Variable Star: Extrinsic, Intrinsic Stars, Cepheids and Observation

Variable Star has a brightness that keeps on fluctuating. Amateurs have contributed to astronomy since a long time ago by maintaining records of the brightness of these variable stars from their backyards via a telescope. A star is referred to as a variable star if the apparent magnitude of the star is in any way changed from our perspective on Earth. These changes in brightness can happen over just a fraction of a second or might even take years and it can range from the one-thousandth value of the magnitude to over 20 times the magnitude of the brightness of the star. In this article, we will have a look at what are variable stars, their types, changes in brightness, and their observation.

Key Terms: Variables, Pulsating Variables, Eruptive Variable Stars, Explosive Variables, Eclipsing Binaries, Rotating Stars

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What are variable stars?

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The stars that change their brightness are popularly known as variable stars. The variation in the brightness of the star is a result of the change in the light that is emitted by them. The changes can also be a result of the partial blocking of light by some object. We have identified over 1,50,000 variable stars and yet many more are left to be identified. Most of the stars in space do have at least some amount of variations in the energy output of the sun, also referred to as luminosity. Even our sun is a variable star. The first-ever variable star was identified by David Fabricius when he observed in the year 1596, that over a period of 11 months, the Omicron Ceti, also known as Mira was seen pulsated in a cycle. This discovery helped verify that the stars were not eternal or invariable. Variable stars are observed over a longer period of time to identify their behavior. The rotating stars are the variable stars and their shape can be ellipsoidal or non-spherical.

Variable Stars or Variables

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Varying Brightness of the Variable Stars

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Variable stars are known to change their brightness over time. This happens for a variety of reasons such as changes in mass, star luminosity, and obstruction in the way of the starlight towards Earth. Some of the reasons are:

• The pulsating variable stars swell and shrink due to some internal forces.
• In the same way, an eclipsing binary star will get dimmer when it is covered by a faint partner and will then brighten when that covering partner moves away.

The different reasons for the changing brightness of the star are responsible for classifying them into various categories.

Observed data of Variable Stars

Check out: Difference between Stars and Planets

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Types of Variable Stars

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Variable Stars are can be classified into two categories which are as follows:

• Intrinsic Variable Stars

The intrinsic variable stars are the ones that have changes in their brightness due to their physical properties such as swelling, shrinking, or pulsations. Intrinsic Variable Stars are further sub-divided into various categories which are:

• Pulsating Variables 

These stars are formed due to periodic contraction and expansion that happens on the surface of the stars. The stars actually decrease and increase periodically in size. The changes in the radius of the star also known as the stellar radius cause corresponding changes in the brightness of the star. Some examples of these stars are Orionis, Antares, and Betelguese.

• Cepheid Variables:

The large pulsating stars which are yellow in color, are the first star identified out of the stellar group Delta Cephei. This star rapidly rises to the maximum level of light and then gradually falls back to a minimum level of light. The whole cycle takes approximately 5.4 days to complete one cycle. Over several hundreds of cepheid variable stars are known in our very own Milky Way Galaxy. Most of the cepheid stars have their periods which are in the range of about 3 to 50 days. The luminosity of these stars is about 1000 to 10,000 times more than the luminosity or the light of the sun which also refers to its brightness.

The North Star, also known as the Polaris is a kind of cepheid variable that has varied in terms of its brightness or luminosity for about one-tenth of its magnitude which is about 10 percent of the visual luminosity. This whole cycle takes place in just four days. Being a cepheid variable star is a stage in the life cycle of any star. 

• Eruptive Variables 

These stars have changes in brightness due to the violent processes that happen on the surface of the star. The violent processes include flares and can really lead to rapid and significant changes in the brightness or the luminosity of the stars. The eruptive variable stars have irregular or semi-regular levels of variations in brightness that are caused by the lost material of the stars. A few examples of these types of stars are flare stars, supernovae, and novae.

• Cataclysmic or Explosive Variables

Explosive Variables are the stars that have violent and sharp outbursts that are caused as a result of the thermonuclear processes which take place either on the surface of the Earth or deep inside it. Within the general variable, over 800 cataclysmic or explosive stars are recognized. Examples of these types of stars include recurrent novae and dwarf novae.

• Extrinsic Variable Stars

The extrinsic variable stars are the stars that have changes in brightness due to external stellar properties such as eclipses or rotations. They are sub-divided into two categories which are:

• Eclipsing Binaries

This includes a binary system wherein the orbital plane exists on the edge in a way that the stars of the system eclipse each other periodically. The light curves of the eclipsing binaries have a characteristic periodic dip in the brightness levels whenever the component stars are eclipsed. For example, present and algol.

• Rotating Stars

Rotating Stars are the type of variable stars that display changes in luminosity or light due to the patches of spots of light on their own surfaces. Stars are not solid bodies, they can undergo some differential motion. Stellar rotation refers to the angular rotation of the stars about their own axis. At the magnetic poles, they may exhibit bright spots.

Types of Variable Stars

Read More: Why do the stars twinkle?

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Observation of Variable Stars

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Many people have collected and maintained the data on the changing brightness of the variable stars. This is useful for a variety of purposes such as:

• The information on variables can be used to study the properties and characteristics of other stars.
• The long-term behavior of these variable stars is identified from this data.
• The data on these stars also provides a lot of information on various properties of stars such as their composition, mass, radius, and luminosity.

Observation of Variable Stars

How to observe variable stars?

Variable stars can be observed by anyone. The types of equipment you need for the same are given below:

• A telescope or a pair of binoculars
• A variable star chart for easy navigation and identification

The variable stargazers may require some instructions and a lot of patience but with time one can observe the variable stars across the night sky even without the astronomical charts.

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Things to Remember 

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• The stars that change their brightness are popularly known as variable stars.
• A star is referred to as a variable star if the apparent magnitude of the star is in any way changed from our perspective on Earth
• The changes in luminosity may happen for a variety of reasons such as changes in mass, star luminosity, and obstruction in the way of the starlight towards Earth.
• The intrinsic variable stars are the ones that have changes in their brightness due to their physical properties such as swelling, shrinking, or pulsations.
• The extrinsic variable stars are the stars that have changes in brightness due to external stellar properties such as eclipses or rotations.
• Eclipsing Binaries include a binary system wherein the orbital plane exists on the edge in a way that the stars of the system eclipse each other periodically.
• Rotating Stars are the type of variable stars that display changes in luminosity or light due to the patches of spots of light on their own surfaces.
• The variable stars are observed and the data help in the research of other stars and stellar properties.

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