Types of Stars in the Galaxy

Astronomers classify stars according to the Morgan-Keenan system, which categorizes them by surface temperature, from hottest (O) to coolest (M). They may be further subdivided by temperature and brightness. The hottest stars are also the most luminous and the largest, but are far less abundant than the coolest stars. Hot stars also have shorter lives: Class M stars, or red dwarfs, can theoretically last for some 200 billion years, but those of Class O have a lifetime of “just” 10 million years.

Class O
Rare, hot stars (30,000 kelvin) around 100,000 times brighter than our Sun.
Class B
A lifetime of c. 100 million years. Surface temperatures reach c. 15,000 kelvin.
Class A
Temperatures reach 10,000 kelvin, with a mass around twice that of our Sun.

Most common type of star

By far the most numerous class of star is the red dwarf, which accounts for around 80% of all stars in our local neighborhood. These are weak, dim stars with no more than 40% of the mass of the Sun, and the brightest of them shine at only 10% of the Sun’s luminosity. Because they burn their fuel
so slowly, they have lifespans of at least 10 billion years.

Largest star with a planet

Astronomers using the Hobby-Eberly Telescope at the McDonald Observatory in Texas, USA, announced in Oct 2011 that they had discovered a planet orbiting HD 96127, a star around 540 parsecs from Earth in the constellation Ursa Major. It is a K2-type red giant, whose radius is around 35 times that of the Sun. The orbiting planet is believed to possess a mass
four times that of Jupiter, and 1,271 times that of Earth.

Oldest star in the galaxy

HD 140283, some 190 light years from Earth, is believed to have formed soon after the Big Bang, 13.8 billion years ago. When the universe formed, it consisted of hydrogen with some helium. As it evolved, the other chemical elements appeared, formed by nuclear synthesis in stars. HD 140283 has almost no metal content, so must have formed from clouds of gas comprising almost pure hydrogen and helium, when the universe was still
in its infancy.

The oldest stars to form planets are BP Piscium and TYCHO 4144 329 2, both c. 400 million years old. In Jan 2008, astronomers announced their discovery of what seem to be dusty discs around these stars. Such discs formed the planets orbiting the Sun, but are usually seen around recently
formed stars.

These two stars probably threw out stellar material to form the discs after swallowing another star or an earlier system of planets.

Fastest star in the galaxy

On 8 Feb 2005, a team from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA, announced their discovery of a star travelling at more than 2.4 million km/h (1.5 million mph). Named
SDSS J090745.0+24507, it was probably accelerated by an encounter with the supermassive black hole at the centre of our Milky Way galaxy slightly less than 80 million years ago.

The fastest-moving star in the sky is Barnard’s star, which moves at 10,357.704 mas/yr (milliarcseconds/year). To put that in perspective, an arcminute is just one-sixtieth of an angle of one degree. In 170 years, Barnard’s star moves a distance in the sky equivalent to the size of a full Moon. This shift in the apparent positions of stars in the sky is termed “proper motion”.

Most stars in a star system

The greatest number of stars in a single star system is six. There are several known examples, but the best known is Castor, the second-brightest star in the constellation of Gemini. From a distance, Castor appears as a single entity, but it actually consists of three pairs of stars.

Smallest stars

Neutron stars, which may have a mass three times that of the Sun, only have a diameter of 10–30 km (6–19 mi). By contrast, the Sun’s average diameter is 864,000 mi (1,390,473 km). Neutron stars are also the densest stars in the universe. A sand-grain size of neutron star material would have the mass of a skyscraper.

Largest star

UY Scuti has an estimated radius 1,708 times that of our Sun. This corresponds to a diameter of 2,276,511,200 km (1,414,558,478 mi), but owing to the difficulty in measuring the sizes of stars accurately, the margin of error could be as high as +/- 192 solar radii. The inset above shows the Sun in relation to UY Scuti.

Flattest star

The least spherical known star in our galaxy is the southern star Achernar (Alpha Eridani). Observations made using the VLT Interferometer at the European Southern Observatory’s Paranal Observatory in Atacama, Chile, have revealed that Achernar is spinning so rapidly that its equatorial diameter is more than 50% greater than its polar diameter.

Most luminous star in the galaxy

A “Wolf-Rayet star” is a very hot and bright star in its early stages. WR 25 (seen near the centre of the image above) is a Wolf-Rayet star in the Trumpler 16 cluster within the Carina Nebula, some 7,500 light years from Earth. The presence of a companion star makes it difficult to ascertain the absolute luminosity of WR 25, but it is believed to be around 6,300,000 times more luminous than the Sun.

Class F
Surface temperatures of up to 7,400 kelvin. Live for c. 3 billion years.
Class G
Includes our Sun. Temperatures may reach 6,000 kelvin.
Class K
Surface temperature of c. 5,000 kelvin. May live for more than 20 billion years.
Class M
Most common and, at c. 3,000 kelvin, coolest of the main star types.

What about our Sun? – Read here

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