Reading: Chapters 19 (The Nature of Stars) and 5 (The Nature of Light)

Astrophysics is a difficult observational (experimental) science for several reasons. Two of the most important ones are:

• Stars are very far away which makes ordinary experiments impossible. (That is, it is difficult to perform experiments like we do on Earth.) For example, the nearest star is Proxima Centauri. It is located around 4.3 light years ~ 40 trillion kilometers from the Earth. Traveling at the speed of the typical interplanetary probe, ~ 20 kilometers per second, it would take 70,000 years to travel to Proxima Centauri! It is clear that we will not be able to travel to stars and get close-up views and to perform experiments in situ.

  We will only be able to perform experiments remotely

• Further, we cannot control stars. This means that we cannot design arbitrarily the types of experiments we would like to perform. We simply observe what nature gives to us. Even though we must take what we get there are many interesting things which occur. e.g., the impact of comet Levy-Shoemaker on Jupiter last summer. However, note that this event was fortuitous in that this was a transient for which we had advance warning (and so we were ready). In general, interesting things like this go unnoticed and under utilized because we aren't expecting them.

How Do We Study Stars?

Traditionally, most observations of Celestial Objects were made using Optical Telescopes. I will define optical formally in a little bit, however, for now, take optical light to mean the type of light to which our eyes are sensitive. Much information has been gleaned from optical observations, however, there is much more information about the Universe than is contained in only its optical light. There are many other forms of light which the objects in the Universe produce. Until recently, this information was not utilized. Today, we study stars across all portions of the electro-magnetic spectrum (to be defined more precisely later), e.g., we study stars using their radio and micro-wave radiations, their infrared radiations, their ultraviolet radiations, their x-ray radiations, and their gamma-ray radiations. There are types of stars which, in fact, radiate predominantly in the x-rays; they are very faint in optical light!

Furthermore, we also study stars using the matter that they emit. For example, the Solar Neutrino Experiment studies the particle emission from the Sun (see 4 Jan lecture notes). In addition, there are active experiments which try to detect the gravitational radiation from stars and other Celestial Objects.

The opening up of the electro-magnetic spectrum and the other advances in technology have substantially enhanced our understanding of not only exotic stars, but also our understanding of more or less normal stars like our Sun.

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