Astro Quest

Transcription

1 Astro Quest Study Material Class 5th and 6th Scientific Development and Research Organisation, New Delhi

2 1 Topic: The Stars When we talk about the Universe, we usually think of a few things: planets, moons, black holes. Yet, one thing stands out, so central to our understanding of space. Stars! Stars are massive balls of plasma found throughout much of outer space. As we ll see, they are incredibly important to the structure of the Universe. They re also intensely interesting (and hot)! What are Stars Made of? The thing that makes up stars is truly out of this world. It s called plasma, and we rarely find it on earth. Plasma is a clump of atoms whose electrons have been taken from them. This makes it kind of like a gas. In fact, you ll probably hear people say that the Sun is made out of gas. Yet only the outer parts of a star contain any. What else is made of plasma, for reference? On Earth, we find it naturally in lightning. Otherwise, it s mostly found in man-made things. The simplest are neon signs. These signs energize electrons on neon atoms, pulling them away to form plasma!

3 2 You might ask: Why is plasma so rare? In reality, it s not. Plasma is the most common state of matter in the universe. But, it only exists in extreme heat. That s something that stars can easily provide, but which is quite rare on Earth. How do Stars Form? Our discussion of plasma raises another question. How do these high energy fireballs (stars) get there in the first place? Well, star formation is quite a complicated subject. We can t go over it all in this article, but you can read more about it here. That said, here are the basics. Stars need some sort of material to form. This comes from what are called nebulae, or dense clouds of gas in space. Sometimes, gas in these nebulae clumps up. This creates a point with high gravity, kind of like how a dense planet has strong gravity. The newly formed center of gravity will start to attract all of the gas particles around it. This happens faster and faster until we have what s called a collapse. All at once, lots of things get sucked inwards! Boom! Most of the matter flies back outwards. Yet some of it remains clumped together tightly, forming a protostar. Around the protostar, nebular matter forms a disk. The stuff in the disk will keep trying to collapse over and over again.

4 3 The protostar will push back, but every time it grows a little. Many collapses later, our star becomes really dense and heavy. At this point, it can begin doing fusion, and we consider it a real star. How Many Stars are There? When you look up into the night sky, how many stars can you see? 100, 500, perhaps even 1000? No matter how many you might find, it s only a tiny fraction of how many there are in the Universe. To be clear, scientists aren t totally sure about the exact number. But, their best guess is let s give this number some space: 700,000,000,000,000,000,000,000 Stars! Given how huge stars are, that s an unbelievable figure! Our sun is only a small star, and one of many, many others. Lucky for us, there s always a lot to look for up in space. Why Do Stars Twinkle? Twinkle, twinkle little star Oh, we re still learning!? Shux. Well, on that subject, let s talk about why stars twinkle. Or rather, what makes them look like they do. That s to say, stars don t really twinkle. The thing we call twinkling is actually an effect of our atmosphere. Air in the sky is actually made up of winds blowing in many different directions. Their interaction creates what is called turbulence. This is when the air gets rough. You might ve felt it if you ve ever been on a plane. In essence, turbulence is responsible for twinkling. As the light from stars passes through rough air, it gets distorted. The distortion depends upon how turbulent the wind is. Differences in distortion are what make stars look like they re twinkling. Light waves come to us constantly, but they re almost always changed in some way by our atmosphere. Stars and Their Planets

5 4 Stars and their planets are found all over the Universe. Together, a star and all of its planets are called a solar system. The Earth is part of a solar system with the Sun and seven other planets. In a solar system, planets circle around a central star because of its strong gravity. Heliocentrism vs. Geocentrism Today we know that in a solar system, the planets orbit around a star. That means that the star is the center of the system. In our solar system, for example, the Earth and all the other planets circle around the Sun. This concept is called heliocentrism. However, scientists didn t always know this. Hundreds of years ago, people believed in a theory called geocentrism. This theory stated that the Earth was the center of the universe. People believed that the Sun, the planets, and the Moon all revolved around the Earth. In 1543 a scientist named Nicolaus Copernicus challenged the geocentric theory and argued that the Sun was the center of the solar system. Other scientists, like Galileo Galilei, also agreed with this idea. The Catholic Church, which was very powerful in Europe at this time, fought against the heliocentric theory. The Bible supported the idea that the Earth was the center of the Universe. Therefore, the Church felt that heliocentrism was a threat to their religion. Because of this, the Church had Galileo arrested for his claims! Even though the Church fought against heliocentrism, scientific evidence has proven it to be true. The Earth circles around the Sun in our solar system just like planets all across the Universe circle around their own suns. Planetary Orbits Now that you know that planets orbit stars in solar systems, it s time to learn more about their orbits. You might think that a planet orbits a sun in a perfect circle. But that s not true! Planets move around their sun in the shape of an ellipse. An ellipse is like a circle that has been stretched out a little bit in one direction. This is caused by the forces of gravity.

6 5 The elliptical orbits of planets were discovered by a scientist named Kepler in The idea that all planets orbit their sun in the shape of an ellipse is called Kepler s First Law of Planetary Motion. Does the Sun Move? We ve just talked about how the planets move around the sun. But you might be wondering: does the sun also move? The answer is yes! Not only does the sun move, but our entire solar system is constantly moving. Our solar system is part of a galaxy called the Milky Way. The sun with the rest of our solar system orbits around the center of the Milky Way Rotation vs. Revolution The movement of planets around a sun is called revolution. However, revolution is not the only kind of movement that planets experience. Planets, including the Earth, also have rotation. Have you ever seen someone spin a basketball on their finger? Even though the ball is staying in one place, it is rotating around its central axis. That movement is called rotation. As planets move around their sun, they also spin around their own axis. This rotation can have important effects. On Earth, for example, a full rotation takes 24 hours. The rotation of the Earth causes day and night. When a place on Earth is facing the Sun, it is daytime at that place. Then, when the Earth rotates and that place is facing away from the Sun, it is nighttime. Galaxies Galaxies are huge groups of stars orbiting a central point. Almost always, that point is a supermassive black hole. Its pull keeps stars clustered into a galaxy which is central to the structure of the Universe.

7 6 How are Galaxies Formed? Whenever we see something spectacular mountains, planets, etc. our first question is how it got there. For galaxies, the answer is pretty simple: gravity. But to understand why this is our answer, we actually have to go back in time. Most galaxies are already well-formed. So, into the time machine we go. Early Universe Matter Clusters Together Welcome to the early Universe! Enjoy your time here, although there s not much to do. Really, there s not much to see either. Sure, there s a ton of matter, but it s not concentrated. It exists mostly in the form of dense clouds, called nebulae.

8 7 Admittedly, this place is kind of boring. But, let s fast forward a bit. Over time, cloudy nebulae began to cluster around central points with strong gravity. As more things arrived at these centers, their gravity got even stronger. Soon, they became enormous! So enormous, in fact, that all the stuff would collapse into a black hole! This would act as a more stable center of gravity, allowing things to orbit from a distance. And so, we have our galaxy. Stars Form But what about all the things in it? At the same time as the galaxies were beginning to take shape, stars started to appear slowly. Yet, they weren t like the ones we recognize (at first). Those like our sun formed late, and on the outer part of galaxies. In the Milky Way, that means in the spiral arms. Types of Galaxies There are two main categories of galaxies. The kind we know best are spiral galaxies (the Milky Way is one). Spiral The main feature of spiral galaxies is, as their name suggests, their spiral shape. At their center is a very bright circle with tons of old stars. Yet, their spiral arms are full of young and forming ones. These stars undergo drastic changes as they grow, generating a lot of light. This is actually what makes the spiral arms of a galaxy visible. Elliptical The second kind of galaxy is one we re maybe less familiar with. It s shaped like an elliptical, or a stretched-out circle. The stars in

9 8 it orbit the center in random directions. Among them are actually some of the oldest stars in existence. Notably, elliptical galaxies can be enormous. The largest, IC 1101, is 6 million light years across. That means that it takes light 6 million years to get from one side of IC 1101 to the other. How Many Galaxies in the Universe Now, I d hate to throw another big number at you. But to answer this question, I have to. That s because there are at least 200 billion galaxies in our Universe. 200 billion! We can hardly get out of our own atmosphere and there s that much else to explore. This should cause us to wonder: What might we still discover? The overwhelming size of the Universe means that it s full of opportunity. Who knows, maybe one day you will explore it! If that sounds interesting, consider becoming an astronomer or an astronaut. Although, whether or not you do, the future is bright. Space holds so much for us to see. Colliding Galaxies In fact, it holds SO much that those things often collide. Galaxies are no exception. Our very own galaxy is set to collide with another (the Andromeda Galaxy).

10 9 But relax, not any time soon! This collision won t happen for another 4 billion years. Still, other galaxies are crashing into each other all the time. When they do, they might merge to form an even larger galaxy. The result is almost always elliptical which is actually what allows elliptical galaxies to be so large. They just keep adding more and more stars through repeated collisions. The Properties of Stars The Universe is full of stars. But not all are the same, there are many different types of stars. If you look more closely at them you will see differences. The first difference we notice is that some stars shine brighter than others. Sometimes that is because some stars are farther away than others. Now some stars are brighter to us because they are so close. Other stars may appear brighter because of the amount or kind of light they give off Color and Temperature

11 10 To the human eye, all stars look white. But they aren t. In fact, stars are many colors. If you look at them through a special telescope or take pictures with a special camera you can see their colors. Some appear white, red, orange, or blue. For a long time, scientists didn t know why this happened. Stars are hot, dense balls of gas. But it gets a little more complicated than that. They have atmospheres, thin layers of gas above the inner layer. These gases absorb light at different wavelengths depending on the elements in them. This results in the star to containing a few gaps where different elements absorb different colors. Nowadays, we use a system invented by Cecelia Payne-Gaposchkin to describe these spectra. She showed the spectrum of a star depended on the temperature of the elements in their

12 11 atmosphere. This discovery allowed scientists to understand a star s composition and so much more. For a long time, we believed that stars had the same composition as the Earth. Cecelia Payne- Gaposchkin s discovery shows that stars are actually mostly hydrogen. Helium is the second most abundant element in stars. This classification system arranges stars by their temperature. As we said earlier cool stars are more red and hotter stars are more blue. Stars come in almost every color except green. This is because of the way we see color.. You see, if a star gives off a green color it will also give off red, blue and orange. Our eyes mix those colors together to form other colors. Even if the star emits green more than any other color we will still only see it as white. Our own Sun actually gives off more green light than any other color but our eyes only see white. Solar Luminosity You can measure how bright a star appears to be in a telescope. Then, by using the distance you can calculate how much energy its actually giving off. This is the star s luminosity. Faint light can look bright if its nearby but so does a very bright light that is far away. By knowing the distance you can correct for that and figure out how luminous they actually are. This is the key to understanding stars. Solar luminosity is a unit of radiant flux, the power given off in the form of photons. Astronomers use this measure of units to understand how much luminosity a star has. The luminosity of a star depends on its size and temperature. If two stars are the same size but one is

13 12 hotter, the hotter one will be more luminous. If two stars are the same temperature but one is bigger, the bigger one will be more luminous. Solar Mass Massive stars and low mass stars age differently. Our sun is a pretty low mass star. Let s clarify what we mean when we say massive. Commonly, when we say massive we are referring to something very big. The words big or large refers to the physical size of an object. In this situation, massive means its an object that has a lot of mass. Generally, it s referring to weight. For example a star can be small in size but can be massive. Meaning it has a great deal of mass, it is very dense. Mass tells everything about a star from what kind of fusion process can occur to its life cycle. The mass of a star is also the hardest thing to measure. Normally the bounds on star mass are from 8% of the sun s mass to a hundred times the size of the sun.

14 13 Younger Types of Stars Take a look up at the night sky! You should see a lot of small, shining stars. Notice, though, that not all stars look the same. Our most important star is actually the Sun, which doesn t look like a normal star at all! There are two main reasons why stars look different. The first is that stars are different distances from the Earth. The second is that there is not one, but many different types of stars! Stars are different in color, brightness, and even age! Here, you will learn about some younger types of stars in our universe. Stars and The Spectral Classes Stars are categorized into 7 spectral classes: O, B, A, F, G, K, M. There s an easy mnemonic that you can use to remember the star classes: Oh Boy, An F Grade Kills Me! The spectral classes are chosen based on a couple of different properties. A few main properties are color, mass, luminosity, and temperature. For example, O stars are the hottest and the brightest stars in the galaxy! Spectral classes are different from types of stars, and are used to better divide different star types. Star s Life Cycles Stars are not immune to the effects of time. Like humans, stars can age! Where the star is in its life cycle is very important. The life cycle of a star interacts with its size. This means that depending on the star s solar mass, the star will go through different stages in its life cycle!

15 14 For stars that are on the smaller size, the star life cycle looks like this. Our Sun is one of these smaller stars! Protostar Main Sequence Red Giant White Dwarf Black Dwarf For stars that are bigger, the star life cycle is a little different: Protostar Main Sequence Red Supergiant Supernova Neutron Star OR Black Hole Younger Star Types Younger star types are main sequence stars and all the types of stars before that in the star life cycle. Here is a breakdown of younger star types and their characteristics! Protostars A protostar is basically the beginning form of a star! A protostar starts off as a huge cloud of molecular gas (within a nebula). As gravity develops, the cloud collapses down into a protostar. The heat inside a protostar comes from gravitational energy, not nuclear fusion like in main sequence stars. The protostar phase lasts around 100,000 years a long time for us, but a very short time for stars! T Tauri Stars This is the next stage of star evolution after the protostar stage! A T Tauri s heat comes from the gravitational pressure that holds the star together. In other aspects, T Tauri stars are more similar to main sequence stars! They tend to be as hot as main sequence stars, but can be bigger in size.

16 15 Brown Dwarfs Brown dwarfs are a little like undeveloped stars. Imagine that you stopped growing after 1st grade! This is kind of what happens to brown dwarfs they never get their growth spurt. The dwarf category in general includes stars that are smaller in size. Brown dwarfs, however, are even smaller. Brown dwarfs are pretty dim as stars go. In the beginning, they may glow red the lowest level of luminosity there is. Afterwards, they cool down very quickly. Brown dwarfs are very difficult to see, and are probably too small for you to find in the night sky! Main Sequence Stars Stars spend the longest time in the main sequence phase around 90% of their life cycle. About 90% of the stars in our universe are in the main sequence right now! Main sequence stars undergo a special process called nuclear fusion. During nuclear fusion, a main sequence star takes two atoms of hydrogen and fuses them into one helium atom in its core. Nuclear fusion releases heat. This is how the main sequence stars get so hot and bright! Main sequence stars contain several subcategories of stars. Red dwarfs are the smallest type of main sequence stars. They aren t very hot and bright, but they actually have a longer life than other main sequence stars! Red dwarfs are the most common stars in the galaxy. For stars that are older than main sequence stars, head on over to Older Types of Stars! Older Types of Stars People like to joke that time doesn t actually exist. But we all feel the effects of time. Even celestial bodies age, let alone us humans! Stars, in particular, go through lots of different stages as they get older. In their life cycles, stars that go beyond the main sequence are older than most! Here, you will finish learning about all the different star types: from younger to older stars!

17 16 Spectral Classes Primer You ve probably already been introduced to spectral classes in some earlier articles. If you want a memory nudge, though, here s some basic information about spectral classes! Stars are categorized into 7 spectral classes: O, B, A, F, G, K, M. There s an easy mnemonic that you can use to remember the star classes: Oh Boy, An F Grade Kills Me! The spectral classes divide stars into categories based on properties like mass, luminosity, temperature, and color. Spectral classes are different from star types and are used to make it easier to categorize types of stars. Older Types of Stars Here is a list of older types of stars in our universe! Red Giants: When main sequence stars get old, they go through some important changes! Stars that are around the size of our sun become red giants. Instead of nuclear fusion, red giants burn helium to produce oxygen and carbon. Red giants are not as hot on the surface as they once were, but they expand a lot in size! Even though red giants tend to have a low mass, they are still pretty bright. Blue Giants: Blue giants are very big, very bright, and very hot blue stars. Like red giants, blue giants are also post main sequence stars. Blue giants burn helium to create heat energy. But because of how hot they are, blue giants burn out very quickly.

18 17 Supergiants and Hypergiants: Supergiants and hypergiants are some of the largest stars in our universe. While supergiants are extremely big, hypergiants are even more so! Main sequence stars that are a lot bigger than our Sun later develop into supergiants. Supergiants burn helium as their fuel. When they re done, they start shedding their outer layers and explode in supernovas. Hypergiants are the biggest stars of all, but are very unstable. White dwarfs: After giant stars have burned out completely, they leave behind the cores. These cores are known as white dwarfs, or dead stars. White dwarfs are extremely dense and very, very hot. But they aren t very bright or big at all. White dwarfs remain hot for very long periods of time. Black dwarfs: Black dwarfs are actually still hypothetical. This means that their existence hasn t been fully proven! Astrology experts think that when a white dwarf cools down completely, a black dwarf remains. However, it takes very long for a white dwarf s heat to go fully away. Our Universe isn t actually old enough for black dwarfs to exist yet!

19 18 Neutron stars and Black holes: Neutron stars are the leftovers of supergiants. After supergiants explode in supernovas, they either become neutron stars or black holes. Neutron stars are called this because the cores of these stars are made mostly of neutrons. Neutrons are the neutral parts of an atom. These stars are extremely dense. They often exist as pulsars, or rapidly spinning neutron stars. Black holes are not quite stars. You ve probably seen a black hole if you like watching space movies! Black holes have the incredible yet scary power to suck everything in even light! This is because of their extreme gravity. As an black holes are one of the most dangerous astrological phenomenon.

20 19 Credit: Noah Louis-Ferdinand. References: 1. The University of Oregon on Star Formation: 2. Fun Facts about Stars by Ducksters: 3. Even More on Star Formation: 4. The ESA on Black Holes: 5. Even More Black Hole Facts: 6. NASA Simple Explanation: