Stars: everything you need to know



Harriot Tubman

Stars, they light the night sky, hundreds and thousands of them with each one contributing a twinkle, making the night sky just so mesmerizing.

It turns out that the number of stars that we see is just a tiny fraction of stars scattered throughout our universe. There are possibly countless of them. Huge fiery balls of gas and dust, that’s how we describe them.


Before jumping into the article we shall first take a look at some of the basic units in astronomy that will be frequently used.

  • Light year – it is the distance that light travels in one year. 1 light-year = 9.4607 × 1012 km
  • Astronomical unit (A.U) – it is the average distance between the earth and the sun. 1 AU = 150 million km
  • Parsec – distance from the sun to an object which has a parallax angle of one arc second.
A parsec
  • Solar mass – the mass of the sun. equals to approx. 2×1030 kg

Life stages of a star

Birth of a star

the helix nebulae
Helix Nebulae
Image credit – NASA

Large clouds of gas and dust accumulate, to form clumps. These clumps get pulled by their gravity. This is a Nebulae, the birthplace of all-stars.

Ready for its first step – the shine

Now as more and more clumps of gas and dust get together, it bounds so tightly that it gets hot. But not hot enough to make the star glow.

Taking the first step – the glow

When it reaches 10 million degrees Celsius, nuclear fission begins. When the clump condenses it becomes a glowing star.

Stable shine

shining star

The outward expanding force is balanced by the inward pull of gravity of the star. Hence the star stays in the same size and burns for billions of years.

Red and huge

When all the hydrogen fuel of the star is burned, the core of the star starts to shrink and begins the process of fusing helium atoms. The outer layers cool and swell turning the star into a gas giant.

The collapse

After all the billions of years for the formation of the star, it takes just seconds before the star collapses. Bigger the star, faster it runs out of its fuel and the star collapses inwards, taking in all the energy instead of emitting it.


Kepler supernova
NASA/ESA/JHU/R.Sankrit & W.Blair, Keplers supernova, marked as public domain, more details on Wikimedia Commons

When a huge star collapses, it looks like a super-giant nuclear explosion called supernovae. For a few weeks, it shines with the brightness of millions of suns.

Death of a star

neutron star
Neutron star
Image credit – NASA

When the supernovae cool, it condenses into a pulsar, a rapidly spinning star made mostly of neutrons.

A smaller star, smaller than our sun, cools and shrinks into a white dwarf.

When a very huge star collapses, it continues to shrink until all its matter is compacted into a single point of infinite density. These are called “black holes”.

For a star to collapse into a black hole, its mass must be 2.7 solar mass, 3.0 solar mass or should exceed 3.2 solar mass. To know more about black holes check our article on – a beginners guide to black holes.

Well, that was just some basics. And we aren’t here for basics, are we?

So let us take a closer look at each level.

Types of stars

Stars are classified based on their spectral characteristics


As the name suggests, a Protostar is what we have before the full formation of a star.

To understand this stage, let us first understand how stars are born.

We know that galaxies have interstellar gas and dust scattered throughout the space. So when this gas clumps together, it collapses under its gravitational force.

Now, as it collapses, the material in the center experiences a great force, as a result, the material at the center stars to heat up. at this stage, the star will be called a protostar.

It is this hot core that one day takes the shape of a star.

The protostar phase of the star lasts for about 100,000 years to 10 million years depending on the size of the star that is about to form.

T Tauri Star

T Tauri stars fall just between the stage of a protostar and a low mass main-sequence star.

T Tauri star
drawing of T Tauri star

These are newly formed stars that are less than 10 million years old and have a low mass that is less than 3 solar masses.

The fact that it has not entered the main sequence phase clears out that the temperature at the center of a T Tauri star is insufficient for the initialization of nuclear fusion.

This type of star tends to exhibit periodic and random fluctuations in their brightness.

But wait! Nuclear fusion hasn’t started yet, so how is the star radiating?

Well, these emitted radiations are entirely a result of the gravitational energy that is released as the star contracts under its gravitational force.

A star can remain in this phase for about 100 million years.

Main Sequence Star

internal view of a main sequence star
An internal view of a main-sequence star
I, SakuramboSolar internal structureCC BY-SA 3.0

After the accumulation of gas and dust to form a clump that collapses under its own gravitational force to form a protostar, if the clump of gas and dust had sufficient mass, that is greater than 0.08 times the suns mass than the collapsing material in the core gets more and more hot and finally reaches a stage where the temperature is enough to fuse hydrogen to helium.

These types of stars are common and in our galaxy, they are a majority. Our sun, for instance, is a main-sequence star.

Similarly, our nearest neighboring stars, Sirius and Alpha Centauri A are main-sequence stars.

They vary in size, mass, and brightness but they all function the same way that is converting hydrogen fuel into helium in their cores and releasing a tremendous amount of energy.

Now since this conversion of hydrogen to helium produces an outward pressure that is released as heat, there should be an inward pressure that balances this outward pressure.

This is the force of gravity that counteracts the outward pressure eventually stabilizing the star.

So what happens to the stars that have a mass less than 0.08 that of the suns?

Well, these stars do not enter the main sequence phase but instead end up in becoming what are called the “Brown Dwarfs”.

Red Giant Star

This is like the old age of a star.

Just like when we get old, we are basically out of energy, the stars that reach this stage have completely run out of hydrogen in their core.

So now there is no fusion taking place in the core. As a result, the outward pressure that balanced the inward gravitational pull vanishes.

Now since there is no force to counteract the inward pressure, gravity compresses the star even tighter, making it smaller in size.

But this is not the end.

As the star is compressed more and more, the temperatures rise rapidly until a level is reached when the helium starts fusing into carbon.

Also, as the temperatures rise, a shell of hydrogen that surrounds the core of the star heats up. This heating continues until there is enough heat to start hydrogen fusion in the shell.

Interestingly, this hydrogen fusion gives off more energy than what it used to give when it was a main-sequence star.

As a result of this process, the star expands enormously in size. This expanded form of the star can be 100 to 1000 times larger than the main sequence star.

This phase lasts for about a hundred million years.

Our sun as we know is in the main sequence phase. And it is estimated to continue its shine for another 5 billion years after which it will enter the red giant phase.

And when in this phase, it will sweep away the planets – Mercury, Venus and probably the Earth (we will be gone till then, so chill).

White Dwarf Star

white dwarf
white dwarf

This too is the old age of a star. A stage followed by the red giant.

So as we know, when the main-sequence star of sufficient mass runs out of its hydrogen fuel it gets compressed more due to gravity.

And as a result of that, now there is more pressure exerted on the core which makes helium fuse to carbon.

When this happens, the star expands in diameter by shedding its outer layers far in space. This expanded star loses about half of its total mass by shedding its layers into a ring – planetary nebula.

But what about the other half?

The core of such a star will be what’s left, this will be called the white dwarf.

Let us take our sun for instance.

After it has gone through its red giant phase, shedding all of its layers to the outer space, the core of the sun which is now a white dwarf will be containing about half the mass of the sun.

But here is when things get interesting!

Even when the white dwarf will contain half the mass of the sun, it will be just the size of our earth. Which means that it will be super dense.

In fact, white dwarfs are considered to be one among the densest celestial objects that are there, after black holes and neutron stars.

Higher density also means the gravity will go crazy in number. In the case of a white dwarf, the gravity can be 100 times more than that of the earth.

This means if you plan on landing one of them, you’ll end up melting and steaming.

The heat of such a white dwarf can be about 40 times more than the sun.

So in the case of white dwarfs, what is the force that counteracts the pressure of gravity? White dwarfs, as mentioned above are so tightly packed, that the electrons in it get completely smashed together.

They form what is called a degenerate matter. This packing is so tight, that the electrons themselves cannot be pushed further and exert an outward force that is enough to counteract the pressure of gravity.

Due to the temperature of the universe, the star now cools from its hot state. This process takes hundreds of billions of years. In the process, it will give out a very stable shine.

Red Dwarf Stars

red dwarf
red Dwarf
NASA/Walt Feimer [Public domain]

Coming to one of the most interesting types of stars – a red dwarf.

These are formed just like the main sequence stars. You remember right? A swarm of gas and dust gets together accumulating layer by layer.

And when the pressure due to gravity increases to a critical point, to counteract the pressure, hydrogen fusion starts in the core.

The difference is noticed in the way hydrogen fuel is consumed. In the case of the main-sequence star, hydrogen fusion takes place only in the core before reaching its later stages, whereas in a red dwarf, hydrogen fusion takes place both – on the inside and outside the core of the star.

What is the advantage you ask? Since hydrogen fusion takes place all over the star, the heat given out by such a star will be very less as compared to the actual main sequence stars.

This means that hydrogen burns less rapidly. Such stars can have a lifespan ranging from 1 trillion to 100 trillion years! Compare it to sun-sized stars who have a lifetime of just 10 billion years.

Red dwarfs are very common stars. In fact, they make up the largest population of stars in the galaxy. 20 out of 30 that are close to earth are red dwarfs.

But even after that, red dwarfs are very difficult to see. The reason behind this – they burn at a temperature of just 3,500 degrees Celsius (6,380 degrees Fahrenheit).

Compare it to the sun which burns at 5,500 degrees Celsius (9,900 degrees Fahrenheit). This extreme temperature difference makes them appear so dim that it is impossible to see a red dwarf with your naked eyes.

The smallest red dwarfs are 0.075 times the mass of the sun and the largest ones can have a mass that is half the mass of the sun.

Neutron star

neutron star
Casey Reed – Penn State University [Public domain]

To understand this very interesting type of stars, let’s have a look at the stages of fusion processes that a star of sufficient mass undergoes.

First comes the very common hydrogen fusion-where the hydrogen atoms fuse to form helium atoms.

The star continues to crunch under its gravity, posing enough pressure for the next fusion – helium fusion. Helium is fused to carbon. still massive, and carbon gets fused to neon. Furthermore – neon gets fused to oxygen. Oxygen to silicon. And lastly, silicon is fused to iron.

These fusion reactions layer up as concentric shells in the star as depicted in the diagram.

When it reaches the stage where silicon is completely fused to iron, there is no further fusion taking place anymore as the iron cannot fuse to further elements.

So now when the fusion reaction stops, there is no outward pressure to counteract gravity. The star loses its radiation pressure.

Now here comes an even interesting part,

If the mass of the core exceeds 1.4 solar masses, the core collapses catastrophically at the center of the star with the outer parts of the core reaching velocities of up to 70,000 km/s

Now as the core is collapsing the only force that’s there to combat the inward gravitational pull are the fundamental forces of an atom.

A rule in quantum mechanics says that the electrons strongly resist being squeezed together but even this rule fails when the core has sufficiently huge mass (1.4 solar mass) and the core continues to collapse.

The quantum mechanical repulsion of electrons is overcome as a result, the electrons and protons fuse into neutrons and get packed super densely as an atomic nucleus.

Now the core is approximately containing about 1 to 3 solar mass and is mostly made up of neutrons and very few protons and electrons that survived. All this packed in a ball that is 20 km (12 miles) in diameter.

The next stage. Just like electrons, even neutrons resist being squeezed too tightly. But in the case of neutrons, the resistance offered is way stronger than that of electrons.

And if the core is less than about 2.8 times the mass of the sun than the collapse of the star comes to a halt.

As a result of that, the star generates a huge shock that releases a stream of energetic subatomic particles called neutrinos into space.

What happens to the outer layers? They get launched into space with a giant supernova.

Finally, what’s remaining is a core that is entirely composed of neutrons, called a neutron star.

The mass of a neutron star can be between 1 – 3 solar mass but due to the super high density, it will be just 20 – 25 km wide.

And the magnetosphere of such stars is around 8 trillion times that of our earth’s magnetic field.

A very surprising property of neutron stars includes its spin rate.

A neutron star spins very very rapidly!

especially if there is another star nearby, that is if it survives the supernova or is just a captured star, then the neutron star will draw matter from the star towards itself and peak its spinning rate, going up to hundreds of rotations in just a second.

Take the star PSRJ1748-2446ad. This neutron star spins approx. at the rate of 252,000,000 km/h. Yes, you read it right. This is 25% of the speed of light.

Super Giants

Now, these are some of the largest stars in the universe. They have a mass that is dozens and dozens of times the mass of the sun.

if we look at our sun, it is a relatively stable star, converting the hydrogen fuel at a stable rate. But Super Giants, they don’t work this way.

They consume their fuel at an enormous rate and hence they consume all their fuel in just a few million years. They die as supernovae, completely disintegrating themselves.

Star systems

Just like our solar system, where all the planets, asteroids, etc. revolve around a central mass, stars have their system.

Here the center can be a massive star or they could be gravitationally bound to each other and revolve around a common point in space.

Do you notice during clear skies (free from light pollution) we see these thousands of stars so close to each other? Some may even think that they are star systems together, you can be right sometimes, but mostly that is not the case.

We know that stars are separated from us by really huge distances, therefore many times it may occur to us as if two stars are very close to each other when in reality they are far apart. This happens because of the closeness of those two stars, they appear to take the same point in space.

To understand it better, take a look at this diagram.

apparent distance

So know we can have two classifications based on how we observe the star systems.

Either they could just appear to be a part of a system by appearing very close together, in which case they will be termed as optically multiple stars.

Or they could be close together for real and form a stellar system. These are called physical multiple stars.

There are two types of star systems –

Hierarchical system

These are very well organized stellar systems having quite stable orbits. These types of systems show nested orbits.

8 star hierarchical system

Most of the stars tend to show this form of arrangement where there is very little interaction between the orbits. As the star moves around the system’s center of mass, it continues to form a stable Keplerian Orbit around it.

16 star hierarchical system


Contrasting to the hierarchical system, this one is chaos! With highly unstable orbits, the stars have a strong interaction.

The reason why such a system exists is explained by the young age of the stars in the system. We know for a multiple star system to form, we require a very huge stellar nursery.

A nebula as we call it. These newly formed stars then quickly fragment into stable multiple stars. Therefore such a system can be commonly found close to or within bright nebulae.

But this type of stars do not remain together. As time passes, in the end, it usually happens that amongst many stars that once formed the system, only two of the stars remain in orbit around each other as binaries with distant companions.

All other stars get ejected outwards with very high velocities.

In a star system, there can be just two stars that revolve around each other at a common center, where they will be called binary stars. There can be three stars – ternary and so on. A stellar system, with a large number of stars, is called a cluster.

Binary stars

It was in the 18th century when astronomers had started to realize that some of the stars that appear to acquire a commonplace in the space were actually orbiting each other.

Later it was known that most of the stars are binary. In fact, nearly 1/2 or 1/3rd of the stars in the sky are binary. One famous example I must give – Mizar and Alcor. In the handle of the big dipper (Ursa Major constellation), these two stars form a binary system.

But also, these stars are members of the even larger – sextuple system. As viewed from earth, they appear to be so close to each other that you require very sharp eyesight to differentiate the two.

Due to the close distance, the stars were used as an eye test in ancient times.

Binary stars form together unlike single stars like our sun.

Now, these binary stars can be further classified based on the way we see them.

Visual Binaries

these are simply the stars that can be resolved by a telescope.

Yes I know what you are thinking, telescopes are getting better and better so it’s not a good classification to rely on.

And that is exactly right. As the angular resolution of the telescope keeps on increasing we get more and more visual binaries added on the list.

Further, depending on the brightness of the stars, we have primary and secondary stars. Primary are those with greater brightness, and secondary are those that are comparatively less bright.

If the two stars happen to have the same brightness than the designation of primary and secondary star is customarily accepted.

Eclipsing Binaries

These are the star system who’s plain of orbit lie very near to our line of sight.

So all that you will see as the two stars orbit each other will be an eclipse over and over again.

These stars appear to be so close to each other that to an observer it appears as a single source of light. But if you see its brightness variations in spectroscopic observations, it will be quite clear that they are two stars that in close orbit with one another.

What is the brightness variation?

We know that generally in a binary system there is one star that is brighter than its companion.

So as they both orbit each other, in the case of eclipsing binaries, what happens is,

brightness variation

When both the stars are visible we get the maximum brightness.

brightness variation
brightness variation

As they orbit, the relatively smaller star comes in the sight of the primary star. When this happens, there is a small dip that is seen in the brightness of the star.

brightness variation
brightness variation

As it completes its revolution, there comes a point when the secondary star is on the far side, and all that you see is the primary star all alone.

brightness variation

When this happens, the brightness drops dramatically. There comes a huge dip in the brightness levels.

brightness variation
brightness variation
brightness variation

The best example of such a star system is Algol, a triple star system in the constellation of Perseus.

Spectroscopic binaries

Contradicting to the visual binaries, these are the stars that cannot be resolved using a telescope because of their proximity to one another.

But wait! If you cannot see them, how do we even know they exist? Well here is when spectroscopy comes into play.

Let me quickly tell you. As two stars orbit each other, as an observer, you’ll see one star moving away from you and the other moving towards you.

Remember something? Exactly! The Doppler Effect. It states that there is a change in the frequency or the wavelength of a wave concerning an observer who is in motion relative to the wave source.

Apply it here. The star that is moving away, undergoes a redshift. And the star that is moving towards will undergo a blue shift. This spectrum reveals that the stars are binary.

Doppler effect
the Doppler effect

Single lined spectroscopic binaries.

These are the spectroscopic binaries where only one of the stars spectrum is seen.

So in here, astronomers will see just one star undergoing a redshift as it goes to the far side and a blue shift as It comes closer.

Double-lined spectroscopic binaries

Here, the spectra of both the stars are known and they both keep switching the Doppler shift alternately as they orbit each other.

Non-eclipsing binaries

We now know how to spot the eclipsing binaries. But what about the binaries that do not eclipse?

Photometry saves the day.

It is a technique used by the astronomers to determine the intensity of light radiated by the targeted celestial object (in this case – a star). It probably came in the 18th century.

to know more on photometry click here

Astrometric binaries

These are the stars that appear to orbit all alone around a point in space.

There are various reasons why it appears to be alone. It can be that the companion of the star is very dim.

Secondly, it can be because the companion star emits little or possibly no electromagnetic radiation.

Also, the companion can be masked by the glare of the primary star.

So here, all that you’ll see is a star that happens to revolve around a point in space.

The way to detect such types of stars is by taking repeated measurements of the position of that star relative to the distant stars. In this way, as the star performs its revolution, you’ll notice the periodic shifts occurring in its position.

This method is only applicable to stars that in the range of 10 parsecs.


Based on position and proximity, binary stars can be classified into three groups.

But before looking at the groups, let us first understand the term, Roche Lobe.

Every star has some amount of region surrounding it, rising at a certain distance above the star. The material stays in orbit around the star as it is bounded by its gravity in this region. This region forms a lobe like structure around a star. This structure is termed as a Roche Lobe

3 simple configurations:

Detached binaries –

detached binaries
detached binaries

Each of the two stars remains within its Roche lobe. Most of the binary stars that we know of, fall in this category.

As both stars stay undisturbed, there is not much effect caused by the two stars on one another.

Semidetached binaries

semi-detached binaries
semi-detached binaries

Here, one of the two stars comes in contact with its companion’s Roche Lobe whereas the other does not. As a result of this, one star (the one that fills its companions Roche Lobe) acts as a donor, transferring its gas and dust to the companion.

Subsequently, the companion generates a disk of gas and dust swirling across it, called the “Accretion Disk”.

Contact binaries

contact binaries
contact binaries

In here, both the stars fill the Roche lobes. If fact they may even end up merging in the process.

Different measurements


Determining the age of a star is quite a difficult task for astronomers. Moreover, if the star is alone, the difficulty rises even more.

When the star is in a cluster, it gets relatively easier for astronomers to determine their age. It is simply assumed that all the stars in a cluster begin their life together.

Through decades of observations, it is known to them that if you plot the brightness and the color of a star, then the pattern that you see helps to identify the age of that star.

But this method is valid only for those stars that are in a cluster.

Now let us look at how the age of an isolated star is calculated.

Have you ever tried spinning a top? Notice how its rotation gradually slows down with time. The same is true for a star. As star ages, its rotation speed decreases. This decrease in the speed can be used as a clock to determine the age of that particular star.

But before deriving a star’s age based on its rotational speed, we have to know the exact relationship as to what rotational speed will mean a particular age.

To get this relationship, astronomers observe the stars that are in clusters, whose age can be easily derived. After they have found the star’s approximate age, now they can observe the rotational speed of that star and generate a relationship between both quantities.

This relationship will aid astronomers to know the age of stars that are isolated. By just obtaining the rotation speed of that star.

For example – NGC 6811 is an open cluster in the constellation of Cygnus which has an angular size equal to that of the full moon. It has about 1000 stars having roughly the same magnitude.

Astronomers, using the Kepler telescope have measured the rotation rates of the stars in this 1 billion-year-old cluster. This gives them knowledge on the relationship that is shared between a star’s rotation speed and its age.

Now that they have a link between both the quantities, to obtain the age of an isolated star, all they have to do is get the rotation speed and calculate its approximate age with the help of that relation.

Most of the observed stars are between 1 billion and 10 billion years old.

A birth certificate that will give you chills!

it is about the star HD 140283 also known as the Methuselah star, about 190.1 light-years away. Astronomers estimated the star to be 14.5 billion years old. That is older than the age of the universe.

The method of determining the age of a low-mass star like the sun from its rotation speed is called Gyrochronology.

Spin rate

Now that we know for calculating the age of a star it’s spin rate plays a very vital role, let us understand how it is measured.

To measure the spin rate of a star, the most important quantity is its brightness. Astronomers keep a constant eye to record the brightness of the star.

So how does the brightness of a star tell us its spin rate?

For this, we shall do one experiment.

Take any light source that has a sufficiently large area that is lit and is properly insulated or better still, completely disconnected from the AC mains (like a torch or an LED bulb that operates on batteries).

Now stick a small (approx. 1cm in radius) circular piece of paper that is colored black on the surface of the light source that you are using. Now start rotating the bulb, notice how the brightness of the source falls as the paper comes right in between the illuminant part and the wall.

You’ll also notice that the brightness revives again as the surface covered by the paper faces the opposite side of the wall.

The same goes when observing a star.

Just like sunspots, stars have their dark spots that remain fixed on the surface.

When these dark spots are encountered on the face of a star, the brightness of that star will fall slightly. And as the star spins, the spot will shift to the side reviving the brightness bit by bit.

Eventually, as the spot reaches the far side of the star, the brightness gets completely restored. Again after a fixed interval of time, the dark spot appears and the cycle repeats.

By measuring the time taken by the spot to complete one rotation, astronomers calculate the spin rate of the star.

But this process as it sounds isn’t that easy.

This comes down to the fact that the dark spots tend to be very very very small compared to the size of a star. This means that the decrease in the brightness is so slight that we require very sensitive instruments to observe it.

Moreover, if the star is old, then the dimming gets even lower, which is almost negligible. The diming of stars that are older than half a billion years cannot be measured from earth because of atmospheric interference.

Kepler space telescope
Kepler space telescope
NASA [Public domain]

For such stars, we have a special instrument in space, designed especially for measuring stellar brightness with ultra-precision. The Kepler space telescope.


The mass of a star is a very important quantity. This is mainly because it tells about the evolutionary past, present, and future of the star.

So, how is the mass of a star measured? Let us see how.

Well basically, there are several methods to know the mass of a star.

The method we shall look at is called Gravitational Lensing. In this method, the basic ideology is that everything in the universe has mass and it exerts a particular amount of gravitational pull on other objects.

Even you and me, have this force inside us but since our mass is so less as compared to the celestial objects, the force exerted becomes negligible (otherwise who wouldn’t enjoy humans colliding like meteors right?).

The concept of Gravitational lensing was first suggested by Albert Einstein in his “theory of General Relativity”. He derived the math for how the light is deflected as it passes through the sun’s gravitational field in 1912. And in May 1919, his idea was tested during the total eclipse of the sun.

Light, as we know, travels in a straight path until and unless it is disturbed. Now, when the light from a more distant object passes by, it gets caught in the gravitational pull of massive objects.

By the time it reaches the observer it is bent and is refocused. These objects can just be anything with considerable mass (galaxy, star, planet, black hole, etc.).

By making a note on exactly how much of the light is distorted by the object (which in this case is a star) in the background, we can tell how massive the object can be.

The relation between the two is direct. That is – the more massive the object, the more gravity it will have, and more gravity means high potential for the star to bend the light in its background.

But this idea of gravitational lensing came in the 1900s. So how did the astronomers deal with finding the mass of a star before the introduction of Gravitational Lensing?

So this is what they did. Before G.L (gravitational lensing) was introduced – astronomers were depended on measurements of stars that shared a common center of mass like binary or multiple star systems (more about star system is covered in this article).

They measured orbits of all the stars in that system along with the orbital velocity, radius, and the orbital period. After getting all the unknowns, a formula is used to derive the mass of stars.


Where M is the required mass, r is the radius of the star, G is gravitational constant (6.67408 × 10-11 m3 kg-1 s-2) and v is the orbital velocity.

But this technique was not so efficient and was not found applicable to all-stars.

So they had another way to resolve it. The Temperature – luminosity relationship.

Astronomers observed that the mass of a star, varied with its temperature and luminosity. This gave rise to a famous diagram – The Hertzsprung-Russell diagram. It is a plot of temperature vs luminosity.

A star with a particular temperature and luminosity (absolute brightness) secures a particular place in the graph. Depending on where the star is placed, what color and temperature it emits, astronomers can deduce its mass. For instance, suppose the star lies in the main sequence curve, then astronomers already know that the star will neither be too small nor will it be a giant.


The next important thing after mass is the temperature of a star. There are several methods to find it. Let us see a few.

The first method is based on its color. It was observed that a cool star tends to emit more red/orange than blue / violet. For example – Betelgeuse has a surface temperature of just 3,500 k and produces a bright orange color.

On the other hand, hot stars are more towards the blue/violet than red/orange. For example – Rigel is one such supergiant that emits blue light. It has a surface temperature of 15,000 k.

So, with that said, can you tell me what is the color of the star in our solar system, the sun?

for those who said red or orange, well, you are not entirely wrong because that is the apparent color that is visible to us during the sunset or sunrise. But the actual color of the sun is white! Yes, it is a hot star with a surface temperature of 5,778 k.

So how is this done exactly? Recording the color of a star is more precisely, recording the wavelength at which the star emits the maximum amount of light. Once we get the required wavelength, we can then use the formula –


Where T is the required temperature and λmax is the maximum wavelength.

Another way to measure the temperature is by making use of the “OBAFGKM spectral classes”.

The concept is that stars can be classified based on the strengths of their hydrogen absorption lines (OBAFGKM classification is explained further in the article).

That is, A – stars will be the one with the strongest hydrogen absorption lines B – stars are the next strongest and so on.

This was found in the 19th century. Then you may wonder why the order was jumbled? It is because late after this classification, in the 20th century, as advancements started to follow in the physics of absorption lines, it was observed that the spectral classes had something to tell about the temperature.

They noticed a sequence – OBAFGKM. With the O – stars being the hottest followed by the B – stars, next hottest and so on till we reach the M – stars which are the coolest in comparison to the rest.

Have a look at the numbers –

O-40,000k B-20,000k A-9000k F-7000k G-5500k K-4500k M-3000k.
(The sequence was later updated with two new spectral classes: L-objects having a temperature around 2000k and T-around 1500k. But the objects that fall in this category are not considered as stars because the temperature is too low to start fusion in the core).


When it comes to the composition of the star, a very useful method that can be used is Spectroscopy (more applications of spectroscopy are given further in the article).

In ancient times when there weren’t any techniques to determine the composition of a star or the planets, it was believed that the heavenly bodies are made up of some ethereal elements.

But as advancements took place in the optical field, it was soon realized that they are made up of almost the same materials that are found on our planet, in you and me.

The basic principle in spectroscopy is that every element in the periodic table produces a series of bright line that is unique to that element.

The line produced by hydrogen will be completely different than the lines produced by helium which in turn will be completely different from that of carbon, and so on.

Just like the bright lines, there is something called the dark lines, more precisely the absorption lines. These are caused when the light emitted by the source (in this case a star) passes through the material before reaching the observer.

This material can be a gas layer that covers the star. Astronomers get the spectra of the stars and look for these bright and dark lines.

These lines are like fingerprints of stars. Unlike our fingerprints, which are used to unlock our phones, only here, in the case of stars, they tell almost everything about them (applications discussed further in the article).

spectrum of the sun
solar spectrum

Take our sun for instance. Its spectrum will give you the light of all the different wavelengths going from blue to red.

Notice the dark lines in the spectrum? Yes exactly, these are the absorption lines. Every element saves a unique place in the spectrum.

For example, the hydrogen lines will always appear at approx. 656.3 nm. So if we are looking at a dark line at around 656.3 nm on the spectrum of any star, we know that the star has hydrogen in its chemical composition.


For the distance of a star, we have two methods.

The first method is called triangulation, also called parallax.

In this method, a star is observed on one day, and its position in the sky is noted. This process is then repeated after exactly 6 months.

So this is how it works. Since the earth revolves around the sun with an orbital diameter of 300 million km (186 million miles), by recording the difference in the viewing angles at the two positions, and by making use of a little trigonometry we can get the distance of that star.

But there is one limitation to this technique. That is, the method is applicable only on stars that are within the distance of 400 light-years from us.

So how do you measure the distance of those stars that are still further away? For that, we have another brilliant technique.

Measuring the brightness. In this method what astronomers do is, they obtain a color spectrum of that star and get its absolute brightness (the actual brightness of that star) and compare it with its apparent brightness (brightness as seen from earth).

By doing this they know how dim the starlight gets on its way to us. This relation helps them determine the distance of the star.


To measure the size of a star, you may think you can just point your telescope and measure the angular space taken by the star, right? Well, not exactly. To do that you will have to first measure the angular resolution of your telescope which is given by θ =≈(1.22 λ )/ D

here θ is the required size, λ is the wavelength and D is the diameter of the telescope.

Since all the stars are so far away from us, measuring the apparent size, as viewed from the earth doesn’t make any sense.

So instead, the amount of space that the star takes in the sky is measured, i.e. the angle it takes in space. Now, this angle is compared with the distance it is separated. This gives the true diameter of that star.

Another very interesting method is when a star gets blocked by the moon.

Though the technique yields a precise diameter, it applies to only a few stars.

In this technique, the dimming of the light is observed as the moon passes right in front of it.

Here, the time taken for the star’s brightness to completely vanish is recorded as the edge of the moon passes over it.

Now since we know how rapidly the moon is moving around us, and we also know the distance of the star from us, we can easily calculate the angular diameter i.e. the size of the star.


This is the apparent brightness of any celestial object.

This terminology dates to the ancient times when astronomers Ptolemy and Hipparchus had made star catalogs that were listed by their magnitudes.

According to this scale, the brightest of the celestial objects were the ones with a magnitude 1 and the faintest objects had the magnitude 6.

Brightness factor of one magnitude. Astronomers, adding precision to this scale have calculated that a 6 magnitude star is 100 times dimmer than a magnitude 1 star or a magnitude 1 star will be 100 times brighter than a magnitude 6 star.

So to calculate the factor of one magnitude, we have to just take the fifth root of 100 which is 2.5118. This means as we go from magnitude 1 to magnitude 2, the brightness is increased by a factor of 2.5118. From 2 to 3 – it will be 2.5118 * 2.5118 and so on.

brightness factor

With the advancement in optical astronomy, we were now able to view much fainter bodies, and also there were some stars and planets that outshine the magnitude 1 objects.

Therefore the scale had to be revised, with even dimmer stars taking their position beyond the 6th magnitude and the brightest stars would go in the negative number line.


So we now know that the apparent brightness of a star is called its magnitude, but what about the absolute brightness? well, the absolute brightness, i.e. the actual brightness of a star is called its luminosity.

Hertzsprung-Russell Diagram

The diagram is a graph of the temperature of the star vs the luminosity.

Named after the two people who independently developed it in the early 1900s, Ejnar Hertzsprung, and Henry Norris Russell. This diagram is very important in the study of stellar evolution as it can tell us the evolutionary stage of a star, also the internal structure by just locating the position of the star on the diagram.

Let us see how it works,

There are 3 sections to the diagram.

The main sequence.

This is a thin section stretching from the upper left to the bottom right of the diagram.

Here, you will find all the main sequence stars that burn hydrogen to helium in their cores. The hot and luminous stars are found in the upper part of the section and the cool and much fainter stars in the lower section.

Stars spend about 90% of their life in this phase.


This section lies in the upper right corner just above the main sequence. It comprises of red giants and also supergiants.

As discussed earlier, giant stars tend to have low surface temperatures. Stars that enter this phase have already run out of hydrogen in its core and have begun with helium fusion.

The stars that fall in this section are having large diameters along with high luminosity.

White dwarf

As we know, white dwarfs have a very low brightness but they are impressively very hot. So in the diagram, they will lie in the bottom left.

The OBAFGKM classification of stars

In this classification, stars are ranked in the order of their decreasing temperatures.

So the stars that fall in the O class are the hottest followed by the stars in the B class and so on.

classeffective temperaturechromaticity
O ≥ 30,000 Kblue
B10,000 – 30,000 Kdeep blue white
A7,500 – 10,000 Kblue white
F6,000 – 7,5000 Kwhite
G5,200 – 6,000 Kyellowish-white
K3,7000 – 5,200 Kpale yellow orange
M2,400 – 3,700 Klight orange-red

Now within these classes, there are subclasses. These go from 0 to 9 depending on the position it stands in that class.

And lastly, there are Yerkes Luminosity classes. This class is to differentiate between the sizes of a star.

luminosity classes description
O or Ia+ hypergiants or extremely luminous supergiants
Ialuminous supergiants
Iabintermediate-size luminous supergiants
Ibless luminous supergiants
IIbright giants
IIInormal giants
IVsub giants
Vmain-sequence stars (dwarfs)
sd (prefix) or VIsubdwarfs
D (prefix) or VIIwhite dwarfs

Let us take our sun for instance. The spectral type of our Sun is G2V. This means that our sun belongs to class – G and the subclass – 2. And the final notation V tells us that it is a main-sequence star.

congratulations, now you can break any spectral type!

Spectrum: absorption and emission lines

Before understanding them, we must first look at what is a spectrum and why it is produced.

So a spectrum is what you’ll see when you pass white light through a prism.

You must have seen the largest possible representation of a spectrum produced by nature, a rainbow.

Photo by Alex from Pexels

White light consists of a combination of different wavelengths.

The Violet – Indigo – blue – green – yellow – orange – red (VIBGYOR). Arranged in the order of increasing wavelengths, with violet having the least wavelength and red having the max wavelength.

Beyond red, however, lies the infrared and before violet lies the ultraviolet but these wavelengths are not sensitive to our eyes.

Now let us understand what absorption and emission lines mean.

Joseph Von Fraunhofer, a Bavarian physicist, and optical lens manufacturer observed the sun’s spectrum very closely by expanding the spectrum on a large wall.

He saw thousands of slices that were missing in the spectrum. What were these dark lines? And why did they appear at very specific places on the spectra?

All the questions were resolved in 1859 when a German physicist – Gustav Kirchhoff and a German chemist – Robert Bunsen experimented with chemicals heated in a Bunsen burner.

They found that when this was done, each chemical would have characteristic bright lines. When compared with the previous spectrum it was seen that some elements had the bright lines at points on the spectrum that exactly matched the Fraunhofer’s dark lines.

hydrogen - emission and absorption spectrum
hydrogen – emission and absorption spectrum

The conclusion –

absorption spectrum
absorption spectrum

Absorption lines – the tendency of the cool atmospheric gasses to absorb the specific lines of light

emission spectrum
emission spectrum

Emission lines – glowing hot gasses produce bright lines at specific points in the spectrum

Applications of a spectrum

Now since each element in the periodic table can appear in a gaseous form, it can produce a series of bright lines that will be unique to that element.

So the emission line produced by hydrogen will be completely different than those produced by helium which will not be the same as carbon and so on.

By obtaining the spectrum of the stars, astronomers can identify what kind of elements the star is made of. This is called spectroscopy.

But the science of spectroscopy doesn’t stop there. Besides knowing the element, it has some bonus applications like,

The width of the lines in the spectrum tells us how fast the material is moving eventually giving us data about the winds on the object.

Temperature and density of the element.

If the star is orbit around another star, then the lines will shift back and forth. This gives us an estimation of the mass of the star.

The magnetic field can be obtained from the data.

We can also learn about the material that surrounds a star. This material can be an accretion disk or just the gas and dust that fills the space between stars.

We can learn about the physical changes in the star by observing glow and fade in the spectral lines.

Famous stars


You may know it as the North Star or the guiding star.

It is called such because of the purpose it served in the past, navigation. The star was used to navigate in the northern hemisphere.

The reason it worked that way was that Polaris appears to be very close to the northern celestial pole. So even when the earth rotates, there is very little displacement of the star’s apparent position in the sky, hence making it point towards the north.

Coming to the characteristics of the star.

It is the 45th brightest star in the night sky.

It is a part of the constellation Ursa Minor. Scientifically, it is known as Alpha Ursae Minoris. As it is the alpha star in Ursa Minor.


Also known as the Dog Star, Sirius is the brightest star in the night sky with an apparent magnitude of -1.46 and an absolute magnitude of +1.4.


The name Sirius itself means glowing in Greek. Since the star was so bright, it doesn’t require the use of any device to observe it, hence it was known to the ancients.

It is 8.6 light-years away and has a mass of approx. 2 suns, and in the competition of brightness, Sirius is 20 times brighter than our sun.

Its companion star, Sirius B is about 10,000 times dimmer than Sirius. To spot the star, just look up at the constellation Canis Major. The brightest star in the constellation will be Sirius.


This 450 million-year-old star, can be seen in the constellation of Lyra.

It ranks as the 5th brightest star in the sky and the 2nd brightest in the northern hemisphere.

Its name means falling or swooping. The star is about 25 light-years away from us.

A very interesting fact about this star is that it was once the North Star, i.e. it was very close to the northern celestial pole for several thousand years.

How is that even possible?! These are all the doings of our dear earth’s motions.

Earth wobbles around its axis which changes our perception of north and south. But this change is not sudden as it takes about 26,000 years for one complete cycle.

A piece of good news to the star is that it will retrieve its position in the north after around 12,000 years. Till then, we will keep loving our dear Polaris.


Image credit – NASA

This is an interesting one!

The star belongs to the constellation of Aquilae (the Eagle) and shines the brightest amongst all the stars in that constellation.

Separated by a distance of 16.7 light-years making it the closest star that is visible through naked eyes. Also, it ranks as the 12th brightest star in the sky.

It has a mass of one and a half suns, so it’s also massive.

Finally for the interesting part. The star has an unusually high rotation speed. It rotates at around 300km (185 miles) per second! And guess what, according to calculations, if the star would rotate just 10% faster than its current speed, then it would be blown off.

Rigel A

Coming to this giant that ranks 7th in brightness.

In case you wonder, here is why I called it giant.

Rigel is a three-star system in which Rigel A is the most massive, accompanied by 2 distant companions that are much much dimmer.

Rigel has already run out of hydrogen in its core. This massive star has 74 times the radius of our sun (don’t be shocked, things get creepier for other stars).

That huge radius means if we were able to place Rigel A in the sun’s position, its boundaries will reach somewhere near the orbit of mercury.

The fact that it is the 7th brightest star even after it is separated by a distance of 870 light-years should mean that it has some crazy numbers when it comes to the luminosity.

It is 47,000 times more luminous than our sun.

The name Rigel comes from an Arabic phrase, “Rijel Juazah al Yusra” which means – left leg of the giant. The giant here is referred to the constellation that Rigel is a part of – the Orion.

Lastly, due to its huge size, the fate of this star is probably a supernova.

Proxima Centauri

Chances are, that you have already heard about this star, for it is famous for being the closest star to the sun.

How close? Just 4.244 light-years (“just” on a cosmological scale. Otherwise that distance is crazy).

But if you haven’t heard of it, don’t worry we are here. Firstly let me tell you, this star, in spite of being so close to us, is not visible to the naked eye.

Why so? This comes down to various reasons.

One, its mass is very low (1/8th the mass of the sun). Second, it is just too dim (500 times dimmer than the sun).

Now the question of why it is so dim has an interesting answer. Proxima Centauri is a red dwarf (if you haven’t read about red dwarfs yet in the article, please check it out).

This star is a family member of the triple star system – Alpha Centauri.


A red Supergiant. This star is in its last phase of the life cycle.

As the clock ticks, the star is sure to end in a spectacular supernova. And when that happens, it will radiate all its matter to space which will probably serve as building blocks for new stars.

This star is among one of the largest known stars. It has about 700 times the diameter of our sun which means, place it in the sun’s position and it will probably reach the orbit of Mars or Jupiter.

It is, however, a variable star. So its diameter changes anywhere between 550 to 920 times the sun’s diameter.

Coming to its brightness, it is the 9th brightest star in the sky. The distance of separation – 800 light-years.


Also called Alpha Piscis Austrini, it is the 18th brightest star in the sky.

You can see it in the constellation Piscis Austrini. The star is 25 light-years away and is one of the closest neighbors to the sun. The luminosity of the star is 16 times that of the sun.

VY Canis Majoris

This one is a red hypergiant.

size of VY Canis Majoris compared to the sun
Oona Räisänen (User:Mysid) [Public domain]

With a radius of about 1,800 times that of our sun, it is the largest known star.

It ranks in the top 50s in the list of extremely luminous stars.

The star burns alone, 4,900 light-years away from us. It appears in the constellation – Canis Majoris.

Gamma Aquilae

Kevin Heider / NASA, ESA. Credit: A. Fujii, Altair-Tarazed-heic0720c, marked as public domain, more details on Wikimedia Commons

Also known as Tarazed falls in the constellation – Aquilae, hence the name.

It is separated by a distance of 395 light-years from the sun.

This is a very young star, about 100 million years old but has already consumed all the hydrogen in the core and has stared the fusion of helium to carbon.

The fate of this star – is going to be a white dwarf.


there are two types of star systems –

  1. hierarchical – ordered
  2. trapezia – unordered

binary stars can be classified into various types

  1. visual binaries – which can be resolved by a telescope
  2. spectroscopic binaries – cannot be resolved by a telescope
  3. eclipsing binaries – appear to eclipse each other over and over again
  4. Non-eclipsing binaries
  5. astrometric binaries – appear to orbit alone around a point in space

there are different techniques to measure the age, composition, spin rate, mass, temperature, distance, size, and magnitude of a star.

some of the techniques that help in measuring the above quantities include –

  1. HR diagram
  2. OBAFGKM spectral classes
  3. absorption and emission lines

lastly, we would love to hear from you – which is your favorite star?

sahil asolkar

Sahil Asolkar

writer and co-founder

Sahil Asolkar is a writer, poet, and shows a good interest in astronomy. His work can be seen in the articles he writes for Astronomiac.

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