Star Formation
Stars form when hydrogen gases in nebulas are slowly pulled together by their own gravity. Eventually, they become so condensed that the hydrogen atoms start fusing into helium atoms and the star is born. Stars that to not gain quite enough mass (generally a mass lower than 176 quintillion megatons or 160,000,000 yottagrams) to start fusing hydrogen become brown dwarfs. However, brown dwarfs are able to fuse deuterium at some point in their lives; stellar objects that cannot even fuse deuterium are classified as sub-brown dwarfs.
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Low-Mass Stars
Red dwarfs, which are small stars with a mass of around 2.19 sextillion megatons (199 million yottagrams), are the longest-lasting stars. Scientists believe that they will slowly fade away over a course of trillions of years. Once their hydrogen supply gets very low, they will become a blue dwarf, and will eventually become a white dwarf, which is only about as large as Earth, despite being incredibly dense. White dwarfs usually have a density of about 1,000,000 g/cm³. Despite this being an incredibly high density, it is very low compared to the density of a neutron star. At this point, the star will be made almost completely out of helium. Over the course of around a quadrillion years, the white dwarf will slowly cool down and become a black dwarf. Because the universe is not old enough, there are not any blue or black dwarfs yet.
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Medium-Sized Stars
Main-sequence stars are stars with a mass of between 1.31 sextillion megatons to 21.9 sextillion megatons (1.193 billion yottagrams to 19.89 billion yottagrams), such as the Sun. These stars usually have a surface temperature of between 9,440 and 10,340 °F (5,227 to 5,727 °C). These stars usually last about 10 billion years until running out of hydrogen in its core to fuse. After this, the star will form a shell of hydrogen around its core that will be fusing into helium. After fusing more and more hydrogen, the outer layers of the star will float off and become a planetary nebula, leaving a white dwarf the size of Earth. Inside of these nebulas, more stars will form and process will start all over again.
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Massive Stars
Stars with a mass of over 21.9 sextillion megatons (19.89 billion yottagrams) are considered massive stars. After running out of hydrogen to fuse into helium, the star will begin fusing helium into heavier elements such as carbon, oxygen, and neon to stop itself from exploding. After reaching a temperature of 1,979,999,540 °F (1,099,999,727 °C), the neon in the core will collapse and start being fused into even heavier elements, including sulfur and magnesium, and finally, silicon. Over the course of about 24 hours, the silicon will be fused into an iron core. Once the star attempts to fuse the iron, it will collapse and explode in a supernova, releasing massive amounts of neutrinos.
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Black Holes
When a massive star explodes in a supernova, there are two things that its core may become. If its core has a mass of over 5 sextillion megatons (4.54 billion yottagrams), it will collapse to a size smaller than an atom and become a black hole. Black holes are a type of singularity: an object that is infinitely small but also infinitely dense. The black area around the singularity is the event horizon; at this point, gravity is so strong that even light cannot escape, which is why it is black. Black holes are classified into different categories based off of their size: Theoretical micro black holes are black holes with a mass lower than the Moon, and their event horizon would only have a radius of about 0.003 inches (0.1 mm). Stellar black holes have masses of up to 10 times the Sun's mass. Intermediate-mass black holes have masses of up to 1,000 times the Sun's mass, and supermassive black holes have masses up to 10 billion times the Sun's mass. Any black holes that are more massive, such as TON 618, are classified as ultramassive black holes.
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Neutron Stars
If the star's core has a mass lower than 5 sextillion megatons (4.54 billion yottagrams) when it explodes, it will become a neutron star. These stars are very small, being less than 18 miles (32 km) in diameter. This is smaller than some cities. However, they are also more massive than the Sun. This is because they are incredibly dense; a single teaspoon (5 mL) of a neutron star's material would have a mass of over 6,063 megatons (5.5 petagrams). At their cores, their density is up to a thousand times higher than this. Neutron stars also have extremely strong magnetic fields, and some neutron stars, known as pulsars, spin extremely fast (sometimes hundreds of times per second).
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Star Classification
Stars are classified into seven spectral types based off of their temperature. These spectral types are not necessarily related to the size of the star, as Type M stars can be tiny red dwarfs or red supergiants, the largest stars in the universe. These spectral types are, from hottest to coolest, O, B, A, F, G, K, and M. Type O stars are over 30,000 K, making them the hottest stars in the universe. Main-sequence stars (not giants or dwarfs) in this spectral type are generally blue in color and over 30,000 times as bright as the Sun and at least 16 times as massive. Type O main-sequence stars are very rare, making up only about 0.00003% of all main-sequence stars. Type B stars are between 10,000 and 30,000 K in temperature and appear pale blue. Type B main-sequence stars are less than 16 but more than 2 times as massive as the Sun, and are between 25 (not 25,000) and 30,000 times as bright. Around 0.13% of main-sequence stars are Type B. Type A stars have temperatures of between 7,500 and 10,000 K and are very pale blue. Type A main-sequence stars are 1.4–2.1 times as massive as the Sun and 5–25 times as luminous. Approximately 0.6% of main-sequence stars are Type A. Type F stars are white in color and account for 3% of main-sequence stars. Type F main-sequence stars are 1.5–5 times as luminous as the Sun and are 1.04–1.4 times as massive. Type G stars, such as the Sun, are yellow and include 7.6% of main-sequence stars. These stars have temperatures between 5,200 and 6,000 K and range from 0.8–1.04 solar masses and 0.6–1.5 solar luminosities. Type K stars, which appear orange and are usually 0.08–0.6 times as bright as the Sun, make up 12.1% of main-sequence stars. Type K main-sequence stars have temperatures between 3,700 and 5,200 K and masses between 0.45 and 0.8 solar masses. The coolest and most common stars are Type M, which account for 76.45% of main-sequence stars, appear red, have temperatures below 3,700 K. Type M main-sequence stars, such as Proxima Centauri, are between 0.08 and 0.45 solar masses and under 0.08 solar luminosities.
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Special Spectral Types
In addition to types O–M, there are special spectral classes that do not appear on Hertzprung-Russell diagrams:
- Type C: Type C stars, also known as carbon stars, are old red giants that have fused most of their lighter elements and now contain very high amounts of carbon but low amounts of oxygen.
- Type W: Type W stars, also known as Wolf-Rayet stars, are old stars which have fused 100% of their hydrogen and are now completely composed of heavier elements such as helium. These stars are generally very hot.
- Type L: Type L stars are stars that are even cooler than Type M stars. Some of these stars are large enough to fuse hydrogen, making them true stars, but others are brown dwarfs. They appear as very dark red.
- Type T: Type T stars, also known as methane dwarfs, are very cool brown dwarfs with temperatures between 550 and 1,300 K and contain methane.
- Type Y: Type Y stars are even cooler than Type T stars, with temperatures often below 400 K. Their mass is between 9 and 25 times the mass of Jupiter, meaning that some Type Y stars can fuse deuterium.
- Type D: Type D stars (white dwarfs) are very small stars that are similar in size to Earth but incredibly dense and hot.