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Cepheid variables can be used to measure distances out to about 30 or 40 megaparsecs with the help of the Hubble Space Telescope. There are also two different types of Cepheid variables, and these two types have different period-luminosity relationships. Nevertheless, these stars are the best tools we have now to measure cosmic distances.

Cosmic rays are high-energy radiation, mainly originating outside the Solar System and even from distant galaxies. Upon impact with the Earth’s atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are of mysterious origin. Data from the Fermi Space Telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Active galactic nuclei probably also produce cosmic rays.

Globular clusters are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense open clusters, which are found in the disk of a galaxy.
Globular clusters are fairly common; there are about 150 to 158 currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered.

Tidal locking is the name given to the situation when an object’s orbital period matches its rotational period.

The sun certainly does rotate – it has a rotational period of about 25 days. This is apparent if you have ever used a telescope to project images of the sun onto a piece of white card so that you can see the sunspots. If you do this over several days, the sunspots appear to move across the face of the sun. Not only does the sun rotate, but it also orbits the centre of our galaxy, completing one orbit every 226 million years or so.

In one calculation, the Milky Way has a mass of about 100 billion solar masses, so it is easiest to translate that to 100 billion stars. This accounts for the stars that would be bigger or smaller than our sun, and averages them out. Other mass estimates bring the number up to 400 billion

According to NASA, the Roentgensatellit (ROSAT) was a joint German, U.S. and British X-ray astrophysics project.

ROSAT carried a German-built imaging X-ray Telescope (XRT) with three focal plane instruments: two German Position Sensitive Proportional Counters (PSPC) and the US-supplied High Resolution Imager (HRI).

A planetary nebula is a shell of material ejected from a dying star. Located about 2,000 light years from Earth in the constellation Lyra, the Ring Nebula is also known as Messier Object 57 and NGC 6720. It is one of the best examples of a planetary nebula and a favorite target of amateur astronomers

The apparent paradox that if stars are distributed evenly throughout an infinite universe, the sky should be as bright by night as by day, since more distant stars would be fainter but more numerous. This is not the case because the universe is of finite age, and the light from the more distant stars is dimmed because they are receding from the observer as the universe expands.

The angular separation of a planet from the sun or of a satellite from a planet, as seen by an observer.
Ecliptic – A great circle on the celestial sphere representing the sun’s apparent path during the year, so called because lunar and solar eclipses can only occur when the moon crosses it.
Epicycle – a small circle whose centre moves round the circumference of a larger one .

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