In the vast emptiness of space, there are floating clouds of gas and dust called nebulae. These clouds are stellar nurseries, filled with material that will one day become multiple solar systems. 
When part of a nebula is slightly more densely packed than the rest, gravity is stronger in that region. It begins to pull the surrounding gas inwards. This increases gravity, which pulls even more stuff in. As this happens, the gas molecules rub against one another, heating up due to friction. When the gas heats up, it begins to give off light, illuminating the nebula with brilliant colors. Eventually, this region of the cloud takes on a rough spherical shape. The gas begins to ionize, separating electrons from their nuclei as the sphere heats up even further as it continues to collapse due to the massive gravitational forces at play. This spherical shape is known as a protostar! 
This protostar will continue to collapse and heat up, eventually becoming a main-sequence star. For that to happen, nuclear fusion must begin in the core. When it starts, outwards radiation pressure is so strong that it prevents gravity from causing the star to collapse any further. We’ll talk about the life — and death — of main-sequence star in a future blog post, so keep your eyes open! Written By: Scott Yarbrough
Tag Archives: Gravity
Black Holes Explained (Sort of)
Black holes are a confusing topic in astronomy. You’ve heard about them starting from a young age, but whenever you ask someone for more information about them, there’s a whole lot of “I dunno”s. The truth is, black holes have been romanticized by science fiction, when in fact they are nothing more than an oddity of physics (albeit still pretty cool). Once you understand a few basic rules of physics, learning about black holes is easy.
Step 1: Gravity
The first step on the road to understanding black holes is understanding how gravity works. An object with mass will cause a bend in spacetime, affecting other objects around itself. This effect is what causes gravity. The more mass something has, the more gravity it produces. The force of gravity gets weaker the farther you get from the center of mass proportional to the inverse of its distance squared. Yeah, I know, that sounds confusing. Maybe it’s easier if you see the equation for calculating the force of gravity.
In this equation, M is the mass of the larger object, m is the mass of the smaller object, G is the universal gravitational constant, and r is the distance between the two objects. If you double the distance between two objects, then the force of gravity is ¼ what it was before. If you triple it, then the force of gravity is only ⅑ the original.
Step 2: Escape Velocity
In order to escape the gravitational field of a massive object, you need to attain a specific speed, called its escape velocity. It’s calculated using the force of gravity, taking into account the mass of the large object and how far away you are. Once something is going at minimum the escape velocity, then it is no longer captured by the massive object.
Step 3: Singularity (or, infinite density)
Density is an easy calculation, as seen below.
Where m is mass and V is volume. As mass increases or as volume decreases, density goes up. In a black hole, the volume is essentially 0, which causes the density to approach infinity. This creates what is called a singularity.
Putting it all Together
When the most massive stars in the universe go supernova, the force of the explosion causes the core of the star to get smaller and smaller, essentially packing it all into a volume of 0. This point of infinite density creates such a huge bend in spacetime that it creates a singularity. At a certain distance away from this singularity, the force of gravity the escape velocity reaches the speed of light.
As defined by the theory of relativity, nothing in the universe can go faster than the speed of light. So if the escape velocity of this object at a certain distance is higher than that, then not even light can escape the singularity. With no light escaping, the object and the space around it gives off no light.
This is the black hole.
There’s a lot more that goes into defining black holes, such as its mass, spin, etc., but that’s just icing on the cake. There you have it. There’s so much more to learn about black holes, but knowing even this much gives you the tools you need to understand them at a fundamental level.
Written By: Scott Yarbrough
Busting the Misconception Between Gravity and Atmospheric Pressure
A common misconception we see at AstroCamp is how gravity and atmospheric pressure work. It makes sense! The two seem interchangeable at a glance, however they both get weaker as altitude increases, and there can’t even be an atmosphere without gravity. Despite their similarities, the two are very different.
Gravity is a property of anything with mass. It’s one of the four fundamental forces in the universe, and it’s caused by an object bending spacetime around itself. The larger the object, the greater a force it will exert.
Pressure isn’t even a force (technically). It’s a measurement of how much force is being applied per unit area. A lot of force can be spread out over a large surface area so that the pressure is overall small, and a small force can be focused onto a small enough point to cause a high pressure.
When talking about atmospheric pressure, we’re talking about the average force per unit area the gas molecules are exerting on objects in the air. The air molecules are zooming everywhere and bouncing into everything. Every time they impact, they push with a tiny force.
Atmospheric pressure changes with 1) the rate of collisions and 2) the force of impact. More molecules mean more collisions, which leads to a higher air pressure. Similarly, heavier gases or gases moving at higher speeds will cause a higher impact force, also increasing the atmospheric pressure.
Air at sea level is being compressed by all of the air above it, weighing it down and increasing its density. When you increase altitude, the less air you have above you, so pressure goes down. This is why atmospheric pressure gets weaker the higher in altitude you go.
After learning how they both work, it’s easy to see why the effects of gravity and atmospheric pressure can get confused. But their differences are also prevalent enough that you should be able distinguish between the two!
Written By: Scott Yarbrough
What is Escape Velocity?
Escape Velocity is probably something you’ve heard on a TV show, or maybe you learned about it from NASA talking about their newest spacecraft. It is a commonly discussed term, but it isn’t the easiest thing to understand. Imagine you’re in a strange universe where only the Earth exists. The only gravity comes from its center, and it extends infinitely far away, getting weaker and weaker the further away you get. In order to get to that infinite point before you get pulled back by the Earth’s gravity, you need to be going at least 11.2 km/s (25,000 mph). This speed is the escape velocity!
This value depends on both the distance from the gravitational center of the object you’re escaping from and the mass of the object. The closer you are to a heavier object, the faster you need to go to reach escape velocity. For example, escape velocity from the Earth at a distance of the moon’s orbit is only 1.3 km/s (3,000 mph), but to escape from the sun’s gravity at the distance of the Earth is a whopping 44.7 km/s (100,000 mph)!
But since there’s no such thing as a universe where only the Earth exists, we have to worry about the gravity of other celestial objects! Once you escape from the Earth’s gravity, you’ll then be captured by the sun. If you escape that, then you’ll be captured by the Milky Way’s gravity! So the hypothetical infinite point is just that: hypothetical! No matter what, there’s always going to be something pulling on you with gravity.
Written By: Scott Yarbrough
The Tale of Sir Isaac Newton
There’s an old story of Newton sitting underneath a tree during his studies, when an apple fell and hit him on the head, leading him to begin his adventures into solving the mysteries of gravity. While it’s doubtful that this fanciful tale actually happened, it is true that Newton had an incredibly significant impact on our understanding of gravity.
English mathematician and physicist Sir Isaac Newton (1642 – 1727) contemplates the force of gravity, as the famous story goes, on seeing an apple fall in his orchard, circa 1665. (Photo by Hulton Archive/Getty Images)
Isaac Newton was born in England on Christmas in 1642 to a widowed mother. His father, after whom he was named, had died three months prior. When his mother remarried, Newton went to live with his grandmother and began his education. In 1661, he began attending Trinity College in Cambridge.
It was there where he began his great works.
Influenced by modern philosophers and astronomers, Newton began to develop his own mathematical methods that would later evolve into a rough form of what we know today as calculus. Inspired by the works of astronomer Johannes Kepler, he began developing theories on gravity. When Cambridge shut down temporarily as a response to the Great Plague, Newton spent two years at home developing these theories, along with many others.
In 1687, he published his Mathematical Principles of Natural Philosophy, which set out his famous three laws of motion. The papers went on to combine these with his law of universal gravitation to investigate and explain Kepler’s work on orbital mechanics, explaining how the planets move about the solar system.
Newton continued to contribute to the world of physics and mathematics until his death. He built telescopes, advanced the field of optics, formulated many laws and theories, and even developed ways to calculate the speed of sound. It’s been said that his work helped to advance every subject of mathematics and physics that was being studied at that time, and even today we still use his works in our understanding of the world.
Written By: Scott Yarbrough
Pendulums and Gravity
In the video above, I talk about how pendulums actually work. If you haven’t watched it, the principle is simple: an object is suspended from a fixed point and allowed to swing back and forth – the mass of the object and the time it takes to swing back and forth are independent of each other, relying only on the length of the string and the strength of gravity.
Normally, you’d think of gravity on Earth’s surface as being constant, but the Earth isn’t a perfect sphere, meaning that the force of gravity near the equator is slightly weaker than at higher or lower latitudes. And how did we discover this fact? Pendulums!
In the year 1671, a French scientist named Jean Richer travelled to French Guiana. Among several experiments and astronomical observations during his two-year trip was to take measurements with a clock pendulum.
He set up the pendulum in the same way I did in my video, but he adjusted the length of the pendulum so that one half-swing took exactly one second, a common technique at the time. What he found was that the pendulum length needed to be slightly shorter than it did back in Paris, by about 3 millimeters. Though a small difference, it was significant enough to begin a discussion about the varying gravitational field of Earth.
This was later proved by Isaac Newton by determining that due to the Earth’s rotation, it was thicker at the equator, meaning the surface was further away from Earth’s center of mass. This was further supported by Newton’s idea that gravitational force decreases as the distance between two objects increases.
Scientists started to use pendulums to take measurements of the gravitational field in other locations and began to create a model of the Earth’s true oblong shape. Since then, we’ve developed more accurate methods to measure the same thing, but they were pioneered by those first efforts.
Written By: Scott Yarbrough
Video Music: Funky Chunk Kevin MacLeod (incompetech.com)
Licensed under Creative Commons: By Attribution 3.0 License
The Life and Death of Stars
Did you know that stars live and die just like other living things?…Okay, maybe not just like them. But they do have a beginning, middle, and end. All stars start out the same way, from a nebula. A nebula, otherwise known as a “star nursery”, is a cloud of gas and dust out in space. Nebulae will then start to clump up due to the massive amounts of gravitational pull. This clumping creates protostars, which are basically spherical masses of the gas and dust that are collecting even more gas and dust from the nebula.
Once the gravity of the protostars becomes great enough, the process of fusion will begin, turning the protostar into a star. A star is defined to be a self-luminous gaseous spheroidal celestial body of great mass which produces energy by means of nuclear fusion reactions. Fusion is the act of turning lighter elements into heavier ones which can only occur under great pressures.
Depending on the original mass of the nebula and protostar, a star can be of any number of sizes. For our purposes, let’s stick with an average sized star (like our Sun), a massive star, and a supermassive star.
For a Sun-like star, once it has completed fusing hydrogen into helium it will become unstable and swell in size, becoming a red giant. As a red giant, it will have a thin outer shell of some hydrogen gas, and an inner core of mostly helium. Once the helium runs out, it will become extremely unstable and puff out it’s shells of hydrogen and helium, becoming a planetary nebula. One example of this is the Ring Nebula (M57). Left in the center is a white dwarf star, which is named so due to how hot and luminous it is. When the white dwarf radiates its energy away, it will fade, becoming a brown (or black) dwarf star.
For a massive and supermassive star: they will go through the fusion process, become unstable, puff out a shell, swell to a red supergiant, start the next round of fusion, and so on and so forth, creating heavier and heavier elements. Once the massive and supermassive star become extremely unstable they will go supernova. A supernova is the largest explosion in space, which is very bright and ejects most of its mass.
When this happens for a massive star, a neutron star will be left behind. A neutron star is a celestial object with very small radius (typically 18 miles/30 km) and very high density, composed mostly of closely packed neutrons. Neutron stars also tend to rotate extremely quickly and emit regular pulses of radio waves and other electromagnetic radiation, earning them another name, pulsars.For a supermassive star, it will follow the same path of a massive star, but with one key difference. Instead of leaving a neutron star behind after the supernova, it will leave behind a black hole. A black hole is simply a region of space having a gravitational field so intense that no matter or radiation can escape.
Supernovae create the heaviest elements in our universe, which are the building blocks to life as we know it. Without this constant cycle of creation and destruction, we would have nothing. So the next time you look up in the sky, be thankful to that glowing orb of incandescent gas and all of the gas and dust that came before it.
Gravitational Lensing
Most of us have experienced using a lens in some way, whether it was using glasses, cameras, or our favorite, telescopes. A lens is a piece of glass or other transparent substance with curved sides for concentrating or dispersing light rays. They have the ability to bend light! Did you know that gravity can bend light in the same way? We call this gravitational lensing; when a large massive object, like a blackhole or galaxy, passes in front of our view of a distant light source and a distant galaxy. It bends the distant light in different ways, sometimes creating two or more images or creating an Einstein ring, a complete ring of the image.
The glass shape at the bottom of the stem of a wine glass, and looking straight through the glass has almost exactly the same optical properties as a massive galaxy or blackhole!
When you pass the glass over a light source or picture, it distorts the light into an Einstein Ring. This is one way Astronomers know about dark matter! According to Einstein’s theory of general relativity, the presence of matter (energy density) can curve spacetime, and the path of a light ray will be deflected as a result.
Written by: Mimi Garai
Wrap Your Head & Hands Around Density
Heat rises, boats float, clouds hang listlessly in the sky, and icebergs aimlessly bob around the ocean. All of these things happen because of the same scientific property: density!
Density is a rather simple property. It depends on two things, mass and volume. Mass is basically how much something weighs, but something still has the same amount of mass weather it is on Earth, Jupiter, or out in space where its weight would change dramatically. Volume doesn’t have anything to do with how loud something is, but rather with how much space it takes up. Mathematically, an object’s density is equal to its mass divided by its volume.
The demonstration here using the pipette in a bottle of water is a great way to buoy your own understanding of density. The pipette is carefully balanced to barely float in the water. However, by squeezing the bottle, water is forced up into the pipette. This makes the pipette heavier, which raises its density. Releasing the bottle down the opposite. For another cool example of density, click here!
By simple comparing the density of different things, we can tell if they would float or sink in water or air or anything else that has a known density. This is incredibly useful and can lead to some fun facts. For example, Saturn is less dense than water. That is to say it would float in a bathtub, but it might leave some rings! Red giant stars, like the famous Betelgeuse, are actually less dense than air! They are incredibly heavy–Betelgeuse weighs in at about 18 suns worth of mass–but they take up so much space that they are essentially GIANT balloons!
The sun would not float like a balloon now, in fact it is currently much more dense than water. However, given 5 billion years it will enter the red giant phase of its own life and grow in volume by about 800 million times! This will decrease its density so much that it goes from 40% higher than water to lower than air! As you can clearly see, density is not necessarily a constant property of something, it is actually something that can change.
One of the most commonly accepted consequences of density is that heat rises, but that isn’t technically true. It would be more correct to say that heat rises on Earth. This is because as things warm up, their volume increases which lowers their density. However, if we leave the friendly confines of Earth, this is no longer true! In the microgravity environment aboard the International Space Station, there isn’t really an “up” or “down”, gravitationally speaking. That makes it hard to decide what “heat rises” even means! Fire gets equally confused. Check out this comparison between fire on Earth and aboard the ISS from NASA.
Where Do Stars Come From?
Where do stars come from? The short answer is: gravity. All objects with mass experience gravitational attraction to each other. That includes you & the Earth, you & me, every person on our planet, and every star in the cosmos. So, why aren’t we pulled in all directions? Gravity depends on two things: mass and distance. The bigger an object is, the stronger its influence. The farther it is from you, the less you’re affected by its pull. Stars have plenty of mass, but they’re so far away that their gravitational effect on us is negligible. You and I could stand next to each other, but we just aren’t massive enough to pull significantly on each other compared to the much stronger attractive force Earth exerts on our bodies.
The Great Orion Nebula, visible to the naked eye as part of Orion’s dagger, is a massive space-cloud system and a productive stellar nursery. Image credit: UMass/2MASS/IPAC
In deep space, it’s a different story. In the absence of larger gravitational influences, even dust particles pull each other closer. It’s this quiet, dark, and delicate dance that gives rise to the stars we see in the night sky. Nebulae are massive space clouds of gas and dust. Within these clouds, turbulence makes some areas denser than others. When a dense knot of dust reaches critical mass, it collapses under its own gravitational pull, heating up and becoming a protostar. This spinning collection of hot, dense debris drags nearby particles along with its rotation, and an embryonic solar system is formed.
Einstein’s theory of relativity describes spacetime as a three-dimensional fabric that can be stretched in a fourth dimension. The way that massive objects stretch the fabric of spacetime explains their gravitational pull on each other. That’s tough to visualize, but a two-dimensional model that stretches in 3D helps us to wrap our heads around the idea.
Particles of water vapor model the behavior of dust in a nebula.
When we place a heavy sphere in the center of a sheet of stretchy fabric, it pulls the surface down. Drop marbles or BBs onto the fabric and they spiral around and around before coming to rest at the lowest point. The same thing happens with water vapor: it spirals to the spot where the gravitational potential is lowest– the heavy object in the middle.
This model of spacetime provides a simple, fairly accurate illustration of the way individual stars form in nebulae.
Written By: Caela Barry
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