This unit follows stars from cold molecular clouds through the main sequence to their ends as white dwarfs, neutron stars or black holes. You learn why mass decides the outcome, how supernovae work, where the chemical elements were made, and how a close companion can rewrite the whole story.
Preview: the beginning of the unit
1 Gas, dust and the birth of stars
1.1 The interstellar medium
The space between the stars of the Milky Way holds gas and dust with about a tenth of the mass that is in stars. By mass this interstellar medium is 70% hydrogen and 28% helium, and the heavier elements that make up the rest sit partly in dust grains smaller than a micrometer. An average cubic centimeter contains one atom, far fewer than the best vacuum chambers on Earth can reach.
Most of this gas is far too hot and thin to form anything. Stars form only in the coldest and densest part, the molecular clouds, where the temperature is 10 to 20 K, hydrogen exists as the molecule \(\mathrm{H_2}\), and a cubic centimeter contains \(10^2\) to \(10^6\) molecules. A giant molecular cloud is tens of parsecs across and has \(10^4\) to \(10^6\,\Msun\). Its dust blocks visible light, so such clouds are studied in the infrared and at radio wavelengths (Unit 4).
1.2 The Jeans mass
Gas pressure pushes a cloud apart and its own gravity pulls it together. The thermal energy of the gas grows in proportion to the mass \(M\), the gravitational energy grows with \(M^2\), so above some mass gravity must win. James Jeans worked out this critical mass in 1902.
While a core is transparent to its own infrared radiation it stays at 10 K, because the heat of compression is radiated away. The density rises, \(M_{\mathrm{J}}\) falls as \(\rho^{-1/2}\), and the collapsing cloud breaks into smaller pieces. This fragmentation is the reason stars are born in groups, and it ends when the pieces become opaque and warm up. Most of the pieces are small: red dwarfs of 0.1 to \(0.5\,\Msun\) are the most common stars, and for each star above \(8\,\Msun\) a few hundred lighter ones are born.
Jeans left out turbulence, rotation and magnetic fields, which all resist collapse. This is why the Milky Way, with \(10^9\,\Msun\) of molecular gas and free-fall times of a few million years, forms only 1.5 to \(2\,\Msun\) of stars per year.
1.3 Protostars and the way to the main sequence
Without pressure, a cloud falls together in the free-fall time, and Kepler’s third law is enough to find it.
At a central density near \(10^{-10}\unit{kg\,m^{-3}}\) the gas traps its own radiation. It heats up, the pressure rises, and the center stops falling. This object, a few au across and hidden in a cocoon of dust, is a protostar. Gas keeps raining onto it for a few hundred thousand years. Since the core has shrunk a millionfold, even a slow initial rotation has become fast, and most of the gas lands on a rotating accretion disk that feeds the star and later forms the planets (Unit 5).
Once the infall stops, a pre-main sequence star becomes visible. It shines because it contracts: gravitational energy is turned into heat (Unit 9). Its interior is convective throughout, which pins the surface temperature near 4000 K, so the luminosity drops as the radius shrinks. In Figure 1 this is the dotted, nearly vertical line. When the center reaches about \(10^7\) K, hydrogen fusion supplies as much energy as the surface radiates and the contraction ends. The star has arrived on the zero age main sequence, after about 40 million years for \(1\,\Msun\). An object below \(0.075\,\Msun\) never gets hot enough and becomes a brown dwarf (Unit 9).
Where the nearest molecular clouds lie in space was worked out only in 2020, with the help of the dust that makes them dark.
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