Cosmology: The Star Factory

Nebulae

Nebulae or Nebula arguably offer the most breathtaking sights of the universe. The countless pictures of these colourful nebulae fill their viewers with awe. A nebula by its Physics and definition is so simple and elegant, it’s hard to contemplate its simplicity when placed beside its other counterparts. The word Nebula comes from the Greek word for cloud and like its name is a giant cloud of dust and gas in space.

As we have earlier seen, as far as the baryonic matter goes, about 99.9% of it in our Universe is composed of Hydrogen and Helium – the two simplest atoms conceivable, with a contribution of 75% and 24% respectively. These atoms are scattered across the Universe with an average density of 5 hydrogen atoms per m³. This density seems too insignificant for us to even fathom that they can interact or create something. But, thanks to the Universe, these atoms are not uniformly distributed. Some spaces accumulate them. These might have wandered and collected due to their combined gravitational pull. These atomic clusters have densities similar to that of gases. Essentially forming vast quantities of free-floating interstellar and intergalactic gases.

Gravity is one of the most fascinating forces of nature. It’s the weakest of the four forces and yet the predominant one. Whenever it is brought into the mix, it gives rise to all the structures that make up the Universe.

The large clouds of hydrogen and helium gas start concentrating under their own gravity giving rise to large opaque clouds. With time, these clouds become denser and denser prohibiting even light to escape. Thus, the basic structure is formed. Now, the question arises how do we see these opaque clouds that do not let light pass through them? Well, let’s see.

All the observable bodies that emit light do so either by emitting light of their own, or by reflecting light from another luminous body. When these opaque gas clouds called the Dark nebulae get concentrated enough, they start fusing by a process called nuclear fusion. This results in the formation of the core of stars and essentially turns the dark nebulae into a star factory. These stars emit light making the nebulae visible.

Light emitted by the stars is reflected back by the surrounding gas, illuminating them in the process. This type of nebulae is called Reflection nebulae. These stars emit radiation ranging from infrared, and visible light to ultraviolet. This light excites the surrounding gas and ionizes the hydrogen and helium atoms. These atoms when de-ionize produce a photon with a wavelength corresponding to the difference between the energy states. These nebulae are now essentially emitting light giving rise to Emission nebulae. A simple question that may arise is, can a nebula be both – an emission and a reflection nebula? And the answer is absolutely YES! These nebulas are what we call Diffuse nebulae. They do not have any strictly defined shape.

Reflection Nebulae

Emission Nebulae

Diffuse Nebulae

The aforementioned nebulae form one type of Nebulae amongst many other varieties. Let’s look at others, shall we? The secret to finding them lies in a very basic question. What happens to these nebulas after billions of years have passed? Well... the stars begin to die. The further classification of nebulas depends upon the mass of the dying star.

Stars with intermediate masses ranging from 0.7 M_s – 8.0 M_s(1 M_s = 1.99*10³⁰ Kg) form our next two kinds. A living star maintains a delicate balance between the radiation pressure pushing the mass outwards and the gravitation pull moving the mass inwards. When a star begins to die, it appears to enlarge due to mass leaving the surface. A star in its final moments collapses under its own gravity sending ripples that scatter the mass around it. These intermediate-mass stars do not have enough mass to produce a supernova. The star dies in a whimper and the core becomes a white dwarf. This white dwarf then illuminates the surrounding gas and forms what we call the ejected nebulae or the protoplanetary nebulae. These nebulae can be easily identified by their two protruding lobes. Its unique shape involves Physics that goes beyond this article. We may come back to it later but for now, let’s keep moving forward.

Menzel 3 - The Ant Nebulae

NGC 2346 - The Twin Jet

Given enough time, these nebulae settle into shape. The dying star’s unique characteristics determine its shape. The outer gas forms an elliptical or spherical shape giving it the appearance of a planet. Historically, in the absence of good telescopes, many of these nebulae were identified as planets giving it its misdirecting name planetary nebulae. These planetary nebulas are also referred to as stellar nebulae.

NGC 7293 - The Helix nebulae

NGC 1501

If the mass of the dying star is more significant than 8.0 M_s, the stars die in a giant explosion called a supernova. The supernovae cause most of the mass to be distributed at once. The supernova remnants form a ring-like structure at the exterior. At the core of these supernovae nebulae is a neutron star or maybe even a black hole depending on the mass of the dead star.

SN 1006

SN 1604 - Kepler's Supernova

Now let’s take a step back and revisit the dark nebulae. They are formed by dense gases which block the light going out or passing through them. They are only visible when they are in the foreground of a bright nebula - dark highly concentrated clouds present at the centre of brighter, larger, less concentrated clouds.

These dark nebulas are the most active region of a nebula. With time, it clears and reveals a cluster of young stars. This nebulae essentially eats itself inside out.

The Pillars of Creation

The Horsehead Nebulae

As we near the end of this article, let’s have a fun exercise for us. At night, look at the sky and try finding the Orion constellation. Try finding the three central horizontal stars. Now, go diagonally, and identify the Orion tail. Do you see three stars forming a line? Well, psst, the middle one is not a star. It’s a nebula! Awe-struck, aren’t you? I was too. Now, today let us all be a part of it and be united in our little Cosmo nerds community.