Light

There are a number of ways to quantify an object’s “brightness”. It may seem simple, but knowing how bright a star, galaxy, or nebula is can be extremely useful data for astronomers trying to understand an objects other properties. When studying object’s at great distances light becomes a valuable way of gathering information. Knowing how much light is emitted and at what wavelengths is an effective way of deriving the physical properties of the object being observed. In astronomy, there are three commonly used terms referring to “brightness”: luminosity, flux, and magnitude. Although all these terms describe how bright an object is, they differ in how they are obtained and what they can tell us.

Once a star reaches the white dwarf stage of its evolution, nuclear fusion in the star’s core no longer provides the gas pressure needed to counteract gravitational contraction and maintain the star’s radius. In this state, the star is kept from collapsing under its own gravity only by the electron degeneracy pressure. However, relativity requires that the maximum speed of the degenerate electrons can never exceed the speed of light. This means there is a limit to the degeneracy pressure, and a limit to the mass of a white dwarf. This mass limit (about 1.44 solar masses) is known as the Chandrasekhar limit, above which the electron degeneracy pressure cannot prevent gravitational contraction. Therefore, any star that reaches the final stages of its evolution with mass greater than the Chandrasekhar limit will continue shrinking until it eventually collapses under its own gravity, and becomes a neutron star (the residual of a super nova explosion) or black hole.

Behind the shutter of a digital camera lives a Complimentary Metal Oxide Semiconductor (CMOS) detector. These detectors are made from an array of linked Metal Oxide Semiconductor (MOS) capacitors (figure 2) (Chromey 2010, 219-221). Each capacitor is a photo sensitive element or “pixel” in the array. When a photon strikes one of these pixels, it produces an analogue signal that can be interpreted and converted to a digital number by on-board electronics (Howell 2006, 8-14). In many ways these detectors are similar to the Charge Couple Device (CCD), however, CMOS detectors have several electrical components for converting analogue signals into digital numbers built into each pixel. This produces faster readout times, which is why digital cameras are able to save images as a matrix of digital numbers and display them on a review screen almost instantly.