Hydrogen-Alpha All-Sky Survey
Hydrogen-Alpha All-Sky Survey
| Group Name | mwHalpha |
| Reference | Doug Finkbeiner (Princeton) |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | No |
| Files | mw-halpha.speck |
| Dependencies | mw-halpha.sgi |
| Wavelength | 656 nm |
| Frequency | 457,000 GHz |
Hydrogen-alpha, or H-alpha, is a term that describes light from the ground state of the hydrogen atom. When an electron in an atom moves from one energy level to a higher one, we say the atom is excited. But the electron does not move to this higher energy level without the atom absorbing energy from either another atom or a passing photon (packet of light).
Once the atom is excited, it cannot remain in that state for long before it wants to return to its ground state. When the electron moves back down to the lower energy level, a photon is released at a wavelength commensurate with the energy between the two levels. For the H-alpha line, this energy difference translates to a wavelength of 656 nanometers and is in the extreme red end of the visible spectrum.
This survey of the sky is a snapshot of light from this wavelength. We can see this light with our eyes, but we also see the integrated light from the entire visible spectrum. If we could block the rest of that light so that we could see only light at 656 nm, we would see this picture of the Milky Way in the night sky.
Image Features
One feature of the sky at this wavelength is the presence of large, spherical bubbles surrounding hot stars. One of the most prominent is the bubble around Lambda Orionis. This is an O star and is among the hottest we see. It lies about 1,000 light-years away and is so hot that it ionizes the surrounding hydrogen gas, causing it to glow.
We can perform a crude size determination on the ionization cloud. If the star is 1,000 light-years away and the cloud has about a 3-degree radius in the sky, then, using a simple triangle, we can determine that the cloud extends about 60 light-years in radius. This HII region, Sharpless 264, is one of many that glow bright in the spiral arms of our Galaxy. Zeta Ophiuchi is another hot star, about 450 light-years away.
Other nebulae and HII regions are visible, like the California Nebula, and what could be called the Orion nebluplex. Among the sights in the lower part of Orion, the Great Nebula of Orion is among the most beautiful star-forming regions in our neighborhood. Just above it is the Horsehead Nebula, an emission nebula with a small, obscuring dust cloud in the shape of a horse's head. Surrounding this is the extended supernova remnant called Barnard's Loop.
We also see galaxies that emit in H-alpha, including the Andromeda Galaxy and the faint M33, the large face-on spiral in Triangulum. In the southern sky are the Large and Small Magellanic Clouds (LMC and SMC), two nearby (about 130,000 light-years) satellite galaxies that have collided with our Galaxy.
This H-alpha view of our sky reveals where the hot, ionized hydrogen is. For the most part, it lies in the plane of the Galaxy but is above or below the plane when objects are relatively nearby in the Galactic foreground.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott