Diamonds that stud the night skies!
How we wondered what they were!
Now we know a little more about them.
The study of stars is the core of astronomy
Stars are the building blocks of a galaxy. There are billions of stars in a galaxy and there are billions of galaxies in the universe. On any given clear starry night we see only about three thousand stars.
Stars are engines that produce light, ultraviolet rays, gamma rays, x-rays and other forms of radiation. They are composed of subatomic particles.
When we look at the stars some appear brighter than the rest. This could be either because those stars are closer or it could also be because they put out more energy. The more energy a star sheds, the brighter the star. This amount of energy a star puts out is called Luminosity.
Stars also have different temperatures and that can be determined by their colors. The hottest stars appear as white or blue and colder ones appear orange and red.
Stars come in many sizes too. They range from dwarfs to supergiants. Supergiants may be thousand times larger than our own sun.
Picture below shows comparative size of our Sun to other huge stars.
Lifecycle of a star
A star is born
The space between stars is full of gases in various forms – ions, atoms, and molecules- and is known as the Interstellar Medium (ISM). Hydrogen is the primary building block of stars. The gas circles through space in cosmic dust clouds called nebulae (singular: Nebula). Stars are born within these nebulae and are scattered throughout most galaxies.
The nebulae that stars form in are known as stellar nurseries. These stellar nurseries are made of clouds of dust and gas called molecular clouds. All stars are born out of nebulae.
However, not all nebulae are clouds of star formation, while eventually their gases and dust do gather together to form stars under the right conditions.
There are different types of nebulae which we will learn about later.
Elephant trunk nebula – a stellar nursery
The Elephant’s Trunk Nebula. Within the globule, protostars can be spotted easily as the bright red-tinted objects.
Where do the gas and other cosmic dust clouds come from in the first place?
The first original material came when the universe came into existence.
Atoms came into existence. As more atoms formed they got closer and closer and soon gathered together as the original nebulae and the first set of stars came from them.
The second kind of nebulae are those that are produced by supernovae and exploding stars. For example the crab nebula.
However a nebula can be a mixture of the original (primordial) matter and material from exploding stars.
A lot of disturbances known as Turbulence take place deep in these clouds. This results in the formation of small knots that have enough gravity to attract the surrounding gas and dust.
What causes these disturbances or movement of the clouds? A passing star or an explosion of a nearby star can trigger a disturbance of the clouds of a nebula thus starting the formation of a new star.
As the clouds start gathering and getting closer, a process known as accretion, the knot begins to spin faster.
Have you seen a skater spin on ice? As the skater brings the arms close to the body he/she spins very fast.
This is called conservation of angular momentum. This is what happens in space too and as the gas and dust get closer it begins to spin faster. The faster it spins the more gravitational pull is generated, attracting more and more gas and dust. This core keeps attracting the surrounding clouds by its gravity and will one day become a star.
The tighter the clouds get, more pressure builds up in the material at the center, and the core gets hotter. When the temperature goes up to 15million degrees Celsius nuclear fusion begins. This core is called the protostar.
Image of a protostar
Most of the time these clouds, as they spin and collapse, start breaking up into two or more blobs of clouds. That’s why, in the Milky Way Galaxy we can find many pairs or groups of stars. Though our sun is a standalone star, almost ¾ of the stars in our galaxy exist as pairs known as the binary system.
Not all of this material ends up as part of a star — the remaining dust can become planets, asteroids, or comets or may remain as dust.
Stars’ Youth and Adulthood
The next stage of a star’s life is known as main sequence star.
As the protostars develop they accumulate mass from the clouds around them and grow into what are known as main sequence stars.
Our sun currently is a main sequence star. It is emitting energy for billions of years converting hydrogen to helium. This energy builds pressure in the star and it is also this energy that makes it shine. The pressure keeps the star from collapsing under its weight.
A star the size of our Sun requires about 50 million years to mature from the beginning to adulthood. Our Sun will stay in this mature phase for approximately 10 billion years.
Star sizes and colors
Main Sequence stars vary in size, color, and luminosity. They are classified based on these characteristics.
The smallest stars, known as red dwarfs, may be as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3,000-4,000Kelvin.
It is because of this temperature range that they look red.
These Red Dwarfs are the most abundant in the Universe and have the longest life spans – tens of billions of years.
Our Sun looks yellow because its surface temperature is about 5,800K
On the other extreme are huge stars, 100 times bigger than our Sun and have temperatures as high as 30,000 – 40,000 K.
There high temperature stars look blue. They are called Hypergiants and emit thousands of times more energy than the Sun.
However, their lifespan is shorter, about a few million years.
In the beginning of the formation of the Universe, Hypergiants were abundant. Now they are rare, only a few in our Milky Way Galaxy.
The main sequence phase comes to an end when:
- All the hydrogen in a star’s core is used up, nuclear reaction comes to an end.
- Now the core has no energy to support it and so all the helium starts to sink into the core, becoming hotter and hotter.
- This raises the star’s temperature and causes the outer layers to expand.
- These swollen stars are known as red giants.
Up to this point all the stars undergo the same phases.
What happens after this is determined by the size of the core.
An average star like our Sun will continue to shed its outer layers till the core is exposed. This core, though dead, is extremely hot, like a coal cinder, and will be known as a White Dwarf.
White Dwarfs will be small, almost the size of the Earth, but will contain the entire mass of the star it once was.
So the smaller the White dwarf the denser it is.
Scientists used to wonder how the dead core with its enormous mass did not collapse within itself under its own weight. They found out that pressure from the fast moving electrons in the core kept the white dwarfs from collapsing.
White dwarfs, due to lack of energy production start cooling down and eventually fade away.
Stars up to 1.4 times the mass of our sun become white dwarfs.
Cats’eye nebula – in the process of becoming a white dwarf.
White Dwarfs in binary/multiple star systems
If a white dwarf is close to a companion star it starts pulling the companion star’s outer layer matter on to itself. This is mostly hydrogen.
When enough hydrogen accumulates to trigger a nuclear fusion, the surface explodes, the white dwarf brightens and the some of its material is thrown off.
After sometimes the surface cools the cycle starts again.
These stars are called Nova
At first when folks observed this brightening of the white dwarf they thought it was a new star and called it Nova which means “new” (novae-plural) in Latin. Now, though it is understood that these are in fact old stars, very old indeed, the name still stuck.
Sometimes when the white dwarfs are close to the 1.4 solar mass limit, they start gathering so much mass that they may collapse and explode completely, due to the intense inner pressure. These are called Supernovae.
Let us look into the stages of collapse when a star is above eight times our solar mass.
A supernova does not mean that it is just bigger than a nova. In a nova only a star’s surface explodes; in a supernova the entire star explodes.
In massive stars, a number of complex nuclear reactions take place.
Heavy elements also begin to form in the core. Formation of heavy elements causes depletion of all the energy to support nuclear fusion instead of creating energy.
The moment all nuclear fusion stops, the heavy core collapses.
In just a matter of seconds the core shrinks from roughly 5000 miles across to about 12 miles (20 km)
Now the temperature goes way up high to 100 billion degrees or more.
The entire star now rebounds and explodes with enormous and unimaginable amount of energy
For a period of days to weeks, a supernova may outshine an entire galaxy.
All the naturally occurring elements and a rich array of subatomic particles are produced in these explosions.
Kepler’s supernova remnant
This supernova was discovered by Kepler in 1604.
What happens after a Supernova explosion?
Supernovae Leave Behind Neutron Stars or Black Holes
If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star.
(Will learn more about neutrons later)
Neutron star – Cassiopeia A
If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. (more on black holes, later)
Rising from the dust, literally
The dust and cosmic material of atoms, sub atoms, ions, and molecules, eventually get closer and form another stellar nursery where new stars with planetary systems of their own form and the cycle is repeated.