If you have Read my previous The Constellation Guides You probably Noticed a particular Type of stars called Variable Star They Were not covered in those guides but Now I'm making a guide which is totally Focused on this topic Of course this is very Huge subject. That's Why I didn't Went any further details and guide could be very long.

A Variable star is a star which periodically Changes its brightness if observed by Scientists, observers or Satellites Even by normal folks.

These Variations Caused By Following Reasons :

1) flowing Material in star. eg Plasma.

2) Due to Strong Magnetic Field.

3) Some powerful Explosions which Deforms The Actual Shape of Star.

4) Exceptionally Active Chromosphere.

5) Due to Ageing or Low gravity.

6) External Factors like Binary Star or a large planet nearby.

7) Nebula or Black Hole in Vicinity which Distorts its Brightness or star itself.

8) Gases which surrounds Stars can Change their Actual Properties.

9) There are Numerous Objects and Structures That changes Stellar Luminosity by Capturing or Ionising Emitted Photons by Star, Like Molecular Clouds or Nebula, Stellar Nursery or Anything Which Can Interact with light.

There is one thing to note The Star is Dangerously Balanced By its Gravity and Nuclear Fusion which is Generally Not Stable So Star Change its Shape, Rotation, Brightness, Spectrum, Temperature or Even Location. Sometimes they Vibrate or Generate a Star quake. Various Things can happen Due to Variable Nature of stars.

There is Two Major Types of Variable Star.


The Brightness Indeed Changes By the Star and Its motions like Shrinking or Expanding Periodically.


This is caused by Light Which generally Changes Due to Binary Systems or other Factors but Not by Star itself.

Some people Think Our Sun Always Look Stable and Uniform But In Reality It is Very dangerous Stormy Star. you can Watch On Youtube How Solar Activity like CMEs are dominant on sun.
But Anyone Can prove it as a lie but There are some Evidences Found after Centuries of Observations, Spectroscopy and Satellite Based Researches That Shows Our Sun is a Variable Star.


The Sun's Main Component is Plasma Which is Ionised Hydrogen Gas That means This Hydrogen has More or less Electrons than Usual in their Nuclei. the Plasma is a fourth State of matter.
If you Give No or very low temperature Then The Gas will become Solid, As the Temperature increases it becomes Liquid then Gas, if you heat the Gas it will become Plasma. Which is very hot Highly Electrically Conductive Viscous soup But Not like Normal Fluid Dynamics. So they Show Different behaviour like Interacting with Magnetic Fields and If these Magnetic Lines Cloak, twist or overlap each other then A large Amount of plasma Explodes along with Magnetic Fields. This called CME.


These are Energetic Flashes of Light which is actually a very powerful Burst on sun.

Besides there is Sun spots, Flipping Magnetic Fields on every 11 years etc.

That Proves Sun is variable Star.

If a Variable Star Shrinks and Swells by its own Factors such as Gravity, Explosion and Energy etc. Then its called Intrinsic Variable Star
Again This is Divided in Many Categories and Sub Categories.


These Stars Normally Contract and Expand in Regular, Semi- Regular, Irregular Time Intervals.
It is Broadly split into Two Types :

(a) Radial = If The Entire Star Swells and Shrinks

(b) Non - Radial = Where One part of star Shrink and Whereas other Swell


There Are Many Types of Stars in this category But Here Only Few of them is Described Below :

Classical Cepheid Variables

In this Group Delta Cephei is a Proto Type or Ideal Variable it means Their Variability Behave like Delta Cephei also they are Population I Stars. Their Pulsate in Very Regular Periods on Days To months.

Type II Cepheids

these Variables Belong To Population II Stars, Before this Group Was known as W Virginis Stars. They Have Very Regular Pulsations.

SX phoenicis Variables

These Stars Found mainly in some Certain Clusters, Their Spectra Lies A2 to F5, 1 or 2 hours is Pulsation Period.

Mira Variables

These Are Long period Pulsating Stars and Also They are Red Giants. The Ideal Star Mira or Omicron Ceti's Pulsation period is about 332 days.

Beta Cephei Variables

They Change their luminosity about 1 to 30 %. their general Pulsation period is 0.1 to 0.6 Days.

RV Tauri Variables

These Are Post - AGB Stars (we can create a Guide on it too), whose General Pulsation period is about 30 to 100 Days. When They are at maximum Sizes their spectra appear like an F or G Stars and When Smallest Sizes their spectra looks like K or M Stars.

Sun like Variables

Our Sun Actually Oscillates in around 5 minutes, Caused by Convection in its outer layers. Variable stars with same Mechanisms Falls in this Group.


These Stars Have Large Material Ejections on their surface However They aren't Explosions.

Protostars

Before Main sequence Phase of star is Called Protostar Phase. You can Read in detail Here

Giant and Supergiant Stars

Very Large stars Loose their Material Very Easily as compare to others, This phenomenon is common in Such Stars.

Wolf - Rayet Variables

these Are Hot massive Population I Stars, Their Variability Caused by Binary Systems and Sometimes Rotating Gas clumps Around Star.

Flare Stars

these Stars Glow Very much about Their 6 times of their usual. There Are Proxima Centauri and Wolf 359 Which are comes in this group.


Supernovae

These Are The one of the most powerful Events in the Universe and I don't Think Telling about it would be necessary Because Most of us Know about it.

Novae

It is also a Powerful Event But Its Star Doesn't Die and Unlike supernova it can Occur Suddenly in some certain Conditions. It happen in Binary Systems Where one member is a White Dwarf Being Accumulating its Partner's Material and Forming an Accretion Disk. its period is about Some decades to Millennia.

Z Andromedae Variables

This is caused by Binary System Formed by a Red giant and Hot Luminous blue Star Which Are Covered By Gas and Dust together and they undergo Nova Like Outbursts.

When Star is not Variable Means Mostly Looks Unchanged By the time But other Factors like Binary, Eclipse etc. make it variable then We call it Extrinsic Variable.

This is Divided in Two Major Types :


Stars with Huge Stellar Spots When Rotate It show Slight Changes in Brightness.

Ellipsoidal Variable Stars

Occurs in Close Binary Systems Where Either Star's Shape Becomes Non Spherical Due to Tidal Interactions, As the Star Rotates its Brightness Changes as seen From the Earth.

SX Arietis Variables

These Stars have Variable Brightness in account to Their Immense Magnetic Fields and High speed Rotation.

Optically Variable Pulsars

We Know That a Pulsar is Neutron Star, Which Spins Very Fast Hence Their Emitted light will be Variable too. Mostly Their variations lasts From Milliseconds to Few Seconds. Some Pulsars have detected in Visible light. Its well Known Example is Crab Pulsar.


In Binary systems and Certain Angle from Earth, One Stars Comes toward us and other goes away But Sometimes they can Eclipse each other. Just like Solar and Lunar Eclipses. Therefore Their Brightness will also Changed.

Algol Variables

These Binaries Eclipses in regular Time. Prototype of this class is Algol star In Perseus Constellation.

Beta Lyrae Variables

If the Commencement and Ending of Eclipses is almost Impossible To detect. it will come in this category. This happens in Extremely Close Binary Systems.

W Ursae Majoris Variables

This Group of Stars Show Period of less than a Day, Stars Are so Nearly Situated to each other that it seems Their surface is in Touch with each other.

Planetary Transits

Star With planets Sometimes show Brightness Variations when their Planet pass Between Earth and Star. However These Changes are only detectable in Extremely Accurate Observations. Except of HD 209458 and All the Exoplanets Discovered by Transit methods.