Today, we start a new series to cover the captivating story of star evolution. We will cover (or uncover) the mysteries of how stars are born, evolve, and ultimately meet their fates.
Let’s start!
Nebula
Nebulae are vast clouds of gas and dust floating in the space between stars. They are distinctly visible due to their own luminescence or interaction with their surroundings. Often they are the star-forming regions but can also form at the end of a star’s life.
They come in different shapes and sizes, and scientists classify them into three main types:
Emission Nebulae: These nebulae emit light of their own. The source of light is usually ionized gas coming from a nearby star. They often shine brightly in vibrant colors, like the famous Orion Nebula.
Reflection Nebulae: These nebulae don't produce their own light but instead reflect the light of nearby stars. They appear bluish because they scatter the blue light more efficiently than other colors.
Dark Nebulae: These nebulae are made of dense clouds of dust that block out the light from objects behind them. They appear as dark patches against the backdrop of stars.
A nebula can also have molecular clouds which are characterized by their composition of cold, dense molecular gas and dust, and their role as star-forming regions.
How are they formed?
A nebula can be formed from gases that are already in the interstellar medium, from supernova explosions, or from a planetary nebula, which is the last stage of a low-mass star.
One of the most famous images associated with nebulae is the Pillars of Creation, located within the Eagle Nebula around 7000 light-years away from the Earth. The pillars are composed of cool molecular hydrogen and dust that are being eroded by photoevaporation — a process where energetic radiation ionizes and disperses the gas. The leftmost pillar is about four light-years in length.
Protostar
A protostar marks a critical phase in the formation of a star. It is the very early stage of a star which is still collecting material from its molecular cloud. It emerges from the gravitational collapse of a dense region within a giant molecular cloud composed of gas and dust in the interstellar medium.
It is important to note that protostars are not yet powered by nuclear fusion, the process that fuels mature stars. Instead, they generate energy through gravitational contraction, which causes their cores to become incredibly hot. The radiation liberated at the surface due to this contraction gets absorbed by the surrounding interstellar dust and reradiated at a longer wavelength. This makes it difficult to observe in the optical wavelengths.
As the protostar continues to collapse, its temperature rises until it becomes hot enough for nuclear fusion to ignite in its core. At this point, the protostar graduates to become a main sequence star, like our Sun.
The duration of the protostar stage varies depending on factors such as the mass of the protostar and the surrounding environment. Some protostars evolve into stars relatively quickly, within a few hundred thousand years, while others may take millions of years to fully develop. For a small star like the Sun, it can last for around 500,000 years. This is considered a very short time frame in the field of astronomy!
How much mass a protostar can gather in this phase will play a crucial role in determining what type of star it becomes, and the trajectory of its evolutionary path, topics we will discuss in subsequent posts.