HOW DO WE STUDY STARS? Chaisson & McMillan, Chs 3 & 4

Astrophysics is a difficult observational science. Why?

• Stars are distant which makes ordinary experiments impossible. The nearest star is Proxima Centauri at a distance of 4.3 light years ~ 40 trillion kilometers. Traveling at the speed of the typical interplanetary probe, ~ 20 kilometers per second, it would take 70,000 years to reach Proxima Centauri!

• We cannot control stars, in particular, we cannot design the types of experiments we would like to perform on stars; We must take what nature gives to us.

Most observations of Celestial Objects are made using Optical Telescopes located at ground-based (e.g., Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, National Solar Observatory, Mauna Kea Observatory, ...) and space-based orbiting observatories (Hubble Space Telescope). Optical light will be defined in a second. For now, take it to mean the type of light to which our eyes are sensitive.

A ton of information about the Universe has been gleaned from optical observations, however, much more can be learned if we consider more than just optical light [objects in the Universe produce many other forms of radiation. Collectively, the overall radiation phenomenon is referred to as Electromagnetic radiation (EM radiation)]. Until recently the complete EM spectrum was not utilized because



that is, most types of EM radiation cannot penetrate the Earth's atmosphere. Today, because we can place telescopes into orbit about the Earth, we are able to study stars across many portions of the EM spectrum.

We also study stars using the matter (particles) they produce, e.g., the Solar Neutrino Experiment studies the particle emission from the Sun.

There are also experiments designed to detect the gravitational radiation from compact stars and other Celestial Objects.

The opening up of the EM spectrum and the study of other forms of Celestial emissions have substantially enhanced our understanding of Celestial objects of all kinds.