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Timothy Gehm
A high-mass star forms and dies quickly. These stars form from protostars in just 10,000 to 100,000 years. While on the main sequence, they are hot and blue, some 1,000 to 1 million times as luminous as the sun and are roughly 10 times wider. When they leave the main sequence, they become a bright red supergiant and eventually become hot enough to fuse carbon into heavier elements. After some 10,000 years of such fusion, the result is an iron core roughly 3,800 miles (6,000 km) wide, and since any more fusion would consume energy instead of liberating it, the star is doomed, as its nuclear radiation can no longer resist the force of gravity.
When a star reaches a mass of more than 1.4 solar masses, electron pressure cannot support the core against further collapse, according to NASA. The result is a supernova. Gravity causes the core to collapse, making the core temperature rise to nearly 18 billion degrees F (10 billion degrees C), breaking the iron down into neutrons and neutrinos. In about one second, the core shrinks to about six miles (10 km) wide and rebounds just like a rubber ball that has been squeezed, sending a shock wave through the star that causes fusion to occur in the outlying layers. The star then explodes in a so-called Type II supernova. If the remaining stellar core was less than roughly three solar masses large, it becomes a neutron star made up nearly entirely of neutrons, and rotating neutron stars that beam out detectable radio pulses are known as pulsars. If the stellar core was larger than about three solar masses, no known force can support it against its own gravitational pull, and it collapses to form a black hole.
In 1937, the first radio telescope was built, enabling astronomers to detect otherwise invisible radiation from stars. The first gamma-ray telescope launched in 1961, pioneering the study of star explosions (supernovae). Also in the 1960s, astronomers commenced infrared observations using balloon-borne telescopes, gathering information about stars and other objects based on their heat emissions; the first infrared telescope (the Infrared Astronomical Satellite) was launched in 1983.
Microwave emissions were first studied from space in 1992, with NASA's Cosmic Microwave Background Explorer (COBE) satellite. (Microwave emissions are generally used to probe the young universe's origins, but they are occasionally used to study stars.) In 1990, the first space-based optical telescope, the Hubble Space Telescope, was launched, providing the deepest, most detailed visible-light view of the universe.
Astronomers now often use constellations in the naming of stars. The International Astronomical Union, the world authority for assigning names to celestial objects, officially recognizes 88 constellations. Usually, the brightest star in a constellation has "alpha," the first letter of the Greek alphabet, as part of its scientific name. The second brightest star in a constellation is typically designated "beta," the third brightest "gamma," and so on until all the Greek letters are used, after which numerical designations follow.
Since there are so many stars in the universe, the IAU uses a different system for newfound stars. Most consist of an abbreviation that stands for either the type of star or a catalog that lists information about the star, followed by a group of symbols. For instance, PSR J1302-6350 is a pulsar, thus the PSR. The J reveals that a coordinate system known as J2000 is being used, while the 1302 and 6350 are coordinates similar to the latitude and longitude codes used on Earth.
In recent years, the IAU formalized several names for stars amid calls from the astronomical community to include the public in their naming process. The IAU formalized 14 star names in the 2015 "Name ExoWorlds" contest, taking suggestions from science and astronomy clubs around the world.
Astronomers measure star temperatures in a unit known as the kelvin, with a temperature of zero K ("absolute zero") equaling minus 273.15 degrees C, or minus 459.67 degrees F. A dark red star has a surface temperature of about 2,500 K (2,225 C and 4,040 F); a bright red star, about 3,500 K (3,225 C and 5,840 F); the sun and other yellow stars, about 5,500 K (5,225 C and 9,440 F); a blue star, about 10,000 K (9,725 C and 17,540 F) to 50,000 K (49,725 C and 89,540 F).
The surface temperature of a star depends in part on its mass and affects its brightness and color. Specifically, the luminosity of a star is proportional to temperature to the fourth power. For instance, if two stars are the same size but one is twice as hot as the other in kelvin, the former would be 16 times as luminous as the latter.
Astronomers generally measure the size of stars in terms of the radius of our sun. For instance, Alpha Centauri A has a radius of 1.05 solar radii (the plural of radius). Stars range in size from neutron stars, which can be only 12 miles (20 kilometers) wide, to supergiants roughly 1,000 times the diameter of the sun.
The size of a star affects its brightness. Specifically, luminosity is proportional to radius squared. For instance, if two stars had the same temperature, if one star was twice as wide as the other one, the former would be four times as bright as the latter.
Three generations of stars may exist based on metallicity. Astronomers have not yet discovered any of what should be the oldest generation, Population III stars born in a universe without "metals." When these stars died, they released heavy elements into the cosmos, which Population II stars incorporated relatively small amounts of. When a number of these died, they released more heavy elements, and the youngest Population I stars like our sun contain the largest amounts of heavy elements.
In the radiative zone, energy from these reactions is transported outward by radiation, like heat from a light bulb, while in the convective zone, energy is transported by the roiling hot gases, like hot air from a hairdryer. Massive stars that are more than several times the mass of the sun are convective in their cores and radiative in their outer layers, while stars comparable to the sun or less in mass are radiative in their cores and convective in their outer layers. Intermediate-mass stars of spectral type A may be radiative throughout.
To explore the stars of our universe for yourself, you can use NASA's Skymap tool. Additionally, to see images of stars taken by the Hubble Space Telescope, browse the European Space Agency's (ESA) image archive.
Starring a repository also shows appreciation to the repository maintainer for their work. Many of GitHub's repository rankings depend on the number of stars a repository has. In addition, Explore GitHub shows popular repositories based on the number of stars they have.
Most of the stars in our galaxy, including the sun, are categorized as main sequence stars. They exist in a stable state of nuclear fusion, converting hydrogen to helium and radiating x-rays. This process emits an enormous amount of energy, keeping the star hot and shining brightly.
By plotting these and other variables on a graph called the Hertzsprung-Russell diagram, astronomers can classify stars into groups. Along with main sequence and white dwarf stars, other groups include dwarfs, giants, and supergiants. Supergiants may have radii a thousand times larger than that of our own sun.
The red giant phase is actually a prelude to a star shedding its outer layers and becoming a small, dense body called a white dwarf. White dwarfs cool for billions of years. Some, if they exist as part of a binary star system, may gather excess matter from their companion stars until their surfaces explode, triggering a bright nova. Eventually all white dwarfs go dark and cease producing energy. At this point, which scientists have yet to observe, they become known as black dwarfs.
Today astronomers use constellations as guideposts for naming newly discovered stars. Constellations also continue to serve as navigational tools. In the Southern Hemisphere, for example, the famous Southern Cross constellation is used as a point of orientation. Meanwhile people in the north may rely on Polaris, or the North Star, for direction. Polaris is part of the well-known constellation Ursa Minor, which includes the famous star pattern known as the Little Dipper.
This false-color infrared image captured by the Spitzer Space Telescope shows the Henize 206 Nebula, a massive cloud of gas and dust in which hundreds and possibly thousands of new stars have formed over the last ten million years. The nebula, located just outside the Milky Way in a galaxy called the Large Magellanic Cloud, offers astrophysicists a celestial ringside seat on the death and rebirth of stars.
An infrared image of the Rosette Nebula shows super-hot O stars (blue dots inside spheres) amid a torrent of gas and dust (green and red). This star-forming nebula, which lies 5,000 light-years away in the constellation Monoceros, is named for its rosebud-like shape when seen using only optical light.
This true-color mosaic captured by the Hubble Space Telescope shows a small portion of the Orion Nebula. The image provides unprecedented detail of the nebula, revealing elongated objects oriented on the region's brightest stars, rapidly expanding plumes of material around young stars, and protoplanetary disks.
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