What is a light-year and how is it used?

A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,500,000,000,000 kilometers.

Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine. It is a few hundred kilometers from New York City to Washington, DC; it is a few thousand kilometers from California to Maine. In the universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.

In our solar system, we tend to describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km (93 million miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our solar system.

For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec . The light-year we have already defined. The parsec is equal to 3.3 light-years. 

Using the light-year, we can say that :

• The Crab supernova remnant is about 4,000 light-years away.
• The Milky Way Galaxy is about 150,000 light-years across.
• The Andromeda Galaxy is 2.3 million light-years away.

What is Fraunhofer Spectram?

Fraunhofer Spectrum also called as Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer. The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

In 1802, a scientist called W.H. Wollaston noticed that the visible spectrum from the Sun had several dark lines in it. Not long afterwards, Joseph von Fraunhofer built the first spectrometer. This focused sunlight from a small telescope onto a narrow slit. The light then passed through a prism, which produced the spectrum. Fraunhofer later invented the diffraction grating, which is used in most spectrometers today. Fraunhofer not only confirmed Wollaston's results, but also found that there were far more dark lines in the spectrum than Wollaston had suspected. Fraunhofer showed that these were a feature of sunlight and not an illusion nor an optical effect, and he labelled them with letters of the alphabet (A,B,C etc.). We now call these dark lines Fraunhofer lines.