Stars
H-R Diagram
The Hertzsprung-Russel (H-R) Diagram is a classification/comparison chart for stars. Using the surface temperature and absolute magnitude (brightness or luminosity), the stars fall into distinct groups. It turns out that these groups correspond to the life cycle of stars. We will discuss stellar evolution next, but let's examine the H-R Diagram more closely. You will see that the stars come in different colors and that these colors are related to the surface temperature. The temperature portion of the diagram or graph is slightly backward from regular graphs. The lower temperatures are on the right and higher temperatures are on the left. Moving from right to left, the stars get hotter. Moving from right to left, the colors change from red (the coolest) to orange to yellow (the Sun) to white to blue-white and to finally blue (the hottest). This is also where you find the class of the star. Moving from left to right, the star classes are O, B, A, F, G, K, and M (mnemonic device: Oh, Be A Fine Girl. Kiss Me.). Most stars are class M stars. As you move up the graph from bottom to top, the brightness of the stars goes from dim to very bright. The categories of stars thus fall among these two criteria. Most stars lie in what is called the Main Sequence. We also have Giant stars that are bright, but not very hot. Supergiants are very bright (they're also really big) and sometimes hotter. White dwarfs are hot, but dim. We can learn a lot about a star just by plotting its surface temperature and brightness.
H-R Diagram simulation
The Hertzsprung-Russel (H-R) Diagram is a classification/comparison chart for stars. Using the surface temperature and absolute magnitude (brightness or luminosity), the stars fall into distinct groups. It turns out that these groups correspond to the life cycle of stars. We will discuss stellar evolution next, but let's examine the H-R Diagram more closely. You will see that the stars come in different colors and that these colors are related to the surface temperature. The temperature portion of the diagram or graph is slightly backward from regular graphs. The lower temperatures are on the right and higher temperatures are on the left. Moving from right to left, the stars get hotter. Moving from right to left, the colors change from red (the coolest) to orange to yellow (the Sun) to white to blue-white and to finally blue (the hottest). This is also where you find the class of the star. Moving from left to right, the star classes are O, B, A, F, G, K, and M (mnemonic device: Oh, Be A Fine Girl. Kiss Me.). Most stars are class M stars. As you move up the graph from bottom to top, the brightness of the stars goes from dim to very bright. The categories of stars thus fall among these two criteria. Most stars lie in what is called the Main Sequence. We also have Giant stars that are bright, but not very hot. Supergiants are very bright (they're also really big) and sometimes hotter. White dwarfs are hot, but dim. We can learn a lot about a star just by plotting its surface temperature and brightness.
H-R Diagram simulation
Stellar Evolution
Stars are born and stars die. What happens in between and at the end will vary depending on the mass of the star. All stars start as a nebula which is a cloud of gas and dust. Remember from the Nebular Theory that this cloud will collapse due to gravity. Why it collapses could be because of a supernova shockwave or the gravitational attraction of a slightly denser area of dust or something else. Either way, the nebula collapses (protostar) and eventually sparks nuclear fusion. A star is born. It is when the star begins nuclear fusion that it enters the Main Sequence. It will stay on the Main Sequence as long as it is fusing atoms in its core. Hydrogen fuses into helium which then fuses into carbon, then oxygen, then every element until iron. When iron is fused, it does not release energy. So, in the bigger stars, iron keeps building up while the star gets cooler and cooler. In low mass stars, they don't get to that point.
When almost all of the hydrogen, the core will collapse, raising the temperature to the point where helium will begin fusing. The heat energy from helium fusion pushes the outer layers of the star further from the core, making the star get bigger. Being far away from the core, the outer layers will cool and turn red. We have a red giant. The Sun will become a red giant someday (about 4 billion years from now). It burns through the helium fast and the core collapses more and starts fusing carbon. It moves through these last stages quickly. Eventually, the energy from the core throws the outer layers away from the star creating a planetary nebula (another horribly descriptive name given to us by William Herschel...it has nothing to do with planets). At the center of the planetary nebula is a small, white star: a white dwarf. This the leftover core from the star. A star with the mass of the Sun will turn into a white dwarf about the size of the Earth. No longer creating energy, it's very slowly cooling off. This process may take a few dozen billion years or up to 100 billion years...we don't really know. So, the Sun will not blow up. But the Earth will have an end. At the red giant stage, the Sun will most likely be big enough to overtake Earth's orbit and, thus, obliterating the planet.
A high mass star will do everything above (except, of course the planetary nebula and white dwarf steps) but make it all the way to iron. With the iron building up and the core collapsing more and more, the outer layers push further and further out. This is a supergiant. A supergiant placed in our Solar System would have a diameter that would take it past Jupiter's orbit. The iron builds up more and more and the core finally completely collapses. The energy and heat created by the total collapse pushes out in a massive explosion called a supernova. The supernova produces enough energy to fuse iron into all of the other elements known in nature. The leftovers continue collapsing. If the mass is on the low side, the atoms in the core will collapse until nothing but neutrons remain: a neutron star. These super-dense stars spin very rapidly and send out beams of energy which makes it look like a stellar lighthouse. We call them pulsars. If the mass is high, the gravitational collapse overtakes any atomic force and crushes everything down into a singularity: a black hole. Black holes are said to have such high gravity due to the incredibly high mass and density that not even light can escape it. But as atoms are pulled down into the black hole, they smash into each other and release x-rays. We can see x-rays. So, we can see evidence of black holes. Research is constantly coming out about black holes that modify our understanding of these objects. But the closest one is very far away, so we'll probably never visit one.
Stars are born and stars die. What happens in between and at the end will vary depending on the mass of the star. All stars start as a nebula which is a cloud of gas and dust. Remember from the Nebular Theory that this cloud will collapse due to gravity. Why it collapses could be because of a supernova shockwave or the gravitational attraction of a slightly denser area of dust or something else. Either way, the nebula collapses (protostar) and eventually sparks nuclear fusion. A star is born. It is when the star begins nuclear fusion that it enters the Main Sequence. It will stay on the Main Sequence as long as it is fusing atoms in its core. Hydrogen fuses into helium which then fuses into carbon, then oxygen, then every element until iron. When iron is fused, it does not release energy. So, in the bigger stars, iron keeps building up while the star gets cooler and cooler. In low mass stars, they don't get to that point.
When almost all of the hydrogen, the core will collapse, raising the temperature to the point where helium will begin fusing. The heat energy from helium fusion pushes the outer layers of the star further from the core, making the star get bigger. Being far away from the core, the outer layers will cool and turn red. We have a red giant. The Sun will become a red giant someday (about 4 billion years from now). It burns through the helium fast and the core collapses more and starts fusing carbon. It moves through these last stages quickly. Eventually, the energy from the core throws the outer layers away from the star creating a planetary nebula (another horribly descriptive name given to us by William Herschel...it has nothing to do with planets). At the center of the planetary nebula is a small, white star: a white dwarf. This the leftover core from the star. A star with the mass of the Sun will turn into a white dwarf about the size of the Earth. No longer creating energy, it's very slowly cooling off. This process may take a few dozen billion years or up to 100 billion years...we don't really know. So, the Sun will not blow up. But the Earth will have an end. At the red giant stage, the Sun will most likely be big enough to overtake Earth's orbit and, thus, obliterating the planet.
A high mass star will do everything above (except, of course the planetary nebula and white dwarf steps) but make it all the way to iron. With the iron building up and the core collapsing more and more, the outer layers push further and further out. This is a supergiant. A supergiant placed in our Solar System would have a diameter that would take it past Jupiter's orbit. The iron builds up more and more and the core finally completely collapses. The energy and heat created by the total collapse pushes out in a massive explosion called a supernova. The supernova produces enough energy to fuse iron into all of the other elements known in nature. The leftovers continue collapsing. If the mass is on the low side, the atoms in the core will collapse until nothing but neutrons remain: a neutron star. These super-dense stars spin very rapidly and send out beams of energy which makes it look like a stellar lighthouse. We call them pulsars. If the mass is high, the gravitational collapse overtakes any atomic force and crushes everything down into a singularity: a black hole. Black holes are said to have such high gravity due to the incredibly high mass and density that not even light can escape it. But as atoms are pulled down into the black hole, they smash into each other and release x-rays. We can see x-rays. So, we can see evidence of black holes. Research is constantly coming out about black holes that modify our understanding of these objects. But the closest one is very far away, so we'll probably never visit one.
Galaxies
A large collection of stars, solar systems, planets, nebulae, and other celestial objects all orbiting a central area is known as a galaxy. There are three types of galaxies: irregular, elliptical, and spiral. Irregular galaxies have no defined shape and that may indicate its youth. Elliptical galaxies are just that: elliptical in shape. An ellipse is an oval, so they have a lens shape to them. Spiral galaxies may be the oldest of galaxies. The Milky Way (our galaxy) is a spiral galaxy. Actually, it's a specific type of spiral galaxy called a barred-spiral. From the central portion of the galaxy, two arms extend and then the spirals come off these arms. The central portion of almost every galaxy is a supermassive black hole. These behemoths of gravity hold billions of stars and everything with them in orbit. However, recent calculations of the speeds at which galaxies spin suggest that some other material might be keeping the galaxy together. The speed at which the outer arms of a spiral galaxy move are enough to tear the galaxy apart, but that does not happen. Astrophysicists believe it may be due to something called dark matter that keeps the galaxy in one piece.
A large collection of stars, solar systems, planets, nebulae, and other celestial objects all orbiting a central area is known as a galaxy. There are three types of galaxies: irregular, elliptical, and spiral. Irregular galaxies have no defined shape and that may indicate its youth. Elliptical galaxies are just that: elliptical in shape. An ellipse is an oval, so they have a lens shape to them. Spiral galaxies may be the oldest of galaxies. The Milky Way (our galaxy) is a spiral galaxy. Actually, it's a specific type of spiral galaxy called a barred-spiral. From the central portion of the galaxy, two arms extend and then the spirals come off these arms. The central portion of almost every galaxy is a supermassive black hole. These behemoths of gravity hold billions of stars and everything with them in orbit. However, recent calculations of the speeds at which galaxies spin suggest that some other material might be keeping the galaxy together. The speed at which the outer arms of a spiral galaxy move are enough to tear the galaxy apart, but that does not happen. Astrophysicists believe it may be due to something called dark matter that keeps the galaxy in one piece.
Constellations
Constellations are pictures in the sky made by an imaginative stellar dot-to-dot in an attempt to create order in the night sky. I admit, I have a hard time with constellations. I am great with patterns and identifying patterns, so I can see many of the 88 officially recognized constellations that are visible in my area of the world. However, I have a hard time bringing my imagination to the level of the ancients who took a pentagon of stars and saw a man holding a baby goat while driving a chariot. That kind of leap of imagination is too much for my logical mind. Some constellations, I admit, are easy to see the picture the ancients thought the stars made. But most are absurd at best. Take a look at this website to get an idea of some of the constellations with their corresponding images.
Constellations are pictures in the sky made by an imaginative stellar dot-to-dot in an attempt to create order in the night sky. I admit, I have a hard time with constellations. I am great with patterns and identifying patterns, so I can see many of the 88 officially recognized constellations that are visible in my area of the world. However, I have a hard time bringing my imagination to the level of the ancients who took a pentagon of stars and saw a man holding a baby goat while driving a chariot. That kind of leap of imagination is too much for my logical mind. Some constellations, I admit, are easy to see the picture the ancients thought the stars made. But most are absurd at best. Take a look at this website to get an idea of some of the constellations with their corresponding images.