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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
informative
ReplyDeleteAs this is going to be my subject in masters ... Indeed helpful šš
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DeleteExcellent presentation ma'am
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DeleteWonderful blog Astha...but you know to develop a theory about something, its effects need to be experienced and observed first. So when it comes to stars, our experience and observations are still at a very preliminary stage. I think to come to s stage of developing a theory or even s hypothesis,we need to observe, interact with and know about the stars a lot more than what we know today, and the only way to do that is through better and better observation.
ReplyDeleteDefinitely! Observations are crucial for our understanding. But yeah, being a physics student it often comes in mind why do we need Theory when we can have all the information by mere observation?!It's just to illustrate that observation and theory have a separate place in Astronomy and both are needed to have a full understanding of the subject :)
DeleteNo, we need a theory indeed as that's what pure physics comprises of. I mean everything we are discovering in the universe today came from theory.So we have our views about star formation and evolution as well...however they haven't evolved into full fledged thory primarily because it's still the early days in our understanding of the cosmos.But you are right, initial steps in this direction need to be taken sooner than later.
Deleteyes that's what I mean when I said that observation and theory have a separate place in Astronomy and both are needed to have a full understanding of the subject :)
DeleteAlso, it's not that everything we are discovering in the universe today came from theory
Theoretical and observational astrophysics/physics/science go hand in hand .For ex- we first observed some unexpected v vs r curves for galaxy which lead to development of the concept or theory of dark matter. While, Einstein first predicted about gravitational lensing and after that we observed it:)
Yes, and sometimes 'the obvious' is just there,waiting to be discovered. Like soon after gravitational lensing was theorised, Einstein was able to prove it by looking at the light from a star behind the Sun reaching the observer bending round the sun during a solar eclipse. So theory stated the obvious, and observation validated it. In case of stellar structure, i think the generic theory we have has been found to be adequate to proceed with more advanced theories like formation of black holes and different kinds of stars. No one has actually tried to prove it with that much of commitment maybe. Lack of resources, or motivation?
DeleteYes, in case of Stellar Structure some theoretical issues have been solved but are precisely discovered some are waiting for some genius insight- like Einstein had in his time - to remove this saturated curve of information( there is no lack of interest definitely.. Tons of articles /research papers are produced daily in an Attempt to solve the unsolved , we have arrived to a saturation like in Newton's time and we need a breakthrough)
DeleteComing to stellar Astrophysics-
This is a subject in which theory and observation have been intimately combined to build up an imposing edifice.Eddington ,who played a key role in establishing theoretical discipline of Stellar structure has imagined physicist on a cloud bound planet and who has never seen stars but makes theoretical predictions of stars on basis of observation. One day veil is removed and physicist is able to see through. Important trends in observational data were discovered before theory and as illustrated in blog provided a motivation for building a theory.( I portrayed it in more dramatic way like a war hehe but there is nothing literal like that- both theory and observations supplement each other ) . So first we will discuss some basic theoretical ideas and then discuss whether our result is confirmed by observations. It will be seen in future blogs that it is necessary to delve into deeper theory to have more complete picture
Thankfully we don't have to (and cannot) rely on observation of life of a single star to study Stellar evolution. As you brought out correctly most stellar changes occur too slowly to be detected within an average human life time. So the only way to understand how stars evolve is by observing numerous stars at various points in their lifetime (and thankfully there are many like that), and by simulating stellar structure using computer models. As you can see since this process is not seamless, theory has to step in to fill in these gaps. And that's the reason perhaps that the theory of stellar structure is not as pure as that of blackholes maybe....because we could see the stars before the theory was evolved.
DeleteYes! 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.
DeleteBut it's not like -'theory of stellar structure is not as pure as that of blackholes ' as theory of stellar structure and evolution also has concept of black holes, white dwarfs in it :)Although they both have different courses to cover and offer variety of grounds to study. Theory of black holes is more about space- time and takes general relativity and quantum mechanics into account while stellar structure and evolution focuses understanding structure and evolution of stars using laws of physics. A stellar evolution is a mathematical model( I will be covering equations in next blogs so stay tuned )that can be used to compute the evolution of a star from its formation until it becomes a remnant. The mass and chemical composition of the star are used as the inputs, and the luminosity and surface temperature are the only constraints.
So, I will say they both are entangled, have their own special characteristics and grounds to cover than separate or one being superior over other as proper understanding of one isn't possible without the knowledge of the other :)
Agree...looking forward to next blog from you. I like the way you've structured your thought process and put accross such complex issues in simplest manner possible. Kudos
DeleteThat was indeed a fruitful discussion! New blog is coming soon :) Thank you so much for your appreciation. I hope I was able to convey whatever was intended.
DeleteCheers !
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ReplyDeleteI really enjoyed The Astrosoul and Anil Singru's #astrodiscussion
ReplyDeleteKeep it up!
Amazing work @theastrosoul