Celestial Objects

Earth is 1 of 8 planets that orbit around a small star we call the Sun. Although first proposed in 5th century BC, the idea that Earth was one of many planets orbiting a central point wasn’t accepted until the 17th century. 

Our Sun formed approximately 4.5 billion years ago. It’s classified as a yellow dwarf sun. A middle–aged celestial object, astronomers estimate that the Sun should remain stable for at least another 5 billion years. 

Over 150 known moons orbit the planets and a few small moons even orbit asteroids. The asteroid belt located between Mars and Jupiter is estimated to contain almost 2 million asteroids larger than half a mile in diameter.

Our Solar System has 8 objects that are classified as planets and 5 dwarf planets. The first 4 planets are rock–based, due to the fact that they were able to withstand the heat when the Solar System was young. 

The Oort Cloud, a cloud of mountain–sized ice chunks, completely surrounds our Solar System. It’s incredibly thick — at least 100,000 times wider than the distance between Earth and the Sun. Scientists believe many comets originated from the cloud. 

Our galaxy, the Milky Way, is nestled within a group of 30 or so galaxies called “The Local Group.” Over two-thirds of all galaxies discovered so far are spiral–shaped like ours. Researchers estimate that the Milky Way alone contains over 100 billion stars.

Our Solar System is located on the outer edges of one of the arms forming the Milky Way. All the stars orbit a supermassive black hole in the center of the galaxy, 28,000 thousand light years away from Earth.

Elliptical galaxies are shaped more or less like a sphere. These galaxies contain mostly older stars, and new stars rarely form within them because they don’t have very much star–making gas and dust clouds. 

Irregular galaxies don’t have very much structure, and don’t have the usual spiral or elliptical shape. These types of galaxies are generally small and contain predominantly dust and gases. Some areas of the galaxy may support star formation.

All galaxies are in motion, traveling across the universe. Smaller galaxies often lose their stars to larger galaxies and galaxies themselves collide with each other. The Milky Way and neighboring Andromeda galaxy are due to collide in the next 4 billion years.

Comets are frozen objects that orbit the Sun, usually in an elliptical pattern. They consist of pieces of solar system material that go all the way back to the formation of the Sun and the planets. Astronomers have discovered over 3,500 comets so far. 

Comets generally measure around 6 miles wide. They consist of frozen water as well as super cold methane, ammonia, and carbon dioxide ices. Those elements mix with rock, dust, and other metallic bits of the solar system.

As a comet approaches the Sun, particles begin to melt and stream away, forming a dust tail. Dust tails can be over 600,000 miles long. A plasma tail forms when a comet interacts with solar wind, but this tail isn’t visible to the naked eye.

Some astronomers believe that Earth’s water was deposited by comets impacting the planet. Some comets, also called sungrazers, have orbits that take them very close to the Sun. They can eventually crash into the Sun and be swallowed up by it.

Because comets orbit the Sun, we can predict when they will arrive. For hundreds of years, Chinese astronomers kept detailed notes of comets when they appeared in the sky and these notes are still used by astronomers today to predict various comets’ paths. 

Asteroids are rocky objects that orbit the Sun. Different from comets, they primarily consist of rock and metal, as opposed to gas and dust. Asteroids number in the millions, and they are often called minor planets or planetoids.

Approximately 1.5 million asteroids make up the asteroid belt located between Mars and Jupiter. While some asteroids are very large, most of them are about the size of pebbles. Gravity can influence asteroids and move them out of the belt.

Asteroids are too small to hold any gases. Many of the larger asteroids with a gravitational pull have 1 or 2 moons that orbit them. Asteroids of the same size can create a binary system and orbit around each other.

Ceres is the largest asteroid in the asteroid belt. Considered a dwarf planet, it has a diameter of 587 miles with a surface composed of rock, possibly iron-rich clay, and minerals. Researchers speculate that water lies under the surface.

Despite the high number of asteroids in the asteroid belt, collisions and near-passings are extremely rare. The average distance between 2 asteroids measures 600,000 miles – around 2.5 times the distance between Earth and the Moon.

Black holes are areas in space where the gravitational pull is so strong that nothing, not even light, can escape. The most common types of black holes are stellar-mass and supermassive. A stellar-mass black hole forms when a star explodes, and the supermassive black hole can be found in the heart of a galaxy.

Black holes are formed after a giant star explodes into a supernova. The mass of these huge stars becomes compressed into a very tiny space. These stellar-mass black holes have the mass of just a few suns.

The extremely strong gravitational pull of a black hole means that even light falls into it and cannot escape. The boundary of a black hole, where light can be seen falling in, is known as the event horizon.

Scientists don’t know what is actually inside a black hole. Our modern understanding of physics suggests that all matter gets crushed into a singularity, but what that looks like and how it behaves is still unknown.

If a person was to fall into a black hole, they would die long before the event horizon. Scientists calculate that going over the event horizon would cause the body to “spaghettify” upon entering the hole. Gravity would stretch and squeeze every atom.

Stars, massive balls of plasma drawn together by gravity, consist mostly of hydrogen. Our Sun is classified as a yellow dwarf star. Within the Milky Way’s trillions of stars, stars exist that are millions of times bigger than our Sun.

Suns go through a cycle. They form, they consume energy, and once that energy is depleted, they explode into a supernova and die. When stars die, their elements scatter into space and become seed materials for the next generation of stars and planets.

Supergiants like Betelguese are massive suns that burn through energy at astronomical rates. Their high consumption of energy makes them extremely luminous, and they become supernovas quickly — in the time span of millions of years instead of billions.

Astronomers calculate that in a few billion years our Sun will become a white dwarf sun. These suns, which are around the same size as Earth, have the same mass as our Sun. Only neutron stars and black holes are denser forms of matter.

Supernovas are the outward explosion of a dying sun’s materials. It’s thought that these explosions help spread the sun’s elements throughout the universe within clouds of gas and dust. These explosions only last a few years.