their highly elliptical orbit, which prevents it.
If you want to know more about them, you can visit this
link, where we've explained those topics in much more detail.
Planet:
According to the International Astronomical Union (IAU), a planet must
satisfy these 3 rules:
1. Must orbit the sun, no other celestial body.
2. Acquire Hydrostatic equilibrium, meaning the object must be
massive enough that gravity makes it nearly a sphere.
3. Must clear its neighborhood. The planet candidate should clear most of
the Trojans, asteroids, and other floating rock or ice
objects from its path.
We see that the definition is solar system-centric; it strictly
applies to the objects of the Solar System or those that orbit the Sun.
Since Planets are not unique to our system, there are trillions of planets
in our galaxy. So those 3 rules can be mandated in our solar system, but
in other exoplanetary systems, those rules can be broken or partially
applied. We'll see them in other posts. For now, let's continue to learn
about planets.
Mass criterion:
Since we are not looking at the Solar system mainly, instead we're
talking about all the planets, including Exoplanets and Solar system
planets. It's necessary to forget most of those rules and focus on the
most common features.
We'll find that all planets are either as small as little spheroidal moons
or as large as Jupiter, so we almost have a clear boundary of sizes. But
in astronomy, Mass matters more than size, radius, or volume. The most
reputable criterion for defining planets is that the planet should not
exceed the 13 Jupiter masses limit, or it should not be heavier than 13
times Jupiter's mass.
Let's find out quickly what happens if any object crosses its upper or
lower limits. All the objects we see around are made by the arrangement of
molecules; in solid objects, their structure is well-organized.
If any object becomes larger and achieves almost 600 km in size,
then it will gain enough mass so that gravity will crush into a spheroid;
it may not be a complete or smooth globe, but we can call it nearly
sphere. If you look at the Solar System's largest asteroids and moons,
especially those with sizes around 600 km, they are almost round instead
of rock-like irregular shapes.
The round objects like the Earth's diameter, is about 12,750 Km,
the Moon's 3474 Km, and the spheroidal objects like Neptune's moon
Proteus' diameter is about 420 Km, Asteroid 4 Vesta's
525 Km and dwarf planet Haumea's diameter is about 1500 Km.
Note that the Lone diameter can't do much: it's also dependent on
the composition of the body. it's clear that around 600 km in
diameter, an Asteroid or large rocks can become spheroid. Also, this rule
doesn't guarantee that any object was formed as round and later
another large rock smashed into it and lost its shape.
On the other hand, if any planetary mass object gets more mass, it won't
be solid until special conditions are met. For example, our Earth is the
largest Rocky planet in our Solar System, which is a balance of Sea and
land. However, it's mostly solid and the sea is on the crust mainly. If
you look at some other planets with only 1.5 times heavier
than Earth, in exoplanets, they are water or liquid worlds.
At the masses of Neptune or sub-Neptune about, the liquid or semi-liquid
nature dominates, while the gaseous nature starts to show in minimal ways,
though. Again, this is dependent on chemical compositions. If we increase
more mass, let's say Jupiter or Saturn like then The Gaseous nature will
dominate mostly. It also depends on the Chemical composition, like Those
Gas giant planets are made of Gases like Hydrogen, Helium and other gases.
A planet can be as massive as 13 times Jupiter's mass, yet its size will
be almost the same. At this mass, the planet will no longer remain a
planet; it will take a step into Star's Realm.
Moon:
Moons are natural satellites orbiting planets or dwarf planets, varying
widely in size. This discussion covers moons in general, not just those
in our Solar System.
In our Solar System, moons are typically smaller and lighter than their
planets. They are often captured asteroids, dwarf planets, or rocky
bodies. Massive objects attract smaller ones, so planets can capture
these smaller bodies into orbit.
Imagine a large planet orbiting its star. A smaller object nearby may be
pulled by the planet’s gravity. If it doesn’t fall into the planet but
slows down enough, it can enter a stable orbit, becoming a moon.
This capture can happen regardless of the composition of the planet or the
smaller object. The key factors are the smaller object's mass, trajectory,
and being trapped by the planet’s gravity. While not all objects are
captured, many moons orbit gas giants like Jupiter, showing this process
is common.
Another way moons form is by co-evolving with their planets from the
protoplanetary disk. If both survive early collisions and grow large
enough, they can form a planet-moon system. However, most moons form
later or are captured. Asteroids, belts, centaurs, and Trans Neptunian
objects are remnants or fossils of planets and moons.
Let’s explore some unique Planet-moon relationships.
Earth and its moon form a remarkable pair. Our moon stabilizes Earth's
rotation axis, preventing extreme tilts that could drastically change
climate, unlike Mars, whose small moons Phobos and Deimos cannot do this.
Being the 5th-largest moon in the solar system, Earth's moon exerts strong
gravitational influence, maintaining Earth's axial tilt and thus stable
weather, though it doesn't affect human-driven global warming.
Pluto and its largest moon Charon, in the Kuiper Belt beyond
Neptune, form an unusual duo. Pluto has five moons:
Charon, Nyx, Styx, Kerberos, and Hydra. Charon is half Pluto’s
size, creating a barycenter outside Pluto itself, causing both to orbit
this shared center of mass. The other four moons orbit this barycenter,
making them circumbinary moons.
Beyond our solar system, 8200 light-years away in Cygnus, star
Kepler 1625 hosts a Jupiter-like planet, Kepler 1625b, with a
Neptune-sized moon, Kepler 1625b I. This gas giant moon orbiting another
gas giant was unexpected but shows any mass can become a moon under the
right conditions.
Stars:
From the Planet Section, you know that if gas giants like Jupiter exceed
13 Mj (Mass of Jupiter), they enter the Stellar Realm. We'll briefly
explore stars, though each could fill volumes, and you might not have
much time to read them.
Stars are self-luminous objects; their light comes from nuclear fusion
regardless of the elements fused or radiation emitted. Even if a star
isn’t visible but emits infrared, radio, or other
radiation
through fusion, it’s classified as a star. In reality, no true star
lacks visible light emission; all emit various electromagnetic
radiations.
These giant spheres are mostly made of Hydrogen and helium. The hydrogen
is not a gas or fog there instead its found in the 4th form of matter
called Plasma. The layer which emits light called Photosphere regardless
of Star type and size, Every Star contains Corona, Chromosphere,
Photosphere, Convective/Radiative layer and finally the core where the
Nuclear fusion takes place which releases enormous energy, equivalent to
power of millions of Nuclear weapons.
You might be thinking why the star can’t explode, The gravity which
compresses the star has enough pressing force from all side to center of
the star, The gravity has almost equal strength to the energy released by
nuclear fusion. Therefore, Star is balanced. Stars are like explosions
which can’t explode gravity constantly pushes them.
Let’s explore some types of stars briefly. For more details, explore our
Blog by clicking the Stars Button above.
Protostar
A gas cloud collapsing under gravity, glowing from thermal reactions. The
central point forms a proto core where gas accumulates, eventually
creating a brown dwarf or star if conditions allow.
Brown Dwarf
If gas accumulation slows or stops due to external factors, the object
becomes a brown dwarf—more massive than 13 Mj but less than 80 Mj,
considered a failed star. Click
Brown Dwarfs.
for more info.
Main Sequence Stars
Stars fusing hydrogen in their cores, representing the longest life phase.
Massive stars are larger, luminous, and short-lived (a few million years).
Less massive stars are smaller, dimmer, and live longer. Blue stars are
hot and massive; orange or red stars are cooler and less massive, true
mainly for main sequence stars.
Giant Star
Evolved stars whose outer layers expand due to fusion processes and core
changes. This expansion cools the photosphere, making the star appear red
or orange while increasing luminosity. Types include red, orange, yellow,
white, or blue giants.
Low Mass Star
Stars from about 0.08 to 1.3 solar masses, glowing red, orange or
yellowish white in the main sequence. Their fusion relies on the slow
proton-proton chain (PP chain), effective at lower temperatures.
Intermediate Mass Star
Star Stars from 1.5 to 8 solar masses, glowing yellowish white to
blueish white. Their fusion uses the fast, temperature-sensitive CNO
cycle, with efficiency increasing with mass.
Massive Star
Stars above 8-10 solar masses, especially over 16 solar masses, with no
upper mass limit. Powered by the CNO cycle.
Wolf-Rayet Star
Evolved stars over 20 solar masses that lose outer layers, exposing
deeper layers. They have strong stellar winds and are the hottest star
types.
Population I Star
Current generation stars with high metallicity—elements heavier than
hydrogen and helium like carbon, iron, and nitrogen.
Population II Star
Older stars with lower metal abundance, differing slightly in evolution
and life cycle.
White dwarf:
|
| Artist impression : White dwarf inside a planetary Nebula |
These are gravitationally compressed cores of stars with masses between
about 0.45 and 8 solar masses. Their outer layers are removed by
stronger winds and activities in later stages, forming a planetary
nebula. Gravity compresses their cores into spheres about 20,000 km in
size. These cores are supported by electron degeneracy pressure, which
prevents further collapse.
White dwarfs are extremely hot when formed, but their spin and temperature
gradually decrease. Their color changes from blue white to cooler hues
like whitish yellow, yellow, red, or brown. Eventually, they become very
cool, non-rotating black dwarfs, possibly made of coal or diamond.
Neutron Star:
|
| Artist impression : Neutron Star |
Like white dwarfs, these are dead cores of stars with masses between 8
and 25 solar masses. Their outer layers shed at enormous rates and
speeds, similar to the later stages of low-mass stars. When their core
forms iron, fusion energy stops suddenly, and gravity takes control. The
star first shrinks slightly, then explodes with tremendous energy in a
supernova.
In white dwarfs, gravity stops compressing at a planet-sized sphere due to
electron degeneracy pressure. Here, compression goes further because
electron degeneracy can’t stop the collapse. Neutron degeneracy pressure
supports the collapsed core, which shrinks from millions of kilometers to
about 15-20 km in size. The degenerate neutrons and some degenerate
electrons halt gravity’s compression.
Neutron stars are compressed cores with high-speed rotation and immense
magnetic fields. The material is so dense that a tablespoon would weigh as
much as Mt. Everest. The compressing gravity is about one-thousandth that
of a black hole. This gravity is strong enough to curve spacetime, bending
incoming light from distant sources around neutron stars.
Black Hole:
|
| Artist impression : Black hole |
Like White Dwarfs and Neutron Stars, these are compressed cores of stars
with more than 25-30 solar masses. Their outer layers explode as a
supernova, but their cores compress even further than neutron stars.
While a neutron star's gravity is halted at about 15-20 km, in more
massive cores, gravitational compression is unstoppable by any
degeneracy pressure. The core continues compressing until it reaches a
point called the singularity, an extremely dense point—not truly
infinite as sometimes described.
The large black sphere around the singularity is called the Event
Horizon. It’s not a surface but a region where spacetime curvature is so
strong that local spacetime falls inward. This creates a zone with an
escape velocity exceeding the speed of light. If you fall here, even
moving outward at light speed won’t let you escape. This is often called
the point of no return.
Since black holes lack defined boundaries, there is a space called the
Accretion Disk where objects, dust, and gas cannot fall directly but orbit
the black hole first. Over time, they spiral inward toward the Event
Horizon. They don’t fall directly due to centrifugal force, which acts on
rotating systems and pushes objects away from the center. Gravity is
slightly stronger than this force here and increases as objects orbit
closer to the Event Horizon.
All objects are aggressively pulled by the immense gravity of the black
hole and begin to orbit. Particles like atoms, molecules, gas, and
dust—remnants of dead stars—orbit in this zone and rub against each other.
This friction generates enormous heat, causing the accretion disk to glow.
Temperatures reach much higher than a star’s surface.
Almost 40% of matter never reaches the Event Horizon. Sometimes
centrifugal and other forces create a kind of jam or stoppage point around
it. Incoming material wants to fall in but can’t, causing pressure and
temperature to rise. Some matter focuses on the black hole’s poles and is
ejected at nearly the speed of light, escaping into space. These are
called Relativistic Jets. Note that accretion disks and jets might not be
present around all White Dwarfs, Neutron Stars, and Black Holes; their
formation depends on the availability of surrounding material. these dead
entities still consume gas and dust, which might not be detected.
Supermassive Black Hole (SMBH):
In the last section, we discussed stellar mass black holes, which form
when massive stars die. Now, we explore a different kind of entity often
labeled as black holes but quite distinct in nature.
Supermassive black holes (SMBHs) are typically found at the centers of
galaxies. Recent observations reveal a deep connection between the mass of
a galaxy and its SMBH. These black holes can influence star formation by
powerful jets or radiation, either triggering or halting it.
Don't think that a stellar mass black hole simply grows by eating and
eventually becomes the galaxy's boss. Stellar mass black holes are limited
to about 3-10 solar masses, roughly the mass of their progenitor stars. In
contrast, SMBHs can be millions to trillions of times more massive than
our sun—no single black hole can consume that much.
It is suggested that SMBHs evolve alongside their galaxies and grow by
merging with intermediate mass black holes (10,000-100,000 solar masses).
The SMBH Phoenix A* is the largest known single object, with a mass
equivalent to about 100 billion suns. Previously, TON 618 held the
record with 66 billion solar masses.
Moving beyond single objects, we will next explore structures formed by
multiple entities, like many houses forming a town.
Structures:
As we said before that we’ll talk about complexes or groups of single
celestial bodies, they are way larger than the largest know Supermassive
Black hole. As always, we’ll start with the most basic then move to
complicated ones.
Ring system:
Many other Astronomers might not categorize it in the Astronomical
Structures, in reality, a ring system is a group of millions of rocks or
ice fragments much like asteroids. In our Solar system, All the Gas
giants Jupiter, Saturn, Uranus and Neptune have Diverse Ring systems. In
theory, Any Gravitational source can form a ring, the ring formation
mechanism is the almost same as the smaller planet gets trapped by
massive object’s gravity. Only difference is Sometimes Planets or
captured moons collide with each other, their fragments first orbit the
massive planet and slowly they all align to parent object’s equator
therefore every rock or ice piece in a ring system behaves like a moon.
Around the equator such particles are mostly safe from other large body
interactions. In other words ring system’s particles are fragments of
neighborhood bodies.
Accretion Disk and Jets:
Accretion disks form around strong gravitational sources like
protostars, neutron stars, white dwarfs, or black holes. These objects
pull in gas and dust, but the process isn’t straightforward. Gravity
pulls matter from all sides, but high-speed rotation flattens it into a
disk with some thickness. Infalling material orbits the source, creating
friction that heats and makes the disk glow. Centrifugal force balances
gravity, preventing matter from falling straight in. Some matter loses
energy as heat and light or is ejected.
If the source pulls in matter at very high rates, the disk glows
intensely and emits radiation. Crossing the Eddington limit triggers two
jets aligned with the rotation axis. Without jets, the source may
explode like a small supernova.
Together, the jets and disk prevent the source from swallowing all
matter, returning some to the universe.
Cluster:
A cluster in space is a group of stars or galaxies. Stellar clusters
form when a large nebula breaks into pieces, each collapsing under
gravity to create stars over time. Galactic clusters form when galaxies
are close enough to influence each other.
Stars in a cluster come from the same nebula, so they share similarities.
This makes stellar clusters useful for studying star properties since
their ages are similar. Clusters can contain massive blue stars,
intermediate, or low-mass stars depending on available gas and dust.
Massive stars form quickly, in a few hundred thousand to a million years,
while low-mass stars take tens of millions of years. Newly formed blue
stars can disrupt local star formation with their strong radiation.
Stellar clusters help observe star activity, evolution, and interactions.
There are two main types of stellar clusters: open and close. Close
clusters have densely packed stars within a few tens of light-years, while
open clusters have stars more spread out. The Arches Cluster in
Sagittarius is the densest known stellar cluster. Omega Centauri is the
largest, containing nearly 10 million stars.
Galactic clusters differ from stellar clusters; they are groups of
galaxies close enough to affect each other, including both small and large
galaxies.
Nebula
|
| Artist impression : The Nebula |
Nebulae are diverse clouds in the interstellar medium that can create
stars, coexist with them, or be remnants of dying stars.
They interact with light by emitting, reflecting, absorbing, or blocking
it, depending on ionization and molecular properties. Like Earth’s
clouds reflect sunlight, nebulae can reflect starlight by scattering
photons. Dense dust and gas regions can block light, especially near
active star-forming areas, making parts of nebulae appear darker.
Supernova shockwaves, ionizing radiation, or jets from supermassive black
holes can trigger or halt star formation in nebulae. Not all nebulae form
stars continuously; some are ready, some are forming stars, others have
stopped due to external factors, and many are stellar remnants. Thus,
nebulae coexist with stars and maintain their presence in space without
all turning into stars.
Galaxy:
 |
| Artist impression : The Spiral Galaxy |
Imagine a vast region spanning at least 30,000 light years, containing stars, planets, asteroids, black holes, brown dwarfs, protostars, and many other objects and their groups. This is a galaxy, regardless of its combined shape.
A galaxy is a system of stars, planets, asteroids, nebulae, stellar clusters, and dark matter reservoirs. Everything coexists in this massive realm, where objects are born, evolve, interact, eliminate others, and eventually die. Think of galaxies as nations with various infrastructures and departments contributing to their overall fate.
Galaxies usually come in three shapes: irregular, spiral, and elliptical. When galaxies group together in large collections, we call them galactic clusters. Based on the behavior of their supermassive black holes (SMBHs), galaxies can be classified into types like inactive, radio, or active galactic nuclei (AGNs). A dedicated post will explore these types in depth.
In the end:
That's all for today's post, we'll see you soon. Have a nice day.