## Glossary

Hot Spot

An HII Region found in a galactic nucleus. May or may not have similar properties to disk HII Regions.
HII Region

A cloud of iniozed hydrogen gas in a galaxy. These are known to be regions of heavy starbirth, and consequently of great interest to astronomers.
Spectrum

The result of breaking light into component wavlengths. This can be accomplished by sending the light through a prism, by sending the light through a defraction grating, or by using a blazed mirror. The result is a breakdown the intensities of the light's components by wavelength.

Spectra can be used to determine many things in astronomer, from redshift and compositions to temperature and density.

Long Slit Spectrum

A spectrum with a spacial dimension. One axis of the resulting image represents wavelength, while the other represents some physical dimension of length. Using longslit spectra, we can see the spectra of many regions at one time, and compare them.
Redshift

The shifting of the spectrum of a star or galaxy due to radial velocity (meaning velocity toward or away from the observer, in this case, us). The shift is caused by the Doppler effect, and through Doppler's equation, we can convert shift to radial velocity.

While this is commonly called redshift, the spectrum could also be shifted the other way, blueshifted, if the star or galaxy were heading toward us rather than away.

The redshift is useful in exploring other galaxies because it can give us an idea of the distance to the galaxy via Hubble's Law.

Hubble's Law

Named for astronomer and discoverer Edwin Hubble, Hubble's Law states that galaxies are receeding from us (for the most part), and that further galaxies are moving away faster. The speed of recession can be expressed as V=H D where V is the velocity, H is Hubble's parameter, and D is the distance. Hubble's parameter lies somewhere between 50 km/sec/Mpc to 100 km/sec/Mpc, probably near 75 km/sec/Mpc. The exactly value is still very much in debate, as it is exceedingly difficult to get distances to galaxies via non-Hubble's Law means.

The reason for this recession, by the way, is that the universe is expanding, and the rate of expansion is uniform everywhere in space. It does not mean that we are at the center of the universe. Think of space as the surface of a balloon and u there galaxies as pennies taped to the balloon. As you blow the balloon up, the pennies move apart, and if you were and ant on one penny, you'd see the others moving away, the futher ones moving faster thant the near.