Nebulae Simplified
You may have seen some stunning, colorful pictures of nebulae from the Hubble Space Telescope or the James Webb Space Telescope. But what exactly are these things? In this article, we will see what these nebulae are made of, how they are formed, what the different types of nebulae are, and what their function is.
What Are Nebulae Made Of?
Nebulae are clouds of gas and dust. To be more specific, they are made up of hydrogen or helium gas. These gases and dust particles typically come from dying stars. This dust — often referred to as cosmic dust — consists of silicates (compounds of silicon and oxygen); ices of water, ammonia or methane; and heavy elements (elements heavier than Helium). These dust particles usually come from supernova explosions (explosions at the end of a star’s life cycle), or could come from stellar winds in giant stars.
How Do Nebulae Form?
Nebulae could form through numerous different ways. Nebulae are categorized depending their properties or their formation, so let us look at some common types of nebulae.
Emission Nebulae
Emission Nebulae are clouds of ionized gas that emit light of various colors. These gases can be ionized through ultraviolet photons emitted from nearby stars. There are various types of emission nebulae, and we will look at two of these in detail — supernova remnants and planetary nebulae.
Supernova Remnants
After a massive star (typically over 8–10 solar masses) exhausts all of its hydrogen, it begins to fuse heavier elements like helium and carbon. This fusion continues till iron is formed, after which no further energy is released, and the core collapses. This is because there is no outward force counteracting gravity, so the core succumbs to the high gravitational force. This rapid collapse of the star results in extremely high temperatures and pressures, resulting in a rebound effect. Gases expand outward from the core, causing the star to explode, releasing a large amount of energy. This expanding layer of gases eventually forms a structure known as a Supernova Remnant (see Fig. 3).
Planetary Nebulae
Before I tell you what planetary nebulae are, I need to tell you that they are NOT related to planets in any way (weird name, I know).
When a star of about 1–8 solar masses reaches the end of its cycle and explodes (just like what we saw in supernova remnants), the structure it creates when ionized gas is expelled from its core is called a planetary nebula (see Fig. 4). Our own sun will likely form a planetary nebula at the end of its life.
Reflection Nebulae
As the name suggests, reflection nebulae reflect the light of nearby stars (see Fig. 5). The energy from these stars is not enough to ionize the gas in these nebulae. This is what differs them from emission nebulae — reflection nebulae do not emit their own light. Since shorter wavelengths of light (blue and violet) are scattered most effectively by dust particles, these nebulae have a characteristic blue color (this is similar to why the sky appears blue — dust and gas particles in our atmosphere scatter the blue wavelengths of sunlight most effectively, creating a wonderful, vibrant light blue!).
Integrated Flux Nebulae
Similar to reflection nebulae, these nebulae also reflect light from stars. However, they are located away from the main body of the galaxy and reflect light from multiple different stars, instead of a single nearby bright star (like a reflection nebula). This makes them incredibly dim and hard to detect (see Fig. 6). These nebulae were detected rather recently. In 2004, Steve Mandel observed a dust complex that wasn’t very well known, and created a catalog of integrated flux nebulae.
Dark Nebulae
Unlike the nebulae we have seen so far, dark nebulae are dense clouds of gas that block light from objects behind them. Due to this, they appear as dark patches against the background of bright stars or emission nebulae (see Fig. 7). They appear dark due to the presence of frozen carbon monoxide or nitrogen in their tiny dust particles, which block visible light.
Nebulae Do More Than Just Look Pretty
These pretty structures are some of the most important features of our universe. Let us look at some of their functions.
Star Formation
Nebulae are often referred to as “star nurseries”, and for good reason. Some regions of these nebulae are incredibly dense, so they collapse due to gravity. This gives rise to cores that eventually evolve into stars. The remaining material may also form planets or other objects in the new solar system. In fact, this is how our own solar system was formed, 4.5 billion years ago!
Element Reservoirs
Nebulae contain vast amounts of heavy elements ejected from the supernovae that created them. These materials are incorporated into the new stellar bodies they create, completing the cosmic cycle of element synthesis and stellar evolution!
Summary
Nebulae, therefore, are some of the most important stellar bodies. They are birthed from dying stars, and in turn, assist in the birthing of hundreds of new stars. These vast clouds of gas and dust serve as a constant source of inspiration for new scientific discovery, propelling us forward in the exploration of our universe’s wonders.