H II regions come in an enormous variety of sizes.

H II regions are ionized by large numbers of extremely bright and hot young stars.

H II regions may give birth to thousands of stars over a period of several million years.

The young stars in H II regions show evidence for containing planetary systems.

H II regions vary greatly in their physical properties.

A large spiral galaxy may contain thousands of H II regions.

The full details of massive star formation within H II regions are not yet well known.

A few of the brightest H II regions are visible to the naked eye.

During the 20th century, observations showed that H II regions often contained hot, bright stars.

Typically H II regions are found among areas of star formation in spiral galaxies.

These lines are important to the energy balance of such things as planetary nebulae and H II regions.

Young, hot stars ionize the gas (mainly hydrogen) around them creating H II regions.

Chemically, H II regions consist of about 90% hydrogen.

In this sense, the stars which generate H II regions act to destroy stellar nurseries.

Twenty first century observations have shown that a very small number of H II regions exist outside galaxies altogether.

They form from H II regions such as the Orion Nebula.

In many cases, the stars and the H II regions will be invisible to optical observations due to high levels of extinction.

Optical observations detected around 15 H II regions situated mainly along the spiral arms.

However it does not have significant H II regions and does not actively form stars at the present moment.

This makes H II regions more complicated than planetary nebulae, which have only one central ionising source.

The strongest hydrogen emission line at 656.3 nm gives H II regions their characteristic red colour.

The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form.

Supernova explosions may also sculpt H II regions.

There are four H II regions powered by short-lived, O-class stars.

H II regions are caused when stars are forming and are very hot, causing the plasma to heat up.