Star

For further reference: A light year is the distance travelled by light in a vacuum in one year (approximately 9,460,730,472,580.8 kilometers or 5,878,625,373,183.6 miles). In this article, distances will be in light years unless otherwise specified.

Stars viewed from the Hubble Space Telescope. Retrieved from the U.S. Public Domain.

A star is an astronomical object that consists of a sphere of luminous plasma held together by its own gravity. The Sun is the closest star to Earth, but numerous others are visible to the naked eye from Earth during the night, appearing as fixed, luminous points. However, although thousands of stars can be viewed from Earth, most of the stars in the universe, including all stars outside our galaxy, are invisible even with the most powerful telescopes.^[1]^[2]^[3]

For the majority of the life of an average star, thermonuclear fusion of hydrogen and helium occurs within the star's core, releasing energy that transverses the outer layers of the star and radiates into outer space, causing the star to shine and emit various types of electromagnetic radiation. During a star's lifetime, almost all of the 92 naturally occurring elements are produced within the star by stellar nucleosynthesis and, for high-mass stars, by supernova nucleosynthesis when the star explodes.^[1]

Astral Measurement
Stars within the Universe
Stellar Classification and Evolution
Characteristics and Structure of Stars
The Hertzsprung-Russell Diagram
References

Astral Measurement

Although astral parameters can be expressed using SI units, it is most convenient to represent characteristics of stars, including mass, luminosity, and radii, based upon solar units, which are based upon characteristics of the Sun. In 2015, the International Astronomical Union defined the set of nominal solar values, which can be used to describe astral parameters:^[1]

nominal solar luminosity: $L_{\bigodot} = 3.828 \times 10^{26} \text{ Watts}$
nominal solar radius: $R_{\bigodot} = 6.957 \times 10^8 \text{ meters} = 6.957 \times 10^5 \text{ kilometers}$
nominal solar mass: $GM_{\bigodot} = 1.3271244 \times 10^{20}\text{ m}^3\text{/s}^{2}.$

As described by Everipedia,^[1]

Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit, approximately equal to the mean distance between Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 meters.

Stars within the Universe

Astronomical Objects Within the Observable Universe (approximate):

There are approximately

10 million superclusters
25 billion galaxy groups
350 billion large galaxies
7 trillion dwarf galaxies
and 30 sextillion (30 billion trillion) stars

within the universe.^[4]

Closest Stars to the Solar System:^[5]^[6]^[7]

	Star Name(s)	Distance from Earth		Star Name(s)	Distance from Earth
1	Proxima Centauri	4.24 light years	21	DX Cancri	11.83 light years
2	α Centauri A and α Centauri B	4.37 light years	22	Tau Ceti	11.89 light years
3	Barnard's Star	5.96 light years	23	GJ 1061	11.99 light years
4	Luhman 16A and Luhman 16B	6.59 light years	24	WISE 0350−5658	12.07 light years
5	WISE 0855−0714	7.27 light years	25	YZ Ceti	12.13 light years
6	Lalande 21185	8.29 light years	26	Luyten's Star	12.37 light years
7	Sirius A and Sirius B	8.58 light years	27	Teegarden's Star	12.51 light years
8 $\hspace{5mm}$	Luyten 726-8 A and Luyten 726-8 B $\hspace{10mm}$	8.73 light years $\hspace{30mm}$	28 $\hspace{5mm}$	SCR 1845-6357 A/SCR 1845-6357 B $\hspace{10mm}$	12.57 light years
9	Ross 154	9.68 light years	29	Kpteyn's Star	12.78 light years
10	Ross 248	10.32 light years	30	Lacaille 8760	12.87 light years
11	Epsilon Eridani	10.52 light years	31	WISE 0535−7500	13.00 light years
12	Lacaille 9352	10.74 light years	32	Kruger 60 A/Kruger 60 B	13.15 light years
13	Ross 128	10.92 light years	33	DEN 1048-3956	13.17 light years
14	WISE 1506+7027	11.01 light years	34	UGPS 0722-05	13.26 light years
15	EZ Aquarii A/EZ Aquarii B/EZ Aquarii C	11.27 light years	35	Ross 614A/Ross 614B	13.35 light years
16	Procyon A/Procyon B	11.40(2) light years	36	Wolf 1061	13.82 light years
17	61 Cygni A/61 Cygni B	11.40(3) light years	37	Van Maanen's Star	14.07 light years
18	Struve 2398 A/Struve 2398 B	11.53 light years	38	Gliese 1	14.23 light years
19	Groombridge 34 A/Groombridge 34 B	11.62 light years	39	Wolf 424 A/Wolf 424 B	14.31 light years
20	Epsilon Indi A/Ba/Bb	11.82 light years	40	2MASS J154043.42-510135.7	14.40 light years

Stellar Classification and Evolution

Classification of Stars

Stellar Formation within Nebulas

Mass-dependent Evolutionary Stages

Characteristics and Structure of Stars

The Hertzsprung-Russell Diagram

The Hertzsprung-Russell diagram and the specific regions where certain star types (e.g. giants, supergiants, etc.) are found. Retrieved from the U.S. Public Domain.

Various color spectrums relating to certain stars (labeled). Retrieved from the U.S. Public Domain.

The Hertzsprung-Russell diagram is a 2-dimensional scatterplot showing the relationship between stars' absolute magnitudes (also known as luminosities or brightnesses) and their effective temperatures (which correlates to the stars' colors). The diagram was created independently by Ejnar Hertzsprung and Henry Norris Russell (hence the name "The Hertzsprung-Russell diagram") in 1910 and represented a major step towards the complete understanding of stellar evolution.^[6]^[8]

The horizontal axis of the Hertzsprung-Russell diagram represents a star's temperature, which directly correlates to the spectral type or spectral class (color) of the star. Astronomers recognize 7 different spectral types or temperature divisions of stars, which are also used in the Hertzsprung-Russell diagram:^[9]

Spectral Type	Color	Surface Temperature	Mass (Sun = 1)	Luminosity (Sun = 1)	Examples
O	Blue	Over 25,000 K	60	1,400,000	10 Lacertra
B	Blue	11,000 - 25,000 K	18	20,000	Rigel
A	Blue	7,500 - 11,000 K	3.2	80	Sirius
F	White	6,000 - 7,500 K	1.7	6	Canopus
G	Yellow	5,000 - 6,000 K	1.1	1.2	Sun, Capella
K	Orange	3,500 - 5,000 K	0.8	0.4	Arcturus
M	Red	Under 3,500 K	0.3	0.04	Antares

References

Everipedia, E. Star. Retrieved April 7, 2018, from www.everipedia.org/wiki/Star/
The Gale Group, I. Star - UXL Encyclopedia of Science, Encyclopedia.com. Retrieved from www.encyclopedia.com/science-and-technology/astronomy-and-space-exploration/astronomy-general/star
Fernie, J. Star - Encyclopædia Britannica. Retrieved from www.britannica.com/science/star-astronomy
Atlas of the Universe, I. The Universe within 14 Billion Light Years - The Visible Universe. Retrieved from www.atlasoftheuniverse.com/universe.html
Tate, K. The Nearest Stars to Earth (Infographic) - Space.com. Retrieved from www.space.com/18964-the-nearest-stars-to-earth-infographic.html
University of Wisconsin-Madison, I. The Closest Stars to the Earth - The 26 Nearest Stars. Retrieved from www.astro.wisc.edu/~dolan/constellations/extra/nearest.html
Atlas of the Universe, I. The Universe within 12.5 Light Years - The Nearest Stars. Retrieved from www.atlasoftheuniverse.com/12lys.html
Wikipedia, I. Hertzsprung-Russell diagram - Wikipedia. Retrieved from https://en.wikipedia.org/wiki/Hertzsprung–Russell_diagram
Enchanted Learning, I. Star Classification - Zoom Astronomy. Retrieved from http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml

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