Starry Starry Night: A Basic Understanding of Stars in Our Universe

By: Aerin Choy

The night sky has always fascinated us, from ancient Arabian astronomers tracking the stars to quilts of the mappings of where stars were. Space calls for us in a unique and addicting way. The beauty of space masks its hidden complexity. Understanding the world requires many years of study. In an attempt to consolidate my learning as a space enthusiast and share my love for astronomy this is a basic compilation of the origins and types of stars.

The birth of stars

In space there are giant clouds of gas of different densities and clouds of dust, this is where stars are born. These regions where there are dense clouds of gas and dust covering hundreds of light years are called stellar nurseries. Turbulence in these clouds create high density areas called knots, when they contain enough mass the gas and dust begin to collapse due to gravity. 

Collapsing clouds according to astronomers have points of no return, where they become so dense and cold that they collapse under their own gravity, when this is achieved, stars form. The pressure caused by gravity heats up the material at the center creating a protostar. Over the course of millions of years the protostar becomes hotter and hotter until the center of the star or the core is hot enough for nuclear fusion to occur. Comets, planets, asteroids and other objects are also made from this stellar dust. The true formation of stars is still unclear, some scientists study supernovae and speculate it could be pressure from radiation, stellar winds or the accumulation of dust.

Classification of stars

Stars are categorized according to their mass and temperature. There are seven categories or classes ranging from O, B, A, F, G, K to M. ‘O’ stars are the biggest and the hottest while ‘M’ stars are the coolest and the smallest. Visually but counterintuitive, hotter stars glow blue and cooler stars grow red. The colors transition from red to yellow and then white as the stars get hotter, eventually the metallic elements become hot enough to glow blue. Our sun in the Milky Way is a G star with a temperature of 5,800 Kelvins, overall really hot stars like O types are 1 in 3 million stars.

Even further, astronomers split the class into subclasses numbered 0 to 9. 0 is hotter than 1 and so on, so a G2 star (our sun) will be hotter than a G7 star. Some interesting examples of the different types of stars are Pollux, a K class star, and Betelgeuse, a M class star. 

Another way stars are classified is through a Hertzsprung-Russell Diagram. It is a chart with luminosity or energy output on the vertical axis and the temperature of a star on the horizontal axis.

Life stages of stars

The initial mass of stars determine how long it lives, the larger the mass of the star, the shorter its lifespan is. This is because a star's mass is a measure of the ‘fuel’ it burns, according to the Webb Telescope website it is how much hydrogen gas is brought together by gravity during the formation of the star.  The larger the star, the faster the fuel can be used up.  This in terms of human perception of time seems like forever, but for these large stars, they only last a few hundred thousand years. Their smaller counterparts last several billion years. When large stars have no more fuel to generate energy, the star begins fusing hydrogen into helium and so on to form heavier elements, as the fuel runs out the stars either explode into supernovae or implode forming neutron stars or black holes. 

Neutron stars are incredibly dense. According to NASA, one sugar cube of neutron star material would weigh 1 trillion kilograms on Earth, this is because a star with twice the mass of our sun is squished into a sphere the diameter of a city. The gravity is so powerful that all the protons and electrons in the center are combined into neutrons, hence the name. Some neutron stars called pulsars emit light and cause scientists to see flashes as the star rotates, pulsars also emit radio waves which scientists detect. 

A star may have not enough mass to end in such an energetic matter, these stars become red giants. The core, after running out of fuel, collapses on itself and the plasma shell begins to be hot enough to fuse hydrogen, as the fusion outside the core happens, the star begins to expand, the energy begins to spread out more in the star rather than be extremely concentrated in one area causing the white to turn to yellow and then to red. This process can take millions of years and massive stars can form red supergiants before exploding into supernovas as well. 

The next step in the process of a life cycle of a star is when the dying star releases out layers to form a nebula, leaving a small inner core called a white dwarf, the embers of dead stars like the Sun. White Dwarfs are fascinating in the sense that they are the size of Earth but contain the same amount of mass as the sun, the high density suggests that a mass of about 2000 kg will weigh 4,000 pounds in Earth’s gravity. Some of the coolest white dwarfs crystallize and are compared to diamonds due to their composition of carbon or oxygen.

Planetary nebulae are also connected to white dwarfs, the UV light emitted by white dwarfs illuminate material surrounding the outer layers of the stars, it ionizes the gas recombining ions in the gas and emits photons. It creates various spectral lines that form an emission spectrum. In the end, the white dwarf becomes a black dwarf, an object that still has mass but no longer emits light. This is a hypothetical star which is older than the Universe itself, technically it would not exist until a long time in the future. 

There are various other types of stars and the knowledge of space is expansive and constantly increasing. If you love constant exploration and mystery, exploring the cosmos may be for you. Wishing you the swiftest journey as you swim among the stars. 

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