Re: If the universe is so cold, harsh and black, how did our Sun evolve?

Hi Nicole,

The Sun and other stars were able to form from a cold dark universe because gravity is always acting to pull things together.

The universe is expanding, which means that all objects in the universe are essentially trying to "fly away" from each other. However, where enough matter is concentrated, such as in our Galaxy, gravity is strong enough to hold it together locally. Galaxies formed early in the history of the universe, from regions which had a more than average amount of matter concentrated in them. The excess matter caused stronger than average gravity locally, and so the overdense regions collapsed, even though the universe as a whole expanded.

Stars form via a somewhat similar process, but on a smaller scale. Clouds of gas form within galaxies, again due to gravity. The gas clouds within a galaxy feel a gravitational pull (due to the matter that makes up the clouds) which will attempt to cause the clouds to collapse inwards. In most clouds, the inwards collapse is prevented by the heat of the clouds, which tends to cause them to expand. Many clouds also contain magnetic fields, which also act to prevent collapse.

Some clouds, however, can become very dense and cold. By cold, I mean about 10 degrees above absolute zero (room temperature is about 300 degrees above absolute zero on the Celsius scale). Within such clouds, gravity may be able to dominate, causing regions of the cloud to collapse.

Initially, as the clouds collapse, they are still dense, dark, and cold. Eventually, however, they heat up as they release gravitational energy. You can release gravitational energy yourself, simply by dropping a book onto a tabletop. In that case, the gravitational energy is released in the form of a sound wave plus some heating of the tabletop. In the case of a collapsing cloud, it can only be converted into heating of the cloud.

The cloud, which is now called a protostar, continues to heat up until its core becomes hot enough to ignite nuclear fusion reactions. Now that it can generate energy and heat to support itself by means other than gravitational contraction, it stops contracting, and begins to evolve as a star.

So, the short answer is that overall the universe is cold and dark, and has a pretty low density. But, gravity acts so as to continue the growth of any density enhancements that occur, allowing the formation of things like galaxies, stars, and planets.

You can find a much more thorough answer in most introductory astronomy textbooks. One fairly good discussion is in Chapter 19 of "Astronomy Today" by Chaisson and MacMillan.