A Trip Through The Universe (Episode 8)

by Carson
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Being in the interstellar neighbourhood, there are a lot of bright objects in the sky. These are stars. Today, we’ll talk about them.

Nuclear Fusion Powers Stars

Stars are ‘nuclear bombs’ instead of a hot fireball, balancing itself by gravity and the force pulling it out by nuclear fusion. Stars are powered by nuclear fusion, which is the process of lighter elements creating heavier ones by the temperature and pressure inside their cores.

On the main-sequence, where the majority of a star’s life is past, it fuses hydrogen to helium. Although high-mass stars have more ‘fuel’, they use all of them up quickly because of the immense pressure inside the core compared to low-mass stars.

Giant Stars

If all their hydrogen fuel uses up, it depends on the star type. For stars having less than a quarter of a solar mass, it collapses into a helium white dwarf. For higher mass stars, it becomes a giant that continues to fuse helium and heavier elements.

Because of the pressure inside the core which is contracting, a giant star can swell for tens or even hundreds of times in diameter. The increasing of heat cannot catch up with the increasing of size, which cools them down. So a star like a yellow dwarf will become a red giant, which is much cooler but carries more heat.

If all their helium fuel uses up, it also depends on the star type. For supergiant stars, that means giant stars having more than 8 solar masses, it will continue to fuse carbon. For stars having less than 8 solar masses, when carbon builds up, it will shrink and become a white dwarf because no more nuclear fusion can keep the balance.

Supernovas

Eventually, supergiant stars will finally fuse silicon to iron. When iron builds up, it cannot fuse anymore. The star collapses and exploded. A supernova explosion is created. When it happens, heavier elements are created to make us alive. But a nearby supernova is fatal because of the gamma rays and X-rays it created. Supernova explosions are so bright that they can outshine an entire galaxy.

Sometimes supernova explosions can be created by a white dwarf when its companion feeds on it and it reaches a mass of 1.4 solar masses.

Spectral Types

How can astronomers determine a star’s temperature? By its spectral type. From the hottest to the coolest spectral types are O-B-A-F-G-K-M-L-T-Y. Sun has a spectral type of G2V while Proxima Centauri is an M5.5V star. The letter after the number depends on its luminosity class. Ia+ represents a hypergiant, Ia and Ib represent a supergiant, II represents bright giants, III represents giants, IV represents subgiants, V represents main-sequence stars and VI represents subdwarfs.

For example, if a star is white or green and it is a main-sequence star, its spectral type is A?V, just like Sirius A for A1V. If a star is red and it is a giant, its spectral type is M?III, like Mira or Omicron Ceti for M7III.

Image Source: wikipedia.org

Star’s formation

We’ve already talked about the death of stars, now we’ll talk about the birth of them.

Let’s take the Solar System for example. 4.6 billion years ago, the Solar System is a nebula, filled with collapsed lumps of gas. The most massive one became a star, which is the Sun. The other four formed Jupiter, Saturn, Uranus and Neptune.

At that time, some microscopic solid particles made by heavier elements such as carbon, silicon and iron, collided with each other and formed protoplanets. They continue to collide into larger bodies. Those failed to become circular are called asteroids. Others are dwarf planets and terrestrial planets.

Still, the Solar System hasn’t fully formed, or mature yet. A lot of planets and protoplanets overlapped their orbits and collided with each other since they aren’t dwarf planets, just like the theoretical historical planet Theia which formed Moon. Comets and asteroids also collided with Earth to provide it a lot of metal and water that are necessary for life.

Finally, the Solar System ended up with 1 star, 8 or 9 planets, hundreds of dwarf planets, millions of comets and billions of asteroids. They are still colliding with each other but not as frequent as at the beginning of the formation. In fact, Jupiter and Saturn are suffering from tens to hundreds of asteroids collisions every day. Earth is also bombarded by asteroids daily but Earth prevented it by its atmosphere.

Talking about dense lumps, they are called protostars. If it exceeds 80 Jupiter masses, it becomes a star. If its mass is between 13-80 Jupiter masses, it becomes a brown dwarf where deuterium can fuse but hydrogen can’t. Others become Jovian or Neptunian planets.

Star Systems

Some dense lumps form stars with companions, stars that are gravitationally connected to each other. Some are called binaries where 2 stars are involved and even more. Sirius and Procyon are 2 examples of binaries. Sometimes more are involved up to 7. Castor (Alpha Geminorum) is a sextenary, which is a 6-star system.

Sometimes binaries can have very tight orbits that they have contact. These are called contact binaries. VFTS 352 in Large Magellanic Cloud is a contact binary, composed of two massive, hot stars.

Star Highlights

R136a1 in Large Magellanic Cloud is the most luminous and most massive star known.
There is a gigantic nebula called Tarantula Nebula, which consists of R136a1 and a lot more massive stars.
Red Dwarfs account for an unusually high 75% of all stars in the Universe.
There are at least one planet for almost every star or star system but currently, we have only found thousands.
HD 140283 is a star ‘older’ than the Universe. It is impossible so the data on either side is wrong.
Some hot, massive stars can develop a spectral type of Wolf-Rayet Stars, which R136a1 is an example.
Some cool giant stars can be an S-type star, which S Cassiopeiae is the coolest example, just 1,800 K.

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