A star is born (Part 2: H-alpha emission nebula)

In this post I will write about a special type of light produced by hydrogen atoms which is extremely important for astronomers in quest of finding star forming regions and newly born stars. By observing the star formation process at different stages in the universe, astronomers can understand how stars are born and how they change from one stage to another.

H-alpha emission is produced by an excited hydrogen atom in which an electron jumps from the n=3 energy level down to n=2 (see picture below on Balmer series source: Balmer series). H-alpha light is visible in the red part of the electromagnetic spectrum (H-alpha emission has a wavelength of 656.281 nm), and is the easiest way for astronomers to trace the ionized hydrogen content of gas clouds. Here is a visualization of Bohr’s model of the Hydrogen atom, n >=2 represents different level quantified/energized electron states while n=1 is called the “ground” state:



Since it takes nearly as much energy to excite the hydrogen atom’s electron from n = 1 (or the “ground state”) to n = 3 as it does to ionize the hydrogen atom (to strip it from its electron), the probability of the electron being excited to n = 3 without being removed from the atom is very small. Instead, after being ionized, the electron and proton recombine to form a new hydrogen atom. In the new atom, the electron may begin in any energy level, and subsequently cascades to the ground state (n = 1), emitting photons with each transition. Approximately half the time, this cascade will include the n = 3 to n = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line always occurs where hydrogen is being ionized.

So, in the image below, warm (about 8000 K) interstellar gas glows in H-alpha. H-alpha emission from much of the northern sky, reveals how complex the structure of the ISM is. The faint diffuse emission in the picture comes mostly from warm, ionized hydrogen gas. The especially bright spots, on the other hand, are quite different, these are regions where intense ultraviolet radiation from massive, hot, luminous stars is able to ionize even relatively dense clouds. These bright regions are called H-II (“H-two”) regions, signifying that they are made up of the second, or ionized, form of hydrogen (The first form of hydrogen is the neutral hydrogen atom or simply H-I). H-II regions are the fingerprints of star forming regions.


H-II regions have extremely diverse shapes, because the distribution of the stars and gas inside them is irregular. They often appear clumpy and filamentary, sometimes showing bizarre shapes such as the Horsehead nebula here below. H-II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H-II region, leaving behind a cluster of birthed stars.

The Horsehead nebula, is a dark molecular cloud, roughly 1,500 light years distant, is visible only because its obscuring dust is silhouetted against another, brighter nebula. The prominent horse head portion of the nebula is really just part of a larger cloud of dust. This picture show that the dark clouds in visible light of the Horsehead (left) is a H-II region because when astronomers looked at it in the far infra red (right) they see intense radiation in regions that are opaque in visible light.


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