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.

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

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. Most stars will live for billions of years by fusing hydrogen into helium. But slowly, as stars run out of fuel, they will pass through several phases and eventually die out.

Features

: NASA's Hubble Space Telescope has discovered a new population of stars isolated deep in the core of M13 star cluster. The stars are among the hottest stars observed in the core of a globular cluster.(Courtesy of G. De Marchi (STScI and Univ. of Florence, Italy) and F. Paresce (STScI)/NASA, ESA)

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 4000 degrees Fahrenheit, yellow are hotter (around 10,000 F), and the blue are the hottest (over 70,000 F).

The size of a star can be 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 can be measured in comparison to the Sun. The least massive stars are only 1/12 the Sun's mass while the most massive stars can be nearly 100 times the Sun's mass.

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.

Missions

: Chandra X-Ray Observatory (Courtesy of NASA)

Despite his best telescope-building efforts, Galileo was never able magnify stars so that they appeared to be more than tiny points of light in his telescope. From this "failure," Galileo correctly reasoned that the stars must be much farther away than anyone had previously thought. In fact, even with modern telescopes, only a handful of stars have been observed as something larger than points of light.

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.

Myths, Stories, and More

: Ursa Major, the Great Bear,as published in a set of constellation cards published in London c. 1825.

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". Patters like "The Big Dipper" are often called asterisms.

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

Earth Matters

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

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 can 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 occurred 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 participate 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.