Which is the biggest galaxy?
One of the the largest known galaxies is IC 1101. This image was taken by the Hubble Space Telescope in 1995.
Short answer: It depends on how you measure the size of a galaxy. Alcyoneus is the largest known galaxy in size, while galaxy IC 1101 has the most stars.
Long answer: Alcyoneus measures 16 million light-years across, making it easily the largest known galaxy. This assumes that we include the jets and two giant radio lobes at either end. The true size of this galaxy was discovered in 2022, putting Alcyoneus ahead of the previous record-holder IC 1101.
IC 1101 is a supergiant elliptical galaxy. It is located 1.04 billion light-years away, in the Abell 2029 galaxy cluster.
The outer halo of IC 1101 has an enormous radius of about 2 million light-years. If the Milky Way was this big, it would extend most of the way to the Andromeda Galaxy. Some astronomers don't think that this reflects the real "size" of the galaxy though, because the outer halo contains a relatively small amount of mass compared to the inner region that has a radius of only around 200,000 light-years (still twice the size of the Milky Way).
There are probably around 100 trillion stars in IC 1101.
IC 1101 was discovered in 1790 by British astronomer Frederick William Herschel, although he didn't know what it was (this was long before astronomers understood the nature of galaxies).