Region of sky that is obscured at optical wavelengths by our own Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical
, the Milky Way. It covers roughly 20% of the sky and is centered along the galactic equator, the ZoA is irregular in shape, varying in height with the widest point located towards the center of our Galaxy in the constellation Sagittarius. Extragalactic objects (red dots in figure) do exist in this region of sky, but the In astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions through the Galactic plane is very high (Galactic dust and dense gas absorb light from background sources). The high concentration of stars also makes faint extragalactic objects difficult to detect within the ZoA. To penetrate the ZoA, we must observe this region at wavelengths that are not as affected by dust (such as infrared, radio, and High-energy electromagnetic radiation, with short wavelength (~10-0.01 nm) and high frequency (greater than ~1016 Hertz). Although the boundaries are somewhat arbitrary, wavelengths shorter than 0.01 nm are called gamma-rays and those longer than 10 nm extreme ultraviolet (EUV). X-rays would be produced by blackbody radiation at temperatures in excess of).