The Summer Triangle comprises Deneb, Vega & Altair
Credit:
Andy Steere

Type A Stars

Sirius
Sirius, more accurately Sirius A, the Dog Star, is about twice the mass of the sun, about 1.7 times the size, and 25 times brighter.  It is the brightest star in our skies, and one of the closest at 8.6 light-years away.  It is quite young; about 200 to 300 million years old.  It is a binary system comprising Sirius A & B.  They revolve around each other every 50 years.  Sirius B is a tiny, but very dense, white dwarf about the same mass as the sun, but slightly smaller than the Earth!  Both are visible in the photograph to the left.  Note that the "rays" are artifacts, not real elements of Sirius. 

Deneb
Deneb is about 1425 light years away. It is a true giant between 108 and 114 times the size of the Sun.  If it were our star, it would reach half way to the Earth, and we would be burned to a crisp!  It weighs, perhaps, 20 times as much as the Sun, and is about 54,400 times as bright.  Deneb has stopped fusing hydrogen in its core, but it is not clear what is happening in its interior.  It may have started to expand and cool, starting to become a red supergiant, or it may have started to fuse helium.  Being so large, its life expectancy is short.  It is only ten million or so years old, having started life as a hot type "B" star; possibly even a class "O".  In a relatively short time, perhaps a million years or so, it will probably explode as a supernova.  Depending upon how much mass it sheds in the process, it may become a black hole.  18,000 years ago, its location in the sky was only 7 degrees from the north celestial pole.  It will again move to a similar position in around 8,000 years time. 

Vega
Vega is a very important star, and probably the most studied after the Sun.  Nearby, at only 25 light-years, it is spinning very rapidly.  In fact, it is spinning at 93% of the rate at which it would break up.  This results in a pronounced equatorial bulge so the polar radius is some 2.26 times that of the Sun, but the equatorial radius is 2.78 times that of the sun.  It weighs about 2.1 times the mass of the Sun, but is nearly 40 times brighter.  It is about 400 million years old, and about half way through its hydrogen burning stage; rather like the sun.  Being much heavier, it does everything much faster, so only has another 400 million or so years before it starts helium burning and becomes a red giant. 

Altair

Altair is a class A main sequence dwarf star, fusing hydrogen into helium in its core, with a temperature of about 7550 degrees Kelvin, It is about 16.8 light years away, and some 10.6 times brighter than the Sun.  It is a rapidly moving star with an apparent motion of about one degree against the background sky in about 5000 years. Altair is also a very rapid rotator. Its equatorial spin speed is 210 kilometers per second, which is nearly half its break-up speed, compared to the Sun spinning at 2 kilometers per second. With a radius around 1.7 times that of the Sun, the star rotates in only about 10 hours compared to nearly a month for the Sun.  This has caused it to flatten at its poles by about 14% of its equatorial diameter.  Altair is a Dwarf Cepheid (or "Delta Scuti" variable) star, its magnitude varying by a few tenths of a percent of a magnitude over periods that range up to 9 hours.  It is somewhat less than a billion years old. 
Astronomy & Cosmology

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Stars - Stellar Classes

WILLIAM & DEBORAH HILLYARD
Class A stars are blue-white to white, and are relatively common in the local neighborhood.  Generally, they weigh between 1.4 and 2.1 times the mass of the Sun, but Deneb, for example, shows how much they can vary from the norm!  
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