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Key Facts & Information

  • Stars are astronomical objects that consist of a luminous spheroid of plasma held together by its own gravity. Stars are huge celestial bodies mostly made of hydrogen and helium which produce light and heat from the churning nuclear forces in their cores.
  • The Sun is the nearest star to the Earth. Stars appear as dots of light in the night sky as they are light years away from us and most stars are invisible to the naked eye.
  • We can see around 2,000-2,500 stars without the use of the telescope and no green stars are perceived by the human eye.
  • The life cycle of stars depends on their initial mass.
  • Observations concluded that stars with high mass usually have shorter life spans and last for billions of years in general.
  • The majority of observed stars are red dwarf stars.
  • Stars are classified according to their spectra and temperature. The Morgan-Keenan (MK) system classifies the stars in decreasing order, namely, O, B, A, F, G, K, and M.

Observation History

  • Stars have been observed, dated and analysed for long as man could gaze into the night sky.
  • Ancient astronomers grouped the stars into constellations and used them to track planets and the inferred position of the Sun.
  • Stars play an important role in celestial navigations and religious practices.
  • Medieval Islamic astronomers gave Arabic names to a lot of stars that are still used today. They were the first to build large observatory research institutes.
  • 1543 BC – One of the oldest star charts appeared in Ancient Egyptian astronomy.
  • 185 AD – Chinese recorded a supernova which is now classified as SN 185.
  • 1838 – First direct measurement of 61 Cygni, a distant star, using the parallax techniques
  • 1913 – the Hertzsprung-Russell diagram was developed
  • 1921 – First measurements of a stellar diameter through the use of an interferometer.
  • 1925 – Idea that stars were primarily made of hydrogen and helium.

Star Formation

  • Stars are usually formed in nebulae which are huge hydrogen-based clouds of gas and dust.
  • Gravity causes these clouds to contract, result in the gases drawing closer.
  • Density and pressure increase as these materials accumulate in the centre.
  • This causes the matter to heat up and glow as its mass increases.
  • Temperature and pressure continue to rise until the hydrogen can be fused.
  • The heat from the nuclear fusion causes the gas to expand and a star is born when the hydrostatic equilibrium is reached.
  • Most stars form in groups known as star clusters but many stars are ejected from these.

Types Of Stars

  • A lot of star classification systems are developed today.
  • Among these, the Morgan-Keenan system is the easiest to understand.
  • Stars are classified using the letters O, B, A, F, G, K, and M with O being the hottest and M the coolest. 
  • The temperature of each spectral class is then subdivided by the addition of a number, 0-9, from hottest to coolest.
  • Blue Stars
    • The spectral types of blue stars are either O or B.
    • Their temperatures are around 30,000 Kelvin and are 100 to 1 million times more luminous than the Sun. They have a mass around 2.5 to 90 times that of the Sun and would last for about 40 million years.
    • Blue stars are usually found in the arms of the spiral galaxies and are characterised by the strong Helium-II absorption lines in their spectra. They have weaker hydrogen and neutral helium than B-type stars.
    • Their mass and temperature cause them to have short lifespans that end in a supernova explosion resulting in black holes or neutron stars. One example is Delta Circini.
  • Yellow Dwarfs
    • Yellow dwarfs have around 10% prevalence, with a spectral type G.
    • They have temperatures between 5,200-7,500 Kelvin and are 0.6-5 times more luminous than the Sun.
    • They have a solar mass of around 0.8-1.4 and would last about 4 to 17 billion years.
    • These stars are usually mistakenly referred to as G-type stars, like our Sun, which convert hydrogen into helium and usually evolve into red giants when their hydrogen fuel is exhausted.
    • One example is Alpha Centauri A.
  • Orange Dwarfs
    • Orange dwarfs have a prevalence of around 10%, with a spectral type K.
    • They have temperatures between 3,700-5,200 Kelvin and are 0.08-0.6 more luminous than the Sun.
    • They have a solar mass of 0.45-0.8 and would last for about 15 to 30 billion years.
    • These stars emit less ultraviolet radiation and remain stable for long periods of time, thus, are favourable for exoplanets that might reside in their habitable zone.
    • One example is Alpha Centauri B.
  • Red Dwarfs
    • Red dwarfs have a prevalence of around 73%, with spectral types either K or M.
    • Their temperatures are usually around 4,000 Kelvin and are 0.0001-0.8 more luminous than the Sun.
    • They have a solar mass of 0.08-0.45 and would last for several trillion years.
    • Though they are very dim, they shoulder the bulk of the stellar population in the Milky Way.
    • They can convert hydrogen into helium throughout and in their core if they are more massive than 0.35 solar masses.
    • Due to this, the nuclear fusion process is slowed down and even prolonged.
    • One example is Proxima Centauri.
  • Blue Giants
    • Blue supergiants are also rare, with a spectral type OB. Their temperatures are around 10,000-50,000 Kelvin and are 10,000 to 1 million times more luminous than the Sun.
    • They have a solar mass of around 20-1,000 and would last for around 10 million years.
    • The OB supergiants have luminosity classifications of I, and spectral classifications of B9.
    • These stars are smaller than red supergiants and usually leave their main sequence in a few million years.
    • Some of the OB supergiants evolve directly into Wolf-Rayet stars, skipping the normal blue supergiant phase.
    • One example is UW Canis Majori.
  • Red Giants
    • Red giants have around 0.4% prevalence, with spectral types M and K.
    • They have temperatures of around 3,300-5,300 Kelvin and are 100 to 1,000 times more luminous than the Sun.
    • They have a solar mass of about 0.3-10 and would last for around 0.1 to 2 billion years.
    • Red giants are much smaller than the supergiant ones and much less massive.
    • In these stars, the RBG-branch is the most common, with hydrogen still being fused into helium.
    • The red-clump giants use the helium and fuse it into carbon, while the AGB branch burns helium in a shell around a degenerate core of carbon and oxygen.
    • One example is Aldebaran.
  • Red Supergiants
    • Red supergiants have a prevalence of around 0.0001% and spectral types K and M.
    • They have temperatures of around 3,500-4,500 Kelvin and are 1,000-800,000 times more luminous than our Sun.
    • They have a solar mass of about 10-40 and would last for around 3 to 100 million years.
    • These stars have already exhausted their supplies of hydrogen at their cores, resulting in the huge expansion of their outer layers as they evolve from the main sequence.
    • They are one of the biggest stars in the universe but not one of the most massive or luminous. Some of them still create heavy elements and eventually explode as type II supernovas.
    • One example is Betelgeuse.
  • White Dwarfs
    • White dwarfs have a prevalence of around 0.4%, with spectral type D.
    • They have temperatures of around 8,000-40,000 Kelvin and are 0.0001-100 times more luminous than our Sun.
    • They have a solar mass of about 0.1-1.4 and would last for around 100,000 to 10 billion years.
    • These stars no longer produce energy in order to counteract their mass and cannot exceed 1.4 solar mass, theoretically.
    • One example is Sirius B.
  • Neutron Stars
    • Neutron stars have around 0.7% prevalence, with spectral type D.
    • They have temperatures of around 600,000 Kelvin and very low luminosities.
    • They have a solar mass of about 1.4-3.2 and would last for around 100,000 to 10 billion years.
    • They consist of neutron particles which are a bit more massive than protons with no electrical charge.
    • These stars can further collapse into black holes with more than 3 solar masses, which only those with high spin rates can.
    • One example is PSR J0108-1431.
  • Black Dwarfs
    • Black dwarfs are more hypothetical and are theorised to be white dwarfs that radiated their leftover heat and light.
    • No black stars had enough time to form heat since white dwarfs have relatively high life spans.
  • Black Holes
    • Stars with high masses become black holes after a supernova explosion.
    • Remnants will continue to collapse into a gravitational singularity and eventually become a black hole since it has no outward pressure.
    • Such a strong object can trap light.
    • One example is Sagittarius A.
  • Brown Dwarfs
    • Brown stars have a prevalence of around 1% to 10%, with spectral type between M, L, T, and Y.
    • They have temperatures of around 300-2,800 Kelvin and very low luminosities.
    • They have a solar mass of about 0.01-0.08 and would last for trillions of years.
    • They usually fill the gap between the least massive stars and the most massive planets.
    • They have a mass of about 13-80 Jupiter masses and mostly don’t emit visible light.
    • One example is Gliese 229.

Did You Know?

  • The twinkle of stars is caused by the Earth’s turbulent atmosphere.
  • The Milky Way contains an estimated 300 billion stars.
  • A lot of stars come in pairs and orbit a common barycenter.
  • Main-sequence stars are powered by the fusion of hydrogen into helium inside their cores.
  • About 90% of the stars in the universe are main-sequence stars and are one-tenth to 200 times the mass of the Sun.
  • Most of the stars in our galaxy and in the universe are main-sequence stars. Our Sun, Sirius, and Alpha Centauri A are main-sequence stars.
  • Red dwarfs live for so long that none of them has reached an advanced stage of evolution since the universe was created.
  • A star begins to burn its helium when it runs out of hydrogen, transforming into either a giant or supergiant. Its core collapses and gets hotter, causing the outer layer to expand outwards.
  • Stars with low or medium mass evolve into red giants, while stars with high mass, around 10 times bigger than our Sun, evolve into red supergiants.
  • Dead stars no longer have fusion processes at their cores.
  • Black holes are so strong, not even light can escape from them.
  • Failed stars do not have sufficient mass to ignite and fuse hydrogen gas and shine. Brown dwarfs are typically known as failed stars.
  • Icarus, a blue supergiant, is the most distant individual star, 14 billion light years away.
  • R136a1, a Wolf-Rayet star, is the most massive and luminous star discovered with 315 solar masses and 8.7 million solar luminosity.
  • VY Canis, a red supergiant, is the largest star discovered with an estimation of around 17 solar masses.
  • HE 15230901, a red giant star, is the oldest star discovered in the Milky Way, about 13.2 billion years old.
  • Sirius was already seen in the night sky 8 years ago.
  • Jupiter could turn into a star if it were around 79 times more massive.