Altair

Altair is a brilliant star located in the constellation Aquila. It is the brightest star in that constellation and the twelfth-brightest star in the night sky.

Astronomers call it Alpha Aquilae, which is its Bayer designation. It is also a key part of the Summer Triangle. This is a famous group of stars that includes Deneb and Vega. Altair is an A-type main-sequence star. This means it is in a stable stage of its life. It currently sits inside the G-cloud. This is a nearby interstellar cloud made of gas and dust.

One of the most amazing things about Altair is how it moves. The star rotates incredibly fast. At its equator, it spins at a velocity of about 286 kilometers per second. This speed is a large fraction of its estimated breakup speed of 400 kilometers per second. Because it spins so quickly, the star is not a perfect sphere. Instead, it is oblate, which means it is flattened at the poles. The middle of the star bulges out significantly.

In fact, its equatorial diameter is over 20 percent larger than its polar diameter. This makes the star look more like a squashed ball than a round one.

This rapid rotation causes several other interesting physical effects. One major effect is known as gravity darkening, or the von Zeipel effect. Because the star spins so fast, its surface gravity and effective temperature are lower at the equator. This makes the equator less bright than the poles. Scientists confirmed this phenomenon using the Navy Precision Optical Interferometer in 2001. Other studies in 2004 and 2006 also analyzed these temperature differences. Additionally, Altair is a Delta Scuti variable star. This means its brightness fluctuates slightly. In 1999, the Wide Field Infrared Explorer found these changes happen in periods of less than two hours.

Humans have studied Altair for many centuries. The name Altair is an abbreviation of the Arabic phrase Al-Nasr Al-Tair. This translates to "the flying eagle." In 2016, the International Astronomical Union organized a group to standardize star names. They officially included Altair in their catalog. For a long time, people have looked at this star and seen different things. Ancient Babylonians and Sumerians likely called it "the eagle star." In medieval Europe, people used astrolabes to map the sky. Some even drew Altair and Vega as birds on these instruments.

Many different cultures have beautiful stories about Altair. In China, it is known as the Cowherd Star. It is part of a legend about a man named Niulang. He is separated from his wife, Zhinü, by the Milky Way. They can only meet once a year when magpies form a bridge. In Japan, the star is called Hikoboshi. The Koori people of Victoria saw it as Bunjil, the wedge-tailed eagle. In Micronesia, it was called Mai-lapa, meaning "big breadfruit." These stories show how much the star has inspired people throughout history.

Scientists have used advanced technology to see Altair more clearly. In the 1960s, R. Hanbury Brown measured its diameter at 3 milliarcseconds. Later, the Palomar Testbed Interferometer showed its flattened shape in 2001. This was a major discovery for understanding stellar shapes. In 2006 and 2007, researchers used the CHARA array to do something even more special. They produced the first-ever image of the surface of a main-sequence star other than our Sun.

This was a massive achievement for the field of astronomy.

Comparing Altair to our Sun helps us understand how different stars can be. Altair is much larger and more powerful. It has about 1.8 times the mass of the Sun. It is also 11 times more luminous, meaning it shines much brighter. While the Sun takes about 25 days to rotate once, Altair rotates in under eight hours. Its equatorial radius is about 2.03 solar radii. Its polar radius is much smaller at 1.63 solar radii. This massive difference in size shows the incredible power of stellar rotation. Studying Altair helps scientists learn how speed and gravity work together in the universe.