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Stars are luminous spheres of plasma held together by their own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. 

A white dwarf star in orbit around Sirius (artist's impression). image: wikipedia/NASA

Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. 

Most stars are invisible to us

Most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful telescopes.

Life of stars

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. 

Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. 

Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. 

Mass of  the star is the determining factor of its life

The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars.

 Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence.

 In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. 

The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V) image: wikipedia

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. 

When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy (mainly through electromagnetic radiation)  in the process. 

The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. 

A star with mass greater than 0.4 times the Sun’s (which includes our Sun) will expand to become a red giant when the hydrogen fuel in its core is exhausted. 

The current size of the Sun (now in the main sequence) compared to its estimated maximum size during its red-giant phase in the future. image: Oona Räisänen/wikipedia

In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. 

Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. 

Stars can form part of a much larger gravitationally bound structure, such as a star clusters or galaxies.

Source adapted from: Star. (2017, February 21). In Wikipedia, The Free Encyclopedia. Retrieved 22:09, February 23, 2017, from

The first stars

Here's a brief outline of the different populations of stars: Population I, II, and III.

- Population III stars:

  - These are theorised to be the first generation of stars, formed shortly after the Big Bang, around 13.6 billion years ago.

  - They consisted almost entirely of hydrogen and helium, with trace amounts of lithium and beryllium, as heavier elements had not yet formed.

  - They were likely extremely massive, leading to short lifetimes, so none are believed to exist in the current universe.

- Population II stars:

  - These are the second generation of stars, formed a few hundred million years after the Big Bang.

  - They contain small amounts of heavier elements (known as "metals" in astronomical terms), which were formed in the first generation of stars and distributed across the universe via supernovae.

  - Many globular clusters contain Population II stars, and these stars are generally found in the halo and bulge of galaxies.

- Population I stars:

  - These are the youngest generation of stars, including our Sun, and are still forming today.

  - They contain the highest amounts of heavier elements, as these have been continuously produced and distributed by earlier generations of stars.

  - These stars are typically found in the spiral arms of galaxies, and are associated with the galaxy disk.

  - Population I stars often form planetary systems, due to their metal-rich composition.

The order of these populations, from earliest to latest, is therefore: Population III, Population II, Population I.