Whenever we look at the night sky, all the stars appear alike, and everything seems very calm and peaceful. But wait! this is not the exact picture of what lies and what is happening in the heavens above. The actual picture is quite different. Each and every star is significantly different from one another. And contrary to the peaceful mask, some vigorous reactions and explosions are taking place beyond our imagination. The 19th article of Basics of Astrophysics series aims to study these astonishing events, The Novae and Supernovae in detail.
Most of the time, the words novae and supernovae are confused to mean the same. But in reality, both these words have a different meaning and represent different phenomenons. The word nova refers to an astronomical event that causes the sudden appearance of a bright, apparently new star, that slowly fades away over a period of a few weeks or months to return to their original state.
Whereas, Supernova refers to a kind of explosive event that generally occurs upon the death of a certain type of stars. Novae are actually one of the several types of supernovae. Now, let’s dig deeper into these terms and the different classifications of these terms one by one.
Classification of Supernovae
Scientists usually classify supernovae on the basis of spectra emitted by them. This spectrum shows what elements the original star created and thus released into space after it exploded. If a supernova’s spectrum contains hydrogen lines, then it is classified as “Type II supernova”, otherwise as “Type I”. We will study the type II supernovae first so that it becomes easier to understand type I supernovae later.
Type II Supernovae
As said earlier, type II supernovae differ from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies. For a star to end it’s life as type II supernova, it must be about 8 to 15 times heavier than our sun.
As we studied in the article on nuclear reactions in stars, these massive stars can fuse heavier elements at their core, switching from hydrogen to helium, and then carbon, neon, etc, all the way up the periodic table. When it reaches heavier elements the fusion reaction takes more energy than it produces. Thus, when these reactions stop, the star starts collapsing under its own gravity.
We have already studied this process in detail in our earlier articles of the series. The outer layers of the star collapse inward in a fraction of a second with about 23 percent the speed of light. Since, the neutron degeneracy pressure halts the collapse of the core, so the incoming outer layers rebound back after colliding with core causing shock-waves, detonating as a Type II supernova.
A collapsed core is left behind by a type II supernova explosion. If the mass of the core is less than 2 or 3 solar masses, it becomes a neutron star, but if more than 2 or 3 solar masses remain, then neutron degeneracy pressure is also not enough to hold the object up, and it collapses into a black hole.
Type I Supernovae
These lack the famous Balmer series (hydrogen lines) in their spectra. The type I supernovae are further of three types, type Ia, type Ib and type Ic.
Type Ia Supernovae
These occur in a binary star system, where one companion has to be a white dwarf, whereas the other one can be any other type of star, like a red giant, main-sequence star, or even another white dwarf. Novae are known to be caused by a star briefly re-igniting itself after staying dormant for many years. We know that stars are fusing elements in their core to sustain themselves. These fusion reactions occur up to different extents in the stars due to the mass variations.
When Sun-like stars use up all their hydrogen and helium, then they start sloughing off their outer envelopes and become very small, very hot white dwarfs. These white dwarfs are the inert cores of dead stars that have used up all of their available fuel.
Now, in any binary system, if one of the stars is a white dwarf, and the other one starts evolving into a red giant, then due to more mass and strong gravitational pull, the white dwarf begins gravitationally attracting some of the gas from the atmosphere of the red giant towards itself. Most of this sucked gas is hydrogen. When the hydrogen reaches the surface of the incredibly hot white dwarf, it rapidly ignites, creating a large nuclear explosion on the surface of the star. This is what we call a nova or a type Ia supernova.
Type Ib and Ic
Type Ib and Ic supernovae also undergo core-collapse just as type II supernovae do. The only difference is that they have lost most of their outer hydrogen envelopes, eventually, they do not show hydrogen lines in their spectra. Also, they do not show a strong ionized silicon absorption line like the Ia type. Apart from these, some other types of supernovae also occur. But, their spectra are not in sync with either type I or II. However, the probability of occurrence of these events is very low.
Novae is the Latin for “new star”. During a nova, a star that could not be seen earlier with the naked eye becomes one of the brightest objects in the sky. It starts appearing like a new star and then slowly fades away. These are characterized by a silicon absorption feature in their maximum light spectra. Also, they can eject material at speeds of the order of 10,000 km/s.
Supernovae can help a lot to study the complete life story of a star. These explosive events release a tremendous amount of energy. Some of this energy helps to fuse elements even heavier than iron! This is where such heavy elements like gold, silver, zinc, uranium, etc. come from. The material that gets ejected into space as a result of the supernova becomes part of the interstellar medium.
New stars and planets form from this interstellar medium. Type Ia supernovae are used as standard candles to measure the distance to their host galaxies. In short, the study of novae and supernovae is one of the most happening and useful tools to unravel some of the greatest secrets of our universe. I hope this article helped you to have a basic understanding of these widely useful phenomenons.
Image credits for the cover of this article – NASA, ESA, N. Smith (University of Arizona) and J. Morse (BoldlyGo Institute)