Star Birth

Making a star is not simple, and yet most galaxies make them all of the time. Perhaps this is because the recipe is simple: You need hydrogen, lots and lots of hydrogen. You need gravity. And you need time. Stir until lit.


Hydrogen is the most basic atom to make – it is one proton and one electron. Recall that an atom is the smallest fragment of an element – any smaller, and it ceases to be that element and is just a community of particles.

The first stable protons, electrons, and neutrons were forming in the early, super hot Universe about 13 billion years ago -- just a few seconds after the Big Bang.

Super hot means that these particles were moving around at great speed, enough so that some hit each other and got stuck together. From these combinations, helium, lithium, and beryllium nuclei were made in small quantities. These nuclei were large enough for electrons to hang on to without being blasted off.

It took the Universe hundreds of thousands of years of cooling before its electrons could stably hover around single protons. This most simple arrangement of one electron to one proton became the most dominant atom in the Universe: hydrogen.

Galactic hydrogen was first discovered by radio astronomers using a horn-shaped antenna sticking out of the side of a laboratory at Harvard. Physicists Harold Ewen and Edwin Purcell detected a radio hiss broadcasting on hydrogen’s frequency channel throughout our Galaxy.

Since then, sophisticated giant radio telescopes such as the GBT have continued to hunt the skies with greater detail to map the distribution of this critical element. Astronomers estimate that 74% of the atoms in the Universe are hydrogen.

Floating around with the hydrogen between the stars are other atoms, molecules, and dust. These materials all give off radio waves, and our telescopes discover, map, measure, and clock these regions in our Galaxy and thousands of others.

When there is in a dense gathering of this matter, gravity can make mountains out of molehills.


Gravity is the force that allows matter to find each other. It may be the result of a fabric of particles called Higgs Bosons and a carrier particle known as a graviton; physicists are still trying to solve this mystery. Regardless of how it works, gravity brings nearby particles together, be they atoms, molecules, or even dust particles.

It takes a long, long time for enough matter to swirl itself into the hefty ball of stuff that will turn into a star. We use radio telescopes to watch thousands of young stars and protostars to assemble a timelapse of events.


As a pile of matter grows, its increasing mass can tug harder on particles around it and grab material farther from it. The pile grows darker and darker as more cosmic dust is drawn into it. This is where radio telescopes are critical to continuing the story of star formation, for ours are the only type of instrument that can see into the dust.

Incoming matter slams into the growing pile, sending loads of energy into it, and heating it up. The dense pile of atoms gets so hot that it becomes a plasma, meaning that electrons become too energized to stay attached to their atomic nuclei and are freed again.

As the cloud of material collapses, the pressure inside increases. The exposed nuclei in the cloud’s core smash into each other and fuse into heavier nuclei. Lots of energy is released when nuclei fuse like this, and the pile becomes one huge fusion reactor.

The energy from the fusing core is released as Gamma rays, but atoms and molecules in the collapsing cloud absorb much of this powerful energy. When those atoms and molecules relax again, they release the energy, and light waves of longer wavelengths finally find their way out of the cloud. It is at this point that we say a star is born.

Dwarf stars like the Sun need about 50 million years to go from a collapsing cloud to a shining star. Giant stars, like Sirius in the constellation of Orion, can turn on in 60,000 years.

To optical telescopes, however, these events remain dark. Infrared telescopes see the warm blankets of dust around the baby stars, but cannot see past them. ALMA picks up waves of light that are slightly longer than infrared, so with it we can see deeper into the shroud to watch how gas feeds onto the new star. ALMA also shows us the dust particles and helps us find evidence of planets growing around young stars.

Eventually, most stars will blow away the extra gas and dust that obscure them, and they will become visible to optical telescopes. High-resolution radio telescopes like the VLA watch this adolescent stage as these young stars shove the excess material from them. The VLA also maps the growing magnetic fields that drive this material off of these stars.

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