The Life-Cycle of Stars Part 1

The term "star" has often be implanted in our vocabulary ever since we were young children. But what exactly is a star? According to the Merriam-Webster Dictionary, a star is any that "self-luminous" body that "produces energy by the means of nuclear fusion" (n.d.). While many people may often think of stars as the same, there are actually many different types. In order to understand the different types of stars though, one must be able to comprehend vital definitions, such as nuclear fusion and solar mass. Nuclear fusion is a process in which "nuclei of atoms are joined" together, causing a release in energy (Merriam-Webster, n.d.). Solar mass, on the other hand, is a unit that "expresses [...] the masses of other celestial objects" by using the mass of the Sun as its base unit (Merriam-Webster, n.d.). Also please note that not all pictures presented in this blog are "real" images, and may be instead images created by artists and computers.

Nebula

A nebula is defined as a collection of gas (hydrogen and helium) and dust, and can be found in the space between stars, often referred to as interstellar space. The closest nebula to Earth is the Helix Nebula, which is located about 700 light years away from Earth. Astronomers can often spot nebulae using powerful telescopes, such as NASA’s Spitzer Space Telescope and the Hubble Space Telescope. Some nebulae (the plural of nebula) can come from the explosion caused by a dying star, such as a supernova. A nebula is a region in which a new star begins to form. Gas and dust are usually spread out in a nebula, but gravity starts to bring them closer to each other, until they form clumps of gas and dust. Gravity continues to pull Gravity continues to pull these clumps closer together until the clumps end up “collapsing from its own gravity” (NASA, 2021). This collapse causes the materials at the center to heat up into a core, which begins to form a star.

Protostar

While a protostar may look like a star, it is actually not one yet because its core is not hot enough for fusion to take place. A protostar emits light through the “heating of the protostar as it contracts” (Las Cumbres Observatory, n.d.). It also is surrounded by dust so a protostar may be harder to observe in using the visible light spectrum (light that the human eye can detect). A protostar also sends out high speed jets of gas due to its rotation, which generates a magnetic field. Hydrogen fusion can start in a protostar once its “core temperature exceeds 10 million K” , which is also when it be classified as a star (Las Cumbres Observatory, n.d.). The amount of time a protostar takes to form into a star depends on its mass. For example, our Sun took about 50 million years to form into a star, while smaller stars may take at least a hundred million years to form into stars.

Brown Dwarf

A protostar that is less than 0.08 solar masses can never reach a temperature of 10 million K, so it never becomes a star. They are instead called failed stars, or Brown Dwarfs. Brown Dwarfs usually have a mass of about 0.075 solar masses and are intermediate between planets and stars. Brown dwarfs are not actually brown in color, and are instead a “deep red to magenta depending on their temperature” (Encyclopedia, n.d.).

Medium-sized star

A medium-sized star is also known as a yellow dwarf or a G-type main sequence star. It is about 0.84 solar masses to about 1.15 solar masses in size, with our Sun being exactly 1 solar mass in size. Surface temperatures of a medium-sized star can range from about 5300 degrees K to 6000 degrees K. They continue hydrogen fusion for about 10 billion years until the nuclear fuel is exhausted, in which the yellow dwarf becomes bigger and bigger until it turns into a red giant.

Red Giant

To become a red giant, a star must have about somewhere between half of our Sun’s mass to eight times our Sun’s mass. A red giant is also known as a low or intermediate star and a star may stay as a red giant for about a billion years, before inevitably becoming a white dwarf. The outer layers of an expanding star start to get farther away from the core, which results in the temperature “dropping from 6000-3000 degrees K down to 5000 degrees K”- which also results in the star's light becoming more red (EarthSky, 2021). Some examples of Red Giants that can be seen in our very own night sky are Aldebaran (located in the Taurus constellation) and Betelgeuse (location in the Orion constellation).

Planetary Nebula

As time progresses, the red giant will submit to gravity and compress on itself, exploding into what is known as a ‘planetary nebula’. The layers of the star will dissenerate into gas and dust, leaving a hot, dense core behind

White Dwarf

A white dwarf is composed of “hot, dense remnants” of dead stars (EarthSky, 2020). They are essentially the cores that have been left behind once a star has consumed its hydrogen fuel and shed its outer layers. The light comes from a slow, steady release of energy that has been left over from nuclear fusion.

Black Dwarf

As the remaining energy of the white dwarf continues to escape, the white dwarf begins to cool and dim until eventually it becomes an “inert lump of carbon and oxygen” that continues to “float invisibly in space” (EarthSky, 2020). The universe does not have any black dwarfs that we know of because it isn’t old enough for any to have formed.

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Thank you for taking time to read through the end! We hope you learned something new about stars and are looking forward for part 2! To learn more specifically about our very own star, the Sun, check out the link below! Make sure to like, comment, and share, so more people can learn just as much as you did. As always, keep gazing skywards!

Sun Blog: https://mid-night-eclipse.wixsite.com/astronomyblog/post/the-sun

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Bibliography

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https://www.merriam-webster.com/dictionary/star

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https://lco.global/spacebook/stars/protostar/

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https://earthsky.org/astronomy-essentials/what-are-red-giants-definition/

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