---
title: "The Interstellar Medium and How Stars Form"
description: "The space between stars holds gas and dust in phases from cold molecular clouds to hot ionised bubbles, and dust reddens and dims everything seen through it. A cloud collapses when its own gravity bea"
canonical: https://lightmysky.com/learn/science/the-interstellar-medium-and-how-stars-form-mt_i2HyLf_Emy
source: https://lightmysky.com/learn/science/the-interstellar-medium-and-how-stars-form-mt_i2HyLf_Emy.md
retrieved: 2026-09-12
---

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# The Interstellar Medium and How Stars Form

The space between stars holds gas and dust in phases from cold molecular clouds to hot ionised bubbles, and dust reddens and dims everything seen through it. A cloud collapses when its own gravity beats its internal pressure, which sets a minimum mass for the collapse and starts the next generation of stars.

Subject: Science · Area: Astronomy & Astrophysics · Ages 21 to 22
Page: https://lightmysky.com/learn/science/the-interstellar-medium-and-how-stars-form-mt_i2HyLf_Emy

## Ready when they can

- Names the phases of the interstellar medium by temperature and density and says which wavelength each is observed at
- Explains extinction and reddening by dust and what they do to a distance measured from brightness
- Argues the collapse condition by comparing gravitational to thermal energy and says what happens to the excess angular momentum

## Lesson: The gas between stars, and how it becomes stars

The space between stars is not empty. It holds gas and dust in phases set by temperature and density. Cold molecular clouds are dense and chilly. Warm neutral and ionised gas fills much of the volume. A hot thin component spreads through the rest.

**Example.** Suppose you point three telescopes at one cloud. The radio dish maps cold molecules. The optical camera catches warm glowing gas. The X ray satellite sees only the hot thin parts. Each window shows one phase, so naming a phase means stating its temperature, density, and window together.

Dust grains are tiny solids that block and redden starlight. Blue light scatters away more strongly, so a star behind dust looks redder and fainter than it is. A distance read from uncorrected brightness overshoots and puts the star too far. Measure the reddening, restore the true brightness, and only then convert to distance. The stolen light returns as heat glow, which is why stellar nurseries shine brightest in the infrared.

**Tip.** To judge a cloud, ask who wins: gravity or heat. Cooling, crowding, or gathering more gas hands victory to gravity, and that is the Jeans argument for collapse. Heating or stirring defends the cloud. As collapse runs, spin-up threatens to fling gas apart, so the excess spin drains away through disks, jets, and magnetic braking, and what remains settles into a young star plus a disk.

**Recap.** Cold dense clouds collapse when gravity beats heat, dust reddens and dims everything behind it, and each phase owns its window.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Stellar Spectra, Spectral Classes and Surface Composition](https://lightmysky.com/learn/science/stellar-spectra-spectral-classes-and-surface-composition-mt_4FBh5G7Gdc)
- [Hydrostatic Equilibrium and the Structure of a Star](https://lightmysky.com/learn/science/hydrostatic-equilibrium-and-the-structure-of-a-star-mt_RKeMW8iX56)