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Bartholomeus Jan "Bart" Bok (April 28, 1906 – August 5, 1983) was a Dutch-American astronomer, teacher, and lecturer.He is best known for his work on the structure and evolution of the Milky Way galaxy, and for the discovery of Bok globules, which are small, densely dark clouds of interstellar gas and dust that can be seen silhouetted against brighter backgrounds.
Eta Ursae Minoris is about one billion years old and has an estimated 1.35 times the mass of the Sun. [7] It has a high rate of spin with a projected rotational velocity of 84.8 km/s. [ 3 ] These coordinates mark a source of X-ray emission with a luminosity of 11.5 × 10 28 erg s −1 . [ 6 ]
The age of the Earth (actually the Solar System) was first accurately measured around 1955 by Clair Patterson at 4.55 billion years, [10] essentially identical to the modern value. For H 0 ~ 75 (km/s)/Mpc, the inverse of H 0 is 13.0 billion years; so after 1958 the Big Bang model age was comfortably older than the Earth.
In physical cosmology, the age of the universe is the time elapsed since the Big Bang: 13.8 billion years. [1] Astronomers have two different approaches to determine the age of the universe . One is based on a particle physics model of the early universe called Lambda-CDM , matched to measurements of the distant, and thus old features, like the ...
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One billion years may be called an eon in astronomy or geology. Previously in British English (but not in American English), the word "billion" referred exclusively to a million millions (1,000,000,000,000). However, this is not common anymore, and the word has been used to mean one thousand million (1,000,000,000) for several decades. [5]
Since the universe has been expanding for 13.8 billion years, the comoving distance (radius) is now about 46.6 billion light-years. Thus, volume ( 4 / 3 πr 3 ) equals 3.58 × 10 80 m 3 and the mass of ordinary matter equals density ( 4.08 × 10 −28 kg/m 3 ) times volume ( 3.58 × 10 80 m 3 ) or 1.46 × 10 53 kg .
The age of the oldest known stars approaches the age of the universe, about 13.8 billion years. Some of these are among the first stars from reionization (the stellar dawn), ending the Dark Ages about 370,000 years after the Big Bang. [1] This list includes stars older than 12 billion years, or about 87% of the age of the universe.