What Are Protostars? The Birth And Formation of New Stars

Introduction

Stars, those brilliant points of light scattered across our night sky, all begin their lives in a similar way: as protostars. But what exactly are they? How do they form, and what role do they play in the birth of a star? Let's dive into the fascinating world of protostars to uncover their secrets.

A protostar is a young star that is still in the process of forming. It represents an early stage in a star's development. This happens after a dense region in a molecular cloud collapses. After the protostar phase, it will start the main sequence, where it will spend most of its life.

During this early stellar formation phase, a protostar goes through many changes. It gathers mass, gets hotter, and eventually starts nuclear fusion in its core. These processes determine the star's characteristics, like its mass, temperature, and brightness, which will shape its entire life.

In this article, we will explore the life of a protostar, from its formation in nebulae to its transformation into a main-sequence star. We will examine the physical processes involved, the changes it goes through, and the importance of this stage in the bigger picture of star and planet formation.

Protostar in Herbig-Haro 46/47.

The Birthplace of Stars: Nebula

Protostars form in nebula, vast clouds of gas and dust in space. These regions, often called "stellar nurseries," are rich in hydrogen, the primary fuel for stars. Within these clouds, something unique happens: gravity begins to pull the gas and dust together.

Nebulae are fascinating and beautiful structures visible in various forms across the universe. Some, like the Orion Nebula, are easily seen with telescopes, while others are more hidden. These clouds can be hundreds of light-years across and contain enough material to form thousands of stars. The gas in the nebula is mostly hydrogen, but it also contains other elements and dust particles.

Formation of a Protostar

1. Gravitational Collapse: This process starts when a region within a nebula becomes dense enough that gravity pulls the material inward. Events like a nearby supernova explosion, which compresses the gas and dust, can trigger this process. As gravity pulls the gas and dust together, they begin to form clumps. Over time, these clumps become denser and hotter as more material falls in.
2. Fragmentation: As the nebula contracts, it can break into smaller, denser clumps. Each of these clumps can potentially form a new star. This process is called fragmentation, which is essential for forming multiple stars within the same nebula. Each fragment will continue collapsing under its own gravity, becoming hotter and denser.
3. Core Formation: The material continues to fall inward within these clumps, forming a dense core. This core is the protostar. As more material accumulates, the core temperature rises because of the pressure and friction of the infalling gas. This heating causes the protostar to shine, although it is not yet a true star.

The energy emitted by the protostar is primarily in the infrared part of the spectrum, making these objects difficult to see with regular telescopes.

Characteristics of Protostars

1. Temperature and Luminosity: Protostars are not yet hot enough for nuclear fusion in their cores, but they are still quite warm because of the energy from gravitational collapse. They often emit infrared light, which makes them visible with infrared telescopes. As they continue to gather material, their core temperature rises. Nuclear fusion will start if the core becomes hot enough, and the protostar will become a main-sequence star.
2. Accretion Disk: A spinning disk of gas and dust forms around the protostar, called an accretion disk. Material from this disk falls onto the protostar, making it grow larger. The accretion disk is essential for forming planets and other objects in a star system. As the dense clump gets bigger, material from the disk can come together to create planets, moons, and other celestial bodies.
3. Outflows and Jets: As the protostar develops, it can shoot out material in strong jets along its spin axis. These jets help to clear the surrounding gas and dust, making the young star visible. The jets form when the protostar's magnetic fields interact with its accretion disk, speeding up particles away from the star. These jets appear as bright streaks of light in images of protostars and are a crucial sign of star formation.

The Evolution of a Protostar

The protostar phase is relatively short in the lifespan of a star, lasting only a few hundred thousand to a few million years. During this time, several important changes occur:

1. Heating Up: As more material falls onto the protostar, its core temperature rises. The temperature increases because gravitational energy converts into heat. This process continues until the core temperature reaches about 10 million degrees Kelvin.
2. Ignition of Nuclear Fusion: When nuclear fusion starts, the protostar becomes a main-sequence star. This marks the end of this life phase and the start of the star's long, stable period of hydrogen fusing into helium. Nuclear fusion is the process that powers stars, creating the light and heat we see from Earth. For a star like our Sun, this phase will last for about 10 billion years.
3. Shedding of Outer Layers: The protostar's strong stellar winds and jets eventually blow away the remaining gas and dust, making the new star visible in the visible light spectrum (meaning it's now visible to the human eye). This shedding of material helps to clear the surrounding area, allowing the young star to emerge fully. The remaining material can form planets, asteroids, and other objects, creating a new star system.

What Are Protostars?, Source: https://www.youtube.com/watch?v=a-rUtp7Alh8

The Role of Protostars in Stellar Evolution

Protostars are a crucial early stage in the formation of stars. Without this phase, stars as we know them wouldn't exist. This stage sets the foundation for the star's characteristics, including its mass, composition, and potential lifespan. Understanding protostars helps astronomers learn more about how stars form and evolve, which in turn provides insights into the formation of planets and even the conditions necessary for life.

Studying protostars also helps scientists understand the processes that lead to the formation of different types of stars. For example, massive protostars can evolve into massive stars that end their lives in spectacular supernova explosions. In contrast, smaller ones form stars like our Sun that have longer, more stable lifespans.

Conclusion

1. Protostars are the early stages of a star's development, and they form in nebulae, vast clouds of gas and dust in space.
2. The formation of a protostar begins with gravitational collapse, which creates dense cores within the nebula.
3. Protostars are warm because of the gravitational collapse and emit mostly infrared light. They have accretion disks of gas and dust, as well as powerful jets and outflows that help clear away surrounding material.
4. As protostars evolve, their temperature rises. When core temperatures reach 10 million degrees Kelvin, nuclear fusion begins, marking their transition to main-sequence stars. Stellar winds and jets eventually clear away the remaining gas and dust and become visible to people from Earth.
5. Protostars are essential in star formation, determining a star's characteristics and future evolution. Studying them provides insights into star and planet formation and the conditions necessary for life.

References

1. Dunham, M. M.; et al. (2014). The Evolution of Protostars in Protostars and Planets VI. University of Arizona Press.
2. Myers, P. C. & Benson, P. J. (1983). "Dense Cores in Dark Clouds: II. NH3 Observation and Star Formation". Astrophysical Journal. 266: 309.
3. NASA - "Star Basics"