Stars
Stars are astronomical objects comprisng of luminous spheroid of plasma held together by their own gravity. The closest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, emerging as a multitude of fixed luminous points in the sky due to their enormous distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the most dazzling of which gained proper names. Astronomers have gathered star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 1×1024 stars, but most are undetectable to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.
Star Formation
Stars are born within the clouds of dust and dispersed throughout most galaxies. A well-known example of such as a dust cloud is the Orion Nebula. Commotion deep within these clouds gives rise to clusters with enough mass that the gas and dust can start to collapse under its gravitational pull. As the cloud collapses, the matter at the center begins to heat up. Known as a protostar, it is this fiery core at the heart of the collapsing cloud that will one day become a star. Three-dimensional computer models of star structure predict that the spinning clouds of collapsing gas and dust may break up into two or three blobs; this would explain why the bulk the stars in the Milky Way are paired or in groups of multiple stars.
As the cloud collapses, a thick, hot core forms and begins gathering dust and gas. Not all of this stuff ends up as part of a star — the residual dust can become planets, asteroids, or comets or may remain as dust.
In some cases, the cloud may not fall at a steady pace. In January 2004, an amateur astronomer, James McNeil, noticed a small nebula that appeared suddenly near the nebula Messier 78, in the constellation of Orion. When watchers around the world pointed their instruments at McNeil’s Nebula, they found something unusual — its brightness appears to vary. Observations with NASA’s Chandra X-ray Observatory provided a likely explanation: the synergy between the young star’s magnetic field and the neighboring gas causes episodic rises in brightness.
Main Sequence Stars
A star the size of our Sun needs about 50 million years to mature from the start of the collapse to adulthood. Our Sun will stay in this mature phase (on the main sequence as shown in the Hertzsprung-Russell Diagram) for approximately 10 billion years.
Stars are fed by the nuclear fusion of hydrogen to form helium deep in their interiors. The flow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines.
Main Sequence stars traverse a wide range of luminosities and colors, and can be classified according to those characteristics. The smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K. Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years.
On the other hand, the most extensive stars, known as hypergiants, may be 100 or more times more massive than the Sun, and have surface temperatures of more than 30,000 K. Hypergiants emit hundreds of thousands of times more energy than the Sun, but have existences of only a few million years. Although extreme stars such as these are believed to have been common in the early Universe, today they are remarkably rare – the entire Milky Way galaxy contains only a handful of hypergiants.
Stars and Their Fates
In general, the more massive a star, the shorter its life, although all but the most bulky stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still accessible outside the core, so hydrogen fusion proceeds in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to swell and cool, remodeling the star into a red giant.
If the star is adequately massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a mixture of heavier elements up to iron. However, such reactions offer only a release. Slowly, the star’s internal nuclear fires become more unstable – sometimes burning furiously, other times dying down. These mutations cause the star to pulsate and throw off its outer layers, wrapping itself in a cocoon of gas and dust. What happens next? Wait for our next article to find out.
