Life Cycle of a Star: Journey From Nebula to Black Hole
Twinkle, twinkle, little star,
How we wonder what you are.
Up above the world so high,
Like a diamond in the sky!
The poem of Jane Taylor, written back in 1806, made our childhood. This English lullaby is phenomenon from another perceptive. It has evidently sown the seed of curiosity in innocent children minds about stars!
Actually it is pretty much impossible to find a man who haven't wondered how Stars work at least once in his/her life. The stars have always been the centre of human's curiosity. While many civilizations developed associated myths with various combinations of stars in the sky, some also associated specific stars with their myths. However, since the development of galilean telescope, the point of view has changed. Truth began to show up and human explored bound their vision. Eventually human began to understand how much contribution they, stars, make to keep the universe away from breaking down! Can you even think what if the Sun goes out for just 8 minutes?
So what is Star? A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. Astronomers had estimated that the observable universe has more than 100 billion galaxies. And only in our galaxy, Milky Way, estimated number of star is 250 billion ± 150 billion! Sirius, Bellatrix, Alpha Andromeda, Orion, Sun, Arcturus, Vega, Bellatrix, Capella, Proxima Centauri, Alpha Crucis, Pollux, Alnilam, Betelgeuse, Rigel, Deneb are JUST few of the stars of our universe (Personally, I believe in multiverse!) Now can you even imagine the number of stars in the universe?
Hydrogen, Helium and few other inert gases are considered as the primary elements of stars. the surface temperature of the Sun is 5,500 °C, with a core temperature as high as 15 million °C. For other stars, the surface temperature can range from 3,000 to 50,000 °C. Stars are predominantly composed of hydrogen (71%) and helium (27%) gases, with traces of heavier elements such as oxygen, carbon, neon and iron. Many of these stars are visible to the naked eyes in the night sky, shaped like sometimes dot or a tiny long luminous object. Star has been a matter of curiosity of man since ancient age. Greeks used to worship Stars. However, there are only about 5,000 stars visible to the naked, average, human eye, MinutePhysics points out. And, because the Earth itself gets in the way, you can only see about a half of those from where you stand. Astronomers have assembled star categories that identify the known stars and provide standardized stellar designation. 300 sextillion stars in the universe are invisible to the naked eye from Earth, including all stars outside our Milky Way.
How Stars Shine?
Let’s compare stars with colossal football. All stars are hot footballs of glowing plasma held together by their own gravity. And the gravity of a star is very intense due to its huge mass. For example, the Sun is a mere 5,800 Kelvin at its surface, but at its core, it can be 15 million Kelvin!
The intense pressure and temperature at the core of a star allow nuclear fusion reactions to take place. These fusions take place in the core of the stars. Here the atoms of hydrogen are fused into helium through several reactions. And these reactions releases a colossal amount of energy in form of radiation like gamma rays. These gamma rays get trapped inside the stars and they push outward against the gravitational contraction. That’s why stars hold to a certain size the gamma rays jump around in the star, trying to get out. They’re absorbed by one atom, and then emitted again. This can happen many times a second, and a single photon can take 100,000 years to get from the core of the star to its surface.
Stellar evolution is a description of the way that stars change with time. On human timescales, most stars do not appear to change at all, but if we were to look for billions of years, we would see how stars are born, how they age, and finally how they die.
The primary factor determining how a star evolves is its mass as it reaches the main sequence. The following is a brief outline tracing the evolution of a low-mass and a high-mass star
Different Stages of Stars
A star's life cycle is determined by its mass. The larger its mass, the shorter its life cycle. A star's mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust from which it was born. Over time, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin. As the gas spins faster, it heats up and becomes as a protostar. Eventually the temperature reaches 15,000,000 degrees and nuclear fusion occurs in the cloud's core. The cloud begins to glow brightly, contracts a little, and becomes stable. It is now a main sequence star and will remain in this stage, shining for millions to billions of years to come. This is the stage our Sun is at right now
Stars form due to the gravitational collapse of gaseous nebula. Stars from inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K, just above absolute zero). At these temperatures, gases become molecular meaning that atoms bind together. CO and H2 are the most common molecules in interstellar gas clouds. The deep cold also causes the gas to clump to high densities. When the density reaches a certain point, stars form.
Since the regions are dense, they are opaque to visible light and are known as dark nebula. Since they don't shine by optical light, we must use IR and radio telescopes to investigate them.
Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have masses around 104 solar masses in the form of gas and dust. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostars and the whole process takes about 10 million years.
In Brief: Solar Mass
The Solar Mass is a standard unit of mass in astronomy, equal to approximately 2×1030 kg. It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies.
Once stars that are 5 times or more massive than our Sun reach the red giant phase, their core temperature increases as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull carbon atoms together as the temperature increases and additional fusion processes proceed, forming oxygen, nitrogen, and eventually iron. A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. A red giant star is a dying star in the last stages of stellar evolution. In only a few billion years, our own sun will turn into a red giant star, expand and engulf the inner planets, possibly even Earth. But hey, do not panic now! It may take more than 5 billion years for Sun to turn into a Red Giant.
But as the temperature reduces, the stars shrink. It generally takes 1 million years for a red giant to convert into white dwarf. In fact, the fate of a star depends on its mass. The larger stars become neutron stars or black holes. On the other hand, vast majority of the stars become hot balls of glowing matters that shines. These stars generally cools down and disappears from the view. Since the source of energy of a star is nuclear fusions, when it stops the star begins to die. But they doesn’t die immediately. As fusion no longer generating the outer contraction, the stars begins to lose its shape. Gradually it take the size of earth. But the lights still do not go away, it is still boiling hot! And the hot things glow. This white hot star is called white dwarf. Due its previous nuclear fusions, the star still has got huge mass. In 1930, Chandrasekhor determined that their mass can be as high as 1.4. it is called Chandrasekhor’s Limit
Tension is building up! The star is just one step away from being a Black Hole! When the mass of star reaches 1.5 to 3 solar mass, more massive than sun, it runs out of fuel in the end of life. Its core begin to collapse while outer shell layers are blow off in a supernova explosion which leaves behind nothing but its original mass. Neutrons stars are extreme objects that measure between 10 and 20 km across. They have densities of 1017 kg/m3(the Earth has a density of around 5×103 kg/m3 and even white dwarf have densities over a million times less) meaning that a teaspoon of neutron star material would weigh around a billion tonnes. The easiest way to picture this is to imagine squeezing twice the mass of the Sun into an object about the size of a small city! The result is that gravity at the surface of the neutron star is around 1011 stronger than what we experience here on Earth, and an object would have to travel at about half the speed of light to escape from the star.
These stars are called Neutron Stars. A more massive star becomes Black hole. Unlike black holes, neutron stars are observable as Pulsar if its magnetic field is favourably aligned with its spin axis. Pulsar are the lighthouse of cosmos. Neutron stars are primarily made of neutrons, which are neutral particles. The neutrinos easily escape the contracting core but the neutrons pack closer together until their density is equivalent to that of an atomic nucleus. At this point, the neutrons occupy the smallest space possible. If the core is less than about 3 solar mass they exert a pressure which is capable of supporting a star. A star supported by neutron degeneracy pressure is known as a ‘neutron star’, which may be seen as a pulsa.
How Pulsars Work?
Born in a core-collapse supernova explosion, neutron stars rotate extremely rapidly as a consequence of the conservation of angular momentum, and have incredibly strong magnetic fields due to conservation of magnetic flux. Neutron stars spin between seven and forty thousand times per minute and form incredibly strong magnetic fields. Rapid spin and intense magnetic fields drive powerful beam of electromagnetic radiation. Neutron stars that have lost energy through radiative processes have been observed to rotate as slowly as once every 8 seconds while still maintaining radio pulses. Observations also reveal that the rotation rate of isolated neutron stars slowly changes over time, generally decreasing as the star ages and rotational energy is lost to the surroundings through the magnetic field. Even though they slow down as they age but some of the oldest pulsars spin hundreds of time a second! Each of these milliseconds pulsars orbit a normal star. Over time the impact of gas pulled from the normal stars has spun the pulsar up to incredibly speeds. This acceleration is the cause of their weaker magnetic fields. Despite this, pulsars emit gamma rays for which they are observable from distance.
The End: Black Hole
The talk of the week! Scientist finally captured event horizon of a Black Hole M87. Now it has got a name too. It has been named Powehi - a Hawaiian phrase referring to an "embellished dark source of unending creation." The Event Horizon Telescope or EHT, presented what Physicists call a ‘Ground-Breaking result’. But what are Black Holes actually?
Black Holes are the last stage of heavy mass stars. That’s something Einstein predicted 110 years ago. A black hole is a region of space-time having such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform space-time to form a black hole. Due to its heavy mass and gravitation field, its space time curvature is deeper than any other stars. A black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape. Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.) If the total mass of the star is large enough (about three times the mass of the Sun), it can be proven theoretically that no force can keep the star from collapsing under the influence of its own gravity. However, as the star collapses, a strange thing occurs. As the surface of the star nears an imaginary surface called the "event horizon," time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object.
let's watch this movie on Stellar Evolution!
When the density of an object is equalled to the Sun’s and its radius is about 500 times to Sun’s, tantamount to Schwarzschild radius which determines the final fate of massive stars , then the escape velocity from the surface of the object would reach the speed of light or more. In this circumstance, the object would pull everything, including the light to itself. These objects are Black holes.
The Schwarzschild radius (Rg) of an object of mass M is given by the following formula, in which G is the universal gravitational constant and c is the speed of light:
Rg = 2GM/c2
For a mass as small as a human being, the Schwarzschild radius is of the order of 10-23 cm, much smaller than the nucleus of an atom; for a typical star such as the Sun, it is about 3 km (2 miles). The Schwarzschild radius is named for the German astronomer and physicist Karl Schwarzschild, who investigated the concept in the early 20th century.
Black Holes are monsters of universe. They just gulp everything that comes near to their event horizons. According to theory, within a black hole there’s something called a singularity. A singularity is what all the matter in a black hole gets crushed into. Some people talk about it as a point of infinite density at the center of the black hole, but that’s probably wrong — true, it’s what classical physics tells us is there, but the singularity is also where classical physics breaks down, so we shouldn’t trust what it says here. And that's where the world of quantum mechanics and relativity begins
In a very specific mathematical case, the singularity in a spinning black hole becomes a ring, not a point. But that mathematical situation won’t exist in reality. Others say that the singularity is actually a whole surface inside the event horizon. We just don’t know. It could be that, in real black holes, singularities don’t even exist. Wormholes are theoretically possible, given the right conditions.
Wormholes are the doorway to another universe. May be they do exist. That's what I believe. Even though Physicists are still arguing about the existence of Multiverse
Black Hole and Stephen Hawkings
When the first ever photograph of black hole was published last week, the first thing that popped up in my mind was what would Professor Hawkings say in his reaction? That man has serious contribution in the ground breaking success indeed. In the moment I just wished he was alive..
© 2019 Yusrat Sadia Nailat