What is a Star?

Stars, the dazzling celestial candles lighting up the night sky, have always fascinated us. Captivating our imaginations, these navigators have been showing us the direction for millennia. 

Let’s have a quick overview of what a star is, together. 

What is a Star?

Stars are spherical celestial bodies consisting of helium and hydrogen. They are the sky’s self-luminous shining orbs that dazzle and are superheated by radiating energy through a nuclear fusion reaction between hydrogen and helium. There are around 200 sextillion stars in the observable universe, of which only  a very small number (5,000 to 6,000) can be seen with the naked eye on a clear night sky. If you are stargazing, how many of these celestial diamonds can you enjoy depends on a range of factors: the time of year, the clearness of the sky, and the eyesight of the stargazer. 

Here are some key physical features of stars:

• Size: From dwarf stars to gigantic luminaries, stars come in various sizes to awe-inspire you. Did you know that the lifespan and brightness of a star largely depend on its size?  
• Temperature: As we know, stars are sizzling, huge stellar marvels that look blue when they are superhot. Comparatively cooler stars appear red.
• Brightness: Not all stars are equally bright. How bright a star would appear is a factor of its temperature, size, and distance from Earth. The brightness of a star is measured in luminosity, which indicates the amount of energy it emits per second.
• Color: From the chilly blues to the vibrant reds, stars, the cosmic artists, smudge the night sky with a vivacious palette of hues. The colour of stars varies due to their temperature differences and to the release of energy at different wavelengths.
• Mass: Another critical feature of stars, mass significantly impacts the lifespan of these luminous celestial bodies.

Lifecycle of a Star: From Birth to Death

Stars sparkle, directing celestial explorers through the timeless expanse of the cosmos. It’s like a tapestry woven with endurance and eternity.

Now let’s come to the lifecycle of a star. The rule of thumb is the heavier the star, the shorter its life. Here’s a simplified explanation of the evolution of a star:

1. Stellar Birth in Nebulas: The process of bringing stars to life starts inside nebulae—a massive gas and hydrogen-based dust cloud. Over thousands of years, dense matter in a dust cloud collapses under gravity, forming a spinning disk called a protostar. This contracting cloud of gas marks the initial stage in the birth and formation of a star.
2. 2. Main Sequence Stars: Over time, a protostar accumulates more gas and, due to its contracting nature, it gets heated up. Once the temperature and pressure at the core reach a critical point, the protostar triggers hydrogen fusion inside and converts it into helium. A significant amount of energy gets released in this process. It is the most significant and longest step in the formation of a star. The Sun, Earth’s ultimate energy source, is now in this stage. The critical temperature to for nuclear fusion in a star is around 10 million degrees Kelvin, and the corresponding critical pressure is several billion pascals.
3. Red Giants and Supergiants: With hydrogen fusing into helium, the main-sequence star runs out of this core gas and starts collapsing under its own weight. This is because nuclear fusion is the only source of energy opposing the gravitational force that pulls everything at the core. The core contracts and gets superheated, and the decreasing pressure causes the outer layers to expand. With the expanding outer layer, the temperature at the star’s surface decreases, causing it to radiate more in the red band of the spectrum. At this stage, the star appears red, known as a red giant (for smaller stars like the Sun) or a supergiant (for larger stars).
4. Stellar Death: Eventually, the red giant becomes unstable and starts throbbing, extending and ousting some of its atmosphere. With aging, they start expelling their outer layers and form striking, small nebulae. This process depends on their mass:
5. When stars of low to medium mass shed their outer layers, planetary nebulae and white dwarfs come to life. This white dwarf, which was once the hot core of a star, cools down for billions of years and fades into a black dwarf. In contrast, high-mass stars undergo more fusion reactions and keep producing heavier elements until a significant amount of iron is formed. The core eventually turns into iron, indicating a stoppage of fusion reactions. The star collapses in its own weight and triggers a supernova explosion. The remnants of this event collapse further and can transform into a neutron star, or a black hole if the star is extremely massive.

Looking Up

Stars blanket the night velvet sky with their twinkling jewels. The urban areas may sometimes disappoint you by not showing the exact number of stars twinkling above due to dust clouds or light pollution. But, in some special spots, you can enjoy the celestial light display with these stellar dancers pirouetting in the cosmic ballet.

Long ago, our ancestors used to see the stars as the ultimate cosmic beacons, guiding them through the darkness. Long ago, they used constellations—patterns formed by stars—to identify the best seasons for farming and sowing seeds. Tracking the movements and patterns of stars is still used by adventurers and astronomers to navigate through the night sky. 

How is a Star’s Brightness Measured

The brightness of a star is measured using “apparent magnitude“. The lower the number it shows on the scale, the brighter the star is, and vice versa. The scale measures a star’s distance from Earth using photometry, which assesses the light received from the star.