Saturday, March 16, 2024

ASTRONOMERS DISCOVERED THE BRIGHTEST OBJECT IN THE UNIVERSE ! A QUASAR 500 TRILLION TIMES BRIGHTER THAN OUR SUN!

Imagine a cosmic light source so powerful it could bathe the entire observable universe in its glow, its brilliance reaching us after billions of years of travel through the inky blackness of space. Believe it or not, astronomers haven't just dreamed this up – they've actually found it! This celestial object is a quasar named J0529-4351, boasting a luminosity that's a mind-boggling 500 trillion times greater than our very own Sun!


                                                            Source : NASA Images



Cracking the Quasar Code


Quasars, short for quasi-stellar objects, are the energetic powerhouses at the cores of distant galaxies (see Fig.1). These powerhouses are fueled by supermassive black holes, millions to billions of times more massive than our Sun. As these gravitational giants eat up surrounding gas and dust, a swirling disk of superheated material forms around them. The intense friction within this disk releases tremendous amounts of energy across the entire electromagnetic spectrum, making quasars some of the brightest objects in the cosmos.


J0529-4351: A Feeding Frenzy on a Cosmic Scale


The newly discovered J0529-4351 exemplifies this phenomenon on a scale unlike anything ever witnessed before. Situated 12 billion light-years away (remember, a light-year is a whopping 5.8 trillion miles!), the light we see from J0529-4351 has been traveling for an incomprehensible amount of time before reaching our telescopes.

The Black Hole Behind the Brilliance





The secret behind J0529-4351's extraordinary light show lies in its central supermassive black hole. According to a study published in the prestigious journal Nature Astronomy, this black hole devours a sun's worth of material every single day! This phenomenal rate of growth fuels the quasar's immense light output, making it the fastest-growing black hole ever observed.

A Hidden Colossus Emerges


Source : Eso.org


The discovery of J0529-4351 is particularly intriguing because it remained undetected for so long. It was first spotted back in 1980 by the European Southern Observatory's Very Large Telescope (VLT) during a sky survey, but it was mistakenly classified as just another star ! It wasn't until recent observations led by an Australian National University team using a 2.3-meter telescope and subsequent confirmation by ESO's VLT that J0529-4351's true nature as a quasar was revealed
. This hidden giant, despite being one of the brightest objects in the sky, managed to evade our detection for decades!


A Cosmic Time Capsule






Studying quasars is like peering back in time. Since their light takes billions of years to reach Earth, these objects offer a glimpse into the early universe, a time when galaxies were just starting to take shape. J0529-4351 acts as a unique time capsule, allowing us to witness the processes that led to the birth and evolution of supermassive black holes and galaxies.

Pushing the Boundaries of Knowledge



This record-breaking quasar presents a fascinating puzzle for astronomers. Its extreme luminosity and environment push the very limits of our current understanding of black hole physics and galaxy formation. Studying J0529-4351 might unlock secrets about these extreme environments, leading to breakthroughs in our comprehension of the universe's most powerful objects.

The discovery of J0529-4351 serves as a reminder of the universe's hidden wonders and our ongoing quest to unravel its mysteries. With advancements in technology and our insatiable curiosity, who knows what other incredible cosmic behemoths await to be unearthed in the vast expanse of space? This discovery is a stepping stone on the path to a deeper understanding of the universe's grand story.

Wednesday, April 13, 2022

Living In International Space Station

How do astronauts live in this Space-House?
Is there any difference in the day to day activities they perform ?
What do they eat? Can they eat Pizza ?
How do they sleep?
...
..

WHAT IS THE ISS

The ISS or International Space Station is the third brightest object in the night sky and clearly visible when there’s no cloud cover. It moves much faster than anything else you are likely to see up there, too. While a typical aeroplane travels at around 965 km/h, the ISS moves at 28,000 km/h. Unlike a plane, there are no flashing lights on the ISS and it travels in a perfectly straight line. A cool thing about ISS that makes it different from Earth is that gravity over there is approximately 0. 

How is Gravity Almost 0 on the ISS ?



Figure 1: The path of the ball being thrown changes with increasing the initial speed.


Let's consider a thought experiment- Let's suppose you are throwing a ball on the surface of Earth. It will take path like path A mentioned in the figure 1, and afterwards, will fall on ground due to gravity. If you can keep a ball throwing hard enough, it will take more and more curved paths until eventually the it will start to orbit the planet ( path C,D). What's happening now ? The ball is still constantly falling towards the ground but is also moving side wards fast enough to miss it. Thus, ball or similarly ISS is in orbit around the earth.

Now, let's consider another experiment - Take a rock and a feather, hold them at a height and note the time each of them takes to hit the ground. As also common sense will suggest, Rock will hit the ground first, thus it falls faster. Now take a vacuum chamber and then repeat the same experiment. Now, you will notice that despite of the mass difference between them, both the rock and feather hit the ground at the same time. It was resistance due to air that was slowing down feather in initial setup! 
Damn! What you just did is the experimentation of one of the central laws of classical physics given by legend Galileo - All Objects of different masses fall at the same speed . Thus, both the space station and the astronauts are in falling or as we discussed above are in 'free fall together', which creates the notion of weightlessness. This weightlessness is essentially responsible for making the life of space station different from Earth.

Even the most basic activities like eating, sleeping, writing and even pooping are affected by it. How ? Let's have a look at them one by one -

1. Writing in Space


 Writing isn't as simple as it looks. Whenever you write something using your pen, it's the force of gravity that is responsible for pushing the ink downwards.( Sounds unbelievable ? Give it a try yourself by writing with tip of pen upwards ;) ) In ISS, there is essentially no gravity so normal pens don't work there, thus initially Astronauts used Pencils to write. But using pencils is kind of unsafe as tips can easily break and drift freely in ISS threatening other sensitive equipments. Moreover, pencils are also flammable, a quality that NASA wanted to avoid after Apollo 1 disaster. 

Figure 2: Fisher's Space Pen

Writing Sounds like a problem now, but not to Paul Fisher, a private investor who reportedly spent 1 million dollars all by himself to create a 'Space Pen' all by his funding. This pen can write in essentially 0 gravity, in extreme temperature limits( -50 to 400 ° F) and even underwater. Instead of gravity the cartridge of this pen is pressurised with Nitrogen at 35 pounds per square inch. The ink also stays gel-like until movement of ball point pen turns it into fluid. In February 1968, these pens were first ordered by NASA ( at $2.39 per pen) and since then astronauts are using this Anti-Gravity pen for writing. Fun fact : According to the Fisher Space Pen Company, the Apollo 11 astronauts also used the pen to fix a broken arming switch, enabling their return to Earth. 


 2. Eating in Space


Figure 3 : Space Food

Eating is less trickier than writing. Swallowing and digestion are possible in space. Free-floating droplets of water can be problematic as they could find their way into sensitive equipment. The same is true of small particles too, and that has implications for eating in space. Food for Astronauts is prepared at Space Food Systems Laboratory at Johnson Space Centre in Houston, Texas. Earlier, Astronauts used to consume food from tubes and in form of dry cubes. Now, the variety and quality has increased as astronauts get to select the meals they like. Currently the variety of food ranges from a fruit salad to spaghetti, from Chinese to Swedish. The lab creates food ensuring peak health and performance for the astronauts. 

Pizza Party in Space
Credit: nasa.gov

The procedure of preparation involves a 'freeze - drying process'. The food is first frozen to -40° F. Then it is put into vacuum chamber and heated so that water content of the food sublimates. This process is repeated multiple times to remove almost all the water content. Exposing cans or pouches to extreme heat and temperature, by a process called thermostablisation (heat treatment) is also applied to make the shelf life of food longer. All these processes aim to prevent the bacteria from multiplying and spoiling the food along with drastically reducing the weight of food, allowing a lighter payload. Space food normally comes in plastic packaging or cans. Tapes, magnets are also used to attach items to surface. Eating liquid is easy as it remains attached to the surface (due to surface tension ). Water used for re-hydration of food is available in pouches.

After eating, they use disinfectant wipes or apply some soap and water solution which is wiped out using a towel.  
 
 

3. Defecating in Space


 On earth, the force of gravity pushes the waste away from the body which is not the case in space and that is what exactly makes using bathroom in space a tricky business. Any loose drops or dribbles can float around. Thus, the toilets today rely on suction techniques. 

Figure 4: Space Toilet
Credit: nasa.gov


The space is known as bathroom and hygiene compartment. For peeing, astronauts use a funnel ( yellow coloured in Fig 3) attached to a hose that uses a fan to pull the urine into a tank. Urine is normally recycled to be used again. Today's pee is tomorrow's coffee. 

For defecating, there is a toilet which consists of a seat and a container below it which holds about 30 deposits. Astronauts poop by sitting on toilet that relies on the same fan to pull their business. The seat is about 5-6 inches in diameter and there is a plastic bag there where the deposit goes. When done, they seal the bag, push it down the container and install a new one. The solid waste is finally sent back to earth where it burns in atmosphere. 

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.nasa.gov%2Ffeature%2Fboldly-go-nasa-s-new-space-toilet-offers-more-comfort-improved-efficiency-for-deep-space&psig=AOvVaw1c3Uaz9hiF15i7ihj9bqk1&ust=1648152901359000&source=images&cd=vfe&ved=0CAsQjRxqFwoTCPC4oZWG3fYCFQAAAAAdAAAAABAD
Figure 5 : NASA's New Space Toilet 
Credit : nasa.gov


However, in the view of the new Artemis 2 mission, which will go to the moon, NASA wanted a new toilet which is more compact and efficient. For this, a brand new space toilet - Universal Water Management System has been built and recently sent to space station for testing. This has been installed next to the adjacent one.

This new space toilet that is more energy efficient, roughly 65 percent smaller and 40 percent lighter than the ones currently in use. In many ways, the new toilet essentially works the same way as its predecessors. It also deals well with 'dual ops' as it allows astronauts to poop and pee at the same time by having urine funnel next to the seat. It also features a 3D-printed titanium dual fan separator, that creates a strong air flow which helps to pull the astronauts’ urine and waste into the toilet. It also has a built-in system that pre-treats urine before it's passed off to the station’s life-support system. Urine can occasionally contain solid material that gets stuck inside the toilet, to mitigate that, there is a highly acidic solution to break down any deposits that might be in the urine. 

4. Sleeping in Space 


Figure 6: Sleeping in space


Astronauts in ISS get 16 sunrises and sunsets in a day, so to schedule the time of sleeping they use cues from the Earth. Sleeping is important as it regulates proper body functioning. For sleeping, astronauts are allotted crew quarters. They are about the size of refrigerator and unlike the space station, it is quite there. Switches regulate the mode of light ( dim, bright, off ) in quarters. For sleeping, there is a sleeping bag in which they sleeping just standing on with the help of chords attached to the bag which keep astronauts attached to the wall, so that they do not break the instruments, or be affected by sir currents on ISS in general. 


This was a mini tour of some of the most basic activities on ISS. Clearly, living in ISS is dominated by the weightlessness. In addition to the complex science experiments that take place there, simple human like activities on ISS also feature high technological requirements and advancements. "Ideally, the ISS program is an incremental step on an expanding, incredible journal of exploration and understanding, taking us higher and farther"






Fun fact

Uber Eats makes its first food delivery to space in December 2021

The delivery to the ISS included boiled mackerel in miso and simmered chicken with bamboo shoots. 

And and, if interested 

You can also order Space food from here :

https://www.shopnasa.com/collections/space-food






Sunday, October 3, 2021

Stellar Structure And Evolution: Part 2: Basic Assumptions and Accuracy of Assumptions

In the previous blog I explained why do we need a theory of Stellar Structure and Evolution when we can get information about stars by just observing them. In this blog I will cover-

1. The basic assumptions of theory of Stellar Structure and Evolution

2. Accuracy of Assumptions of theory of Stellar Structure and Evolution


To understand above mentioned aims more clearly, let's first understand( answer these questions - that I generally don't see many writers stress about:


What are the assumptions for a theory ?

Assumptions are basically the foundation stones for a theory. These are taken as the postulates that are generally assumed to be true throughout the explanation.

 


Now, What is the need for assumptions to a theory?

-First of all, because they are reasonable - they offer observational or at least mathematical verification.

-- Secondly, Making assumptions simplify our work in terms of crucial/critical understanding and mathematics.

- Lastly, although it is harsh but true: We only have limited information about things. Thus, as long as these are not true in general, we choose not to include them (esp while teaching) instead of including it with ad-hoc suppositions that lack any observational verification.


Now that we know what are assumptions and why do we need them for building a theory let's see what are the basic assumptions for the theory of Stellar Structure and Evolution


1. Stars are isolated in space - This is a fairly reasonable assumption for most of the single stars in galaxies as this condition is satisfied to a high degree - compare the distance of sun to its nearest star Proxima Centauri. We are ignoring binary stars and stars in dense clusters.

2.Stars are formed with a homogeneous composition- it is again reasonable as the clouds from which stars are formed are well - mixed.

3.Stars have no magnetic field - This is fully reasonable as for most of the stars magnetism plays a notable role only in phenomena related to surface of stars but in overall life cycle they don't play any significant role.

Stellar evolution is fully determined by internal physical processes which take deep inside the star near it's core.

4. Stars are rotating slowly- This one is a lot harder to justify as most of the stars rotate at a considerable fraction of their critical velocity(*1). Since we do not have a theory that shows how Stellar interior rotates at the birth of the star and making this assumption causes a huge mathematical simplification we are going to hold it.

5. Stars are in mechanical equilibrium(*2) : Majority of stars are in such long lived phases of their evolution that no structural changes are observed for them for most period of times. This implies that there is no noticeable acceleration and all the forces balance each other perfectly.

For an isolated, slowly rotating, homogeneous composition star with no magnetic field these forces are gravity and pressure. Thus, all the stars are in hydrostatic equilibrium.

All of the above mentioned assumptions can be tested just by testing for accuracy of hydrostatic and spherical symmetry assumptions.

Accuracy of Hydrostatic Assumption -

Let's first understand the equation of Hydrostatic support using simplest Newtonian dynamics

Balance between gravity and pressure is called hydrostatic equilibrium.

For a given time t, let's consider a spherical mass shell with infinitesimal thickness δr at a distance r from the centre. 

 



Mass of the element δm at this distance is δm= ρ(r)δs δr

ρ(r) = density at radius r


Outward force = pressure exerted by stellar material on lower surface

P(r)δs

Inward force on mass element= Pressure exerted by stellar material on upper surface and gravitational attraction of all stellar material lying within r

P(r+δr)δs+ GM(r)δm/r2 = P(r+δr)δs+ GM(r)ρ(r)δs δr/r2


For hydrostatic equilibrium, inward force= outward force

P(r)δs=P(r+δr)δs+ GM(r)ρ(r)δs δr/r2

so, P(r+δr)-P(r)= GM(r)ρ(r)δr/r2

 For infinitesimal element:

P(r+δr)-P(r)/δr = dP(r)/dr

Thus, dP(r)/dr=-GM(r)ρ(r)/r2

which is the equation of Hydrostatic support.


Accuracy of hydrostatic assumption

To answer how valid is that assumption let's consider a situation where inward force and outward force aren't equal which gives rise to acceleration a.


P(r+δr)δs+GM(r)ρ(r)δs δr/r2 -P(r)δs = ρ(r) δs δr a

»dP(r)/dr +GM(r)ρ(r)= ρ(r) a

 

this is the generalized form of equation of hydrostatic support.


Now consider there is a resultant force on element with the sum being a small fraction of gravitational term(β)

Inward acceleration a = β.g

Spatial displacement from rest after time t = d= 0.5.a.t2 =0.5.β.g.t2


If if allow star to collapse or expand, by setting d=r we obtain

t=(2.r3/G.M.β)1/2

Assuming beta =1 we obtain

t=(2.r3/G.M)1/2


This is the dynamical time scale of the star.

Of course each mass shell will be accelerated at different rate so this should be taken as an average value for star to collapse at radius R.

Since average density is we can also write this t to be

½.(G.ρ)1/2

For sun we obtain a value of 1600 sec or about half an hour. Thus, any significant departure from hydrostatic equilibrium should lead very quickly to an observable phenomenon (sudden collapse or explosion of the star ) . But age of sun is already


This is much smaller than the age of sun - 10^17 secs - by 14 orders of magnitude. Thus if this assumption have been wrong we would have noticed a significant collapse or explosion of sun much earlier.

This assumption is very much accurate.


Accuracy of spherical symmetry assumption


Let's see if average rotation rate of stars is causing significant departure from spherical symmetry.

Consider a star of mass M and radius R with an element of mass δm near the surface of the star.


star of mass M and radius R with an element of mass δm near the surface rotating at angular speed w

Gravity supplies the extra centripetal force to make the object move around in a circular path.

Thus, for mass δm, centripetal force = gravitational force


There will be a departure from spherical symmetry if there will be any difference between gravitational and centripetal force i.e.


mω2r)/(GMδm/r2) <<1

or ω2<<GM/r3

note RHS of last equation is similar to t

>> ω2<<2/t2 (ω=2π/T) where T =rotation period

 for spherical symmetry to be valid T>>t

for example, for sun t~2000 sec and T~21 month


Thus, for majority of stars, departures from spherical symmetry can be ignored.

With this we complete the basic assumptions of theory of Stellar Structure and Evolution and see that these assumptions are qualified to build a theory of Stellar Structure and Evolution.

 

 

*1.Definition of Critical velocity : Stars have a maximum speed at which they can spin. If stars exceed the critical rotation, the outward force caused by their spinning will overcome the inward gravitational force that keeps the star together.

If stars get to that limit, they will begin to fly apart.

*2. Mechanical equilibrium : A state of rest or unaccelerated motion in which sum of all the forces acting on a particle is 0. In case of stars, this state is reached when pressure forces are balanced by gravity. In astronomy, this is called hydrostatic equilibrium.



Wednesday, August 18, 2021

Stellar Structure and Evolution Part : 1

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.


                                         


3. Spectroscopy (Spectroscopy is the study of the interaction between matter and electromagnetic radiation as a function of the wavelength or frequency of the radiation.) at sufficiently high resolution gives detailed information about the physical conditions in the atmosphere. With detailed spectral-line analysis using stellar atmosphere models one can determine the photospheric properties of a star ( like effective temperature, surface gravity , rotation velocity etc.)
 

4.Mass of stars -one of its most fundamental properties can 'only' be measured indirectly by using binary stars (spectroscopic binaries)

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


Monday, July 26, 2021

How to destroy a Cluster of Stars/ Globular Cluster

 This blog includes - 

1.1 What are Globular clusters ?

1.2 What are Binary Stars ?

1.3 What is escape velocity ?

1.4 How to Destroy a Globular Cluster ?

1.5 Summary of Topic 

In this blog we will destroy a Cluster of Stars called Globular Cluster!

The main focus of this Blog is to make you understand how you can destroy a Globular cluster for which we will run from 1.1 to 1.3 to gather all the material to understand the topic. If you know about all of this already you can directly skip to section 1.4 to know the real tea 🍵

1.1) What are Globular clusters ? 

The name is derived by Latin word - Globulus - which means a small sphere (they are really big though- approximate size of a Globular cluster is - 300 light years) 

Globular clusters are a group of stars all formed at approximately same time and held together by gravity. ( Fig 1) They are called 'museums of stars' as they have held the stars intact since they were formed and thus help astronomers in studying age and properties of various stars( Bonus : As the stars in clusters are all formed at same time, the globular clusters help in comparing the properties of different mass stars formed at approximately same time, which is an excellent opportunity for astronomers ! ) 

fig 1

GLOBULAR CLUSTER

Although they look dense, collisions between stars in globular clusters are very rare as distances between individual stars are greater than approximate distance between stars. Moreover,  velocities of stars at any point inside the cluster are fairly random, in direction as well as size. The stars form what is called a 'collision less gas'.

Soon we will see how to boil away or destroy these museums of stars 😈


1.2) What are Binary Stars ? 

A binary system consists of two stars orbiting each other around a common centre of mass.

BINARY STARS

Given the stars in a binary system are themselves spherical, both of their orbits will be ellipses, just as for planets in the Solar System ( Bonus : for a spherical star system, the gravitational field is as such as if all of the mass is concentrated in center, when this is not the case - like when stars come close to each other tidal forces might deform the shape and resultant orbits will be complex then )



Escape velocity from a binary system : 

Let's consider a binary system of two stars orbiting around a common center of mass as shown below

Two binary stars (in purple) in orbit around a common center of mass

For simplicity, let's consider orbits to be circular and masses of both stars to be equal = M

Radius of circle = R 

In order for one of star to escape from gravitational field of other star, the kinetic energy of star should become equal the gravitational potential of the other star.

(1/2)*m*v^2 = (G*M*M)/(2*R)  (Kinetic energy = potential energy)

So, v= sqrt(GM/R)

Therefore, energy of 1 star when it has this speed

is 1/2 * m *v^2 = (G*M*M)/(2*R)   ------------ (1) 

We need to learn only this much about binary stars to understand today's topic :)


1.3) What is escape velocity ? 

The minimum velocity which is needed by a non- propelled body to escape from gravitational field of another object is called it's escape velocity. 



Formula - sqrt( 2*GM/ R ) where,  ----------(2)

G - Gravitational Constant = 6.674 x10−11 m3⋅kg−1⋅s−2

M = Mass of object which is to be escaped 

R = Radius of object which is to be escaped

Now that we are equipped with all the necessary tools to destroy a Globular cluster let's dive into it ! 


1.4) How to destroy a Globular cluster?


Let's understand what is meant by destruction of a Globular cluster. It means kicking out every star from the cluster so that it no longer remains a Globular cluster. 

Here's is the recipe, let's do it together 

Step 1. Let's calculate how much energy is needed to kick out stars from a Globular cluster?

 To do this let's take a sample cluster. 

* Mass of cluster Mcl      = Mass of 10^(6) stars, where avg. mass of 1 star = 1 Mo ( 1 solar mass ) 

* Radius of Cluster Rcl   = 50 parsecs ( pc),where 1 parsec = 3.086e+13 km 

* Escape velocity for a single star =  v_escape = sqrt( 2*G*Mcl/ Rcl ) ~ 13 km/s  ----from(2)

* Escape energy for a single star, 

   E_escape = 0.5*mass of 1 star * (v_escape)^2

                   = 2*(10^38) Joules

* Escape or destruction energy for whole cluster! 

   E_escapecluster = 

   Escape energy for a single star × total number of stars =2 × 10^38 × 10^6 

                                                                                          = 2 × 10^44 Joules


To destroy whole cluster, we need to provide 2 × 10^44 Joules energy to it. But how can we do it ?

Answer: Step 2: By forming binary pairs of stars!


Formation of a binary pair releases energy, as the two stars become bound. The energy required to form a binary pair is the same that is required to split it again. 

Thus, From (1) we know that energy released when a binary pair is formed is - (G*M*M)/(2*R)

 Suppose the stars each have one solar mass and are separated by 

the radius of a white dwarf, about 5 × 10^6 m.  Formation of such a system releases energy equivalent to 3 × 10^43 Joules.( using (1) )

Wow! only 10% of the binding energy of the cluster is provided by only one binary system. The formation of a handful of such systems could easily provide enough energy to expand the cluster or even disrupt it!!!

The energy released in this way goes to the third star by Newton's third law ( Newton's Third Law: every action has equal and opposite reaction)  and the third star now has so much energy that it simply shoots straight out of the cluster. ( This is not out of some magic, when two stars form a binary pair energy is released in a same way it is released when a particle is falling onto the star. If third star were not present, they couldn't form a binary pair ) - see the last section of this Blog ( * )to understand more accurately how the energy is actually transferred :)



Finally! We have successfully destroyed the Globular Cluster


Summary of Topic 

Globular clusters are a group of stars all formed at approximately same time and held together by gravity.

Destruction of cluster is possible by formation of a handful of binary pairs. The energy released in this way goes to the third star by Newton's third law and the third star now has so much energy that it simply shoots straight out of the cluster.

Thus one by one, shooting the stars out we can completely destroy the Globular Cluster !

___________________________________________________

                                     ( *)

(Let's take 3 stars in a Globular cluster with masses say 1, 2 and 3 solar mass. We arrange them in mercury - sun distance and give them some small initial velocity so that no star has energy to escape the three - body system. After some time, two stars become bound and the third one is expelled. 

Now how does that happen? 

The answer is in gravitational energy.When two stars become more tightly bound, they release energy, just as a particle falling onto a star releases energy . This energy goes into the motion of the third star by Newton’s third law which says that every action has equal and opposite reaction. By this law, the force exerted on the pair by the third star is exerted back on the third star by the pair: which results in binding the pair more tightly and expelling the third. If the third star were not present, the first two could not form a tight binary pair: they would fall towards one another and then recede to the same distance. )

Thursday, June 24, 2021

Dark Matter Slowing Down Milky Way

Topics of this Blog include- 

1.1- A brief about Milky Way Spiral Galaxy and Dark Matter 

1.2- Details about Topic -Dark Matter Slowing down Milky Way

1.3 - Conclusion and Learnings from this discovery 

1.4 - Summary of Topic



1.1) A brief about Milky Way galaxy and Dark Matter

       Milky Way Galaxy  

Milky way ( or Akashganga ) is our home galaxy. Approximately two-thirds of all spiral galaxies contain a bar, so does our galaxy - It is a barred spiral galaxy which means that it has a central bar with spiral body structure ( Fig. 1and 2) 

Fig 1


Fig 2
                                                                      

This bar-shaped core region is surrounded by disk of gas, dust and stars


       Dark matter 

- To know more about dark matter you can refer to this link : Dark matter 

Dark matter constitutes 27 % of content of whole universe. It is made up of 'something' that cannot be detected by electromagnetic force ( basically by touch or sight ). It's gravitational effects are responsible for flattening of velocity vs radius curves in the outer regions of galaxies.  According to theories It forms a spherical halo around the galaxy. 



1.2) Details about Topic - How dark matter is slowing down Milky Way

"Astrophysicists have long suspected that the spinning bar at the center of our galaxy is slowing down, but we have found the first evidence of this happening," study co-author Ralph Schoenrich, in a statement.

Galaxy models have long predicted that galactic bars slow down by losing angular momentum to their postulated dark haloes.

Hercules stream is a group of stars currently rotating away from center of Milky Way. This stream is in resonance with the central bar - i.e. it is gravitationally trapped by the central bar and thus, revolves around at the same rate as bar's spin ( like Jupiter's Trojan ) 

If the bar's rotation slows, the stars in the stream move outward to have orbit in sync with rotation of bar. Researchers investigated the chemical composition of stars in Hercules stream and found them to be rich in heavier elements which is possible only when they have been formed close to the center of Galaxy.

Thus they have been sweeping out of the center of galaxy which implies that galactic rotation has been slowing down and has slowed down by 24%.

 The one possible and seemingly correct explanation is that dark matter has been slowing down the spin by dynamical friction. Loss of the bar's angular momentum has been attributed to dark matter.

This possible explanation, although seeming the most correct till now has run into rivals.  Schoenrich said "Our finding also poses a major problem for alternative gravity theories — as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar ". There are also alternating theories of gravity which propose tweaks in Einstein's relativity and disregard dark matter possibility. But Einstein's theory has passed all of the tests till now ( Our GPS works on basis of that, the famous black hole picture that we have obtained is also consistent with his theory ) 


1.3) Conclusion and Learnings from this discovery 

• This model supports the presence of Dark Matter, while physical detection is yet to be done.

•It again put a shade on alternating theory of gravity and dark matter. ( Like MOND )

• We have got new constraints on galactic history and the unique opportunity to differentiate between different dark matter models.

• Due to position of  Sun’s position far from the center we currently see only the outer region of the resonance of spin of bar and Hercules stream . By performing the analysis at a spatial coordinate closer to the Lagrange points, we could probe deeper into the inner region of the resonance, where we may find traces of events that happened in the early epoch of bar formation and also determine the size of the initial core of the resonance which stems from the formation of the bar. However author mentions that this will be possible in the future with extended data covering the full range of resonance and a proper chemo-dynamical model predicting the age-dependent effects. 


1.4) Summary of Topic 

Analysing data from Gaia, a European Space Agency Mission to map position of stars in milky way, researchers at University College London (UCL) and the University of Oxford have shown that the spin of the Milky Way's galactic bar has slowed by about 24 % or a quarter since its formation. Most plausible explanation is Dark matter acts as a counterweight slowing the spin. 



Also Check :

Milky way : https://en.m.wikipedia.org/wiki/Milky_Way

Dark matter: https://en.wikipedia.org/wiki/Dark_matter



Sunday, September 20, 2020

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.

source: wikipedia
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.


SOURCE: https://youtu.be/wqwGljLDcjM



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 . 

SOURCE: https://youtu.be/wqwGljLDcjM



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!"