 To understand how old a star is, we need to take a closer look at how stars form. We cannot observe the whole formation process for a single star, it takes millions of years. But we can learn a great deal by observing star formation at different stages, starting with the molecular clouds that hold the matter that is transformed into the stars. As a giant molecular clouds exist in the disc of our galaxy, each has hundreds of thousands to a few million solar masses of material. We've seen some of these clouds in the Starbirth Nebula segment of the How Far Away Is It video book. Giant molecular clouds can be as large as 600 light-years wide. This illustration is 200 light-years in diameter. They contain mostly hydrogen and some helium, but they are also seeded with some heavier elements, such as oxygen, carbon, iron and others, from the dusty remains of earlier generation stars that ended as planetary nebula or supernova remnants. The clouds are dense relative to the rest of the gas between the stars, the interstellar medium, but are still much less dense than the atmosphere of a planet. Typical cloud densities are around a billion molecules per cubic meter. That might sound like a lot, but each cubic meter of air at the surface of the Earth has 10,000 trillion times more than that. To determine the temperature of a molecular cloud, we measure the incoming radiation power from a sizeable area and use the Stefan-Boltzmann law to calculate the temperature. Let's show that clouds like these are around 10 Kelvin. That's extremely cold. At these low temperatures, atoms combine to form molecules, such as molecules of hydrogen that's H2 or water H2O. Over 80 other molecules have been observed in these clouds. These clouds are in hydrostatic equilibrium and ruled by the ideal gas laws. At every point inside the cloud, the weak outward gas pressure is equal to the weak inward gravitational force. We don't know what triggers a collapse. One thought is that it happens when clouds collide with other clouds. Another theory has it that supernova remnant wave fronts can do it. Here we see the remnant exert a force that compresses a cloud to the point where it creates an imbalance in the gas pressure versus gravitational force in favor of the gravitational force. Another theory suggests that collapse is triggered when a cloud passes into a galaxy's spiral arm. In any case, once started, the collapse becomes extremely chaotic and the chaos will continue until a new hydrostatic equilibrium is established. In our example, the cloud has collapsed to a 180 light-year diameter. Observations and computer simulations indicate that such a compression would lead to the cloud breaking up into fragments of various sizes and shapes within 2 million years. Each of these fragments continue to collapse. Over a 20 million year period, they form planet-sized objects, brown dwarfs and stars of all masses. In this way, collapsing giant molecular clouds create star clusters. The new hot stars, radiation and shock waves push away lighter surrounding gas and dust and illuminate denser surroundings, creating a site like this one. NGC 602 is at the center of the star birth emission nebula N90 in the small Magellanic cloud orbiting the Milky Way. Hubble detected over 5,600 stars. A key point to remember for our How Old Is It Purposes is that these molecular cloud collapses always create star clusters, with star counts that range from a few to hundreds of thousands of stars depending on the amount of matter in the original collapsing molecular cloud. We never see single stars being formed. All the stars in the cluster will be approximately the same age. It will be a globular cluster like M80. If the gravitational force, due to the total mass of all the stars, is enough to bind them together. Or it will be an open star cluster, like depleides. These stars still have some of their cloud fragment material in their vicinity. These stars create this reflection nebula. These stars are all the same age. The fact that they are still close together indicates that they are very young. Over the next 250 million years, these stars will drift so far apart that observers viewing these stars at that time won't be able to link them together. Stars not associated with any cluster are called field stars, just stars in a field of stars. Our sun is a field star. It has drifted far from the cluster it was formed in.