What Is #1: A Star

I started this blog over six years ago and in this time it has changed and evolved as would be expected. One of the new changes I've been thinking about for awhile and am implementing now, is a new series titled "What Is". To start this series, I've chosen an astronomical object category that most people have at least some familiarity, a star.

What is a star? Simply put, a star is a large ball of gas giving off a great deal of light and producing energy through nuclear fusion.

Star Merope (23 Tau) in Pleiades (M45) by Henryk Kowalewski

This barely scratches the surface (no pun intended, or maybe it is intended) of a star. Let's start with the gas. Stars are formed from large clouds of gas and dust and this gas is primarily hydrogen and helium. A star like the Sun is 75% hydrogen and 25% helium with trace amounts of other materials. Over a star's lifetime it converts hydrogen into helium in the core through a nuclear fusion process. It is the nuclear fusion process that differentiates a star from a brown dwarf (failed star). The star ends its life with just a bit less hydrogen than it started with and a bit more helium than it started with.

Stars come in a variety of categories. Stars can be much smaller than the Sun, as small as 8% the mass of the Sun. The smaller the star, the cooler and redder it will be. Stars can have masses upwards of a 100 times that of the Sun. These larger stars are much hotter and much bluer in color. Once a star uses up its hydrogen supply in the core, it enters the end stages of its life and at this point a lot of weird things happen. Large, hot, blue stars can suddenly become cool and red. I'll cover this in much greater detail in a future post in this series.

Stars are often found in binary (or more) systems with other stars. Estimates suggest 50% or more of stars are part of multi-star systems in which the two (or more) stars are in orbit about each other. Our Sun is a single star system so we'll never know what it's like to live in a binary system. 

There is so much more to discuss on the topic of stars, but to avoid a very long post, I'll leave you with the basics and dig much deeper into the details with future posts on red giants, white dwarfs, neutron stars, pulsars, black dwarfs, brown dwarfs, etc.