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Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within globular star clusters. [2] Two main divisions were defined as Population I star and population II , with another newer, hypothetical division called population III added in 1978.
Additionally, astronomers have found 6 white dwarfs (stars that have exhausted all fusible hydrogen), 21 brown dwarfs, as well as 1 sub-brown dwarf, WISE 0855−0714 (possibly a rogue planet). The closest system is Alpha Centauri , with Proxima Centauri as the closest star in that system, at 4.2465 light-years from Earth.
HD 140283 (also known as the Methuselah star) is a metal-poor subgiant star about 200 light years away from the Earth in the constellation Libra, near the boundary with Ophiuchus in the Milky Way Galaxy.
It is the 2nd/3rd nearest individual star to the Solar System, and the fourth-brightest individual star in the night sky. Has one candidate exoplanet. Tau Ceti: 11.912 ± 0.007: G8V [76] 0.793 ± 0.004 [120] [121] 0.800 ± 0.008 [121] 3.5 [120] 5.68 [120] Also the 20nd nearest star system to the Solar System. Eta Cassiopeiae A (Achird) 19.42 ...
The fact that the Harvard classification of a star indicated its surface or photospheric temperature (or more precisely, its effective temperature) was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally suspected to be true. [15]
Theoretical isochrones for near-solar metallicity and a range of ages. In stellar evolution, an isochrone is a curve on the Hertzsprung-Russell diagram, representing a population of stars of the same age but with different mass. [1] The Hertzsprung-Russell diagram plots a star's luminosity against its temperature, or equivalently, its color ...
Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants.
In massive stars (greater than about 1.5 M ☉), the core temperature is above about 1.8×10 7 K, so hydrogen-to-helium fusion occurs primarily via the CNO cycle. In the CNO cycle, the energy generation rate scales as the temperature to the 15th power, whereas the rate scales as the temperature to the 4th power in the proton-proton chains. [2]