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Marcin Legeżyński The mystery of the size of the universe.

While looking at the mysterious night sky, such as the one in the picture above, have you ever wondered how many billions of stars there are, or what lies beyond what we are able to see from our planet? Most of the celestial bodiescelestial bodies [celestial body]celestial bodies you can observe from Earth are part of our galaxygalaxygalaxy called the Milky WayMilky WayMilky Way. The name related to the fact that it appeared to ancient observers to be a milky band of lightband of lightband of light – like a cosmic roadway – stretching across the dark sky.

Our Milky Way spiral galaxyspiral galaxyspiral galaxy is just one of the billions of galaxies in the universeuniverseuniverse. Within it, there are at least 100 billion stars, and on average, each star has at least one planet orbitingorbitingorbiting it. This means there are potentially thousands of planetary systemsplanetary systems [planetary system]planetary systems like our solar systemsolar systemsolar system within the galaxy! Our Sun is one of at least 100 billion stars in the Milky Way, which measures about 100,000 light years across.

And where are we in the Milky Way? Our Sun lies near a small, partial armpartial armpartial arm called the Orion Arm, or Orion Spur, located between the SagittariusSagittariusSagittarius and PerseusPerseusPerseus arms. The stars of our galaxy are arranged in a pinwheel patternpinwheel patternpinwheel pattern, with four major arms. We live in one of them, about two‑thirds of the way outwardoutwardoutward from the centre. Most of the stars in our galaxy are thought to hosthost (verb) host their own families of planetsfamilies of planets [family of planets]families of planets. Thousands of these planets have been discovered so far, with thousands more candidates being detected all the time and awaiting confirmationawaiting confirmationawaiting confirmation. Many of these newly discovered planetary systems are quite different from our own. All of the stars in the Milky Way orbit a supermassive black holesupermassive black holesupermassive black hole at the galaxy's centre. Sagittarius A*, as the black hole was named, is estimated to be four million times as massive as our Sun. Fortunately, it is a safe distance from Earth, at around 28,000 light years away.

Clearly, the universe is a vast expanse of spacevast expanse of spacevast expanse of space that contains absolutely everything in existence: all of the galaxies, stars, and planets. Scientists calculate that there are at least 100 billion galaxies in the observable universeobservable universeobservable universe, each one full of stars. On a very large scale, they form a bubbly structurebubbly structurebubbly structure. And from what we know, galaxies are not distributed randomlydistributed randomly [distribute randomly]distributed randomly throughout the Universe. Instead, they are clustered togetherclustered together [cluster together]clustered together in a very particular and intricate way.

All in all, the universe seems to be hiding its secrets well - the exact size of it being the biggest mystery. However, scientists believe that the universe is still expanding outwardexpanding outward [expand outward]expanding outward. Will we ever be able to uncover all mysteries held by the universe? Considering the size of it, and the unimaginable number of stars and their planets that it contains, it is unlikely that we may ever learn everything about it.

1 Źródło: Marcin Legeżyński, licencja: CC BY-SA 3.0.