Question

From the section of the birth, life, and death of stars, what did you learn about how stars are born and how long they live and how they die that was new to you and interesting? Do you understand it or is there something that still is hard to grasp? Are there topics that you would like to know more about? Do you understand when a protostar finally becomes a Main Sequence star, what does it need to achieve to be called a main sequence star and why does it take light stars such a long time to achieve this? What is a Herbig-Haro object? Do you understand why light stars live longer than heavy stars; is that fair??   Do you understand the difference between planetary nebulae and type 2 supernova; what type of stars experience the Helium Flash and why? Why do light mass stars experience 2 Red Giant stages whereas Heavy stars experience only 1 Red Giant Stage? Which stars make the heavier elements of the periodic table and why? Do you understand the difference between type 2 supernova, type 1 supernova and nova and how they are created? What are some famous supernova observations in history, see if you can come up with some other than the ones mentioned in the lecture??   How can we harness the energy of a supernova as a source of clean energy and do you think one day humanity will find a way of doing this? What impressions does this section leave on you on the lifecycle of stars and the analogy with human lifecycle, how does it change your worldview and perspective of life and inspire you?

Answer 1

Birth, Life and Death of stars: Matter in space is unevenly distributed. In between the voids of emptiness, there are regions of gas and dust clouds, called the interstellar medium, which are denser than their surroundings. Usually, the kinetic energy of the clouds’ particles will be balanced by the gravitational force of the cloud. However, if the cloud is disturbed, perhaps by a nearby supernova, the balance is broken and the cloud may become denser in certain areas.

When they reach a certain critical mass, the densest parts of the cloud can contract under the influence of their own gravitational attraction, causing the cloud to fragment into smaller and denser sections. This process takes a few million years. As the cloud contracts, the temperature and the density increases. Eventually, the new object obtains a spherical shape and becomes what is called a protostar. Due to its gravitational pull, matter from the cloud still falls into it, continuously raising the temperature and density until they become so large that nuclear reactions start where the hydrogen fusions to make helium. This is how a new star is born.Most stars are born within the arms of a spiral galaxy, where there is more gas and dust. Sometimes, several stars can form within the same molecular cloud, and we have what is known as a star cluster. There are two types of cluster; open clusters, which tend to contain a few hundred relatively young, hot stars that quite spaced out, and globular clusters, that tend to contain thousands of much older stars, more densely packed together.

For roughly 90% of a star's life, the star will be relatively stable and have roughly the same luminosity, surface temperature and size. At this point in its evolution, the star is in hydrostatic equilibrium.In this state, the star is a Main Sequence star. Stars can stay in this phase for a very long time. However, the timescales for stars are not all the same. Larger stars tend to burn their fuel much faster and, therefore, run out much quicker. Smaller stars don’t need to use as much energy to counterbalance their gravity, hence have longer lives.

During the Main Sequence, stars use hydrogen as their fuel. When the hydrogen begins to run out, the star produces less energy to support its weight and the core begins to contract. This increases the temperature and density in the core and the luminosity of the star increases as a result. Through the enhanced heat released, the radius of the star increases by 100 to 1000 times its original size but with more of the surface to heat up and less fuel to do it with, the surface temperature can decrease as much as 50%, and the star becomes redder. Such stars are called Red Giants.

Death of a star can happen in two ways and is determined by its mass:

For small stars (that is less than 8 times the mass of the Sun), at the end of the Red Giant phase, the star can’t contract enough to generate the temperatures needed for further nuclear fusion. With no nuclear processes to power it, the outer layers of the star become unstable and the stellar wind produced by the star blows them away. This cloud moving away from the star is called a planetary nebula.The leftover core is small but very dense and hot. They’re called White Dwarfs after their small size but white-hot surface.

For a stars with mass more than 8 times the mass of our Sun, death results in a gigantic explosion:supernova.After a supernova, depending on the mass of the original star, there are two possible outcomes for the star’s core. For smaller stars, the core becomes a Neutron Star. However, if the star's core has more than about 2.5 times the mass of the Sun, what remains is a Black Hole.

A protostar becomes a main sequence star when its core temperature exceeds 10 million K. This is the temperature needed for hydrogen fusion to operate efficiently. The length of time this takes depends on the mass of the star. The rate of this formation depends on the density of the star.More massive stars will need more thermal pressure to hold it up against gravity, hence will reach this temperature faster than the lighter stars.

Herbig–Haro objects are small patches of nebulosity associated with newly born stars, and are formed when narrow jets of partially ionized gas ejected by said stars collide with nearby clouds of gas and dust at speeds of several hundred kilometres per second.

The more massive a star is, the greater its gravitational influence. As such, the more massive the star is, the more hydrogen it needs to fuse in order to produce sufficient pressure to withstand its own gravity. As massive stars burn their fuel at a much higher rate, their lifespans are substantially shorter. They do shine with a much greater luminosity however. This is why light stars can live longer than heavier stars.

Difference between planetary nebula and type 2 supernova:

For small stars (that is less than 8 times the mass of the Sun), at the end of the Red Giant phase, the star can’t contract enough to generate the temperatures needed for further nuclear fusion. With no nuclear processes to power it, the outer layers of the star become unstable and the stellar wind produced by the star blows them away. This cloud moving away from the star is called a planetary nebula.

For a stars with mass more than 8 times the mass of our Sun, death results in a gigantic explosion: during the first second it can be as bright as a whole galaxy with hundreds of billions of stars. Such explosions are called Type-II Supernovas.

A helium flash is a very brief nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 to 2.0 solar masses) during their red giant phase. In triple alpha process three alpha particles(⁴₂He) will fuse to form a carbon(¹²C₆) atom. It is caused by the ignition of helium fusion in a core that contains degenerate electrons. The degenerate electrons control the pressure of the core and because they are degenerate, do not readily expand with an increase in temperature.

In case of a lighter star, transition to Red Giant happens in two stages. First, the star will appear to cool slowly and will undergo a modest increase in luminosity.Stars in this phase are usually referred to as subgiants.  Next, the star will grow to as much as, or even more than, 100 times its original size, which will cause a significant increase in luminosity with only a small decrease in temperature. Stars in this stage are referred to as red giants. However in case of a more massive star, it skips the "subgiant" stage and directly transforms into a Red giant as due to its already high temperature and size it can expand rapidly into a Red Giant stage.

The supermassive stars which are capable of undergoing supernova can make elements heavier than iron or nickel. These elements are believed to form as a result of such explosions.

Difference between type-1, 2 supernova and nova:

A type 1 supernova is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.

A Type II supernova results from the rapid collapse and violent explosion of a massive star.

A nova is an astronomical event that causes the sudden appearance of a bright, apparently "new" star, that slowly fades over several weeks or many months. Nova involve an interaction between two stars that results in the birth of this new star that is much brighter than the stars involved.

Some of the famous supernova observations are: SN185(observed by chinese astronomers in 185AD), SN1006( observed by islamic astronomers in 1006AD), SN1054( resulting in origin of crab nebula), SN 1604( observed by keppler in 1604AD)