Before diving into the deep, confusing content of the Universe, we first need to look at a few basic terms and concepts:
A Nebula: A nebula is an interstellar cloud in outer space that is made up of dust, hydrogen and helium gas, and plasma. A Nebula, or nebulae (plural) can also be called a molecular cloud. They condense down to form stars, planets, galaxies, etc.
A nebula is a truly wondrous thing to behold. Named after the Latin word for “cloud”, nebulae are not only massive clouds of dust, hydrogen and helium gas, and plasma; they are also often “stellar nurseries” – i.e. the place where stars are born. And for centuries, distant galaxies were often mistaken for these massive clouds.
Alas, such descriptions barely scratch the surface of what nebulae are and what there significance is. Between their formation process, their role in stellar and planetary formation, and their diversity, nebulae have provided humanity with endless intrigue and discovery. For some time now, scientists and astronomers have been aware that outer space is not really a total vacuum. In fact, it is made up of gas and dust particles known collectively as the Interstellar Medium (ISM). Approximately 99% of the ISM is composed of gas, while about 75% of its mass takes the form of hydrogen and the remaining 25% as helium. The interstellar gas consists partly of neutral atoms and molecules, as well as charged particles (aka. plasma), such as ions and electrons. This gas is extremely dilute, with an average density of about 1 atom per cubic centimeter. In contrast, Earth’s atmosphere has a density of approximately 30 quintillion molecules per cubic centimeter (3.0 x 1019per cm³) at sea level.
Even though the interstellar gas is very dispersed, the amount of matter adds up over the vast distances between the stars. And eventually, and with enough gravitational attraction between clouds, this matter can coalesce and collapse to forms stars and planetary systems.
We will be focusing on Planetary Nebulae & and Interstellar Mediums (Star forming Nebulae)
Stellar objects that can be called Nebula come in four major classes. Most fall into the category of Diffuse Nebulae, which means they have no well-defined boundaries.
Planetary Nebula: Involves a low-mass star entering the final stage of its life. In this scenario, stars enter their Red Giant phase, slowly losing their outer layers due to helium flashes in their interior. When the star has lost enough material, its temperature increases and the UV radiation it emits ionizes the surrounding material it has thrown off.
This type of nebula earns its name because some astronomers of the 18th Century believed that they looked like giant worlds through the eyepiece of small telescopes. Here’s a tricky question; how do you think that these nebulae are made? Here’s a clue - it’s not through the collapse of the Interstellar Medium and it’s something to do with the fuel of a star. Have you had a guess? Read on to see if you’re right! Planetary nebulae are made when a star runs out of fuel to burn. What happens next is amazing. What do you expect to happen when a star runs out of fuel? Have a quick think and write down some answers. It’s not quite the same as when your car runs out of gas, where it stops moving - what happens to a star is quite a bit different; it blows off its outer layers of gas in the shape of a ring or bubble. When stars do this, astronomers say that a star is dying. But it’s not a sad ending for the star, it’s a beautiful colorful one!
What we mean in terms of a dying star, at least when it comes to stars just like our Sun (called Sun-like stars), is that it’s changing into a red giant star. A red giant is a huge star that can swell to a size that swallows up everything in its path. After spending millions of years as a heavy weight giant, it shrinks again, pushing off the outer layers that we mentioned earlier. Planetary nebula are usually visible for around 50,000 years before starting to mix with the space that surrounds it - so there’s plenty of time to get out your telescope to have a look. Maybe you can decide for yourself if it looks like a planet!
Interstellar Medium: The space between stars is not ‘empty space’ it actually contains gas and dust that makes up around 15% of the visible mass in the Milky Way. This is known as the Interstellar Medium or ISM. Even though matter in the ISM is abundant it is highly dispersed over vast areas, it is in fact far more dispersed than any vacuum that can be created on Earth. The ISM has an average density of only 1 atom for every cubic centimeter, compare that to Earth’s atmosphere which contains a 100 billion billion atoms per cubic centimeter.
The vast majority of the ISM is composed of the most abundant gas in the Universe, hydrogen, which makes up around 90% of its mass, around 9% is composed of other gases, mostly helium. The remaining 1% of the ISM is made up of dust particles, this is not like the dust you find in your home, these tiny particles are only 1 to 20 millionth of a centimeter in size.
The nature of gravity means that over time these materials will be attracted to one another, forming into clumps of matter which in turn will join other clumps of matter eventually forming into huge dark nebula clouds that can be seen against the bright backdrop of the bright Milky Way (picture above). These nebulae can eventually give birth to stars, planets and moons such as we see in our own solar system.
A stellar nebula is a cloud of superheated gases and other elements formed by the explosive death of a massive star. Stellar nebulae emit brilliant waves of infrared light generated by dust particles within the cloud. Within the nebulae, you have something called a Stellar Nursery.
These dark molecular clouds of gas can become stellar nurseries, providing an environment which allows stars to be born. When pockets of the dark nebula have sufficient density, hydrogen molecules will begin to condense, this is possibly triggered by the shockwaves of a nearby supernova explosion. Clumps of hydrogen will grow large enough to begin collapsing under their own weight, heating up and creating the early stage of a star called a Protostar. Over thousands of years the young star continues to grow and heat up until its core is hot enough to allow nuclear fusion, this is when hydrogen atoms are fused together producing enormous amounts of energy. The star is now in its main sequence and will remain in this state for most of its life.
Around 5 billion years ago such a process produced a yellow dwarf star which was located several thousand light years from the center of our galaxy. Out of the large disk of gas and dust that formed around the new star a small rocky planet was created which was in just the right location to allow water to flow on its surface. This planet was our very own Earth and through the process of evolution these conditions ultimately allowed life to flourish on our world.
Star formation begins in a nebula when particles within the cloud begin to bond with each other due to their gravitational pull, the increased mass of these particles in turn gaining more gravitational pull. As they attract more of the surrounding dust and gas, the pressures at the core of the construct reach such an extreme that nuclear fusion results, sparking the earliest stages of star development.
Star:
A cloud of hydrogen and helium gases held together by gravity
Constantly undergo nuclear reactions and emit heat, light, UV rays, X-rays, and other types of radiation
Star Cluster:
Stars formed from the same interstellar gas properties which have similar properties of:
Age
Distance
Composition
Galaxy:
Large systems of stars, containing several million to several trillion stars
Held together by millions of light years in distance
Galactic Cluster:
The largest known objects held together by gravity
Forms the densest part of the structure of the Universe
We reside in the Laniakea Supercluster
Universe:
All existing matter and space considered as a whole
Includes planets, stars, galaxies - all matter and energy
Approximately 10 billion light years in diameter
Was created approximately 14 billion years ago
Measurements in Space:
Direction The first measurement type we will discuss taken in space is measuring the direction stars are moving (towards or away from us on Earth. This uses Visible Light (ROYGBIV) with a Red Shift and a Blue Shift.
Astronomers use the light spectrum (ROYGBIV) emitted from stars to determine movement (red - 700nm) (blue - 400nm). With this, they use the doppler shift of the visible light spectrum to calculate precisely how fast stars move towards or away from Earth.
The first thing you have to understand is that all objects will either emit light or absorb light. Us on Earth, we do not emit light, therefore we absorb the light our star (the sun) emits. Every object which emits light is a star in space. They emit light in many different wavelengths encompassing ROYGBIV - Red, Orange, Yellow, Green, Blue, Indigo, Violet.
In the visible light spectrum, Red (at 700 nanometers) light waves are longer than blue (at 400 nanometer) light waves.
Here is an image of the Electromagnetic Spectrum. As humans, we can only understand a certain wavelength size (from around 400-700nm). All others are not able to be understood by our brains. We do get affected by them, but we can not see them. That is why we keep saying "light emitting" stars. Not all stars will emit light, some will emit Infrared waves only, or Microwaves only. If they emit them, we can not see them with our naked eye. We have to have advanced filters to view them.
When viewing light emitting stars, we have what is called a red shift or a blue shift.
A red shift is an increase in wavelength from light-emitting stars. It occurs when a light source moves away from the observer. Galaxies which are further away or moving away from Earth emit light toward the Red (longest wavelength).
A blue shiftis a decrease in a wavelength from emitting stars. It occurs when a light source moves away from the observer. So galaxies that are closer or moving towards Earth emit light towards the blue (shorter wavelength).
However, we do not see the actual wavelengths, we see emission and absorption lines. Emission lines are the lines of what a light-emitting objects sends out. We convert emission lines into absorption lines. Absorption lines are what is absorbed from that light emitting object. Think of it this way, if an object is emitting blue light to us on Earth, then we on Earth are going to absorb that blue light.
Make sense?
Astronomers often use the term redshift when describing how far away a distant object is. To understand what a redshift is, think of how the sound of a siren changes as it moves toward and then away from you. As the sound waves from the siren move toward you, they are compressed into higher frequency sound waves. As the siren moves away from you, its sound waves are stretched into lower frequencies. This shifting of frequencies is called the Doppler effect.
A similar thing happens to light waves. When an object in space moves toward us it light waves are compressed into higher frequencies or shorter wavelengths, and we say that the light is blueshifted. When an object moves away from us, its light waves are stretched into lower frequencies or longer wavelengths, and we say that the light is redshifted.
If you look at the image above, compare the middle and bottom absorption lines to the top. A star's emission and absorption lines are like a person's finger print. Look at how the lines stay the same, but they shift up the color either closer to red or closer to blue. The top absorption lines have shifted more towards the blue color therefore, the object is blue shifting - or moving towards Earth. The bottom absorption lines have shifted more towards the red color therefore, the object is red shifting - or moving away from Earth.
In the visible portion of the electromagnetic spectrum, blue light has the highest frequency and red light has the lowest. The term blueshift is used when visible light is shifted toward higher frequencies or toward the blue end of the spectrum, and the term redshift is used when light is shifted toward lower frequencies or toward the red end of the spectrum. Today, we can observe light in many other parts of the electromagnetic spectrum such as radio, infrared, ultraviolet, X-rays and gamma rays. However, the terms redshift and blueshift are still used to describe a Doppler shift in any part of the spectrum. For example, if radio waves are shifted into the ultraviolet part of the spectrum, we still say that the light is redshifted - shifted toward lower frequencies.
Distance The second measurement method we will discuss is measuring directions in Space (how far away or close you are). We have two methods of measuring distance: Astronomical Units (AU) and Light Years (LY)
Astronomical Units are the distance from Earth to our Sun. 1 AU is equal to approximately 93 million miles. Usually this measurement does not span past our solar system.
Astronomers measure large distances in space in units of Light Years - the distance light travels in one year (about 6 trillion miles or 10 trillion kilometers)
Brightness The brightness is measured in magnitude: it is the life cycle of stars - how old the stars are around us.
One way Astronomers try to determine how far away on object is in space is by its magnitude.
Apparent Magnitude is how bright objects appear to an observer on Earth. Apparent Magnitude depends on how close they are to Earth and how brightly they shine (luminosity.
Absolute Magnitude is a measure of how bright the star actually is. Magnitude is measured on number scale (from + to -). The lower the magnitude, the brighter the star.
If you look at the two images above, the first shows a person looking into the night sky. It appears that star #3 is the brightest in the night sky. This is what the observer sees. Therefore, this is the apparent magnitude. When the stars are actually measured, you see the image on the right. The image on the right shows that star #3 is NOT the largest, actually star #5 is the largest. Now, #5 was not even seen in the night sky on the first image. This is because it was so far away that it can not easily be seen in the sky. The only reason #3 appears as large as it does is due to it being closer to Earth.
It's kind of like the Sun. When you look at the Sun, it appears to be the brightest star we have in sky, however; we know that it is not. It only appears to be the brightest due how close it is to Earth.
Location: We determine location of a star in space by using Parallax. It helps determine where the star is actually located at. Astronomers measure distances to the nearest star using a stellar parallax.
A parallax is the apparent shift in nearby objects (less than 100 ly) against more distance ones when viewed from different vantage points.
By measuring the angle labelled p on the diagram, we can use our orbital distance from the Sun and a little trigonometry to calculate the distance to the nearby star. However, even for the nearest stars, the angle is very small, but if you are very careful, you can measure the distance to stars several hundred light-years away.
The Big Bang:
The Big Bang Theory
There are multiple theories about how the Universe was developed. Nonetheless, it all starts with a star.
The Big Bang Theory is not the only concept, but it is the most accepted at this time.
Back to the Big Bang
The Universe is expanding. What does this mean? Are your ears gradually getting further apart? Fortunately no, even though your head is part of the Universe. Nearby, forces like the gravitational field of the Earth, or indeed our Galaxy, overcome the "universal" expansion. Only for things at least as isolated as galaxies themselves does the ever-increasing separation become obvious. A good image for this is to imagine what happens as you blow up a balloon with some stickers attached to the surface. All the stickers get further apart from each other, and incidentally to each sticker it appears that the others are receding symmetrically, the further ones faster. The stickers don't expand, and neither does your head.
Run this expansion backwards in time and you arrive at a moment when the distances between all objects were zero. That is the instant of the Big Bang. These everyday phrases have to be handled very carefully in this context! For example, you might want to say that, at the time of the Big Bang, everything was in one place. However it is possible (even likely) that the Universe was infinite even at the instant of the Big Bang. One way of seeing that this could happen is to remember from high school math that infinity times zero isn't necessarily zero (or infinity). If the distance between each point in the universe is zero, but there is an infinite number of points, the size of the Universe is, roughly, infinity times zero. This illustrates why it's a mistake to think of the Big Bang as happening "somewhere". It happened everywhere!