Listening to the Cries of Baby Stars

We are familiar with the spectacle of birth here on Earth. We see seeds turning into apple trees, eggs hatching into tadpoles, and, of course, humans giving birth to babies. No matter how familiar, we are awed by the majesty and mystery of each birth. This drama, however, is not limited only to the terrestrial inhabitants but is also played in the celestial heavens.

Stars are born in large clouds of hydrogen gas. Far from being uniform, some parts of these clouds contain more hydrogen than the others and it is from these denser areas where stars eventually form. A nudge from a neighboring gas cloud is sometimes all that is needed to trigger the collapse of these dense hydrogen pockets leading to the formation of young stars. Thus, depending on its size, each cloud can give birth to hundreds to thousands of stars.

Our Sun was born in such a cloud roughly four and a half billion years ago. It had many siblings. While close at first, the gravitational binding was never strong enough to hold this family of stars together. Our Sun is a middle-age star today and its siblings are somewhere lost in the billions of stars that make up our galaxy, the Milky Way.

The cycle of birth, however, has not stopped and there are many hydrogen clouds in our galaxy that are forming stars even today. On a clear winter night, even a binocular can spot a fuzzy spot in the direction of the constellation of Orion the Hunter. This fuzzy patch, known as the Orion nebula, is glowing because of the light of a thousand baby stars “only” one million years old. To humans, one million years may seem like an eternity, but it is only a blink of an eye in a universe that is 14 thousand million years old. Orion nebula is a spectacular stellar nursery and there are many such places in the Milky Way.

The Milky Way is not unique. Other galaxies are forming stars too and there are more than a hundred billion galaxies spread out in the universe. There must be countless clouds turning hydrogen into stars every second somewhere in the universe.

Can we detect stellar nurseries, like the Orion nebula, in other galaxies? Fortunately there is a way, and it involves the detection of hydrogen gas surrounding baby stars. Just like fingerprints, each element in the universe has a unique signature based on its structure.

Young stars announce their presence by lighting up the leftover hydrogen gas surrounding them, resulting in a signature light that astronomers call Hydrogen-alpha or H (for readers familiar with physics, this is the light that results when an electron jumps from level 3 to level 2 of the hydrogen atom – amazing that a jump of an electron can yield secrets of star birth). The light from a galaxy, however, is a combination of young and old stars. We use filters at telescopes that can separate out hydrogen-alpha light by blocking light from old stars. As if we have silenced all noise except for the cries of baby stars, we are left with a picture of only the stellar nurseries.

This snapshot of baby stars can reveal past secrets as well as predict the future course of a galaxy. For example, in the galaxy by the name of NGC-1398, the thirteen hundred and ninety eighth object in the New General Catalogue, we find that most of the young stars are located in two beautiful rings, thousands of light years across. These rings may not last forever, but at the present time, it appears that gas clouds are bumping into each other at those locations in some kind of a cosmic traffic jam resulting in the formation of new stars.

Why are there almost no new stars in between the rings? How long before NGC-1398 runs out of gas to form new stars? What will it look like in the future – in the next billion years? The picture of young stars is only the beginning of the inquiry.

By studying the cycle of birth and death in other galaxies, we are trying to understand our own home, the Milky Way. Our galaxy appears to have enough gas (both unused and also recycled from the death of other stars) to sustain the birth of new stars for billions of years in the future. Our Sun will eventually run out of fuel and will return most of its gases to the galaxy – to be used again in the formation of new stars. We will be gone, but the cycle of life and death will continue.