Stars are the most widely recognized astronomical objects. Those enigmatic luminous spheres in the night sky, are almost unimaginably immense balls of plasma glowing with their own light due to intense gravitational pressure and nuclear fusion.

Among the billions in our Milky Way and the trillions across the universe’s countless galaxies, our own Sun stands as a typical example, a medium-sized star midway through its life cycle. Just beyond our solar system lies Alpha Centauri, the next nearest star to us, a distant 25 trillion miles away, illustrating the vastness of space even within our stellar neighborhood.

But what is a star? Let’s take a look under the hood of a star: how they’re born, live and die, and how we, as astronomers, classify them.

The Lifecycle of a Star

There are three distinct phases of a star’s life: its birth, evolution, and death. We look at each of these in more detail below.

A Star is Born

Have you ever looked up at the night sky and wondered, “How are stars formed?”

The birth of a star is a fascinating process that begins in a rotating cloud of gas, mainly hydrogen and helium. This isn’t just a puff of gas, though. We’re talking about a colossal amount, the kind that can take up to 100,000 years to collapse under its own gravity and ignite to become an approximately spherical body of plasma, or what we call a star.

And when we say ‘ignite’, we’re not talking about striking a match. We mean the start of nuclear fusion, the process that lights up a star and keeps it burning. This requires a temperature of about one million degrees Celsius, a heat only attainable under immense pressure from gravity.

Meanwhile, the outer reaches of the spinning disc of gas that collapsed to form a star condenses and coalesces into planets that orbit the star in the same direction as the spin from the original gas cloud.

Idealised artist impression of a star forming from a gas cloud.
Star formation in a gas cloud

The Evolution of a Star

Once a star ignites, its life cycle is primarily determined by its mass. The rule of thumb is this: the larger the star, the hotter it burns, and consequently, the shorter its lifespan.

Our sun, for instance, is considered an intermediate-mass star, a term given to stars from half the sun’s size to more than eight times its size. Low-mass stars are those with a mass less than half that of the sun, down to about a tenth of its mass. Anything smaller, and you don’t have a star, but a ‘brown dwarf‘ or a ‘failed star’ like Jupiter.

A newly ignited star, known as a T-Tauri star, fluctuates in brightness as it continues to contract under gravitational forces for about ten million years. After this period, it matures into a main-sequence star, maintaining this status for as long as it continues to fuse the hydrogen into helium in its core.

Larger stars, despite having more hydrogen, burn at a higher rate due to the increased pressures and temperatures at their core, resulting in a shorter stay on the main sequence.

The Death of a Star

This phase sets in relatively quickly when the hydrogen in the star’s core gets used up. After being stable for billions of years, the star begins to fuse helium and heavier elements. What’s important to note here is that every star spends its life fielding a battle between being crushed by gravity and being blown apart by fusion.

As the fusion begins to subside towards the end of a star’s life, there is less energy fighting the force of gravity, so the core is crushed, causing it to heat up dramatically. This intense heating fights back against gravity and causes the outer layers of the star to expand and cool, transitioning the star from a white or yellow phase to a red giant.

The core contracts and heats up again after all the helium has been fused into heavier elements, leading to the ejection of the cooled outer layers of gas. What’s left behind is a white dwarf, the core of the old star, made up of the heaviest elements created – carbon and oxygen. White dwarfs cool over billions of years, before ending up as an undetectable black dwarf.

This is the fate that lies in store for our own sun, and every other star up to eight times its mass.

Low-mass stars burn their fuel supply so slowly that they can live for possibly a trillion years. They have life spans several times the age of the universe, so no such star has yet completed its life cycle. Scientists believe that these red dwarf stars will never have the mass to fuse anything denser than hydrogen and will eventually die by gradually cooling down to become black dwarfs.

High-mass stars, on the other hand, form quickly – in as little as 10,000 years – and are between one thousand and one million times more luminous than the sun on the main sequence. An entirely different end-of-life path awaits them…

Going Supernova

When high-mass stars leave the main sequence, they become Red Supergiants. Their extra mass causes additional gravitational pressure to act on the core, making it hot enough to fuse carbon into heavier elements.

The countdown to one of the most cataclysmic events in the universe begins when the core becomes iron-rich. Iron doesn’t undergo fusion reactions and so, over 10,000 years, the fusion reactions that have sustained the star for so long come to an end.

Very suddenly, gravity has all the power and, in less than a second, collapses the core from 5,000 miles wide to just six! That tremendous wave of energy bounces back out from the center of the star with a truly colossal velocity and causes the whole thing to explode: the star has gone supernova!

The remaining core has a choice of two fates, depending on its mass. If it is less than three solar masses, it becomes a neutron star. This is a star composed entirely of neutrons and is one of the densest materials in the universe. If the leftover core is larger than three solar masses, it collapses in on itself and forms a black hole.

Learn more about star formation here.

Unraveling the Layers of a Star

Have you ever considered stars as cosmic onions? Much like an onion, stars are layered, and each layer contributes to the mesmerizing spectacle we witness in our night sky. As we delve deeper into the structure of a star, we’ll peel back these layers, from inside out, to reveal their secrets.

The Core: The Heartbeat of a Star

At the heart of a star, we find the core. Here, temperatures and densities reach staggering heights, creating the perfect environment for fusion to occur. It’s like the engine room of a giant spaceship, powering the star and giving it life.

The Radiative Zone: Riding the Waves of Heat

Next up, we encounter the Radiative Zone. If you’ve ever felt the comforting warmth of a bar heater, you understand radiation. In this zone, heat moves in a similar way, transferring energy through the emptiness of space.

The Convective Zone: A Cosmic Hotpot

Picture a simmering hotpot. The heat rises, and cooler liquid sinks, creating a loop of movement. This is convection. In a star, the Convective Zone is where electromagnetic energy follows this same pattern, moving from the hotter areas to the cooler ones.

Interestingly, the order of the Radiative and Convective zones can vary according to the star’s size. For instance, our sun has the Radiative Zone sandwiched between the Core and the Convective Zone. However, more massive stars can have these zones reversed. And those stars with a mass somewhere between our sun and the universe’s giants might only have Convective Zones.

The Photosphere: The Face of a Star

You know those breathtaking pictures of our sun, like the one below, showcasing sunspots and other solar features? Those images capture the Photosphere – the outer surface layer of a star. It’s like the skin of an apple, forming the visible surface we gaze upon.

Detailed view of sunspots in the sun's photosphere.
The sun’s photosphere. Credit: Marty Wise, AstroBin

The Chromosphere: A Hydrogen Haven

As we move outwards, we come across the Chromosphere. This layer is a gas haven, rich in hydrogen. It’s like a celestial sponge, soaking up and emitting energy.

The Corona: A Solar Crown

Finally, we reach the Corona. This layer is a super-heated plasma, much hotter than the star’s surface, and can stretch millions of miles into space. Catching a glimpse of our sun’s Corona is a rare treat, only possible during a total solar eclipse.

The Characteristics of Stars

We’ve talked about stars being categorized as main sequence stars, but have you ever wondered what makes each star in our night sky unique? Just as people have different traits, stars too are characterized by certain distinctive features. Let’s dive into these fascinating celestial bodies, shedding light on their individuality.

Brightness: The Twinkle in the Night Sky

One of the most noticeable traits of a star is its brightness, which is measured in terms of magnitude and luminosity. The term ‘magnitude’ refers to the star’s brightness as perceived from Earth, while ‘luminosity’ is the total amount of energy a star emits in all directions.

Color: The Hue of Stars

Star color is another key identifier of a star. It’s not just for aesthetics, but it’s a significant indicator of a star’s temperature. Blue stars are among the hottest, while red stars are cooler. It’s a bit like the flame of a gas burner, where the blue part of the flame is hotter than the red part.

Size, Mass, and More

Size and mass are two more critical parameters. Generally, the larger a star is, the greater its mass. However, this isn’t always the case, as density also plays a role. For example, a small but dense neutron star might be heavier than a larger, less dense red giant.

Stars also possess magnetic fields, much like Earth. These fields are responsible for spectacular phenomena such as solar flares and star spots, which are akin to our planet’s magnetic storms and the auroras they produce.

Lastly, let’s not forget ‘metallicity’, a term astronomers use to describe the abundance of elements heavier than hydrogen and helium in a star. This trait is vital for understanding a star’s age and its potential to host planets. For example, a star with more metals is more likely to have a gas giant orbiting it due to the composition of the original gas cloud that created both of them.

Into the Stellar Spectrum: The Morgan-Keenan Classification

Now that we’ve outlined the various traits of stars, let’s delve into how astronomers classify them. This is where the Morgan-Keenan (MK) system comes into play. Think of it as a filing system for stars, segregating them based on their surface temperature and luminosity.

The MK system comprises seven spectral classes, designated by the letters OBAFGKM. The ‘O’ class stars are the hottest and ‘M’ class the coolest. Each letter is further subdivided with a number from 0 to 9, with 0 being the hottest in each class.

But, temperature is only half the story. The MK system also groups stars based on their luminosity, as follows:

  • Ia – Bright supergiants
  • Ib – Supergiant
  • II – Bright giant
  • III – Giant
  • IV – Subgiant
  • V – Main sequence or dwarf stars

To give you a familiar example, our Sun falls under the G2V category in the MK classification system. That means it’s a relatively cool, main-sequence star.

Discover More About Stars for Yourself

Now you know a good deal about the life and composition of stars and how we as astronomers classify them, but the best thing you can do is go and look at them for yourself.

We invite you to explore further, maybe even starting with your own observations of the night sky. Our list of best telescopes for observing stars might be a great place to kick off your stellar journey!