O-type stars are rare but luminous, so they are easy to detect and there are a number of naked eye examples.
The rarest type of star is the type O star. It is the largest of the main sequence stars. In the entire milky way galaxy there are estimated to only be 20,000 of these stars or one in 20,00,000.
M stars are the coolest stars. They are so cool (2000-3000K) that molecules, including water, carbon monoxide, Vanadium Oxide (used in sunblock) and Titanium oxide (the base of white paint) are visible.
According to Science Alert, researchers recently confirmed their discovery of a white dwarf pulsar 773 light years away from Earth, and they are understandably excited because this is officially one of the rarest stars in the entire galaxy. What is this?
Located almost a billion light-years away, IC 1101 is the single largest galaxy that has ever been found in the observable universe. Just how large is it? At its largest point, this galaxy extends about 2 million light-years from its core, and it has a mass of about 100 trillion stars.
Of the various types of exotic star proposed, the most well evidenced and understood is the quark star. Objects dense enough to trap any emitted light, yet are not actually black holes, are called dark stars, however the same name is used for hypothetical ancient "stars" which derived energy from dark matter.
And some are even older than life. The oldest star in the known universe is the Methuselah star, also known as HD 140283, a subgiant star. Methuselah is located in the constellation Libra, close to the Milky Way galaxy's Ophiuchus border, and around 190 light-years away from the Earth.
Quick Answer: The dimmest stars are called brown dwarfs, and they are orange or red when they are the hottest, and acquire a magenta color as they cool. Brown dwarfs are proof that star formation does not always result in a traditional star. But this is not to be confused with planets being too small to become stars.
Supergiant Stars:
The largest stars in the Universe are supergiant stars. Giants and supergiants form when a star runs out of hydrogen and begins burning helium.
The Sun is a class G star; these are yellow, with surface temperatures of 5,000–6,000 K. Class K stars are yellow to orange, at about 3,500–5,000 K, and M stars are red, at about 3,000 K, with titanium oxide prominent in their spectra.
A Class M star is the type that consists of some of the dimmest of the main sequence stars. Such main sequence Class M stars are known as red dwarfs, though the category can also include very late Class K stars that may be nearly convective and early Class L stars able to fuse hydrogen.
A star's class is determined by its surface temperature, which we see as its color. G stars are in the middle of the temperature scale, so they shine yellow to yellow-white. Most class G stars are in the prime of life — a span that puts them on the main sequence.
The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye, all within the Milky Way galaxy.
Only 1-in-10,000 galaxies fall into the rarest category of all: ring galaxies.
This extraordinary bubble, glowing like the ghost of a star in the haunting darkness of space, may appear supernatural and mysterious, but it is a familiar astronomical object: a planetary nebula, the remnants of a dying star.
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen (1H) into helium in their cores, unlike a main-sequence star.
Algol is a variable star
The early stargazers surely knew about its changing brightness. This probably led them to name the strangely behaving star in the sky for a mythological demon. There are many variable stars known throughout the heavens, but Algol might well be the most famous of them all.
A star's death throes have so violently disrupted its planetary system that the dead star left behind, called a white dwarf, is siphoning off debris from both the system's inner and outer reaches.
NASA's Hubble Space Telescope snapped a view of what may be the youngest galaxy ever seen. This "late bloomer" may not have begun active star formation until about 13 billion years after the Big Bang. Called I Zwicky 18 [below, left], the galaxy may be as young as 500 million years old.
The red dwarf stars are considered the smallest stars known, and representative of the smallest star possible.
Explanation: What's the most dangerous star near earth? Many believe it's Eta Carinae, a binary star system about 100 times the mass of the Sun, just 10,000 light years from earth.
Alpha Crucis, also called Acrux, brightest star in the southern constellation Crux (the Southern Cross) and the 13th brightest star in the sky.
According to a new study, a star discovered 75 light-years away is no warmer than a freshly brewed cup of coffee. Dubbed CFBDSIR 1458 10b, the star is what's called a brown dwarf.