Stars


by SpaceGYAN team

This article was first published in 2020; later it has been revised in March 2021. Sources are - Wikiped

IN THIS ARTICLE

Basics of stars

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What are stars?

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity due to which fusion reaction occurs. 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.

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Requirements of a star to be formed

There should be a source of fuel which is Hydrogen (H) which will be acquired from a nebulae. The right mass is required for the star to be formed so that the fusion reaction starts in the core of the steller object. With a mass only 93 times that of Jupiter (MJ), or .09 M☉, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 MJ.

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How are stars formed?

Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds or nebulae. Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have masses around 104 M☉(solar mass) in the form of gas and dust. When the pressure inside increases to a level that the heat reaches to the point where fusion reaction takes place. This temperature is 1.5 x 107 °C (15 million degrees elsius). This approximate temperature is the core temperature of our sun.

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What are stars composed of?

Stars are made of very hot gas. This gas is mostly hydrogen and helium, which are the two lightest elements. Stars shine by burning hydrogen into helium in their cores, and later in their lives create heavier elements.
Theses heavier elements are formed in the core of the star and also some heavy metals during supernovae.

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Death of a star

There are two possibilities of a death of a star. A star of mass below the Chandrasekhar limitChandrasekhar limit. The Chandrasekhar limit is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about 1.4M☉ (2.765x1030 kg). will end being a white dwarf with no explosion and will form a planetery nebulae around it. If the star is above the Chandrasekhar limit then it will go under supernovaeA supernova is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. and will either form a neutron star or a black hole if the mass is even more.

Types of stars

The classification of stars

Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M.
O and B stars are uncommon but very bright; M stars are common but dim.

Hertzsprung - Russell Diagram

The Hertzsprung-Russell diagram, abbreviated as H-R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures. In a nutshell, this diagram compares the suraface temperature of the star to the luminosity level. Mostly, 3 types of it is considered which are more common to see.

On the basis electromagnetic energy release or spectrum classes

Star type Colour Approx. surface temperature Average mass (in M☉) Average radius (in R☉) Average luminosity (in L☉) Main characterstics Examples
O Blue ≥ 30,000 K ≥ 16 ≥ 6.6 ≥ 30,000 Singly ionized helium lines (H I) either in emission or absorption. Strong UV continuum. O7V - S Monocerotis, O9V - 10 Lacertae
B Blue-white 10,000 - 30,000 K 2.1 - 1.6 1.8 - 6.6 25 - 30,000 Neutral helium lines (H II) in absorption. B0V - Upsilon Orionis
B0Ia - Alnilam
A White 7,500 - 10,000 K 1.4 - 2.1 1.4 - 1.8 5 - 25 Hydrogen (H) lines strongest for A0 stars, decreasing for other A's. A0Van - Gamma Ursae Majoris
A0Va - Vega
F Yellow white 6,000 - 7,500 K 1.04 - 1.4 1.15 - 1.4 1.5 - 5 Ca II absorption. Metallic lines become noticeable. F0IIIa - Zeta Leonis
F0Ib - Alpha Leporis
G Yellow 5,200 - 6,000 K 0.8 - 1.04 0.96 - 1.15 0.6 - 1.5 Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium). G0V - Beta Canum Venaticorum
G0IV - Eta Boötis
K Light orange 3,700 - 5,200 K 0.45 - 0.8 0.7 - 0.96 0.08 - 0.6 Metallic lines, some blue continuum. K3III - Rho Boötis
K5V - 61 Cygni A
M Orange red 2,400 - 3,700 K 0.08 - 0.45 ≤ 0.7 ≤ 0.08 Some molecular bands of titanium oxide. M0IIIa - Beta Andromedae
M1-M2Ia-Iab - Betelgeuse
Hope it helps. This information isn't less of course, but are you confused by the alphabetical digits I, II, and many other? Well, these are Yerkes luminosity classes. What is this? Well here is a defination paragraph from Wikipedia about it.

Yerkes spectral classification

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory. This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity, which is related to luminosity (whilst the Harvard classification is based on just surface temperature). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the Morgan-Keenan classification, or MK, and this system remains in use.

Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.

A number of different luminosity classes are distinguished, as listed in the table below.

Yerkes luminosity classes
Luminosity class Description Examples
0 or Ia+ hypergiants or extremely luminous supergiants Cygnus OB2#12 - B3-4Ia+
Ia luminous supergiants Eta Canis Majoris - B5Ia
Iab intermediate-size luminous supergiants Gamma Cygni - F8Iab
Ib less luminous supergiants Zeta Persei - B1Ib
II bright giants Beta Leporis - G0II
III normal giants Arcturus - K0III
IV subgiants Gamma Cassiopeiae - B0.5IVpe
V main-sequence stars (dwarfs) Achernar - B6Vep
sd (prefix) or VI subdwarfs HD 149382 - sdB5 or B5VI
D (prefix) or VII white dwarfs van Maanen 2 - DZ8

Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used:

  • A slash (/) means that a star is either one class or the other.
  • A dash (-) means that the star is in between the two classes.

For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant.

Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than the main sequence).

Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs.

Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly more luminous than typical may be given a luminosity class of IIIb.

A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the Vz designation. An example star is HD 93129 B.

Brief information we should know about stars :-

Nearest star to Sun:  Alpha CentauriThe closest star to Earth are three stars in the Alpha Centauri system. The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are an average of 4.3 light-years from Earth. The third star is Proxima Centauri.

Brightest star after Sun:  Sirius ASirius, also known as the Dog Star or Sirius A, is the brightest star in Earth's night sky. The name means "glowing" in Greek - a fitting description, as only a few planets, the full moon and the International Space Station outshine this star.

Largest star:  VY Canis MajorisVY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. With a size of 2600 solar radii, it is the largest known star and also one of the most luminous known. It is located about 1.5 kiloparsecs (4.6x1016 km) or about 4,900 light years away from Earth.

Most massive star:  R136a1R136a1. The star R136a1 currently holds the record as the most massive star known to exist in the universe. It's more than 265 times the mass of our Sun.

Oldest star still found:  Methuselah starThe star designated at HD 140283 is currently the oldest known star in the galaxy. HD 140283 is so old that it has been dubbed the Methuselah star by the media.

Fastest spinning star:  VFTS 102The star, rotates at a dizzying 1 million miles per hour, or 100 times faster than our sun. It lies in a neighboring dwarf galaxy, about 160,000 light-years from Earth. Though this speed is not actual. In a star rotating speed depends on the place where you are measuring it.

Hottest star:  WR 102WR 102 is a Wolf-Rayet star in the constellation Sagittarius, an extremely rare star on the WO oxygen sequence. It is a luminous and very hot star, highly evolved and close to exploding as a supernova. Its surface temperature is mind boggling 210,000 K.

About our star, Sun

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth.It is a yellow dwarf star.
Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest particles of debris in its orbit. Electric currents in the Sun generate a magnetic field that is carried out through the solar system by the solar wind-a stream of electrically charged gas blowing outward from the Sun in all directions.
The connection and interactions between the Sun and Earth drive the seasons, ocean currents, weather, climate, radiation belts and aurorae. Though it is special to us, there are billions of stars like our Sun scattered across the Milky Way galaxy.

  • EQUATORIAL INCLINATION:  7.25 with respect to the ecliptic
  • EQUATORIAL RADIUS:  6,95,508km
  • EQUATORIAL CIRCUMFERENCE:  43,70,005.6km
  • VOLUME:  1.40927256905986 x 1018 km3
  • DENSITY:  1.409g cm-3
  • MASS:  1.9891 x 1030 kg
  • SURFACE AREA:  6.078747774547 x 1012 km2
  • SURFACE GRAVITY:  274 m s-2
  • ESCAPE VELOCITY:  2,223,720 km/h
  • SURFACE TEMPERATURE:  5,778 K