HELIOSEISMOLOGY : SOUND OF SUN

Listen to the sound of sun( shifted in audible range)- click here 

Source - nasa.gov

"The Sun is playing a secret melody, hidden inside itself, that produces a widespread throbbing motion of its surface. The sounds are coursing through the Sun's interior, causing the entire globe, or parts of it, to move in and out, slowly and rhythmically like the regular rise and fall of tides in a bay of a beating heart." - Kenneth R. Lang

Study of stars is a tedious task because you have only a beam of light to decode everything. What what if I say that we are learning about the stars , or I should say the whole universe by listening to the

 'Star - Song ' !
Welcome to the world of Asteroseismology .
Astro - Relating to space and
Seismology -Oscillations in stars
It is a scientific word for the studying Stars by listening to it's heartbeat .
In this blog we are going to talk about Helioseismology . That is , the study of sun by listening to the surface oscillations 

Why sun 🌞? 

Because it is the only closest star to us . We can easily get more information from Sun than what we can get from very distant stars - our lives depend on it.
We are building a model for sun first and then we will extend it to other stars - Logical Approach .

Before starting with Helioseismology , let's me share some important information about interior of Sun:

Sun is not as peaceful as it looks . Home of complex magnetic field, source of solar flares and solar winds💥, (capable of disrupting communication, and generating auroras here on earth ) this hot ball of plasma is 27 million Fahrenheit🌡in the core and 10,000 Fahrenheit on the surface. It's a dense body , with density around 100 g / cm^3 ( the density of rock is about 5 g / cm^3) .

Moving out of the core , we have radiative zone - which can be thought about as inserting an individual in a room jam-packed with people . The person moved in will transfer his energy to the other person , and that person to others in room . This chain of energy transfer will continue taking place among people in the room , but no one is going to move very much . Similarly , in radiative zone , we have energy moving throughout the zone , with very little movement of actual material.

Then we have Convective Zone - Where the bulk motion of material takes place . The hot material rises up , cools and sinks back. It generates magnetic field.
Hydrogen is being stably burnt in the core since the past 4.6 billion years and the process will continue till coming 5 billion years , after which the star will start burning Helium to Carbon . We know all of this about Sun but , for far away distance , if we take two stars , one that has ignited Helium and one that has not , they can look exactly same on the surface - same temp , luminosity , colour , etc . That is very annoying for astronomers , for they might predict the phenomenon but it won't be visible in these observations .

This shows that we aren't fully aware of all the information we need to describe a star
Let's look at how do we know the internal structure of earth ?

 We came to know about it by studying earthquakes or seismology .When an earthquake hits an area that earthquake injects some energy that is measured using seismographs placed at specific locations to detect them . Earthquake waves use different paths and reach the detector traveling through different layers of planet . Once we know about the time and path , we can decode the interior structure of Earth. We use the similar procedure for Sun , but we don't place seismographs on Sun ( we cannot of course) ,and we don't need too . The observation of surface of sun is enough .

Stars are the musical instruments 🎸- a set of vibrations gives rise to a particular sound .Solar oscillations first came into notice in 1961 when the observations by Mt. Wilson Observatory discovered that the Surface of Sun is slowly puffing with a period of about 5 mins . Material in the star is getting compressed and rarefied , hotter and cooler as the temperature changes , so you get intensity variations , which lead to change in the brightness of star. This flickering is so subtle (a few parts per million) that it is barely visible to naked eye. 

Vibration in stars is similar to vibration of cowbells . As big bells vibrate with deep low frequency , so do the big stars and opposite is true for small stars. So, knowing the frequency of vibration can help you figure out the size of the star. Isn't that amazing? 

Calculating the frequency was not an easy task . These few parts per million fluctuations aren't easily detectable .Hold on! Kepler satellite had got a role to play here . The satellite which was originally meant to find earth like planets near sun like system required long time observation of a patch of sky(4 years)with the precision of a few parts per million to detect the passing of planet in front of the star by detecting the change in the brightness of the star . This made the long time high precision measurement of the star possible and hence , we were able to detect the oscillations. 

Lets take a closer look into the oscillations:

It can be shown that the oscillations are separated into two categories: interior oscillations and a special category of surface oscillations. These are called p(pressure, g(gravity) and f(surface gravity modes)

p modes: 

The dominant restoring force in this oscillation is pressure. Pressure modes are basically standing sound waves. Their energy densities vary with radius inversely proportional to the speed of sound, so their resonant frequencies are determined predominantly by the outer regions of the Sun.

Sun is vibrating in a superposition of acoustic normal modes (like the patterns with which a guitar string vibrates, but for a spherical body rather than a string). The period of oscillation is about 5 minutes. The motions of these were originally believed to be due to turbulent convention in solar atmosphere. Later on , it was discovered the the phenomenon was global and are manifestation at the solar surface of resonant waves(pressure waves). About 10⁷ distinct( p modes ) are thought to be excited.

g- modes:

For these type of oscillations , restoring force is predominantly gravity. They are envanescent in the convection zone and due to their tiny energy, they are very difficult to detect. So, no g mode has been directly measured although claims about indirect interactions have been made .

The measurement of even just a few g modes could substantially increase our knowledge of the deep interior of the Sun.

f-waves:

It is essentially a surface mode and it can be expected to provide a diagonistic of flows and magnetic fields present in surface regions.

Illustration of a solar pressure mode (p mode) with radial order n=14, angular degree l=20 and azimuthal order m=16. The surface shows the corresponding spherical harmonic. The interior shows the radial displacement computed using a standard solar model. Note that the increase in the speed of sound as waves approach the center of the sun causes a corresponding increase in the acoustic wavelength.

So, Studying these oscillations or fingerprint of stars can gives us titanic amount of information let's see how :

1. These stars look very different depending upon how they flicker. For big stars , these pulsations are slow and amplitude is big , where as for the small stars, these pulsations are crowded, fast and amplitude is small. So, by clearly looking at these oscillations you can figure out that this buddy is big and this is small. We don't analyse the data as such , we take its Fourier transform- figuring out the sinusoids that the curve is made up  of and then we have a diagram called power spectrum.

2. This is known as the power spectrum of the star- making the typical frequency at which the star oscillates. It won't be wrong if we call this the fingerprint? ultrasound of the star . This curve carries an immense amount of information. Looking at the peaks , and the spacings, mass and size of the star can be predicted. Looking at subtle spacings , you can tell what's going on in the core of the star , how much H it has burnt into Helium , what is it's age . 

These oscillations have helped us in giving answer to following questions:

How big the star is 
How massive it is 

Has it started He burning?

How old a star is?

Are our models correct?

This is an astounding amount of information with such an ease .

Studying these oscillations opens new gateways to study astronomy with precision that is entirely unravelled.

These oscillations have made us sing -"Twinkle twinkle 🌟 little star , now I know what you are!"