How Do Variable Stars Help Us Understand the Universe?
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.
FAQs on Variable Stars: Types, Causes, and Significance
1. What is a variable star?
A variable star is a star whose brightness, or magnitude, as observed from Earth, changes over time. This variation in luminosity is not due to atmospheric effects but is caused by either physical changes within the star itself or external factors, such as being eclipsed by a companion star.
2. Why do variable stars change in brightness?
Variable stars change brightness due to two main categories of causes:
Intrinsic Causes: These are physical changes happening within the star. This includes processes like pulsation, where the star expands and contracts, or eruptions, where the star violently expels mass, causing a sudden increase in brightness (e.g., a nova).
Extrinsic Causes: These are external factors that affect the light we receive. The most common example is an eclipsing binary, where one star in a binary system passes in front of its companion, blocking some of its light from reaching us.
3. What are the main types of variable stars?
The main types of variable stars are broadly classified based on the cause of their variability:
Intrinsic Variables: Stars that vary due to their own physical properties. Sub-types include Pulsating Variables (like Cepheids and RR Lyrae stars) that rhythmically expand and contract, and Eruptive Variables (like novae and supernovae) that undergo sudden, dramatic changes.
Extrinsic Variables: Stars whose brightness changes due to an external reason. Sub-types include Eclipsing Binaries, where brightness dips when one star is blocked by its orbital partner, and Rotating Variables, which have large sunspots or bright patches that cause brightness to vary as the star rotates.
4. What is the difference between an intrinsic and an extrinsic variable star?
The fundamental difference lies in the origin of the brightness change. For an intrinsic variable, the change in luminosity is a result of physical processes occurring within the star itself, such as changes in size, temperature, or energy output. For an extrinsic variable, the star's own luminosity is stable; the observed change in brightness is caused by an external geometric effect, like another object blocking its light or the star's rotation revealing different surface features.
5. What are Cepheid variables and why are they so important in astronomy?
Cepheid variables are a famous type of pulsating intrinsic variable star. They are incredibly important in astronomy because they serve as cosmic 'standard candles'. This is due to a direct and reliable relationship between their pulsation period and their absolute luminosity (their true brightness). This is known as the period-luminosity relationship. By measuring how long a Cepheid takes to pulse, astronomers can determine its true brightness and, by comparing that to its apparent brightness from Earth, calculate its distance with high accuracy.
6. How do astronomers use the period-luminosity relationship to measure cosmic distances?
Astronomers follow a precise method to measure cosmic distances using Cepheid variables:
Step 1: They observe a distant Cepheid star and carefully measure the time it takes to complete one cycle of brightening and dimming. This is its pulsation period.
Step 2: Using the established period-luminosity relationship, they use this period to calculate the star's absolute magnitude (its actual, intrinsic brightness).
Step 3: They measure the star's apparent magnitude (how bright it appears from our perspective on Earth).
Step 4: By comparing the absolute magnitude with the apparent magnitude, they can calculate the distance to the star (and its host galaxy) using the inverse square law of light.
7. How does a star's pulsation mechanism work?
Pulsation in stars is a continuous battle between the inward pull of gravity and the outward push of radiation pressure from nuclear fusion. The cycle works as follows:
In a specific layer of the star (the helium ionisation zone), the gas becomes opaque and traps heat. This increased heat causes the outward pressure to overcome gravity, making the star's outer layers expand.
As the star expands, it cools down. The cooling causes the gas to become transparent again, releasing the trapped energy.
With the outward pressure reduced, gravity takes over again and causes the outer layers to contract and fall inwards.
As the layers compress, they heat up, become opaque, and the cycle repeats. This creates the periodic pulsation and change in brightness.
8. What are eclipsing binaries, and are they considered true variable stars?
An eclipsing binary is a system of two stars orbiting a common centre of mass, where the orbital plane is aligned with our line of sight. This alignment causes one star to periodically pass in front of the other, resulting in a temporary, observable dip in the system's total brightness. They are classified as extrinsic variable stars. While their brightness varies, it is not a 'true' variability in the sense of the stars themselves changing. The stars maintain a constant luminosity; the variation we see is purely a geometric effect of the eclipse.
9. Besides measuring distance, what other astronomical insights do variable stars provide?
Beyond their crucial role as standard candles, variable stars are vital for understanding stellar evolution and physics. Studying their variations allows astronomers to model the internal structure of stars, a field known as astroseismology. By analysing the light curves of variable stars, scientists can determine key properties like their mass, radius, temperature, and composition, providing essential data to test and refine theories about how stars live and die.
