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Summer Triangle
A pattern of three bright stars that stands highest in the sky during the nights of summer and early autumn. Its brightest member is Vega, in the constellation Lyra, which rises first. Deneb, the tail of Cygnus, the swan, is the northernmost point of the triangle, with Altair, in Aquila, the eagle, marking the southern point.
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Sun
The closest star to the Earth and the center of the solar system.
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Sunrise and Sunset
The point in time at which the Sun first appears in the morning sky or sets in the evening sky.There are several ways to define sunrise and sunset. They can be the points at which the Sun is bisected by the horizon (half in view, half not), or the points when the Sun has dropped below the horizon, or the points when the Sun is just out of view.You might think those last two would be the same, but they’re not. Earth’s atmosphere acts as a lens, bending the Sun’s rays, so when you see the Sun standing just atop the horizon, it’s actually below the horizon, but the atmosphere has projected an image of the Sun into view.The official timekeeper for the United States, the U.S. Naval Observatory, defines sunrise and sunset as the moments when the center of the Sun is physically 50 minutes of arc below the horizon, which is a bit less than one degree. That accounts for the size of the Sun itself, and the “bending” properties of the atmosphere. The Naval Observatory's web site provides a tool for calculating sunrise and sunset times at any point on Earth's surface.The atmosphere can bend the Sun’s rays at different angles at different times, though. So the predicted times of sunrise and sunset can be off by a minute or so. And when the Sun rises and sets at a low angle to the horizon, they can be off by several minutes.
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Sunspots and Solar Flares
A sunspot is a cooler, and therefore darker, region of the Sun’s photosphere caused by a solar magnetic disturbance. Strong, dense magnetic fields generated by circulating plasma sometimes become entangled and surge through the photosphere, creating the sunspot. The knot causes the temperature to fall to around 1000 K in the sunspot region, which darkens the photosphere. The dark center of the sunspot is called the umbra, and it is surrounded by a dim filamentary area called the penumbra. Sunspots range from Earth-size “pimples” to swollen scars halfway across the surface. Sunspot activity generally follows an 11-year cycle, called the “sunspot cycle.” A solar flare is a violent eruption of plasma from the chromosphere of the Sun that is whipped up by intense magnetic activity. During the eruption, flares rise thousands of kilometers above the chromosphere, and the plasma temperatures quickly soar to 20 million degrees. Large flares release 10^25 Joules, or about the energy of a few million volcanic eruptions on the Earth. Sunspot and solar flare frequency are strongly related. In addition, flares often disturb the Earth’s atmosphere electrically, thus interfering with radio transmissions. The aurora borealis and aurora australis are results of flare activity that injects energetic particles into Earth’s magnetic field.
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Supergiant Stars
A state of stellar evolution beyond the main-sequence life of a star. A red giant core is degenerate ionized helium, surrounded by a shell of hydrogen fusion, that expands the outer atmosphere in response to higher core temperatures. The hydrogen fusing shell eats through the surrounding atmosphere and deposits helium onto the shrinking core. The ballooning atmosphere cools and glows red; hence red giant. The Sun will become a red giant the size of Earth’s orbit in five to six billion years. Once the helium core reaches 100 million degrees, it explosively begins fusing helium. The birth of the active helium core is called the helium flash. The Sun as a red giant will fuse helium for about 2 billion years after the helium flash.
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Supernova
A supernova is a violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two primary groups: Type Ia and Type II. Type Ia supernovae form as a white dwarf “steals” hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. A Type II supernova is the final stage in the evolution of a star that is at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star’s core to collapse to form a neutron star or black hole. The star’s outer layers “rebound” violently, blasting into space at several percent of the speed of light.
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