UNIT 5 Main Lecture: “How were stars formed?”
A Universe without stars: We’ve all looked up at the stars at night and wondered about them. But could you imagine a Universe with no stars, no points of light, nothing out there. That’s what the early universe was like for almost 200 million years after the big bang. In fact, as the Universe expanded, it got colder and colder and darker and darker and less and less like a place that might eventually produce you and me. Astronomers call this era the Dark Ages.
During the Dark Ages, the Universe was very simple and didn’t have much structure. Flowing through the darkness were vast numbers of atoms. Most, about 75%, were Hydrogen; and the rest were Helium, apart from a tiny sprinkling of other small atoms such as Lithium (with 3 protons), Beryllium (with 4) and Boron
(with 5). (There was also stuff that astronomers call ‘dark matter’ because they don’t really know what it is, but we’re going to ignore it because it doesn’t seem to affect the story much.)
Not only was the matter extremely simple; it was also arranged very simply. Studies of the Cosmic
Background Radiation show that 380,000 years after the Big Bang there was hardly any variety.
Every region looked almost exactly like every other region, with the same temperature, the same pressure, the same types of atoms. This was puzzling because it seemed as if the universe was too uniform for anything new to happen.
And, that was a serious problem. If things stayed that way, we wouldn’t be here today to puzzle over this. So, how did things change? Well, the formation of stars is a key player in this story, so let’s think about how the stars got created.
Goldilocks conditions for making stars: We’ll see over and over again in this course that more complex things seem to appear only when the conditions are just right, where you have the right ‘Goldilocks’ conditions. Not too hot. Not too cold. Not too big. Not too small. Not too close together. Not too far apart. You get the idea.
So, what were the Goldilocks conditions, the perfect conditions for building more complexity in the early
Universe? It turns out that these conditions were very simple and they occurred throughout the Universe.
These early Goldilocks conditions were: lots of matter, gravity, and tiny differences in the way matter was distributed. Let me try to explain.
Recent studies of the CBR have shown that there were tiny differences within it. Some regions were ever so slightly hotter (by just about 1/1000 th of a degree C) and just slightly denser than other regions. And this meant that the Universe was not quite as stable or as uniform as it seemed.
What happened next was that the force of gravity got to work on those tiny differences, magnified them and by doing so created entirely new objects: stars. Here’s how gravity did it.
Gravity gets to work: Gravity is one of the four fundamental forces. It’s a force of attraction that exists between every particle of matter and energy in the Universe. But the closer together two things are and the larger they are the greater the power of gravity.
The earth’s gravitational pull is huge, but if you move away from the earth into space, it gets much weaker.
Now think of that simple, early universe. In slightly denser regions there was slightly more matter than in less dense regions. With more matter gravity is more powerful, so gravity began to pull denser regions together and atoms began to clump into billions of huge clouds.
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UNIT 5 Main Lecture: “How were stars formed?”
As each cloud contracted it got denser and the power of gravity increased. So each cloud contracted faster and faster; and now change began to speed up like a runaway train.
As clouds of atoms contracted there were more and more collisions between particles, so they began to heat up, particularly in the center of each cloud where there were the most atoms. Notice that now, for the first time since the big bang, parts of the Universe are getting hotter again.
The temperature reached about 3,000 degrees. Does that number sound familiar? It’s the same temperature as the surface of cooler stars, which is no accident. At this temperature protons could no longer hold on to electrons so that atoms broke apart re-forming the sort of plasma that existed before the appearance of the
Cosmic Background Radiation, 380,000 years after the Big Bang.
The heat kept rising until the center of each cloud reached 10 million degrees. Now protons and neutrons are colliding so violently they can overcome the repulsion of their positive electric charges and fuse, as the ‘strong nuclear force’ takes over and binds them together.
As they fuse tiny amounts of their matter turn into pure energy, so there is a huge release of energy from the center of the cloud. This is similar to what happens in an H-bomb.
Like a vast furnace, the heat and energy at the center of the cloud begins to push back against gravity and prevents the cloud collapsing any further. Now at last the cloud stops collapsing and we seem to have reached a more stable state as the heat from the center of the cloud balances the force of gravity.
The center of the cloud now begins to shine with light and energy. A star is born! Something so stable that it may last for millions or even billions of years.
We have now crossed our second major threshold! From about 200 million years after the big bang, stars lit up throughout the Universe. All formed in essentially the same way.
So what? Yes, stars make the night sky much more interesting to look at, but they’re far more important than that. Because of stars, we’ve now gone from a Universe made of a largely uniform mush where not much interesting could happen, to a Universe filled with enormous glowing batteries that emanated light and heat, and enabled even more interesting things to occur.
Stars are still forming today inside galaxies. Astronomers can detect many of these ‘star nurseries’ within our own galaxy. They offer some of the most gorgeous and spectacular images in our Universe. You could see these for yourself through a telescope or the Hubble web site or any number or other places. It’s worth checking out.
Stars also added one more thing to the Universe: they gave it structure at a variety of scales – from individual stars, to galaxies, to superclusters. In a word, they made the Universe more complex. Remember we started this unit in a Universe that seemed to have hardly any structure at all!
Let’s look at these structures at different scales. All stars have the same basic structure. At their centers, temperatures are so high that protons fuse together to form helium nuclei. Outer layers contain stores of charged protons (hydrogen nuclei) that will eventually be converted into helium nuclei.
Photons of light and energy from the center may take hundreds of thousands of years to work their way
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UNIT 5 Main Lecture: “How were stars formed?” through the plasma of protons and reach the surface. Then, at the surface, they pour away into the cold space surrounding the star.
Gravity gathered stars together into huge communities of billions of stars. We call these structures galaxies.
Our own galaxy, the Milky Way, contains about 100 billion stars. But ours isn’t the only galaxy. In fact, our
Universe is filled with over 100 billion of these communities of stars.
And the structures get even larger. Gravity can even gather and hold neighboring galaxies together in clusters.
Our ‘local group’ of about 30 galaxies includes the Andromeda galaxy and the ‘Magellanic clouds’, all of which can be seen with the naked eye.
Gravity can even gather and hold several clusters together to form super-clusters, which are strung out through space in huge webs. But gravity is too weak to hold together anything larger. Only at scales larger than super-clusters can we begin to see what Hubble observed: different parts of the universe moving away from each other.
We’ll see throughout this course that complexity builds upon complexity. In a sense, stars prepared the
Universe for the next steps of complexity. Most of the universe was (and still is) cold, almost empty, and from our perspective very very boring. Like camp fires in Antarctica stars create hot spots. And as we’ll see those hot spots are the key to understanding all later forms of complexity. From now on we’ll see that the goldilocks conditions for creating more complexity are found only in very special places and times and always somewhere within galaxies.
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