Variable Stars

Introduction on Variable Stars

Astronomy is a natural science that studies celestial objects such as (sun, moon, stars, planets, galaxies, comets, and other objects). An Astronomer is a scientist in the field of astronomy who focuses on the field outside the earth like(planets, galaxies, and many others). A star whose brightness as seen from earth keeps fluctuating is a variable star. The variation may be caused by something partly blocking the light or change in the emitted light. Our own sun is also a variable star. In this article, we will discuss variable stars, what are the types of variable stars and why do variable stars change their brightness.


Concept of Variable Stars

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A variable star is simply a star that changes brightness. A star is considered as a variable star if its apparent magnitude (brightness) is altered in any way from our perspective. The first identified variable star was Mira(omicron Ceti). It has been described by David Fabricius in 1596. This was an important discovery, as it helped verify that the stars were not invariable and eternal. These stars are observed for many years to determine their long term behavior. Their combining light curves with spectral data often gives a clue as to the changes that occur in a variable star. In very cases, it is possible to take a picture of a stellar disk. These may show darker spots on its surface. Rotating stars are variable stars and it can be nonspherical or ellipsoidal in shape. There are two types of variable stars, intrinsic variables, and extrinsic variables.

  • Intrinsic variables are stars whose brightness changes due to shrinking, pulsations, or swelling.

  • Extrinsic variables are stars that change in brightness because of being escaped by another planet or star.

Different Types of Intrinsic Variables are :

  • Pulsating variables

  • Eruptive variables

  • Explosive or cataclysmic variables


Pulsating Variables

It is caused due to the periodic expansion and contraction of the surface of the stars. This means the star actually increases and decreases in size periodically. These fluctuations in the stellar radius cause corresponding changes in the luminosity of the star.

Example:-  Antares, Betelgeuse, Orionis, etc.


Eruptive Variables

Eruptive variables are stars varying in brightness because of violent processes such as flares that occur on the surface of the star. They can exhibit significant and rapid changes in their luminosity. Eruptive variable stars show semi-regular or irregular brightness variations caused by material being lost from the star.

Example:- novae, supernovae, flare stars, etc


Explosive or Cataclysmic Variables

Cataclysmic variables are also called explosive variables because of sharp or violent outbursts caused by thermonuclear processes, either on the deep or surface inside. There are almost 800 cataclysmic variables identified within the general catalog of variable stars.

Example:- dwarf novae, recurrent novae, etc.


Different Types of Extrinsic Variables are :

  • Rotating stars

  • Eclipsing binaries 

Rotating Stars

These are variable stars that show light changes caused by patches of light spots on their surfaces. Stars can undergo differential rotation, they are not solid bodies. The angular motion of the star about its axis is called stellar rotation. They may have bright spots at the magnetic poles. 

Examples:- stars with extreme sunspots or stars that have fast rotation speeds.


Eclipsing Binaries

A binary system in which the orbital plane lies edge on to us so that the components stars periodically eclipse one another. Eclipsing binaries light curves are characterized by periodic dips in brightness that occur whenever one of the components is eclipsed. 

Example:- algol, present, etc.


Why Variable States Change Their Brightness

Variable stars change brightness due to several reasons. These include changes in star luminosity, mass, and obstruction in the amount of light that reaches earth. The luminosity of the star is the amount of light it emits from its surface. The difference between apparent and luminosity depends on distance. An eclipsing binary will brighten when the occulting partner moves away and dim when observed by a faint partner.

FAQ (Frequently Asked Questions)

1.How do Stars Pulsate?

The steps involved in pulsating stars are:-

  • If the pressure outwards exceeds the gravitational force inwards, the outer layers of a star will expand outwards.

  • The gravitational force inwards diminishes but its outwards pressure also drops at an even greater rate.

  • After that, the star would reach a position at which hydrostatic equilibrium starts. However, momentum is still in the outward Layers so they resist a change in motion. 

  • As the gravitational force acts on the layer it slows down. A point is reached where it stops but now the outward gas and radiation pressure is weaker than the inward-acting gravitational force.

  • Outer layers of the star start to collapse inwards because of the imbalance of forces.

  • And, when the pressure outwards exceeding the inwards gravitational force the collapsing layer slows down and eventually stops.

2. How are Variable Stars Used to Measure Distances?

The intrinsic brightness of these variable stars is strongly tied to their period. So, astronomers can predict the absolute magnitude of any given cepheid by measuring the time it takes to rhythmically charge its brightness. Also, variables like cephids also tend to have a larger radius. Larger radius translates into larger surface areas. At the start where the outwards pressure is greater than the gravitational force the pulsation cycle starts. They start observing the apparent luminosity, dimmed by the inverse square law, and comparing this with the predicted luminosity and that is how astronomers can calculate the distance to that star.