Suns, stars and black holes

By THOMAS HUKAHU
LAST week I wrote about the passing of theoretical physicist the late Prof Stephen Hawking.
While I was going over his life, I was thinking about the wonders of the heavens – the stars, the constellations (groups of stars and structures in a part of the sky) and other matter and the universe itself that kept him interested for many years.
In this article, I will go over some basics and interesting stuff about the suns, stars and black holes – structures out there in space.
My thoughts also went over concepts that I first learned and that slowly got me thinking about studying physics in university decades ago. What I found intriguing then still remains fascinating.
Physics by the way is the natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force.
Matter is defined as anything that has mass and takes up space. Energy – like light and heat – is not matter but since the days of Albert Einstein and his now famous equation it has been shown that one can affect the other. The gravity of massive bodies like the sun can attract light from other stars and cause it to bend, while light can also be described at times to act like particles – which are matter.
A book in astronomy started it
While I was studying science in Grade 12, I borrowed a book on astronomy from the library. (While cosmology is the study of the universe, astronomy is the science which deals with the constitution, relative positions, and motions of the heavenly bodies; that is, of all the bodies in the material universe outside of the earth, as well as of the earth itself in its relation to them. Many physicists in the past were also mathematicians and astronomers.)
As I was reading the book over a few days, I learned some amazing stuff.
That book made my physics lessons appealing when we were learning Newton’s Laws of Motion.
The concept of objects passing through space and being affected or otherwise by external forces was made more fascinating by the things I learned from that book – about matter in space and light from the stars traversing the millions or billions of kilometres to get to us here on earth.
Geography, one of my favourite subjects in Grade 11, had an influence on my choice of physics too. I was particularly interested in physical geography, the study of rivers, mountains and other physical landforms on earth.
As a science student, I did not have the privilege of studying geography in Grade 12. However, when I stumbled over the book on astronomy, it was as if I was still the geography student but now turning my eyes off the landforms on earth and looking at physical structures in space. And it was even more fascinating than the world here on earth.
The amazing stuff learned
Here are some of the things I learned from that book on astronomy.
Firstly, our Sun, the centre of the Solar System, is a star – and all stars are suns and all suns in other systems are actually stars.
Secondly, stars have lives – they are born at a particular instant and will die one day. They have much longer ages though, in fact, stretching over billions of years. Most stars are between 1 and 10 billion years old.
Our Sun will last for about 9 or 10 billion years. It is about 5 billion years old so it is half way through its life.
Our Sun’s energy supports life on earth. The energy (which is composed of heat, light and other forms of electromagnetic energy) that it emits come from a nuclear fusion reaction where two hydrogen atoms merge to form a nucleus of a helium atom.
In nuclear physics, we are told that a fusion reaction releases the most energy, unlike in fission reactions where atoms are split – as in the case of nuclear bombs and reactions in nuclear power generation stations.
So, the energy from the Sun is immense and this nuclear reaction has been going on for 5 billion years and will continue.
The third concept in that book that fascinated me was that the light from stars we see at night take time to travel – a lot of time in fact, to traverse the great distance that separates them from us.
Regardless of that fact that light from stars travels at 300, 000,000 metres/second (300,000 kilometres/second), which is the greatest speed ever known, light still takes time to pass the distance separating the stars from us.
Proxima Centauri is the closest star to our Solar System. Proxima in the constellation of Centaurus, a constellation next to the Southern Cross (Crux) – to the southern part of the sky. (See the photograph on Proxima’s position.)
You may not be able to see Proxima because it is not bright, but you can see the other stars in Centaurus, like Hadar and ?-Centauri.
Proxima Centauri is 4.25 light-years from us. That means, if we are observing light or energy travelling to us, the light that reaches us actually left Proxima 4.25 years ago. A light-year is a distance unit – the equivalent of the distance light covers in travelling for a year, which is about 9 trillion kilometres.
Many stars and galaxies (systems of millions or billions of stars, together with dust and gas, held together by gravitational attraction) are thousands or millions of light-years away.
The thing that dawned on me was: Whenever we are looking into space, we are looking into the past.
Light we are seeing actually left star systems thousands or millions of years ago and finally reached us. On the other hand, there were new stars which are born this very minute but we are not aware of them because light from them is still travelling to us.
The birth and death of stars
Have you heard of structures in space called supernovae (plural for supernova), nebulae or black holes?
Well, those are structures formed as a star goes through different stages in its life, depending on its composition, size and mass. Such objects were of interest to me when I first read that book on astronomy.
Birthplace of stars
A web page of the National Aeronautics and Space Administration (NASA) says that molecular clouds, dense clouds of gases located primarily in the spiral arms of galaxies, are the birthplace of stars.
Dense regions in the clouds collapse and form “protostars”. Initially, the gravitational energy of the collapsing star is the source of its energy. Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a “main sequence” star.
It is important to note that the two forces that keep the star in equilibrium is pressure outwards by fusion – as in burning of hydrogen into helium – and gravity’s pull inwards. An imbalance in these forces determines whether a star swells or contracts as it goes through its life.
Main sequence stars
Main sequence stars are, like our Sun, that fuse hydrogen atoms together to make helium atoms in their cores. Among other things, the amount of energy radiated by the star depends on its mass.
Generally, depending on how massive they are, stars are formed and die in two ways – as an “ordinary” star, or as a massive star.
Stars that are 10 times more massive than the Sun are more luminous than it. The more massive a main sequence star, the brighter and bluer it is.
Sirius, the Dog Star, next to Orion, is like this and appears bluer. Proxima Centauri, on the other hand, is less massive than the Sun and is redder and less luminous.
Death of an ordinary star
Low mass stars, like the Sun, are ordinary stars that after burning up its hydrogen, collapse under its own gravity.
Hydrogen around the core continues to burn though and the star evolves into a red giant. Its size then will be huge so that Earth and its moon, as well as the inner planets will vanish.
More burning will occur, particularly the helium burning into carbon, will last for about 100 million years. When the helium has all been burned up, the red giant will now become a supergiant.
In its supergiant stage, the Sun’s outer envelope will stretch further out as far as Jupiter. It will continue in this phase for a few tens of thousands of years and lose its mass, as more helium is burned up.
Finally, the Sun will lose it mass in the outer envelope and leave behind a core of carbon inside a nebula of expelled gas.
Radiation from this hot core will ionise the nebula giving it the beautiful colours like the nebulae often seen around the remnant of other stars.
The carbon core will cool and become a dwarf star, the dense dim remnant of a once bright star.
Death of a massive star
Massive stars burn brighter than our Sun and die faster – they have shorter lives.
When a massive star exhausts the helium in the core, it contracts further and reaches temperatures where carbon is burned to oxygen, neon, silicon, sulphur and finally to iron.
Because iron is a stable form of nuclear matter, no burning occurs and the iron core collapses until it reaches very high densities where the collapsing matter may “bounce” off the core. This bouncing effect (which includes the release of energetic neutrons from the core) produces a supernova explosion.
For a month or more, a single star (in this phase) burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other elements into interstellar space.
It is also in this region where heavier elements are produced. (This material can later be incorporated into the formation of other stars and planets.)
The death of the neutron core depends upon the mass of the progenitor star. If its mass is 10 times the mass of the Sun, the core will form a neutron star which is detectable as a pulsar – an object that emits radio waves. (A neutron star can have a diameter of about 11-11.7km but its mass can be twice that of the Sun. It is the smallest and densest of all stars.)
If the core of the progenitor star is so heavy that nuclear forces cannot resist the pull of gravity, the core collapses to form a black hole.
A black hole is believed to have a very strong gravitational field that it attracts everything close to it – matter as well as energy, like light. A black hole cannot be seen but astronomers can study how strong gravity affects stars and gas near a black hole.
Moreover, the late Stephen Hawking showed that black holes have a temperature and they glow and they radiate out into space – and ultimately over a long time evaporate away. This is called Hawking radiation.
Research to learn more
If you are interested in astronomy as I was decades ago, go online and learn from sites such as NASA’s.
In our country astronomy is not offered as a separate discipline and students who are interested in the field may have to study physics and later go abroad to study it.
l Next week: Our traditional people knew the stars. – The author is a freelance writer.