Stars
Constellations
Ancient Greeks, Romans, and other early cultures observed patterns of stars in the sky called constellations and imagined that they represented mythological characters, animals, or familiar objects.
From Earth, a constellation looks like spots of light arranged in a particular shape against the dark night sky. It also shows how stars in the constellation have no relationship to each other in space.
Stars in the sky can be found at specific locations within a constellation. For example, you can find the star Betelgeuse (BEE tul jooz) in the shoulder of the mighty hunter Orion. Orion's faithful companion is his dog, Canis Major. Sirius, the brightest star visible from the northern hemisphere, is in Canis Major.
Ancient Greeks, Romans, and other early cultures observed patterns of stars in the sky called constellations and imagined that they represented mythological characters, animals, or familiar objects.
From Earth, a constellation looks like spots of light arranged in a particular shape against the dark night sky. It also shows how stars in the constellation have no relationship to each other in space.
Stars in the sky can be found at specific locations within a constellation. For example, you can find the star Betelgeuse (BEE tul jooz) in the shoulder of the mighty hunter Orion. Orion's faithful companion is his dog, Canis Major. Sirius, the brightest star visible from the northern hemisphere, is in Canis Major.
Stars (Absolute and Apparent Magnitudes)
When you look at constellations, you'll notice that some stars are brighter than other. For example, Sirius looks much brighter than Rigel. Is Sirius a brighter star or is it just closer to Earth, making it appear to be brighter? As it turns out, Sirius is 100 times closer to Earth than Rigel is. If Sirius and Rigel were the same distance from Earth, Rigel would appear much brighter in the night sky than Sirius would.
When you refer to the brightness of a star, you can refer to its absolute magnitude or its apparent magnitude. The absolute magnitude of a star is measure of the amount of light it gives off. A measure of the amount of light received on Earth is called the apparent. A star that's rather dim can appear bright in the sky if it is close to EArth, and a sstar that bright can appear dim if it is far away. If two stars are the same distance away, what might cause one of them to be brighter than the other?
When you refer to the brightness of a star, you can refer to its absolute magnitude or its apparent magnitude. The absolute magnitude of a star is measure of the amount of light it gives off. A measure of the amount of light received on Earth is called the apparent. A star that's rather dim can appear bright in the sky if it is close to EArth, and a sstar that bright can appear dim if it is far away. If two stars are the same distance away, what might cause one of them to be brighter than the other?
Measurements in Space
How do scientists determmine distances from the solar system that Earth is part of? One way is to measure its parallax-the apparent shift in the position of an object when viewed from two different positions. Extend your arm and look at your thumb first with your left eye closed and then with your right eye closed. Your thumb appears to change position with respect to the backhround. Now do the same experiment with you thumb closer to your face. What do you observe? The nearer the object is to the observer, the greater its parallax is.
Astronomers can measure the parallax of relatively close stars to determine their distances from Earth. Knowing the aangle that the stars's position changes and the size of Earth's orbit, astronomers can calculate the distance of the star from Earth.
Because space is so vast, a special unit of measure is needed to record distances. Distances between stars and galaxies are measured in light-years. A light-year is the distance that light traavels in one year. Light travels at 300,000 km/s, or about 9.5 trillion km in one year. the nearest star to Earth, other than the Sun, is Proxima Centauri. Proxima Centaurii is a mere 4.3 light-years away, or about 40 trillion km.
Astronomers can measure the parallax of relatively close stars to determine their distances from Earth. Knowing the aangle that the stars's position changes and the size of Earth's orbit, astronomers can calculate the distance of the star from Earth.
Because space is so vast, a special unit of measure is needed to record distances. Distances between stars and galaxies are measured in light-years. A light-year is the distance that light traavels in one year. Light travels at 300,000 km/s, or about 9.5 trillion km in one year. the nearest star to Earth, other than the Sun, is Proxima Centauri. Proxima Centaurii is a mere 4.3 light-years away, or about 40 trillion km.
Evolution of Stars
Classifying Stars
When you look at the night sky, all stars might appear to be similar, but they are quite different. Like people, they vary in age and size, but stars also vary in temperature.
In the early 1900s, Ejnar Hertzsprung and Henry Russell made some improtant observations. They noticed that in general, stars with higher temperatures also have brighter absolute magnitudes.
Hertzsprung and Russell developed a graph to show this relationship. They placed temperatures across the bottom and absolute magnitudes up one side. Agraph that shows the relationship of a star's temperature to its absolute magnitude is called a Hertzsprung-Russell (H-R) diagram.
The Main Sequence
As you can see, stars seem to fit into specific areas of the graph. Most stars fit into a diagonal band that runs from the upper left to the lower right of the chart. this band, called the main sequence, contains hot, blue, bright stars in the upper left and cool, red, dim stars in the lower right. Yellow, main sequence stars, like the Sun, fall in between.
When you look at the night sky, all stars might appear to be similar, but they are quite different. Like people, they vary in age and size, but stars also vary in temperature.
In the early 1900s, Ejnar Hertzsprung and Henry Russell made some improtant observations. They noticed that in general, stars with higher temperatures also have brighter absolute magnitudes.
Hertzsprung and Russell developed a graph to show this relationship. They placed temperatures across the bottom and absolute magnitudes up one side. Agraph that shows the relationship of a star's temperature to its absolute magnitude is called a Hertzsprung-Russell (H-R) diagram.
The Main Sequence
As you can see, stars seem to fit into specific areas of the graph. Most stars fit into a diagonal band that runs from the upper left to the lower right of the chart. this band, called the main sequence, contains hot, blue, bright stars in the upper left and cool, red, dim stars in the lower right. Yellow, main sequence stars, like the Sun, fall in between.
Dwarfs and Giants
About 90 percent of all stars are main sequence stars. Most of these are small, red stars found in the lower right of the H-R duagram. Among main sequence stars, the hottest stars generate the most light and the coolest ones generate the least. What about the tem percent of stars that are not part of the main sequence? Some of these stars are hot but not bright. These small stars are located on the lower left of the H-R diagram and are called white dwarfs. Other stars are right of the H-R diagram are called giants, or red giants because they are usually red in color. The largest are called supergiants.
About 90 percent of all stars are main sequence stars. Most of these are small, red stars found in the lower right of the H-R duagram. Among main sequence stars, the hottest stars generate the most light and the coolest ones generate the least. What about the tem percent of stars that are not part of the main sequence? Some of these stars are hot but not bright. These small stars are located on the lower left of the H-R diagram and are called white dwarfs. Other stars are right of the H-R diagram are called giants, or red giants because they are usually red in color. The largest are called supergiants.
How do stars shine?
When the H-R diagram was developed, scientists didn't know what caused stars to shine. Hertzsprung and Russell developed their diagram without knowing what produced the light and heat of stars.
For centuries, people were puzzled by the questions of what stars were made of and how they produced light. Many people had estimated that Earth was only a few thousand years old. The Sun could have been made of coal and shined for that long. However, when people realized that Earth was much older, they wondered what material possibly could burn for so many years. Early in the twentieth century, scientists began to understand the process that keeps stars shining for billions of years.
Generating Energy
For centuries, people were puzzled by the questions of what stars were made of and how they produced light. Many people had estimated that Earth was only a few thousand years old. The Sun could have been made of coal and shined for that long. However, when people realized that Earth was much older, they wondered what material possibly could burn for so many years. Early in the twentieth century, scientists began to understand the process that keeps stars shining for billions of years.
Generating Energy