Probably each of us once wondered about what's up there in the sky? We have all observed the moon, the stars, and the sun, wondering what are they, how big are they, and what are they made of?... But not only the moon, or the stars, or the sun are there in the sky. There are many more objects, like planets, comets, asteroids, and meteors, some of them visible occasionally.
The Sun, the moon, the stars, the planets, and the other objects we mentioned before, together are called celestial objects. They can be also named celestial bodies or astronomical objects. In this lesson, we are going to learn more about these CELESTIAL OBJECTS. We are going to answer some basic questions about:
In astronomy, an astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms object and body are often used interchangeably.
Each natural object which is located outside of Earth's atmosphere is considered a celestial object. Such objects are moons, the Sun, asteroids, planets, comets, meteors, stars, etc. What does it mean when we say a natural object?
If you think about airplanes, they are objects that can be found outside of Earth's atmosphere. But, the difference is that they are man-made. So, they are not celestial objects. Planets, moons, sun, asteroids are not made by man. That means they are natural objects, and that is why they are considered celestial objects.
Now, let's take a closer look at each of the mentioned celestial objects.
Stars are luminous spheres made of plasma (a superheated gas threaded with a magnetic field). They are huge celestial bodies made mostly of hydrogen and helium. Stars produce light and heat from the churning nuclear forges inside their cores. The nearest star to our planet Earth is the Sun (yes, the Sun is actually a star!). Many other stars are visible to the naked eye at night, but they appear as fixed points of light in the sky. That is because of their immense distance from Earth. Stars get so hot by burning hydrogen into helium in a process called nuclear fusion. This is what makes them so hot and bright. As light from a star races through our atmosphere, it bounces and bumps through the different layers, bending the light before we see it. Since the hot and cold layers of air keep moving, the bending of the light changes too, which causes the star's appearance to twinkle or wobble.
There are an estimated one hundred billion (100,000,000,000) stars in our own Milky Way galaxy, although some estimates range up to four times that many.
Some of the stars have their names. The largest known star (in terms of mass and brightness) is called the Pistol Star. It is believed to be 100 times as massive as our Sun, and 10,000,000 times as bright.
Sirius, also known as the Dog Star or Sirius A, is the brightest star in Earth's night sky.
The Sun is a star that is located at the center of our Solar System. It is a yellow dwarf star. It gives off energy as light. The Earth and the other components of the Solar System revolve around it. It is the dominant body of the system. The Sun is like a hot ball of gases that gives off great amounts of energy. Almost every basic need of living organisms depends on the Suns' light and heat. All life on Earth depends on the Sun.
The Sun, like other stars, is a ball of gas. In terms of the number of atoms, it is made of 91.0% hydrogen and 8.9% helium. By mass, the Sun is about 70.6% hydrogen and 27.4% helium.
The visible part of the Sun is about 5,500 degrees Celsius (10,000 degrees Fahrenheit ), while temperatures in the core reach more than 15 million Celsius (27 million Fahrenheit), driven by nuclear reactions.
A planet is a celestial body that is in orbit around the Sun. Planets are smaller than stars, and they do not produce light. Planets are shaped like a spheroid, which looks like a slightly squashed ball.
Eight planets orbit the Sun. In order from the closest to the Sun, these planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The four planets closest to the Sun: Mercury, Venus, Earth, and Mars, are called the inner planets, or terrestrial planets.
The rest of the planets in our Solar system are called outer planets. These are the gas giants Jupiter and Saturn and ice giants Uranus and Neptune.
The Earth is the only planet known to support life.
All of the planets in our solar system orbit around the Sun. Planets that orbit around other stars are called exoplanets. Exoplanets are very hard to see directly with telescopes.
The Moon we see in the night sky is the Earth's only natural satellite (satellite means that orbits a planet or star). It is a large round object that circles the Earth and that shines at night by reflecting light from the Sun. The moon is a reflector of light, rather than a source, meaning it creates no light but redirects it from reflected sunlight.
But, this moon is not the only moon in our Solar System. There are many moons, actually more than 200 moons. Most of the planets have moons. Only Mercury and Venus don't have moons. Saturn and Jupiter have the most moons. Moons come in many shapes, sizes, and types.
Asteroids are rocky worlds revolving around the sun that are too small to be called planets. They are also known as planetoids or minor planets. There are millions of asteroids, ranging in size from a few meters to hundreds of kilometers. In total, the mass of all the asteroids is less than that of Earth's moon.
The asteroids are orbiting the Sun, each one traveling around the Sun, fast enough for the orbits not to degrade. If something slows an asteroid, it may "fall" towards the Sun, towards Mars, or Jupiter.
It is not uncommon for asteroids to hit even the Earth. Hundreds of meteorites reach the surface of our planet every year, most too small to be of any concern. But occasionally, large rocks can hit and cause damage.
Meteors are often referred to as shooting stars or falling stars. Simply put, a meteor is a meteoroid that has entered Earth's atmosphere. A meteoroid is a small body of matter usually composed of dust or rock that travels through outer space. A meteor that reaches Earth's surface is called a meteorite.
Most meteoroids are made of silicon and oxygen (minerals called silicates) and heavier metals like nickel and iron. Iron and nickel-iron meteoroids are massive and dense, while stony meteoroids are lighter and more fragile.
Comets are cosmic snowballs of frozen gases, rock, and dust that orbit the Sun. When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. Sometimes comets are referred to as “dirty snowballs” or “cosmic snowballs”.
Comets contain dust, ice, carbon dioxide, ammonia, methane, and more.
Most of the comets are never bright enough to be seen by the naked eye in the sky. They generally pass through the inner Solar System unseen by anyone except astronomers. On average, every five years, one can expect to see a major comet visible from the Earth.
Halley's Comet or Comet Halley is a short-period comet visible from Earth every 75–76 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and the only one that can appear twice in a human lifetime.
Another visible comet is the Comet Hale–Bopp. It is perhaps the most widely observed of the 20th century and one of the brightest seen for many decades.
