# General Astronomy/Stars/Introduction

Stars, by astronomical definition, are massive luminous balls of plasma. They exist in numbers too large to comprehend, and most are believed to be accompanied by a system of planets. The planet Earth is part of the Sun's solar system of 8 planets, the Sun being the nearest star to Earth. The Sun provides most of Earth's energy, and is critical to the survival of all forms of life on the planet.
Over the history of mankind, human views of the stars have differed greatly. Historically, the stars have existed as major figments in the mythological canon of many cultures, and were seen as objects of mystery and speculation. The Sun in particular has been the object of great cultural, religious and theoretical importance and speculation and has, more specifically, been a central point of debate in arguments relating to the topography and geography of the solar system itself.
The stars have also been great use. Seafarers and fisherman have historically used the patterns of the stars on the night sky as a vast and accurate kind of map. Many civilizations tracked their movements and channeled their findings into calendars, which in turn would determine agricultural practices.
In recent centuries, scientific views of the stars have become much more advanced, following the invention and continual advancement of the telescope, and the discovery of various techniques and ways to assess the various properties of a star. This study of stars is built upon the only two attributes that are directly observable from Earth: a star's location and its brightness. Although the stars appear to move across the sky over the course of a night and over the course of a year, the relative positions of the stars are almost constant relative to each other. However, the careful measurement of stellar positions (astrometry) shows small motions. Three of the most significant contributors to stellar motion are:
* Parallax changes, due to the earth motion around the sun,
* Proper motion, due to the star's journey around the center of the galaxy, and
* Wobbles in the proper motion, due to the star's gravitational "dance" with a companion, either another star (or stars) or one or more planets orbiting the star.
Measurement of star brightness provides a wealth of information about the star. In particular:
* Measurement of how the star's brightness changes over time (its lightcurve) can reveal the presence of companion stars (in binary or triple systems), companion planets, sunspots, flares, explosions, and recurring (sometimes even periodic) fluctuations in the star's structure. Collectively, we refer to any star whose brightness fluctuates as a variable star.
* Measurement of how the star's brightness varies with the wavelength (color) of light can be used to describe the star's color, estimate its temperature, and detect specific wavelengths where the star's light is either brighter or dimmer than average. These spectral lines can be used to identify the elements (and sometimes even molecules) within the star's atmosphere.
A star's apparent brightness is influenced by three primary factors:
* Its intrinsic brightness (luminosity), a measure of the amount of energy being released by the star,
* Its distance from the earth, and
* The presence of any clouds of absorbing material between the earth and the star.
Because the dimming of starlight because of distance from the earth is well-defined and easily calculated and because the apparent brightness of a star is easily measured, knowing (or estimating) the star's luminosity allows its distance to be calculated and vice versa.
In the following topic regarding stars, we explore several key aspects of stars, their properties, the continual advancement of our knowledge of these heavenly bodies, and what their place is in the universe.

## Table of contents

* Mass
* Luminosity
* Temperature
* The H-R Diagram
* Star Clusters as Cosmic Laboratories