Of Humans and Stardust
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| Ramjas College grounds; University of Delhi, Delhi, India. Courtesy: Uddeepta Deka |
I
7.6 billion! That’s quite a big number, isn’t it? Well, this is the human population on our planet, and is ever increasing. But how many people can Earth really support? While attempting to calculate this number, one should take into account the constraints of the biosphere, since there is a limit on how far the available resources can stretch. Thomas Malthus, the late 18th century philosopher once wrote, “The power of population is so superior to the power of the Earth to produce subsistence for man that premature death must in some shape or other visit the human race”. Nevertheless, the journey of life on Earth has made the curious minds wonder and at the same time appreciate the alluring craftsmanship of nature.
II
When I was a kid, I would often gaze at the night sky - at the stars. I found a sheer serenity in the vastness. As I was told that stars twinkle and planets don’t, so most of my star-gazing would go into finding planets and stars apart. Today, we know from estimates that there are more than 100 billion galaxies in the ‘observable’ universe. Each of these galaxies may contain about 100-200 billion stars.
Stars. These twinkling beauties glow because of the nuclear fusion reactions taking place in their cores. Just 3 minutes after the universe was formed (as a result of Big Bang, about 13-14 billion years ago), only the lightest elements (hydrogen and helium, and some miniscule amounts of lithium) existed. Gravity made the atoms of these elements to come together and condense into huge clouds. This gravitational pressure on the centers of these clouds made the temperatures to increase up to millions of degrees, leading to the nuclear fusion of hydrogen into helium. Thus, the early stars (or the generation I stars) were born. These stars were more than 50 times the size of our Sun. The hydrogen fuel of the stars keeps on burning and eventually all of hydrogen turns into helium. The bigger the star, the faster the fuel burns. This process of burning lighter elements to form heavier elements (helium is heavier than hydrogen) is termed as nucleosynthesis. These generation I stars burned their fuel rapidly and were able to make only a few elements heavier than hydrogen and helium.<\p>
When stars run out of fuel, they ‘die’. They essentially go into a ‘supernova explosion’, that is, they blow up. When this happens, all the elements generated inside the star gets swept out into space. These elements make up the next generation of stars. Other heavier elements are then produced by this new generation of stars. This process of going into supernova and generation of heavier elements goes on and on leading to the synthesis of nearly all the elements in the periodic table.<\p>
III
Our planet (with 7 other planets) revolves around a star, the Sun. This star system, known as the Solar System is part of the Milky Way galaxy, which itself is part of a group of galaxies known as the ‘Local Group’. The formation of our planet dates back to about 4.6 billion years. Although, the details of how the Earth came into being are yet being worked out by scientists but there are two widely accepted theories about its formation.
While one theory – the theory of core accretion – works well with the formation of the terrestrial planets like Earth, the other theory – the disk instability method – may account for the creation of giant planets like Jupiter. In both the theories, however, one considers that approximately 4.6 billion years ago, the solar system existed as a cloud of dust and gas known as a solar nebula. Collisions in this cloud of gaseous materials with the assistance of gravity led to the formation of Sun and the other planets. An impact by a large body on the young Earth’s mantle made pieces of Earth to fly off. Gravity made these pieces come together and form the moon.
IV
Fossilized lifeforms on Earth have been found in iron and silica-rich rocks, which are as old as 4.28 billion years. If this turns out to be the oldest record of life on Earth, then it’d mean that life on Earth came into being just after oceans were formed 4.4 billion years ago. Although there is no standard model explaining how life started, but scientists are more or less convinced that the basic chemicals which must have contributed in forming the first life forms are – ammonia, carbon dioxide, carbon monoxide, hydrogen sulphide, methane, phosphate and water. These chemicals originated from the building blocks of life, often abbreviated as CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.
Every element in the periodic table has a unique ‘fingerprint’. Solids, liquids and dense gases emit all colors (wavelengths) of visible light from violet till red. This is called a ‘continuous spectrum’. Thin gases, however, emit only certain colors of light. This is known as ‘emission spectrum’. If a source of light is present behind a thin gas, it will absorb light of the same color it emits. This is known as ‘absorption spectrum’. Using an instrument called a ‘spectrograph’, one can find the spectrum of an element. It is found that every element has a unique spectra (the fingerprint!). In other words, one can study the spectra to identify the materials which is emitting or absorbing light. Astronomers use this technique to look at the spectra of different stars and estimate their chemical composition.
A study by researchers using observations from the Sloan Digital Sky Survey’s (SDSS) Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectrograph in New Mexico shows that ‘Humans are, indeed, made of stardust’. The study consisting of a survey of 150,000 stars indicated that humans and Milky Way galaxy have around 97 percent of the same kind of atoms.
Thus, most of the elements that we are made of were formed in stars over the course of billions of years and multiple star lifetimes. Any element in our body, heavier than iron, has travelled through at least one supernova. It is also possible that some of the hydrogen (which makes up about 9.5 percent of our bodies) and lithium, which is present in our body in trace amounts, originated from the Big Bang itself!
So, the next time your mom tells you – “tu meri aankhon ka taara hai”, look up and tell her how correct she literally is.
Uddeepta Deka
A wanderer on the landscape of physics problems.