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Aim of this Blog is to explain why we need a theory of Stellar Structure and Evolution- A war for need of theory over observation.
Before coming to the central question let us look at the answers to some simple looking but very crucial questions:
Q1. What is meant by Stellar Structure and Evolution?
To understand structure and evolution of stars using laws of physics.
Q2. What are stars ?
A star is an object that -
1. Radiates energy from an internal source
2. It is bound by its own gravity
3. Star should evolve ? But why ?
Stars are born within the clouds of dust scattered throughout most galaxies.
Evolution is the change in properties of star with time. In stars it occurs due to burning of fuel to balance the forces of gas and pressure. This evolution is highly dependent on mass of stars - On average, Greater the mass shorter it's life
Now coming to the central question -
What is the need for theory for stellar evolution when we can have so much information by just observation of Stars ?
To answer this let's understand what all we can gather by the known observational techniques that we use to study stars -
1. Photometric measurements (Photometry, in astronomy, the measurement of the brightness of stars and other celestial objects) yield the apparent brightness of a star, i.e. the energy flux ( f ) received on Earth, in different wavelength bands.
(I have covered more of the terminologies here )
2. Distances to nearby stars can be measured with the help of parallax. As Hipparcos satellite has measured parallaxes with 1 milliarcsec accuracy of more than 105 stars.
4.Mass of stars -one of its most fundamental properties can 'only' be measured indirectly by using binary stars (spectroscopic binaries)
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| Spectroscopic binaries |
As you might have noticed, all of these( Mass, Temperature , Rotation Velocity, Distance etc.) are only surface properties. Thus, we need to build a theory of Stellar Evolution to derive internal properties of stars as observational techniques seem to fail in that !?
Well Game isn't over yet ! Observation always provides astronomers a window to interior of stars like -
1. Neutrinos: which escape from interior of stars without any interaction. But neutrinos interact little with matter regardless of energy. Moreover, beyond certain temperature, there is a decrease in relative flux of neutrinos
2. Oscillations: Yes, I am talking about Seismology here. Stars are musical instruments. You can refer my blogpost on Helioseismology to know more. Here is a brief - The surfaces of stars oscillate with a particular time period and this can give us valuable information about size, age and mass of stars
Why we need a theory for stellar structure and evolution when we can just decode information from observation?
It is true that Astronomical observations can yield information about important stellar parameters. But these are like snapshots of the life of star as timespan of these observations is much smaller than the age of stars. Thus, any of these observations cannot give us a complete picture of Stellar Evolution.
Moreover, a theory is also need to explain some of the most important results of Astronomy such as mass - luminosity relationship and mass - radius relationship that we get from HR Diagram of stars. (I am going to cover HR Diagram in my future Blogs so don't worry about that :)
Thus overall, we see that the observational techniques we use cannot provide us with 'all' of the necessary information about stars.
Thus a theory is needed to explain Stellar evolution and results of Stellar Observation.
In the next blog I will cover -
The basic assumptions of theory of Stellar Structure and Evolution
Accuracy of Assumptions of theory of Stellar Structure and Evolution
stellar evolution
