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What is a Star? – Everything You Need To Know

Stars are among the most beautiful and incomprehensibly large formations in our galaxy. Bound by gravity, these gigantic plasmatic spheres could contain millions of Earths within them. In fact, the closest star to us, our very own Sun, is large enough to fit 1.3-million Earths in it by volume—and, compared to other stars in the galaxy, that’s on the smaller end. With an estimated one-hundred-thousand-million stars in the Milky Way galaxy, there is a massive variety of formations close-by and unquantifiably more in the deep reaches of space. And, much like people, stars come in all shapes and sizes, possessing rich histories and complex natures beneath their luminous surfaces—far more peculiar and unique than the nearly indistinguishable pin-points of light seen from Earth through clear night skies.

To Make a Star

While it may be easier to imagine stars as solitary formations, floating alone in space and relatively distant from other interstellar objects—this skewed perception is a result of our own relative place and time in the universe. Stars are born amongst countless others, inside incomprehensibly large clouds of gas and dust known as nebulae. Scientists often refer to these regions of space as ‘stellar nurseries.’ A place where hydrogen and helium swirl within, as gravitational forces slowly amalgamate matter together over the span of thousands of years, eventually packing everything tight enough together to collapse under its own weight, and form a protostar.  

star in the sky

The Protostar

The protostar phase is the earliest in the process of stellar evolution, and marks the moment a core begins to form within the spinning mass of gas and dust. This stage can last millions of years, and is marked by the gradual absorption of surrounding materials that are caught within the protostar’s expanding gravity-well. As the protostar itself gets smaller, it naturally grows denser, and begins to spin faster due to the conservation of angular momentum.

To provide a classic example, this is the same principle that allows a spinning figure skater to increase the speed of their spin by bringing their arms close to their body.  

The Main Event

This entire process forces hydrogen molecules closer together, gradually increasing pressure and thus heat in the core. When the core temperature reaches 15-million-degrees Celsius (27-million-degrees Fahrenheit) the forces are great enough to fuse the hydrogen within the core of the star, igniting nuclear fusion and transitioning it into a main-sequence star.  

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How to Measure a Star

Despite a shared origin story, there are several characteristics that astronomers use to measure stars and determine their type. For instance, luminosity is a measurement used to determine the amount of energy a star is putting out, based on its brightness and distance from earth. Another indicator used is color, which can show the relative temperature of a star. While hot stars tend to burn white or blue, cooler stars burn red, or orange.  

These, along with other factors, are employed in a graph called the Hertzsprung-Russell diagram, which is used to classify stars based on their defining characteristics.

Below are a few examples of the various forms a star can take:

Class M (Red Dwarfs) 

The vast majority of main-sequence stars fall in this category. Appearing a reddish color, they are the longest living stars, due to their low temperatures and slow rate of nuclear fusion. However, despite their common nature, they can be difficult to locate due to their low luminosity and small size.

Class G (Yellow Dwarf)

The classification of our own Sun is a Yellow Dwarf, though the name is a misnomer. While this type of star can have a slight yellow tinge in lower temperature variants, our own Sun is actually a very distinct white due to the temperature it burns at. Its yellow appearance is due to effects from our atmosphere, and not reflective of the Sun’s actual appearance. 

Class A 

The most common type of star visible to the naked eye. Class A stars are hotter than the G-Class, though only 1.4 to 2.1 times the Sun’s mass, and appear as white or bluish-white in color. 

Class O

These exceptionally rare stars are incredibly hot and unbelievably luminous. They are between forty-thousand and one-million times the luminosity of the Sun, burning so brightly that they appear a blue or white color in the visible spectrum—but actually output the majority of their light in the ultraviolet range.  Additionally, their mass can vary between fifteen and ninety times that of the Sun. 

what is a star

Death of a Star

All main-sequence stars are defined by two forces fighting for control within them: the energy released by the fusion reaction in the core trying to force its way out, and the weight of the star’s own gravity attempting to collapse its mass inward, to the core. For 90% of their existence, stars exist in this state of equilibrium, but over the millions and billions of years it all begins to break down—and they start to die.

Which stage of stellar evolution a star moves in to next is defined by the mass of the star: 

Red Dwarfs

The least dramatic in their old, dying ages, the Red Dwarfs burn hydrogen slowly, and gradually expend their fuel until fusion stops entirely—resulting in a tiny remnant of their core known as a White Dwarf. Interestingly, this process is completely theoretical and has never actually been observed, as the lifespan of Red Dwarfs is longer than the age of our universe—so none have had the time to reach ‘old age’ yet. 

Sun-like Stars

As stars of this size run out of hydrogen to fuse, the helium produced from the reaction will begin to sink into the star’s core. This will cause the external layer of the star to swell, increasing in size exponentially until it takes the form of a Red Giant. Gradually, the Red Giant will shed away its exterior gases, until only a White Dwarf remains. 

Supergiant Stars

These stars begin their deaths in the Red Giant phase, but their expanding exterior obscures a contracting core. Eventually, gravitational forces will overpower the core, causing it to collapse in a massive explosion known as a supernova. In some cases, the remnants will be a super-dense neutron star—but, in the most extreme cases, a black hole will form.  

New Beginnings

While the death of stars can be depressing to observe, there is some beauty in their demise. When a star dies, it releases a rich cloud of basic and complex elements into the surrounding space. Over time, these star remnants can mix with other elements in the universe, forming a new nebula and resuming the process of stellar creation. Even the carbon, nitrogen, and oxygen atoms in our own bodies were all released by the deaths of ancient stars—so, to quote the late Carl Sagan, ‘We're made of star stuff.’