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Astronomy is a natural science that deals with the study of celestial objects (such as stars, planets, comets, nebulae, star clusters and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion (physics) of celestial objects, as well as the formation and development of the universe. Astronomy does not concentrate on the origin of the universe but it usually studies the aspects of the celestial objects in it. If gravity were very slightly weaker, or electromagnetism very slightly stronger, (or the electron slightly less massive relative to the proton), all stars would be red dwarfs(au73)

In late 1994, however, a version of the MACHO theory was dealt a blow when the Hubble Space Telescope, scanning the Milky Way galaxy for red dwarf stars, found fewer of these dim stars than expected(be151)

In the 1920s, observations at Mount Wilson and elsewhere began to convince astronomers that those nebulas were not simply clouds of gas in our own Milky Way galaxy, as had been thought, but rather were galaxies in their own right(bhtw167)

Radio galaxies emit radio waves not only from their giant double lobes, one on each side of the central galaxy, but also from the core of central galaxy itself. Perhaps a single engine in the galaxy's core was responsible for all the galaxy's radio waves? Such electromagnetic beams cannot penetrate through the galaxy's interstellar gas, no matter how hard they try. Hot, magnetized gas rather than electromagnetic waves, though, this kind of gas jet would do the job(bhtw343)

In a radio galaxy, by contrast with a quasar, the central accretion disk presumably is rather quiescent(bhtw353)

It is still possible to explain all the observed properties of radio galaxies and quasars using an alternative, non-black hole engine: a rapidly spinning, magnetized, supermassive star, weighing millions or billions of times as much as the Sun- a type star that has never been seen by astronomers, but that theory suggests might form at the centers of galaxies(bhtw353b)

Although the youngest of radio galaxies and quasars might be powered by supermassive stars, older ones are almost certainly powered, instead, by gigantic holes(bhtw354)

We cannot predict whether the Earth and Sun will wind up, ultimately, inside the galaxy's central hole, or will be flung out of the galaxy(bhtw356)

Estimates place the age of the sun at a little greater than that of the Earth, which accords well with current astronomical theories that the solar system formed together as a single unit(gnp11)

If the mass of the halo of dark matter is ten times the mass of all the bright stars put together, it stabilizes the spiral pattern and makes it last much longer, before eventually changing its shape to form a bar across the center of the galaxy(itb205)

Since the galaxy is about 100,000 light-years in diameter, it would thus take about 600,000 years to colonize the entire Milky Way galaxy(pi55)

Pulsar observations showed no trace of the variations you would expect if were actually coming from a planet in orbit around a star(uet86)

Although calculations of the pulsation periods of white dwarfs had been carried out by theorists in 1966, the basic periods they came up with were no lower than eight seconds, a little too big to explain the pulsars. On the other hand, even a simple calculation showed that neutron stars would vibrate with periods much shorter than those of the first pulsars discovered, around a few thousandths of a second(uet87)

Pulsars are indeed neutron stars(uet89)

The pulsars we see are relatively young and active neutron stars, and as they age they slow down and radiate less energy, eventually fading away into invisibility(uet112)

Astronomy is the oldest of the sciences, and the contemplation of the heavens, with their periodic regularities, gave men their first conceptions of natural law(hk23)

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