If you want REALLY big, look no further than VY Canis Majoris; a red hyper-giant.  It is thought to be between 1,800 & 2,100 times the size of the sun, but only up to about 30 to 40 times its mass.  As it is nearly 5,000 light-years away, it is difficult to be more exact.  If it were to replace the sun, it would reach beyond the orbit of Jupiter, almost to Saturn; Earth would certainly be engulfed!  It started life on the main sequence as a type "O" star, and hs evolved into a red giant between type M3e Ia and M5e Ia.  In the image to the left, VY Canis Majoris ejects huge amounts of gas during its outbursts. Credit: NASA, ESA, and R. Humphreys (University of Minnesota).  The star does not have much longer to live, and will explode in a supernova, or possibly a hypernova depending on its mass, and become a neutron star or a black hole.  The image to the right shows a comparison between the size of VY Canis Majoris, and our Sun. 
Type M stars are the most common type of stars.  Astronomers define them by their optical spectrum which tends to be dominated by absorption bands of titanium oxide and vanadium oxide.  While most of the stars in this class are main sequence and red dwarf types, it also includes a number of giant stars as well as some super-giants & hyper-giants; including VY Canis Majoris the largest known star. 

Type M Stars

Betelgeuse (Alpha Orionis) is a red super-giant, type M2Iab, being about 1,000 times the diameter of the sun, about 20 times its' weight, and about 100,000 times as bright.  It is about 650 light-years away from the sun.  At about 8.5 to 10 million years old, it is old for it's type, and between 1993 and 2009, its diameter shrank by around 15% from 1,185 to 995 stellar radii.  This means that the surface of the star is collapsing at approximately 1,000 km (625 miles) per hour, but all this is highly dependent on the accuracy of the distance and size estimates.  Remember the heavier the star, the quicker it burns out.  Even though Betelgeuse is only about ten million years old, it has finished burning all its hydrogen and moved off the main sequence.  It is now fusing helium into carbon and oxygen.  It will then start to fuse neon, magnesium, sodium, and silicon all the way to iron. Soon after this, with no nuclear reactions at the center to resist gravity, the core collapses, causing the star to blow up as a supernova, resulting in a neutron star.  It is likely to go super-nova some time in the next, say, million years, but it could be within a thousand years.  When it does go, it will be spectacular as it will outshine the moon for several months.  The star is one of the "shoulders" in the well known constellation Orion, as shown in the image to the left, so after it goes supernova, a familiar constellation will change forever!  The right hand picture is an actual image of Betelgeuse taken in UV light by the Hubble Space Telescope. 
Mira is a red super giant star that was estimated in the past to be about 220 light-years away.  The latest data from Hipparcos puts it closer to about 420 light years away.  It is a binary system comprising the type M7 IIIe red supergiant Mira, and a type K companion, Mira B, that is estimated at about 70% the mass of the Sun.  Mira itself weighs only about 18% more than the Sun, but is around 350 times its diameter.  Mira is an oscillating variable star of the Mira Variables type, for which it is the prototype.  All of the six to seven thousand known Mira variables are red giants, weighing much the same as the Sun, whose surfaces oscillate resulting in marked increases and decreases in brightness over periods between about 80 to 1,000+ days.  After the sun becomes a red giant, it will probably become a Mira Variable.  Being so large, but light weight, the star easily looses its outer envelope to a strong solar wind, so the Mira Variable stage lasts for only a million years or so.  A planetary nebula then forms  around the star from the ejected matter, while what remains of the star collapses into a white dwarf.  The image to the right shows the "tail" of matter trailing from Mira's envelope that is 13 light-years long.  Mira travels at high speed, some 130 km/second, through space and creates a "bow wave" as it interacts with interstellar gas and dust from which material is stripped to form the tail. 
NASA/JPL - Caltech
Mira
VY Canis Majoris
Betelgeuse
Barnard's Star is a very dim red dwarf, type M4Ve, only around 6 light-years from the sun.  It is only about 18% the mass of the sun, and about 0.04% of the sun's luminosity.  Around 10 billion years old, it may be one of the oldest stars in the universe, and is considered an intermediate Population 2 star.  It has a very large proper motion relative to the sun, moving laterally at around 65 miles/sec.  It is also approaching the Sun at nearly 90 miles/sec.  Its closest approach will be 3.8 light-years in a little less than 10,000 years.  Click to see a series of photographs taken between 1985 and 2005, showing how it has moved in our sky relative to the other stars we see. 
Barnard's Star
Astronomy & Cosmology

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Stars - Stellar Classes

Kapteyn's Star
Kapteyn's Star is a BY Draconis variable type M1V star between 25% and 33% the size and mass of the Sun.  It has the second highest known proper motion after Barnard's Star, and orbits the Milky Way in retrograde.  It is one of 14 stars moving in a rough group, all of which are thought to have originated in the globular cluster Omega Centauri, which is now about 16,000 light years away, while  Kapteyn's Star is less than 13 light years away. 
WILLIAM & DEBORAH HILLYARD
Mira bow wave and tail
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