Only when night falls, you begin to see them again. When we look into the night sky we are seeing the light from billions of stars that are at many distances from the earth. Depending upon the distance, some of the light that is shining could have come from stars that gave off that light millions of years ago.
We are actually seeing the moment that each sun released the energy that had waited and fought to get outside of the sun and was carried through the universe to us. If we had the chance to actually travel to where that star is located, we would notice that many things have changed from the moment it released the energy until the time it traveled to reach our site.
Part of this confusion was because while all sky objects appear bright, some stars and galaxies collections of stars radiate light, while the others only reflect light. Stars are huge celestial bodies made mostly of hydrogen and helium that produce light and heat from the churning nuclear forges inside their cores. Aside from our sun, the dots of light we see in the sky are all light-years from Earth. From Earth, the Sun looks like it moves across the sky in the daytime and appears to disappear at night.
This is because the Earth is spinning towards the east. The Earth spins about its axis, an imaginary line that runs through the middle of the Earth between the North and South poles. Stars on the Main Sequence that are hotter than the Sun are also larger than the Sun.
So hot blue stars are more luminous and therefore appear higher in this diagram for two reasons: they are hotter, and hot objects are more luminous than cool objects, but they are also larger. The Earth is an example of a planet and orbits the sun, which is a star. A star is usually defined as a body of gas which is large enough and dense enough that the heat and crushing pressure at its center produces nuclear fusion. This is a fancy way of saying that it glows or burns, like our sun.
Stars that go supernova are responsible for creating many of the elements of the periodic table, including those that make up the human body. When we look into the night sky we are seeing the light from billions of stars that are at many distances from the earth.
Depending upon the distance, some of the light that is shining could have come from stars that gave off that light millions of years ago. We are actually seeing the moment that each sun released the energy that had waited and fought to get outside of the sun and was carried through the universe to us.
If we had the chance to actually travel to where that star is located, we would notice that many things have changed from the moment it released the energy until the time it traveled to reach our site.
If we were actually there and looked back, we might also see the light from our own sun, but that would be light that was sent out millions of years before. So the answer to the question as to why stars shine is really that they are a powerhouse of energy, with gigantic cores of fusion reaction that causes energy to be released and sent out into the universe as light.
Where do "young" stars get their energy from if nuclear fusion has not yet taken place? When does it finally take place? Young stars get their energy from gravity. They are slowly contracting, and as they squeeze together that generates energy which gets radiated away as light. Once the center of the star is hot and dense enough millions of degrees! It takes a star the size of our sun about 20 million years to reach this point. Once it starts nuclear fusion, the star can shine for about 10 billion years.
The first method is to look at the star's spectrum formed when we spread out the light from the star into the various colors, like a rainbow. Using special instruments, we can find dark lines in the spectrum that correspond to the elements in a star. The element lithium can be used to get an age for a star because the amount of lithium in a star decreases with time.
This is because it gets converted to other elements by nuclear reactions. So if we can measure the amount of lithium in the star, we can get its age the less lithium, the older the star. The second method is to find the age of a cluster, or group, of stars. Many stars form together in clusters, so they all have the same age. We know from our calculations that very big, massive stars burn up their nuclear fuel very fast and have short lifetimes, while smaller stars use up their fuel much more slowly and can continue giving off light for much longer.
By looking at the various stars in the cluster we can see which ones have used up their fuel and become red giants and which ones are still shining as usual. Then we can figure out that all the stars in the cluster must be the same age as the age of the stars that just recently used up their nuclear fuels.
For instance, if all the stars that are greater than three times as massive as our sun have run out of nuclear fuel and have become red giants, then we know that all the stars in the cluster are million years old. Well of course here on Earth we see the sun brighter than anything else!
That is because the sun is so much closer than the other stars. But if you could line up a bunch of stars, including our sun, all at the same distance, you would see that some stars are brighter and some are fainter than our sun.
The biggest, heaviest stars can make more energy and shine more brightly than the sun. The smaller, lighter stars make less energy and shine less brightly than our sun. So it all depends on how big and heavy a star is. It's all due to gravity. The nebula consists of gases, mostly hydrogen, and also dust. The dust is just what you would expect, tiny rocky particles. If the nebula is cold and dark, denser blobs can form in it.
Those blobs have gravity and can put surrounding gas and dust into them. As they get bigger, they have stronger gravity and can pull more and more gas and dust to them. The inner layers of gas and dust start to warm from the pressure of the gas and dust above. The dust is vaporized and turned into gas. When the inner gases get hot enough, the blob — now a protostar, or very young star — begins to glow.
At first it can be seen only in infrared light, but as it warms up and brightens it can be seen as a red star. Now the star's heat and light blows away the surrounding gas and dust, and the new star can finally be seen in the nebula. It sure can. In fact over half the stars in the sky are actually binary or triple stars, in which two or three stars formed together and are in orbit around each other.
In addition, stars tend to form in large groups within huge dark clouds of gas and dust. There can be hundreds of stars forming in these big dark clouds. People have seen the stars overhead for thousands of years. Even though they didn't know what they were or had some odd ideas about that!
The ancient Greeks said "aster" from which we get the word astronomy , while the Romans said "stella. We give stars names so we can talk about them with each other. This is just like giving people names, so you can call your classmate "John" instead of "red-haired boy with freckles wearing blue jeans.
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