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Stars
Quick Facts
 
Closest Star to the Earth: Sun
 
Example of a Red Star: Antares
 
Example of a Blue Star: Rigel
 
Hottest Star: Blue Star
 
Coolest Star: Red
 
Color of Our Sun: Yellow
 
Oldest Known Star: Yellow
HE 1523
Stars

Twinkle, twinkle, little star. How I wonder what you are. Well, wonder no more! Stars are large balls of extremely hot gas or, more accurately, hot gas and a gas-like substance called plasma. They are primarily composed of the two most common elements in the universe: hydrogen and helium. The "twinkle" comes from turbulence, or movement, in our atmosphere. It's similar to watching an object from the bottom of the swimming pool; the ripples and movement of the water distort what you see on land. In space, outside of our atmosphere, the stars don't twinkle.

Stars form within clouds of gas, dust, and debris, known as molecular clouds. Read more about the conditions for star formation on our nebulae page. Over the course of billions of years, they pass through several phases and eventually die.

Image at right: White dwarf stars (courtesy of NASA and H. Richer, University of British Columbia).

DID YOU KNOW?
Every second, the Sun radiates enough energy to power the entire United States for millions of years.

Cluster of blue stars (Courtesy of NASA)
Features

There are five main characteristics that scientists use to classify stars: brightness, color, surface temperature, size, and mass.

Brightness is described in magnitudes or luminosity. The current scale of magnitude is based on a system invented by the Greek astronomer Hipparchus in approximately 125 B.C. His scale ranged from one (the brightest stars) to six (the faintest). Today, the scale can dip into negative numbers for very bright stars and upwards of twenty for very faint stars.

Even without a telescope, you can see differences in the colors of stars. They range in color from red to yellow to blue. The color is determined by surface temperature. Red stars are the coolest at around 4,040 degrees Fahrenheit, yellow are hotter, and blue are the hottest.

The size of a star is measured in terms of the Sun's radius. So the Sun is one solar radius, while Antares, for example, is 776 solar radii. The Sun's radius is 432,000 miles. Likewise, the mass of a star is measured in comparison to the Sun.

Did you know that most stars are born with "siblings"? Lots of stars are born in clusters! Thanks to modern telescopes that can look into clouds of gas and dust, scientists have discovered that most stars are born alongside other stars, forming a "Star Cluster." Scientists think that our own star, the Sun, was born in a cluster of stars. Our Sun has no companions today, but scientists have collected evidence that it once was part of a cluster with stars that were far more massive than the Sun. Many star clusters do not stay gravitationally bound throughout their entire lifetimes because of interactions between the stars and dusty clouds out of which they were born.

Some Star Clusters do stay bound together, however. Globular Star Clusters are an example of star clusters that do stay bound together. Globular Star Clusters are ball-shaped groups of old stars that contain hundreds of thousands of members! That's a lot of stars!

Learn more about clusters of stars on NASA's IYA website >>


Chandra X-Ray Observatory (Courtesy of NASA)
Missions

While a number of missions have been sent to study the Sun, we haven't yet sent any missions to other stars. However, missions like the Chandra X-Ray Observatory are expanding our knowledge of the stars from afar.

Launched in the middle of 1999, Chandra was placed outside of the Earth's atmosphere to study x-rays from high-energy regions of the universe, for example, the remains of exploded stars. The observatory is named in honor of Subrahmanyan Chandrasekhar, the late Indian-American Nobel laureate who is widely regarded as one of the top astrophysicists of the twentieth century.

Learn more about Chandra at the mission website.


Ursa Major (Courtesy of NASA)
Myths, Stories, and More

For millennia, humans have been inspired to create stories about the stars. Constellations, the stories of the stars, are numerous and as varied as the cultures on Earth.

One of the most well known constellations is Ursa Major or "Great Bear" in Latin. The back of the "Bear" is often called "The Big Dipper". Unofficial names like these are called, asterisms. Other asterisms for the Great Bear are Plough, Butcher's Cleaver, Charles' Wain (wagon), Northern Dipper, and the Casserole.

Learn how to find the constellations at NASA's Space Place website.

The view from space - Hubble Space Telescope multicolor image of the inner 36 light years of M13. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
The view from space - Hubble Space Telescope multicolor image of the inner 36 light years of the Hercules Globular Cluster. [Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]
Earth Matters

Studying stars can tell us a lot about how life formed on Earth. Stars create the key ingredients for life such as carbon, nitrogen, oxygen, and iron. Exploding stars, or supernovae, cast these materials into space and cause new stars to form.

In May of 2008, Chandra's X-Ray Observatory and the National Radio Astronomy Observatory's Very Large Array, discovered the most recent supernova in our galaxy. The supernova explosion occured about 140 years ago. Before that the most recent known explosion was thought to of happened in 1680. Discoveries like these can help us understand how often supernovae explode in the Milky Way Galaxy.

If you don't have access to an observatory, you can partcipate in star viewing with your naked eye or with a pair of binoculars. One the brightest and closest globular clusters is the Hercules Globular Cluster. With a pair of binoculars you can easily see the Hercules Globular Cluster, which is 25,000 light years away and 13 billion years old! It is located in the constellation Hercules. On a clear night, in a dark place, it will look something like a fuzzy patch of light in the sky.

Learn more about how to view the Hercules Globular Cluster on NASA's IYA website >>

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