Space and the Origin of the Universe (Junior Cert Science): Revision Notes

Space and the Origin of the Universe

Introduction to astronomy

Astronomy is the branch of science that explores the universe beyond Earth's atmosphere. When we look up at the night sky, we're observing what astronomers study every day.

The universe encompasses everything that exists in space. This includes all naturally occurring objects beyond Earth's atmosphere, from tiny specks of dust to enormous galaxies spanning billions of light-years.

A celestial body is any natural object found outside Earth's atmosphere. These objects include stars, planets, moons, asteroids, and comets.

Types of celestial bodies

Understanding the different types of celestial bodies helps us make sense of what we observe in space. There are five main categories you need to know.

Stars

A star is a celestial object that produces its own light and heat. Stars are massive spheres of gas held together by their own gravity. Deep within a star's core, nuclear fusion reactions occur, converting hydrogen into helium. This process releases enormous amounts of energy, which we see as light and feel as heat.

The Sun is the star at the centre of our solar system and the brightest object in Earth's sky. It's the most important source of energy for life on Earth. Through nuclear fusion, hydrogen atoms in the Sun's core are constantly being converted into helium. The Sun is so bright that objects visible in the night sky are actually distant stars. Proxima Centauri, the nearest star to Earth after the Sun, is located $4.2$ light-years away. This means light takes $4.2$ years to travel from that star to Earth.

Planets

A planet is a celestial body that orbits around a star. For an object to be classified as a planet, it must meet three specific criteria:

• It must be in orbit around a star
• It must have enough mass to form an almost round shape due to its own gravity
• It must have cleared the region around its orbit of other celestial bodies

You might sometimes see a planet in the night sky because planets reflect light from the Sun.

Moons (natural satellites)

A moon is a natural satellite of a planet. The term satellite refers to any object that orbits a celestial body of larger mass while revolving around it.

We're all familiar with Earth's Moon, the only natural satellite of our planet. However, other planets have their own moons too. Jupiter holds the record with more than $50$ moons, whilst Saturn has more than $20$ moons. Uranus has more than $10$ moons, and Neptune has more than $5$ moons. These numbers continue to change as astronomers carry out further studies and discover additional moons.

Asteroids

An asteroid is a small rocky body in space. Most asteroids are made of rock that was left over after the planets formed about $4.5$ billion years ago. Their diameters range from a few metres to a few hundred kilometres.

Sometimes asteroids can be knocked out of their orbit due to a collision with another asteroid or a planet. If they travel through space and come close to another celestial body, such as a planet, that planet's gravity can pull them towards it, potentially causing a high-energy collision.

Comets

A comet is a dusty snowball made of ice and dust that orbits a star. Comets orbit the Sun, but unlike planets, their orbital path isn't circular. They spend most of their time in the outer regions of the solar system, far from the Sun. When comets come close to the Sun, some of the ice and dust particles are released, creating a tail pointing away from the Sun. This tail makes comets visible in the night sky.

The most famous comet is Halley's Comet, which has orbited around the Sun for thousands of years. It revolves around the Sun every $76$ years. You may have seen the comet Neowise, which was very clear in the night sky in July 2020.

Our solar system

Let's first discuss what we mean by the term "solar system."

A solar system is a group of planets, asteroids, comets, and other celestial bodies that orbit around a star. Our solar system is only one of many millions of planetary systems in the universe. The size of any solar system is limited by the distance at which the gravity of the star can attract other celestial bodies.

Our solar system consists of the Sun, planets, moons, comets, asteroids, and other celestial bodies, all under the influence of the Sun's gravity. Gravity is often referred to as the "glue" of the universe because it holds everything together.

The Sun is a huge celestial body – a star at the centre of a system of eight planets. Our Sun contains $99.9$ per cent of the mass of the solar system. As we've already mentioned, it generates its own heat and light through nuclear reactions. Due to its huge mass, the gravity of the Sun attracts all other objects in the solar system.

The planets revolve around the Sun in a nearly circular pathway called an orbit. The force of gravity between each planet and the Sun helps the planets remain in orbit.

Due to the fact that the planets are orbiting around the Sun, astronomers have discovered a large number of rocky objects between Mars and Jupiter. These are asteroids. Most of the asteroids in the solar system are clustered between Mars and Jupiter. This region is known as the asteroid belt. However, some wander out of this region and are in danger of striking other planets.

At the edges of the solar system, there exists a region of gas clouds called the Kuiper belt. It contains occasional lumps of rocky ice up to $8$ km in diameter in orbit around the Sun. They are called comets. Occasionally, the orbits of comets around the Sun may be disturbed. They may begin to move towards the Sun and may orbit the Sun in a different orbit to the Kuiper belt. The Sun melts some of the ice and a tail of gas, ice, and dust is formed. The sunlight reflects off the tail when we see a comet.

Galaxies and space

A galaxy is a large system of stars held together by their gravity.

Our solar system is part of the Milky Way Galaxy. Galaxies are isolated from similar systems by vast regions of empty space. Our Sun is just one of $100$ billion stars in the Milky Way Galaxy. Many of these stars have their own planetary systems. Our nearest neighbouring large galaxy is called Andromeda.

Outer space, or simply space, is the empty volume that exists between celestial bodies. It is not completely empty but consists of a vacuum containing mostly hydrogen and helium, as well as radiation.

Historical models of the universe

For centuries, people have wondered how the Sun and planets are organised. Were we right about the way we've come to understand it? Several historical models were proposed.

One such model was put forward by Ptolemy, an Egyptian astronomer, about $2,000$ years ago. His model is called the geocentric model of the universe. In this model, he proposed that the Earth was at the centre of the universe and that the Sun, Moon, and planets were in orbit around Earth.

For $1,300$ years, this model was accepted as an explanation for how the universe worked.

However, in 1543, a Polish priest and astronomer named Nicolaus Copernicus proposed an alternative model of the universe. He called it the heliocentric model (the word "Helios" comes from the Greek word for Sun). In this model, the Sun is at the centre of the solar system and the planets are in orbit around it. This model was supported by the work of Greek mathematicians who came before him.

He knew that his findings would go against the teachings of the Catholic Church, and only published his theory just before he died. The Church resisted the findings of Copernicus. It was not until almost $100$ years later that Galileo Galilei read the works of Copernicus and sought to prove Copernicus's theory.

Gravity – the glue of the universe

Let's revise what we learned about gravity in an earlier chapter.

If any two masses are placed near each other, they will be attracted to each other by a force of gravity. If one of the masses is very large, for example, a moon, a planet, or a star, the force of gravity is much larger. In essence, gravity is the force of attraction for large masses in the universe. The smaller the distance between two masses, the greater the force of attraction.

Comparing the Earth to other celestial bodies

For your examination, you will be expected to be able to interpret data to compare Earth with other planets and moons in the solar system with respect to properties including mass, gravity, size, and composition. You do not have to memorise any data on the planets or moons, as it will be presented to you in the exam as a table.

The gravity on the surface of a celestial body (surface gravity) can be calculated using information about the body's mass and radius.

In tables comparing celestial bodies, the gravity on other celestial bodies is compared to the surface gravity on the Earth. This is called relative surface gravity (relative means "compared to"). In this case, we are comparing gravity to that on Earth.

If you compare the relative mass to the surface gravity, you should notice that the greater the mass of the planet, the greater the surface gravity.

The composition of all the planets (what they are made of) is not the same. The first four planets from the Sun (Mercury, Venus, Earth, and Mars) are mostly made of rock. The next two (Jupiter and Saturn), called the gas giants, are mostly made of the gases hydrogen and helium. Finally, the last two (Neptune and Uranus) are known as the ice giants.

Here is a table comparing some of the largest objects in our solar system, including the Sun, the planets, some dwarf planets, and some moons:

Object	Type of object	Relative mass	Relative radius	Surface gravity
Sun	Star	$333,000$	$109$	$28.0$
Jupiter	Planet	$318$	$11.0$	$2.53$
Saturn	Planet	$95.2$	$9.14$	$1.06$
Uranus	Planet	$14.5$	$3.98$	$0.90$
Neptune	Planet	$17.1$	$3.86$	$1.14$
Earth	Planet	$1$	$1$	$1$
Venus	Planet	$0.815$	$0.950$	$0.905$
Mars	Planet	$0.107$	$0.532$	$0.38$
Ganymede	Moon of Jupiter	$0.0248$	$0.413$	$0.15$
Titan	Moon of Saturn	$0.0225$	$0.404$	$0.14$
Mercury	Planet	$0.0553$	$0.383$	$0.38$
Callisto	Moon of Jupiter	$0.0180$	$0.378$	$0.126$
Io	Moon of Jupiter	$0.0150$	$0.286$	$0.183$
Moon	Moon of Earth	$0.0123$	$0.273$	$0.166$

The size of the planets of our solar system varies greatly from planet to planet. This is shown in the table by the relative radius.

In the table, the mass of the celestial bodies is compared to Earth. A relative mass of $0.5$ means a body has half the mass of Earth.

The Big Bang theory and the formation of the universe

The branch of astronomy that deals with the formation of the universe is called cosmology.

Astronomers believe that the universe began about $14$ billion years ago in a Big Bang. At that time, the entire universe was inside a bubble that was much smaller than the size of a pinhead. This was extremely hot and dense, and suddenly exploded. The universe began to be formed. Time, space, and matter all began with the Big Bang.

In a fraction of a second, the universe grew from the size of a single atom to bigger than the size of a galaxy. It kept on expanding at an ever-decreasing rate.

Edwin Hubble ($1889$–$1953$) was one of the leading cosmologists of the twentieth century. He discovered that there are more galaxies outside of our own galaxy, the Milky Way. He also discovered that the further away a galaxy is, the faster it is moving away from Earth. This was later used as evidence for his theory. In 1990, NASA placed a very powerful telescope in space, which has made many new discoveries. It is named the Hubble Space Telescope in his honour.

Evidence for the Big Bang

Evidence for the Big Bang comes from the following observations:

Remember!