Big Numbers in Time
The average lifespan of a human being is approximately 80 years. This number pales in comparison to the lifespans of celestial bodies such as the stars, luminous spheres of plasma that illuminate the night sky (Figure 1).
The lifespan of a star depends on its mass. Interestingly, it is found that the more massive the star, the faster it fades away. The Sun, for example, is about 4.6 billion years old, and will last another 9 billion years. Stars that are 10 times the mass of the Sun burn only for 100 million years. On the other hand, stars one-tenth the mass of the Sun burn 100 billion years or longer. Those are some large numbers being thrown about, but in astronomy, these orders of magnitude are quite common. To put this in perspective, our ancestors have been around for about six million years, but the modern form of humans, or homo sapiens evolved only about 200,000 years ago. Human civilization is a recent enterprise of 6000 years, with industrialization beginning only in the 1800. Our existence is a fleeting instance compared to the lives of stars, a rich history woven through several evolutionary stages of enormous extravagance, which begs the question, what happens when a star dies?
Colors of an Explosion
Stellar evolution is the process by which a star changes over the course of time. As mentioned earlier, the more massive a star, the faster it burns out. All stars are born from nebulae, clouds of gas and dust, and over the course of millions of years, these proto- or infant stars settle down, and transform into what is known as a main-sequence star. The Sun is a typical main-sequence star (Figure 2).
The death of a star is intrinsically related to the energy source that powers a star for most of its life: nuclear fusion. If one were to peel open a star, they would find a ring structure like that inside an onion. Initially, the energy generated by a main-sequence star is through the fusion of hydrogen atoms at its core. Hydrogen atoms fuse to produce Helium resulting in an abundance of the latter, and the depletion of the former fuel. Eventually, the star begins to fuse the Hydrogen fuel along a spherical shell surrounding a mostly Helium core. This process causes the star to grow in size, and evolve into what is known as a Red Giant. Stars with half the mass of the Sun can also generate Helium fusion at their core, while more massive stars fuse heavier elements in a series of concentric shells. In general, the more onion rings, the more massive the star (Figure 3).
Once this nuclear fuel has been exhausted, a star like the Sun collapses into a dense, small body known as a white dwarf, with much of the outer layers of the Sun being expelled into a planetary nebula (Figure 4).
We learn about the stars by receiving and interpreting the messages which their light brings to us. The message of the Companion of Sirius when it was decoded ran: “I am composed of material 3,000 times denser than anything you have ever come across; a ton of my material would be a little nugget that you could put in a matchbox.” What reply can one make to such a message? The reply which most of us made in 1914 was—”Shut up. Don’t talk nonsense.” – Sir Arthur Eddington
The word “planetary nebula” is a misnomer. It does not mean clouds of gas and dust consisting of planets. The word originated in the 1780s when astronomer William Herschel viewed these objects through his telescope, naming them so because they resembled the rounded shapes of planets.
Stars more massive than the Sun can explode in a supernova releasing much of the material that they were composed of in a shock-wave into the vacuum that is space. By releasing the bulk of the chemical elements that they had originally sustained in their core (the list includes: Hydrogen, Helium, Carbon, Neon, Oxygen, Silicon, and Iron), stars enrich the interstellar medium. The resulting shock-wave produced from a supernova also helps trigger the formation of new stars. The cores of such massive stars collapse into an extremely dense neutron star, and in certain cases, a black hole (Figure 5).
A normal-sized matchbox containing neutron-star material would have a mass of approximately 13 million tonnes, or a 2.5 million m3 chunk of the Earth (a cube with edges of about 135 metres).
The decisive factor is always the mass of the star, which in simple terms, is proportional to the strength of its gravity.
Gravity is a one-dimensional force, in that it is always attractive, and tries to pull things together. We are held to the surface of the Earth by the planet’s gravitational force. A black hole is born when an object is unable to withstand the compressing force of its own gravity. Stars use their nuclear fusion to maintain a tremulous balance, for several million years, in an exhaustive fight against gravity. The Sun will never become a black hole, as its gravity isn’t sufficient to overpower the force produced by its nuclear furnace. But in more massive stars, gravity ultimately wins.
Then what are hypernovas?
Even Bigger Explosions
Simply put, hypernovas are pretty much the same thing as supernovas, just on a much grander scale. Hypernovas are extremely energetic supernovas, and though their formation is similar, they are both distinct phenomena (Figure 6).
In a supernova, a star sheds its outer matter leaving behind a dense core in a neutron star. In a hypernova, the force of the explosion tears the inner star apart as well. Hypernovas only occur in stars with greater than 30 times the mass of the Sun. Like in a supernova, the star runs out of fuel, unable to support itself under the weight of its own gravity. As it collapses, the star subsequently explodes, spewing matter in all directions. The energy released within mere seconds of this explosion is greater than the energy that the Sun will release in its entire lifetime.
Time for an analogy. The sun radiates ~ 3.83 x 1026 W of energy. The standard light bulb for a table lamp has a wattage of 60 W. Thus, the sun radiates energy equivalent of 7 x 1024 light bulbs. Supernovas shine with the brightness of 10 billion suns (1 sun = 7 x 1024 light bulbs, then 10 billion suns = 10 x 109 x 7 x 1024 light bulbs = 7 x 1034 light bulbs), their total energy output being ~ 1044 J, which is the total energy output of the sun in its 10 billion year lifetime. Hypernovas release energy in excess of this amount. That’s a lot of light-bulbs!
Two plausible reasons currently conceived on the formation of a hypernova include:
- A massive star (rotating at a very high speed or encased in a powerful magnetic field ) exploding, resulting in the inner core being ripped apart.
- Two stars in a binary system colliding, forming one gigantic mass, and exploding.
The result is ultimately clear: a black hole is produced, and a huge amount of energy is released in the form of a gamma-ray burst, one of the brightest known events in the universe. The light released in a hypernova is several million times greater than all the light of the stars in the Milky Way galaxy put together.
Introducing the Universe
The world is very old, and human beings are very young. Significant events precede our appearance on Earth in what is an awesome vista of time. But in our vanity, we find the stubborn pride which motivates our claims, and actions as a higher organism on this planet. Thus, we are blind to the overwhelming reality that our existence on Earth, and the very existence of the planet itself, is nothing more than a single thread woven into the rich tapestry that is the universe. There, beyond the confines of our world and among the stars, lies unfathomable mysteries of great wonder. Hypernovas are a small shade of that enormous spectrum of amazing phenomena in our universe.