Module 1 / Lecture 1 : Introduction to the Universe

Summary

This lecture offers an insightful journey through cosmic scales, from Earth to the universe's expanse. It begins with the Hubble Telescope's view of distant galaxies, then delves into our solar system, highlighting Earth's position and the planets' arrangement. The script explains Pluto's demotion, introduces celestial bodies like asteroids and comets, and describes the Sun's role in our galaxy, the Milky Way. It further explores the universe's structure, from local groups to superclusters, and emphasizes the importance of light years and the observable universe's limits. The lecture concludes with the universe's history, the Big Bang, and a poetic reflection on our cosmic origins.

Takeaways

• 🌌 The universe is vast, with the Hubble Space Telescope capturing a small piece of the sky that contains about 10,000 galaxies, each with billions of stars.
• 🌍 Earth is the third planet in our solar system, averaging 93 million miles from the Sun, and it would take 152 years to travel this distance at highway speeds.
• 🔠 A mnemonic to remember the order of the planets is 'Mary's Velvet Eyes Make John Stay Up, Nights', but Pluto is no longer considered a planet due to not clearing its orbit.
• 🚀 The solar system is made up of the Sun, planets, their moons, and smaller objects like asteroids and comets, which are remnants or 'dirty snowballs' in space.
• 🌠 Comets have highly elliptical orbits, and when Earth passes through their paths, it results in meteor showers from the debris left behind.
• ☀️ The Sun is a typical star that formed 4.5 billion years ago and will continue to burn for another 5 billion years, being just one of hundreds of billions in the Milky Way.
• 🌌 The Milky Way is a disk-shaped galaxy with our solar system located about two-thirds of the way out from the center, making the band of the Milky Way visible as a result of our positioning within the galaxy.
• 🌌 The universe contains billions of galaxies that form groups and clusters, like the Local Group which includes the Milky Way and about 40 other galaxies.
• 📏 Astronomers use the astronomical unit (AU) for smaller distances and the light-year for larger scales, with a light-year being the distance light travels in a year.
• 💫 The finite speed of light means we see objects as they were in the past; for instance, the light from the Andromeda Galaxy left 2.5 million years ago, so we see it as it was then.
• 💥 The universe is expanding, with galaxies moving away from each other, implying an initial point of expansion known as the Big Bang about 14 billion years ago.
• ✨ Every atom on Earth, including the elements in our bodies, originated from stars, echoing Carl Sagan's sentiment that 'We are all stardust'.

Q & A

What is the significance of the Hubble Space Telescope image shown in the lecture?

The Hubble Space Telescope image, capturing a small piece of the sky, reveals approximately 10,000 galaxies, each containing billions of stars. This underscores the vastness of the universe and the immense number of galaxies, even within such a tiny field of view.

How far is Earth from the Sun on average, and what is the time it would take to travel this distance at highway speeds?

Earth is on average about 150 million kilometers (or 93 million miles) from the Sun. At highway speeds, it would take approximately 152 years to travel from Earth to the Sun.

What mnemonic is mentioned in the script to remember the order of the planets in our solar system?

The mnemonic provided in the script to remember the order of the planets is 'Mary's Velvet Eyes Make John Stay Up, Nights'.

Why is Pluto no longer considered a planet according to the International Astronomical Union's definition?

Pluto is no longer considered a planet because it fails to meet the third criterion of the International Astronomical Union's definition of a planet: it has not 'cleared the neighborhood' around its orbit due to its small size.

What are the main components of a comet, and how do they differ from asteroids?

Comets are made of rock, dust, water ice, and frozen gases like carbon dioxide, carbon monoxide, methane, and ammonia, essentially 'dirty snowballs'. Asteroids, on the other hand, are minor planets thought to be remnants of planetesimals that never grew large enough to become planets.

What causes a meteor shower, and how is it related to comets?

A meteor shower occurs when Earth passes through the orbital path of a comet, encountering the rocky debris left behind by the comet. These small comet pieces burn up in our atmosphere, creating the meteor shower.

How is the Sun described in the context of other stars, and what is its estimated lifespan?

The Sun is described as an average star, not exceptional, having formed about 4.5 billion years ago. It is expected to continue providing energy through nuclear fusion in its core for another approximately 5 billion years.

What is the Milky Way, and how does its structure relate to our view of the night sky?

The Milky Way is our galaxy, resembling a thin circular disk. Our solar system is located within this disk, about two-thirds of the way out from the center. Looking in the direction of the galaxy's disk, we see the band of the Milky Way, which is a result of many stars appearing close together.

What are the two main units of distance used in astronomy, and what are their respective scales?

The two main units of distance used in astronomy are the astronomical unit (AU), which is Earth's average distance from the Sun (about 150 million kilometers or 93 million miles), and the light year, which is the distance light can travel in one year (approximately 10 trillion kilometers or 6 trillion miles).

Why do we see objects in the universe as they were in the past, and how does this relate to the observable universe?

Due to the finite speed of light, the light from distant objects takes time to reach us, meaning we see objects as they were when the light left them. The observable universe is the portion of the universe we can potentially observe, limited by the distance light can travel in the universe's age, which is about 14 billion light years.

What is the Big Bang, and how does it relate to the observed expansion of the universe?

The Big Bang is considered the beginning of the universe, a point in time when the expansion of the universe began. Observations of distant galaxies moving away from us, with increasing speed the farther they are, imply that the universe is expanding. By tracing this expansion back in time, astronomers estimate that the Big Bang occurred about 14 billion years ago.

How are new stars formed in the Milky Way, and what is the ultimate fate of our Sun?

New stars in the Milky Way are formed in the disk of the galaxy, where there is material for star formation. When stars like our Sun run out of nuclear fuel, they will die, potentially in supernovae explosions, and their material will be recycled to form new stars. Our Sun and solar system were created from the remains of other stars.

Outlines

00:00

🌌 Introduction to the Cosmos

This paragraph introduces the vastness of the universe, starting with a Hubble Space Telescope image of a small patch of sky filled with thousands of galaxies, each with billions of stars. It emphasizes the scale of the universe, from Earth's position in the solar system to the Milky Way and beyond. The paragraph also explains the solar system's composition, including planets, asteroids, and comets, and touches on the IAU's definition of a planet, which excludes Pluto. The Sun is described as an average star among billions in the Milky Way, setting the stage for a grand journey through astronomy.

05:03

📡 Understanding Our Galaxy and the Universe's Scale

The second paragraph delves into the structure of the Milky Way, describing our position within it and the visibility of the galaxy's band from Earth. It discusses the concept of nebulae, the Local Group of galaxies, and galaxy clusters, emphasizing the universe's large-scale structure. The paragraph introduces astronomical units of distance, such as the astronomical unit (AU) and the light-year, explaining their significance and how they relate to the speed of light. It also explores the concept of observing objects as they were in the past due to the finite speed of light, using the Andromeda Galaxy as an example.

10:04

🌠 The Observable Universe and the Big Bang Theory

The final paragraph discusses the observable universe, explaining its boundaries defined by the 14 billion light-years distance, which corresponds to the age of the universe. It describes the expansion of the universe, with galaxies moving away from each other, and introduces the Big Bang as the beginning of this expansion. The paragraph also touches on the life cycle of stars, supernovae, and the recycling of stellar material to form new stars, including our Sun. It concludes with a poetic reflection on humanity's cosmic origins, echoing Carl Sagan's notion that 'We are all stardust,' highlighting our intrinsic connection to the universe.

Keywords

💡Hubble Space Telescope

The Hubble Space Telescope is an optical space observatory that was launched into low Earth orbit in 1990. It is one of the most well-known and productive astronomical tools, capturing high-resolution images of distant galaxies and nebulae. In the video, it is mentioned as the source of an image showing a small piece of the sky filled with thousands of galaxies, illustrating the vastness of the universe.

💡Galaxy

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, gas, dust, and dark matter. The video emphasizes the sheer number of galaxies visible in the Hubble image, each containing billions of stars, to convey the scale of the cosmos. The Milky Way, our own galaxy, is also discussed, highlighting its structure and our position within it.

💡Planet

A planet is a celestial body orbiting a star that is massive enough to be rounded by its own gravity, has cleared its neighboring region of planetesimals, and has not become hot enough to cause thermonuclear fusion. The script lists the eight planets of our solar system and discusses the declassification of Pluto as a planet due to its failure to 'clear the neighborhood' around its orbit.

💡Solar System

The solar system encompasses the Sun and the celestial bodies that are gravitationally bound to it, including planets, their moons, asteroids, and comets. The video script describes the solar system's composition and provides a mnemonic to remember the order of the planets, emphasizing the interconnected nature of these bodies.

💡Asteroid

An asteroid is a small rocky body in the solar system, often considered remnants of planetesimals that never coalesced into planets. The script mentions that most asteroids are thought to be the shattered remnants of these early solar system bodies, contributing to the debris in our solar system.

💡Comet

A comet is a small celestial object consisting of a nucleus of ice and rock, which, when close enough to the Sun, displays a visible coma and sometimes also a tail. The video describes comets as 'dirty snowballs' and explains their highly elliptical orbits and the phenomenon of meteor showers when Earth passes through their debris.

💡Milky Way

The Milky Way is the galaxy that contains our solar system, with a flattened disk-like structure and a halo of globular clusters. The script explains that from our vantage point within the disk, we see a band of light in the sky, which is actually the dense concentration of stars in the galaxy's plane.

💡Nebula

A nebula is a large cloud of gas and dust in space, often the site of star formation. The script uses the Orion Nebula as an example of a nebula, describing it as a vast cloud of dust and gas where new stars are born, and emphasizing its scale relative to the solar system and the galaxy.

💡Light Year

A light year is a unit of distance used in astronomy, equal to the distance that light travels in one year, approximately 9.461×10^12 kilometers. The video uses the concept of a light year to explain the vast distances between celestial objects and the time it takes for light to reach us from them, thus connecting the concepts of distance and time in the universe.

💡Big Bang

The Big Bang is the prevailing cosmological model for the observable universe's origin, suggesting that the universe began as a singularity and has been expanding ever since. The script references the Big Bang as the beginning of the universe, approximately 14 billion years ago, and uses the observed expansion of the universe to support this theory.

💡Stardust

The term 'stardust' metaphorically refers to the idea that the elements that make up the Earth, and by extension, life, were created in the hearts of stars and dispersed into the universe through supernovae. The script concludes with this concept, emphasizing our cosmic origins and the interconnectedness of all matter in the universe.

Highlights

The universe contains billions of galaxies, each with billions of stars.

Astronomy is like a grand journey starting from Earth, the 3rd planet in our solar system.

Earth is 93 million miles from the Sun on average.

Our solar system includes the Sun, 8 planets, satellites, asteroids, and comets.

Pluto was reclassified as a dwarf planet in 2006 due to not clearing its orbit.

Asteroids are remnants of planetesimals that never grew large enough to become planets.

Comets are dirty snowballs of rock, dust, and frozen gases with highly elliptical orbits.

Meteor showers occur when Earth passes through a comet's debris left in its orbit.

The Sun is an average star that formed 4.5 billion years ago and will last another 5 billion years.

The Milky Way contains hundreds of billions of stars and is one of many galaxies in the universe.

The Milky Way appears as a band of light in the sky due to our position inside the galaxy.

Nebulae are vast clouds of dust and gas in space where stars are born.

Galaxies group together in clusters and superclusters, forming the large-scale structure of the universe.

The Andromeda Galaxy, our neighbor, is part of the Local Group with the Milky Way.

Astronomical units (AU) and light-years are used to measure vast cosmic distances.

The speed of light, denoted as 'c', is the universal speed limit and is a constant.

We see distant objects as they were in the past due to the finite speed of light.

The observable universe is limited to 14 billion light-years, the age of the universe.

The universe is expanding, with galaxies moving away from each other over time.

The Big Bang marks the beginning of the universe 14 billion years ago.

Stars go through life cycles, eventually exploding as supernovae and seeding new stars.

We are all made of stardust, as every atom in our bodies was once part of a star.

Transcripts

Hello, in this first lecture, we will take a look at our place in the universe, and try

to wrap our minds around the sizes of our planet, our solar system, our galaxy, and

our universe.

The backdrop here is an image that was taken with the Hubble Space Telescope.

It's a very small piece of sky.

Imagine taking a picture through a 1 millimeter by 1 millimeter square held at a meter away,

and that's what we have here.

But even with such a small field of view, look at all those galaxies!

The whole image contains about 10,000 galaxies, and each individual galaxy contains billions

of stars.

The smaller smudges are galaxies so far away that the light takes billions of years to

reach Earth.

The only objects that aren't galaxies are a few stars in the foreground.

These are part of our own Milky Way galaxy.

The study of astronomy can be likened to a grand journey, and like most journeys, we'll

begin from home.

Home for us is planet Earth, the third of eight planets in our solar system.

Earth is on average 93 million miles (or about 150 million kilometers) from the Sun.

At highway speeds it would take about 152 years to travel from the Earth to the Sun.

Our solar system consists of the Sun, the planets and their satellites, and many other

smaller objects such as asteroids and comets.

The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

There are many mnemonics for remembering the planets.

The one I learned when I was a young sprout is: "Mary's Velvet Eyes Make John Stay Up

Nights".

You've probably heard that Pluto is no longer considered a planet.

And this is why- the definition of planet was set in Prague in 2006 by the International

Astronomical Union.

They say that in the Solar System, a planet is a celestial body which: is in orbit around

the Sun, has sufficient mass to assume a nearly round shape, and has "cleared the neighborhood"

around its orbit.

Pluto fails on number three.

It's just too small.

There are objects besides the Sun and planets like the millions of asteroids, or minor planets.

Most are thought to be the shattered remnants of planetesimals, bodies within the young

Sun's solar nebula that never grew large enough to become planets.

Comets are essentially dirty snowballs made of rock, dust, water ice, and frozen gases

like carbon dioxide, carbon monoxide, methane, and ammonia.

Determining their size is difficult, but comet nuclei with radii of up to 20 kilometers have

been observed.

The tail of a comet can be much larger- thousands or even millions of kilometers across.

The orbits of comets are highly elliptical- that is their orbits look like really squished

ovals.

Periodically Earth will pass through a comet's orbital path.

Because comets leave all sorts of rocky debris behind them, when Earth passes through a comet's

orbit, the debris hit our planet.

You may have seen "shooting stars" in the night sky.

These are meteors- little comet pieces burning up in our atmosphere.

Therefore when we pass through a comet's orbit, we get a "meteor shower".

The Sun is our parent star, and the source of light and life on planet Earth.

As far as stars go, the Sun is nothing exceptional.

It formed approximately 4.5 billion years ago, and will continue to provide energy via

nuclear fusion in its core for another 5 billion years or so.

All of the other stars we see not really different from the Sun.

Our parent star is just one of several hundred billion stars in our galaxy, the Milky Way

Galaxy.

The sky is divided up into 88 official constellations defined by the International Astronomical

Union.

Every star in the sky belongs to a constellation.

From a dark sky site, you may notice a cloud-like band off light arching overhead.

It may be difficult to tell with just your eye, but binoculars or a small telescope will

show that this band of light is a result of many stars that appear close together.

This is the view we have of our galaxy, the Milky Way.

The band of the Milky Way is nearly impossible to see from Rockville or DC unless all the

power goes out.

It may seem strange that we see a band of light like this, but once you think about

how we're situated inside the galaxy, it should make a little more sense.

The Milky Way resembles a thin circular disk, like a pancake.

Our solar system is inside the disk of the galaxy, about two-thirds of the way out from

the center.

If we look straight up or straight down, we don't see as much pancake compared to when

we look into the pancake.

That is, when we look in the direction of our galaxy's disk, we see many more stars-

the band of the Milky way- than when we look in directions above or below the disk.

Every star you see in the night sky is part of our Milky Way galaxy.

There are billions of other galaxies with billions of stars all their own.

We can see a few of our neighboring galaxies from a dark sky sight with the naked eye (they

look like fuzzy cotton balls), but we cannot distinguish the individual stars in any galaxy

with our eyes except our own.

Each galaxy can be thought of as an island of stars.

If you lived in another galaxy, you would see a whole new set of stars in your sky.

Within the disk of the Milky Way are clouds of dust and gas in space called nebulae, like

the beautiful Orion Nebula.

Nebulae can be enormous compared to the size of our solar system, but small relative to

the total size of our galaxy.

There are billions of galaxies in the universe and galaxies tend to group together.

Our Milky Way, for example, is one of two largest galaxies among about 40 galaxies in

what is known as the Local Group.

Groups of galaxies with more than a dozen members are known as galaxy clusters, like

the Coma Cluster shown here.

This is an image of our neighboring galaxy, the Andromeda Galaxy, also part of the local

group.

On a very large scale the universe has structure.

Galaxies and galaxy clusters appear to be arranged in sheets and chains with large voids

of emptiness between them.

The superclusters are where the galaxies are clumped together.

Everywhere we look in the universe we see this overall structure.

Later in the semester we'll talk more about the large-scale structure of the universe.

This is cosmology!

Now, let's take a moment to talk about units of distance.

There are a variety units that are used for measuring distance, but there are two that

are often used in astronomy.

The first is the astronomical unit, or AU.

The astronomical unit is Earth's average distance from the Sun and it's about 150 million kilometers

or 93 million miles.

The AU is useful for smaller measurements.

For larger scales we generally use the light year.

A light year is the distance that light can travel in one year.

Light is pretty darn fast so it can cover a lot of space in a year.

A light year amounts to 10 trillion kilometers or about 6 trillion miles.

It's important to remember that a light year is a measure of distance, not time.

Also, nothing travels faster than light.

It's the universal speed limit and it's a constant.

The speed of light is 300,000 km/s.

Because it's a special number, it gets its own a special letter.

The speed of light is denoted by the letter 'c'.

The 'c' stands for celeritas which is Latin for 'rapid' or 'swift'.

Light has a finite speed, so it means that it takes a certain amount of time for light

to travel from an object to us.

Even for close objects, you never see anything in real time.

Consider the Moon.

It takes light from the Moon one second to reach us on Earth.

It takes light from the Sun about eight minutes to reach us.

This means if something really strange happened right at this moment on the Sun, we would

not know about it for eight minutes.

Sirius is a nearby star.

It takes light 8.6 years for the light to reach us from Sirius.

Light from the Andromeda Galaxy spends two and a half million years traveling to reach

Earth.

This means we see objects as they were in the past, and the farther away we look in

distance, the further back we look in time.

Here is the lovely Andromeda Galaxy again.

The Andromeda Galaxy is actually bright enough that you can see it with your naked eye from

a dark sky site.

It looks a little bit like a fuzzy cotton ball.

From Rockville or anywhere with city lights, we need binoculars or a telescope to see it.

If you could go out tonight and see the Andromeda Galaxy, the light that would hit your eye

left that galaxy two and a half million years ago.

When will be able to see what the Andromeda Galaxy looks like right now at this very moment?

Well, we have to wait two and a half million years.

We estimate that the universe is about 14 billion years old, and we'll talk all about

how we know this later in the semester.

Because of the finite speed of light, the farther away something is, the longer it took

the light to reach us.

At great distances we see objects as they were when the universe was much younger.

The observable universe is the portion of the entire universe that we could potentially

observe.

If we try to look beyond 14 billion light years, we'd be looking to a time more than

14 billion years ago, and this is before the universe existed, so there's nothing for us

to see.

The distance of 14 billion light years therefore marks the boundary or horizon of the observable

universe, and the observable universe is the portion of the entire universe that we could

potentially observe.

Finally, let's take a brief look at the history of our universe.

Observations of distant galaxies outside of our Local Group reveal that they are moving

away from us.

We also find that the farther away a galaxy is, the faster it is moving away from us.

These observations imply that the space between galaxies is increasing with time.

The universe is expanding!

Following this line of thought, we can figure that galaxies must have been closer together

in the past.

If we go back far enough, we must reach the point at which this expansion began.

We call the beginning of the universe the Big Bang.

Astronomers use the observed rate of expansion to calculate that the Big Bang occurred about

14 billion years ago.

The universe as a whole has continued to expand ever since the Big Bang, but on smaller scales,

gravity holds things together.

The stars within galaxies and galaxy clusters do not expand.

Things on Earth (people, rulers, coffee cups) are not expanding.

It's only the space between galaxies that is expanding.

Let's go back to our galaxy for a moment.

The disk of the galaxy is where all star formation takes place, simply because that's where the

material is to make new stars.

We'll talk much more about this later in the semester, but for now know that a star goes

through most of its life generating energy via nuclear fusion reactions in its core.

Eventually every star will run out of nuclear material.

The largest stars use their nuclear fuel the quickest and die in massive explosions called

supernovae.

When any star dies it blows its atmosphere back into space.

Eventually this material will form new stars.

Our Sun and solar system were created from the remains of other stars.

Every atom on Earth was once part of a star.

From the iron in your blood, to the calcium in your bones- every bit of you was once inside

of a star.

The late astronomer Carl Sagan was fond of saying "We are all stardust".

It's true!

We are made of the universe.

We are a cosmic consciousness.

We are truly a way for the universe to know itself!

That's all for the first lecture.

Over the course of the semester we'll explore the details of the topics I touched on today,

as well as many other fascinating things like light and telescopes and exoplanets and dark

matter.

But for now take care, and I'll talk you again soon.