HII Regions
HII Regions
| Group Name | h2 |
| Reference | CO Radial Velocities Toward Galactic H II Regions
(Blitz+ 1982)
Georgelin, Y. M. and Georgelin, Y. M. 1976, Astronomy & Astrophysics, 49, 57 |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | h2region.speck, h2region.label |
| Dependencies | none |
| Census | 261 HII Regions and labels |
HII (pronounced “H-two”) regions are stellar nurseries for newborn stars. Stars are born from condensing clouds of hydrogen gas. As these clouds condense, the densities become high enough to form stars.
Typical gas clouds in the interstellar medium have densities too low to form stars. They need an outside stimulus or perturbation, like a nearby supernova, to compress parts of the cloud. If this occurs, small fragments are compressed, heating up the gas. If the cloud densities continue to increase, the cloud will collapse into a protostar. This protostar will contract under its own gravity, causing it to heat up. Eventually the protostar is hot enough to clear away the gas and dust that enshroud it. When the core temperature is hot enough for hydrogen fusion, the star is born.
HII regions are the surrounding clouds of hydrogen that glow from the stars born within them. It takes ultraviolet light to ionize hydrogen, light that can come only from hot, luminous stars (like O stars). When the star “turns on,” the electrons in the surrounding hydrogen are stripped away. The hotter the star, the farther the ionization radius, creating what astronomers call a Strömgren sphere. An O5 star can excite hydrogen up to 65 light-years from the star.
The result is a glowing nebula that is seen from great distances. One local celebrity among HII regions is the Orion Nebula (M42). About 1,500 light-years away, the wispy cloud can be seen with the naked eye in Orion's sword and resembles a hazy star. At its center are four bright stars that form an asterism called the Trapezium. These stars are surrounded by the cloud, which is about 25 light-years across. The largest of these stars, θ1 Orionis C, is 40 solar masses and has a surface temperature around 30,000 Kelvin (compared with our Sun's 6,000 K). It is also about 300,000 times more luminous that the Sun. The cloud, however, is heated only to about 70 K and has a very low density of 600 atoms per cubic centimeter. Compare this with air at sea level that has 1019 atoms per cubic centimeter.
Source Catalogs
Our catalog of HII regions is composed of two main surveys. The main catalog combines observations of molecular cloud complexes made in the Northern and Southern hemispheres. Observations from the Southern Hemisphere were made between 1975 and 1976 with the Anglo-Australian 3.9-meter optical telescope at a frequency of 115 gigaHertz (or a wavelength of 2.6 millimeters). They were then added to the Northern Hemisphere data in the early 1980s.
The catalog is composed of many characteristics of these clouds, such as temperatures, optical diameters, and carbon monoxide (CO) radial velocities. The CO molecule is important because it acts as a tracer of molecular hydrogen and radiates at an atmospherically transparent region in radio, so we can observe it from Earth. CO is rare in the Universe (relative to hydrogen), as it is broken apart by the ultraviolet light from stars. We observe CO only deep within molecular clouds, where it is protected by dust from the destructive interstellar radiation.
We use additional data from a paper published in 1976 by Y. M. Georgelin and Y. P. Georgelin at the Observatoire de Marseille. This paper is a study of molecular clouds in the Galactic plane and has a table of 100 HII regions. While the table gives Galactic longitude and the distance, it does not give the Galactic latitude, as the clouds all lie close to the plane of the Milky Way. For this reason, we place them all in the Galactic plane. This is not entirely correct from Earth's point of view, but it is valid when you are viewing them from outside the Galaxy.
Labels
HII regions are described by blue polygons. We designated labels from the Georgelin and Georgelin catalog to have a “GG” prefix. Because this is not strictly a catalog, there was no naming convention. The Blitz catalog retained the naming convention from the Sharpless catalog of the 1960s. Most of the data have Sharpless identifiers indicated by an ‘S’ followed by the object number.
Tracing Galactic Structure
From Earth's perspective, you'll notice that the HII regions all lie close to the Galactic plane. This is not an accident of nature. These star-forming regions lie in the plane of the Galaxy because that is where star formation occurs in spiral galaxies such as our Milky Way.
Radio astronomy was born in the 1950s, but it was not until the late '60s that astronomers began using these observations to trace the spiral arms of our Galaxy, forming a picture of the Galaxy we live in. Only 40 years earlier, we were debating the very existence of galaxies, and now we were mapping our Galaxy's spiral arms using radio observations of the CO molecule found in HII regions.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott