We know how bright a star looks, but to know how bright it really is, you have to know how far away it is: is it like a headlight a mile away or an airport beacon 10 miles away? In the dark of the nighttime sky with no reference points, it's pretty hard to tell.
Technical advances, such as the introduction of charge-coupled devices to replace photographic plates for the measuring of stellar distances and brightnesses, are making our observations more secure. How exactly does the energy get from the center of the star, where it is generated, to the surface, where it becomes the light that we see? How important is convection as a means of transporting energy, and how efficient is the convection?
The answer to these questions has some effect on the inferred relationship between mass and surface temperature. Just how much oxygen is in the stars, along with the hydrogen and helium? The relative amount of oxygen present has a modest effect on the efficiency of the central furnace, affecting the relation between mass and brightness and, hence, age. According to our best available estimates, stars having about 90 percent of the sun's mass are just now starting to die in the globulars. These stars are most probably around 15 billion years old, but they could conceivably be as young as 12 billion years or as old as 18 billion years.
It is very unlikely that most of them could be either younger or older than this range. This estimate is already accurate enough to place some very interesting limits on the age and life history of the universe. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. Given that the Universe is only The luminosity of the star is the energy released per unit time. For main sequence stars, the energy comes from hydrogen fusion and we have:. This is the stage our Sun is at right now. As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts.
The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. In the core of the red giant, helium fuses into carbon. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take from there.
The life cycle of a low mass star left oval and a high mass star right oval. The illustration above compares the different evolutionary paths low-mass stars like our Sun and high-mass stars take after the red giant phase. For low-mass stars left hand side , after the helium has fused into carbon, the core collapses again.
As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. While the sun will spend about 10 billion years on the main sequence, a star 10 times as massive will stick around for only 20 million years. A red dwarf , which is half as massive as the sun, can last 80 to billion years, which is far longer than the universe's age of This long lifetime is one reason red dwarfs are considered to be good sources for planets hosting life , because they are stable for such a long time.
More than 2, years ago, the Greek astronomer Hipparchus was the first to make a catalog of stars according to their brightness , according to Dave Rothstein, who participated in Cornell University's "Ask An Astronomer" website in In the early 20th century, astronomers realized that the mass of a star is related to its luminosity , or how much light it produces. These are both related to the stellar temperature.
Stars 10 times as massive as the sun shine more than a thousand times as much. The mass and luminosity of a star also relate to its color. More massive stars are hotter and bluer, while less massive stars are cooler and have a reddish appearance. The sun falls in between the spectrum, given it a more yellowish appearance. This understanding lead to the creation of a plot known as the Hertzsprung-Russell H-R diagram, a graph of stars based on their brightness and color which in turn shows their temperature.
Most stars lie on a line known as the "main sequence," which runs from the top left where hot stars are brighter to the bottom right where cool stars tend to be dimmer.
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