The Star is Dead, Long Live the Star

The Star is Dead, Long Live the Star

Debi Prasad Choudhary

Los Angeles, 4/1/2014

All that is born must die. That is the ancient oriental wisdom. It is always observed for the living creatures on earth, but is true for the stars, galaxies and perhaps for the universe it self? These are big questions. We made considerable progress in past one hundred years to answer some of these questions. In fact, two Nobel Prizes have been awarded for understanding the dead stars!!

So what happens in the end of a star’s life? That was a question pondered by a young Indian student Subramanian Chandrasekhar in 1930 while travelling from Madras to London by boat. He used the newly discovered physical laws that govern the property of matter at very very small and very very large scales in extreme situations.

The energy produced at the center by converting hydrogen to helium flow out and exert pressure on the matter, which is countered by the pressure due to gravitational force during the main part of stars life that keeps it stable. Towards the end of its life, more hydrogen is burnt to produce more energy. As the energy flows out, the outer part of the star expands making the star a bigger sphere. These are called red giants, as they look redder due to relatively cooler surface temperature. Betelgeuse is an example of such a star.  Later, the material from the outer envelops blows away with varying speed. The outer most part is blown away at a slower speed compared to the inner part, which eventually overtakes. This process produces a shell like structure, called planetary nebula, as they can be seen through small telescopes as planets. 

As the outer layer becomes loosely bound to the star and ultimately blown out, the core of the star starts collapsing after the exhaustion of hydrogen to burn and become a dense. Typically a star like the size of sun reduces to an object of the size of earth and become white dwarf stars. They are hot and small, so look dimmer compared to their temperature. They are so dense that the weight of a teaspoon of material from such objects would be equal to the weight of a cruise ship!!

Chandrasekhar worked out the property of matter in such extreme situations. Following the discovery of the “particle” property of light waves by Einstein in 1905, in 1924 a French PhD student de Broglie postulated the wave nature of electrons in his 17-page thesis, which was awarded Nobel Prize. According to this, all particle of matter is associated with a wave, the wavelength of which decreases with its mass and velocity. Using the physics of “small world” or “quantum mechanics”, it becomes apparent that particles cannot be brought closer than their wavelength. When the matter is squeezed enough in a white dwarf stars, they come closer and the velocity increases approaching the velocity of light. The velocity of light has special significance, as nothing can cross this speed limit. As objects approach this speed limit, they become heavier and shorter and the time slows down!! Chandrasekhar showed that in such extreme conditions, if the mass of a star were 1.72 ´ (mass of the sun), the pressure due to the electron gas would play the role of radiation pressure for a normal star and stop the further collapse of the star. This is called Chandrasekhar limit, for which he received Noble Prize in 1983, a year after I joined for PhD and took few classes from him!!

I still remember his simple style of teaching complex mathematics using just chalk and blackboard!! Question may arise, why did it take more than 50 years to award Noble Prize to Dr. S. Chandrasekhar, professor of physics in University of Chicago? My understanding is that, observational verification of his theory was difficult and took a long time. One such star accompanies the brightest star in the sky called Sirius.  The stars like the sun and 3 to 4 times heavier, would die to create a Planetary Nebula, which would eventually disappear, but their core would survive for eternity as white dwarf!!

For heavier stars with masses in the range of 4 to 8 times the mass of the sun, the death is more violent. The electron gas cannot stop the collapse of the core. At the end of the life, the core would collapse suddenly such that it would be denser than the nucleus of an atom. As, that would not be sustainable; the core would immediately relax to a slight bigger size, so that a star like sun shrinks to the size of San Fernando Valley. This would send shock waves to blow away the outer envelop of the star at very high speed. The outer ejecta, known as supernova remnant, would shine an entire galaxy, brighter than 10 billion suns. Chinese astronomers recorded one such event in our galaxy in 1048 AD, which is known as Crab nebula. I witnessed such an event in 1987, in a nearby galaxy Large Magellanic cloud. Recently in Mid January 2014, one such event was observed in a nearby spiral galaxy M82. These ejecta fads away quickly depending on the death of a normal or a binary star with strange companions.

These ejecta prepare elements heaver that iron. We are made out of these elements, so we are star stuff!!

If the core of the dead star is below 5.6 times the mass of the sun and squeezed to a size of 10 to 15 km in size, the matter would be so dense that all of them would be in the form of neutrons. At this stage the neutrons would resist further collapse, similar to the electrons in case of white dwarfs. Neutrons are neutral particles inside the nucleus of an atom and play very important role in nuclear reaction that produce energy in power plants. After receiving a telegram about their discovery by Chadwick in 1932 at University of Cambridge, Lev Landau of USSR immediately stated that there must be neutron stars. Two years later Caltech astronomers Walter Baade and Fritz Zwicky published a paper proposing the core of some of the supernova ruminants to be neutron stars. These dense objects rotate fast with periods of about few seconds and emit light, mostly in radio wavelengths, through cones along the rotation axis. Some such stars are aligned towards us in such a way that we see them as lighthouse beacons and known as pulsars. While in graduate school, I had a rare privilege to work briefly with Dr. Anthony Hewish, who received Noble Prize in 1974 for the discovery of pulsar by his PhD student Jocelyn Bell in 1967 under dramatic situations. It was a pleasure to hear many stories surrounding this discovery.

When the core mass exceeds 3 to 5 times the mass of the sun (the limit is not precisely known), even the neutrons cannot stop the collapse. The star eventually becomes a Black Hole. If the light enters a sphere of about 10 km of a Black Hole of 5 solar masses, it cannot escape. So, anything that happens within this sphere can never be known. This sphere is known as event horizon.

Every thing that is born is certain to die. When and how is not precisely know, but we surely know the rough circumstances of all these events. So, the stars die but their cores long live.

The Living Star

The Living Star

Debi Prasad Choudhary

LAX, Los Angeles, 3/23/2014

A star is born in a pile of dirt; it lives a life witnessing events around and in the end die, at times violently to rest in peace. That is the real life story of a star, not what I heard in my childhood that they attained Godhood, who were noble on earth and they are eternal.

The birthplaces of the stars are seen through telescopes as fuzzy looking cold nebulas objects that contain 1000 to 100,000 atoms per cubic cm at temperature of about 100 degree K, in the sky. Some of these objects look dark as they contain dense dust and gas, some other look brighter due to the reflection of starlight and in some other the gas is excited like in neon lamps to produce their own light. In between the gas clouds and stars, there are very small amount of material, about one atom per cubic meter at temperatures of 1000 degree K. Since the light travels a long distance from the stars to reach earth they look relatively redder as blue light is absorbed, similar to the phenomena of red appearance of evening and morning sun. This void is also filled with hydrogen atom that has one positive proton at center circled by a negative electron. The hydrogen atom has less energy when these protons and electrons rotate in opposite direction and more energy when the direction is same. Many atoms emit a light particle of very low energy and flip the direction of rotation of electron. Since hydrogen is filled in the galaxy, this light is used to map the rotation of our galaxy.

When the gas clouds are disturbed due to passing by shock waves and fragment into vast chunks of clouds. Few years back, I experienced a shock wave as space shuttle was landing in Edward Air Force Base. When shuttle enters the earth, the air in front gets compressed resulting sound waves whose velocity depends on the local density and temperature. The shock waves are produced when the shuttle speed exceeds the sound speed, in which case sonic booms in the form of sound are produced and we experience earthquake-type situation to our home. Among many causes, in case of stellar situation such waves are produced when a near by star die.

As a vast chunk of the gas cloud, produced by fragmentation of bigger clouds of dust and gas, collapse to become a star they start rotating faster and faster.  You will notice a similar effect of fast rotation of a skater when the arms are kept near the chest compared to the stretched arms. The rapidly rotating collapsing cloud becomes a flat pancake like object and ejects gas jets above and below at the center. These jets are produced as puffs of material at the center of rotating disk are ejected at high speed producing shock waves similar to supersonic jets on its path.

As the chunks of cloud collapse, the material gets squeezed and becomes hotter. On midway to become star, they start glowing in infrared light at about 1000 degree K. Finally, the nuclear fusion sets in to produce energy at the center that blow away dust around the star. From dirt to star, the time it takes depend on how heavy it is. The stars that are about 8 to 10 times heavier than sun take about 0.16 million years, where as a sun like star takes about 30 million years to rise from dirt. It takes 100 million years to become a star with half solar mass. Still lower mass stars take longer time to become a real star. Stars are produced in bursts, not alone. There is a fuzzy object in Orion constellation, called Orion Nebula, where such process is going on. With a binocular, you can see at least four young stars at this place.

About 100 years ago, no body knew how the stars glow for such a long time. What is the source of energy? At that time, people knew that energy is produced by burning wood or oil or of similar processes. If that were the mechanism with which stars produce energy, they will become dead in few thousand to few million years. In around 1920, Arthur Eddington, a British Astronomer, thought that there must be some other forms of producing energy than what is known at that time and argued that nuclear source.

Twenty years later, Hans Bethe, a German born, American physicist proved that indeed nuclear fusion fuel the stars that keep them shining for billions of years. The same principle was used to make hydrogen bomb. Nuclear fusion combines smaller elements to produce higher ones and energy. Energy can be produced by combining lighter element such as hydrogen, helium, carbon, neon, oxygen and silicon for fusion. Heavier than iron cannot be combined to produce energy. All stars start by combining hydrogen to produce helium and stop the reaction depending on their mass. A sun like star or 10 times lighter one, combine hydrogen at a temperature of about 4 x 10⁶ degree K, where as tars that are eight times heavier combine Silicon to produce Iron at a temperature of 2.7 x 10⁹ degree K. Star like sun remain in the main sequence for 10 billion years, which is twice it’s present age. Where as a star that is 15 times heavier lives the main sequence in 10 million years. The like of heavier stars is short.

The energy produced at the center flows outward producing pressure due to radiation at each layer. This pressure is balanced by the pressure due to the material or the gravitational force. When these two reassure are balanced at each layer, a star becomes stable. If the radiation pressure exceed gravitational pressure the star would explode, if gravitational pressure exceeds it would collapse. In case of atom bomb or hydrogen bomb, for example, the radiation force is so much that it explodes.

Advanced physics students use their knowledge to understand various stages of their evolution with the aid of modern computers. Since my childhood, I have been hearing how some people can know that they are going die, even though I never encountered such a person. But for stars there are several signs with dramatic events before their death. Stars pulsate; they change their brightness severely due to change in the energy production at the center and changing size before they finally die. They become very big. In Orion constellation, there is a bright red star, called Betelgeuse, which is in such a stage. There are pulsating stars called Cepheids and Lyrae that are at different stages of evolution marching towards death. North star pulsates, which can be observed to change its brightness slightly. As the stars are produced in clusters in different sizes at the same time, they are observed in different stages of evolution at the present time.

The sun is 4.5 billion years old, and shall remain like this for 5 more billion years before swelling to engulf the earth and other planets. Ultimately it will die to rest in peace. What will happen to us? I cannot imagine. We made a lot of progress in past 100 years, human is becoming better. If we manage to survive when sun stars dying, we shall manage to survive, I think optimistically.

The star community

The star community

Debi Prasad Choudhary

Los Angeles 03/16/2013

My childhood fascination to know the stars was based on the fact that many of them govern our lives. But than, I was also suspicious that what the other stars are doing? How far they are? And if they are shaped like us and hanging in the space. What do they eat? I never dared to ask these questions to my elders and wise man, who predicted my future using the stars. But, in my first astronomy class it became clear that these objects are truly powerful even though they have nothing to do with my future.

Twinkling of stars of different brightness and color, mainly blue and red, is a priceless spectacular sight at a dark location under clear sky. Why some stars are brighter than others and what their color mean? Major part of the last century was spent in answering these questions.

Like our sun, the stars are hot balls of hydrogen, helium and other elements glowing at temperatures ranging from 2000 to 30,000 degree kelvin. Kelvin is a temperature scale in which 310.9 degree kelvin is -100 degree Fahrenheit. The red stars are cooler than the blue stars and our sun is a yellow star with temperature of about 5800 degree K. More accurate temperature of the star is determined by measuring the amount of light removed by some elements such as hydrogen, Calcium or Iron in selected wavelengths.

The brightness of the stars depends on their temperature and size. If a star is two times hotter than another star of same size, it would be 16 times brighter as predicted from the simple laws of physics. If the radius of a star is two times larger than another star of same temperature, it would be four times brighter because its surface area would increase by four times. In reality the stellar brightness is a combined effect of temperature and size.

If a star of same brightness to another star were located nearer to us, it would look brighter. A similar effect we might observe as the brightness of the headlight of an approaching car would increase as it becomes nearer to us. So, it is important to know the distance to the stars to determine its real brightness. Among many methods, the distance to the nearer stars is determined by using methods similar to stereopsis with which we gain depth perception. Our depth perception is because of looking at the same object by two eyes located on our face that are laterally separated by a small distance. We can observe this effect by placing our thump in front of the forehead by stretching the hand. When viewed by one eye while closing another the finger would appear to be in front of different background. As our eyes are separated by few inches, we can distinguish depth at horizontal difference of 30 seconds of arc.

Exactly same method is used to determine distances to relatively nearby stars. Photograph of the star field in the sky are taken at different times of the year when the earth is located on the opposite side of the sun. Since earth is at different locations while going around the sun, nearby stars change position with respect to far away stars similar to what we see in case of thump experiment. Since the distance of the earth between two positions is 2 Astronomical Unit we can determine the small angle p and distance to the star as 1/p. Remember one astronomical unit is the distance between sun and earth about 92 million miles We can measure distance to about 100,000 stars using this method. The distance to the stars are so large that we need to devise another unit, which the distance at which a star makes 1 second of arc. To travel this distance light takes about 3.26 years. As we know this distance can also be 3.26 light years.

When we see a source of light such as a lamp or a headlight of a car from a distance it looks dimmer than when we see this from nearby. If the distance is twice far, the lamp appears four times as dimmer. The source becomes dimmer as the square of the increased distance. So, by knowing the distance to the lamp, we can find the true or absolute brightness. Similar way, we can determine the absolute brightness in magnitude scale by knowing its distance. In order to compare the brightness of different stars we determine their absolute brightness at a standard distance of 32.6 light years.

There are two more properties, mass and diameter or size, that are commonly measured by using binary stars. Binary stars circle around a common center similar to the sun and the planets. In our galaxy about 85% of the stars are in binary system. So, we can get the mass and radius of majority of stars using these methods. The spectral lines from these two stars display Doppler shifts that can be used to determine the orbital velocity. When a faint star goes in front of a bright star the brightness of the system change periodically. The amount of time the brightness decreases we determine the diameter of the star using its distance from us. From the period of changes we can determine the mass of the system by using the Kepler’s law.  In practice, however, the planes of stellar orbits around the common center are oriented randomly so determining mass and diameter accurately is a very difficult job. There are several other methods for these measurements that are used to determine one property independently, which gives confidence in the results.

It is possible to study the long-term evolution of a group of human or tree in a forest from their common properties. For example, in a forest you can see some saplings, young, old and dying trees, which are at different stages of their evolution. Without waiting to observe the birth, growth and death of a single tree, properties of trees at different stage are used to study their evolution. Similarly, after gathering the absolute brightness and temperature of many stars, we examine their properties to understand their life story. According to their temperature, stars are classified into groups of O B A F G K M; O being hotter and M being cooler stars. (Astronomers remember this sequence with the rime Oh Be A Find Girl Kiss Me.)

In circa 1910, a Danish astronomer and chemist Ejnar Hurtzsprung and American astronomer Henry Norris Russell, found that most stars fall into a sequence, called Main Sequence, in which cool stars are fainter and hot stars are brighter. There are more faint, cool stars than bright hot stars, because hot bright stars are heavier, and rapidly consume their nuclear fuel that keeps them bright. So, hot bright stars die faster than cool faint stars. There are some stars, which are brighter than what is expected from their temperature if they were Main Sequence stars. These stars, known as giants and super giants, appear brighter because of their large size. Such situation in the life of a star happens when the nuclear fuel runs out just before their death. There also some stars that appear fainter than what would be expected due to their temperature if they were Main Sequence stars. These stars are known as white dwarfs, which are high-density compact hot core of dead stars that no longer produce light.

Stars are born out of huge dust and gas clouds, and spend most of their life producing light by converting one element to another. In the end they die and become dust and gas. From these material new stars are born. These stars also produce the elements in our body, so we may as well be called “star stuff”!! This grand cosmic evolution gives us a sense of belonging to vastness that surrounds us. It makes us generous. Today, I know that even if the stars do not control my life or my future, I have a real intimate relationship with these shining objects in the sky.

Sun – our star

Sun – our star

Debi Prasad Choudhary

Los Angeles, 3/9/2014

I used to pay homage to the rising sun every day before leaving for the Laboratory from my dorm room. When, I decided to work on solar physics for my PhD thesis, my roommate joked, “You worship the sun here and ask questions about it in the laboratory.” Over these years, my respect and amazement for this object have only grown, even though I have written a number of peer reviewed research papers.

Like other stars, the sun was born when huge clouds of dust and gas collapsed almost 4.6 billion years ago. It is a huge sphere almost 109 times bigger and 330, 000 times heavier than the earth, contains mainly hydrogen. The core of the sun at about 1/4^th of it’s radius contains hydrogen denser than 150 times the density of the water and 300, 000 times hotter than boiling water. At such high temperature, the dense matter squeezes so much that two-hydrogen nuclei fuse to form a bit less heavier helium nuclei and produce energy. Every second 600 million tons of hydrogen are converted to Helium to keep the sun shining for all these years. The energy produced at the center of the sun takes about 170,000 years to come out of sun as the dense matter absorb it multiple time. After escaping the sun, it takes only 8 minutes to reach the earth. The energy travel about 70% of the sun above the core in a manner similar to what happens when we heat an iron rod in which the matter does not move but the heat conducts. In about 25% below the surface of the sun, energy is transported similar to boiling water, where the hot water in the bottom of a pot on hot plate moves up carrying heat. So, the light that we see today were created long long ago may be about 200 000 year back. In other words, by the time the light particle created in the center of the sun move only a fraction of a millimeter, we are dead!!! (Even considering a human lifetime of 120 years!!)

This huge blob of hydrogen is a stable sphere due to the delicate balance of inward pressure of the material that produces gravitational force by the outward pressure created by the radiation that propagate out of the sun. If any of these two become greater it will either collapse or explode, which happens at the end of the life of sun like stars.

The giant hydrogen blob intertwined with magnetic field running along north-south direction rotates around its axis at a rate of once in about 25 days. The sun does not rotate like a solid object as earth rotate in which all parts rotate at same rate. But in sun, different parts of the outer envelope rotate at different rate, such that the rotation rate at the equator is about 25 days and at the pole 31 days. As the equator rotates faster than the poles, the magnetic field lines embedded in the solar plasma just below the surface gets entangled in a similar way like spaghetti gets entangled when a fork is rotated to hold them before eating. This process brings more magnetic field lines to one place. As the magnetic field becomes more intense, the material motion is restricted and hence cooled. The matter with magnetic field is similar to a bottle of ice-cold water in a swimming pool. The cold water is isolated from the pool water by the bottle. The cool material is lighter than hot material, so they come out of the sun to float. The cross section of such bundles of magnetic field containing cooler material appear dark on the solar surface and known as sunspots. Sunspots are locations of intense magnetic field that contain cooler solar material.

The intensification of vertical north-south magnetic field in east west equatorial direction happens approximately every 11 years resulting more number of sunspots marking a solar cycle. The sunspots appear in pairs, where in one spot the magnetic field direction is from the sun to out side, and in other spot, the field direction is into the spot. The spots located towards the direction of the rotation, called leaders, are compact, unlike their followers that are more disintegrated. The leader spots in north and south hemisphere of the sun have opposite magnetic polarity, which was discovered by George Hale in Pasadena about 100 years ago. The north south field flips direction in two sunspot cycles. The sunspots of two opposite polarity are not aligned to the local latitude, but tilted due to the force of solar rotation while emerging from below the visible solar surface. In the beginning of a solar cycle, the spots appear in higher latitude and gradually start appearing in low latitudes.

The visible disk of the sun that is the source of most light is called photosphere. The temperature of this surface is about 5800 kelvins that emit most of the light in yellow color. Images of the photosphere obtained in excellent conditions show cell patterns similar to the surface of boiled rice. The sizes of these cells are of about 720 km equivalent to the size of the state of Texas. There is a tenuous colorful atmosphere above photosphere, seen as a red and yellow ring around disk during a total solar eclipse, when photosphere is covered. This is known as chromosphere due to colorful look extending about 2000 km and contains material at a temperature of about 4500 kelvin.  Above the chromosphere a very low density atmosphere with million-degree kelvin temperature called corona is situated. It is puzzling understand that a photosphere of 5800 K could heat the material above it to million degrees. In other words, the question is how a hot plate of 6000 K could heat the material above it to million degrees? The exact mechanisms of this heating process still remain enigmatic.

The chromosphere and corona is full of dynamical structures in the form of arches and loops that connect two regions often with spots of opposite polarity. These structures carry large amount of electric current and often stressed due either to the motion of anchoring foot points at the photosphere or emergence of new magnetic structures at those locations. When these structures are stressed beyond a limit they breakdown leading to a solar flares and coronal mass ejections. These are violent disruption events that produce energy equivalent of 20 million nuclear bombs. They dump about one billion tons of material with speeds of about a million mile per hour – a speed with which you can travel from New York to Los Angeles in 18 seconds.

These events happen at a rate of about three times a day and more during active period. The high speed material reach near the earth and affect the space environment. When this material enters the earth’s atmosphere through magnetic field they produce auroras – colorful light work in high latitudes. These materials, largely electrons, induce electric current in power grids leading to burning of transformers. When they hit the instruments on board space platform, such as communication satellites, they disrupt them. The astronauts get huge radiation doses. We need to know when they will occur, so that intercontinental flights can avoid shorter roots through polar region. So, predicting the occurrence of these events like weather prediction is important.  A major field of study developed in the past two decade is known as space weather that makes effort to understand the circumstances leading to these events.

What happens above the visible surface of the sun is easily observed, but can we know the internal structure of this giant ball of hydrogen? In early 1970s, a scientist from Caltech Bob Leighton, and from University of California Los Angeles Roger Ulrich and from Germany Dr. Deubner found that the sun vibrates like a ringing bell with a predominate period of five minutes as a result of propagating sound waves. This became a powerful technique to study the internal structure of the sun similar to the use of earthquakes to study the structure beneath the earth. The sun has been observed with unprecedented temporal coverage and spatial resolution in past 20 years to record these vibrations precisely. We now know that the outer envelope rotates at a different speed compared to the internal 70% of the material among many other facts.

In past quarter century, I have been actively engaged in understanding the magnetic structure of the source of space weather and other different properties of the sun. I am truly fascinated by the complexity and now in a better position to appreciate this complex object that govern our lives in many ways.