Deep Space
Nebulae and Star-Forming Regions
Updated by Spaceweathertracker staff ·
Vast clouds of gas and dust, glowing or dark depending on what's lighting them from within or hiding them from view.
A nebula is a cloud of interstellar gas and dust. What it looks like depends on how it interacts with light: an emission nebula glows because nearby hot stars ionize its gas; a reflection nebula merely scatters starlight, the way dust scatters headlight beams; a dark nebula is dense enough to block light behind it entirely. Many nebulae are also stellar nurseries — regions where cold, dense molecular hydrogen is actively collapsing under its own gravity to form new stars.
How a cloud becomes a star
Star formation begins when a region of a molecular cloud becomes dense and cold enough for gravity to overcome the outward pressure of the gas. As the region collapses, it fragments into smaller clumps, each a candidate protostar. Conservation of angular momentum spins the collapsing material into a disk, and the star grows by drawing in material from that disk over roughly tens of thousands to a few million years, depending on the star's eventual mass.
Why some nebulae glow and others don't
Emission nebulae like the Orion Nebula sit near young, massive, extremely hot stars whose ultraviolet radiation strips electrons from surrounding hydrogen atoms; the characteristic red glow comes from those electrons recombining and releasing light at a specific wavelength. Reflection nebulae, by contrast, sit near stars too cool to ionize the surrounding gas — the blue color often seen (as in parts of the Pleiades) is starlight scattering off dust grains, the same physical process that makes Earth's sky blue.
What scientists know
The broad physics of gravitational collapse and the basic classification of nebula types by their light-interaction mechanism are well established and consistent with decades of spectroscopic observation.
What remains uncertain
The precise details of exactly how a collapsing cloud fragments into individual stars of different masses — what astronomers call the initial mass function — remains an active area of research, as does the role turbulence and magnetic fields play in slowing or accelerating collapse.
Related questions
What's the difference between a nebula and a galaxy?
A nebula is a cloud of gas and dust within a galaxy — a galaxy is a much larger gravitationally bound system of stars, gas, dust, and dark matter, of which nebulae are just one component.
How long does star formation take?
For a Sun-like star, the collapse phase from dense core to a stable, hydrogen-fusing star takes on the order of tens of millions of years, though the initial protostar phase is much shorter.